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Lead-Free Perovskite Solar Cells

Fundamentals, Fabrication, and Future Prospective
  • Khursheed Ahmad
  • Shaikh M. MobinEmail author
Living reference work entry
  • 9 Downloads

Abstract

Although perovskite solar cells having methyl ammonium lead halide perovskite visible light sensitizer achieved an excellent efficiency of 25% which is close to theoretical efficiency, the simple fabrication procedure and excellent performance of the perovskite solar cells make it most a promising photovoltaic device compared to the other solar cells. However, some major problems have been observed in the perovskite solar cells which are the presence of toxic metal (Pb) and its hygroscopic nature which resulted in poor stability in air. Thus, in last 5 years, research on the development or finding of Pb-free perovskite structures has been accelerated by various research groups around the world. Therefore, Pb has been replaced by other nontoxin or less toxic metal like Sn, Bi, Sb, and Cu. Different architectures of Pb-free perovskite solar cells have been developed to enhance the performance and efficiency of these photovoltaic devices. In this chapter, the origin of different Pb-free perovskite structures-assisted perovskite solar cells, fabrication, challenges, and future prospective of Pb-free perovskite solar cells have been described.

Introduction

Energy demand has been increased rapidly all over the world [1, 2, 3, 4]. The world needs neat, clean, and safe energy with clean environment [4]. In the coming years, this energy demands are further going to increase with increasing population globally [3]. Currently, fossil fuels or other energy sources are fulfilling the requirements, but these sources are limited [5]. Therefore, it is of great importance to develop neat and clean energy sources to fulfill the energy requirements in the future. There are different renewable energy sources present, but solar energy is the best and most promising renewable energy source. Photovoltaic devices were developed to consume solar energy and convert it into electrical energy without emission of any pollutant [6]. Basically, these photovoltaic devices, also called solar cells, convert the solar energy into electricity and work on the photoelectric effect [7]. There were different types of photovoltaic devices developed, but dye-sensitized solar cells (DSSC) showed most promising features. In 1991, dye-sensitized solar cells were invented by M. Gratzel, in which a ruthenium complex was employed as light sensitizer while titanium dioxide as electron transporter. The developed dye-sensitized solar cells have shown good photovoltaic performance [8]. To date, numerous efforts were made by various scientists/research groups to improve the performance of the dye-sensitized solar cells. But the highest efficiency of only ~13% for DSSCs have been reported, which is lower than that of the commercialized solar cells like silicon solar cells [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]. In 2009, Kojima et al. who were working on the improvements of DSSCs have employed a new visible light sensitizer named perovskite (CH3NH3PbI3 and CH3NH3PbBr3) [25]. They have investigated its optical and electrical properties and found the lowest band gap for CH3NH3PbI3 compared to the CH3NH3PbBr3. They developed the DSSCs with these two light sensitizers and reported an interesting efficiency of ~3.1% [25]. This work opened a new window for the scientific community to explore such novel perovskite materials as visible light sensitizers for the development of efficient photovoltaic. Furthermore, Park et al. employed CH3NH3PbI3 quantum dot, whereas Gratzel and coworkers used solid state electrolyte to further increase the efficiency of the perovskite solar cells (PSCs) [26, 27]. The photovoltaic efficiency was enhanced up to 9% which is quite interesting [27]. The use of solid state electrolyte also hindered the dissolution of CH3NH3PbI3 in the electrolyte. The performance of the PSCs depends on various factors such as electron transport layer, surface morphology and film thickness of the perovskite light absorber, hole transport materials, concentration of precursor, spin coating speed, time, annealing temperature and device architectures, etc., and numerous work on PSCs has been published [28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63]. Cao et al. fabricated the PSCs with CH3NH3PbI2Br and CH3NH3PbI3 and the resultant PSCs showed the highest efficiency of 11.03% and 10.51% respectively [64]. Cho et al. introduced (FAPbI3)0.85(MAPbBr3)0.15 perovskite for the development of highly efficient PSCs [65]. The developed PSCs exhibited an excellent efficiency of 21.3%. This efficiency was quite interesting with good open circuit voltage and short-circuits current density. Boopathi et al. used metal halide salts as additive to improve the surface morphology and control the crystal growth during the preparation of perovskite structures [66]. The fabricated PSCs devices showed the highest efficiency of 15.03% with long-term stability up to 50 days with slight degradation in efficiency. The improved performance may be due to the passivation of grain boundaries and recrystallization of small grains by the presence of additive metal halide salts. Kanda et al. introduced perhydropoly (silazane) precursor to improve the stability of the PSCs and the fabricated PSCs showed the remarkably high efficiency of 22.1% with enhanced stability [67]. There are numerous reports available with novel and new strategies to improve the stability of the PSCs, but the presence of highly toxic metal (Pb) remains a significant challenge for the scientific community. In this regard, many research groups have been engaged to eliminate the Pb from the perovskite solar cells. Recently, new perovskite materials with Sn, Ge, Bi, Cu, and Sb have proposed as suitable replacement for Pb. These reported Pb-free perovskite structures showed significant results with promising characteristics.

