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Advanced Fiber Materials

, Volume 1, Issue 2, pp 101–125 | Cite as

Perovskite Solar Fibers: Current Status, Issues and Challenges

  • Andrew Balilonda
  • Qian Li
  • Mike Tebyetekerwa
  • Rogers Tusiime
  • Hui Zhang
  • Rajan Jose
  • Fatemeh ZabihiEmail author
  • Shengyuan YangEmail author
  • Seeram Ramakrishna
  • Meifang Zhu
Review
  • 713 Downloads

Abstract

Perovskite-based solar cells with high power conversion efficiencies (PCEs) are currently being demonstrated in solid-state device designs. Their elevated performances can possibly be attained with different non-standard geometries, for example, the fiber-shaped perovskite solar cells, in the light of careful design and engineering. Fiber-shaped solar cells are promising in smart textiles energy harvesting towards next-generation electronic applications and devices. They can be made with facile process and at low cost. Recently, fiber-shaped perovskite solar devices have been reported, particularly with the focus on the proof-of-concept in such non-traditional architectures. In this line, there are so many technical aspects which need to be addressed, if these photovoltaic (PV) cells are to be industrialized and produced massively. Herein, a well-organized and comprehensive discussion about the reported devices in this arena is presented. The challenges that need to be addressed, the possible solutions and the probable applications of these PV cells are also discussed. More still, the perovskite fiber-shaped PV cells with other fiber PV devices reported in literature in terms of their scope, characteristic designs, performances, and other technical considerations have been summarised.

Keywords

Fiber-shaped solar cells Perovskites Photovoltaics Smart textiles Fiber materials 

Introduction

As the world’s population gradually increases, the global energy consumption similarly rises at almost the same stride. This implies that traditional energy sources such as fossil fuels may not suffice in the near future. As a solution, the existing traditional energy sources need to be either supported or replaced with clean, nearly inexhaustible, renewable and sustainable sources [1]. In the search for these sources, solar energy seems to be the best alternative. This stems from the fact that it is supported by the sunlight, which is readily available. The conversion process is also clean as it does not involve any moving parts and emission of greenhouse gases. Table 1 summarizes the current and the predicted future global energy consumption [2]. Such results have triggered the observations we see in Fig. 1.
Table 1

Current global energy statistics and future projections (

adapted from Ref. [2])

Quantity

Definition

Units

2001*

2050

2100

N

Population

B persons

6.145

9.4

10.4

GDP

GDP§

T $/year

46

140

284

GDP/N

Per capita GDP

$/(person-year)

7,470

14,850

27,320

Ė/GDP

Energy intensity

W/($year)

0.294

0.20

0.15

Ė

Energy consumption rate

TW

13.5

27.6

43.0

C/E

Carbon intensity

KgC/(W·year)

0.49

0.40

0.31

Ċ

Carbon emission rate

GtC/year

6.57

11.0

13.3

Ċ

Equivalent CO2 emission rate

GtCO2/year

24.07

40.3

48.8

*Ė = (403.9 Quads/year) · (33.4 GWyear/Quad) · (10−3 TW/GW) = 13.5 TW; and Ċ (24.072 GtCO2/year) · (12/44 GtC/GtCO2) = 6.565 GtC (as adapted from Energy Information Administration (2005) Annual Energy Outlook (US Dept of Energy, Washington, DC))

Ė = (869 EJ/year) × (106 TJ/EJ)/(60 × 60 × 24 × 365 s/year) = 27.5 TW [adapted from Ref. [3] (Scenario B2), pp. 48–55]

Ė = (1,357 EJ/year) × (106 TJ/EJ)/(60 × 60 × 24 × 365 s/year) = 43.0 TW [adapted from Ref. [3] (Scenario B2), pp. 48–55]

§All in year 2000 U.S. dollars, using the inflation-adjusted conversions: $2000 = 1/0.81590 $1990 [as adapted from Energy Information Administration (2005) Annual Energy Outlook (US Dept of Energy, Washington, DC)], and ‘purchasing power parity’ exchange rates

In year 2000 US dollars: (113.9 T$1990) × (1/0.81590 $2000/$1990) = 139.6 T$2000

In year 2000 US dollars: (231.8 T$1990) × (1/0.81590 $2000/$1990) = 284.1 T$2000

Fig. 1

Energy prospects and design. a The remarkable global growth in PV as compared to other technologies (adapted from “Australia’s renewable energy industry is delivering rapid and deep emissions cuts” available freely from ANU at http://re100.eng.anu.edu.au/). b Global solar cell/modules production for the period between 2010 to 2018 (Reprinted/adapted from Ref. [4]. Copyright The Author, some rights reserved; exclusive licensee EDP Sciences. Distributed under a creative commons Attribution-NonCommercial License 4.0 (CC BY-NC) http://creativecommons.org/licenses/by-nc/4.0/), c Trend of progression of PCE values of fiber-based PSCs (summarized from references [5, 6, 7, 8, 9, 10]). d Smart garment connected to several different portable electronics powered by perovskite fiber solar devices. (Reproduced with permission [11]. Copyright 2019, The Royal Society of Chemistry), included is a zoomed out image of a woven device structure revealing the design of several fibrous solar yarns. (Reproduced with permission [7]. Copyright 2015, The Royal Society of Chemistry)

The sun emits huge solar energy to the earth daily in the form of electromagnetic radiations. It is believed that its daily energy emission is several times greater than the energy required annually to run all the activities on earth [12, 13]. Additionally, research has revealed that covering 1/10 of the Earth’s surface with solar cells having efficiency of 10% would satisfy the present global energy requirements [14]. This makes solar energy a nearly inexhaustible, sustainable and an underutilized energy source, whose harvesting methods still require extensive research and improvement in order to give a nearly conclusive answer to the world’s future energy puzzle [15]. More so, PVs have demonstrated great potential for solving the current problem of climate change [16].

The PV industry is believed to have started in 1883 [17] and has continued to attract more research attention, as well as registering a number of ground-breaking innovations. In 2017, the industry registered a remarkable annual growth as compared to other energy technologies (see Fig. 1a). Generally, the PV industry growth rate has been rising steadily in the period since 2010, as illustrated in Fig. 1b. This is due to the intervention of new production technologies which have managed to lower the production costs of solar panels by almost 80%. With the development of other promising materials such as perovskites which could replace silicon, the prices of solar panels are expected to continue dropping [18, 19]. In 2018, a global annual solar addition of 27% was registered, adding 106 GW to the cumulative balance of 404 GW giving a total global solar energy production of 510 GW [4]. It is further anticipated that between 2018 to 2022, an addition of 621.7 GW will be realized, and hence a cumulative global solar energy production of 1025.4 GW by the year 2022 [20] will be attained. In this discussion, we have considered cumulative values simply because more than 90% of the present solar cells in service are silicon-based [21], and these have been reported to have an average performance life time of over 22 years, with the ability to maintain at least 92% of their maximum power within that period [22].

Why Perovskites?

Without doubt, silicon (Si) is currently the benchmark material for the PV industry. However, Si solar cells are characterized by increased production costs, a non-eco-friendly production process as well as low possibilities of increasing their performance. Thus, new PV cells are under study, with the aim of achieving higher efficiencies than what is possible with Si PVs without cost effects. The new generation of PV cells which are still under study include; dye-sensitized, perovskite, organic, inorganic (CZTSSe, CdTe, CIGS), quantum-dot, and their tandems (with or without Si) [3]. Some of the named alternatives can be processed using low-temperature manufacturing techniques, which results in low production costs compared to Si PVs. However, other than perovskite solar cells (PSCs), the power conversion efficiencies (PCEs) of the rest are far less than those of commercial silicon PV cells.

PSCs are reckoned as the future PV cells due to a number of factors [23]. These include, broad spectral absorption [24], obtainable high PCE (currently up to 23.7% for purely perovskite cells and 28.0% for perovskite/Si terminal tandem cells) [25], high theoretical PCE of 31.4% [26], with low manufacturing costs and ease of fabrication from the earth’s abundant raw materials [27]. Besides, methyl ammonium lead halide (CH3NH3PbX3) and related materials possess excellent optoelectronic traits which surpass those of existing photoactive counterparts, including strong optical absorption due to s-p anti-bonding coupling [28, 29, 30], excellent carrier mobility plus a long diffusion length exceeding 1000 nm [31, 32, 33, 34], superior structural defect tolerance [35] and shallow point defects [36, 37, 38], tunable grain boundaries, ability to minimize the electron–hole recombination [39, 40] and small exciton binding energies [39, 41]. Also, certain perovskite materials have been reported to act as sensitizers as well as hole transporters. Hence, these could eliminate the need for separate hole transport materials [42, 43] toward carrier transport-free PSCs.

