Advanced Functional Materials and Devices (AFMD) Group

The microstructure of organic small molecule (SM) layers in organic solar cells (OSCs) strongly influences device performance by impacting light absorption, charge transport, and recombination. We demonstrate that ellagic acid (EA), a naturally derived templating layer, induces substantial morphological and thus optoelectronic changes in the vacuum thermally evaporated (VTE) donor molecule DCV5T-Me(3,3). Using in situ grazing incidence wide-angle X-ray scattering (GIWAXS) during thin film deposition in the purpose-built MINERVA VTE chamber at Diamond Light Source, we show that a 5 nm EA layer reorients DCV5T-Me from an edge-on to a face-on molecular packing motif. This templating effect persists for up to around 90 nm of film thickness. Through UV–Vis spectrophotometry and photoluminescence (PL) spectroscopy, we observe a shift towards H-aggregation and decreased light absorption in the donor molecule with the EA template. Atomic force microscopy (AFM) shows that the donor morphology changes as a function of thickness from the donor-templating interface. In DCV5T-Me(3,3):C60 bulk heterojunction devices, the EA layer helps retain donor crystallinity and enhances short circuit current (JSC), despite the lower absorption. Maximum power conversion efficiency in our devices is achieved with a 5 nm templating layer, which provides sufficient structural templating while maintaining partial interfacial contact for efficient charge extraction. We hypothesize that the improvement in JSC is likely driven by enhanced charge carrier dynamics due to the orientation change, shift towards H-aggregation, and change in growth mode.

Vacuum-deposited organic solar cells (OSCs) have lagged behind their solution-processed counterparts in achieving high power conversion efficiency (PCE), in particular as result of higher voltage losses. In this study, we demonstrate a bulk heterojunction OSC using SubNc as donor and DCV3T as non-fullerene acceptor, achieving a PCE of 2.6% and a remarkably low total voltage loss of 0.64 V, lower than the typical values exceeding 0.7 V observed in vacuum thermally evaporated fullerene-based systems. The device also exhibits non-radiative voltage losses comparable to leading non-fullerene-acceptor (NFA) based OSCs. Transient absorption spectroscopy confirms efficient Förster resonance energy transfer from DCV3T to SubNc, followed by electron transfer for exciton separation. Morphological Grazing Incidence Wide-Angle X-ray Scattering features suggest both blends have a preferential edge-on orientation of DCV3T molecules, and the blends with higher DCV3T content could suffer from suppressed out-of-plane lamellar crystallinity with possible connection with greater non-radiative losses. Our findings demonstrate the potential of designing low-voltage-loss evaporated OSCs by building on strategies from solution-processed NFA systems, while highlighting the continued need for new evaporable acceptors with optimised optoelectronic and morphological properties.

This paper presents the development and implementation of a high-sensitivity external quantum efficiency (sEQE) measurement system designed to characterize optoelectronic devices, particularly solar cells and photodetectors. Our setup enables precise measurement of the conversion efficiency of photons into free charge carriers, providing crucial insights into device performance and underlying physical mechanisms. The measurement setup is based on a white-light source coupled to a monochromator for wavelength selection, with the diffracted beam focused onto either a calibrated photodiode or the device under test. Mea- surements can be performed at room temperature using a custom sample holder or across a range of temperatures using a cryostat. Signal detection is achieved through Lock-In amplifi- cation, enabling high sensitivity in noisy environments. The incorporation of spectral filters and multiple photodiodes with extended calibration ranges enables a dynamic range span- ning six orders of magnitude, allowing detection of sub-bandgap signals. Beyond describing component modularity and hardware specifications, we provide open-source Python-based control and analysis software to control the sEQE setup and analyze the resulting data. This comprehensive documentation of both hardware and software components contributes to an ongoing effort to increase transparency, standardization, and reproducibility in experimental research and aims to ease access to an important characterization technique for solar cells and photodetectors.