Electric Field Effects on Charge Transport in Polymer/TiO2 Photovoltaic Cells Investigated by Intensity Modulated Photocurrent Spectroscopy

摘要

The electric field effects on the charge transport dynamics in bilayer polymer/TiO2 photovoltaic cells are studied by intensity modulated photocurrent spectroscopy (IMPS), for the first time, where a small bias voltage (V_a) in the range from -0.1 to 0.2 V is applied to a cell during each IMPS measurement. The bilayer cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and nanostructured TiO2 are constructed by using a polymer layer thickness l of 350 nm but varied TiO2 layer thicknesses (d = 40 and 65 nm). All the experimental IMPS responses are satisfactorily fitted to the previously developed IMPS model for the bilayer cells at V_a = 0 V, providing a detailed analysis of charge transport properties under the applied voltages. It is revealed that the external electric field almost exclusively changes the exciton dissociation rate (S) at the donor/acceptor (D/A) interface, but imposes only a very faint effect on the transport dynamics of photogenerated electrons [φ_n(ω), τ_D and D_e]. A linear correlation is found for the dependence of S (IPCE),τ_D (f_(min)), and D_e on the applied voltages, which is not changed remarkably by the TiO2 thickness. Fitted results show that, as the voltage is changed from -0.1 to 0.2 V, which provides the change from a reverse electric field E_r (V_a < 0 V) that sweeps the carriers toward electrodes to a forward electric field E_f (V_a > 0 V) that drives the carriers toward the D/A interface, the S values linearly decreased from 130 to 96 cm/s (d = 40 nm) or 100 to 64 cm/s (d = 65 nm), while the D_e values slightly decreased from 9 x 10~(-5) to 8 x 10~(-5) cm~2/s (d = 40 nm) or 7.5 x 10~(-5) to 6.6 x 10~(-5) cm~2/s (d = 65 nm). The IMPS data suggest that the transport of photogenerated electrons in the bilayer MEH-PPV/TiO2 cells proceeds mainly by diffusion rather than drift, and the transport dynamics is hardly altered by the electric field. Apparently, the field-dependent exciton dissociation at the D/A interface is almost the exclusive determining process for the energy conversion efficiency of bilayer cells.