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 donortemplating interface. In DCV5T-Me(3,3):C-60 bulk heterojunction devices, the EA layer helps retain donor crystallinity and enhances short circuit current (J(SC)), 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 J(SC) 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 F & ouml;rster 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.

Monolithic perovskite-organic tandem solar cells (POTSCs) have attracted considerable attention in recent years due to their compatible fabrication routes and advances in single-cell efficiencies. To further boost the performance of POTSCs, reducing the voltage losses that mainly arise from wide bandgap (WBG, >1.7 eV) perovskite subcells and interconnecting layers (ICLs) is critical. Here, a new ICL with a configuration of C-60/YbOx/Au/MoOx is demonstrated for constructing the monolithic POTSC. The YbOx-based ICL benefits from an ohmic contact and high transparency, resulting in improved POSTC performance. The champion device presents a PCE of 23.2% owing to a high V-OC of 2.11 V (approximately equal to the sum of individual V-OC's of the subcells) without compromising the short-circuit current density and fill factors. This work opens an avenue for developing efficient ICLs in POTSCs.

This paper presents the development and implementation of a high-sensitivity external quantum efficiency (sEQE) measurement system designed to characterize optoelectronic de- vices, 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.

Vacuum-processed organic solar cells (VP-OSCs) possess many advantages for scalability. However, as the academic community focusses on high performing solution-processed OSCs, detailed studies about the relation between morphology and device characteristics in VP-OSCs are rare. Here, we present a study on a model donor/fullerene VP-OSC system deposited at different substrate temperatures. Substrate heating results in increases in current density and fill factor (FF). The changes in morphology are characterised by grazing-incidence wide-angle scattering (GIWAXS) and resonant soft X-ray scattering (RSoXS). The increase in the degree of crystallinity and preferential orientation of the donor molecule in heated samples results in enhanced absorption increasing current density. The exciton and charge separation efficiency studied by transient absorption and photoluminescence quenching show only minor differences. To study the FF differences, charge transport and non-geminate recombination are studied by optoelectronic measurements and device simulations. The charge carrier kinetics are governed by a large density of trap states. While the energetic disorder and non-geminate recombination under open circuit conditions remain largely unchanged, the increased effective mobility and lower transport disorder observed in photocurrent transients explain the increased collection efficiency for heated devices. We relate this to the increased donor phase purity. Our results suggest that charge recombination and transport are governed by different aspects of disorder related to amorphous and crystalline donor phases. Quantitative comparison with high FF solution-processed OSCs reveals that the low mobility limits FF. Finally, drift-diffusion simulations give an outlook for possible performance increases through further optimisation of the deposition control.

Free charge generation after photoexcitation of donor or acceptor molecules in organic solar cells generally proceeds via (1) formation of charge transfer states and (2) their dissociation into charge separated states. Research often either focuses on the first component or the combined effect of both processes. Here, we provide evidence that charge transfer state dissociation rather than formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between singlet and charge transfer states. We investigate devices based on dilute donor content blends of (fluorinated) ZnPc:C60 and perform density functional theory calculations, device characterization, transient absorption spectroscopy and time-resolved electron paramagnetic resonance measurements. We draw a comprehensive picture of how energies and transitions between singlet, charge transfer, and charge separated states change upon ZnPc fluorination. We find that a significant reduction in photocurrent can be attributed to increasingly inefficient charge transfer state dissociation. With this, our work highlights potential reasons why low offset fullerene systems do not show the high performance of non-fullerene acceptors. Fullerene acceptors are relevant for upscaling industrial production of organic solar cells. Here, authors reveal that for donor-acceptor blends with low energetic offset, charge transfer state dissociation rather than charge transfer state formation presents a bottleneck for free charge generation.

Perovskite solar cells (PSCs) comprise a solid perovskite absorber sandwiched between several layers of different charge-selective materials, ensuring unidirectional current flow and high voltage output of the devices 1 , 2 . A ‘buffer material’ between the electron-selective layer and the metal electrode in p-type/intrinsic/n-type (p-i-n) PSCs (also known as inverted PSCs) enables electrons to flow from the electron-selective layer to the electrode 3 – 5 . Furthermore, it acts as a barrier inhibiting the inter-diffusion of harmful species into or degradation products out of the perovskite absorber 6 – 8 . Thus far, evaporable organic molecules 9 , 10 and atomic-layer-deposited metal oxides 11 , 12 have been successful, but each has specific imperfections. Here we report a chemically stable and multifunctional buffer material, ytterbium oxide (YbO x ), for p-i-n PSCs by scalable thermal evaporation deposition. We used this YbO x buffer in the p-i-n PSCs with a narrow-bandgap perovskite absorber, yielding a certified power conversion efficiency of more than 25%. We also demonstrate the broad applicability of YbO x in enabling highly efficient PSCs from various types of perovskite absorber layer, delivering state-of-the-art efficiencies of 20.1% for the wide-bandgap perovskite absorber and 22.1% for the mid-bandgap perovskite absorber, respectively. Moreover, when subjected to ISOS-L-3 accelerated ageing, encapsulated devices with YbO x exhibit markedly enhanced device stability.

Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfill ambitions for net-zero carbon dioxide equivalent (CO _2 eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TW _p in 2021 to 8.5 TW _p by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the PVs community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other.