In this chapter, the basic principle and fabrication of Pb-free perovskite solar cells have been discussed. Moreover the challenges and future perspective have also been discussed.

Device Architecture and Components of Pb-Free PSCs

The fluorine doped tin oxide (FTO)/glass substrate was used as a working electrode to fabricate the Pb-free PSCs. Initially, FTO glass substrate was etched with zinc powder + 2 molar HCl solution and was washed with detergent and cleaned by acetone, 2-propoanol, and D.I. water. Further, electron transport layer (ETL; generally titanium dioxide) was deposited using spin coating method. Further, Pb-free perovskite layer to be deposited was placed on to the ETL and heated at temperature between 70 °C and 100 °C.

Later, a hole transport material layer (HTL) was deposited on to the prepared perovskite film and finally a gold layer was deposited to complete the Pb-free PSCs device (Scheme 1). Basically Pb-free PSCs device composed of different components like electrode substrate (FTO), electron transport layer (TiO2), perovskite (CH3NH3SnI3, CH3NH3GeI3 and (CH3NH3)3Bi2I9 etc.), hole transport materials (CuI), and metal electrode (Ag, Au, carbon or Al etc.). The light strikes at the PSCs device which generates the electron-hole pairs in the perovskite structure. The ETLs transfer this electron to the FTO, while HTL accepts the remaining hole in the perovskite. The current flows in the opposite direction of the electron travel.
Scheme 1

Schematic representation of the fabrication of PSCs

Development of Pb-Free PSCs

In last few years, Pb-free perovskite structures gained much attention because of their aerobic stability and less toxic nature. Few research groups are continuously working on the development of Pb-free PSCs. The recent advances in the Pb-free PSCs have been discussed below:

Tin-Based Pb-Free PSCs

Tin (Sn) is less toxic and have potential to replace the Pb from the CH3NH3PbI3 perovskite structure. In 2014, Snaith and coworkers used CH3NH3PbI3 perovskite as visible light absorber for the fabrication of Pb-free PSCs [63]. The observed results were impressive with good power conversion efficiency. In 2015, Koh et al. introduced HC(NH2)2-SnI3 = FASnI3 perovskite light sensitizer for the development of PSCs [68]. The band gap of the FASnI3 was smaller (1.41eV) compared to the commercial Pb-containing perovskite light absorbers CH3NH3PbI3 (1.47eV) and FAPbI3 (1.5eV). Therefore, the low band gap of this Pb-free perovskite makes it a suitable light absorber toward the development of less toxic or Pb-free PSCs. The authors have prepared FASnI3 perovskite using simple approaches. Moreover, they have investigated the effect of SnF2 additive on the performance of PSCs. The Tauc plot of the FASnI3 perovskite film showed the band gap of 1.41eV (Fig. 1a). However, the X-ray diffraction patterns (XRD) of the FASnI3 and FASnI3 with SnF2 additive have been presented in Fig. 1b.
Fig. 1

Tauc relation plot (a); inset shows digital film image, XRD patterns (b) and XPS (c) of pure and mixed FASnI3 with SnF2 (mol%). Expanded XPS of FASnI3 film (d). (Adapted with permission [68])