The commonly used photoactive materials in PSCs are methyl ammonium lead mono or mixed halides CH3NH3PbX3, where X can be Br or I, whereas chloride (Cl) is rarely used due to its very large bandgap [44]. The bandgap of these organic–inorganic perovskites can be tuned by varying the ratio of the halides in the methyl ammonium mixed halide, for instance in form of CH3NH3PbI3−XClX [45, 46, 47]. Also an important factor in these perovskites is the diffusion length (LD) denoted in Eq. 1. This describes the average length a carrier moves between generation and recombination. It follows that the higher the diffusion length of a PV semiconductor, the better the performance of that material and vice versa [34, 48, 49],
$$L_{D} = \sqrt {Dt}$$
(1)
where D is the diffusion factor in m2/s and t is the charge carrier life time in s.
Subsequently, single and mixed halides perovskites of iodine origin CH3NH3PbI3 and CH3NH3Pb(I1−xBrx)3 have been reported to have increased decomposition, due to increased crystallization of inorganic PbI2 and the corresponding degradation due to light absorption compared to the CH3NH3PbBr3 perovskites [50, 51]. Table 2 compares the values of the diffusion constant (D) and diffusion length of pure (iodide) and mixed halide perovskites [46, 48].
Table 2

Values of carrier diffusion constants (D) and diffusion lengths (LD) for both iodine and [46] bromine based mixed and mono halide perovskites [46, 48]

Perovskite

Charge

D (cm2s−1)

LD (nm)

CH3NH3PbI3 − XClX

Electrons

0.042 ± 0.016

1069 ± 204

 

Holes

0.054 ± 0.022

1213 ± 243

CH3NH3PbI3

Electrons

0.017 ± 0.011

129 ± 41

 

Holes

0.011 ± 0.007

105 ± 32

CH3NH3PbBr3Cl

Electrons

 

360 ± 22a

aUnder illumination. The value changes to 150 ± 50 in darkness [46, 48]

For most perovskite species, under various conditions, the carrier lifetime ranges between 50 and 100 ns [52, 53]. On a different note, recently, there is a vivid debate on the reasons behind the exceptional band structure of perovskite, which offers both high absorption and carrier lifetime. It is argued that due to the peculiar crystal momentum, an additional quasi-Fermi level is formed and hidden in the direct bandgap of perovskite, which protects the excitons from irradiative recombination and consequently prolongs the carrier lifetime [54, 55, 56]. This hypothesis might support better the behavior of the mixed halide specimens which potentially create stronger internal momentum.

Importance of Fibrous Structured Devices?

Fibers being confined in single dimension material structures offer limited space for matter comprising it. This, in turn, makes the perovskites employed in fibrous materials to be nanostructured in size. Nanostructured materials are known to offer properties absent in their bulk counterparts [57]. In this line, different research groups have tried various means to synthesize nanostructured perovskites of different morphologies with interesting optoelectronic properties [58, 59, 60, 61, 62, 63]. And it has been confirmed that different morphologies give different performances. Moreover, what is important to note is that nanostructured perovskites exhibit better optical and electronic properties courtesy of quantum confinement effects which are absent in their bulk [64, 65, 66, 67]. Quite a number of different articles have been quoted here to help the reader to understand the effect of size on the performance of perovskite materials for various high-performance optoelectronic applications [57, 67, 68, 69].

In textiles field, newly developing smart fibers capable of accommodating various miniaturized electronics are at the fore of the next high-tech products for the textile consumers. These miniaturized fiber electronics can be categorized into three major sections: (1) energy harvesting (fiber-based solar cells, piezoelectric-nanogenerators, etc., (2) energy storage (fiber-based batteries, super capacitors, etc.) and (3) energy usage electronics (fiber-based sensors, actuators, controllers, displays, etc.). All these make up the smart textiles which is an emerging industry. It is predicted to have a market worth of over US $9.3 billion by 2024 [11, 70]. Consequently, it is apparent that for these electronics to be powered, some form of electrical energy is required. One of the smartest ways is using fiber-based energy harvesting techniques of which fiber solar cells fulfil this motive.

Traditional rigid solar cells have greater efficiency and energy output. However, their application is very restrictive and are not pliable for easy incorporation in clothing or related textiles. Therefore, finding ways to obtain flexible, lightweight, multi-functional PV materials/devices in form of one-dimensional fibers that could later be converted into fabrics, through weaving, knitting or non-conventional techniques is paramount. Consequently, developing an efficient fiber-shaped perovskite solar cell is a hot focus, as verified by Fig. 1c. There has been great efficiency improvement over the last few years. It is also predicted that the world’s future miniaturized technology will be more based on multi-functional fibers [11, 71, 72, 73]. The average daily energy consumption of each personal device such as a mobile phone falls between 2 and 6 watts when charging (http://energyusecalculator.com) [74, 75]. Based on this and drawing from global demographics, in 2019 and beyond, the annual worldwide energy consumption of such personal electronics will fall between (3416.4 and 10,249.2 GW) which is much greater than the current and the near future annual global solar energy production. However, engineering fiber-shaped solar cells can trigger the fabrication of PV fabrics and garments, which would be used as a source of energy to power various miniature electronics incorporated in smart wearable textiles and garments without need fp grid connections.

The trend for smart textiles is also shifting from simple inclusion of electronics and attachment of small solar cells onto textiles [76, 77] to textiles which are assembled directly out of smart multi-functional fibers such as piezoelectric [78], triboelectric [79] and PV, that can harvest energy, store it and also guarantee additional functionality [80, 81]. The ability of PSCs to be recycled [82, 83, 84], together with a high possibility of increasing their PCE in fiber-shaped devices as is in the planar PSCs [26], implies the feasibility of highly efficient, recyclable and sustainable perovskite PV garments to power the portable devices in the near future. Figure 1d illustrates one approach by which perovskite fiber solar cells can be incorporated into smart fabrics to power various functional electronics. Beyond the predicted demand for smart textiles, the need for near body and on body electronics has also continued to increase tremendously and by 2015, over 300 types of such devices were on the market, of which 40% were fitness trackers, 40% life style/computing and 10% health care adoption [85, 86, 87].

Perovskite Materials and Solar Cells: Basics, Fundamentals and Working Principles

The crystal structure of the perovskite material commonly used in PV cells consists of a halogen, an organic group and a di-valent metal which can be tin, antimony or lead [88]. There are so many compounds which adopt this structure [89, 90]. However, a few of them provide a compelling combination of a high absorption coefficient, optimum bandgap, efficient electron–hole mobility and a long carrier lifetime to be used as the active layer in the solar systems. The main reason why most perovskites cannot harvest light energy over a wide length of the spectrum is having divalent anions and strong electrostatic bonding which make their bandgap improper for optoelectronic performances [90, 91]. The most efficient halide perovskite photon absorbers consist of an organic ammonium ion (CH3NH3+) or formamidinium ion ([H2N–CH=NH2]+), lead ions (Pb2+), and halide ions (I, Cl, Br) [92, 93, 94].

A typical perovskite solar cell is made up of multiple layers including; an active (perovskite) layer sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL), plus a transparent electrode at one end and a counter electrode on the opposite end [95, 96, 97]. In some seminal studies, ETL and HTL were absent, but the direct contact caused enormous charge recombination which instigated low PV performance [98]. Transparent electrode is principally a conducting metal oxide on glass (TCO) substrate; mostly tin oxide (ITO) or fluorine-doped tin oxide (FTO) [99, 100], and electron transport layer is basically a nano (meso)-porous TiO2 or double junction of compact and mesoporous TiO2 (as scaffold atop the perovskite) [101, 102, 103]. For the hole transport material 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMETAD) is commonly used [104, 105], and an ultrathin layer of gold, silver or aluminium plays the role of the counter electrode [106, 107]. So far, several attempts have been made to prepare solar cells with inexpensive all-carbon electrodes, or flexible foundation, i.e. polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), which registered low PCEs [5, 13, 73, 76, 108]. Moreover, the need for improved stability, cost-effectiveness and efficiency, have directed researchers to rethink the architecture and formulation of perovskite devices, by application of various organic composites and co-polymers, such as PCBM (phenyl-C61-butyric acid methyl ester), P3HT (poly(3-hexylthiophene) and PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrenesulfonate) [109, 110], different carbon allotropes (carbon nanotubes, graphene and fullerene) [109, 111, 112], and alternative metal oxides (Al2O3, ZnO, SnO2, NiOx) [113, 114, 115, 116, 117, 118]. Figure 2a displays the typical structure of direct stacked (n–i–p) architecture of a standard thin film perovskite solar cell and the arrangement of molecules in a single perovskite unit. Although the specific mechanism of operation of perovskite solar cell is up to now not clearly known [116, 117, 118, 119], we rely on the general working principle of electron–hole separation which applies to all PV systems (Fig. 2b).
Fig. 2

PSCs structure. a Generic unit cell structure of (ABX3) perovskite crystal structure and stacking form in a standard thin-film perovskite solar cell. b Generic energy diagram and charge transfer pathway (profile of electrical potential), in a thin-film perovskite solar cell under illumination

When the perovskite layer receives a level of photon energy, which is greater than its bandgap energy, electron–hole pairs are excited and disintegrated, creating a quasi-Fermi splitting. Then, the generated charges migrate towards corresponding charge selective layers, proceed through the complete layer, and finally get collected at the electrodes. It should be noted that the separation of electron–hole pairs and their movement towards opposite directions is significantly influenced by an inbuilt electric field which is generated by the p–n junction [117, 118, 119, 120, 121]. A mesoporous under-layer (TiO2 or Al2O3) avails a large surface area for delivery of photo-generated electrons [101, 102, 103, 120, 121, 122, 123, 124, 125]. Besides, an effective charge delivery is only possible if the stacking alignment provides a downward pathway for the electrons from conduction band of perovskite toward the anode, and an upward profile for the holes from the valence band of perovskite toward the cathode (Fig. 2b). Concerning the fiber-shaped PSCs, the coaxial layer arrangement and increasing surface area from inner to outer junctions undisputedly requires more complicated photon-to-current mechanism and causes additional non-idealities to the system, as we discuss later on.

