Bismuth Halide Perovskite Solar Cells

**85**

or Cs+

**Chapter 6**

**Abstract**

*Khursheed Ahmad*

Bismuth Halide Perovskites for

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

Photovoltaic Applications

properties toward the development of perovskite solar cells.

photovoltaics, perovskite solar cells

**1. Introduction**

**Keywords:** methyl ammonium bismuth halide, perovskites, light absorbers,

In the last few decades, researchers/scientists have focused to find out the most efficient and promising technology to fulfill the energy requirements globally [1–7]. 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 (CaTiO3) by Russian mineralogist L.A. Perovski [6]. However, another class of perovskite has also been discovered with molecular formula of ABX3, where A = organic or inorganic cation such as CH3NH3

, B=Pb2+ or Sn2+, etc. and X = halide anions. Miyasaka and co-workers have

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 MAPbX3 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 [8–19]. Later, some other strategies were made to further enhance the photovoltaic parameters of the perovskite solar cells. Lee et al. in 2012 reported

prepared methyl ammonium lead halide (MAPbX3; MA = CH3NH3

+

+

, X = I<sup>−</sup> or Br<sup>−</sup>)

## **Chapter 6**

## Bismuth Halide Perovskites for Photovoltaic Applications

*Khursheed Ahmad*

## **Abstract**

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.

**Keywords:** methyl ammonium bismuth halide, perovskites, light absorbers, photovoltaics, perovskite solar cells

## **1. 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 [1–7]. 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 (CaTiO3) by Russian mineralogist L.A. Perovski [6]. However, another class of perovskite has also been discovered with molecular formula of ABX3, where A = organic or inorganic cation such as CH3NH3 + or Cs+ , B=Pb2+ or Sn2+, etc. and X = halide anions. Miyasaka and co-workers have prepared methyl ammonium lead halide (MAPbX3; MA = CH3NH3 + , X = I<sup>−</sup> or Br<sup>−</sup>) 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 MAPbX3 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 [8–19]. 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. MAPbX3 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 [22–26]. MAPbX3 has been considered the key material for the development of high-performance perovskite solar cells due to its excellent absorption coefficient. MAPbX3 perovskite worked as a light absorber in perovskite solar cells. However, the presence of highly toxic Pb in the MAPbX3 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 ABX3 as discussed above. The optical properties of the perovskite materials can be tuned by changing the A, B or X ion present in the ABX3 structure. The size of the A, B or X should satisfy the Goldschmidt tolerance factor (*t*) equation given below: (t)= \_

$$\mathbf{r}(\mathbf{t}) = \frac{(rA + rX)}{\sqrt{2}(rB + rX.)} \tag{1}$$

**87**

applications.

**Figure 1.**

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

*Schematic illustration for the fabrication of perovskite solar cells.*

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.

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 [29–31]. The performance of these perovskite solar cells was less than 1%. In 2019, Lan et al. used FA3Bi2I9 (FA = CH(NH2)2) perovskite material for perovskite solar cell applications [36].

for perovskite solar cell applications. The X-ray diffraction (XRD) patterns of the FA3Bi2I9 and FA3Bi2I9 were found to be well-matched with the previous Joint

Ghasemi et al. [37] also prepared a new bismuth-based perovskite material phenethylammonium bismuth halides ((PEA)3Bi2I9, (PEA)3Bi2Br9 and

materials under facile conditions. The crystal structures of the (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 have been presented in **Figure 2**. Furthermore, they have been investigated for their physiochemical properties for optoelectronic

(PEA)4Bi2Cl10). The authors have prepared the crystal structures of these perovskite

The crystal structures of (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 showed the monoclinic crystal system with space group of *P21/n, P21/n* and *P21/c*, respectively.

+

) perovskite

**3. Origin of bismuth halide perovskite solar cells**

In this work, the authors also prepared MA3Bi2I9 (MA = CH3NH3

Committee on Powder Diffraction Standards (JCPDS) data.

where rA and rB are ionic radii of the A and B, while rX is the ionic radii of the X present in the ABX3 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 [29–32]. 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 A3Bi2I9 (A = MA+ or Cs+ ) for Pb-free perovskite solar cells [33]. Buonassisi et al. also employed solvent engineering approach using A3Bi2I9 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.