The authors have used different percentage (10%, 20%, 30%, and 40%) of additive (SnF2) to prepare the high performance Pb-free PSCs. To further confirm the formation of FASnI3 perovskite, X-ray photoelectron spectroscopy (XPS) was also employed. The recorded XPS results of the FASnI3 and FASnI3 with SnF2 (20 mol%) have been shown in Fig. 1c; whereas that of the expanded XPS of FASnI3 have been presented in Fig. 1d. The obtained results confirmed the formation of FASnI3 perovskite phase with high purity. The XPS data clearly showed the presence of fluoride in the FASnI3 with SnF2 sample, whereas it was absent in case of pure FASnI3. The addition of SnF2 from 10% to 40% increases the stability of the FASnI3.

Further, scanning electron microscopic (SEM) images of the FASnI3, FASnI3@20%SnF2, and FASnI3@30%SnF2 have been recorded. The obtained results have been shown in Fig. 2a–c. However, the cross-sectional SEM image of the fabricated PSCs has been shown in Fig. 2d. The pure FASnI3 showed the poor coverage of the substrate (Fig. 2a), while the addition of 20% SnF2 increases the coverage of substrate (Fig. 2b). However, with 30%SnF2 showed the nanoplatelet-like structure (Fig. 2c). However bare titanium dioxide (TiO2) can be seen with the nanoplatelets which may reduce the performance. Further the performance of the FASnI3-based Pb-free PSCs was checked under standard conditions by recording the photocurrent density-voltage (I-V) curves. The obtained I-V curves have been shown in Fig. 3. The PSC devices with different thickness of ETL were fabricated. The highest performance of 2.1% efficiency with excellent photocurrent density of 24.45 mA/cm2 was obtained for the ETL with thickness of 500 nm (Fig. 3).
Fig. 2

SEM images of FASnI3 and FASnI3@20%SnF2 (a, b), SEM image of FASnI3@30%SnF2 (c), and cross-sectional SEM image of the PSCs device (d). (Adapted with permission [68])

Fig. 3

I-V curves of PSCs with FASnI3@20%SnF2 with TiO2 (different thicknesses). (Adapted with permission [68])

Hao et al. employed a solvent-mediated crystallization approach for the preparations of CH3NH3SnI3 films for Pb-free heterojunction-depleted PSCs [69]. The CH3NH3SnI3 films were grown using dimethyl sulfoxide or DMSO solution via a transitional SnI2·3DMSO intermediate phase. The obtained high-quality thin films of CH3NH3SnI3 were found to be highly dense, uniform, and pin hole free which may be contributed for the high performance of PSCs. Figure 4a shows the schematic representation for the preparation of high-quality thin films of CH3NH3SnI3 perovskite. The XRD patterns of the CH3NH3SnI3 perovskite before and after annealing have been presented in Fig. 4b.
Fig. 4

Schematic of preparation of perovskite film (a), XRD (b), and FTIR (c) of perovskite films before and after annealing. (Adapted with permission [69])

The Fourier-transform infrared spectroscopy or FTIR was also used for further investigations. The FTIR spectra before and after annealing have been shown in Fig. 4c. The FTIR results for the CH3NH3SnI3 layer prepared from DMSO (before annealing) exhibits that the C–S and C–O stretching vibrations corresponded to the Sn2+-coordinated DMSO solvent respectively. However, after annealing these peaks of DMSO disappeared, which confirmed the complete removal of DMSO and formation of CH3NH3SnI3 phase on the conversion of intermediate.