Developments in Fiber Perovskites

Recently, fiber-based devices such as fiber batteries, supercapacitors, triboelectric nanogenerators, sensors, solar cells, chromic devices and many others are receiving enormous attention with the aim to be integrated into the wearable textiles [79, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141]. For fiber solar cells, perovskite as the active component is a hot spot, courtesy of the material’s known optoelectronic functions. It is worth noting that, from the technical point of view, there is a wide field of information and reports on thin-film PSCs [92, 109, 123], but limited information on perovskite devices which adopt a 1D-fiber (cylinder-type) geometry. This is attributed to technical complication of perovskite functional fiber axial development, curvature interfaces and expected uneven exposure to the light which all kill final device efficiency. Also, despite the same perovskite material, the final overall material system and positioning in fibers should be to some extent different with those of the thin-film PSCs. To clearly understand the architecture and working principle of the complex designs used in perovskite fiber-shape PV cells, we need to first comprehend the roles of each material in the different layers of a single fiber unit, as are itemized in Table 3, followed by a review on perovskite fiber devices history, techniques of production, working mechanism, level of performance and lifetime, as well as the parameters which trigger them.
Table 3

Conductive and semiconductive materials used in fiber PSCs, their potential technical role and optical band gap [5, 7, 8, 10, 142, 143]

Material

Technical role

Materials optical band gap

TiO2 (compact and mesoporous)

Serves to increase the surface area for harvesting of electrons (hole blocking) from the active material.

TiO2 nano-tubes; − 4.2 eV

Acts as the base/scaffold for the deposition of the active material.

Compact TiO2; − 4.0 eV

Carbon nano-tubes (opaque and transparent)

Transparent CNTs are used as top electrodes

− 4.8 eV

Opaque CNT is used as a core electrode (mimics the back contact in thin-film counterparts) electrodes

As the base to fabricate the shape and fiber structure device, favoring flexibility

Spiro-OMeTAD

Serves as a hole transporting material (HTL) or electron blocking layer

− 5.22 eV

P3HT/SWCNT

Hole transporting material or electron blocking layer

− 5.22 eV

Thin gold layer

Top (counter) electrode

− 4.4 to 4.7 eV, in original condition

− 5.3 to 5.4 eV when sputter cleaned [142]

Steel wire, titanium wire

Basic fiber with enhanced resistivity to tensile stress, and core positive electrodes.

Steel wire; − 4.5 eV

Ti wire; − 4.3 Ev

Silver Nano-wires

Doped of positioned at interfaces to heal the junction and bridge the pathway of charge carriers

4.26 eV

Perovskite material (CH3NH3PbI3, CH3NH3PbI3−XClX, CsPbBr3 and so on)

Photoactive element

− 3.75 to − 5.43 eV

Perovskite Fiber Solar Cells

In 2014, Qiu et al. [5] invented the first fiber shaped perovskite solar cell, using a thin, smooth stainless-steel fiber as the core and the positive electrode. The steel fiber was dip coated in TiO2 solution and annealed at 400 °C to form an n-type compact layer of TiO2. This was then followed by a second dip in a TiO2 nanoparticle solution and formation of mesoporous TiO2 layer around. Then CH3NH3PbI3 was added on top of the mesoporous titanium layer, where the CH3NH3PbI3 effectively infiltrated into the mesopores of the nanocrystal TiO2. The hole transport layer was then encrusted on top of the CH3NH3PbI3 layer through a dip-coating procedure. Lastly, transparent multi-walled carbon nanotubes (MWCNTs) with an average diameter of 10 nm, dry-drawn from a spinnable CNT array, were twisted onto the exterior as the cathode [5]. The fiber- shaped perovskite solar cell developed in this work delivered a PCE of 3.3%, VOC = 0.664 V, ISC = 10.2 mA cm−2, FF = 0.487 and could be woven into a perovskite solar cell fabric. Figure 3a exhibits the layer structure and energy diagram of the above-mentioned fiber perovskite solar device. Despite the compact 1D architecture favoring its ease of application and incorporation, the metal core constrains the flexibility and cost-effectiveness. The compact TiO2 layer in this PV structure was used to separate the HTL and CNT sheet from the steel wire anode. However, applying thick layers of compact TiO2 material, was reported by the authors to be associated with quite a number of big challenges such as; increasing the series resistance to the flow of carriers, lowering the fill factor of the composite PV fiber, which in the end instigated the recorded low power conversion efficiency (PCE). Furthermore, thicker films are also reported to be vulnerable to crack formation and propagation, since they easily break into many pieces especially when the fiber is flexed several times. On the other hand, an ultrathin film of TiO2 possesses a large number of defects, which cause shunts, reduce the carrier lifetime and thus poor photovoltaic performance [5, 9, 98].
Fig. 3

State of the art perovskite fiber solar cells. a Schematic layer structure, energy level diagram of the fiber perovskite device. (Reproduced with permission [5]. Copyright 2014, Wiley–VCH). b Schematic layer structure, SEM images (area section and length of fiber) and energy level diagram of fiber perovskite solar cell, suggested by Li et al. (Reproduced with permission [6]. Copyright 2015, Wiley–VCH). c Schematic of deposition steps, layer structure and energy level diagram of fiber perovskite device. (Reproduced with permission [7]. Copyright 2015, The Royal Society of Chemistry). d Schematic layer structure, SEM image (area section and length of fiber) and energy level diagram, developed by Hu et al. Reproduced with permission [8]. Copyright 2016, The Royal Society of Chemistry). e Schematic layer structure and energy level diagram of fiber perovskite solar cell, fabricated by Qui et al. reproduced with permission [9]. Copyright 2016, Wiley–VCH). f The Schematic layer structure, SEM image (area section) and energy level diagram of fiber perovskite device, suggested by Hu et al. (reproduced with permission [10]. Copyright 2018, The Royal Society of Chemistry

With the aim of fabricating a more flexible, corrosion-resistant and durable perovskite solar cell fiber, Li et al. [6] reported a double-twisted fiber-shaped solar cell (Fig. 3b). Their device was engineered using multi-twisted carbon nanotubes, which were coated with TiO2 layers (compact and mesoporous). Then perovskite (CH3NH3PbI3−xCLx) poly(3-hexylthiophene)/single-walled carbon nanotube (P3HT/SWNT), and silver (Ag) nanowire were in-turn coated from the inside out. Lastly the PV fiber shaped device was twisted with a multi spun strand CNT yarn to form a double-twisted structure and coated with polymethyl methacrylate (PMMA), as a protective layer. Silver nanowire facilitates efficient charge transfer towards the outer CNT fiber electrode. This device was reported to show a champion PCE of 3.03%, with a fill factor of 56.4% and, and could withstand 1000 bending cycles with no significant loss in the PCE. Exposure of the device to atmospheric conditions for 96 h revealed that it could maintain up to 89% of its PCE. We predict that the poor contact, excessive series resistivity, surface damage, and corrosion at the contact points of the twisted fibers might be the main reasons for the low PCE [6, 144]. However, twist integration is beneficial in term of miniaturization.

Deng et al. [7] reported a highly elastic, fiber-shape perovskite solar cell, assembled on aligned TiO2 nanoparticles working electrode (Fig. 3c). Fabrication process started by immersing a spring-like Ti wire into a diluted titanium diisopropoxide bis(acetylacetonate) solution (precursor of TiO2), high-temperature sintering, treatment in TiCl4 aqueous solution and dip-coating in TiO2 nanoparticles (average 20 nm) dispersed in ethanol. The modified Ti wire (coated by TiO2) was then dipped in perovskite precursor solution (twice, followed by thermal annealing at 100 °C) and Spiro-MeOTAD precursor solution, consequently. Finally, an elastic conductive fiber made of silicone rubber and aligned CNT was inserted on, to render an elastic nature, and another CNT sheet was twisted around the outer surface to serve as the front electrode. The complete device registered a PCE of 5.22%, while without the outermost CNT sheets, the efficiency dropped drastically to 0.6%. The aligned CNTs sheets are attached stably on the surface of the Si rubbery by van der Waals forces, due to the high surface contact area, making the whole structure sustainable to bending and stretching.

