In the view of a rapid increase in efficiency of organic solar cells, reaching their long-term operational stability represents one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light-induced dimerization of fullerene-based acceptor materials discovered recently is considered to be one of the main causes for burn-in degradation of organic solar cells. In this work, we reveal the mechanism of the light-induced dimerization of the fullerene derivatives and establish important correlations with their molecular structure and electronic properties.

We also show that conjugated polymers and small molecules undergo similar light-induced crosslinking regardless of their chemical composition and structure. In case of conjugated polymers, crosslinking leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times.

Our results, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics.

The fine-structured film morphology in the active layer of all-polymer solar cells is the key to their high performance. Polymeric non-fullerene acceptors offer the potential to restrict the self-aggregation that is typical for small molecule non-fullerene acceptors. But what limits the phase separation that otherwise dominates in polymer-polymer blends? In this study, we employed a blend of the polymeric acceptor PF5-Y5 and the donor polymer PBDB-T to investigate the molecular interactions in solution in a joint experimental-theoretical spectroscopy study. Solar cells prepared of this blend have reached power conversion efficiencies of over 14%.(1) From absorption spectroscopy of the PBDB-T:PF5-Y5 blend solutions at increasing temperatures, combined with concentration-dependent fluorescence spectroscopy and excitation spectroscopy, we could conclude that in addition to temperature-induced disaggregation of both donor and acceptor polymers, donor-acceptor complexes are formed in dilute blend solutions of PBDB-T and PF5-Y5. The formation of the donor-acceptor complexes competes with the donor and acceptor self-aggregation and the solvent environment is found to influence these interactions. Our results show also that the donor-acceptor polymer complexes are stabilized in more polar solvents. The near IR-region of the absorption spectrum could be matched with the calculated electronic excitations of donor-acceptor complexes of PBDB-T and PF5-Y5 oligomers. The results corroborate that van der Waals interaction between segments of the donor and acceptor polymer chains favours the formation of donor-acceptor charge transfer complexes, stabilized by hybridization of the molecular orbitals, which reduces the electronic energy. These pre-formed donor-acceptor complexes in solution can be expected to have important consequences on the resulting film morphology. These insights are also expected to direct the future design of compatible donor-acceptor polymer pairs for high-performance all-polymer solar cells.

OPV cells have a proven efficiency of over 18 % 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, OPV is reaching out to applications that are going beyond the typical niche markets. The first generation of commercially available OPV modules shows lifetimes in the order of 5 years and more under outdoor conditions. Independent of the application, operational lifetime of organic solar cells is not fully understood. Few publications highlight operational lifetimes of over 25000 hrs under lab conditions. Organic solar cell materials being stable under light and oxygen are reported as well. We recently demonstrated solar cells that canbe operated in water and under 1 sun for hundreds of hours – unpackaged. However, all these “best you can do” lifetime values are reported for different material and interface systems.
This talk will analyze the most common degradation mechanisms and outline, how they are overcome in modern OPV materials. Bulk degradation will be distinguished from interface degradation, and, not unexpected, interface degradation or interface related degradation is found to be the currently leading degradation mechanism. All these findings indicate that we can expect a significant improvement in operational stability of OPV in the next few years.

The transition from fullerene to non-fullerene acceptors (NFAs) has dramatically increased the power conversion efficiency of organic solar cells and re-ignited the interest in these devices. Even though, the efficiency of NFA based devices has surpassed 18 % [1], these devices are still challenged by poor device stability. It has been shown, that the popular electron transport layer (ETL), ZnO, causes a photoinduced degradation of the NFA molecule ITIC, which contributes to a poor device stability under continuous light soaking in PCE12:ITIC devices with ZnO as an ETL [2]. Recently, we have shown that replacing ZnO with sputtered TiO_x increases the device stability of high performing PCE12: ITIC devices significantly [3], when the sputtered TiO_x is optimized in terms of composition and microstructure. Photoemission and spectrophotometry measurements have shown that the sputtered TiO_x films possess less surface defects, which to a large extend mitigates the photoinduced degradation of ITIC as opposed to conventional ZnO ETLs. To better understand the mechanisms behind the improved stability of devices that are based on the sputtered TiO_x, electronic structure and the full energy level alignment between TiO_x (ETL) and ITIC (NFA) has been studied using synchrotron-based X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and resonant X-ray photoelectron spectroscopy to uncover any charge transfer, chemical bonding, trap states and interfacial states at the ETL/acceptor interface. These results will be presented during this talk.

