Efficiencies of organic solar cells have doubled since the development of non-fullerene acceptors (NFAs). Despite this, it is still unclear how the acceptor-donor-acceptor molecular architecture, elongated molecular shape, and planar conjugated core of typical NFAs contribute to the observed efficiency boost. We demonstrate that electrostatics of acceptor-donor-acceptor molecules provides barrierless dissociation of excited states, as well as lossless charge extraction. We also show that the donor-acceptor-donor architecture would be an appropriate design for the donor molecule. Our findings are experimentally supported by analyzing various PCE10:NFA solar cells, with NFAs including Y6, IEICO, O-IDTBR, O-IDTBCN, ITIC, and their halogenated derivatives.

One of the great challenges facing organic solar cells (OSCs) for commercialisation, is increasing the active layer thickness without sacrificing the power conversion efficiency (PCE) and the fill factor (FF). Recently, PM6:Y6 as an OSC based on non-fullerene acceptor (NFA) has excited the community because of its PCE reaching as high as 15.9%; however, by increasing the thickness, the PCE drops due to reduction of the FF. This drop is attributed to change in mobility ratio with increasing thickness. Furthermore, this work demonstrates that by regulating the packing and the crystallinity of the donor and the acceptor, through volumetric content of chloronaphthalene (CN) as a solvent additive, one can improve the FF of thick PM6:Y6 device (~400nm) from 58% to 68% (PCE enhances from 12.2% to 14.4%) [1]. Our data indicates that the origin of this enhancement is the reduction of the structural and energetic disorders in the thick device with 1.5%CN compared with 0.5%CN. This correlates with improved electron and hole mobilities (by 1.4 and 3.6 times, respectively) and a 50% suppressed bimolecular recombination, such that the non-Langevin reduction factor is 180 times. This work reveals the role of disorder on the charge extraction and bimolecular recombination of NFA based OSCs.

Reference:

[1] Hosseini, Seyed Mehrdad, et al. "Putting Order into PM6: Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction." Solar RRL.

Organic solar cells based on Y-shaped non-fullerene acceptors, such as Y6, have gained considerable attention because of their high performance in single layer devices [1]. We recently demonstrated that free charge generation in the blend of Y6 with the donor polymer PM6 is barrierless [2]. It was proposed that the efficient charge separation in this blend is related to the high morphological order in this blend. In this work, we compare the charge carrier dynamics of PM6:Y6 with the blend of PM6 with a less-aggregating Y6-derivative, denoted as N4 [3]. Interestingly, the later blend performs fairly well but displays a significantly lower open-circuit voltage (V_OC). This hints at different recombination properties. By performing time delayed collection field (TDCF) and bias assisted charge extraction (BACE) experiments, we found that in both systems, bimolecular recombination is the dominant loss under illuminations equivalent to 1 sun. Surprisingly, the bimolecular recombination rate in PM6:N4 is about five times lower than that in PM6:Y6, in apparent contradiction to the lower V_OC of the blend. To study the reasons behind this discrepancy, quasi-steady-state photoinduced absorption (PIA) was applied to the blend film and to the full device to determine the steady state carrier density without and with the presence of electrodes. These measurements consistently unravel a higher concentration of charge carriers in the PM6:N4 blend under comparable illuminations while at the same time ruling out a significant contribution from recombination losses at the interfaces and across the transporting layers in these devices. Thus, considering the data for bimolecular recombination rates, the carrier concentrations and the negligible surface recombination, we propose that the smaller V_OC of the PM6:N4 blend is due to larger energetic disorder and a lower energy of the charge transfer states – highlighting the need to establish high structural and energetic order to realize efficient devices.

With the development of non-fullerene acceptors (NFAs), organic solar cells (OPV) recently have achieved efficiencies beyond 16 % in both single and double junction devices. Due to the disordered nature of organic semiconductors (OSCs), charge transport in OSCs is generally described by ‘hopping’ within an inhomogeneously broadened density of state distribution (DOS), where the energetic disorder plays a critical role. There are, however, conflicting views regarding the role of disorder on both the free charge generation and recombination. Problems in comparing and interpreting these results arise from the often ill-defined morphology of donor-acceptor systems. This asks for detailed experimental and theoretical studies regarding the role of disorder on the generation, extraction, and recombination of charge in well-defined OPV systems.