Using a similar process as above, Hu et al. [8] prepared a fiber-shaped device based on mixed-halide perovskite and Ti-wire core electrode (Fig. 3d). To this aim Ti-wire was first dip-coated in TiO2 solution, then heated electrically at high temperature in the presence of air to form a compact layer of TiO2, and a mesoporous TiO2 layer was added by dip coating in TiO2 colloids. CH3NH3PbI3−XClX and spiro-OMeTAD (hole conductive layer) were then sequentially coated using solution processing, and the outermost (a gold back contact) was fabricated via magnetron sputtering. The optimized device exhibited an open circuit voltage of 0.714 V, short circuit current density of 12.32 mA/cm2, fill factor of 60.9%, registering 5.35% PCE. With efforts from Qui et al. [9] a fiber-shaped perovskite solar cell was coaxially grown on a titanium wire, serving as core and anode electrode. See Fig. 3e. Through anodization process, an array of TiO2 nanoroads were radially grown on the Ti-wire, creating a mesoporous configuration. A porous, sponge-like PbO layer was then coated on the TiO2 nanotubes using cathodic deposition, followed by addition of hydroiodic acid to form PbI2 crystalline plates, and dip-coating with CH3NH3I to form CH3NH3PbI3 grains, partially infiltrated in the mesoporous TiO2. The pathway of this chemical process is suggested to be as below (Ra and Rb)
$${\text{PbO}}_{{\left( {\text{S}} \right)}} + 2{\text{HI}}_{{\left( {\text{aq}} \right)}} \to {\text{PbI}}_{{2 \left( {\text{aq}} \right)}} + {\text{H}}_{2} {\text{O}}_{{\left( {\text{l}} \right)}} \quad \quad \quad \quad {\text{R}}_{\text{a}}$$
$${\text{PbI}}_{{2 \left( {\text{s}} \right)}} + {\text{CH}}_{3} {\text{NH}}_{3} {\text{I}} + {\text{isopropanol}}_{{ \left( {{\text{annealing at }}80\,^\circ {\text{C}},\;{\text{t}} = 30{ \hbox{min} }} \right)}} \to {\text{CH}}_{3} {\text{NH}}_{3} {\text{PbI}}_{{3 \left( {\text{s}} \right)}} \quad \quad {\text{R}}_{\text{b}} .$$
The whole structure then was wrapped into a transparent CNT Tape (80% transmittance in 400–800 nm spectrum range and 500 S cm−1 electrical conductivity), affording the two-fold function of electron collection and photon reception. Authors remarked that the controlled thickness (~ 50 nm) and desirable uniformity of TiO2 mesoporous layer accelerates electrons transport, barriers shunting between the two electrodes (i.e. Ti and CNTs) and barriers hole injection. Such a fiber-shaped coaxially aligned PSC demonstrated a champion PCE of 6.8%, corresponding to an open-circuit voltage of 0.852 V and short circuit current density of 16.1 mA cm−2. When the outer layer (CNT) was covered by a 5 nm silver coat, conductivity of contact electrode elevated from 1000 S cm−1, and the PCE of device increased to 7.1%, corresponding to a fair fill factor of 0.56 and a suppressed short-circuit current density of 14.5 mA cm−2, which might be stemming from damages on front electrode during measurements [9]. Low fill factor was found to be derived from the high series resistance of the CNT sheet electrode, but it was subsidized by introducing silver to enhance the conductivity of CNTs through thermal deposition. Flexing for 400 cycles reduced the PCE to about 90% which evidences poor mechanical stability. Hu et al. also used a titanium wire (manually polished and cleaned in an ultrasonic bath) as the core electrode to grow a compact and a mesoporous TiO2 by dip coating and electrical heating [10]. The perovskite precursor was prepared according to following chemical pathway (Rc–Rd),
$$\begin{aligned} {\text{Pb}}({\text{CH}}_{3} {\text{COO}})_{2} + {\text{CH}}_{3} {\text{NH}}_{3} {\text{I}} + {\text{DMF}} & \to ({\text{CH}}_{3} {\text{NH}}_{3} {\text{I}})_{\text{x}} - {\text{PbI}}_{2} - ({\text{DMF}})_{\text{y}} + {\text{CH}}_{3} {\text{NH}}_{2} \uparrow \\ & \quad + {\text{DMF}} \uparrow + {\text{CH}}_{3} {\text{COOH}} \uparrow \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad {\text{R}}_{\text{c}} \\ \end{aligned}$$
$$({\text{CH}}_{3} {\text{NH}}_{3} {\text{I}})_{\text{x}} - {\text{PbI}}_{2} - ({\text{DMF}})_{\text{y}} \to {\text{CH}}_{3} {\text{NH}}_{3} {\text{PbI}}_{3} + {\text{DMF}} \uparrow + {\text{CH}}_{3} {\text{NH}}_{3} {\text{I}}\quad \quad \quad {\text{R}}_{\text{d}}$$
To elaborate, the wire was dipped in prepared perovskite solution and heated at 100 °C in nitrogen atmosphere. To form the hole-transport layer, the resulted fiber was dipped in precursor solution of Spiro-OMeTAD. The gold front electrode was finally attached by magnetron sputtering (Fig. 3f). Author reported a new source of lead (lead acetate), to improve the morphology of perovskite lattice, grown on curvature surface (fiber substrates). The complete device delivered 7.53% PCE which is the highest value reported thus far, corresponding to 0.96 V open circuit voltage under AM 1.5 illumination. To easily compare the fiber perovskite devices, the PV metrics of above-mentioned fiber-shape perovskite devices are summarized in Table 4.
Table 4

PV parameters of state-of-the art fiber-shape PSCs, corresponding to Fig. 3a–f

Short circuit current density (Jsc, mA cm−2)

Open circuit voltage (V)

Fill factor (%)

PCE (%)

Corresponding Fig.

References

10.2

0.667

48.7

3.30

3a

[5]

8.75

0.615

56.4

3.03

3b

[6]

3.50

0.630

5.22

3c

[7]

12.32

0.714

60.9

5.35

3d

[8]

14.2

0.85

56.0

7.1

3e

[9]

14.18

0.96

66.0

7.53

3f

[10]

Mechanical Stability and Typical Assessments

Beyond the optoelectronic properties, fiber-based devices must be able to endure the very harsh service conditions, such as repetitive bending cycles, fractions and contacts, compression, ties and stretchings [11]. Due to these requirements, additional characterization techniques have been designed and used to probe the genuinity of these fiber photovoltaic devices, including perovskite SCs [5, 6, 7, 9].

Flexibility tests are carried out to evaluate the mechanical resistance of a single fiber, yarn or textile when subjected to various mechanical deformations. During mechanical testing, the bending, twisting, stretching and flexing in many cases will affect the overall photovoltaic performances of these devices, due to changes induced within the fiber solar cell. In such a scenario, we can get a bird’s view of the possible performance of the solar device during actual and normal use during wear or other functions which involve interruptions. Unlike other fiber devices, solar fibers devices are light-driven. Traditionally, the nature of their round morphology constrains the full-exposure to the light. Hence, the density of the incident photons significantly varies with the angle of exposure, which in-turn changes with type and position of deformation. This makes this characterization critical in these devices.

Concerning the fiber PSCs, a standard procedure has not been suggested, for flexibility assessment (in light and dark), yet. The most informative analysis so far has been reported by Peng’s group [5] where they proved that their mesoporous fiber perovskite SC assembled between a Ti-wire (core) and a transparent CNT sheets (front electrode), could endure over 50 cycles of bending and various angles (Fig. 4a). In their recent effort however authors improved the structure of cell by modification of Ti core wire using CNT nanosheets and cathodic deposition of perovskite (Fig. 4b), which resulted in a steady PV yielding (7.1%) against 400 cycles of twisting [9]. In a similar context Deng et al. [7] particularly focused on the effect of stretching on PV function of an elastic perovskite fiber-shaped cell. Fiber perovskite was grown on a spring-like CNT-doped Ti-wire and closely wrapped with a stretchable conductive (Si rubber-CNT). The photovoltaic output monitored for a single and three connected (series and parallel) in stretched and normal mode showed a minor difference (Fig. 4c). Li et al. [6] also developed a double twisted fibrous perovskite with CNT-fiber support (Fig. 4d) and attempted to discuss PCE-dependence on the incident light together with the bending cycles.
Fig. 4

Flexibility and stretchability analysis of fiber-shape perovskite solar cell. a Photographs of knotted and twisted devices and their corresponding variation in PV parameters versus twisting cycles. Reproduced with permission [9]. Copyright 2016, Wiley–VCH. b SEM image of a bent fiber-shape perovskite and photograph of corresponding textile together with the performance of device under various bending cycles. Reproduced with permission [5]. Copyright 2014, Wiley–VCH. c SEM image of an elastic PSC fiber with a pitch distance of approximately 1.25 mm together and J–V profiles of its device made into textiles using three stretchable PSC fibers of similar nature, connected in series and parallel before and after stretching. Reproduced with permission [7]. Copyright 2015, The Royal Society of Chemistry. d Photograph of the double-twisted fibrous perovskite solar device wrapped onto a capillary tube and the variation in normalized efficiency across various bending cycles of the device. Reproduced with permission [6]. Copyright 2015, Wiley–VCH

Current Shortcomings of Perovskite Fiber Solar Cells

Some of the main challenges facing fiber-shaped PSCs include but are not limited to; chemical and bending degradation, high annealing temperature requirement, low fill factor, material toxicity, limited surface area to light exposure for a uni-directional source, flexibility and low PCE. Each factor has been given special attention in the subsequent paragraphs.

