Bismuth Halide Perovskites for Photovoltaic Applications

In the last decade, energy crisis has become the most important topic for researchers. Energy requirements have increased drastically. To overcome the issue of energy crisis in near future, numerous efforts and sources have been developed. Therefore, solar energy has been considered the most promising energy source compared to other energy sources. There were different kinds of photovoltaic devices developed, but perovskite solar cells have been considered the most efficient and promising solar cell. The perovskite solar cells were invented in 2009 and crossed an excellent power conversion efficiency of 25%. However, it has a few major drawbacks, such as the presence of highly toxic lead (Pb) and poor stability. Hence, numerous efforts were made toward the replacement of Pb and highly stable perovskite solar cells in the last few years. Bismuth halide perovskite solar cell is one type of the replacement introduced to overcome these issues. In this chapter, I have reviewed the role of bismuth halide perovskite structures and their optoelectronic properties toward the development of perovskite solar cells.


Introduction
In the last few decades, researchers/scientists have focused to find out the most efficient and promising technology to fulfill the energy requirements globally [1][2][3][4][5][6][7]. There are various renewable energy sources, but solar energy is a never-ending source. Photovoltaic or solar cell is a device that converts solar energy into electrical energy. In 2009, perovskite solar cells (PSCs) were developed, which exhibited the good open circuit voltage with decent power conversion efficiency (PCE) [6]. The term "perovskite" was created for calcium titanate (CaTiO 3 ) by Russian mineralogist L.A. Perovski [6]. However, another class of perovskite has also been discovered with molecular formula of ABX 3 , where A = organic or inorganic cation such as CH 3 NH 3 + or Cs + , B=Pb 2+ or Sn 2+ , etc. and X = halide anions. Miyasaka and co-workers have prepared methyl ammonium lead halide (MAPbX 3 ; MA = CH 3 NH 3 + , X = I − or Br − ) perovskites and used as visible light sensitizer in dye-sensitized solar cells (DSSCs) [6]. The fabricated device showed an interesting PCE of 3%. However, the use of liquid electrolyte destroyed its photovoltaic performance due to the dissolution of MAPbX 3 in the liquid electrolyte. Later, various strategies were developed to solve this issue, and a solid-state hole transport material was employed to overcome the dissolution of perovskites [8][9][10][11][12][13][14][15][16][17][18][19]. Later, some other strategies were made to further enhance the photovoltaic parameters of the perovskite solar cells. Lee et al. in 2012 reported an improved PCE of 10.9% by introducing solid-state perovskite solar cells [20]. In a short span of time, perovskite solar cells attracted researchers globally. In 2019, the highest PCE of 23.3% was reported for Pb halide perovskite solar cells [21]. This enhanced PCE is really impressive and has the potential for practical applications of perovskite solar cells. MAPbX 3 has also been used in other applications such as photodetectors, light-emitting-diodes, batteries and supercapacitors due to the presence of excellent optoelectronic properties in the perovskite materials [22][23][24][25][26]. MAPbX 3 has been considered the key material for the development of high-performance perovskite solar cells due to its excellent absorption coefficient. MAPbX 3 perovskite worked as a light absorber in perovskite solar cells. However, the presence of highly toxic Pb in the MAPbX 3 structure is a major drawback and a challenge for the scientific community to replace it with less toxic or nontoxic element. The presence of toxic Pb also restricted the practical use of perovskite solar cells. Thus, in the last 5 years, Pb has been replaced with Sn and Ge and showed good performance, but poor stability dismissed their performance [27,28]. The perovskite structures possess general formula of ABX 3 as discussed above. The optical properties of the perovskite materials can be tuned by changing the A, B or X ion present in the ABX 3 structure. The size of the A, B or X should satisfy the Goldschmidt tolerance factor (t) equation given below: where rA and rB are ionic radii of the A and B, while rX is the ionic radii of the X present in the ABX 3 structure. The perovskite materials showed good stability when tolerance factor is equal to 1. Recently, bismuth (Bi), antimony and copper, which are less toxic metals, have been used for the fabrication of Pb-free perovskite solar cells [29][30][31][32]. The Bi is a nontoxic element and has the similar properties and ionic size to those of Pb. The ionic radii of the Bi also satisfy the tolerance factor rule and enhanced the stability of the Bi-based perovskite materials. Moreover, it was found that Bi-based perovskite materials possess higher absorption coefficient, which makes them an efficient light-absorbing material for solar cell applications. Thus, Johansson et al. employed a ternary Bi-based perovskites with a molecular formula of A 3 Bi 2 I 9 (A = MA + or Cs + ) for Pb-free perovskite solar cells [33]. Buonassisi et al. also employed solvent engineering approach using A 3 Bi 2 I 9 perovskite material and the PCE of 0.71% was obtained [34]. Later, various approaches have been made to improve the performance of the Bi-based perovskite solar cells, and the highest PCE of 3.17% was achieved by applying vapor deposition approach [35]. In this chapter, I have reviewed the fabrication of perovskite solar cells. Recent advances in Bi-based perovskite solar cells and future prospective have also been described.

