**3. Advances in perovskite solar cells**

The possibility of merging the properties of inorganic with those of organic solids has inspired intensive research into the versatile properties. Organicinorganic perovskite materials have been widely used in PSCs using different ETLs and HTLs. The optimization of materials and structures is one of the solutions to improve the PCE. **Table 1** shows some representative devices and their architectures and performance.

Methylammonium Lead halide perovskites (MAPbX3) are mostly regarded as promising light absorbers owing their many advantages comprising high absorption coefficients, optimal bandgaps, and long-range exciton diffusion lengths. These perovskites have led to solar cells with PCEs upto15% in combination with meso-structure metal oxides and deposition methods (such as sequential and vapor deposition) [34]. There were few attempts to synthesize new perovskites by changing halide anions (X) in the MAPbX3 structure, but these materials did not result too much improvement in device efficiency. Optical and electronic properties of organo lead halide perovskites have been considered by replacing MA cation with other organic cations such as ethyl ammonium and formamidinium [55].

Dkhissi et al. [32] fabricated an efficiently CH3NH3PbI3-based planar perovskite solar cells on polymer substrates at 150°C or below. The hole blocking layer employed is a TiO2 layer. The devices showed an average efficiency of 10.6 ± 1.2%, and a maximum efficiency of 12.3% for flexible perovskite solar cells, presenting great potential for further enhancement of the low-cost, low-temperature processing solar technology.

In 2014, Choi et al. [55] modified perovskite material with Cesium (Cs) by doping methyl ammonium lead iodide perovskites by Cesium to improve the performance of inverted-type perovskite/fullerene planar heterojunction hybrid solar cells. CsxMA1-xPbI3 perovskite devices achieved improvement in device efficiency from 5.51–6.8% with an optimized 10% Cs doping concentration. The devices exhibited an outstanding increase in efficiency due to increases in short-circuit current density and open-circuit voltage.


#### **Table 1.**

*Comparison of different organic inorganic perovskite materials with different hole and electron TLs in PSCs.*

**79**

*Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material*

transport material for efficient perovskite based solar cells.

CH3NH3PbBr3 and CH3NH3PbI3, were used as sensitizers for TiO2 in a liquid junction solar cell, with open-circuit voltages of 0.61 and 0.71 V were achieved. CH3NH3PbI3 on mesoporous TiO2 showed good charge transport properties, where the perovskite is both the absorber and the hole conductor. Further CH3NH3PbX3 (X = Br, I), mixed perovskite lead halides i.e., CH3NH3PbI2Cl, CH3NH3PbBr3 − xClx,

Giacomo et al. [47] fabricated PSCs using CH3NH3PbI3-xClx with different holetransporting materials. The mostly used Spiro-OMeTAD has been compared to the P3HT. By changing the energy level of P3HT and optimizing the device fabrication, PCE reached to 9.3%. They showed that P3HT can be used a suitable low-cost hole

NiO has been tried as a substitute for organic molecular or polymeric HTMs (spiro-MeOTAD), displaying encouraging results in the TiO2/CH3NH3PbI3 configuration, a PCE of 9.5% was attained with nanocrystalline NiO layer. As the valence band edge (5.4 eV) for NiO is near to that of iodide perovskite (5.3 eV), so posttreatment of NiO film by means of UV light or oxygen plasma is vital to progress hole injection efficiency due to an increase in the work function of NiO by such post treatments. UV-ozone post-treated NiO usually has a greater photovoltaic performance than untreated NiO, due to change in work function and an enhancement in wettability indicating a better chemical interaction between perovskite

The use of perovskites with mixed cations and halides has become significant for PV applications which are mainly MAPbX3, FAPbX3 and CsPbX3 (X = Br or I). on the introduction of MA into FA brings the crystallization of FA perovskite (because MA is slightly smaller than FA) which allows a large fraction of the yellow phase to continue. MA/FA compounds show notable PCEs and therefore the development of these compounds is an opportunity in the advancement of PSCs. Saliba et al. [56] introduces an innovative approach using a triple Cs/MA/FA cation mixture where Cs is used to progress MA/FA perovskite compounds. A small amount of Cs is enough to efficiently suppress yellow phase impurities allowing the preparation of

Song et al. [57] reported that the combination of FA decreases the release of organic species but does not stop the formation of I/HI. Though, the addition of Cs successfully overcomes the release of all volatile gases. The best photostability is found with FA/Cs mixed perovskites, presenting the complete removal of MA from

