Time-resolved study of charge photo-generation in polymer-based solar cells with low driving force
Abstract: Organic polymer-based solar cells recently attracted much attention as an efficient and inexpensive light-to-electricity conversion solution. However, the mechanism behind charge generation processes in organic materials is still not fully understood due to complex nature of carbon-based compounds. Studies of charge generation, recombination and transport processes are essential for revealing the critical power conversion efficiency (PCE) limiting factors. In this thesis, polymer and fullerene bulk heterojunction (BHJ) solar cells are addressed. In particular, charge generation processes were investigated as a function of driving force in two polymer-fullerene systems. The time resolved photoluminescence (TRPL) studies allowed to investigate ultrafast processes occurring within the timescale of picoseconds. Clear evidence of fast excitation diffusion process (~10ps) was found in pure polymer materials, allowing an assumption that most excitons may reach the interface in polymer-fullerene blends. A strong correlation between higher driving force and photocurrent was shown for polymer based solar cells. The driving force of 0.1 eV for P3TI was proven to be sufficient to dissociate polymer excitons, which indicates a surprisingly low exciton binding energy. In combination with faster exciton diffusion, this suggests the P3TI has superior properties for charge generation compared to similar PTI-1 polymer. Further analysis was focussed on investigation of possible “hot” exciton and charge-transfer (CT) state dissociation.
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