Theoretical optimization of defect density and band offsets for CsPbI2Br based perovskite solar cells

The stability of Methyl-Ammonium (MA) and Formamidinium (FA) based perovskite solar cells (PSCs) has al-ways been a matter of concern because of volatile nature of organic cations present in them. So, it becomes necessary to replace organic cations with inorganic non-volatile cations. Recently, lot of efforts have been put in to a new class of PSCs, CsPbX3 (where, X = Br, I, Cl etc.) wherein organic cation is replaced by Caesium, to address the stability issue. Despite being more stable than their organic counterparts, the PCE of CsPbI2Br based PSCs is still low as compared to that offered by Methyl-Ammonium (MA) and Formamidinium (FA) based PSCs. In this paper, we have proposed a new CsPbI2Br based PSC structure having n-i-p architecture: FTO/Zn(O0.3, S0.7)/CsPbI2Br/HTL/Au. The proposed structure has been simulated using SCAPS software by employing new ETL Zn(O0.3, S0.7) and various HTLs (spiro-OMeTAD, CuSCN, CuI and MoO3). Initial simulations reveal that the proposed PSC achieves best PCE of 20.36 %, when CuI is used as HTL. The impact of defect density (Nt) in CsPbI2Br layer has been studied for various HTLs and optimum value of Nt obtained as 1.0 x 1011 cm-3. Defect densities at ETL/CsPbI2Br and CsPbI2Br/HTL interfaces have also been optimised at values of 1.0 x 1015 cm-3 and 1.0 x 1015 cm-3 respectively. Finally, VBO and CBO at respective interfaces have also been optimized and the final proposed structure having Zn(O0.3, S0.7) as ETL and CuI as HTL resulted in the PCE of 21.51 % with VOC of 1.55 V, JSC of 15.21 mAcm- 2 and FF of 90.77 %, which are comparable to the Shockley-Queisser limit for CsPbI2Br perovskite solar cells, thereby, considerably enhancing the efficiency of inorganic perovskite solar cells.