Organic solar cell (OSC) bulk heterojunctions (BHJ) typically feature a rich phase morphology with the phase composition and distribution significantly affecting processes such as charge generation, recombination and extraction, and, in turn, device performance. After deposition, the active layer is in a metastable state including several phases that vary in composition (from pure to mixed) and degree of order (from crystalline to amorphous). This metastable morphology is generally prone to changes that lead to cell degradation. To stall this degradation, it is necessary to identify the different phases and follow their dynamics as a function of composition, processing conditions and temperature. Recently we developed a new staining methodology that offers imaging of the BHJ in electron microscopy. Using this technique we were able to study the temperature-induced morphology evolution of a high efficiency fullerene:polymer blend, as a model system. Based on the results we developed a unified thermodynamic and kinetic mechanism that allowed us to correlate the morphology evolution with OSC degradation during thermal annealing of fullerene based OSCs. We then turned to donor:non-fullerene acceptor (NFA) blends that are more challenging to elucidate because they often display different polymorphs. Moreover, their glassy states can be more complex. Using the developed electron microscopy analysis, we followed the dynamics of the phases as a function of composition and temperature. We identify multiple glassy phases and complex NFA-based BHJ morphology emphasizing the need for a deeper understanding of the phase behavior of such systems.

OPV cells have a proven efficiency of over 19 % while OPV modules have a proven record efficiency
of 13.5 %. Both values are still increasing, towards > 20 % for small area cells and > 15 % for large
scale modules. With these performance values, solution processed emerging photovoltaic
technologies are reaching out to applications that are going beyond the typical niche markets. The
first generation of commercially available printed PV modules showed a lifespan in the order of
beyond 5 years and more under outdoor conditions (OPV). Interestingly, several experiments are
strongly suggesting that solution processed semiconductors like organics can be stable under light
and, to some extent, under oxygen as well. Despite these impressive numbers, one should not forget
that these are “best you can do” lifetime values.
On the other hand, the community did not progress significantly in overcoming the fundamental limitations of OPV. The energy gap law for excitonic materials, the precise microstructure control of binary or ternary composites, the design principles for environmentally stable materials or the Kirchhoff law for multi-junction cells continue to be major barriers for this technology. We briefly introduce into these long-time challenges for excitonic absorbers and then discuss concepts and strategies how to resolve them. Among them, the development of a digital twin for OPV which has inverse predictive power is a most promising concept. “Solar FAU”, an alliance of research partners in the Erlangen-Nürnberg region that is headed by Friedrich Alexander University, is exploring the basic concepts and methodologies how to build a digital twin for emerging-PV technologies. The central and most desired element of the digital twin is the power of inverse design, e.g., inventing molecules, device architectures and processes with tailored properties. Insight from first pieces (agents) of the digital twin strongly supports the assumption that inverse design capability is possible, even in the case of considerable experimental uncertainty. Coupling the digital twin to Material Acceleration Platforms (MAP) reduces experimental uncertainty and allows to learn predictions which otherwise would be impossible. We have recently demonstrated the power of of such coupled systems and demonstrated correlations which were previously unthinkable, like the prediction of performance and lifetime of OPV cells from simple absorption data or the identification of molecular features that determine the environmental operational stability of OPV.

Organic solar cells have witnessed a rapid improvement in device performance over the past few years, now achieving an exceptional power conversion efficiency of over 19% under standard solar irradiation (over 30% under indoor illumination) in a single junction device, rapidly closing the performance gap with competing technologies such as crystalline silicon. This upsurge in performance is primarily driven by the emergence of non-fullerene organic small molecular and polymeric acceptors, surpassing conventional fullerene acceptors due to stronger optical absorption, optimal energy levels and potentially lower cost in synthesis and purification. This is further coupled with major advances in device design (e.g. ternary bulk-heterojunction blends, novel device interlayers), placing organic solar cells in an unprecedentedly promising position for potential large scale commercialisation in multiple application areas.

A further key factor to realise the full commercialisation potential of organic solar cells is stability, that is, a PV device must have a sufficiently long lifespan that exceeds the required operational period for a particular application. For example, it is desirable for a PV device lifespan of >20 years for building integrated applications, whereas a shorter lifespan of ~5-10 years may be sufficient for powering indoor autonomous sensors. The modest device stability has been a widely-recognised and long-standing challenge for conventional fullerene-based organic solar cells, with multiple degradation mechanisms already identified that result in rapid losses of device performance under illumination, ambient air and thermal stress conditions. Nevertheless, the recent transition of organic solar cells from fullerene acceptors to non-fullerene acceptors, as well as the major advances in their molecular and device design, has brought exciting opportunities to fully overcome this challenge.