## **2. 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**.

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

**Figure 1.**

*Bismuth - Fundamentals and Optoelectronic Applications*

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. MAPbX3 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 [22–26]. MAPbX3 has been considered the key material for the development of high-performance perovskite solar cells due to its excellent absorption coefficient. MAPbX3 perovskite worked as a light absorber in perovskite solar cells. However, the presence of highly toxic Pb in the MAPbX3 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 ABX3 as discussed above. The optical properties of the perovskite materials can be tuned by changing the A, B or X ion present in the ABX3 structure. The size of the A, B or X should satisfy the Goldschmidt tolerance factor (*t*) equation given below: (t)= \_

(*rA* + *rX*)

where rA and rB are ionic radii of the A and B, while rX is the ionic radii of the X present in the ABX3 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 [29–32]. 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

) for Pb-free perovskite solar cells [33]. Buonassisi et al.

(1)

√ \_ 2 (*rB* + *rX*.)

also employed solvent engineering approach using A3Bi2I9 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

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

perovskite solar cells and future prospective have also been described.

**86**

illustrated in **Figure 1**.

of A3Bi2I9 (A = MA+

or Cs+

**2. Fabrication of perovskite solar cells**

*Schematic illustration for the fabrication of perovskite solar cells.*

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.

## **3. 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 [29–31]. The performance of these perovskite solar cells was less than 1%. In 2019, Lan et al. used FA3Bi2I9 (FA = CH(NH2)2) perovskite material for perovskite solar cell applications [36]. In this work, the authors also prepared MA3Bi2I9 (MA = CH3NH3 + ) perovskite for perovskite solar cell applications. The X-ray diffraction (XRD) patterns of the FA3Bi2I9 and FA3Bi2I9 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)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10). The authors have prepared the crystal structures of these perovskite materials under facile conditions. The crystal structures of the (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 have been presented in **Figure 2**. Furthermore, they have been investigated for their physiochemical properties for optoelectronic applications.

The crystal structures of (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 showed the monoclinic crystal system with space group of *P21/n, P21/n* and *P21/c*, respectively.

#### **Figure 2.**

*Crystal structures of (PEA)3Bi2I9 (a and b), (PEA)3Bi2Br9 (c and d) and (PEA)4Bi2Cl10 (e and f). Reproduced with permission [37].*

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)3Bi2I9 from the single crystal structure and measured XRD of the powder sample have been presented in **Figure 3a**. The measured XRD of (PEA)3Bi2I9 was well-matched with the calculated XRD pattern. The UV-vis spectra of the (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA) have been plotted in **Figure 3b**. The band gap of the (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 was calculated by Tauc relation as shown in **Figure 3c**. The direct band gap of the prepared (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 perovskite structures was found to be 2.23, 2.66 and 3.28 eV, whereas indirect band gap was 2.38, 2.79 and 3.38 eV, respectively.

The UPS of the (PEA)3Bi2I9 perovskite structure has been presented in **Figure 3d**, which provides better information of the energy levels of the (PEA)3Bi2I9 perovskite structure. Further, perovskite solar cells were fabricated and cross-sectional scanning electron microscopic (SEM) image of the device has been presented in **Figure 4a**, whereas the photocurrent density (I)-voltage (V) curves have been depicted in **Figure 4b**. The SEM image showed the connection between the interlayers of the perovskite solar cell components. The I-V curves of the fabricated perovskite solar cells with (PEA)3Bi2I9 and (MA)3Bi2I9 perovskite light absorbers showed good open circuit voltage, but the performance was poor. The open circuit voltage of the (PEA)3Bi2I9-based perovskite solar cell was high with improved

**89**

**Figure 3.**

**Figure 4.**

*permission [37].*

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

photocurrent density compared to the (MA)3Bi2I9-based perovskite solar cells. The PCE of the (PEA)3Bi2I9-based perovskite solar cells were found to be 0.23% with high open circuit voltage of 614 mV. However, the open circuit voltage for the (MA)3Bi2I9-based perovskite solar cell was found to be 515 mV with PCE of 0.09%.