Fabrication of perovskite solar cells
Generally, perovskite solar cells are composed of different components such as electrode substrates (usually indium-doped tin oxide [ITI] or fluorine-doped tin oxide [FTO] glass substrates), perovskite light absorber layer, electron transport layer, hole transport layer and metal contact layer. Firstly, FTO or ITO glass substrates were cleaned and patterned with zinc powder and 2 molar hydrochloride solution to avoid short circuit in the device. Further, an electron transport layer was spin coated and annealed at ~500°C. Further, perovskite layer was deposited and annealed at temperature of ~ 70-120°C. Later, a hole transport material (HTM) layer was deposited followed by the deposition of metal contact layer of Au or Ag using thermal evaporation. The fabrication of perovskite solar cells has been illustrated in Figure 1. The performance of the fabricated perovskite solar cells is investigated by various techniques such as photocurrent-voltage (I-V), external quantum efficiency (EQE), incident-photon-to-current-conversion efficiency (IPCE), photoluminescence spectroscopy, etc. Generally, the photovoltaic performance of any perovskite solar cells is estimated in terms of power conversion efficiency (PCE), fill factor, open circuit voltage and photocurrent density using I-V measurements. Photoluminescence spectroscopy revealed the electron lifetime of the generated electron in the perovskite structure, which is used to check the recombination reactions. It is believed that the perovskite solar cells with lower recombination reaction rate or high electron lifetime provide better performance in terms of efficiency.

Origin of bismuth halide perovskite solar cells
Since 2009, perovskite solar cells attracted enormous attention due to their excellent performance and simple fabrication process. However, the presence of highly toxic Pb remains a challenge. In recent years, bismuth has been employed to replace Pb from perovskite solar cells. Bismuth has been explored and Pb-free perovskite structures have been synthesized and employed for the development of Pb-free perovskite solar cells. Ahmad et al. employed new Pb-free perovskite light absorbers for perovskite solar cell applications [29][30][31]. The performance of these perovskite solar cells was less than 1%. In 2019, Lan et al. used FA 3 Bi 2 I 9 (FA = CH(NH 2 ) 2 ) perovskite material for perovskite solar cell applications [36]. In this work, the authors also prepared MA 3 Bi 2 I 9 (MA = CH 3 NH 3 + ) perovskite for perovskite solar cell applications. The X-ray diffraction (XRD) patterns of the FA 3 Bi 2 I 9 and FA 3 Bi 2 I 9 were found to be well-matched with the previous Joint Committee on Powder Diffraction Standards (JCPDS) data.
Ghasemi et al. [37] also prepared a new bismuth-based perovskite material phenethylammonium bismuth halides ((PEA) 3 Bi 2 I 9 , (PEA) 3 Bi 2 Br 9 and (PEA) 4 Bi 2 Cl 10 ). The authors have prepared the crystal structures of these perovskite materials under facile conditions. The crystal structures of the (PEA) 3 Bi 2 I 9 , (PEA) 3 Bi 2 Br 9 and (PEA) 4 Bi 2 Cl 10 have been presented in Figure 2. Furthermore, they have been investigated for their physiochemical properties for optoelectronic applications.
The crystal structures of (PEA) 3 Bi 2 I 9 , (PEA) 3 Bi 2 Br 9 and (PEA) 4 Bi 2 Cl 10 showed the monoclinic crystal system with space group of P2 1 /n, P2 1 /n and P2 1 /c, respectively. These perovskite structures were composed of octahedral halide anionic clusters associated with PEA + cationic groups. In the crystal structures, Bi atom is situated in the center of each octahedron, whereas halide anions are in the corner of every face-sharing octahedron. The prepared perovskite structures possess zerodimensional (0D) structure. The calculated XRD of (PEA) 3 Bi 2 I 9 from the single crystal structure and measured XRD of the powder sample have been presented in Figure 3a. The measured XRD of (PEA) 3 Bi 2 I 9 was well-matched with the calculated XRD pattern. The UV-vis spectra of the (PEA) 3 Bi 2 I 9 , (PEA) 3  The UPS of the (PEA) 3 Bi 2 I 9 perovskite structure has been presented in Figure 3d, which provides better information of the energy levels of the (PEA) 3 Bi 2 I 9 perovskite structure. Further, perovskite solar cells were fabricated and cross-sectional scanning electron microscopic (SEM) image of the device has been presented in  Kulkarni et al. [38] also developed the Pb-free perovskite solar cells using (MA) 3 Bi 2 I 9 perovskite structure with novel strategies. Kulkarni et al. have employed N-methyl pyrrolidone (NMP) additive as morphology controller. In this work, different amount of NMP was added to observe the effect of NMP on the surface morphology of (MA) 3 Bi 2 I 9 perovskite.   3 Bi 2 I 9 perovskite structure exposed to air for different time. Reproduced with permission [37].