[1] Q. Liu et al. 18 % efficiency organic solar cells, Science Bulletin, 65, 272-275, (2020).

[2] S. Park et al. Intrinsic photo-degradation and mechanism of polymer solar cells: the crucial role of non-fullerene acceptors, J. Mater. Chem. A., 7, 25830-25837, (2019).

[3] M. Ahmadpour et al., in preparation, (2021).

Organic solar cells (OSCs) have lately received a lot of interest as power sources to drive low power consumption devices for the Internet of Things application by harvesting the energy from indoor artificial light.[1,2] In this work, an enhanced power conversion efficiency (PCE) in indoor LED illuminated OSCs was observed, 60% higher than those of AM1.5 G illuminated OSCs by a cathode interface engineering strategy. Moreover, the photostability study under indoor artificial light, which is rarely investigated, is critical for the practical applications of this emerging technology. Along with the performance enhancement, we observed that OSCs illuminated under indoor LED light had better photostability than OSCs illuminated under 1 Sun scenario. Impedance analysis with the 3RC equivalent circuit models elucidates the mechanisms behind the enhanced performance and improved photostability of indoor OSCs.

Implementation of 2D materials in organic photovoltaic (OPV) cells has emerged as a promising route to modify fundamental device properties, and potentially improve device efficiency and stability. Recently, the 2D material MXene has attracted huge attention and demonstrated a large potential for next-generation solar cells due to exciting optical and electronic properties, and especially the ability to tune work function via surfaces termination routes which is highly desirable in OPV device interlayers. In this work, we employ such 2D MXene, Ti₃C₂T_x, in conventional ETL to develop composite 2D based electron transport layers (2D-ETL), and demonstrate their high performance for non-fullerene acceptor (NFA) based inverted OPV processed with the non-halogenated solvent o-xylene. The PM6:N3 OPV based on the composite 2D-ETL exhibited power conversion efficiencies (PCE) of around 14%, and importantly, a superior device lifetime when compared to conventional 2D-free ETL. In this work, the integration of such 2D interlayers in 2D-ETL is investigated in terms of morphology and optical as well as electrical properties, while the degradation and stability mechanisms are studied by optical spectroscopy techniques and ISOS-L device lifetimes measurements. Here, the usage of 2D MXene is shown to possess a great potential for the development of ambient stable and efficient flexible NFA OPV devices in the future.

The introduction of non-fullerene acceptors (NFA) has provided several recent record efficiencies in organic photovoltaic (OPV) cells, reaching now above 18%¹ for single-junction devices. While these developments have provided a strong boost to the OPV field, more efforts have to be devoted to their application, such as their use in windows to lower the carbon footprint of buildings. To improve the performance of such semi-transparent NFA OPV, a highly reflective Bragg mirror can be formed to reflect selective parts of the sunlight spectrum that match specific parts of the absorption spectrum of the active layer, e.g. the contribution from the near-infrared absorbing NFA molecules. With the progress in the film formation using reactive sputtering, it is possible to tune the thickness, composition, transparency, and uniformity of alternating low and high refractive index oxide thin films, which is needed to form well-performing DBR stacks, making it an ideal technique for this application.  

Here, recent progress in adjusting the reflectance of thin film oxide based DBR, e.g. by fine-tuning composition and thickness of the individual layers in order to match the absorption region of specific high performance non-fullerene acceptor molecules, as well as their integration in efficient semi-transparent NFA OPV devices with low visible transmission loss, is demonstrated. Supported by a variety of surface science characterization studies, the importance of the detailed thin film composition and microstructure on the optical properties² and intrinsic stability of these DBR is discussed. To meet the requirements on scalable OPV development, the up-scaling of these new DBRs is discussed, considering recent results on industrially compatible OPV device development^3,4. This includes Roll-to-Roll (R2R) processing of OPV cells and modules using combined solution and vacuum-based techniques including also the reactive sputtering process on R2R scale utilized for DBR development.