Here the efficient non-fullerene acceptor Y6 ^[1] was blended with different donor materials like PTB7Th, PM6, TQ1 and PCDTBT, chosen to cover a wide range in energetic disorder in the neat polymer film. To determine the energetic disorder for those material systems, the temperature dependent JV curves were measured based on both hole- and electron-only blend devices. Whereas the electron transport has almost constant disorder of around 65 meV for the different donor materials the energetic disorder for hole transport varies significnatly, from 65 to120 meV,. Charge transport and charge carrier dynamics was studied as a function of internal field, temperature, and excitation. TDCF results show that almost all 1:1 donor:Y6 blends exhibit field-independent charge generation, which we assign to the presence of an electrostatic interfacial field which for well-crystallized Y6 is large enough to compensate the Coulomb dissociation barrier ^[2]. To avoid the impact of blended morphologies, blends with a low concentration of the NFA (10 wt%) were studied, using the same set of donor materials with different energetic disorder. The results show that the energetic disorder has deteriorative effect on both free charge generation and recombination, consistent with the results of extensive kinetic Monte Carlo simulations.

The power conversion efficiency and open-circuit voltage of organic photovoltaic devices is determined by the recombination of photo-generated charge carriers. In this process, charge transfer (CT) states at the interface between the electron-donating and electron-accepting materials comprising the photo-active layer, play a crucial role.^[1] These electronic states are characterized by absorption and emission bands within the optical gap of the interfacing materials. Depending on the used donor and acceptor materials, CT states can be very emissive and generate free carriers at high yield.^[2] In this talk, I will discuss the fundamental properties of CT states, with focus on high and low frequency vibrational modes, and link them to organic opto-electronic device performance. Furthermore, using a new device architecture, we introduce strong light-matter coupling, resulting in redshifted and steepened absorption edges, as well as reduced energy losses in organic photovoltaic devices.^[3]

My talk will focus on the charge carrier dynamics which underlie the performance of organic solar cells employing non-fullerene acceptors. I will start by considering the kinetic competition between exciton decay to ground and charge separation, and how this is particularly important for low bandgap non-fullerene acceptors due to their relatively short exciton lifetime in films. I will go consider the impact of charge trapping in increasing energy loss, and in particular evidence that low energetic disorder in PM6:Y6 blends may be a key factor behind the high performance of this blend. Finally I will consider charge generation in low energy offset blends, and the importance of entropy of driving charge separation in such blends.

Organic photovoltaics (OPV), like other thin film PV technologies, is not yet part of this global TW scenario. The first OPV products were launched in 2008/2009 for portable chargers at an efficiency of about 2 %. Despite the rather low performance, these first products already showed the characteristic “OPV features” like flexibility, transparency and colour variability. Since then, the OPV community has concentrated on developing novel material systems for higher efficiency – a development which was outstandingly successful. In the last 10 years, organic solar cell performance was increased from about 8 % in 2009 to about 17 % in 2019. The certified record efficiencies beyond 17% were all achieved with non-fullerene acceptors (NFAs). Certified module efficiencies are typically lagging behind the certified cell efficiencies though our research group recently certified organic solar modules with efficiency as high as 12.6 % Despite the great progress in performance, organic PV is still facing the major challenge of overcoming a relatively large exciton without excessively loosing energy. Operation principles of organic solar cells in the limit of negligible potential driving forces for matched donor – acceptor HOMO levels is the currently most promising concept to drive efficiency of OPV towards the 20 % regime. The traditional concept for organic solar cells (OSC) suggests an offset in energy levels (Eoffset) to provide sufficient driving force to split excitons into free charge carriers. Understanding the factors limiting device operation at very small Eoffset is imperative in order to design material composites operating efficiently under such conditions. In this presentation we show that exciton splitting in highly efficient NFA systems at negligible HOMO level offset still takes place, but on ultra-long timescales, even exceeding the exciton lifetime, which obviously becomes the ultimate limit for efficient systems. Moreover, we analyze the voltage losses and surprisingly, in systems where no charge transfer state is detected, we show that the non-radiative voltage losses still correlate with the small but non-negligible EHOMO offset until reaching the pristine materials´ limit.