Degradation

Similar to thin-film perovskite devices, the degradation problem in fiber PSCs must be solved if these devices are to be industrialized and produced massively. Degradation in these solar cells is mainly triggered by environmental factors such as humidity, temperature, air and UV radiation. According to literature and with reference to the most studied and basic semiconductor perovskite, CH3NH3PbI3, it starts to decompose at 55% relative humidity, and this is observed by a notable color change from dark brown to yellow [145, 146, 147, 148, 149, 150, 151]. Generally, the degradation and formation of methyl ammonium lead halide perovskites can be clearly explained using the chemical equation (Re) below,
$${\text{PbI}}_{2} + {\text{CH}}_{3} {\text{NH}}_{3} {\text{I}}_{{\left( {\text{aq}} \right)}} \leftrightarrow {\text{CH}}_{3} {\text{NH}}_{3} {\text{PbI}}_{{3 \left( {\text{s}} \right)}} \quad \quad \quad \quad {\text{R}}_{\text{e}}$$
To form CH3NH3PbI3, the reaction shifts from left to right as PbI2 reacts with CH3NH3I, and the reverse is true. There are two proposed degradation pathways, independent of the geometry, and in the first mechanism it is proposed that PbI2 and CH3NH3I combine with peripheral molecules such as water molecules to cause breakdown of CH3NH3PbI3. The second model explains that the products formed by the degradation of the perovskite generally depend on the prevailing condition subjected to it [152]. In the first scenario, moisture causes degradation due to the solubility of methylammonium halide in water. In the presence of water traces, CH3NH3PbX3 suffers partial degradation to form an intermediate compound which later breaks down to form HI, CH3NH2 gases and PbI2 (see reactions \({\text{R}}_{\text{f}} \;{\text{and}}\;{\text{R}}_{\text{e}}\)). Excess water in the system causes complete degradation by dissolving HI and CH3NH2 leaving PbI2 as a bright yellow substance (see reactions Rf and Rg) [153, 154, 155, 156]. All the above-mentioned reactions are reversible, and this makes CH3NH3I, HI, CH3NH2 together with the insoluble PbI2 to coexist in the system. If the system is well encapsulated, such that there is minimum loss of these bi-products, they recombine to reform CH3NH3PbI3 according to reactions Rf, Rg and Rh and the performance of the perovskite cell is maintained for a long period of time [152, 156, 157, 158],
$${\text{CH}}_{3} {\text{NH}}_{3} {\text{I}}_{{\left( {\text{aq}} \right)}} \leftrightarrow {\text{CH}}_{3} {\text{NH}}_{{2 \left( {\text{aq}} \right)}} + {\text{HI}}_{{ \left( {\text{aq}} \right)}} \quad \quad \quad \quad \quad {\text{R}}_{\text{f}}$$
The decomposition of HI can proceed by either oxidation or under the influence of UV radiations as shown below,
$$2{\text{HI}}_{{\left( {\text{aq}} \right)}} \leftrightarrow {\text{H}}_{{2 \left( {\text{g}} \right)}} + {\text{I}}_{{2 \left( {\text{s}} \right)}} \quad \quad \quad \quad \quad \quad \quad \quad \,\;{\text{R}}_{\text{g}}$$
$$4{\text{HI}}_{{\left( {\text{aq}} \right)}} + {\text{O}}_{{2 \left( {\text{g}} \right)}} \leftrightarrow 2{\text{I}}_{{2 \left( {\text{s}} \right)}} + 2{\text{H}}_{2} {\text{O}}_{{ \left( {\text{l}} \right)}} \quad \quad \quad \quad {\text{R}}_{\text{h}} .$$
In addition to the discussed reversible reactions, moisture, UV and temperature can initiate irreversible perovskite degradation reactions at normal solar cell operating temperatures of 40–80 °C as demonstrated by reaction Ri. Thus, this explains the phenomenon whereby a slight loss in efficiency persists even when the perovskite solar cell is sealed inside a polymer [154, 159].
$${\text{CH}}_{3} {\text{NH}}_{3} {\text{PbI}}_{{\left( {\text{aq}} \right)}} \to {\text{NH}}_{{3 \left( {\text{aq}} \right)}} + {\text{CH}}_{3} {\text{I}}_{{\left( {\text{aq}} \right)}} + {\text{PbI}}_{{2 \left( {\text{s}} \right)}} \quad \quad \quad \quad {\text{R}}_{\text{i}} .$$
In the effort to stabilize perovskites, some standard sealant approaches such as encapsulation under inert conditions have been employed and proved to be good in stopping the overall device degradation from moisture and oxygen [157, 158, 160, 161]. Sealing/encapsulation of the perovskite solar cell not only prevents its exposure to the moist air, but also prevents the leakage and evaporation of volatile products resulting from perovskite degradation and thus makes them available to react and reform the perovskite in the system. According to Li et al. [6], their elasitc mixed halide perovskite fiber (double twisted with CNT fiber) lost 92% of its original PCE when was stored in ambient condition for 96 h, without encapsulation. In an identical storage condition, coating by Polymethyl methacrylate (PMMA) helped to maintain 89% of initial PCE. Figure 5a compares the storage behavior of double twisted CNT-CH3NH3PbX3 fiber with and without PMMA encapsulation [6].
Fig. 5

a Stability test of CH3NH3PbIXCl3−X fiber device (double twisted with CNT fiber), with and without PMMA cover, during 96 h storage in ambient condition. Inset is device photograph. (Reproduced with permission [6]. Copyright 2015, Wiley–VCH). b Theoretical correlation between the voltage, current, and fill factor in a solar cell. Inset is an illustration of the imbalanced carrier profile due to the varying surface area from outermost to the core layer. c Different potential light exposure for the thin film and fiber SC. d SEM images (layer and cross-section) and efficiency versus bending of a CH3NH3PbI3−xClx fiber SC with stainless steel support and CNTs front electrode. (Reproduced with permission [5].Copyright 2014, Wiley–VCH). e SEM image, efficiency versus bending, and the bending strategy of double-twist perovskite fiber SC, with CNTs yarn support and wrapping electrodes. (Reproduced with permission [6]. Copyright 2015, Wiley–VCH). f (i–iv) Efficiency versus bending, strain, stretch and side-angle SEM image of CH3NH3PbI3–xClx fiber SC, with Ti wire support, twisted onto a super elastic CNTs fiber. (Reproduced with permission [7]. Copyright 2015, The Royal Society of Chemistry)

Alternative techniques for improving the stability of PSCs include the use of carbon protective (hydrophobic) layers and chemically inert scaffolds and electrodes [162, 163, 164, 165]. Also, the use of mixed hybrid perovskites was reported by Seok et al. [147] to boost solar cell stability and performance. In their work, they found out that CH3NH3Pb(I1−xBrx) where (x > 0.2) demonstrated good stability when it was kept at 55% relative humidity for a period of 20 days. The notable resistance versus moisture-initiated degradation was thought to be caused by the compact and stable structure which resulted due to partial replacement of iodine with smaller bromine atoms in CH3NH3Pb(I1−xBrx). This led to a reduction in lattice constant and a transition to the cubic phase [147]. The application of an insulating mesoporous layer, comprising of Al2O3 nanoparticles was testified to prevent metal electrode migration, while giving room for accurate control of HTM thickness and preventing solar cell degradation for a period of 350 h of operation [166]. Beyond encapsulation, TiO2 doped with Al2O3 has been reported to be a suitable scaffold for perovskite, protecting it against rapid degradation [152, 167, 168, 169]. Though, using Al2O3 alone as a scaffold may even accelerate the degradation due to the formation of covalent bonds between the alumina and the perovskite on interfaces, leading to the dissociation of perovskite molecules [168].