In this talk I will give a summary of the recent research progress of my group in understanding the degradation mechanisms of non-fullerene organic solar cells. I will highlight the distinct roles of several molecular and device design factors in the resulting degradation behaviour, and propose potential strategies to overcome these degradation mechanisms toward achieving long-term stability of organic solar cells.

The organic photovoltaics (OPV) field has seen a boost in performance with the introduction of small-molecule electron acceptors, such as Y6, and their copolymer derivatives, leading to power conversion efficiencies above 18% [1]. The device operational lifetime depends critically on the photochemical stability of the materials and the film morphology. Here we report the evolution of optical properties, composition, and energy levels, during one-sun (AM1.5) illumination in air of thin films of the donor polymer PBDB-T, the small-molecule acceptor Y5 [2], its copolymer counterpart PF5-Y5 [3], as well as their blends. UV-Vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Near-edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM) were used to monitor the thin film properties. The absorption spectra show that the PBDB-T polymer and the copolymer PF5-Y5 undergo rapid photobleaching, while the Y5 film spectrum remains, surprisingly, almost intact even after 30 hours of light exposure in air. The corresponding blend films of PBDB-T with Y5 and PF5-Y5 show similar losses in absorption as their components. New carbonyl peaks emerge in the FTIR spectra of PBDBT, PF5-Y5 and blend films, but are absent in those of Y5, indicating that the bulk of the Y5 film is resistant to photooxidation, while the copolymer containing a Y5-moiety undergoes photochemical degradation reactions. The faster photodegradation of PF5-Y5 compared to Y5 raises the question about the role of the copolymer’s BDT moiety in the photooxidation. However, the effect of film packing on the rate of degradation should also be considered. Angle-resolved NEXAFS spectra reveal a stronger linear dichroism in the spin-coated PF5-Y5 films compared to Y5 films, confirming a preferred orientation of the PF5-Y5 polymer backbone, while the random average orientation of Y5 molecules suggests a multi-crystalline Y5 film. Surface analysis by core level XPS indicates, moreover, that the surface of blend films is donor-enriched. These new insights on the effects of intentional photodegradation on donor and acceptor materials properties are expected to contribute to the design of stable acceptors as well as the development long-lived OPV devices.

Our research group is focused on the development of functional polymer interlayers for various kinds of optoelectronic devices, including thin-film transistor (TFT), (photo-)memory, light-emitting diode (LED), and solar cell devices. We are particularly interested in exploring the structure-performance relationships of polymers. In addition to advances in the controlled synthesis of organic semiconductors, we also explore innovative interfacial and device engineering to optimize the device performance. Herein, we highlight our recent works on interface engineering for polymer solar cells. In this presentation, an integrated study of combining material synthesis, interface engineering, and morphology analyses will be introduced and discussed to explore the full promise of the devices, with a particular focus on long-term device stability, including thermal stability, photostabiltiy, and underwater-stability.[1,2] We first present robust interface engineering of graphene oxide nanosheets (GNS) to improve the thermal and photostability of non-fullerene bulk-heterojunction (NFA BHJ) OPVs to practical levels. Afterwards, we demonstrate the great potential of all-polymer blends and np-TiO2 ETL to improve the durability of unencapsulated OPVs in high humidity environments and even underwater immersion.

Non-fullerene acceptors (NFAs) have attracted a lot of attention within the organic PV community. Several NFAs are available today outperforming fullerene acceptors in bulk heterojunction solar cells and power conversion efficiencies > 18% have been reported for several different donor acceptor systems. In addition, NFA-based solar cells show also very promising device stabilities under standard operation conditions. In this contribution, I will review the status and the potential of non-fullerene acceptors. I will discuss the charge generation and recombination processes and the role of triple states in NFA-based absorbers. Photoluminescence experiments performed at low temperatures and high magnetic fields reveal the efficient generation of triplet states in non-fullerene acceptors like Y6. Different recombination processes in bulk-heterojunctions containing fullerene or non-fullerene acceptors are found. The role to triplet states in NFA-based bulk-heterojunction blends will be discussed. Especially the role of triplet-triplet annihilation and singlet fission processes will be considered in detail.

