Maximizing Performance and Stability of Organic Solar Cells at Low Driving Force for Charge Separation

Thanks to the development of novel electron acceptor materials, the power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices are now approaching 20%. Further improvement of PCE is complicated by the need for a driving force to split strongly bound excitons into free charges, causing voltage losses. This review discusses recent approaches to finding efficient OPV systems with minimal driving force, combining near unity quantum efficiency (maximum short circuit currents) with optimal energy efficiency (maximum open circuit voltages). The authors discuss apparently contradicting results on the amount of exciton binding in recent literature, and approaches to harmonize the findings. A comprehensive view is then presented on motifs providing a driving force for charge separation, namely hybridization at the donor:acceptor interface and polarization effects in the bulk, of which quadrupole moments (electrostatics) play a leading role. Apart from controlling the energies of the involved states, these motifs also control the dynamics of recombination processes, which are essential to avoid voltage and fill factor losses. Importantly, all motifs are shown to depend on both molecular structure and process conditions. The resulting high dimensional search space advocates for high throughput (HT) workflows. The final part of the review presents recent HT studies finding consolidated structure-property relationships in OPV films and devices from various deposition methods, from research to industrial upscaling.

Power-Law Density of States in Organic Solar Cells Revealed by the Open-Circuit Voltage Dependence of the Ideality Factor

The density of states (DOS) is fundamentally important for understanding physical processes in organic disordered semiconductors, yet hard to determine experimentally. We evaluated the DOS by considering recombination via tail states and using the temperature and open-circuit voltage (V_{oc}) dependence of the ideality factor. By performing Suns-V_{oc} measurements, we find that the energetic disorder increases deeper into the band gap, which is not expected for a Gaussian or exponential DOS. The linear dependence of the disorder on energy reveals the power-law DOS in organic solar cells.

Understanding Causalities in Organic Photovoltaics Device Degradation in a Machine-Learning-Driven High-Throughput Platform

Organic solar cells (OSCs) now approach power conversion efficiencies of 20%. However, in order to enter mass markets, problems in upscaling and operational lifetime have to be solved, both concerning the connection between processing conditions and active layer morphology. Morphological studies supporting the development of structure-process-property relations are time-consuming, complex, and expensive to undergo and for which statistics, needed to assess significance, are difficult to be collected. This work demonstrates that causal relationships between processing conditions, morphology, and stability can be obtained in a high-throughput method by combining low-cost automated experiments with data-driven analysis methods. An automatic spectral modeling feeds parametrized absorption data into a feature selection technique that is combined with Gaussian process regression to quantify deterministic relationships linking morphological features and processing conditions with device functionality. The effect of the active layer thickness and the morphological order is further modeled by drift-diffusion simulations and returns valuable insight into the underlying mechanisms for improving device stability by tuning the microstructure morphology with versatile approaches. Predicting microstructural features as a function of processing parameters is decisive know-how for the large-scale production of OSCs.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Traps and transport resistance are the next frontiers for stable non-fullerene acceptor solar cells

Stability is one of the most important challenges facing material research for organic solar cells (OSC) on their path to further commercialization. In the high-performance material system PM6:Y6 studied here, we investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires the presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits the open-circuit voltage and lowers the charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.

Long operational stability is essential to commercialisation of organic solar cells. Here, the authors investigate the thermal degradation of inverted photovoltaic devices based on PM6:Y6 non-fullerene system to reveal that trap-induced transport resistance is primarily responsible for the drop in fill factor.

Molecular Oligothiophene-Fullerene Dyad Reaching Over 5% Efficiency in Single-Material Organic Solar Cells

A novel donor-acceptor dyad, 4, in which the conjugated oligothiophene donor is covalently connected to fullerene PC₇₁ BM by a flexible alkyl ester linker, is synthesized and applied as photoactive layer in solution-processed single-material organic solar cells (SMOSCs). Excellent photovoltaic performance, including a high short-circuit current density (J_SC ) of 13.56 mA cm^-2 , is achieved, leading to a power conversion efficiency of 5.34% in an inverted cell architecture, which is substantially increased compared to other molecular single materials. Furthermore, dyad 4-based SMOSCs display excellent stability maintaining 96% of the initial performance after 750 h (one month) of continuous illumination and operation under simulated AM 1.5G irradiation. These results will strengthen the rational molecular design to further develop SMOSCs for potential industrial application.

Oligomer-Assisted Photoactive Layers Enable >18 % Efficiency of Organic Solar Cells

Although ternary organic solar cells (OSCs) have unique advantages in improving device performance, the morphology assembly in the ternary-phase would be more uncertain or complex than that in the binary-phase. Here, we propose a new concept of oligomer-assisted photoactive layers for high-performance OSCs. The formed alloy-like phase of the oligomer : host polymer blend enabled the oligomer-assisted OSCs to fuse the advantages of both binary and ternary devices, exhibiting substantially enhanced performance and stability compared to the control devices. With the addition of oligomers, outstanding efficiencies of 17.33 % for a PM6 : Y6 device, 18.32 % for a PM6 : BTP-eC9 device, and 17.13 % for a PM6/Y6 pseudo-bilayer device were achieved, all of which are one of the highest values in their corresponding fields. The improved performance originated from the downshift energy levels, enhanced light absorption, optimized blend morphology, favorable charge dynamics, and reduced non-radiative energy loss.

Assembling Mesoscale-Structured Organic Interfaces in Perovskite Photovoltaics

Mesoscale-structured materials offer broad opportunities in extremely diverse applications owing to their high surface areas, tunable surface energy, and large pore volume. These benefits may improve the performance of materials in terms of carrier density, charge transport, and stability. Although metal oxides-based mesoscale-structured materials, such as TiO₂ , predominantly hold the record efficiency in perovskite solar cells, high temperatures (above 400 °C) and limited materials choices still challenge the community. A novel route to fabricate organic-based mesoscale-structured interfaces (OMI) for perovskite solar cells using a low-temperature and green solvent-based process is presented here. The efficient infiltration of organic porous structures based on crystalline nanoparticles allows engineering efficient "n-i-p" and "p-i-n" perovskite solar cells with enhanced thermal stability, good performance, and excellent lateral homogeneity. The results show that this method is universal for multiple organic electronic materials, which opens the door to transform a wide variety of organic-based semiconductors into scalable n- or p-type porous interfaces for diverse advanced applications.