*Cross-sectional SEM images of perovskite solar cell device (a), I-V curve (b) of perovskite solar cells and XRD patterns of the (PEA)3Bi2I9 perovskite structure exposed to air for different time. Reproduced with* 

*Calculated and experimental XRD (a) of (PEA)3Bi2I9, UV–vis spectra of (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 (b), Tauc relation of (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 (c) and ultraviolet photoemission spectroscopic (UPS) curve of (PEA)3Bi2I9 (d). Reproduced with permission [37].*

Kulkarni et al. [38] also developed the Pb-free perovskite solar cells using (MA)3Bi2I9 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

The obtained results were found to be poor in terms of PCE.

morphology of (MA)3Bi2I9 perovskite.

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

#### **Figure 3.**

*Bismuth - Fundamentals and Optoelectronic Applications*

These perovskite structures were composed of octahedral halide anionic clusters

ated 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)3Bi2I9 from the single crystal structure and measured XRD of the powder sample have been presented in **Figure 3a**. The measured XRD of (PEA)3Bi2I9 was well-matched with the calculated XRD pattern. The UV-vis spectra of the (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA) have been plotted in **Figure 3b**. The band gap of the (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 was calculated by Tauc relation as shown in **Figure 3c**. The direct band gap of the prepared (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 perovskite structures was found to be 2.23, 2.66 and 3.28 eV, whereas indirect band gap was

*Crystal structures of (PEA)3Bi2I9 (a and b), (PEA)3Bi2Br9 (c and d) and (PEA)4Bi2Cl10 (e and f).* 

The UPS of the (PEA)3Bi2I9 perovskite structure has been presented in **Figure 3d**, which provides better information of the energy levels of the (PEA)3Bi2I9 perovskite structure. Further, perovskite solar cells were fabricated and cross-sectional scanning electron microscopic (SEM) image of the device has been presented in **Figure 4a**, whereas the photocurrent density (I)-voltage (V) curves have been depicted in **Figure 4b**. The SEM image showed the connection between the interlayers of the perovskite solar cell components. The I-V curves of the fabricated perovskite solar cells with (PEA)3Bi2I9 and (MA)3Bi2I9 perovskite light absorbers showed good open circuit voltage, but the performance was poor. The open circuit voltage of the (PEA)3Bi2I9-based perovskite solar cell was high with improved

cationic groups. In the crystal structures, Bi atom is situ-

**88**

associated with PEA+

*Reproduced with permission [37].*

**Figure 2.**

2.38, 2.79 and 3.38 eV, respectively.

*Calculated and experimental XRD (a) of (PEA)3Bi2I9, UV–vis spectra of (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 (b), Tauc relation of (PEA)3Bi2I9, (PEA)3Bi2Br9 and (PEA)4Bi2Cl10 (c) and ultraviolet photoemission spectroscopic (UPS) curve of (PEA)3Bi2I9 (d). Reproduced with permission [37].*

#### **Figure 4.**

*Cross-sectional SEM images of perovskite solar cell device (a), I-V curve (b) of perovskite solar cells and XRD patterns of the (PEA)3Bi2I9 perovskite structure exposed to air for different time. Reproduced with permission [37].*

photocurrent density compared to the (MA)3Bi2I9-based perovskite solar cells. The PCE of the (PEA)3Bi2I9-based perovskite solar cells were found to be 0.23% with high open circuit voltage of 614 mV. However, the open circuit voltage for the (MA)3Bi2I9-based perovskite solar cell was found to be 515 mV with PCE of 0.09%. The obtained results were found to be poor in terms of PCE.

Kulkarni et al. [38] also developed the Pb-free perovskite solar cells using (MA)3Bi2I9 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)3Bi2I9 perovskite.

The XRD patterns of the (MA)3Bi2I9 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)3Bi2I9 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)3Bi2I9. Therefore, it can be said that the prepared (MA)3Bi2I9 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)3Bi2I9 without NMP (**Figure 6a**) showed the hexagonal flakes with high exposure of TiO2 and nonuniform surface. However, in case of (MA)3Bi2I9 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)3Bi2I9. Moreover, (MA)3Bi2I9 with 25% NMP covers the whole surface of the electrode substrate uniformly.

Further, perovskite solar cells were fabricated using (MA)3Bi2I9 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 (Jsc) 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)3Bi2I9 without and with different amount of NMP. # peak for FTO and \* for TiO2. Reproduced with permission [38].*

**91**

light absorber.