Organic solar cells (OSCs) are flexible, semi-transparent and environmentally friendly devices which can be installed in areas where silicon panels are not suitable (such as glass windows on buildings). Conventional OSCs are based on fullerene acceptors as a key component. However, fullerene-based OSCs can only achieve modest efficiency of 12% at best, due to their large voltage loss (above 0.8V), and poor device stability.

Recently, there has been a major revolution in the OSC field, as researchers developed many high-performance non-fullerene acceptors that can overcome the limitation of traditional fullerene acceptors and open a new era for the OSC field. One of the unique features of non-emerging non-fullerene OSCs is the surprisingly small voltage losses of the devices (~ 0.5V). Since 2016, our team at HKUST has developed a range of non-fullerene systems that can simultaneously generate high photocurrent (near 100% yield) with small voltage losses (first published in Nature Energy). Moreover, we have recently achieved record-breaking OSCs based on a state-of-the-art non-fullerene acceptor, achieving an unprecedented efficiency of 16.7% in single-junction OSC device. Our work clearly indicate that OSCs have the potential to reach the high efficiency of inorganic solar cells.

Our results show that the key factor is the long charge transfer life-time that allows for efficient charge separation despite of a small energy offset. In the other aspect, we study structure-property relationship of high-performance donor and non-fullerene acceptor materials and reveal the key structure features that enable highly efficient non-fullerene organic solar cell devices with over 16% efficiency. With these understandings of mechanism and structure-property relationship, it is feasible to further increase the efficiency of organic solar cells to the range of 18 to 20% in near future.

Ladder type fused aromatic monomers have been at the forefront of conjugated semiconductor development, finding use as the active component in both transistor and solar applications. Here I discuss our recent efforts to develop flexible synthetic routes to a range of such monomers which allow the ready manipulation of the solubilizing sidechains, as well as the aromatic heterocycle in the fused unit. We show that the nature of the sidechain is important for both donor polymers and non-fullerene acceptors. Changing from commonly used arylalkyl to simple alkyl sidechains is shown to have a positive impact on the performance of materials in single junction solar cells. We also demonstrate that the nature of the fused heterocycle has an important impact on the optoelectronic properties and device properties, highlighting that fused electron rich heterocycles are attractive building blocks for both donor and acceptor materials.

Over more than two decades of research, organic solar cells have achieved tremendous progresses in materials & device engineering and applications. For further advance, the power conversion efficiencies (PCEs) of organic solar cells need to be substantially improved. 

Inspired by the recent success in non-fullerene electron acceptors (NFAs), we have developed a design strategy defined as “A-DA¢D-A” to obtain a series of high-performing NFAs, called as Y series. D = electron donor unit while A and A¢ = electron acceptor unit. The key to this molecular innovation is introducing an electron-deficient moiety (A¢) such as benzotriazole or benzothiadiazole into the central fused ring. Generally, these electron acceptors show extended absorption in the NIR region and provide considerably low energy losses in organic solar cells, hence having set new records for the certified power conversion efficiencies by National Renewable Energy Laboratory (NREL).

It is worth mentioned that our research on these newly designed electron acceptors has attracted extensive attention. For instance, the research paper on the Y6 acceptor (Joule, 2019, 3, 1140) was cited over 800 times by the others within a very short time since its publication. More importantly, the certified power conversion efficiency of more than 17% has been reported by our fellow researchers based on the commercially available Y6. The underlying role of these acceptors has been actively investigated at home and abroad. While first achieving the 15% PCE in the single-junction solar cells, Y6 appears to be a universal electron acceptor and contributes to developing semi-transparent and flexible organic solar cells.

Keywords: Organic solar cells; Power conversion efficiency; Electron acceptor; Bulk heterojunction

References

1. Joule, 2019, 3, 1140-1151;

2.Adv. Mater., 2019, 31, 1807577;

3.Joule, 2019, 3, 3020-3033;

4. Nat. Photon., 2020,14,300-305;

5. Nat.Commun, 2019, 10, 570.

Solution-processed organic solar cells display low thermal stability largely because the nanostructure of the active layer blend changes upon heating. While photovoltaic blends based on non-fullerene acceptors such as indacenodithienothiophene-based ITIC derivatives are touted as more thermally stable than those based on fullerenes, they readily crystallize even far below their nominal glass transition temperature . This can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. We study two halogenated ITIC derivatives that readily co-crystallize upon mixing, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization which results in a fine-grained ternary blend with nanometer-sized domains that do not coarsen due to a high  ~ 200 ºC. As a result, annealing at temperatures of up to 170 ºC does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. Our results indicate that the ternary approach enables the use of high-temperature processing protocols, which are needed for upscaling and high-throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 ºC for at least 205 hours, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability.

