Spectral Characteristics of Light Harvesting, Electron Injection, and Steady-State Charge Collection in Pressed TiO2 Dye Solar Cells
journal contributionposted on 10.04.2008, 00:00 by Janne Halme, Gerrit Boschloo, Anders Hagfeldt, Peter Lund
The factors that limit photocurrent in dye solar cells (DSC) were studied by incident-photon-to-collected-electron efficiency (ηIPCE), optical, and photovoltaic measurements. Nanostructured TiO2 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 (ηAPCE) 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 (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. 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 ηAPCE data into ηINJ and ηCOL parts and subsequent estimation 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. ηINJ 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 composition. L increased systematically with d in both types of cells. Consistent with the increase of ηIPCE 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.