The schematic graph of the fabricated PSCs devices has been presented in Fig. 5a, whereas the recorded I-V curves of the PSCs have been shown in Fig. 5b. The photovoltaic performance of the Pb-free PSCs was compared with the CH3NH3PbI3-based PSCs. The photocurrent density for Pb-free PSCs was higher, whereas the open circuit voltage was high for the CH3NH3PbI3-based PSCs. The incident-photon-to-current efficiency (IPCE) of the CH3NH3SnI3- and CH3NH3PbI3-based PSCs have been shown in Fig. 5c. The CH3NH3SnI3-based PSCs showed better absorption compared to the CH3NH3PbI3-based PSCs. The photocurrent density as a function of time of the fabricated CH3NH3SnI3- and CH3NH3PbI3-based PSCs have been depicted in Fig. 5d. An efficiency of 3.15% and 5.79% were obtained for CH3NH3SnI3- and CH3NH3PbI3-based PSCs respectively. The lower efficiency for CH3NH3SnI3-based PSCs may be attributed to the strong dark carrier density present in CH3NH3SnI3.
Fig. 5

Schematic of PSCs device (a), I-V curves (b), IPCE curves (c), and comparison of photocurrent density versus time of PSCs (d). (Adapted with permission [69])

Song et al. [70] also used CsSnI3 perovskite (CsI to SnI2 ratio ~0.4) for the fabrication of PSCs. The authors also used piperazine to suppress the p-doping of CsSnI3.

Different amounts (0, 5, 10, 15, and 20%) of piperazine were used to prepare the CsSnI3 films. The XRD patterns of the CsSnI3 with different amounts of piperazine have been incorporated in Fig. 6a. The absorption spectra of CsSnI3 with 0, 10, and 20% piperazine have been shown in Fig. 6b. The observation revealed that the introduction of piperazine suppresses the crystallization of SnI2 and improves the surface coverage with reduction in the conductivity of CsSnI3. The digital pictures of 0.4 CsSnI3 with piperazine (0–25 mol%) from left to right in white light have been shown in Fig. 6c.
Fig. 6

XRD (a) of 0.4 CsSnI3 with piperazine (0–25 mol%), absorption spectra (b) 0.4 CsSnI3 with piperazine (0, 10, and 20 mol%), and digital pictures of 0.4 CsSnI3 with piperazine (0–25 mol%) from left to right in white light. (Adapted with permission [70])

The performance of the fabricated PSCs devices were investigated by recording I-V curves and external quantum efficiency (EQE). As shown in Fig. 7a, the best performing PSCs device exhibited the efficiency of 3.83% with excellent photocurrent density. The fabricated PSCs also showed good quantum efficiency (Fig. 7b). The overall results showed that the introduction of piperazine enhances the photovoltaic performance of the developed PSCs. Piperazine not only improves the performance but also increases the surface coverage and reduces the chances of p-type doping of CsSnI3.
Fig. 7

I-V curve (a) and EQE curve (b) of the best-performing PSC device. (Adapted with permission [70])

Wang et al. [71] also explored CsSnI3 perovskite quantum dots for photovoltaic applications. In their work, they have prepared CsSnI3 quantum dots by benign approach. Figure 8a shows the schematic graph for the preparation of CsSnI3 perovskite quantum dots.
Fig. 8

Synthetic rout for CsSnI3 quantum dots (a) and schematic view for the production process of CsSnI3 quantum dots. (Adapted with permission [71])

The authors have introduced novel antioxidant solvent additive or ASA (triphenyl phosphite-TPPI) to overcome the issue of oxidation of CsSnI3 structure. The prepared CsSnI3 quantum dots in absence of TPPI oxidized, while in presence of TPPI remained stable upto 3 months. The mechanism or the formation procedure for the CsSnI3 quantum dots in absence and presence of TPPI have been depicted in Fig. 8b. Further, XRD patterns and absorption spectra for the CsSnI3 with different percentage of TPPI (0, 2, 4, 6 vol%) were recorded and presented in Fig. 9.
Fig. 9

XRD (a) and normalized absorption spectra (b) of CsSnI3 with different percentage of TPPI (0, 2, 4, 6 vol%). (Adapted with permission [71])

The schematic device architecture and cross-sectional SEM image of the fabricated PSSCs have been depicted in Fig. 10a, b respectively.
Fig. 10

Schematic (a) and cross-sectional SEM (b), I-V (c), and EQE (d) of the PSCs. (Adapted with permission [71])

There were different PSC devices fabricated with CsSnI3 quantum dots and nonquantum dots (with and without ASA). The PSC device with nonquantum dots and in absence of ASA exhibited the least performance, while the quantum-dot-based PSCs showed highest efficiency (Fig. 10c). The obtained EQE (Fig. 10d) also exhibited a similar trend and was consistent with I-V measurements. In this work, the highest power conversion efficiency of 4.13% was recorded with photocurrent density of 23.79 mA/cm2. The integrated photocurrent density has also been inserted in Fig. 10d.

