The Role of Charge Transfer States in Free Charge Generation in Organic Donor-Acceptor Solar Cells

• Applicant: Professor Dr. Dieter Neher
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2014 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 256605806

The aim of this proposal is to use time delayed collection field (TDCF) experiments to quantify the field and temperature dependence of several polymer:fullerene blends as a function of excitation wavelength, and to clarify the role of charge transfer- (CT-) states in polymer-based organic solar cells.Current applicants and other groups work indicated that charge separation in organic donor-acceptor bulk heterojunction solar cells involves electronically relaxed and vibronically thermalized CT-states. Thus, the solar cells behavior is significantly affected by the efficiency of split-up of these states. We expect that the exact energetic position of thermalized CT-excitons in relation to the energy of free charges is of vital importance for the photovoltaic function of organic solar cells.Therefore, the central point of this proposal is to measure generation efficiency as function of several parameters with emphasis on exciting only low energy CT-states. These experiments will be conducted on polymer:fullerene blends with variable, tunable position of the ionization energy and electron affinity of the donor and acceptor, respectively.With TDCF experiments as a function of the electric field and temperature we like to examine whether the often observed field dependent generation is a result of mainly thermally activated separation of CT-states or of field-assisted tunneling. In addition to these measurements, we will investigate whether the presumption, drawn from experiments at room temperature, that free charge generation is mainly mediated by thermalized CT states is generally valid, even in the case at low thermal energies. TDCF experiments with selective CT-excitation on ternary polymer:fullerene1:fullerene2 blends will be conducted to show whether CT-states are delocalized over several fullerenes or not. Finally, detailed experiments on binary polymer:fullerene blends with low polymer concentrations are planned. In such blends, we expect the formation of a two-phase structure with polymer:fullerene blend domains and pure fullerene domains. We will investigate if and under which circumstances activationless separation of CT states, as now seen in several established polymer:fullerene blends, does also occur in such two-phase systems. Findings of this work will be important for a comprehensive understanding of charge generation in organic solar cells and will form the base for further improvement of solar cell efficiency.

• DFG Programme: Research Grants