Emergence and Evolution of Organometal Halide Perovskite Solar Cell

Abstract

Since the first report on long-term durable perovskite solar cell in 2012, a surge of interest in perovskite solar cell has been received due to its superb photovoltaic performance exceeding 20%. MAPbI 3 (MA = CH 3 NH 3) perovskite film is able to be prepared simply by solution processes of either sequential two-step or single step procedure. Since MAPbI 3 shows balanced charge transport property with micrometer scale charge diffusion length, it can be applied to any kind of junction structures. Mostly studied structure is mesoscopic structure employing mesoporous oxide layer in perovskite film. Photovoltaic performance is primarily influenced by the quality of perovskite film but interfaces are equally important. In this mini review, emergence and evolution of perovskite solar cell are described. Perovskite solar cell based on methylammonium lead halide, MAPbX 3 (MA = CH 3 NH 3 , X = Br, I) was first reported by Miyasaka group in 2009 using dye-sensitized solar cell structure [1]. A power conversion efficiency (PCE) of about 3-4% was demonstrated. An improved PCE of 6.5% was reported by Park group in 2011 [2]. Spin coating of the polar aprotic solvent such as N,N-dimethylformamide (DMF) or gamma-butyrolactone (GBL) solution containing MAI and PbI 2 on nanocrystalline TiO 2 film led to semi-spherical dot morphology of MAPbI 3. However, perovskite dots sitting on TiO 2 surface tends to be readily dissolved in polar solvent in electrolyte. Such instability hinders further development of perovskite solar cell. Park et al. eventually solved dissolution problem by employing solid state hole transport material (HTM), spiro-MeOTAD, which was reported in October 2012 [3]. Submicrometer-thick mesoporous TiO 2 film was decorated with perovskite dots and the pores were filled with spiro-MeOTAD, which demonstrated a PCE of 9.7%. It was found that PCE was improved as the TiO 2 film thickness decreased, which was mainly due to high absorption coefficient of more than 10 4 cm-1 [2, 3]. Transient absorption spectroscopic study showed effective charge separation occurred between perovskite and hole transport spiro-MeOTAD, which indicates that valance band of perovskite and HOMO level of spiro-MeOTAD is well matched for injection of holes from perovskite to HTM. In Figure 1, structure of MAPbI 3 and energy levels among TiO 2 , MAPbI 3 and spiro-MeOTAD are schematically depicted, where methylammonium cation is located in cubo-octahedral site and Pb-I octahedra are corner shared. Since oraganometal halide perovskite is ionic crystal, it can be prepared by solution process under mild condition. Since lattice energy of halide perovskite is lower than that of covalent oxides due to low Madelung constant [4], MAPbI 3 can be formed even at ambient temperature without heat treatment. MAPbI 3 is formed via either one-step or sequential two-step procedure. For one-step spin coating, polar aprotic solvent containing PbI 2 and MAI is used to form MAPbI 3. PbI 2 layer is first formed in two-step procedure, which is followed by spin-coating of MAI. This leads to MAPbI 3 according to eq 1. PbI 2 (s) + MAI (l) → MAPbI 3 (s) (1) Morphology of one-step coated MAPbI3 is different from that of two-step one. Without any specific process involved in two methods, photovoltaic performance of perovskite solar cell prepared by one-step is found to be inferior to that of two-step method [5]. Figure 1. Local structure of MAPbI 3 perovskite showing lead-iodide octahedral and cubo-octahedral MA. Band positions of perovskite along with those for TiO 2 and spiro-MeOTAD. However, one-step method is expected to be superior to two-step procedure if crystal growth is controlled. Regarding two-step procedure, it was found that crystal size was controlled by varying MAI concentration, where relatively large crystal formed from low concentration but small size formed from high concentration [6]. Figure 2. SEM images showing dependence of crystal growth on MAI concentration of (a) 0.038 M and (b) 0.063 M.