New Scheme for Efficient Photovoltaic Response in Ferroelectric Oxides

Abstract

Ferroelectric materials have been widely investigated as a promising candidate for a solar cell active layer owing to their above bandgap (Eg) photovoltages. In conventional p-n junction solar cells, open circuit voltage (Voc) cannot exceed the bandgap of active layer semiconductor and built-in potential inside p-n junction separates photo-generated electron-hole pairs. In contrast, in thin-film ferroelectric photovoltaics (FPVs), Voc exceeds the bandgap of the ferroelectric material and photo-generated electron-hole pairs are separated by the depolarization field along the direction of polarization in homogeneous layer. While several experiments have demonstrated the increased power conversion efficiency (PCE) in FPVs including bandgap tuning, domain structure manipulation, electrode engineering, and multilayer structure fabrication, however, FPVs have remained as a scientific curiosity rather than practical applications because of the low output photocurrent and the low PCE. The low efficiency of FPVs is known to be attributed to the wide bandgap and the low absorption coefficient of ferroelectric materials. Recently, FPVs are attracting renewed attention by several breakthroughs, such as achievement of high PCE of 8.1% by tuning bandgap in multilayer ferroelectric structure, and attainment of PCE that exceeds the Shockley-Queisser limit from single bulk ferroelectric crystal. The highest PCE in single layer thin film FPV was observed in Bi(Fe,Cr)0.5O3 (BFCO) based solar cell. Compared to BiFeO3 (BFO) which has 2.67eV bandgap, BFCO has bandgap range from 1.5eV to 2.5eV. The bandgap of BFCO highly depends on the atomic disordering of B-site ions, Fe and Cr, which is determined by the deposition condition. As an active layer in FPV, a homogeneous single layer BFCO can exhibit short circuit current (Jsc) up to the order of one to ten mA and PCE up to 3.1%, while BFO only shows Jsc of order of a few hundreds of nA to a few μA and PCE below 1%. The main reason of significantly increased Jsc and PCE in BFCO compared to BFO is claimed to the decreased bandgap and the increased solar absorption. However, quantitative analysis of the effect of increased solar absorption has not been demonstrated yet and the main reason for the increased Jsc is still an open question. Herein, we discuss the quantum mechanical origins of high Jsc and PCE in thin-film BFCO ferroelectric solar cell based on the Density Functional Theory (DFT) first-principles calculation. Based on the DFT analysis, we suggest new promising materials for efficient ferroelectric photovoltaic devices. First, we calculated electronic structure of BFCO and confirmed the existence of efficient charge separation which does not exist in single perovskite material, BFO. Increased photocurrent in BFCO is closely related to the efficient charge separation in the media. Second, based on electronic structure analysis, we screened possible combinations of transition metal double perovskite oxides. Four other double perovskite materials showed similar or better degree of charge separation than BFCO, which indicates that those materials are promising to be used as active material. Finally, we found that disordered domain provides efficient electron mobility than ordered region, indicating the importance of disordered domain as efficient charge transport path provider.