2D hybrid perovskites (C6H5(CH2)2NH3)2MA(Pb3I10) where (MA = CH3NH3) and other variants such as 2D/3D (HOOC(CH2)4NH3)2PbI4/CH3NH3PbI3 perovskite junctions, have been reported to extend the lifetime of PSCs. Actually, 2D/3D hybrid perovskites have managed to maintain their efficiencies up to a record time of 1 year [170], under normal operating conditions of a solar cell. This level of endurance was attributed to the stable crystalline structure, compared with the mono-halide counterparts [170, 171, 172, 173]. The use of an Sb2S3 interlayer between the mesoporous titania and CH3NH3PbI3 has also been referred as one of the trending strategies to overcoming UV light instigated degradation [152]. Furthermore, as explained by Komarala et al. the use of UV filters such as europium doped yttrium vanadate prolongs the active lifetime of the perovskite cell [174], despite the high cost of these rare earth material compounds. The fiber PV cells have cores (metal or metal oxide, doped polymer or any other conducting material) onto which different layers are chronologically deposited. Therefore, any stress caused by flexing the core material and the fiber in general, could damage the compact TiO2 layer and also weaken the various interfaces, thereby instigating degradation and eventually cell failure [24]. To overcome this problem, more flexible layers should be used to replace the traditional crack vulnerable layers.

High Annealing Temperatures Involved

A close examination of the annealing process indicates that highly efficient n-type compact TiO2 layers can be produced at high annealing temperatures. Elghniji et al. [175] reported that Fe3+ doped TiO2 demonstrated superior photo current generation properties in comparison with the undoped material. However, annealing at elevated temperatures comes along with production of iron oxide in the Fe3+–TiO2 composite which highly affects the process of electron transport, leading to low power conversion results [9]. High annealing temperatures definitely increase the cost of production of the PV fibers and almost make the whole design process impossible, which at the end makes them uncompetitive on the market. More still, the high-temperature annealing of n-TiO2 at 450–500 °C limits deposition of perovskites on light and flexible supports [6, 92, 175, 176]. Also, the annealed TiO2 layer on highly curved surfaces such as fiber PV tends to peel off, causing surface defects especially if conventional annealing methods are used [39]. To overcome the intensive effect of crack formation and propagation associated with annealing of metal-oxides, especially in fiber-based devices, we further suggest its replacement in these PV devices with polymers which have high electron affinities such as poly(p-phenylene vinylene) (PPV) [177]. Other materials such as (6,6)-phenyl C61-butyric acid methyl ester (PCBM), may also be suitable electron transport materials for this application [178]. It would also be appropriate to replace other layers of these PV fibers with bending resistant and low-temperature processable materials such as the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) [179]. However, these replacement alternatives are constrained by many challenges. Firstly, the semi conductive polymers are electrically resistive, chemically vulnerable, and to the best of our knowledge so far, they have not been incorporated in the fiber devices. This might have risen from process difficulties and the poor performance of semi-conductive polymers. Moreover, most of the low temperature (room temperature) annealing techniques which have been discovered are not technically applicable for fiber shaped cells [109, 117, 180].

Low Fill Factor and Low PCE

Generally, the fill factor values of reported fiber PSCs are still low, with the highest report value being 60.9% [8]. This is caused by the structural defects which are either intrinsic such as lattice imperfections in every individual layer, or extrinsic such as lack of interfacial contacts. The former results from the uneven co-axial growth of the layers which is barely controllable, while the latter is mainly caused by the poor interlayer contacts, particularly between twisted yarns, and also originates from breaking, pilling off and segregation of layers during measurement or when the device is in service [5, 6, 7, 8].

Fill factor is expressed by Eq. 2 and graphically illustrated by Fig. 5b, where FF stands for the fill factor (%), Jm and Vm respectively indicate the current density and voltage corresponding to the maximum output power. Jsc and Voc respectively denote the short circuit current (maximum current when the voltage is zero) and open circuit voltage (maximum voltage when current density if zero),
$${\text{FF}} = \frac{{{\text{J}}_{{\text{m}}} {\text{V}}_{{\text{m}}}}}{{{\text{J}}_{{\text{sc}}} {\text{V}}_{{\text{oc}}}}}.$$
(2)

From this expression, it is inferred that the fill factor can be readily limited by a poor overall current density and resulting overall voltage, which are dramatically susceptible to the molecular and bulk structure of layers and interfaces. What makes the issue more serious for the fiber solar cells, including the fiber PSCs, is the decreasing active surface area (co-axial stacking) from outermost to the core. As shown in the inset of Fig. 5b, such an alignment would increase charge accumulation in the junctions. In other words, the fiber architecture, potentially creates larger trap densities and recombination rates, compared with the flat (thin film) architecture. For the PSC fibers, which are typically composed of at least five layers, this phenomenon leads to even further complications. For an acceptable quality of solar cell, the fill factor is supposed to be between 0.7 and 0.8, and for a poorly performing cell, it may be 0.4 or even less [181]. This value for the perovskite fiber solar cells, lies between 0.48 and 0.66, as tabulated in Table 4. It seems to be possible to control this issue partly, by passivation of trap sites at every individual junction, using oxidation [182], assembly of an electrically active mono-layer [183] and doping with monovalent ions (Li+, Cl) [184].

Toxicity

Human health and comfort are the foremost considerable items in design and materials system of wearable devices. Application of fiber perovskite in wearable electronics is severely constrained by the presence of lead (Pb) which is a vital part in perovskite stoichiometry [185, 186, 187, 188, 189]. Lead is considered as a heavy metal which causes a number of health hazards not only to human beings, but also to other organisms in the ecosystem [190, 191]. And hence, this can deter the acceptance of the final products on the market, triggering researches on replacing the lead by some other elements which are not toxic. Appropriate materials which have been proposed so far include tin (Sn), bismuth (Bi), germanium (GE) and copper (Cu) [30, 145, 192, 193]. Of all the above listed elements, tin (Sn) possesses closely similar chemical properties with lead and therefore it has been suggested as the best candidate for replacing lead in perovskite [194, 195, 196]. In 2016, the highest PCE was registered from a mixed-cation mixed- metal (Sn/Pb) perovskite delivering 15% PCE in a planar model [197]. Also, there are hundreds of Pb-free formulations, but their PCE (below 9.1%) and stability still lag behind the lead-based perovskites [188, 198, 199, 200]. Configurations of mixed metal/mixed halide perovskites such as (MA2CuClxBr4−x) and Cs2AgBiX6 (X = Br, Cl) offer multitude advantages of superior efficiency and chemical stability in addition to their non-toxicity. Thus these stand to be potential photo-active material for fiber shaped solar cells. Lastly, despite of all the achievements in making perovskite lead free, currently no work has been reported about lead-free perovskite fiber solar cell.

Limited Surface Area for Unidirectional Light Exposure

The nature of the arched outward surface of fibers limits the access to the source of photons when used as solar device. Figure 5c schematically compares the photon-available surface in flat and fiber solar cells. With unidirectional illumination, a significant part of the fiber system always remains in the dark. That is why, the conversion efficiency of fiber PSCs is technically lower than the anticipated values. Fibre shaped PSCs could compete favorably with their planar counterparts in terms of efficiency, if would be illuminated from all angles. Theoretically, in a full-area exposure the fiber solar cells should be able to produce a higher PCE, compared with its flat counterpart, owing to the large surface area of the cylinder-shaped fibers. However, factually this wide area is always partially illuminated. This can more or less explain why there is a huge difference between the reported top efficiency in fiber and thin film solar cells, including the PSCs. Beyond the functional problems due to the reducing surface area from the outside-to-inside, as mentioned above, accumulation of thermal energy in perovskite layers could potentially be a key factor in cell degradation and low-stability. To partially surmount this shortfall, modified highly absorptive perovskite compounds must be formulated specifically to be use in the fiber shape. Additionally, a highly transparent front electrode and an anti-refractive layer (like carbon allotropes) must be accommodated at the top layer [201].

Degradation Due to Bending

Wearable electronics are steadily exposed to deformation. Cracks and fractures which are created by bending not only cause local malfunctions but also permeate the oxygen and water or act as the original points for unwanted chemical reactions [5, 6, 7, 107, 123, 202]. That is why, incorporation of flexible and stress-sustaining materials in fiber solar cells is a key issue in the field. Particularly, the functional layers of a perovskite SC (including the active and charge selective layers) need to be ultrathin, as their formation inevitably comes along with numerous tiny defects which enlarge upon flexing and eventually disturb the efficiency of the device. Figure 5d presents the SEM images (layer and cross-section) and degradation profile versus bending, for a perovskite (NH3CH3PbI3−xClx) fiber cell [5]. In this case, a relatively stable performance is observed, which might be owing to the structure sturdiness, offered by the stainless steel support, CNTs fiber front contact and a double (compact/mesoporous) TiO2 ETL under perovskite. Nevertheless, formation of mp-TiO2 with 400 °C annealing process is a manufacturing difficulty, and incompatible for highly flexible supports such as conductive polymers. Li et al. [6] attempted to improve the bending stability by designing a double twist perovskite fiber in which CNT yarns which were used for both the core support and the wrapping electrode (Fig. 3b). ETL and HTL were respectively composed of a double (compact/mesoporous) TiO2 and P3HT covered by single-wall (SW) carbon nanotubes. This fiber configuration could remain at original performance along with 1000 cycles of bending (Fig. 5e). It was claimed that SWCNTs control the degradation due to bending, by filling in the initial cracks. Deng et al. [7] grew the CH3NH3PbI3−xClx on a Ti-wire support between TiO2 nanotube or nanoparticles (ETL) and Spiro (HTL), and twisted the whole fiber onto a super elastic fiber which was wrapped in CNT fibers (Fig. 3c). The Influence of bending, strain and stretch on efficiency, and SEM image of this fiber solar system displayed on Fig. 5f shows a desirable mechanical endurance particularly when TiO2 nanoparticles are embedded as ETL [8].