In other reported work, Cao et al. [73] tuned the cationic part of the CsSnI3 perovskite structure. They introduced formamidinium iodide in place of Cs and prepared a formamidinium tin iodide (HC(NH2)2SnI3 = FASnI3) perovskite and investigated its optoelectronic properties.

Authors also introduced ammonium hypophosphite additive in the precursor solution to suppress the oxidation of Sn2+. The pictures of the precursor solutions have been shown in Fig. 11a, whereas the SEM images of the FASnI3 with different mol% of ammonium hypophosphite (AHP) additive have been shown in Fig. 11b–e. The SEM results revealed that the introduction of AHP improved the surface coverage, produced large grains, suppressed pin holes, and controlled the crystallization process. The schematic diagram of the PSCs has been displayed in Fig. 12a, whereas the power conversion efficiency (PCE) distribution graph with respect to the AHP additive concentrations has been placed in Fig. 12b.
Fig. 11

Digital pictures (a) of FASnI3 precursors and SEM images (be) with different amounts of AHP (0, 3, 5, and 7 mol%). (Adapted with permission [72])

Fig. 12

Schematic of PSCs (a), PCE distributions with different concentrations of AHP (b), I-V curve (c), and stabilized PCE and current density of the best performing device (d). (Adapted with permission [72])

The highest PCE for the developed PSC device was observed at 5% AHP additive. The I-V curve of the best-performing PSC device has been displayed in Fig. 12c, and stabilized PCE and current density of the best performing device have been shown in Fig. 12d. The recorded I-V curve exhibited the highest PCE of 7.37% with good open-circuit voltage of 550 mV. This improved PCE was attributed due to the formation of uniform, large grain size and pin hole FASnI3 film using AHP additive.

Germanium-Based Pb-Free PSCs

Previous reports showed the promising nature and performance of tin-based perovskite light absorbers, but the moisture sensitivity and the rapid oxidation of Sn2+ restrict its use for the development of PSCs. In 2015, Krishnamoorthy et al. [73] employed a new Pb-free light absorber for photovoltaic applications. Authors have proposed CsGeI3, FAGeI3, and MAGeI3 (MA = CH3NH3) perovskite materials as suitable Pb-free light absorbers. The Pawley fit of PXRD of CsGeI3 (a), MAGeI3 (b), FAGeI3 (c), and crystal structure of AGeI3 (d) have been displayed in Fig. 13a–d. The crystal structure suggested the presence of rhombohedral symmetry (R3m) in the perovskite structures.
Fig. 13

Pawley fit of PXRD of CsGeI3 (a), MAGeI3 (b), and FAGeI3 (c), and crystal structure of AGeI3 (d); where A = Cs, MA or FA. (Adapted with permission [73])

Further, the optical properties of the CsGeI3, MAGeI3, and FAGeI3 perovskite materials were investigated using UV-vis absorption spectroscopy. The absorption spectra of the CsGeI3, MAGeI3, and FAGeI3 perovskite materials have been shown in Fig. 14a.
Fig. 14

Absorption Tauc spectra (a) of CsSnI3 (blue), CsGeI3 (black), MAGeI3 (red), and FAGeI3 (yellow) perovskite materials, density of states of CsGeI3 (b), photoemission spectroscopy in air or PESA of CsGeI3, MAGeI3, and FAGeI3 perovskite materials (c), and schematic energy level diagram (d). (Adapted with permission [73])

The band gap of the CsGeI3, MAGeI3, and FAGeI3 perovskite materials were found to be 1.63eV, 2.0eV, and 2.65eV, respectively (Fig. 14a). The density of states of CsGeI3 and band structure is shown in Fig. 14b. Further, photoemission spectroscopy in air or PESA was also employed to calculate the band energy level of CsGeI3, MAGeI3, and FAGeI3 perovskite materials (Fig. 14c). The energy level values of the CsGeI3, MAGeI3, and FAGeI3 perovskite materials with other PSCs components have been displayed in Fig. 14d.