Recent advancements in material design and related power conversion efficiency (PCE) improvements; organic photovoltaics (OPVs) are positioned as a very promising technology for the needed green energy transition. Particularly, the development of non-fullerene acceptors (NFAs) is continuously raising the bar for PCE and device stability. However, translating these findings to industrially applicable techniques, while maintaining high performance and stability parameters, remains a significant challenge. The aim of this study is to facilitate a paradigm shift in the development of OPV by introducing a device structure which can maintain its morphology over a longer period of time. Sequentially depositing the donor and acceptor materials to create bilayer architecture, where the morphology governing nano interfaces throughout the bulk is eliminated, provides an efficient method to develop morphologically robust OPVs. Here investigation on different active layer thicknesses using PM6 and Y7 materials have been carried out to see how varied donor and acceptor layer thicknessess effect charge transport and recombination behavior, and how the device's performance can be tweaked further. Combined with the electrical and morphological characterizations, the device performance of bilayers has also been compared with the conventional bulk-heterojunctions (BHJs). To further illustrate the significant advantages of bilayer structure over BHJs, the photo and thermal degradation measurements have not only been conducted for PM6 ̶ Y7, but for a number of other NFA systems like PM6 ̶ IT4F, PM6 ̶ Y6, PM6 ̶ N3, PBDB-T ̶ ITIC, TPD-3F ̶ IT4F,  PCE10 ̶ O-IDTBR, and more demonstrating the robustness of the structure.

Lately, non-fullerene acceptors (NFA’s) have received increasing attention for use in polymer-based bulk-heterojunction organic solar cells, as improved photovoltaic performance compared to classical polymer-fullerene blends could be demonstrated. In this study, polymer solar cells based on high-efficiency donor polymers are used in combination with a number of different electron-accepting materials. Both donors and acceptors are either fluorinated and/or chlorinated (or, in more general terms, halogenation) and compared with the non-halogenated version. To see the influence of end-group fluorination of ITIC in a photovoltaic application, the well-known donor polymers PBDB-T (PCE12) and its fluorinated and chlorinated representative, PBDB-T-2F (PM6), PBDB-T-2Cl (PM7) were used to process bulk heterojunction solar cells. Different thermal annealing during device processing is investigated from their photovoltaics parameters. Charge generation and recombination mechanism upon annealing were investigated via several optoelectronic characterization methods. Especially, Time-Delayed Collection Field (TDCF) measurements demonstrate that field-dependent charge generation and bimolecular recombination processes affect the fill factor and, thus, the efficiency of devices.

Agrivoltaics is an encouraging application area for organic photovoltaics (OPVs) that merges solar power generation with the energy demands of crop growth, thus providing dual functionality to dedicated farmland. When integrated on greenhouses, semitransparent OPV technologies are expected to improve the sustainability of crop growth and turn net zero energy greenhouses a reality. In this work, we screen all-polymer and polymer:small-molecule, donor:acceptor blends for their use in organic agrivoltaics. The optical requirements of the blends are benchmarked according to a novel greenhouse figure-of-merit (gh-FoM) to account for the -simultaneous- transparency requirements of crops and humans. Modeling of the gh-FoM reveals that photoactive layer thicknesses between 25-125 nm and acceptor-enriched ratios (such as 1:4, w:w, with the acceptor being the lower band gap material) are more suitable for the optical constraints imposed by most types of crops. Nevertheless, such ratios are found to be less thermally stable, thus imposing an undesirable trade-off between the required device semitransparency and its long-term stability.

The optimized blends are then upscaled to form 25 cm² active area laminated modules processed entirely from solution, on flexible substrates, via roll-to-roll compatible methods (slot-die and blade coating) and in ambient conditions. These modules are installed on a domestic greenhouse in Sweden, where their outdoor stability is tracked 24 hours per day over the spring and summer periods while in compliance with the ISOS-O-2 protocol. We accordingly design an autonomous, Arduino-based IV-tracing setup that automatically collects, analyses and uploads relevant device data to the cloud for remote tracking worldwide, at very low cost. Our outdoor stability study shows degradation modes undetectable under laboratory conditions such as decreased ideality factors and detrimental module delamination, which accounts for 10-20% loss in active area as per photocurrent imaging. Harsh thermal and humidity cycling conditions are correspondingly tracked during the day-night cycles, which are found to contribute to the amplified degradation pace of the agrivoltaic modules. Among the active layers tested in this work in the form of air-processed laminated modular architectures, polymer:small-molecule blends are the most stable and position as prominent photoactive layers toward sustainable organic agrivoltaic technologies.