In recent years, organic solar cells (OSCs) have shown a tremendous increase in power conversion efficiencies (PCEs) to over 18% thanks to the development of novel non-fullerene acceptors (NFAs) and corresponding donor polymers. While the performance of OSCs has reached the level of commercial applications, the stability of the devices needs to be improved and is now the focus of research. This presentation will review recent progress in evaluating and understanding the intrinsic photostability of high-efficiency NFAs including ITIC derivatives, IDTBRs, Y family, and the resulting blends with PBDB-T (PCE12) or PBDB-T-2F (PM6) as the donor polymer by addressing impact of film crystallinity and molecular structure.

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 18% under standard solar irradiation (over 30% under indoor illumination) in a single junction device, rapidly closing the performance gap with potentially competing technologies such as crystalline silicon and halide perovskite solar cells. 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 the donor and acceptor materials as well as the device organic/inorganic interfaces in the degradation, and propose potential strategies to overcome these degradation mechanisms.

Organic solar cells (OSCs) using non-fullerene acceptors (NFAs) have garnered a lot of attention during the last years and showed dramatic increases in the power conversion efficiency (PCE). PCEs higher than 19% for single-junction systems were achieved, but these high-performance organic photovoltaic cells are often processed with halogenated solvents. To accelerate the mass fabrication of OSCs, green solvent processing is crucial to reduce the harmful effect of halogenated solvents to human health and our environment. In this talk, I will discuss the design, synthesis, and performance of organic semiconductors processed from green solvents such as xylene and 2-methyl tetrahydrofuran (2-MeTHF). 2-MeTHF can be obtained from furfural, which is derived from agricultural by-products. It fits within the class of solvents sought from renewable resources and the concept of capitalizing on waste to generate useful chemicals.  Moreover, the toxicity of 2-MeTHF is lower than that of commonly used halogenated and aromatic solvents, such as chlorobenzene and chloroform.  A combination of characterization methods were employed to gain insight into the film morphology and solar cell performance.

Organic conjugated materials have many favourable properties that make them interesting for a variety of electronic applications. The aim of my group is to understand the fundamental processes underlying their functionality. We use ultrafast spectroscopic techniques, such as transient absorption (TA) and time-domain terahertz (TD-THz) spectroscopies, to investigate charge carriers in organic semiconductors. While femtosecond TA measurements bring insights to the nature and evolution of the photoexcited species, we use TD-THz spectroscopy to gain information about the charge transport properties on the nanoscale. After presenting an overview of our experimental techniques, I will show results about charge generation in highly efficient solar cell materials based on organic polymer:fullerene [1] polymer:non-fullerene blends [2] with negligible driving force for interfacial charge transfer. Efficiencies beyond 18% have recently been achieved by combining low bandgap conjugated polymers with small-molecule non-fullerene acceptors (NFAs). I will discuss how charge generation and recombination processes depend on parameters such as the charge transfer driving force, the short-range charge mobility and the morphology.

In a molecular photovoltaic device, charge separation and energy conversion result from the evolution of a photogenerated exciton into a charge separated state, in competition with recombination to ground. The efficiency of charge separation is a function of the molecular packing and energy level alignment near the interface, and of disorder in these properties. We need to understand and isolate the effects of chemical structure, molecular packing, energetics and disorder on the competition between charge separation and recombination in order to identify the factors controlling device efficiency. Electro- and photo-luminescence have proved to be valuable tools to probe the energy and dynamics of excited states involved in photoinduced charge separation, and to identify structural and energetic disorder at molecular interfaces. Here, we use luminescence and other spectroscopic probes along with transient electrical measurements to study charge generation and photovoltage in molecular donor: acceptor solar cells. We explore how the properties of the intermediate charge-transfer state influence recombination losses and show, with the aid of a numerical models how control of these molecular properties could benefit performance [1]. We study the effect of hybridisation of charge-transfer and local exciton states [2] and of disorder in CT state energies [3] on non-radiative voltage losses. We then develop an integrated modeling framework in which excited state dynamics are combined with a one-dimensional device model that accounts for spatial variations in charge density. The integrated model allows different experimental measurements to be reconciled within a single picture and helps to show how the energies and dynamics of interfacial states influnce the overall device performance. We use our results to consider the ultimate limitations placed on solar to electric conversion by the molecular nature of the materials.