Can Fiber Shaped PSCs be Knitted or Woven into Photovoltaic Fabrics?

Knitting is considered as one of the most typical techniques for converting yarns into fabrics because; it is fast, cost effective, diverse and free of chemical treatments such as sizing [128, 203]. The unsuitable yarns for knitting can be used as in-lays in the fabrics, and the knitting systems can be adjusted to form garment parts which minimizes fabric wastage caused by cutting activities during sewing operations [204, 205]. Finally, the method is easily adjustable to produce textiles for clothing as well as for technical applications [206]. A suitable yarn for either weft or warp knitting should be highly bendable by even the finest of needles, and allow formation of loops with a small average diameter, such that the resulting fabric is compact with a reasonable cover factor. The action of needles during loop formation and inter-meshing imposes massive bending and initiates a number of forces and cracks, together with length constraints which results in excessive stress concentration with in the yarn [203]. Fabrication theory, motion constraints, possible stresses and features of crack formation during knitting of yarn are illustrated in Fig. 6a. The increased level of stress associated with loop formation and intermeshing, leads to massive crack formation and propagation within different layers of the fiber device. On this basis, fabrication of a suitably knitted perovskite solar cell fabric by warp and weft knitting technologies seems to be unsuitable, unless for strongly resistive and bendable material on a highly flexible support (core fiber).
Fig. 6

Major fabric manufacturing methods. a Techniques, stress points and the crack propagation (SEM image) in a knitted fabric. (Reproduced with permission [203]. Copyright 2005, association for computing machinery, SEM image is reproduced with permission.[6]. Copyright 2015, Wiley–VCH). b Techniques of a weaving, and SEM images of knitted PSCs fibers with titanium wire support weaves, inset are photograph and high resolution SEM, and with stainless steel wire supports [optical image (i) reproduced with permission [7]. Copyright 2015, The royal society of chemistry and (ii) Ref. [5] Copyright 2014, Wiley–VCH)

Most weaving techniques are also applicable only on highly robust material with ability to withstand the weaving tension generated in the warp yarns during shedding, and resist the abrasion resulting from yarn to yarn friction in the beat-up process. In this latter process, the weft yarn is pushed to the cloth fell to form a new fabric surface [207]. A close look at the ultimate tensile strength of some of the frequently used materials in fabricating fiber solar cells such as; titanium wires (434 MPa) [208], carbon Nano-tubes (11-63 GPa) [209] and steel wires (400–550 MPa) [210] among others, indicates that they are strong materials and hence the strength of the resulting fiber shaped PV cells will be sufficient to withstand the tension during weaving. Figure 6b depicts the techniques, and the possible stresses in a woven fabric, and the SEM images of weaving assembly of PSC fiber [128, 132]. In both cases an excessively strong core support (titanium or a stainless-steel wire) serves as either central foundation or as the positive electrode. Clearly weaving fabrication imposes less number and density stresses to the subjected filaments, as proved in reported perovskite SC fabrics, that have been assembled by weaving [5, 7].

Perovskite Fiber-Shaped Integrated Energy Harvesting Systems

The output energy of a PSCs (fiber or film) is entirely dependent on the presence of the photon stimuli. In the absence of the sun light, the relevance of these photo-electric converters ceases to exist [201]. On the other hand, the limited surface of illumination, as mentioned earlier, and the current degradation due to charge trapping in the defective junctions [211, 212, 213, 214] seem to be the unsolvable issues in this area. Therefore, in order to fabricate perovskite textiles, with continuity in power supply, we must integrate the fiber solar system with an energy storage system such as a capacitor to form a perovskite solar capacity or a battery [215, 216, 217, 218]. Moreover, an integrative design with the other power suppliers such as a thermoelectric or a piezoelectric, may partly compensate the voltage losses and malfunction due to the defective structure and the curvature front electrode. Figure 7a exhibits the layout and working principle of a proposed blue print for a perovskite fiber solar-capacitor. The layout shows a carbon nanofiber being employed as either the core (support) or the electrode of an asymmetric fiber capacitor. A solid or hydrogel dielectric and a Ti tube (grown or wrapped) are added respectively, to complete the capacitor segment. The Ti tube handles the double function of electron collection for the solar cell and electrode of capacitor (shared layer). Then mesoporous (mp)-TiO2, perovskite, Spiro-OMeTAD (HTL) and ultrathin gold layers (back contact) are respectively grown. This hybrid fiber-perovskite pack can afford the dual service of photo-charge, galvanostatic discharge or charge generation and storage, when it is exposed to illumination and simultaneously placed in a circuit with an external load.
Fig. 7

a Cross-section and alignment of suggested layout for a perovskite fiber solar capacitor. b Function mechanism of suggested photo-piezoelectric fiber perovskite device. c Dye synthesized solar cell. (i) Schematic diagram of generic working mechanism of dye synthesized solar cells. (Reproduced with permission [223]. Copyright 2017, Elsevier). (ii) Cross-sectional alignment and photograph of a DSSC fiber based on organic dye. (Reproduced with permission [226]. Copyright 2017, Elsevier). (iii) Photon-to-current generation pathway in a generic dye synthesized solar cell. (Reproduced with permission [240]. Copyright 2014, Elsevier) and (iv) design of a fiber DSSC in which CoNi2S4 nanoribbons deposited on carbon fibers as electrode. (Reproduced with permission [225].Copyright 2015, Elsevier). d Polymer solar cell. Charge generation and conduction pathway in a typical hetero (bi)-junction polymer solar cell. (i) Fabrication steps with layer arrangement (Reproduced with permission [234]. Copyright 2015, The Authors, some rights reserved; exclusive licensee Springer Nature. Distributed under a Creative Commons Attribution Non-commercial License 4.0 (CC BY-NC) http://creativecommons.org/licenses/by-nc/4.0/) and (ii) current-to-voltage trend (inset is SEM image) of a fiber polymer solar cell, based on steel wire support and CNTs fiber back contact. (Reproduced with permission [239]. Copyright 2012, American Chemical Society)

A wearable perovskite solar-piezo fabric potentially exploits the photo and piezo generated electrical voltages, constantly independent of location, climate and day-cycle. To the best of our knowledge, the one-dimensional or fiber-shape piezo-phototronic perovskite devices have not been reported, thus far. However there are plenty of approaches toward enforcement of piezoelectric performance of perovskite, including poling in a strong electrical field [219]. As mentioned above, application of a highly piezoelectric junction, like a metal oxide wire [220], and combination with piezoelectric polymers such as PVDF (Polyvinylidene fluoride) [221, 222] could be reasonable approaches. However, the latter approach seems to be associated with technical challenges. Whereas the conjugating polymer induces polarity and improves flexibility of perovskite lattice, it may disturb photon absorption especially if mixed with the photoactive material. As a possible approach, a way to prepare a fully piezo-photoelectric perovskite fiber device has been schematically shown on Fig. 7b. PVDF fibers, doped with metal oxide (ZnO, TiO2) or CNT and graphene, accommodated as layers underneath the perovskite can act as semi conductive (ETL) layers and highly piezoelectric effects can be attained. This fiber device features a two-in-one piezo-photoelectric perovskite system. Integrated and applied as a wearable garment, the piezoelectric element assists the photoelectric element in day-time and replace it in night-times. Also, with a proper engineering, piezoelectric material especially nanowires could advance the electron transfer effect in the resulting device (piezophototronic effect).

