Lead-Free Perovskite Solar Cells
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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 Therefore, Pb has been replaced by other nontoxin or less toxic metal like Sn, Bi, Sb, and Cu.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.
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 . In the coming years, this energy demands are further going to increase with increasing population globally . Currently, fossil fuels or other energy sources are fulfilling the requirements, but these sources are limited . 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 . Basically, these photovoltaic devices, also called solar cells, convert the solar energy into electricity and work on the photoelectric effect . 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 . 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) . 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% . 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 . 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 . Cho et al. introduced (FAPbI3)0.85(MAPbBr3)0.15 perovskite for the development of highly efficient PSCs . 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 . 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 . 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.
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
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.
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.
Song et al.  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.
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.  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.
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
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.
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.  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.  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.
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. , 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 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
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 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.
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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