Cross-sectional SEM images of perovskite solar cell device (a), I-V curve (b) of perovskite solar cells and XRD patterns of the (PEA)
The XRD patterns of the (MA) 3 Bi 2 I 9 without and with different amount of NMP were recorded and the obtained results have been presented in Figure 5. The obtained XRD patterned was well-matched with previously reported JCPDS data. This confirms the formation of (MA) 3 Bi 2 I 9 phase. However, the addition of NMP increases the peak intensity, which resulted in the orientation change. The other parameters also suggested that addition of NMP increases the crystallinity of the (MA) 3 Bi 2 I 9 . Therefore, it can be said that the prepared (MA) 3 Bi 2 I 9 perovskite in the absence of NMP has poor crystalline phase or poor crystallinity. Furthermore, to investigate the effect of NMP on the morphological features, SEM images were recorded and have been presented in Figure 6.
The SEM image of (MA) 3 Bi 2 I 9 without NMP (Figure 6a) showed the hexagonal flakes with high exposure of TiO 2 and nonuniform surface. However, in case of (MA) 3 Bi 2 I 9 with NMP 12.5% (Figure 6b) showed large crystals, but poor uniform surface was observed. When 25% NMP (Figure 6c) was added, the surface morphology further changed. And the addition of 50% NMP drastically changed the surface morphology with larger grains as shown in Figure 6d. This suggested that 12.5% NMP was not sufficient to control the surface morphology of the (MA) 3 Bi 2 I 9 . Moreover, (MA) 3 Bi 2 I 9 with 25% NMP covers the whole surface of the electrode substrate uniformly.
Further, perovskite solar cells were fabricated using (MA) 3 Bi 2 I 9 without and with NMP (at different percentage) under same conditions. The I-V and IPCE curves of the fabricated perovskite solar cells have been presented in Figure 7a and b respectively. The photocurrent density (J sc ) and PCE histograms were also presented in Figure 7c and d, respectively. The perovskite solar cell device fabricated in the absence of NMP showed PCE of 0.19% and open circuit voltage of 530 mV. However, the improved PCE of 0.31% was obtained for the perovskite solar cells fabricated with the addition of 25% NMP. In other cases, perovskite solar cells fabricated with 12.5% and 50% NMP showed lower performance, which may be due to the poor coverage of surface morphology.

