With the advent of non-fullerene acceptors (NFAs) and their record power conversion efficiencies, focus of the academic organic photovoltaics (OPV) research community shifted towards solution processed polymer : non-fullerene solar cells. These performance improvements could not be mirrored in evaporated OPVs since no evaporable NFAs enabling low voltage losses and efficient charge extraction have been introduced to date.^[1] Despite this, evaporated small molecule donor : fullerene solar cells continue to be at the forefront of industrial research due to many practical advantages like an easy route to upscale, high reproducibility, the possibility of multijunction systems and intrinsically high thermal stability.^[2] Also the commercial success of OLEDs provides important lessons for the upscale of evaporated OPVs. In 2016, Heliatek demonstrated evaporated tandem OPVs with 13.2 %. Further improvements in performance will strenghten the prospects of commercial success for evaporated OPVs.

This presentation combines results from optical spectroscopy, transient optoelectronic techniques, and drift-diffusion simulations to elucidate the performance limitations of a state-of-the-art evaporated OPV system compared to its solution processed counterparts. The donor molecule with an acceptor-donor-acceptor structure was paired with C60 and the morphology and active layer thickness were optimised. Through Grazing-Incidence Wide-Angle and Resonant Soft X-Ray Spectroscopy, we connect the device characteristics to the active layer morphology.

We find that heating the substrate during evaporation enhances the crystallinity and phase purity of the active layer and greatly improves performance due to an enhanced absorption, charge separation and charge collection efficiency. Despite the modest improvements in charge collection efficiency with improved crystallinity, its impact on the fill factor remains the main bottleneck compared to solution processed OPVs. This in turn limits the optimum device thickness and absorption. Measurements of the energetic disorder reveal a broad tailstate density in the evaporated OPVs in contrast to the highest performing Y6 based NFA devices but comparable to other solution processed systems.^[3] This disorder leads to highly charge carrier density dependent mobility and recombination as demonstrated by a combination of charge extraction and transient photovoltage measurements. The observations are best described by a recombination mechanism that is limited by the encounter of spatially localised trapped carriers with free carriers of the opposite sign in addition to free-to-free recombination. Further combining current transient measurements with drift diffusion simulations we investigated the role of imbalanced transport and deep traps on the recombination mechanisms. While the recombination rate under device operating conditions is comparable or even lower than in the fullerene and non-fullerene based solution processed references, respectively, the order of magnitude lower effective mobility and the mobility imbalance result in the poor charge collection efficiency even for thin active layers around 50 nm. Using the simulation parameters extracted from a global fit of the experimental data, we give an outlook for possible performance improvements by better transport enabling enhanced absorption for thicker active layers.

Solution-processed blends like the small molecule BTR mixed with PCBM maintain a high fill factor even at active layer thicknesses well beyond 200 nm.^[4] This illustrates that neither a polymeric backbone nor highly crystalline NFAs are prerequisites for good hole or electron transport, respectively. Indeed other evaporated blends have demonstrated better transport properties while suffering from poor absorption and large voltage losses.^[1] Better morphological control and efforts in synthesizing more crystalline evaporable donor molecules while maintaining the demonstrated high absorption coefficients and good charge generation properties of the blend presented herein are instrumental for further performance improvements. Power conversion efficiencies beyond 10 % for single junction evaporated solar cells are well within reach.