Understanding the spatial dynamics of nanoscale exciton energy transport beyond the temporal decay is at the core of photosynthesis and essential to provide a better framework for further improvements of nanostructured optoelectronic devices, such as solar cells. The diffusion coefficient (D) of photovoltaic films has so far only been determined indirectly, from transient singlet-singlet annihilation (SSA) experiments. Here, we present the full picture of the exciton distribution dynamics, adding the spatial domain to the temporal one, by spatio-temporally resolved photo-luminescence microscopy. In this way, we directly track diffusion and we are able to decouple the real spatial broadening from its overestimation given by SSA. We measured the diffusion coefficient, D = 0.017 ≈ 0.002 cm2/s, of the non-fullerene electron acceptor Y6, which combined with an exciton lifetime of τ = 840 ps, gives a Y6 film diffusion length of L = √ Dτ ≈ 35 nm. Thus, we provide an essential tool that enables a direct and free-of-artifacts determination of diffusion coefficients, which we expect to be at the core of further methodical studies on exciton dynamics in energy materials.

The world’s energy consumption is expected to grow vastly over the next few decades. Contributing to this growing energy demand is the advent of the Internet-of-Things (IoT), through which many devices and sensors may one day share immense quantities of information to benefit society as a whole.^[1] Indoor photovoltaics (IPVs) based on easy-to-process, earth-abundant organic semiconductors have recently gained considerable traction as a potential source of power for the IoT.^[2] These IPVs would recycle low-intensity, artificial light generated by LEDs to power small devices and IoT nodes, and they are fast approaching commercial viability. Organic semiconductors, however, are not yet optimised for IPV applications – they are plagued by detrimental non-radiative open-circuit voltage losses, and the majority of previous research has actually been channelled towards outdoor solar cell applications instead.^[3] In this work, a realistic model is used to demonstrate the limiting effects of an intrinsic material property of organic semiconductors – the energetic disorder.^{[4, 5]} The thermodynamic constraints imposed on the power conversion efficiency of IPVs by energetic disorder are explored, alongside the effects of sub-optical gap absorption, non-radiative recombination losses, and the incident light intensity. Finally, a methodology that takes a photovoltaic external quantum efficiency spectrum and one measurement of the open-circuit voltage (under one-sun conditions) is presented for estimating the performance of organic semiconductor-based IPVs under illumination by any spectrum, at any intensity.

The first part shall provide an overview of the development of roll-to-roll processed organic solar modules at Fraunhofer ISE.
The focus is on slot-die coated, laser-processed modules with ITO-free electrode systems.
Results will be presented both from opaque devices as well as semitransparent ones based on visibly transparent electrodes with high NIR reflection.
The second part is on photoluminescence measurements of organic solar cells. These are far more complicated compared to all other PV technologies as the signal is strongly dominated by the radiative decay of photogenerated excitons in the donor and/or acceptor phase [1]. The underlying luminescence signal related to the free charge carriers is therefore hidden and can not be determined quantitatively. I will present a newly developed method that enables separating the contribution of photogenerated excitons from the one related to the radiative recombination of free charge carriers. This way the quasi Fermi level separation within the absorber can be determined. An excellent agreement between the latter and the electrical voltage measured between the terminals of the solar cells is observed for devices with selective electrodes.
This newly developed method enables new optimization strategies, including the quantitative comparison of pure absorber films or half cells with complete solar cells.

We demonstrate a method to manipulate the morphology of thin film semiconducting polymers. The manipulation is achieved by optical excitation of the polymer during roll-to-roll slot-die coating, providing a technique that is viable for large-scale production. Along with establishing the technique, the entire knowledge chain from fundamental insight in polymer physics and structure to application is presented. By correlating X-ray and neutron scattering techniques with density functional theory and molecular dynamics simulations of solvent evaporation, we successfully determine the packing of the polymers in the ground state and excited state. The findings are coupled with measurements of dynamical physical properties and solar cell device performance to pinpoint the structure-property relationship.

