Spectral Characteristics of Light Harvesting, Electron Injection, and Steady-State Charge Collection in Pressed TiO2 Dye Solar Cells

The factors that limit photocurrent in dye solar cells were studied by incident-photon-to-collected-electron efficiency (ηIPCE), optical, and photovoltaic measurements. Nanostructured TiO2 photoelectrodes were prepd. by compression technique on glass substrates, and half of them were subjected to an addnl. heat treatment at 450°. The spectral absorbed-photon-to-collected-electron efficiency (ηAPCE) of the cells was detd. as a function of the photoelectrode film thickness (d) and direction of illumination and analyzed in terms of electron injection (ηINJ) and collection (ηCOL) efficiency. The cells with pressed-only photoelectrodes gave significantly lower photocurrents yet their ηAPCE, and thus ηCOL, increased significantly with increasing d value. To analyze this result quant., methods were formulated based on the std. diffusion model of electron transport in nanostructured photoelectrodes for the factorization of exptl. ηAPCE data into ηINJ and ηCOL parts and subsequent estn. of the effective steady-state electron diffusion length (L). Consistent decoupling of ηINJ and ηCOL was reached in a spectral region where electron generation rate was independent of d value. The ηINJ value was low and strongly wavelength-dependent, which was attributed to a poor energetic matching between dye excited states and TiO2 acceptor states due to unfavorable electrolyte compn. L increased systematically with d in both types of cells. Consistent with the increase of ηIPCE with light intensity, the result was attributed qual. to the electron concn. dependence of L and for a small part to decrease of film porosity with d value. The diffusion model and its predictions were summarized, and its validity in the present case was discussed critically.