Enhanced photovoltaic response of lead-free ferroelectric solar cells based on the (K,Bi)(Nb,Yb)O3 ﬁlms
Herein, we ﬁrstly present the (K,Bi)(Nb,Yb)O3 inorganic ferroelectric photovoltaic (FPV) ﬁlm by conﬁrming a nearly ideal bandgap of ∼1.45 eV for solar spectrum center and the co-existance of oxygen vacancy as well as ferroelectric polarization. Furthermore, a novel cell structure of combining with the charge-transporting TiO2 nanoparticles, perovskite-absorbing sensitizer and light-absorbing oxide hole p-type NiO conductor, is successfully fabricated to realize a 1 V open circuit voltage, which can be increased to 1.56 V by adjusting the test bias near the coercive voltage. Additionally, under simulated standard AM 1.5G illumination, a ﬁll factor of 86% and a power conversion efﬁciency of 0.85% are achieved via the oxygen vacancy electromigration and polarization switching modulation. It has shown that the obtained power conversion efﬁciency is one to three orders higher than those of pure BiFeO3 and Pb(Zr,Ti)O3. The enhanced PV effects are well elucidated using the transformation from Schottky-like barrier to Ohmic contacts caused by polarization switching and oxygen vacancies. Building upon the above studies, the deep insights into the bandgap tunability and PV effects in ferroelectric ﬁlms with high oxygen vacancy concentration have been ﬁrstly provided, which will facilitate a new versatile route for exploring high PV performance based on inorganic ferroelectric ﬁlms.