Different processing was applied to match energies below and above the excitation energy levels of P3HT in solution and in thin film [1]. Specifically, either no illumination (i.e., dark), or LEDs emitting red, green, or blue light illuminated the films while being coated. It is known from density functional theory (DFT) calculations [2] that, when exciting P3HT with visible light, i.e. going from the ground state to the excited state, the electron density changes to form double bonds between neighbouring thiophene rings. This change in the bonding pattern planarizes the polymer backbone. Consequently, the final morphology is affected when exciting the polymer during coating. To probe the morphological changes caused by the four treatments, we use Grazing-Incidence Wide-Angle X- ray Scattering (GIWAXS) on the coated films. To elucidate the physical origin of the changes, DFT and solvent evaporation molecular dynamics (MD) are used to simulate the effect of excitation on single polymer chains and thin films. The full picture of morphological changes is followed by a thorough investigation of the change of physical properties in the thin film. Here, the UV-VIS absorption spectra are analysed, followed by THz spectroscopy measuring the thin-film photo- conductivity. Finally, the consequence of light treating a complete OPV device (P3HT:O-IDTBR) is discussed.

The experimental and computational results demonstrate that exciting P3HT with visible light during deposition serves as a tool for manipulating the packing behaviour of P3HT and can be used to modify the final thin film morphology. It can be considered a new processing parameter for achieving the desired performance of organic thin films. Particularly, the capability to increase the out-of-plane mobility by light treatment can be used for transistor applications where directional mobility and patterning is essential [3]. Application in OPV devices will require an optimization process for the right crystal packing and domain size of donor and acceptor constituents. For the specific combination P3HT:O- IDTBR, the domain size decreases with light treatment and still results in a decrease in power conversion efficiency. Understanding how light treatment during fabrication influences the final morphology of a film can enable major improvements for specific materials systems or other technologies in flexible electronics and photovoltaics.

In conclusion, we report a method to manipulate the morphology of P3HT thin films through illumination with visible LED light during roll-to-roll slot-die coating. Optical polymer excitation temporarily constrains large sections of the chains into a planar geometry that is the minimum of the excited state potential energy surface. This structural effect is strong enough to affect the aggregation behaviour and, thereby, the final morphology of the P3HT film. The light-treated films are less crystalline overall, display a higher degree of face-on orientation, shorter conjugation length, and a change of the unit cell dimensions with less efficient packing. Consequently, the in-plane photoconductivity decreases and the out-of-plane conductivity increases drastically with light treatment.

The global trend towards automatization and miniaturization of smart devices has triggered the development of reliable off-grid power sources with low economic and environmental impact. Such autonomy can be provided when a photovoltaic cell is integrated with an electrochemical storage device in one monolithic device. This work demonstrates a reliable and straightforward approach to monolithically integrate high-performance single and multijunction organic solar cells with mesoporous nitrogen doped carbon nanosphere-based supercapacitors or lithium-organic batteries in a single device with a three-electrode configuration. To assess the efficiency of these devices, a novel approach is presented that relies on the direct monitoring of both integrating parts during illuminated and dark phases and accounts for possible losses. This versatile approach is applicable for all kinds of integrated multifunctional photoconversion-storage systems. For the photosupercapacitors, the evaluation with the standard literature approach showed an outstanding performance with a peak photoelectrochemical energy conversion efficiency of 17 %. However, in our opinion this type of efficiency does not properly represent the real overall device efficiency. Based on our newly developed efficiency determination, a more modest overall cycle efficiency of 2 % is obtained. For battery-based devices, a higher output voltage is achieved, but at the cost of a lower, 0.3% cycle efficiency. In our view these values represent the real overall performance of the integrated device in a precise manner and will thus enable meaningful direct comparisons among different photoelectrochemical storage systems.  