The donor:acceptor ratio in organic solar cells has a strong influence on the device performance. Such feature mostly determines the fraction of light harvested and the efficiency of charge photogeneration and transport to the electrodes in archetypal bulk heterojunction devices. Accordingly, the donor:acceptor ratio requires careful screening each time a novel blend breaks into the state-of-the-art of the organic photovoltaic community. While traditional experimentation relies on the intense prototyping of devices with well-controlled blending ratios to tailor performance, that is still a highly resources-inefficient approach incompatible with the massive screening of donor:acceptor blend combinations, especially when only small synthetic batches of the materials under study are readily available.

Here, we present a novel thin-film processing approach that merges microfluidic technologies with blade coating to generate controllable lateral compositional libraries in solid-state. [1] This approach serves to realize controllable, lateral compositional gradients in films of polymer:small molecule and all-polymer organic photovoltaic blends, thus demonstrating an outstanding versatility. These gradients are exploited to perform the high-throughput optimization of composition in organic solar cells by adopting a measuring-intensive screening scenario. As a result, the optimization process takes place more than one order of magnitude faster than classical Edisonian experimentation. Furthermore, the procedure demonstrates an unbeatable, efficient use of resources as it only requires ca. 2 mg per material to complete the compositional optimization study.

In the future of printable electronics, organic solar cells (OPV) – due to their low cost, light weight and mechanical flexibility - hold as the technology with the greatest potential. The latest recorded performances of 18.2 % for single junction and 17.3 % for multijunction cells, disclose the great potential of this technology for future commercialization and filling of the gap left by their silicon counterparts. However, besides a multitude of great advantages, a weak point of OPV is their stability. Among the different stabilization routes, photochemical degradation of organic solar cells can be inhibited by blending stabilizing additives such as antioxidants, radical scavengers, singlet oxygen quenchers, UV absorbers in the active layers [1], resulting in drastically improved cell lifetimes. The presence of such additives can reduce and slow down the degradation processes arising from singlet oxygen and radical chain oxidation processes, which are pronounced under illumination in the presence of oxygen. The importance of this approach is underlined by the fact that encapsulation can only partially block oxygen from entering the cells. Once the oxygen permeates the devices, its diffusion is related to the morphological properties of the layers which are strongly affected by the mechanical stressing they are exposed to. In real working conditions, in addition to oxygen and illumination, the flexible OPV devices are exposed to mechanical stresses. These different degradation mechanisms are expected to interact and foster each other, e.g. mechanical crack formation and delamination, formation of macro radicals and photochemical degradation. This, however, has not been thoroughly studied to date. 

The presented work reports on the implementation of a naturally occurring additive in the active layers of two different bulk heterojunction solar cell systems, and elucidates their photochemical stabilization mechanisms [2, 3]. Furthermore, combined photochemical and mechanical stabilization of the cells is investigated. A new additive is designed, combining functional groups that inhibit interaction of the active layer materials with oxygen and the ones that promote mechanical elasticity. Relatively small loss in initial PCE and improvements in the device lifetime upon such combined stabilization is achieved. The reported results are supported with a study of the intrinsic mechanical properties of the active layer (tensile modulus) combined with its intrinsic adhesion properties (cohesive energy measurements) upon addition of stabilizing additives. The improved mechanical and photochemical stability sets out a promising direction for highly flexible and stable OPV devices.

The introduction of non-fullerene acceptors (NFA) has significantly improved the power conversion efficiency of organic photovoltaic (OPV) cells recently, reaching now above 18% for single-junction devices. While these developments have provided a strong boost to the OPV field, more efforts have to be devoted to the stability and up-scaling of such high performance NFA OPV devices, which includes development of interlayers that deliver in these areas. Metal oxide thin films have been widely used in OPV devices where they act as contact layers selective to either hole or electron transport (HTL and ETL, respectively), and thus support efficient carrier extraction. Well-known examples are titanium, TiOx, and molybdenum, MoOx, oxides, used in both organic and perovskite solar cells, with new variations appearing as the technologies develop further. In recent work, we have demonstrated that sputtered metal oxides thin films may act as interlayers in organic photovoltaic (OPV) devices, supporting both efficient and stable carrier extraction from such cells.

