Comparison of performance and optoelectronic processes in ZnO and TiO2 nanorod array-based hybrid solar cells

The reported efficiencies of pristine ZnO nanorod array (NRA)-based polymer-inorganic hybrid solar cells (HSCs) are normally lower than those of their pristine TiO2 NRA-based counterparts. This difference typically results from the lower short-circuit current density (J(sc)) of the ZnO NRA device. This paper presents a comparative study of pristine ZnO and TiO2 NRA-based HSCs. We investigate the morphological structure (length, diameter, number density, area of nanorod laterals), photovoltaic performance (current density-voltage J-V, external quantum efficiency EQE), and optoelectronic processes related to electron transfer (electron mobility mu(e), electron diffusion length L-D, electron lifetime tau(e) and electron transit time tau(t) related electron collecting efficiency eta(cc),( )electron injection surface potential SP, photoluminescence PL, bound charge pairs BCP) in HSCs, with ZnO and TiO2 NRA as electron acceptor. Our comparative investigations reveal that the factors relating to the interface area, mu(e), L-D, and eta(cc) are not the key factors responsible for the difference in the value of J(sc) in ZnO and TiO2 NRA-based HSCs with the same device structure. In fact, the crucial step for a lower J(sc )in ZnO NRA-based HSCs than in TiO2 NRA-based HSCs is attributed to the less efficient transfer of photo-generated electrons at the charge separation interface in ZnO NRA-based HSCs. Dynamic characterizations indicate that the transfer of interfacial photo-generated electrons in TiO2 NRA-based HSCs is more efficient than ZnO NRA-based HSCs, and is confirmed by Kelvin probe force microscopy (KPFM) and PL studies. The reason for the better interface charge transfer property in MEH-PPV/TiO2 NRA than that of in MEH-PPV/ZnO NRA is further investigated by Marcus model, we find that more trapped BCP states are generated in the ZnO NRA based HSCs, which resulting in lower interfacial electron injection efficiency from polymer to ZnO NRA.