Furthermore, the PSCs were fabricated and the I-V curves of the CsGeI3- and MAGeI3-based PSCs have been presented in Fig. 15a.
Fig. 15

I-V curves of PSCs device with CsGeI3 and MAGeI3 perovskite materials (a) and IPCE of MAGeI3-perovskite-materials-based PSCs device (b). (Adapted with permission [73])

The best MAGeI3-based PSCs device showed a better open-circuit voltage compared to the CsGeI3-based PSCs device (Fig. 15a). The IPCE curve of the MAGeI3-based PSCs have been placed in Fig. 15b. The PSC devices with CsGeI3 and MAGeI3 exhibited the highest PCE of 0.11% and 0.20% respectively.

Antimony-Based Pb-Free PSCs

From the obtained results of the germanium-based PSCs, it can be clearly seen that such devices exhibited poor PCE. Moreover, the other major drawbacks of germanium-based perovskite materials are the rapid oxidation of Ge2+ in air.

Ahmad et al. [74] introduced two-step sequential deposition approach for the preparation of high-quality thin films of (CH3NH3)3Sb2I9 perovskite, but the performance of the fabricated PSCs was less than 1%. However, the fabricated PSCs device showed a good open-circuit voltage.

In another approach, Karuppuswamy et al. [75] developed the Pb-free PSCs by employing zero-dimensional (0D) methyl ammonium antimony iodide dimer [(CH3NH3)3Sb2I9] for the Pb-free PSCs applications. The authors have introduced antisolvent approach to enhance the morphological features of the (CH3NH3)3Sb2I9 by controlling the nucleation growth. A more suitable and effective interlayer, which worked as scaffold layer, was also introduced to improve the grain size. This scaffold layer improved the (CH3NH3)3Sb2I9 film quality with reduction in the number of voids.

In this work, pyrene and perylene were used as scaffold layer, while chlorobenzene was used as antisolvent to improve the film quality and control the crystallization process. The formation of the perovskite film and device architecture has been displayed in Fig. 16. The XRD patterns of the Sb-based perovskite structures with HI additive (black), with HI:CB (red), with perylene:HI and CB (blue), and with pyrene:HI:CB (green) have been presented in Fig. 17a, while an expanded view of XRD patters have been shown in Fig. 17b. The XRD spectra of the HI-(CH3NH3)3Sb2I9 showed the (003) diffraction plane which indicates the hexagonal structure with point group P63/mmc and the orientation along the c-axis. However, the introduction of scaffold layer changed the preferred orientation of the crystal growth or the formation of more random aligned crystals.
Fig. 16

Schematic view for the preparation of HI-assisted (CH3NH3)3Sb2I9 by using antisolvent treatment, cross-sectional SEM image, and device architecture of the PSCs. (Adapted with permission [75])

Fig. 17

XRD (a) of Sb-based perovskite structures with HI additive (black), with HI additive and chlorobenzene (red), with perylene, HI, and CB (blue), and with pyrene, HI, and CB (green). Enlarged view of XRD patterns (b). (Adapted with permission [75])

The morphological feature and the effect of additive, scaffold layer and antisolvent were investigated by recording SEM images of the prepared Sb-based perovskites. The HI-(CH3NH3)3Sb2I9 and HI-CB-(CH3NH3)3Sb2I9 revealed the presence of indistinct hexagonal structures, whereas the introduction of scaffold layers to the Sb-based perovskite formed distinct hexagons with larger grain size. The introduction of scaffold layer increases the grain size of the (CH3NH3)3Sb2I9 crystals which is beneficial for solar cell applications. The photovoltaic performance of the fabricated PSCs was checked by I-V and EQE measurements. The recorded I-V and EQE results have been shown in Fig. 18. The PSC device with pyrene exhibited the highest efficiency compared to the other fabricated PSCs devices. The quantum efficiency was also high for the pyrene-assisted PSCs device. The highest efficiency of 2.8% was reported for the pyrene-assisted antimony-based PSCs. This efficiency is quite interesting and high among the other reported Sb-based PSCs.
Fig. 18

I-V curves (a) and EQE curves (b) of the Sb-based PSCs. (Adapted with permission [75])

This high performance was attributed to the high-quality thin films and larger grain size of (CH3NH3)3Sb2I9 perovskite using pyrene scaffold layer.