Other Fiber Solar Cells

Dye Sensitized Fiber Solar Cells

Fiber shaped dye sensitized solar cells (DSSCs) have registered a superior maximum PCE of 8–9%, comparatively greater than that of PSC fibers. They also benefit from ease of integration with energy storing devices and provision of toxic free organic dyes as active materials [223], as well as many other ground-breaking innovations, including formation of flexible devices with a length of up to 30 cm [224], engineering of platinum free functional materials as counter electrodes [225], advanced engineering of photo anodes [226] and high voltage out puts [227]. Despite the fore mentioned merits, DSSC fibers are technically unable to compete with wearable perovskite electronics. This setback is caused by their complicated material system and association with the liquid medium (electrolyte), which in the case of fiber should be filled in a flexible and transparent capillary, made of plastic [223, 224, 225, 228]. Beyond the focus of this article, we attempt to give a generic picture of the morphology and mechanism of power generation in fiber dye sensitized solar cells. Principally, a dye sensitized solar cell consists of a photo-active material (organic or inorganic dye), a semi-conducting layer usually (TiO2, ZnO, etc.), a redox electrolyte such as \({\text{I}}^{-}_{ 3}/{\text{I}}^{-}\) and a counter electrode which may be fabricated from carbon, Pt, etc. When light with photon energy greater than the band gap energy of dye molecule is made incident to the dye, electrons from the excited state of the dye enter the conduction band of a semi-conductor material such as TiO2 nanoparticles. The electrons received by the TiO2 material flow towards the transparent conducting oxide (TCO) through an external circuit and then reach the counter electrode [229, 230, 231]. Figure 7c(1) is a schematic illustration of the working mechanism and cross-sectional alignment of a fiber DSSC based on an organic dye, producing 3% PCE [223]. In Fig. 7c(2) a photograph and schematic alignment of a fiber DSSC are presented, in which the photo-anode is composed of a Ti wire and a uniform TiO2 (nanoparticle) blocking layer, that led to a 7.4% PCE [226]. Figure 7c(3) displays the photon-to-current generation pathway in a generic dye synthesized solar cell [229], and Fig. 7c(4) depicts the current-to-voltage trend along with a design schematic (inset) of a fiber DSSC in which CoNi2S4 nanoribbons deposited on carbon fibers replaced the platinum counter electrode, producing an overall PCE of 7.03% [225].

Polymer Fiber-Shaped Solar Cells

owing to their increased flexibility and low production costs polymer fiber shaped solar cells are considered as some of the potential materials for making self-powered fibers devices, suitable for weaving and knitting into wearable textiles. Typical architecture and mechanism of carrier delivery in a polymer solar cell mimic those in a perovskite solar cell, except that here the active layer is a donor–acceptor system (double/hetero-junction), consisting of a polymer and usually a carbon derivative (fullerene or PCBM). Once photons reach the active layer, excitons (electron–hole pair) are created and get to the donor–acceptor boundaries, where they dissociate to free electrons and holes, and diffuse toward the corresponding electrodes [232, 233, 234, 235, 236, 237]. Figure 7d(1) illustrates the charge generation and conduction pathway in a typical hetero (bi)-junction polymer solar cell. Despite the potentially flexible nature, abundance of related materials and cost-effectiveness, research attention is slowly getting away from this type of solar cells, due to their demonstrated low PCE and limited active lifetime. Particularly, the polymer solar cells, which are formed and applied as textile, might be further susceptible to the mechanical triggers [232, 233, 234, 235]. One of the latest works reported a champion PCE of 3.27%, which is one of the highest reported so far [238]. Figure 7d(2, 3) exhibit the fabrication steps, layer arrangement, SEM image (inset) and the current-to-voltage trend of a fiber polymer solar cell, based on a steel wire support and CNT fiber back contact [239].

Perovskite Fibers in Other Devices

Besides photon harvesting, perovskite (nano)-fibers have found numerous uses in sensing, catalytic reactions, purification and piezoelectric generators, owing to their large specific surface area (area-to-volume ratio), little dimension, low expenses, ease of deformation and incorporation into the other systems [241]. Table 5 summarizes some of the reported works in which perovskites fibers, have been used as part of different functional devices. It can be seen that beyond photo-activation, perovskite fibers have registered considerable yielding, as catalyst in oxidation reactions [242, 243, 244, 245, 246]. Figure 8 briefs the structure and function of some perovskite nanofiber samples, which are applied in different categories other than PVs [242, 244, 245, 247].
Table 5

Perovskite fibers applied in various devices, beyond the PV field

Composition of perovskite

Nature and fabrication method

Application

References

Perovskite type oxides (LaSrCOFeO3)

Nano-fiber webs(electro-spinning)

Catalyst in soot oxidation

[242]

La0.6Sr0.4 CoO3−δ (LSC)

Hollow fiber membranes (combined phase inversion/sintering)

Oxygen permeation

[243]

Perovskite type oxides (La1−xKxFeO3−δ)

Nano-tubes (Electro-spinning)

Soot oxidation

[244]

BaTiO3- perovskite

Nano-fibers (sol–gel based electro spinning)

Piezo electric devices

[248]

Ag-(PrBa)0.95Mn2O5+δ

Nano fiber (electro-spinning)

Oxygen reduction reaction (ORR) catalysts

[245]

SmBa0.5Sr0.5Co2O5+δ (SBSCO) layered perovskite oxide

Nano-fiber web (electro spinning)

Cathode material for a low temp operating solid oxide fuel cell

[246]

LaFeO3 perovskite oxide

Perovskites/carbon/cotton fibrous composite (sol–gel and thermal treatment)

Adsorption of Rhodamine B

[247]

Fig. 8

Structure and function of perovskite nanofibers/nanotubes, applicable in different processes other than PV devices: a Yielding and catalytic function of 3D La1−xSrxCo2Fe8O3 perovskite fibrous webs (LSCF) with different Sr doping levels, for trapping and oxidation of soot. (Reproduced with permission [242]. Copyright 2016, Elsevier). b Structure and performance of macro/mesoporous fibrous perovskite La1-xKxFeO3-δ as catalyst for soot combustion. (Reproduced with permission [244]. Copyright 2018, Elsevier). c Nanofibers-based Ag-PbMO5. SEM and HRTEM images. ORR LSV performance before and after 1000 CV cycles and the proposed mechanism of ORR performance. Reproduced with permission [245]. Copyright 2017, Elsevier). d Structure and performance (together with the schematic detailing mechanism of adsorption of RhB onto the material) of LaFeO3 perovskite oxide deposited on carbon fibers (LFO-ACFs) (synthesized through sol–gel loading and thermal treatment. (Reproduced with permission [247]. Copyright 2019, Elsevier)

Concluding Remarks

Solar power harvesting for portable electronics is one of the key strategies that must be considered so as to satisfy the growing global energy demands, particularly with the increasing number of miniaturized, mobile electronics and high-tech personal devices. To realise this, there is need for flexible and light weight solar cells whose geometries can be manipulated into different structures to suit distinctive technological applications. The best required geometry and size for most of these devices can be provided by fibers. Although several examples of fibrous solar cells have been reported, the future for perovskite-based fiber cells looks to be much brighter than any other types due to the countless number of engineering adjustments, which could be effected on perovskite-based fibers to shoot up their performance. The field of fiber shaped PSCs is quite novel and continues to be keenly considered by many research groups. Potentially, the PV function of the fiber-shape perovskite is drastically increasing toward that observed in planar PSCs. There is also increased attention towards lead free perovskite fiber shaped solar cells, in view of eliminating the toxicity risk which is associated with perovskite PV systems, mainly composed of highly toxic lead containing material. Moreover, if this technology is to be industrialized, more research is needed to develop flexible, durable, and light weight PSCs that can be knitted, because of the endless advantages and few preparatory processes involved in the production of knitted structures. Therefore, more innovative and advanced engineering designs for different layers of these solar cells are required so that most of the stated goals will be attained. We further anticipate cheap, light, more sturdy conducting polymeric materials will be increasingly adopted for HTL/ETL and also for other functional layers. Finally, it is worth believing that with advancements in perovskite fiber-shaped solar cells, more efficient and high potential PV textiles (fabric solar panels) will drive smart self-powered wearable electronics, lowering prices and partly solving the current energy crisis.

Notes

Acknowledgements

The authors thank the National Key Research and Development Program of China (2016YFA0201702/2016YFA0201700), the Shanghai Natural Science Foundation (19ZR1400900), the Science and Technology Commission of Shanghai Municipality (16JC1400700), the Fundamental Research Funds for the Central Universities (Grant No. 2232018A3-01), the Program for Innovative Research Team at the University of Ministry of Education of China (IRT_16R13), the International Joint Laboratory for Advanced Fiber and Low-dimension Materials (18520750400), and the (No. 111-2-04). R. J. acknowledges the Flagship Leap 3 (RDU 172201) of Universiti Malaysia Pahang (http://ump.edu.my). M. T. also acknowledges the research support of the Australian Government Research Training Program (RTP) Scholarship at the Australian National University, Canberra.

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Copyright information

© Donghua University, Shanghai, China 2019

Authors and Affiliations

  • Andrew Balilonda
    • 1
  • Qian Li
    • 1
  • Mike Tebyetekerwa
    • 2
  • Rogers Tusiime
    • 1
  • Hui Zhang
    • 1
  • Rajan Jose
    • 3
  • Fatemeh Zabihi
    • 1
    Email author
  • Shengyuan Yang
    • 1
    Email author
  • Seeram Ramakrishna
    • 4
  • Meifang Zhu
    • 1
  1. 1.State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, International Joint Laboratory for Advanced Fiber and Low-dimension Materials, College of Materials Science and EngineeringDonghua UniversityShanghaiPeople’s Republic of China
  2. 2.Research School of Electrical, Energy and Materials Engineering, College of Engineering and Computer ScienceThe Australian National UniversityCanberraAustralia
  3. 3.Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences and TechnologyUniversiti Malaysia PahangPekanMalaysia
  4. 4.Centre for Nanofibers and NanotechnologyNational University of SingaporeSingaporeSingapore

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