Sensitive external quantum efficiency measurements for studying charge transfer in organic solar cells

Charge transfer (CT) states, which form at the interface between donor and acceptor molecules in organic solar cells (OSCs), strongly influence device performance, in particular the open-circuit voltage and the photogeneration of free charges. However, the connection between molecular properties, the resulting energetics at the donor-acceptor interface, and ultimately device performance is still incompletely understood. To address this knowledge gap, this thesis studies the impact of (1) molecular energy levels and donor:acceptor mixing ratios on exciton separation, and (2) blend morphologies on interfacial energetic disorder and voltage losses. CT state properties are studied via a custom-built sensitive external quantum efficiency (EQE) setup and fit from the resulting spectra. In the first part of this thesis, we establish a reliable and reproducible fitting methodology that considers both singlet and CT state absorption contributions in the EQE. We first apply this methodology to study the impact of molecular energy levels on exciton separation in low donor content blends of ZnPc or its fluorinated derivatives (FxZnPc, with x = 0, 4, 8, 16) and C60. We find that the fluorination-induced shifts of the donor energy levels result in an increasing CT state energy with C60 and a decreasing offset between local excitation (LE) and CT states. Through in-depth characterization of molecular and blend energy levels, we draw a comprehensive picture of how LE, CT, and charge separated (CS) energy levels and the energetic transitions between states change upon fluorination of ZnPc. We find that the CT → CS transition presents a major bottleneck for free charge generation in the low offset F8ZnPc and F16ZnPc blends. This study is further extended beyond the low donor content blends by gradually varying the donor:acceptor mixing ratio from neat FxZnPc to neat C60. We find that with increasing F8ZnPc or F16ZnPc content, the CT dissociation efficiency is further reduced, leading to very low photocurrents in these blends. The dependence of CT dissociation on donor concentration is attributed to charge-quadruple induced electrostatic shifts, caused by the positive quadrupole moments of F8ZnPc/F16ZnPc, which increase the CT dissociation barrier with increasing donor content. In comparison, ZnPc/F4ZnPc, which have negative quadrupole moments, experience the opposite electrostatic shifts and show efficient CT dissociation across all donor:C60 mixing ratios. These results not only motivate further studies of the CS state, but also show that improving the CT → CS transition, for instance via the design of molecules with suitable quadrupole moments, is an effective route for increasing the photocurrents of OSCs. Finally, we study the impact of interfacial energetic disorder on device performance and voltage losses for five small molecule donor:C60 systems. Through temperature-dependent EQE measurements, we disentangle the contributions of static (i.e. temperature independent) and dynamic (i.e. temperature-dependent) disorder. Contrary to what is often stated for evaporated blends of small molecules, static disorder is found to be significant, and often exceed the contribution of dynamic disorder in the CT linewidth. This motivates future studies of interfacial disorder in OSCs, linking bulk and interface morphologies to device performance, and finding ways to reduce disorder to decrease voltage losses.

Keywords: Marcus theory
charge transfer states
organic solar cells
organic photovoltaics
voltage losses
Marcus-Levich-Jortner theory
sensitive external quantum efficiency
Advisor: Riede, M