As CH3NH3PbI3 has ambipolar properties and is slightly more p-type than n-type and is satisfactory to develop p-n junction-like devices without an HTM, known as HTM-free photovoltaic cells. CH3NH3PbI3 could act both as light absorber and hole transporter in a CH3NH3PbI3/mesoporous TiO2 heterojunction device with a PCE of 5.5%. It was observed that HTM-free perovskite solar cells had a poor FF and a low Voc as compared to those with an HTM, which is related to the larger shunt current

Different proportions of inorganic (Pb*,* Sn) cations, organic cations and halide anions (I*,* Br*,* Cl) can be combined in mixed perovskites, permitting their properties to be fine-tuned [35]. Tuning of bandgap of MAPbX3 has been attained through the substitution of I with Cl/Br ions, which occurs from a dependence of electronic energies on the effective exciton mass. The optical absorption can be tuned by bandgap engineering to comprise the whole visible spectrum. In the meantime, the combination of Cl/Br into iodide-based structure has markedly advanced the charge transport and the separation kinetics within the perovskite layer. Hence, by tuning the composition of perovskite resulted in improved efficiency and the stability of PSCs. It was observed that an increase in the size of perovskite cation materials

mixed-cation perovskite is favored for more photostable perovskites.

*DOI: http://dx.doi.org/10.5772/intechopen.94104*

and CH3NH3PbI3 − xClx were studied [27].

and NiO [36].

pure, defect-free perovskite films.

along with a lower IPCE for these devices [2].

#### *Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material DOI: http://dx.doi.org/10.5772/intechopen.94104*

*Perovskite and Piezoelectric Materials*

tures and performance.

ing solar technology.

current density and open-circuit voltage.


Li-TFSI

Li-TFSI

TiO2-Al2O3 Spiro-OMeTAD

SnO2 QD Spiro-OMeTAD-

SnO2 QD Spiro-OMeTAD-

**3. Advances in perovskite solar cells**

The possibility of merging the properties of inorganic with those of organic

inorganic perovskite materials have been widely used in PSCs using different ETLs and HTLs. The optimization of materials and structures is one of the solutions to improve the PCE. **Table 1** shows some representative devices and their architec-

Methylammonium Lead halide perovskites (MAPbX3) are mostly regarded as promising light absorbers owing their many advantages comprising high absorption coefficients, optimal bandgaps, and long-range exciton diffusion lengths. These perovskites have led to solar cells with PCEs upto15% in combination with meso-structure metal oxides and deposition methods (such as sequential and vapor deposition) [34]. There were few attempts to synthesize new perovskites by changing halide anions (X) in the MAPbX3 structure, but these materials did not result too much improvement in device efficiency. Optical and electronic properties of organo lead halide perovskites have been considered by replacing MA cation with

Dkhissi et al. [32] fabricated an efficiently CH3NH3PbI3-based planar perovskite

In 2014, Choi et al. [55] modified perovskite material with Cesium (Cs) by doping methyl ammonium lead iodide perovskites by Cesium to improve the performance of inverted-type perovskite/fullerene planar heterojunction hybrid solar cells. CsxMA1-xPbI3 perovskite devices achieved improvement in device efficiency from 5.51–6.8% with an optimized 10% Cs doping concentration. The devices exhibited an outstanding increase in efficiency due to increases in short-circuit

**ETL HTL Perovskite PCE References** Gr/ZnO-QDs Spiro-OMeTAD CH3NH3PbI3 9.73 [18]

ZnO-NPs Spiro-OMeTAD CH3NH3PbI3 10.2 [34] TiO2 Spiro-OMeTAD CH3NH3PbI3-xClx 11.7 [47] TiO2 Spiro-OMeTAD CH3NH3PbI3 15.4 [48] TiO2 — CH3NH3Pb Br3−n 8.54 [49]

SnO2 Spiro-OMeTAD Cs/MA/FA perovskite 20.7 [51] SnO2 QD Spiro-OMeTAD CH3NH3PbI3 19.12 [52] TiO2 Spiro-OMeTAD (FAI)0.81(PbI2)0.85(MAPbBr3)0.15 21.02 [53, 54]

*Comparison of different organic inorganic perovskite materials with different hole and electron TLs in PSCs.*

Cs0.05(MA0.17FA0.83)0.95Pb (I0.83Br0.17)3

— 10 [24]

CH3NH3PbI3 19.73 [50]

20.79 [50]

solids has inspired intensive research into the versatile properties. Organic-

other organic cations such as ethyl ammonium and formamidinium [55].

solar cells on polymer substrates at 150°C or below. The hole blocking layer employed is a TiO2 layer. The devices showed an average efficiency of 10.6 ± 1.2%, and a maximum efficiency of 12.3% for flexible perovskite solar cells, presenting great potential for further enhancement of the low-cost, low-temperature process-

**78**

**Table 1.**

CH3NH3PbBr3 and CH3NH3PbI3, were used as sensitizers for TiO2 in a liquid junction solar cell, with open-circuit voltages of 0.61 and 0.71 V were achieved. CH3NH3PbI3 on mesoporous TiO2 showed good charge transport properties, where the perovskite is both the absorber and the hole conductor. Further CH3NH3PbX3 (X = Br, I), mixed perovskite lead halides i.e., CH3NH3PbI2Cl, CH3NH3PbBr3 − xClx, and CH3NH3PbI3 − xClx were studied [27].

