Since the advent of low bandgap non-fullerene acceptors (NFAs), the performance of organic solar cells (OSCs) has improved significantly. A critical parameter is the offset between the relevant frontier orbitals at the DA heterojunction, which in most NFA-based blends is the difference in HOMO energies. Here we combine a wide range of methods, from femtosecond transient absorption to steady state photoluminescence and electroluminescence, spectroscopy to study the mechanisms and efficiency of free charge generation and recombination [1]. For a wide series of NFA-based OSCs, we find that the singlet exciton decay is the main competing pathway for free charge generation while reformation of singlet excitons from reformed CT states dominates the radiative recombination in EL. To explain our data as function of the HOMO offset, we set up a 5-state model which includes singlet and triplet excitons. Our results show that the optimal range of the energy offset for achieving optimal performance is quite narrow and that without additional means for efficient photon harvesting, the power conversion efficiency is limited to around 20 %.

Recent progress, particularly with Y-series non-fullerene acceptors (NFAs), has propelled the power conversion efficiency (PCE) of single-junction organic photovoltaics (OPVs) beyond 19%.[1] This improvement pushes the need to design new donor polymers that effectively pair with NFAs. However, many high-performing donor polymers are expensive due to complex synthesis processes and costly raw materials. Thus, the design of low-cost donor polymers is crucial for large-scale production and commercial adoption. In this study, we explored the charge generation mechanism in new low-cost donor polymer FO6-T blends, which presents high crystallinity challenges and new potentials for efficient OPVs. When blended with the NFA L8BO, FO6-T achieves a notable PCE of 15%.[2] We identified two distinct polaron states in FO6-T: one associated with disordered domains and the other with crystalline domains, as evidenced by cyclic voltammetry (CV) and spectroelectrochemistry (SEC) measurements. These findings align with the reported FO6-T’s semi-crystalline morphology3. To gain deeper insights, transient absorption spectroscopy (TAS) was employed across timescales from femtoseconds (fs) to seconds (s) to investigate charge dynamics within these domains. Upon photoexcitation, FO6-T excitons rapidly converted into polaron states within a few picoseconds (ps). Polaron states in the disordered domains near the donor/acceptor (D/A) interface transferred to crystalline phases via an energy cascade within 20 ps. The crystalline phases played an important role in stabilizing the separated polarons, which exhibited lifetimes extending into the microsecond (μs) range, as shown by μs-TAS measurements. Then we studied the high-performing FO6-T:L8-BO. The two polaron states of FO6-T persist in the FO6-T/L8-BO blend, revealed by fs-TAS. Furthermore, Förster resonance energy transfer (FRET) was observed under donor excitation, supported by the TAS kinetic analysis. The highly overlapping emission spectrum of FO6-T and the absorption spectrum of L8-BO also support the FRET observation. To investigate the impact of findings discussed above on device performances, a series of D/A ratio studies was conducted. The FO6-T/L8-BO blend exhibited exceptional tolerance to D/A ratios ranging from 2:1 to 1:10. These findings underscore the potential of low-cost, simple donor polymers like FO6-T in achieving high-efficiency and semi-transparent OPVs, paving the way for broader commercialization and innovative applications.

The inverse design of tailored organic molecules for specific optoelectronic devices of high complexity holds an enormous potential but has not yet been realized. Current models rely on large data sets that generally do not exist for specialized research fields. We demonstrate a closed-loop workflow that combines high-throughput synthesis of organic semiconductors to create large data sets and Bayesian optimization to discover new hole-transporting materials with tailored properties for solar cell applications. The predictive models were based on molecular descriptors that allowed us to link the structure of these materials to their performance. A series of high-performance molecules were identified from minimal suggestions and achieved up to 26.2% (certified 25.9%) power conversion efficiency in perovskite solar cells.
That milestone underlines the feasibility of developing autonomous research strategies that discover materials tailored for specific applications. That requires a highly interconnected workflow including synthesis, purification, characterization and device optimization. Such lines could specifically develop optimized interface materials for perovskite cells with various bandgaps, but also discover optimized interfaces for LEDs, photodetectors or X-Ray detectors. The outlook will summarize the advantages but also the limitations of data driven methods and will give further examples of such campaigns searching to find optimized materials for very different applications

Whilst organic solar cells (OSCs) have now reached efficiencies necessary for commercialization, their current high cost is prohibitive for most applications. One exception to this is semi-transparent OSCs, where the tuneable absorption of the active layer can be uniquely exploited to create power generating, semi-transparent energy sources. These hold great potential for building integrated photovoltaics, and net-zero architecture. A common way to achieve this transparency is via reduced donor content, in so called ‘dilute donor’ cells.