Organic bulk heterojunction photodiodes (OPDs) attract wide attention for light sensing and imaging but their detectivity is typically limited by a substantial reverse bias dark current density J_d. Recently, the J_d in OPDs was attributed to thermal charge generation mediated by mid-gap states. The evidence of such trap states, however, comes from rather indirect techniques such as thermal admittance and/or photocurrent measurements. In this work, we study the temperature dependence of J_d in state-of-the-art OPDs that have Jd values down to 10⁻⁹ mA cm⁻². For a variety of donor-acceptor bulk-heterojunction (BHJ) blends we find that the activation energy of J_d is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV. Ultra-sensitive sub-bandgap photocurrent spectroscopy on these diodes show evidence for sub-bandgap states. The minimum energy for optical charge generation from these sub-bandgap states correlates very well with the dark current thermal activation energy. From this, we conclude that the dark current in these OPDs is caused by thermal charge generation at the donor-acceptor interface mediated by sub-bandgap states that create an activation energy that is 0.3-0.5 eV below the bandgap energy. By studying donor- and acceptor-only diodes, we find that such sub-bandgap states are prevalent in polymer (donor) semiconductor materials, and in the fullerene acceptor PCBM, but not in the non-fullerene acceptor materials studied.

The extraordinary increase in efficiency of organic solar cells has rushed the interest towards an industrial scale-up. However, for a realistic lab-to-fab transition, strategies to improve simultaneously the performance and the solar cell lifetime need to be explored. In this aspect, the interface between the photoactive layer and the buffer interlayers is considered critical. As an example, ZnO triggers photodegradation of non-fullerene acceptor molecules at the interface, while surprisingly it is still the benchmark buffer layer in organic solar cells [1]. Recently, SnO₂ nanoparticles have emerged as a high performing, scalable and low-cost alternative to ZnO, and it has already proven its potential for perovskite solar cells. However, although several reports point at SnO₂ as a promising layer also for organic solar cells, most of the well performing systems are still reported with ZnO, suggesting SnO₂ has intrinsic limitations in organic solar cells. On the other hand, several reports indicate a need for surface modification via chemical treatments to improve the interfacial properties [2]. Driven by these subject, in our work we make use of water-based SnO₂ nanoparticles in n-i-p organic solar cells. We prove that the presence of surface cations produce an inherent defective interface with several organic blends, leading to s-shaped J/V curves and we amend it via a very simple an entirely environmentally-safe method. More strikingly, we demonstrate that the device stability is strongly correlated with the concentration of surface ions, and our method raises the operational stability factor T80 under continuous illumination from 8.4h to 244h. Finally, the power conversion efficiency and its standard deviation improve universally for several systems, showing its potential to be systematically applied in both lab scale but also industrial scale.

The inverted (n-i-p) structure is considered more suitable for the industrial production of organic solar cells (OSCs), due to the superior stability of the materials used in this configuration. Transparent conductive oxides are usually employed as electron transport layer (ETL) in OSCs with the inverted structure. Among them, zinc oxide (ZnO) is the most studied and adopted, since it is easy to obtain with relative good crystalline quality and it provides a good band alignment. Tin oxide (SnO₂) could represent a valid alternative to ZnO as ETL, with superior transparency to the visible light and higher electron mobility. Most importantly, unlike ZnO, SnO₂ is reported to have an excellent ambient stability and to not cause any photocatalytic degradation of the organic materials [1].

The main drawback that hinder an extended use of SnO₂ as ETL in OSCs is the difficulty to fabricate films of the material with high crystalline quality. SnO₂ films are often fabricated by solution processing, starting from a nanoparticle colloidal dispersion. This method is lacking in control over the material quality, giving rise to films with high density of defects, especially on the surface. Moreover, organic ligands, used to stabilize the nanoparticle dispersion, often leave residuals on the deposited film, which can compromise the interface with the active layer. As a result, the overall performance of the OSCs with SnO₂ as ETL is often inferior to that of devices with ZnO, due to substantially lower values of fill factor (FF).

An alternative method to fabricate SnO₂ films, with the desired crystalline quality, is the use of atomic layer deposition (ALD). ALD is a deposition technique widely used in the fabrication of electronic devices, since it is scalable and allows obtaining compact layers the material with exceptional quality. SnO₂ deposited by ALD is attracting attention in the research field of photovoltaic and it is currently used in perovskite [2] and tandem solar cells [3]. Nonetheless, its use in OSCs is scarcely reported and its potential in these devices still need to be evaluated.

In this work, we fabricated OSCs with SnO₂ deposited by ALD as ETL. We compared the performance of the solar cells with those of devices with either SnO₂ or ZnO deposited from solution. The scalability of the performance with the device active area and device stability have also been investigated. Our results show the higher efficiency of the OSCs with SnO₂ deposited by ALD, thanks to a superior FF. The improvement of FF has been verified on OSCs with different composition of the active layer. Values of FF close to 80% have been achieved, an exceptional result for OSCs with the inverted structure, which demonstrate the great potential of SnO₂ deposited by ALD as ETL for high-performance OSCs.

