Static and Dynamic Disorder of Charge Transfer States Probed by Optical Spectroscopy

Abstract

Since the key role of charge transfers (CT) states has been identified for organic solar cells (OSCs), research into their properties is a timely topic. Conventionally, their absorption and emission spectra are described in terms of Marcus’ electron transfer theory. This is a single site approach with the essential parameter being the reorganization energy. Thus, it ignores ensemble effects, notably the role of static disorder that is inevitably present in a spin‐coated OSC film. Here time dependent photoluminescence spectroscopy is applied on blends of the polymeric donor MeLPPP with either the non‐fullerene acceptor SF‐PDI2 or with PC61BM within a temperature range from 295 to 5 K. The authors monitor how initially excited singlet states are converted to emissive CT states. Concomitantly, emission from residual singlets on the acceptor is observed rather than hybrid CT‐states. The role of spectral diffusion in this process is discussed. From the temperature and time dependent linewidths of absorption, fluorescence, and CT emission, the static and dynamic contributions to the total disorder are inferred. In both blends, at 295 K, the contribution of static disorder is comparable to the dynamic disorder.

Unraveling Urbach Tail Effects in High-Performance Organic Photovoltaics: Dynamic vs Static Disorder

“…Suppressing the static disorder via improved molecular design is clearly a path for moving forward. 38,48 For Y-series acceptors, our data and analysis indicate that static disorder can be suppressed by optimizing side chains and restricting end-group rotations. Molecular design along this direction should be further explored.…”

“…Although recent advances in Y-series molecules are driving the OSC field into a new phase, gaps remain with regard to the disorder properties between organic and inorganic systems. Suppressing the static disorder via improved molecular design is clearly a path for moving forward. , For Y-series acceptors, our data and analysis indicate that static disorder can be suppressed by optimizing side chains and restricting end-group rotations. Molecular design along this direction should be further explored.…”

“…On the other hand, the linear regions in Figure 3a originate from the thermally induced disorder that may arise from activated rotations of the molecular structure as well as intermolecular motions. 38 Kahle et al reported that the dynamic contribution is intramolecular and intermolecular geometric reorganization (phonon-induced broadening) in organic solids. In our study here, the two thermal-related parameters, E U,t and θ E , are smaller in Y6-O and Y18 based BHJs, in comparison with those of Y3 and Y25.…”

“…These results are consistent with previous observations − ,, that the absence of side chains promotes conformational diversities and the end-group rotations lead to the intrinsic structural disorder in Y-derivatives. On the other hand, the linear regions in Figure a originate from the thermally induced disorder that may arise from activated rotations of the molecular structure as well as intermolecular motions . Kahle et al reported that the dynamic contribution is intramolecular and intermolecular geometric reorganization (phonon-induced broadening) in organic solids.…”

Reducing Voltage Losses of Organic Solar Cells against Energetics Modifications by Thermal Stress

“…We attribute this opposite trend compared to that of PM6:BTP-eC9 systems to a dominated static disorder and suppressed dynamic disorder for this all-polymer system. The slight increase of EL spectra line width with decreasing temperature could be related to broadened distributions of injected charges in the density of states. ,, At lower temperature, the electron–vibration coupling can be suppressed and charge carriers could occupy broader distributions in the density of states, which could lead to slightly increased line width. Intriguingly, the 30 and 140 °C annealed PM6:PY-IT films only slightly differ in the slopes of line width vs temperature, indicating that the static disorder is not much affected by thermal annealing.…”