In this work we explore printing such dilute donor systems, using donor PTQ10 and acceptor Y12, via blade coating; alongside structural analysis to understand the relationship between printing conditions and photovoltaic performance.

We show results from a custom, blade coater setup used to measure in-situ grazing incidence wide angle x-ray scattering (GIWAXS) at the Diamond Light Source synchrotron, under a range of conditions. We see differences in crystallization and drying dynamics depending on stage temperature and donor content, and relate these to device performance for blade coated OSCs. Grazing incidence small angle x-ray scattering (GISAXS) and a range of device based optoelectronic measurements are also shown to provide an overview of structure-function relationships. We believe this work provides an important insight into how blade-coating conditions impact film morphology, and therefore how printing conditions can be designed for optimum semi-transparent OSCs.

Organic solar cells (OSCs) have demonstrated significant improvements in power conversion efficiency, surpassing 20%, largely due to the development of non-fullerene acceptors (NFAs).[1] However, the limited operational device stability continues to present challenges for the organic photovoltaic community.[2,3,4,5] Here, we investigated the photodegradation of thin films of the donor PTQ10, the NFA Y6, and their blends under AM 1.5 illumination using a solar simulator in ambient conditions. We found that the pristine PTQ10 and Y6 exhibit relatively higher photochemical stability compared to the PTQ10:Y6 blend after 45 hours of degradation. When the blend PTQ10:Y6 is exposed to light through a 400 nm long-pass filter, the degradation rate significantly decreases, indicating the influence of UV light on the blends. To address this, we incorporated a third component, PC₇₀BM into the active layer of the PTQ10:Y6 blend. The results show that the degradation rate in white light (AM 1.5) on the ternary blend (PTQ10:Y6:PC₇₀BM) has the same rate as 400 nm long-pass filtered light on the binary blend (PTQ10:Y6), suggesting that the addition of PC70BM does not mitigate UV-induced degradation. These results are confirmed by UV-vis absorption spectra and IR spectra. The atomic force microscopy images show that after 45 hours of degradation, a slight increase in the roughness of the films is observed. The addition of a third component could be an effective way to improve the photostability of the active layer used in OSCs.

Semi-transparent organic photovoltaics (ST-PV) have gathered significant attention due to the unique ability to simultaneously convert sunlight into electricity while allowing visible light transmission, together with their ultra-light weight, flexibility, and process scalability. With this, potential applications such as agrovoltaics, photovoltaic windows or building faceds have been highlighted by researchers and industry.

Microalgae are photosynthetic microorganisms that are able to convert light and nutrients into biomass made of various valuable molecules such as lipids and proteins for the production of sustainable advanced fuels (SAF). In the context of COCPIT project (https://www.cocpit-horizon.eu/https://www.cocpit-horizon.eu/), microalgae biomass productivity could be maximized using thin film photobioreactor (PBR) in solar conditions such as the AlgoFilm technology. Optimization of the use of the sunlight collected at the culture system surface, by filtering the NIR and IR spectrum with a ST-PV, is considered to prevent PBR overheating, while enabling the dual valorisation of solar spectrum for both biomass and electricity production.

In this work, different active layer blends have been proposed to adapt and complement the ST-PV absorbance spectrum of Parachlorella kessleri (i.e. green microalga known for its interest in SAF production) absorbance spectra requirements. Lab-scale OPV devices were fabricated using a scalable deposition technique such as slot-die. Tests were first performed in terms of efficiency and transparency and further evaluated in combination with 6 lab-scale PBRs in parallel. The selection of the final active layer was based on a compromise between transparency, efficiency and microalgae productivity.  

Single-junction organic solar cells (OSC) nowadays have reached promising power conversion efficiencies around 20%. Besides new materials, going beyond the current efficiencies could, in principle, be achieved by multi-junction devices, which promise a reduction in thermalization losses [1]. For this promise to become a reality, two items should be addressed, namely, the multi-junction geometry and the screening of materials with very different gaps.

