The origin of efficiency enhancement of inorganic/organic Hybrid solar Cells by robust samarium phosphate nanophosphors

An effective energy level regulation of acceptor by doping samarium phosphate nanophosphors (SmPO₄ NPs) was reported for inorganic/organic hybrid solar cell applications. SmPO₄ NPs doped TiO₂/P3HT bulk heterojunction (BHJ) solar cell shows an enhanced power conversion efficiency of approaching 3% as compared with that of its counterpart without SmPO₄ NPs (1.98%). The underlying photophysical mechanism was probed by applying femtosecond transient absorption spectroscopy and the results show that the efficiency enhancement was ascribed to the improved hot electron, less energetic electron, hole transports at the interface of BHJ apart from down-conversion photoluminescence of SmPO₄ NPs. It has been evidenced that the hot electron transfer life time was shortened by more than 40% (i.e., from τ_hot-e=30.2 to 17.9 ps) than pure TiO₂ acceptor while the hole transfer lifetime was boosted by almost 20% (i.e., from 6.92 to 5.58 ns). Such charge carrier improvements stem from the efficient energy level regulations by SmPO₄ NPs. In detail, the conduction band (CB) edge of TiO₂ has been elevated by 0.57 eV while the valence band (VB) edge has been elevated by 0.32 eV, thus not only narrowing down the energy offset between CB energy levels of acceptor TiO₂ and donor P3HT, but also meanwhile enlarging the band gap of TiO₂ itself that permits to inhibit electron-hole recombination within TiO₂. This work demonstrates that samarium ions can efficiently facilitate exciton generation, dissociation and charge transport and have an important role in enhancing photovoltaic performance.