Preparation and characterization of narrow bandgap ferroelectric (K,Ba)(Ni,Nb)O_3-δ films for mesoporous all-oxide solar cells

Ferroelectric (K,Ba)(Ni,Nb)O_3-δ films have triggered intense studies for applications in photovoltaic device due to their efficient ferroelectric polarization-driven carrier separation and above-bandgap generated photovoltages. However, they are suffered from a challenge of preparation limiting novel device architectures. Meanwhile, the bandgap for most of ferroelectric materials reported so far is still too large to be considered for desirable spectral absorption. Here, we propose a unique strategy to successfully synthesize the (K,Ba)(Ni,Nb)O_3-δ filmswith the lower bandgap of about 1.45 eV. Anew cell structure of utilizing (K,Ba)(Ni,Nb)O_3-δ as a active layer is explored to interfacewith electrontransporting TiO₂. Such mesoporous-ferroelectric combination solar cell is beneficial for facilitating the extraction of photocarriers. Under standardAM1.5G irradiation, the optimized (K,Ba)(Ni, Nb)O_3-δ filmsolar cell exhibits a higher open-circuit voltage of 1.27Vthan those of previous reports on ferroelectrics. Furthermore, a fill factor of 64% and a power conversion efficiency of 0.2% are achieved via the polarization switching modulation. The present results provide a novel synthetic approach toward developing high performance solar cells based on lead-free ferroelectric films.