In this talk, we will present a multi-junction in-plane spectral splitting geometry that we call Rainbow solar cells [2]. In this geometry, a series of sub-cells are placed next to each other laterally, and illuminated through an optical component that splits the incoming white beam into its spectral components, thus matching local spectrum and absorption for each sub-cell. The fabricated n-terminal devices are capable of extracting the maximum power of each sub-cell without the need for current matching nor processing challenges. We demonstrate the concept for PM6:IO-4Cl and PTB7-Th:COTIC-4F blends, as high and low band-gap sub-cells, respectively. In agreement with simulations, we show an efficiency increase of around 30% of the Rainbow geometry with respect to our best single junction device [2]. Then, we use high throughput methods based on gradients on the parameters of interest and blade coating [3-5] to screen tens of materials exhibiting either wide band gap [4] or narrow bandgap [5], and thus push the efficiency of the Rainbow multi-junction further up. Finally, we evaluate the potential of ternary mixing to further improve the efficiency of rainbow solar cells. Material design rules for this type of device will then be revisited.

Organic photovoltaics (OPVs) have rapidly improved in efficiency, with single-junction cells now exceeding 19% efficiency. These improvements have been driven by the adoption of new non-fullerene acceptors (NFAs) and the fine tuning of their molecular structures. Although OPVs are highly efficient, they often show extremely poor operational stability, primarily owing to the complex interplay between the morphological instability of the blended bulk heterojunction photoactive layers and the intrinsically poor photostability of the organic semiconductor materials themselves. To realize commercialization, it is vital to understand the degradation mechanisms of these organic materials to improve their stability [1]. Efficiency increases have, in part, been driven by the rational molecular design of materials. In this talk, I will discuss key molecular design parameters and show how each parameter impacts different degradation pathways with a particular focus on NFA molecular planarity, rigidity, and end groups [2-4] and polymers [5]. I will also discuss the impact of morphological and photochemical instabilities on OPV device stability. The fundamental understanding of the molecular origin of OPV stability is a key research theme for next-generation OPVs.

Bulk heterojunction organic solar cells (BHJ OSCs) potentially can offer low cost, large area, flexible, light-weight, clean, and quiet alternative energy sources for indoor and outdoor applications. OSCs using non-fullerene acceptors (NFAs) have garnered a lot of attention during the past few years and shown dramatic increases in the power conversion efficiency (PCE). PCEs higher than 20% for single-junction systems have been achieved, but the device lifetime is still too short for practical applications. Thus, understanding factors that affect the OSC long-term stability is crucial. In this talk, I will discuss the impact of different blend materials and device structures on the stability. A combination of characterization methods such as solid state Nuclear Magnetic Resonance (NMR), 4D TEM, GIWAXS, resonant soft X-ray scattering (RSoXS), AFM, X-ray photoelectron spectroscopy (XPS), Electron paramagnetic resonance (EPR) spectroscopy, and capacitance spectroscopy are employed to gain insight into the device stability. We propose strategies to improve the device stability.  

The power conversion efficiency of organic photovoltaics (OPV) has recently crossed the 20% milestone, placing an even larger focus on degradation processes and device stability. Here, recent results on oxide-organic as well as 2D MXene-organic interfaces in scalable OPV will be presented. Transition metal oxides serve as efficient charge carrier selective interlayers for electron and hole extraction in organic photovoltaic devices. However, interlayer related instabilities have been reported as a degradation route for high performing non-fullerene acceptor OPV devices, making a thorough understanding of such interfaces important for the further development of OPV technology. Recent progress made within sputtered oxide charge extraction and transport interlayers for OPV devices will be presented. Supported by a variety of surface science characterization techniques, the effect of e.g. oxide composition, microstructure and defect states on the performance of sputtered oxide interlayers in organic photovoltaic devices will be discussed [1,2]. In addition, the use of 2D MXene to tune interfaces between active layers and transport layers in OPV will be presented [3], demonstrating how such 2D layers can be employed to passivate interface defect states, and from that improve efficiency and in particular device stability. Finally, routes for Roll-to-Roll (R2R) up-scaling and manufacturing of OPV will be presented. This includes development of the hybrid interfaces using industrial-compatible Sheet-to-Sheet (S2S) and R2R techniques, and the scale up from cells to modules using various device configurations. This part also includes examples on development of scalable transparent tandem solar modules reaching power conversion efficiencies of 12.3% for an average visible transmittance of 30%, developed in the EU project CITYSOLAR.

