Light Induced Charge Generation, Recombination, and Transport in Organic Solar Cells

Abstract: Light-to-electricity conversion processes in organic and dye sensitized solar cells have attracted much scientific effort, aiming to understand the fundamental nature of these processes and to improve solar cell efficiency. Power conversion efficiency of such solar cells has progressed steadily in the last few decades, suggesting applicability for commercial products in the nearest future. Yet, the overall light-to-charge carrier conversion processes as well as the factors controlling these processes are not fully understood. Dye sensitized semiconductor solar cells (DSSC) and bulk heterojunction (BHJ) organic solar cells are addressed in this Thesis. The main topics of the Thesis are light-induced charge generation, recombination, and transport in the active materials as well as solar cell devices. Electron injection and recombination in Zn-porphyrin sensitized TiO2 electrode were investigated by means of transient absorption (TA) spectroscopy and surface sensitive sum frequency generation. It was found that the sensitizer molecule is bound to the semiconductor (TiO2) surface at an angle, which is controlled by experimental factors such as natureof the sensitization solvent and time. This tilt angle controls the electron transfers (injection and recombination) between sensitizer and TiO2 and it was shown that transfer occurs through space rather than through the molecular spacer connecting the porphyrin core to the surface. A strong correlation between cell efficiency and electron injection and recombination dynamics was demonstrated. In particular, the role of the electron-Zn-porphyrin cation recombination was shown to be decisive for high solar cell efficiency – a very slow (> 50 ns) recombination resulted in the highest cell efficiency. In functioning BHJ solar cells, charge pair photo-generation and separation as well as recombination, transport and extraction of photo-generated charge carriers were studied by several techniques such as TA spectroscopy, electro-modulated differential absorption (EDA), time resolved electric field induced second harmonic generation (TREFISH), time-of-flight method (TOF), photo-carrier extraction by linearly increasing voltage (CELIV), transient photocurrent and photovoltage. Solar cells made of fullerene blends with several different polymers have been studied. Photo-induced dynamics has been observed over more than ten decades of time and three decades of photo-generated charge concentrations by a combination of various optical and electrical time-resolved methods. Light absorption has been shown to lead to formation of closely spaced bound rather than long-distance separated free charges. Mechanism of the charge pair separation dominated by fast three-dimensional diffusion at earlier time scale and by electric field-induced drift on later times has been purposed. Electric field dependence of charge pair separation and recombination in non-optimized and optimized solar cells was studied. The time dependent carrier mobility over picosecond to microsecond time scale was shown and considered in relation to charge pair separation and extraction in solar cells made of polymer:fullerene blends with different stoichiometry. Dominance of geminateand non-g eminate recombination has been shown for solar cell blends made of different polymers. Detail description of recombination mechanism over broad range of times and charge concentrations have been achieved by means of the combination of different optical and electrical techniques. A critical role of electrodes in functioning solar cells on the charge dynamics has been discussed.