In the past two decades the power conversion efficiency of organic solar cells has increased significantly and is now closing the gap to crystalline silicon. This remarkable advancement finds its origin in the design and synthesis of novel organic and polymer semiconductors with electron donating and electron accepting properties, next to the optimization of the solar cell device architecture. The organic semiconductors used in these devices have to meet several chemical, optical, electronic, and morphological requirements to provide such high efficiencies. The chemical structure, intrachain defects, optical band gap, redox potentials, molecular weight, processing conditions, charge transport layers, and device architectures all exert important roles to reach the intrinsic limits of these materials. Recent progress on developing new donor and acceptor polymers and molecules, combined improved device layouts, will be presented that result in organic solar cells with power conversion efficiencies close to 18%. Detailed analysis of these devices reveals that the feature low energy losses and relatively low energetic disorder at the optical band gap. This analysis provides directions for further advancements.

Polymer solar cells (PSCs) are a promising candidate as a future photovoltaic technology due to their lightweight, flexible and solution processable features. Development of non-fullerene acceptors (NFAs) allows significantly improved power conversion efficiencies (PCEs) of PSCs over 18% with their stronger light absorption properties and a wider absorption ranges than the fullerene acceptors. However, achieving highly efficient and highly stable PSCs is still challenging due to intrinsic disadvantages in the NFA based PSCs such as photochemically active molecular moiety in the photoactive materials and poor miscibility of donor and acceptor. Besides the features of the photoactive layer, interfacial layers are also critical to determine the stability of PSCs. Zinc oxide (ZnO) has been widely employed as an electron transport layer (ETL) in the inverted device structure because of their transparency in the visible spectral range, good electronic conductivity and solution processability. However, ZnO shows strong photocatalytic activity under illumination with UV light and induces the decomposition at the ETL/photoactive layer interface. In this work, we introduce an effective strategy to improve stability at the ZnO/photoactive layer interface by adding fullerene-based self-assembled monolayer(C₆₀-SAM) in the photoactive layer as an additive. It is found that the C₆₀-SAM in the photoactive layer can be spontaneously formed at the surfaces of the ZnO during spin-coating process due to the carboxylic acid group in the C₆₀-SAM. PSCs without and with C₆₀-SAM were fabricated and the devices with C₆₀-SAM show efficiency enhancements of ~4-14% compared to the PSCs without C₆₀-SAM additive. Furthermore, PCE drops of PSCs with C₆₀-SAM are improved of ~26-30% under 1 sun illumination. The C₆₀-SAM in the photoactive layer acts multiple functions to improve the molecular ordering of the photoactive materials and to decrease the photo bleaching rate of the photoactive materials by UV light.

The long-time operational stability is a central challenge for developing organic solar cells (OSCs) to market readiness. However, many state-of-the-art OSCs based on the bulk-heterojunction concept suffer from stability problems caused by severe morphological changes upon thermal or illumination stress [1]. Single-component materials, enabled by the covalently-bonded structure with donor and acceptor in one molecule, present attractive advantages such as a simplification of device fabrication and stabilization of microstructure [2]. Recently, with the rapid improvement of efficiency from 2-3% to 11.3% for single-component organic solar cells (SCOSCs), this class of materials is getting into the research focus of the OPV community. However, reports on their operational stability are still scanty.

In this work, we systematically investigated for the first time the stability under thermal and illumination stress for a series of SCOSCs based on polymeric (SCP3, PBDBPBI-Cl) and molecular (dyad 1, 2, 3, and 4) materials. Under significant thermal stress, double-cable polymer-based SCOSCs exhibited excellent thermal stability with no degradation at 90 ^oC for 3000 hours. Furthermore, in order to compare polymeric with molecular single component materials, we studied the thermal stability among a series of SCOSCs based on D-A small molecules (dyad 1, 2, and 3) with the same donor and acceptor units but differently long alkyl space linkers. Since macroscopic diffusion of molecules is excluded in these dyads, the length of the spacer can only provide the necessary flexibility for sub-nm rearrangements caused by thermal stress. Interestingly, the single dyads showed a distinctly different behavior: dyad 1 with the shortest linker exhibited the highest thermal stability, while dyad 3 with the longest linker showed the relatively lowest thermal stability [3]. This highlights the need for further in-depth studies on optimizing the spacer length in parallel for performance and stability. Moreover, dyad 1-based SCOSCs exhibited exceptional illumination stability, retaining 98% of the initial PCE under concentrated light equal to 7.5-suns for over 1000 hours, which is among the best values for solution-processed OSCs. Based on the outstanding stability, SCOSCs could be an ideal candidate to study the ultimate stability under extremely rugged conditions such as high temperature and concentrated light. Since the morphological evolution is excluded, SCOSCs could serve as a model system to selectively study interface degradation. Shortly, SCOSCs are predicted to see a prospective renaissance with efficiencies over 10% and a lifetime of over 20 years, thus closing the gap towards industrial applications.