Control of the molecular configuration at the interface of an organic heterojunction is key to the development of efficient optoelectronic devices. Due to the difficulty in characterizing these buried and (likely) disordered heterointerfaces the interfacial structure in most systems remains a mystery. Here, we demonstrate a novel synthetic strategy to design and control model interfaces, allowing for their detailed study in isolation from the bulk material. This is achieved by the synthesis of a polymer in which a non-fullerene acceptor moiety is covalently bonded to a donor polymer backbone using dual alkyl chain links, constraining the acceptor and donor units in a ‘through space’ co-facial arrangement. The constrained geometry of acceptor relative to electron rich and poor moieties in the polymer backbone can be tuned to control the kinetics of charge separation and the energy of the resultant charge-transfer state giving unprecedented insight into factors that govern charge generation at organic heterojunctions.

The organic photovoltaics (OPV) field has seen a boost in performance with the introduction of small-molecule electron acceptors (SMA), such as Y5 and Y6, leading to power conversion efficiencies above 19%[1]. For these SMAs new challenges come about due to their tendency to self-aggregate. One strategy to limit aggregation and promote miscibility with the donor is the use of Y-based copolymers as electron acceptors, yielding high-efficiency all-polymer solar cells.[2] We have shown earlier [3] by temperature-dependent optical spectroscopy of solutions of PBDB-T and the polymer acceptor PF5-Y5 that donor and acceptor interact even at low concentrations.

Here we will first present results from morphological analysis of acceptor films by angle-resolved Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, with focus on the orientation of planar SMAs Y5 and Y6 in pristine spin-coated films. We found that the processing solvent is a key parameter to obtain preferentially face-on orientation. Face-on orientation was also found for films of the Y5-based copolymers PF5-Y5 and PYT. Due to the presence of nitrogen in the acceptor N K-edge NEXAFS spectroscopy could be used to probe the orientation of acceptor molecules in the bulk heterojunction films, where they are blended with the donor polymer PBDB-T, where nitrogen is absent. We find that the surface of these blend films is donor-rich and the polymer acceptors PF5-Y5 and PYT retain the face-on orientation that they have in neat films also in the blend films.[4]

Furthermore, we will present our findings on the photochemical stability of the acceptor materials. We studied the evolution of optical properties, composition, and energy levels, during one-sun (AM1.5) illumination in air of thin films of the small-molecule acceptor Y5 and its copolymer counterparts PF5-Y5 and PYT, along with their blends with PBDB-T.[5] We found that the copolymer PF5-Y5 undergoes rapid photooxidation, while the PYT film degrades much slower and the Y5 film properties remain almost intact, even after 30 hours of light exposure in air. For Y5 molecular packing and aggregation may protect the small molecule film from photodegradation. However, the significantly faster photodegradation of PF5-Y5 compared to PYT indicates that the BDT moiety in PF5-Y5 accelerates its photooxidation. These insights on the effects of intentional photodegradation on materials properties are expected to contribute to the design of stable acceptors for long-lived OPV devices. 

Squaraine (SQ) dyes have gained significant attention in the scientific community due to their versatility, tunable structures, photothermal stability, and exceptional photophysical properties, including their ability to exhibit fluorescence under specific conditions.[1] SQ dyes feature a donor-acceptor-donor (D-A-D) architecture, where the donor part is an electron-rich group, typically an aromatic or conjugated structure, that donates electrons upon excitation. The central core, typically a butanedione group, is electron-deficient and serves as the acceptor unit (A), enabling efficient electron transfer from the donor to the acceptor upon excitation.[2,3]  To stabilize the D-A-D system, strong electron-donating groups, such as heterocycles or electron-rich aromatic units like aniline derivatives, are often incorporated into squaraine dyes. Symmetric SQ dyes have two equivalent donor units, while unsymmetrical squaraine (USQ) dyes feature two different donor units.[4]

In this study, several unsymmetrical squaraine (USQ) dyes were synthesized using a simple two-step method involving a condensation reaction between electron-rich aromatic amines and squaric acid, which minimizes byproducts and simplifies purification. The synthesized products were characterized using ¹H-NMR, ¹³C-NMR and FT-IR to confirm their structures. The absorption and emission properties of the dyes were evaluated through UV-Vis and fluorescence spectroscopy.