Figure 5.
XRD of (MA) 3  In another work, Yu et al. [39] have prepared new perovskite solar cell device architecture of perovskite solar cells with mixed halide bismuth-based perovskite light absorber.
The authors have prepared a series of mixed halide-based perovskite structures. They have employed novel and simple strategies to prepare the Pb-free perovskite   [38]. solar cells. The optical images of the Cs 3 Bi 2 I 9-x Br x perovskite series (x = 0, 1, 2, 3, 4, 6 and 9) have been presented in Figure 8a, which show the different colors of the prepared thin films, and this change in the color of the perovskite thin film was due to the change in the halide composition. Further, optical properties were checked by recording absorption spectra of the prepared thin films of Cs 3 Bi 2 I 9-x Br x perovskite series (x = 0, 1, 2, 3, 4, 6 and 9). The band gap of the Cs 3 Bi 2 I 9 was lower than that of the Cs 3 Bi 2 Br 9 as confirmed by the absorption spectra (Figure 8b).
The XRD patterns of the Cs 3 Bi 2 I 9-x Br x perovskite series (x = 0, 1, 2, 3, 4, 6 and 9) have been presented in Figure 8c. The obtained results showed strong and welldefined diffraction peaks, which suggested the high crystalline nature of the prepared Pb-free perovskite structures. The obtained XRD results were well-matched with previous JCPDS card number . This confirms the successful formation of the prepared Pb-free perovskite structures. Further, perovskite solar cell devices were fabricated with these perovskite light absorbers. The schematic graph for the energy level values of the Pb-free perovskite solar cell components has been displayed in Figure 9a.
The schematic diagram showing that the electrons generated in the perovskite structure is transferred to the hole transport layers. The cross-sectional SEM image of the fabricated perovskite solar cells showed all the component layers of the perovskite solar cells in Figure 9b. Further, perovskite solar cells were fabricated by using different light absorbers Cs 3 Bi 2 I 9 and Cs 3 Bi 2 I 6 Br 3 . The I-V measurements (Figure 10a) and external quantum efficiency (EQE) were measured (Figure 10b) of the fabricated perovskite solar cells. The I-V curve of the Cs 3 Bi 2 I 9 showed poor performance, whereas the I-V curve of Cs 3 Bi 2 I 6 Br 3 showed improved performance. The highest PCE of 1.15% was achieved for the Cs 3 Bi 2 I 6 Br 3 -based perovskite solar cells. The integrated J sc for the Cs 3 Bi 2 I 6 Br 3 -based perovskite solar cells has been presented in Figure 10b, which showed the J sc value of 3.11 mA/cm 2 . However, the PCE of 0.23% was obtained for the Cs 3 Bi 2 I 9 -based perovskite solar cell device. The enhanced performance of the Cs 3 Bi 2 I 6 Br 3 -based perovskite solar cells attributed to the lower band gap and higher absorption property of Cs 3 Bi 2 I 6 Br 3 perovskite structure.
These obtained PCE were quite interesting, which is believed due to the tuning of the perovskite structure and better absorption activity of the Cs 3 Bi 2 I 6 Br 3 . Further improvements are still required to enhance the performance of the bismuth-based perovskite solar cells.

Future prospective
It is well understood that Pb-based solar cells could not be commercialized due to the poor stability and toxic nature of Pb. Thus, Bi-based perovskite solar cells were developed under benign approaches as listed in Table 1. Since 2017, various approaches were made to construct the highly stable Pb-free PSCs. In this regard, Ghasemiet et al. [37] prepared (PEA) 3 Bi 2 I 9 perovskite light absorber and constructed the PSC device. The developed device exhibited PCE of 0.3%. Kulkarni   et al. [38] also employed novel and unique approaches to improve the surface morphology of the (MA) 3 Bi 2 I 9 perovskite film, and the developed PSC device showed PCE of 0.3%. Yu et al. [39] also tuned the optical properties of the Cs 3 Bi 2 I 9 perovskite structure. Thus, the designed and synthesized Cs 3 Bi 2 I 6 Br 3 perovskite material-based PSC device showed enhanced PCE of 1.15%. These obtained results showed the excellent optical properties of the Bi-based perovskite materials and suggested their potential as light absorbers in the construction of PSCs. The Bi-based perovskite structures have wide band gap ~2.2 eV, which limits their absorption properties. Moreover, poor surface morphology of the Bi-based perovskite structures has been the other drawback, which resulted in the poor performance. Hence, the following strategies could be the key points to further improve the performance of the Bi-based perovskite solar cells: 1. Introducing new device architecture of the perovskite solar cells may improve the performance of the perovskite solar cells.
2. The performance of the Bi-based perovskite solar cells could be enhanced by developing the novel charge extraction/electron transport layer.
3. Some new approaches can be applied to prepare the high-quality thin films of the Bi-based perovskite structures, which further help to improve the photovoltaic parameters.
4. Solvent engineering and doping of Bi-based perovskite structure may also improve the performance of the perovskite solar cells.

Conclusions
The origin and advances in the field of perovskite solar cells have been reviewed. Perovskite solar cells could be the future energy source with low cost. The replacements of the toxic Pb with different metals have been investigated to produce Pb-free perovskite structures for the development of Pb-free perovskite solar cells. Bismuth, which is a nontoxic element, has been widely employed for the development of highly stable and Pb-free perovskite structures. Bi-based perovskite structures have shown promising performance and their performance may be further improved by incorporating unique approaches and efficient charge extraction layers.