Organic solar cells (OPV) impress with their low energy consumption during production and their low raw material requirements [1]. Since the layer thicknesses are only around 100 nm, around 1 g of an organic semiconductor is sufficient to produce a solar cell area of 10 m². In the future, the energy payback times of organic solar cells will ideally be in the range of just a few days [2]. Over 18% power conversion efficiencies have now been achieved on a laboratory scale [3], however, OPV suffers from limited photostability and hence relatively short lifetimes, which remains a great challenge on the way to the widespread use of this technology.

The lifetime of OPV devices is negatively influenced by light, oxygen and humidity initiated degradation, and also by the morphological rearrangements accelerated by elevated temperatures. The additive assisted photooxidative stabilization of OPV by implementation of a third component into the active layer is one of the most promising strategies to overcome degradation. Recently, our group has reported stabilization of OPV devices by antioxidants which can simultaneously enhance the mechanical properties of OPV. Introduction of naturally abundant carotenoid compounds as photooxidative stabilizers has resulted in drastic improvement of accumulated power generation of the devices, attributed to their singlet oxygen as well as singlet oxygen precursor (e,g, fullerene triplet states) quenching capabilities [4,5]. Our newest results demonstrate the successful application of this approach to NFA-based systems, and we elucidate the dominating stabilizing mechanism via advanced microscopic and spectroscopic measurements.

In the first part of the talk I will present current upscaling activities at Fraunhofer ISE, i.e., from small cells processed by means of spin-coating from chlorinated solvents over larger area cells processed from non-chlorinated solvents to large area roll-to-roll processed modules. Furthermore, this includes also the transition from an Indium Tin Oxide (ITO) based rigid architecture to an ITO-free flexible cell stack.

In the second part I will present results of electro- and photoluminescence studies of high-efficiency donor:acceptor blends as well as of complete solar cells. Using both steady-state and transient techniques, we try to elucidate the principal working mechanisms of organic solar cells. A strong focus is on the relation between charge carrier density, the (internal) quasi Fermi level separation and the (outer) voltage measurable between the terminals. Our aim is to contribute to the answer of the question whether organic solar cells differ from their crystalline inorganic counterparts in more than the fact that in organic solar cells, charge carrier generation is mostly achieved via electron transfer at donor/acceptor interfaces.

With the recent substantial leap in power conversion efficiency of organic solar cells, partly driven by formulations with new non-fullerene acceptors, new challenges has arisen as regards the fundamental understanding of the connection between mesoscale structure and the active layer performance. Unless this connection is understood and controlled, it will not be possible to overcome the recurring lab-to-fab challenge, or the scaling lag, as it is sometimes referred to. Typically, no less than 50% efficiency is lost when scaling up from minute, spin-coated devices to the large areas that are required for commercially viable modules.

We have identified three crucial focus points for overcoming the lab-to-fab challenge: (i) dual temperature control, i.e. simultaneous control of the ink and substrate temperatures during deposition, (ii) systematic in situ morphology studies of active layer inks with new, green solvent formulations during continuous deposition, and (iii) development of protocols for continuous solution processing of smooth, transparent interfacial layers with efficient charge transfer to the active layer. Combining these efforts and in general accompanying such studies with stability analyses and fabrication of large-area, scalably processed devices are believed to accelerate the relevance of organic solar cells for large-scale energy supply [1].

In this presentation, I focus on our efforts in developing an in-line methodology that is supported by molecular dynamics simulation [2] in order to disentangle the scattering fingerprints that may eventually be used to steer mesoscale structure formation to achieve the optimal photovoltaic performance of a bulk heterojunction. As will be shown, the methodology may be applied both on laboratory scale setups [3] as well as on synchrotron beam lines [4], and may even be used in conjunction with coating conditions far removed from ambient, i.e. with fully heated solution, coating head and substrate to handle materials that otherwise quickly aggregate and gelate. In combination with optical probes and machine learning techniques, the methodology is expected to close the lab-to-fab gap that continues to hold back the commercial breakthrough of organic solar cells.