Giacomo et al. [47] fabricated PSCs using CH3NH3PbI3-xClx with different holetransporting materials. The mostly used Spiro-OMeTAD has been compared to the P3HT. By changing the energy level of P3HT and optimizing the device fabrication, PCE reached to 9.3%. They showed that P3HT can be used a suitable low-cost hole transport material for efficient perovskite based solar cells.

NiO has been tried as a substitute for organic molecular or polymeric HTMs (spiro-MeOTAD), displaying encouraging results in the TiO2/CH3NH3PbI3 configuration, a PCE of 9.5% was attained with nanocrystalline NiO layer. As the valence band edge (5.4 eV) for NiO is near to that of iodide perovskite (5.3 eV), so posttreatment of NiO film by means of UV light or oxygen plasma is vital to progress hole injection efficiency due to an increase in the work function of NiO by such post treatments. UV-ozone post-treated NiO usually has a greater photovoltaic performance than untreated NiO, due to change in work function and an enhancement in wettability indicating a better chemical interaction between perovskite and NiO [36].

The use of perovskites with mixed cations and halides has become significant for PV applications which are mainly MAPbX3, FAPbX3 and CsPbX3 (X = Br or I). on the introduction of MA into FA brings the crystallization of FA perovskite (because MA is slightly smaller than FA) which allows a large fraction of the yellow phase to continue. MA/FA compounds show notable PCEs and therefore the development of these compounds is an opportunity in the advancement of PSCs. Saliba et al. [56] introduces an innovative approach using a triple Cs/MA/FA cation mixture where Cs is used to progress MA/FA perovskite compounds. A small amount of Cs is enough to efficiently suppress yellow phase impurities allowing the preparation of pure, defect-free perovskite films.

Song et al. [57] reported that the combination of FA decreases the release of organic species but does not stop the formation of I/HI. Though, the addition of Cs successfully overcomes the release of all volatile gases. The best photostability is found with FA/Cs mixed perovskites, presenting the complete removal of MA from mixed-cation perovskite is favored for more photostable perovskites.

As CH3NH3PbI3 has ambipolar properties and is slightly more p-type than n-type and is satisfactory to develop p-n junction-like devices without an HTM, known as HTM-free photovoltaic cells. CH3NH3PbI3 could act both as light absorber and hole transporter in a CH3NH3PbI3/mesoporous TiO2 heterojunction device with a PCE of 5.5%. It was observed that HTM-free perovskite solar cells had a poor FF and a low Voc as compared to those with an HTM, which is related to the larger shunt current along with a lower IPCE for these devices [2].

Different proportions of inorganic (Pb*,* Sn) cations, organic cations and halide anions (I*,* Br*,* Cl) can be combined in mixed perovskites, permitting their properties to be fine-tuned [35]. Tuning of bandgap of MAPbX3 has been attained through the substitution of I with Cl/Br ions, which occurs from a dependence of electronic energies on the effective exciton mass. The optical absorption can be tuned by bandgap engineering to comprise the whole visible spectrum. In the meantime, the combination of Cl/Br into iodide-based structure has markedly advanced the charge transport and the separation kinetics within the perovskite layer. Hence, by tuning the composition of perovskite resulted in improved efficiency and the stability of PSCs. It was observed that an increase in the size of perovskite cation materials

resulted in a reduction in the bandgap. A tunable bandgap can be obtained (between 1.48 and 2.23 eV) by replacing the methylammonium with a slightly larger formamidinium cation. Significantly, the reduced bandgap led to a PCE of up to 14.2% and high short circuit currents (>23 mA cm−2) [31].

CH3NH3SnI3 is demanded to be a low-carrier-density p-type metal. Theoretical calculations on perovskite recommended that their electronic properties intensely depend on the structure of the inorganic cage and formation of the perovskite octahedral network. By changing the inorganic and organic components and their stoichiometric ratio, it is probable to control the system dimensionality and electronic and optical properties. Furthermore, the presence of weak bonds in the perovskite structures ensures malleability and flexibility that could permit the deposition of thin films on flexible substrates [26].