Bismuth-Based Pb-Free PSCs

Bismuth (Bi) is another nontoxic element which has similar properties to Pb and has potential for use in the preparation of Pb-free perovskite structures. Ahmad et al. employed two different perovskite light absorbers ([(CH3NH3)3Bi2Cl9]n and CH3NH3)3Bi2I9) for the development of PSCs [76, 77]. Ahmad et al. introduced one dimensional (1D) polymer and two-step approach using toluene as antisolvent to obtain the high-quality thin films; but the performance of the fabricated devices was less than 0.5%. The developed PSC devices exhibited an excellent open circuit voltage but lower photocurrent density.

In another approach by Huang et al. [78], an organic electron transport layer (fluorinated perylene diimide or FPDI) was employed in the fabrication of Pb-free PSCs. Huang et al. have proposed new device architecture of Pb-free PSCs.

The schematic graph of the bismuth-based Pb-free PSCs device has been shown in Scheme 2.
Scheme 2

Schematic graph of the Pb-free PSCs. (Adapted with permission [78])

The authors prepared the MA3Bi2I9 perovskite materials with different thickness ratio of MAI and BiI3. The XRD pattern of the pure BiI3 (black) and MA3Bi2I9 with different thickness ratio of BiI3:MAI (150:130, 180:130 and 210:130) were recorded and the results have been depicted in Fig. 19. The XRD results showed a successful formation of the MA3Bi2I9 perovskite structures. Further, the morphological features were investigated using SEM analysis. The recorded SEM images of BiI3 (a) and MA3Bi2I9 with different thickness ratio of BiI3:MAI (150:130 (b), 180:130 (c), and 210:130 (d)) have been depicted in Fig. 20.
Fig. 19

XRD pattern of the pure BiI3 (black) and MA3Bi2I9 with different thickness ratio of BiI3:MAI (150:130, 180:130 and 210:130). (Adapted with permission [78])

Fig. 20

SEM images of BiI3 (a) and MA3Bi2I9 with different thickness ratio of BiI3:MAI (150:130 (b), 180:130 (c), and 210:130 (d)). (Adapted with permission [78])

The SEM image of pure BiI3 exhibited a smooth and compact surface morphology (Fig. 20a). However, in case of MA3Bi2I9 with different thickness ratio of BiI3:MAI showed a larger grain size (Fig. 20b–d). The SEM results shows that some small pin holes on the surface are present which can lead to the leakage of current and low fill factor which resulted to the poor performance of the PSCs device. The fabricated PSCs device showed the efficiency of 0.063% with open circuit voltage of 610 mV. The obtained performance was very poor even after introducing organic electron transport layer (FPDI). Further, improvements are still required to enhance the performance of Bi based PSCs.

Copper-Based Pb-Free PSCs

Copper is also a less toxic element which can be used as a promising replacement for Pb [79]. Li et al. [80] prepared two-dimensional (2D) layered copper (Cu)-based (C6H5CH2NH3)2CuBr4 perovskite under benign conditions. The synthesis of C6H5CH2NH3Br (a) and (C6H5CH2NH3)2CuBr4 crystals has been depicted in Scheme 3.
Scheme 3

Schematic representation of synthesis process of C6H5CH2NH3Br (a) and (C6H5CH2NH3)2CuBr4 crystals (b). (Adapted with permission [80])

Initially, C6H5CH2NH3Br was prepared as shown in Scheme 3a which was further used as precursor for the preparation of (C6H5CH2NH3)2CuBr4 (Scheme 3b). The crystal structure of (C6H5CH2NH3)2CuBr4 was obtained by the reaction of C6H5CH2NH3Br and CuBr2 in stoichiometric amount. HBr was used as solvent for the preparation of crystal of (C6H5CH2NH3)2CuBr4 = (PMA)2CuBr4).

