Spectral Characteristics of Light Harvesting, Electron Injection, and Steady-State Charge Collection in Pressed TiO2Dye Solar Cells

Abstract

The factors that limit photocurrent in dye solar cells (DSC) were studied by incident-photon-to-collected-electron efficiency ( η IPCE ), optical, and photovoltaic measurements. Nanostructured TiO 2 photoelectrodes were prepared by compression technique on glass substrates, and half of them were given an additional heat treatment at 450 °C. The spectral absorbed-photon-to-collected-electron efficiency ( η APCE ) of the cells was determined as a function of the photoelectrode film thickness ( d ) and direction of illumination and analyzed in terms of electron injection ( η INJ ) and collection ( η COL ) efficiency. The cells with pressed-only photoelectrodes gave significantly lower photocurrents yet their η APCE, and thus η COL, increased significantly with increasing d . To analyze this result quantitatively, methods were formulated based on the standard diffusion model of electron transport in nanostructured photoelectrodes for the factorization of experimental η APCE data into η INJ and η COL parts and subsequent estimation of the effective steady-state electron diffusion length ( L ). Consistent decoupling of η INJ and η COL was reached in a spectral region where electron generation rate was independent of d . η INJ was low and strongly wavelength-dependent, which was attributed to a poor energetic matching between dye excited states and TiO 2 acceptor states due to unfavorable electrolyte composition. L increased systematically with d in both types of cells. Consistent with the increase of η IPCE with light intensity, the result was attributed qualitatively to the electron concentration dependence of L and for a small part to decrease of film porosity with d . The diffusion model and its predictions were reviewed, and its validity in the present case was discussed critically.