Organic solar cells are showing great potential to be employed in indoor and outdoor applications. The utilization of portable electronics, internet of things (IoT), and sensors for wearable and healthcare applications that are used in both indoor and 1-sun conditions highlight the importance of the development of photovoltaic devices that can efficiently perform under different light intensities.  In this paper, we fabricated OSCs with polyethyleneimine ethoxylated (PEIE) and a water-based material Poly-lysine as interfacial layers (IFL) and investigated the device's performance under 1-sun and indoor light intensity. Our results show that the choice of the interfacial layer is important and affect significantly the performance of devices. Under 1 sun illumination devices with poly-Lysine as an interfacial layer show the highest average efficiency of 7.72% whereas PEIE-based devices displayed lower average efficiency of 7.13%. Under the low light intensity of 1000 lux Poly-Lysine-based devices due to their low shunt resistance causing leakage current resulting in lower average efficiency of 7.09% while on the contrary PEIE-based devices exhibited increased average efficiency of 12.25%. Our study paves the way toward OPV devices for portable and wearable applications.

The discovery of several new classes of electron-accepting molecule that work well as electron acceptors at molecular heterojunctions had led to a strong increase in the power-conversion efficiency in organic solar cells, from 11 to 19% in the last 5 years. The impressive performance of the new materials and the range of chemical structures has led to interest in whether alternatives to the traditional ‘bulk heterojunction’ architecture for an organic solar cell can be found.

One approach concerns structures where, through chemical design, electron-rich and electron-poor components of a single macromolecular material are constrained to have a particular geometry relative to each other, being electronically coupled either through space or through bond. In a second intriguing development, devices based on a single molecular material have been reported deliver relatively high charge-generation efficiency in the absence of a donor-acceptor interface. This latter example challenges the current understanding of how free charges are generated in organic semiconductors. Achieving efficient energy conversion with a single macromolecular component would be both interesting and practically useful.

In this work, we report on studies of the design, characterisation and modelling of both types of structure, including a variety of non-fullerene acceptors based on alternating donor and acceptor units, and bonded macromolecular compounds with donor and acceptor type domains. By combining experimental characterisation under varying conditions (field, temperature, excitation) with molecular and device-level calculations, we endeavour to relate exciton and charge dissociation efficiency in single-component devices to molecular parameters.

Despite an impressive increase over the last decade, experimentally determined power conversion efficiencies of organic photovoltaic (OPV) cells still fall considerably below the theoretical upper bound for near-equilibrium solar cells, set by the Shockley-Queisser limit. Even in otherwise optimized devices, a prominent yet incompletely understood loss channel is the thermalization of photogenerated charge carriers in the density of states that is broadened by energetic disorder. First, we will show by a detailed comparison between numerical modeling and experiments that the slowness of the mentioned thermalization leads to a 0.1-0.2 eV higher open circuit voltage (Voc) in OPV devices than would be expected for instantaneous thermalization.[1] The latter is commonly assumed when analyzing Voc of OPV devices. Second, we demonstrate by extensive numerical modelling how this loss channel can be mitigated in carefully designed morphologies. Specifically, we show how funnel-shaped donor- and acceptor-rich domains in the phase-separated morphology that is characteristic of organic bulk heterojunction solar cells can promote directed transport of positive and negative charge carriers towards the anode and cathode, respectively.[2] We demonstrate that in optimized funnel morphologies this kinetic, non-equilibrium effect, which is boosted by the said slow thermalization of photogenerated charges, allows to surpass the Shockley-Queisser limit for the same material in absence of gradients and under near-equilibrium conditions. Methodologies by which the proposed morphologies can be realized are discussed, along with the relevance to already published results.

We address here the long-standing quest to gain broadly applicable understanding of the structure formation of donor:acceptor blends that are used in organic solar cells, leading generally to a rich phase morphology where intermixed and neat phases of the donor and acceptor material co-exist. This is especially true for blends with non-fullerene acceptors because NFAs often display different polymorphs; moreover, their glassy states can be highly complex. Here we show that some donor polymer:non-fullerene acceptor blends comprise two different populations of glassy phases: a molecularly intermixed and an ITIC-rich one. This picture assists to understand the performance of some of these systems when used in solar cells; it also allows us to establish a tentative picture of the complex correlation of structure and electronic landscape for the understanding of organic photovoltaic cells. Our works furthermore highlights the challenges that are faced when using next generation materials, especially if long-term stability is sought.