Here, recent progress made on reactively sputtered metal oxide hole¹ and electron² transport layers is presented. In both systems, a strong correlation between initial material composition and annealing condition to the microstructure of the films is given, leading to a pronounced improvement in their carrier extraction capabilities. Supported by a variety of surface science characterization studies, the importance of the energy band alignment, work function, microstructure, oxygen vacancies, optical and electrical properties and intrinsic stability on their performance as contact layers in OPV devices is discussed. A new crystalline TiO_x layer is shown to lead to an efficient electron extraction without s-shape current-voltage characteristics, in striking contrast to established TiO_x interlayers. Furthermore, for both sputtered MoOx HTL and TiOx ETL interlayers, a pronounced improvement on OPV stability is demonstrated, leading to prolonged lifetime for both fullerene and non fullerene based OPV devices. In order to meet the requirements on scalable OPV development, the up-scaling of these new metal oxide interlayer systems is also discussed, considering recent results on industrially relevant OPV device development³

The efficiency of organic solar cells has been increased strongly in recent years, mostly by smart choice of new acceptor materials that show strong complimentary absorption with regard to the donor material.

We have used the absorber material combination D18:Y6 and fabricated solar cells with >1cm² active area. The devices were measured in our lab and encapsulated before being sent over to the calibration lab at Fraunhofer ISE. The certified efficiency we reached is higher than the record value listed in the last version of the Solar Efficiency Tables [1].

To quantify the remaining optimization potential further measurements were carried out. Light beam induced current (LBIC) was used to reveal the homogeneity of current generation over the active area. Further, electro- and photoluminescence (EL & PL) spectroscopy were applied. Due to the HOMO levels of donor and acceptor being very close together [2], hybridization of the charge transfer state (CT state) with the local exciton state of the acceptor occurs [3]. This leads to a strong CT state emission which is advantageous for characterization via PL. Indeed, in contrast to many other absorber materials, here we observed PL and EL emission essentially at the same wavelength. For the PL, our aim was to disentangle the part of the signal that scales with the density of free charge carriers and therefore delivers important information about the operational state of the device.

Molecular based photovoltaics can play a key role in the transition to a sustainable energy paradigm, as is the most sustainable solar cell technology (in terms of energy investment on return), and simultaneously is amenable to be solution processed into any shape, can be as light as a feather, and can be produced with the largest colour pallet and transparency degree imaginable. Indeed, there is an infinite number of molecules that can be synthesised to match our needs. The bottleneck then becomes how to identify and screen potential candidates, or, in other words, how to find the needle in the haystack.

In this talk we will first describe a novel methodology for the fast evaluation of donor/acceptor systems for photovoltaics. The new approach is based on the fabrication of samples with gradients in the relevant parameters of interest that represent a large fraction of the corresponding parameter space. In particular, we fabricate gradients in thickness [1, 2], microstructure [1, 2], composition [2, 3] and apply hyperspectral imaging to correlate material and device properties. The method can be used both, for evaporated [4] and solution processed systems [1-3], is up to 100 faster than conventional optimization protocols, uses less than 50 mg of each active layer material and generates hundreds to millions of data points per system [3]. Then we show how this machinery can be used to find design rules for the optimum composition in non-fullerene acceptors based devices.

Organic solar cells (OSCs) evidence a rapid progress in recent years with the emergence of non-fullerene acceptors (NFAs), reaching a maximum power conversion efficiency (PCE) around 18% and surpasses a PCE below 12% of fullerene based counterparts. The fullerene acceptor PCBM is near-ball shaped and either loosely- or closely- packed in the photoactive layer of OSCs, leaving limited space for structural tuning. The complex chemical structure of A-D-A type NFAs, on the other hand, cast versatile stacking forms among the A, D and side-chain units of NFAs, which further affect their aggregations. Based on molecular dynamics simulations and experimental investigation of optoelectronic properties of NFAs, H aggregation, A-to-A type and A-to-D type J aggregation, A-to-A type and A-to-D type cluster of NFAs has been realized. The H aggregation blue-shifts the absorption spectrum, whilst the J aggregation red-shifts the absorption spectrum and construct three-dimensional p-p stacked network at the molecular level for efficient charge transport. We demonstrate how these different aggregations can be controlled experimentally, in particular the heating and solvent induced aggregation strategies, in different OSC systems. The modulation of molecular stacking and aggregation of NFAs can effectively tune its absorption and optoelectronic properties, and provides a crucial guidance for further developments of high performance OSCs.