The record power conversion efficiency of solution-processed organic solar cells (OSCs) has almost doubled since non-fullerene acceptors (NFAs) replaced fullerene derivatives as the best-performing acceptor molecules. The successful transition from C60 to NFAs is still pending for vacuum-thermal evaporated (VTE) OSCs, not least because most NFAs are too large to be evaporated without breaking. Due to VTE’s relevance in terms of industrial manufacturing, discovering high-performing VTE NFAs is a major opportunity for OSCs. Here, we fabricate evaporated OSCs based on the NFA BTIC-H known from solution processing. This A–D–A molecule has an unfused bithiophene core, 1,1-dicyanomethylene-3-indanone end groups, and hexyl side chains, making it small enough to be evaporated well. We pair BTIC-H with four commonly used evaporated donors—DCV5T-Me(3,3), DTDCPB, HB194, and SubNc—in planar heterojunctions. We observe appreciable photocurrents and a voltage loss of ∼0.8 V, matching that of corresponding C60 devices. Donor:BTIC-H bulk heterojunctions likely face charge collection issues due to unfavorable microstructure. Our work demonstrates one of few NFA based evaporated OSCs with encouraging performance results and gives one potential starting point for molecule design of further NFAs suitable for VTE.

The advent of nonfullerene acceptors (NFAs) enabled records of organic photovoltaics (OPVs) exceeding 19% power conversion efficiency in the laboratory. However, high-efficiency NFAs have so far only been realized in solution-processed blends. Due to its proven track record in upscaled industrial production, vacuum thermal evaporation (VTE) is of prime interest for real-world OPV commercialization. Here, we combine the benchmark solution-processed NFA Y6 with three different evaporated donors in a bilayer (planar heterojunction) architecture. We find that voltage losses decrease by hundreds of millivolts when VTE donors are paired with the NFA instead of the fullerene C60, the current standard acceptor in VTE OPVs. By showing that evaporated small-molecule donors behave much like solution-processed donor polymers in terms of voltage loss when combined with NFAs, we highlight the immense potential for evaporable NFAs and the urgent need to direct synthesis efforts toward making smaller, evaporable compounds.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The performance of organic solar cells is strongly governed by the properties of the photovoltaic active layer. In particular, the energetics at the donor (D)–acceptor (A) interface dictate the properties of charge transfer (CT) states and limit the open-circuit voltage. More generally, energetic landscapes in thin films are affected by intermolecular, e.g., van der Waals, dipole, and quadrupole, interactions that vary with D:A mixing ratio and impact energy levels of free charges (ionization energy, electron affinity) and excitons (singlet, CT states). Disentangling how different intermolecular interactions impact energy levels and support or hinder free charge generation is still a major challenge. In this work, we investigate interface energetics of bulk heterojunctions via sensitive external quantum efficiency measurements and by varying the D:A mixing ratios of ZnPc or its fluorinated derivatives and C60. With increasing donor fluorination, the energetic offset between FxZnPc and C60 reduces. Moving from large to low offset systems, we find qualitatively different trends in device performances with D:C60 mixing ratios. We rationalize the performance trends via changes in the energy levels that govern exciton separation and voltage losses. We do so by carefully analyzing shifts and broadening sEQE spectra on a linear and logarithmic scale. Linking this analysis with molecular properties and device performance, we comment on the impact of charge–quadrupole interactions for CT dissociation and free charge generation in our D:C60 blends. With this, our work (1) demonstrates how relatively accessible characterization techniques can be used to probe energy levels and (2) addresses ongoing discussions on future molecular design and optimal D–A pairing for efficient CT formation and dissociation.

This structural characterisation shows how non-fullerene acceptor packing largely track from single crystals to solar cells. The crystal packing impacts electron mobility, while the domain purity drives the bimolecular recombination and performance.

Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfil ambitions for net-zero carbon dioxide equivalent (CO2eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TWp in 2021 to 8.5 TWp by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the photovoltaics community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other.

We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the gamma-ZnPc polymorph thereafter (thickness approximate to 1.0 nm). As the growth of the bulk gamma-ZnPc progresses, a substantial out-of-plane lattice strain (approximate to 15% relative to gamma-ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of approximate to 13 nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.

The efficiency of Organic Solar Cells (OSCs) has surpassed 18%, after a new generation of Non-Fullerene Acceptors (NFAs), the Y-series, was introduced to the field. These materials are characterised by high electron mobility, which is commonly attributed to its 3-dimensional packing motif in the single crystal. However, the bridge that links the NFA packing from single crystals to solar cells has not clearly been shown yet. In this work, we investigate the molecular organisation of a large body of NFAs, following the evolution of their packing motif in single-crystals, powder, and thin films made with pure NFAs and donor:NFA blends. We identified the most relevant packing motifs and polymorphs for the NFAs, discussing their role in the bulk heterojunction morphology, performance and charge transport, by combining experimental and theoretical approaches.

All-Small-Molecule Organic Solar Cells (ASM-OSCs) have reached 17% efficiency. Unlike polymers, SM synthesis is reproducible with controlled quality, making ASM-OSC highly relevant for commercialization. We perform extensive optoelectronic and microstructure characterization on high-performing ASM-OSC deposited via solution and vacuum thermal evaporation. We quantify absorption, voltage losses, and charge transport – probed via ellipsometry, sensitive EQE, and CELIV, respectively. To rationalize differences in charge carrier mobility, we construct the phase diagram and study phase separation via scattering methods. Example systems include ZnPc:C60, DCV5T-Me:C60, BTR:PCBM, and BTR-Cl:Y6. We find, e.g., that ASM-OSCs can achieve lower voltage losses than corresponding polymer-based OSCs.