The energetic offset between the highest occupied molecular orbital (HOMO) levels of the donor and acceptor components of the organic photovoltaic (OPV) blends is well-known to affect the efficiency of the singlet exciton (S1) dissociation into separated charges (CS) via the chargetransfer (CT) state, however the impact of this offset on bimolecular recombination of free charge carriers has not been explored. In this study, using different non-fullerene acceptors blended with the same donor polymer PM6, we demonstrate that, apart from reducing the driving force for charge generation, diminishing HOMO-HOMO energy offset also activates faster bimolecular recombination of free charges either through the CT state or through exciton reformation as a new channel. Using the comparison between PM6:ITIC and PM6:o-IDBTR, we show that neither morphology, nor carrier mobilities can on their own explain the observed difference in performance, signaling the importance of the energy landscape in controlling the OPV device efficiency, both through generation and recombination of charge carriers.

Triplet formation is generally regarded as an energy loss process in organic photovoltaics. Understanding charge photogeneration and triplet formation mechanisms in non-fullerene acceptor blends is essential for deepening understanding of photophysics in these important organic photovoltaic materials. Here, we present a comprehensive spectroscopy and morphology study on non-fullerene acceptors ITIC, ITIC-Th, ITIC-2F and Y6, both pristine and blended with reference polymer PffBT4T-C9C13. Atomic force microscopy and grazing-incidence X-ray diffraction provided information regarding the morphology of the films while spectroelectrochemistry combined with microsecond transient absorption spectroscopy allowed triplets and charge carriers to be investigated in detail. Crucially, we used triplet sensitisation to determine molar extinction coefficients of the non-fullerene acceptor triplets (2.7 – 6.5 ´ 10⁴ mol L^-1 cm^-1), allowing triplet populations to be quantified in the blends. Intriguingly, no consistent trends were found in the photophysics of the studied blend systems, with each presenting its own unique mechanism. PffBT4T-C9C13:Y6 showed no triplet formation, only charge carriers that decayed rapidly in a relatively crystalline environment, consistent with the observed highly segregated morphology. In contrast, all blends in the ITIC series produced evidence of considerable triplet formation in addition to charge carriers. PffBT4T-C9C13:ITIC-Th blend produced acceptor triplets irrespective of excitation wavelength, and these were formed via intersystem crossing and/or energy transfer. Conversely, both ITIC and ITIC-2F blends displayed triplet formation via non-geminate recombination of charge carriers, with both NFA and polymer triplets observed. However, PffBT4T-C9C13:ITIC-2F produced a substantially higher charge carrier population than the ITIC blend. Because its triplet formation mechanism relies on the presence of charge carriers, PffBT4T-C9C13:ITIC-2F, with the highest charge carrier population, also had the highest triplet population. These results exemplify the prevalence of triplet states across a range of NFA blend systems, despite the varying formation mechanisms. Furthermore, they showcase that triplet populations can reach very high levels, particularly in cases of concomitantly high charge populations. Since high charge carrier densities correlate with large short circuit currents, this has significant ramifications for organic photovoltaic performance.

Impedance Spectroscopy (IS) is a non-destructive characterization technique that has been traditionally applied to inorganic optoelectronic devices, such as standard diodes, LEDs, LASERs and Solar Cells. This technique consists in applying a small sinusoidal voltage superimposed on a bias voltage and measuring the current response. This technique allows to distinguish between the different dynamical mechanisms occurring inside the device at different time scales, from ms to s, such as carrier recombination, carrier diffusion and/or ionic related phenomena. In our research group, we use this technique as a complementary tool to identify the main physical mechanisms responsible of organic solar cells degradation, under different ISOS protocols. In addition, this technique sheds light about where the governing degradation process is taking place, either at the active layer or at the intermediate layers and/or contacts. Here, we will present a degradation study in non-fullerene OSC with layer structure Ag /ZnO /PBDBT-ITIC /PEDOT, using IS. In addition, we will compare devices with and without the processing additive DIO in terms of pristine efficiency and devices stability.