The crystal structure of (PMA)2CuBr4 has been presented in Fig. 21a, while a digital picture of the (PMA)2CuBr4 plates are also shown in Fig. 21b. The crystal structure of (PMA)2CuBr4 has a hexagonal structure (a = 10.588 Å, b = 10.486 Å, and c = 63.473 Å). The SEM results indicated the formation of layered structure of (PMA)2CuBr4 (Fig. 21c). The XRD patterns of the (PMA)2CuBr4, CuBr2, and C6H5CH2NH3CuBr have been displayed in Fig. 21d. The XRD pattern of (PMA)2CuBr4 showed the complete formation without remnants of raw materials. Furthermore, the PSCs were developed with a device architecture shown in Fig. 22a. The band gap of the (PMA)2CuBr4 perovskite was found to be 1.81 eV, which is lower than that of the Bi- or Sb-based perovskite structures.
Fig. 21

Schematic of crystal structure of (PMA)2CuBr4 (a), digital picture of (PMA)2CuBr4 plates (b), SEM (c) image of (PMA)2CuBr4 and XRD of (PMA)2CuBr4, CuBr2, and C6H5CH2NH3CuBr (d). (Adapted with permission [80])

Fig. 22

Schematic graph (a) and cross-sectional (b) SEM image of PSCs device. I-V curves (c), and EQE (d) of best performing PSCs device. (Adapted with permission [80])

The cross-sectional SEM image of the PSCs device was also recorded to show the inner layers as depicted in Fig. 22b. The cross-sectional SEM image clearly showed the presence of different components of the fabricated PSCs device. The performance was also investigated by recording the I-V and EQE measurements. The I-V and EQE curves of the best performing PSCs devices have been depicted in Fig. 22c–d. The best PSCs device exhibited a PCE of 0.2% with good open-circuit voltage of 680 mV (Fig. 22c). The quantum efficiency was also lower which was consistent with I-V results.

Conclusion and future prospectives

Perovskite solar cells with CH3NH3PbI3 perovskite structure have been widely studied and the highest efficiency of more than 25% was reported. However, the toxicity of Pb and lower stability of the CH3NH3PbI3 perovskite structure remains a challenge. Thus, in last few years, the research on the development of Pb-free perovskite solar cells has been increased. Many researchers around the world developed the Pb-free perovskite solar cells using different nontoxic or less toxic metals. Previously, bismuth- and antimony-based perovskite structures have been employed for solar cell applications but showed poor performance which may be due to the wide band gap or poor morphological features. Some researchers also prepared tin- or germanium-based perovskite structures which showed suitable band gap and good performance, but the rapid oxidation in air destroyed their practical applications. Copper-based perovskite structures have also been employed which also showed lower band gap, but their performance was found to be poor. Thus some effective and efficient organic or inorganic electron transport/charge extraction layers are required to improve the performance of Pb-free perovskite solar cells. The design of novel device architecture and use of efficient Pb-free new light absorbers are also essential for the development of efficient Pb-free perovskite solar cells. The introduction of new additives, antisolvents, etc. would also be of great importance toward the development of efficient Pb-free perovskite solar cells.

Notes

Acknowledgments

K.A. would like to thanks UGC, New Delhi (India) for RGNFD. Authors sincerely acknowledged Discipline of Chemistry, IIT Indore. S.M.M. acknowledged to SERB-DST (File No.: EMR/2016/001113), and CSIR (File No.: 01(2935)/18/EMR-II), New Delhi (India) for financial support and IIT Indore for research grant.

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Authors and Affiliations

  1. 1.Discipline of ChemistryIndian Institute of Technology IndoreIndoreIndia
  2. 2.Discipline of Biosciences and Bio-Medical EngineeringIndian Institute of Technology IndoreIndoreIndia
  3. 3.Discipline of Metallurgy Engineering and Material ScienceIndian Institute of Technology IndoreIndoreIndia

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