*NMP. Reproduced with permission [38].*

**Figure 7.**

**Figure 6.**

*permission [38].*

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

*Average I-V curves (a), IPCE (b), Jsc (c) and PCE (d) histogram of devices with and without* 

*SEM image of (MA)3Bi2I9 without (a) and with different amount of NMP (b–d). Reproduced with* 

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

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

#### **Figure 6.**

*Bismuth - Fundamentals and Optoelectronic Applications*

recorded and have been presented in **Figure 6**.

to the poor coverage of surface morphology.

substrate uniformly.

The XRD patterns of the (MA)3Bi2I9 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)3Bi2I9 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)3Bi2I9. Therefore, it can be said that the prepared (MA)3Bi2I9 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

The SEM image of (MA)3Bi2I9 without NMP (**Figure 6a**) showed the hexagonal flakes with high exposure of TiO2 and nonuniform surface. However, in case of (MA)3Bi2I9 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)3Bi2I9. Moreover, (MA)3Bi2I9 with 25% NMP covers the whole surface of the electrode

Further, perovskite solar cells were fabricated using (MA)3Bi2I9 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 (Jsc) 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

*XRD of (MA)3Bi2I9 without and with different amount of NMP. # peak for FTO and \* for TiO2. Reproduced* 

**90**

**Figure 5.**

*with permission [38].*

*SEM image of (MA)3Bi2I9 without (a) and with different amount of NMP (b–d). Reproduced with permission [38].*

#### **Figure 7.**

*Average I-V curves (a), IPCE (b), Jsc (c) and PCE (d) histogram of devices with and without NMP. Reproduced with permission [38].*

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

solar cells. The optical images of the Cs3Bi2I9-xBrx 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 Cs3Bi2I9-xBrx perovskite series (x = 0, 1, 2, 3, 4, 6 and 9). The band gap of the Cs3Bi2I9 was lower than that of the Cs3Bi2Br9 as confirmed by the absorption spectra (**Figure 8b**).

The XRD patterns of the Cs3Bi2I9-xBrx 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 23-0847. 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 Cs3Bi2I9 and Cs3Bi2I6Br3. 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 Cs3Bi2I9 showed poor performance, whereas the I-V curve of Cs3Bi2I6Br3 showed improved performance. The highest PCE of 1.15% was achieved for the Cs3Bi2I6Br3-based perovskite solar cells. The integrated Jsc for the Cs3Bi2I6Br3-based perovskite solar cells has been presented in **Figure 10b**, which showed the Jsc value of 3.11 mA/cm2 . However, the PCE of 0.23% was obtained for the Cs3Bi2I9-based perovskite solar cell device. The enhanced performance of the Cs3Bi2I6Br3-based perovskite solar cells attributed to the lower band gap and higher absorption property of Cs3Bi2I6Br3 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 Cs3Bi2I6Br3. Further improvements are still required to enhance the performance of the bismuth-based perovskite solar cells.

#### **Figure 8.**

*Optical images (a), UV-vis spectra (b) and XRD patterns of Cs3Bi2I9-xBrx (x = 0, 1, 2, 3, 4, 6 and 9). Reproduced with permission [39].*

**93**

**Table 1.**

**4. Future prospective**

**S. No. Light absorbers Jsc (mA/cm2**

*Photovoltaic parameters of Bi-based perovskite solar cells.*

**Figure 9.**

**Figure 10.**

*permission [39].*

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)3Bi2I9 perovskite light absorber and constructed the PSC device. The developed device exhibited PCE of 0.3%. Kulkarni

1. (PEA)3Bi2I9 ~0.81 614 0.3 [37] 2. (MA)3Bi2I9 ~0.4 515 0.09 [37] 3. (MA)3Bi2I9 ~0.87 530 0.3 [38] 4. Cs3Bi2I6Br3 3.11 ~650 1.15 [39] 5. Cs3Bi2I9 ~0.501 ~750 0.23 [39]

**) Voc (mV) PCE (%) References**

*I-V curves (a) and EQE (b) of the fabricated perovskite solar cells. Reproduced with permission [39].*

*Energy level graph (a) and cross-sectional SEM image (b) of perovskite solar cells. Reproduced with* 

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

#### **Figure 9.**

*Bismuth - Fundamentals and Optoelectronic Applications*

played in **Figure 9a**.

perovskite solar cells.

solar cells. The optical images of the Cs3Bi2I9-xBrx 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 Cs3Bi2I9-xBrx perovskite series (x = 0, 1, 2, 3, 4, 6 and 9). The band gap of the Cs3Bi2I9 was lower than that of