Photovoltaic (PV) devices were fabricated by depositing the cell components using thermal vapor deposition (< 5.0x10^-6 Torr) on pre-patterned indium tin oxide (ITO) glasses (c.a. 100 nm).[5] The structure of the assembled cell consist of 8 nm layer of MoO₃ as hole-extracting layer (HEL), a 40 nm co-deposition of the SQ dye with fullerene (C60) as the active layer, a 8 nm layer of BPhen as the electron transport layer (ETL) and 8-hydroxyquinolinato lithium (Liq) as electron extracting layer (EEL) and a final 100 nm aluminum electrode. These devices exhibited homogeneous morphologies that reduced charge recombination. The resulting PV cell achieved an average power conversion efficiency (PCE) of 3.38%, attributed to the optimal alignment of HOMO/LUMO levels and the spectral synergy between SQ dyes and C60 fullerenes. This study provides essential guidelines for optimizing these materials to further enhance the performance of SQ-based photovoltaic cells.

Despite having similar power conversion efficiency limits according to detailed balance, the performance of Y6 in organic photovoltaic blends still outshines that of its narrower bandgap derivatives, as shown in the TOC plot adapted from ref [1]. An understanding of the underlying mechanisms is needed to tap into the potential of low-bandgap non-fullerene acceptors (NFAs) for tandem applications and beyond. Thus motivated, we scrutinize two low-bandgap NFAs – BTPV-eF-eC9 and BTPV-4Cl-eC9 – in binary devices from the perspective of photocurrent generation, and resolve the losses incurred at each step between photon absorption and carrier extraction. With a combination of steady state and time-resolved optoelectronic techniques, we find that in these acceptors, the kinetic competition between charge-transfer state decay and its separation is a major limiting role in the overall photon harvesting capability of the devices. Despite having similar voltage losses as PM6:Y6, which we attribute to efficient charge-transfer formation at lower driving forces, the geminate loss pathway via the charge-transfer state curbs fill factor and photocurrent. This reinforces the notion that suppression of geminate loss pathways can vitally balance recombination losses with improved charge generation, and markedly move the overall internal quantum efficiency towards the detailed balance limit.

Non-fullerene acceptors (NFAs) are exciting molecules allowing high efficiency in organic photovoltaic (OPV) blends with conjugated polymers. Interestingly, charges can also be generated by neat NFA films without additional donor. To understand the origins of exciton dissociation in neat NFAs, we have looked at the impact of aggregation, external electric field and non-linear effects. We used solvatochromism in order to gain insight on charge redistribution after excitation in isolated NFAs. We found that unaggregated NFAs feature a more dipolar excited state, revealing intramolecular charge transfer (ICT) character. This ICT character, however, is not enough to generate separated charges. Aggregation is the key to exciton dissociation in neat NFAs, which we observe with TA of solutions and films of several different NFAs. To explore the impact of an electric field on exciton dissociation in neat NFA devices, we used bias-dependent external quantum efficiency (EQE) and transient absorption (TA) spectroscopy. Electromodulated differential absorption (EDA) measurements then allowed us to observe charge transport under bias. Excitation correlation spectroscopy and fluence-dependent TA finally revealed how non-linear effects can increase the charge yield. Lastly, we comment on whether the neat domain charge generation significantly affects the photophysics of blends or not.

Understanding and minimizing non-radiative recombination pathways is key to enhanced efficiencies in emerging solar cells, such as perovskite solar cells (PSC) and organic photovoltaics (OPV). Non-radiative recombination in any form, i.e. trap-assisted, or surface recombination of minority carriers at the (wrong) electrode will inevitably lead to lower efficiencies [1]. However, given the fast development of the efficiencies, the stability of OPVs is still not satisfactory.

In this talk, I will focus on the charge transport layers (CTL) and their properties in increasing efficiency and stability. I will focus on the passivation of metal-oxides in nip-solar cells for OPVs. The criterion that the CTL is conducting [2] will simultaneously lead to loss of selectivity [3]. We have identified and mitigated an important loss-factor caused by ZnO in OPVs. Surface defects in ZnO cause n-doping in a thin layer close to the cathode, causing a loss of Jsc. By inserting a thin layer of SiOxNy from solution processing, the recombination zone will be removed, with increased efficiency and lifetime as a result [4].