Preparation of well-interconnected TiO₂ electrodes at low temperature is the critical issue for the highly-efficient dye-sensitization solar cells (DSCs) fabricated on plastic substrates^1-4. Herein we took a synergistic approach for the first time to address this issue by combining the chemical sintering and the physical compression method. As a chemical sintering agent, we formulated binder-free TiO₂ paste based on “nanoglue” and constructed photoelectrode, followed by physical compression at 130 ^oC to further improve the connectivity of TiO₂ films. We intensively investigated the factors affecting the photovoltaic performance experimentally and theoretically. It was found that electron transport resistance significantly reduced with compression at 100 MPa, and the diffusion length was as long as 25 micrometer while charge collection efficiency was as high as more than 95% based on measurement of electrochemical impedance spectroscopy in dark, both of which are notable and essential for highly efficient plastic solar cell prepared by low temperature process. Furthermore, the simulations with a combined optical and electrical modeling⁵ were performed to determine an efficient diffuse light scattering design for highly efficient DSCs processed with low sintering temperature. Consequently, a layered film structure with gradient distribution of scatterers in each layer, rather than the diffuse scattering layer acting as back reflector commonly used in high temperature sintered DSCs, was elaborately tailored to enhance light harvesting efficiencyand charge collection efficiency. State-of-the-art conversion efficiency of 8.67% and 7.76% was achieved for the first time on plastic DSC with Pt/FTO/Glass and Pt/ITO/PEN as counter electrode, respectively.
Figure 1 Photovoltaic properties of dye-sensitized solar cells were compared based on titania films with single layer structure which was made of scatters uniformly distributed through films, and multilayer structure which was made of scatters with gradually varied concentration in each layers. (a) presents the photovoltage-current curves of cells, which gives the efficiency of 7.23% and 8.67% for cells with single layer and multilayer structures. (b) indicates the IPCE from both cells, with simulation data from a theoretical modeling consisting of the Monte Carlo approach with Mie theory.
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