The XRD patterns of the Cs3Bi2I9-xBrx 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 23-0847. 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 dis-

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 Cs3Bi2I9 and Cs3Bi2I6Br3. 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 Cs3Bi2I9 showed poor performance, whereas the I-V curve of Cs3Bi2I6Br3 showed improved performance. The highest PCE of 1.15% was achieved for the Cs3Bi2I6Br3-based perovskite solar cells. The integrated Jsc for the Cs3Bi2I6Br3-based perovskite solar cells has been presented

0.23% was obtained for the Cs3Bi2I9-based perovskite solar cell device. The enhanced performance of the Cs3Bi2I6Br3-based perovskite solar cells attributed to the lower band gap and higher absorption property of Cs3Bi2I6Br3 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 Cs3Bi2I6Br3. Further improvements are still required to enhance the performance of the bismuth-based

*Optical images (a), UV-vis spectra (b) and XRD patterns of Cs3Bi2I9-xBrx (x = 0, 1, 2, 3, 4, 6 and 9).* 

. However, the PCE of

the Cs3Bi2Br9 as confirmed by the absorption spectra (**Figure 8b**).

in **Figure 10b**, which showed the Jsc value of 3.11 mA/cm2

**92**

**Figure 8.**

*Reproduced with permission [39].*

*Energy level graph (a) and cross-sectional SEM image (b) of perovskite solar cells. Reproduced with permission [39].*

**Figure 10.** *I-V curves (a) and EQE (b) of the fabricated perovskite solar cells. Reproduced with permission [39].*

## **4. 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)3Bi2I9 perovskite light absorber and constructed the PSC device. The developed device exhibited PCE of 0.3%. Kulkarni


**Table 1.**

*Photovoltaic parameters of Bi-based perovskite solar cells.*

et al. [38] also employed novel and unique approaches to improve the surface morphology of the (MA)3Bi2I9 perovskite film, and the developed PSC device showed PCE of 0.3%. Yu et al. [39] also tuned the optical properties of the Cs3Bi2I9 perovskite structure. Thus, the designed and synthesized Cs3Bi2I6Br3 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:


## **5. 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.

## **Acknowledgements**

K.A. sincerely acknowledged Discipline of Chemistry, IIT Indore. K.A. also thanks UGC, New Delhi, India, for RGNFD fellowship.

**95**

**Author details**

Khursheed Ahmad

Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, M.P., India

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: khursheed.energy@gmail.com

provided the original work is properly cited.

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

## **Conflict of interest**

The author declares no conflict of interest.

*Bismuth Halide Perovskites for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.92413*

*Bismuth - Fundamentals and Optoelectronic Applications*

et al. [38] also employed novel and unique approaches to improve the surface morphology of the (MA)3Bi2I9 perovskite film, and the developed PSC device showed PCE of 0.3%. Yu et al. [39] also tuned the optical properties of the Cs3Bi2I9 perovskite structure. Thus, the designed and synthesized Cs3Bi2I6Br3 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.

improve the performance of the Bi-based perovskite solar cells:

improve the performance of the perovskite solar cells.

the performance of the perovskite solar cells.

voltaic parameters.

**5. Conclusions**

tion layers.

**Acknowledgements**

**Conflict of interest**

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

1.Introducing new device architecture of the perovskite solar cells may improve

2.The performance of the Bi-based perovskite solar cells could be enhanced by

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 photo-

4.Solvent engineering and doping of Bi-based perovskite structure may also

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 extrac-

K.A. sincerely acknowledged Discipline of Chemistry, IIT Indore. K.A. also

thanks UGC, New Delhi, India, for RGNFD fellowship.

The author declares no conflict of interest.

developing the novel charge extraction/electron transport layer.

**94**

## **Author details**

Khursheed Ahmad Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, M.P., India

\*Address all correspondence to: khursheed.energy@gmail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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2018;**1**:3826-3834

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[2] Reddy VS, Kaushik SC, Ranjan KR, Tyagi SK. State-of-the-art of solar thermal power plants. Renewable and Sustainable Energy Reviews.

[3] Kitano M, Hara M. Heterogeneous photocatalytic cleavage of water. Journal of Materials Chemistry.

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Section 4

Bismuth Related Data

Storage Materials

**99**

Section 4