The advantages of Solution processed Organic Photovoltaics (OPVs), such as their light weight, mechanical flexibility in addition to the small energy demand, and low cost equipment requirements for roll-to-roll mass production, characterize them as a dominant candidate source for future electrical power [1]. Over the last few years, the discovery of novel solution processed non-fullerene acceptor electronic materials greatly improve OPVs power conversion efficiency. Despite that, power conversion efficiency is not a “stand-alone” OPV product development target. Lifetime, material cost and printing processing must be equally considered, regarding solution processed OPVs technological progress. Adjusting the properties of electronic materials and device design, is crucial to achieving high performance OPV product development targets. Therefore, a number of high-level objectives concerning printing processing [2], electrodes [3] and interfaces [4] relevant to high performance solution processed OPVs will be presented.

Tandem solar cells are designed as a serial connection of wide-bandgap and low-bandgap sub-cells with complementary absorption spectra, to improve the overlap with the solar spectrum and to minimize thermalization losses.

With the introduction of non-fullerene acceptors in organic photovoltaics, the loss in open-circuit voltage (V_oc) compared to the energy gap of the absorber (E_g/q) can be below 0.5 V and the internal quantum efficiency concomitantly reaches levels of unity.

We will present our recent work on organic cells based on the PM6:Y6 system, which provide an efficiency of > 16% and a high V_oc. While high efficiencies are known for this donor/acceptor combination, stability concerns exist, especially under illumination. We will show the concomitant improvements in efficiency and stability by the addition of small amounts of fullerene to the active layer and, more critically, we will demonstrate that the optimum choice of hole and electron extraction/transport layers is a key that unlocks substantially improved operational stability in NFA solar cells. Furthermore, we found that the stability of the cells is critically affected by the spectrum of the light source. Cells based on MoO₃ instead of PEDOT:PSS on the anode side, and C60/BCP/Ag on the cathode side, sustain continuous operation in the maximum power point for hundreds of hours without notable degradation, if only the acceptor is excited (i.e. for wavelengths > 680 nm;  hn < 1.82 eV), in contrast to the excitation of both donor and acceptor, which results in some substantial decay of efficiency. This finding opens a favorable opportunity to use PM6:Y6 cells in a tandem architecture where short wavelength components of the AM1.5 solar spectrum are absorbed by a suitable wide gap sub-cell. Here, we exemplarily show such a combination with a wide-gap perovskite cell (E_g = 1.8 eV), that as a single junction shows a V_oc of 1.3 V and an efficiency > 15%. The resulting tandem cells show a high V_oc of 2.16 V, which is the perfect addition of the V_oc’s of the two sub-cells, and a high FF of 75%, which combines to a high efficiency > 21%. This is the highest efficiency for a perovskite/organic tandem cell and the highest efficiency of a solar cells, where an organic sub-cell is involved. Further improvements seed the prospect of efficiencies in the range of 26% for these tandem devices.  

Hybrid devices based on a heterojunction between an inorganic and an organic semiconductor have attracted interest as a way to combine the advantages of both classes of materials, but in practice the performance of such devices has often been disappointing. Here, focus on hybrid heterojunctions based on copper thiocyanate (CuSCN), a low-cost p-type inorganic semiconductor as a donor, and a variety of (fullerene and non-fullerene) molecular acceptors  Because CuSCN is transparent in the visible, the absorption properties of such devices are controlled by the choice of acceptor, which enables colour-tuneable and semi-transparent optoelectronic devices. We show that planar hybrid heterojunctions led to visible-transmitting heterojunctions with high voltages and low non-radiative voltage losses (0.21 ± 0.02 V) monitored from the emission of the CT state that is formed at the hybrid heterojunction. This class of devices can serve as power generating windows in greenhouses and in water-splitting photoelectrochemical devices, in both of which the solar spectrum is shared. We propose designs for structures with promising performance for both types of application.