Heterogeneity in dye-TiO₂ interactions dictate charge transfer efficiencies for diketopyrrolopyrrole-based polymer sensitized solar cells

Power conversion efficiency of a solar cell is a complex parameter which usually hides the molecular details of the charge generation process. For rationally tailoring the overall device efficiency of the dye-sensitized solar cell, detailed molecular understanding of photoinduced reactions at the dye-TiO₂ interface has to be achieved. Recently, near-IR absorbing diketopyrrolopyrrolebased (DPP) low bandgap polymeric dyes with enhanced photostabilities have been used for TiO₂ sensitization with moderate efficiencies. To improve the reported device performances, a critical analysis of the polymer-TiO₂ interaction and electron transfer dynamics is imperative. Employing a combination of time-resolved optical measurements complemented by low temperature EPR and steady-state Raman spectroscopy on polymer-TiO₂ conjugates, we provide direct evidence for photoinduced electron injection from the TDPP-BBT polymer singlet state into TiO₂ through the C=O group of the DPP-core. A detailed excited state description of the electron transfer process in films reveals instrument response function (IRF) limited (<110 fs) charge injection from a minor polymer fraction followed by a picosecond recombination. The major fraction of photoexcited polymers, however, does not show injection indicating pronounced ground state heterogeneity induced due to nonspecific polymer-TiO₂ interactions. Our work therefore underscores the importance of gathering molecular-level insight into the competitive pathways of ultrafast charge generation along with probing the chemical heterogeneity at the nanoscale within the polymer-TiO₂ films for optimizing photovoltaic device efficiencies.