The introduction of Y-series non-fullerene acceptor such as Y6 have helped to push the power conversion efficiency of organic solar cells above 18 % in single and 19 % in multiple junction organic solar cells. While the wide absorption spectrum of state of the art organic blends guarantees high short circuit currents (Jsc), the fill factor (FF) and in particular the open circuit voltage (Voc) lack well behind the Shockley Queisser limit.

Here we present the results of detailed investigations of Y-series NFAs blended with the donor polymer PM6. Regarding free charge recombination, we find that the reoccupation of the highly emissive Y6 singlet exciton causes almost all photon emission from of the solar cell, but that this pathway contributes to less than 1 % of the total recombination current [1]. On the other hand, while the decay of the decay of the CT state entirely dominates charge recombination, its optical properties remain hidden under the strong Y6 singlet exciton absorption and emission. Interestingly, it is only for very low CT state absorption combined with a fairly high CT radiative efficiency that the solar cell benefits from the radiative properties of the singlet excitons. We also find that despite the high structural order of the PM6:YY6 blend, energetic disorder causes a substantial loss in the Voc [2]. Further studies suggest that the same energetic disorder accelerates the CT decay and with this non-geminate recombination, being the main cause for the non-ideal FF. We conclude that major performances losses in such blends are connected to the CT state properties and energetic disorder, both being difficult to disclose by conventional spectroscopic technqiues

The design and operation of organic photovoltaic devices (OPVs) rely primarily on the electronic structure of organic semiconductors (OSCs). For instance, the dissociation of the exciton at the donor-acceptor (D-A) interface is determined by the energy level offset between the highest energy occupied molecular orbital (HOMO) and the lowest energy unoccupied orbital (LUMO) between the donor and the acceptor. Consequently, the measurement technique and its accuracy to determine the frontier molecular energies of OSCs are critical for designing efficient devices.

Several techniques, such as cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), low-energy inverted photoelectron spectroscopy (LE-IPES), or photoelectron spectroscopy in air (PESA), are used to measure the ionization energy (IE) and the electron affinity (EA) of OSCs. CV is more commonly used in OPV field for its accessibility and its simple operation. However, differences between the absolute energy levels measured by CV and photoelectron spectroscopy (PES) have been reported. The range of energy level values for identical material measured with CV shows the uncertainties and the inconsistency of the measurement protocol of this method. Moreover, direct measurement of the EA can be challenging when measured with CV, and indirect methods are used to estimate it by calculation using the IE and the optical gap. Since the optical gap been usually lower than the transport gap (difference between the IE and EA of a same material), that kind of approximation can lead to contradictory conclusion. Several high performing blends have been reported to have negligeable energy offset, but other studies are showing that an IE offset of 0.5eV is necessary for efficient charge transfer.

In this work, we highlight the differences in the IE and EA values for different commonly used OSCs between CV and UPS/LE-IPES measurements. The work on OPV devices made with different D-A blends shows correlation between the energetic properties of the pristine donor and non-fullerene acceptor (NFA) and the OPV devices’ parameters. When measured with PES methods, the photovoltaic gap Epv, difference between the IE of the donor and the EA of the acceptor, corelate better with the open-circuit voltage (Voc), compared to CV measurements. This led to a good path to predict the maximum Voc of OPV devices. A further study on blends like PM6:Y6 reported as high-performing blends with low IE offset, based on CV measurement, reveals a bigger energy offset when measured with PES techniques. In contrast, devices with low IE offset measured with PES methods demonstrate a non-efficient charge generation which can be observed with their low short-circuit current (Jsc). This may bring the community to rethink the behaviors and the correct designs of OPVs. This work establishes a solid base for reliably evaluating material energetics and can be relevant to a broader community working on OSCs for electronic applications.

Probing the photoluminescence of free charge carriers in organic solar cells: derivation of the separation of the quasi Fermi levels

Uli Würfel^1,2

¹ Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany

² Freiburg Materials Research Center FMF, University of Freiburg, Stefan-Meier-Str. 21, 79104 Freiburg

Photoluminescence (spectroscopy) is an important characterization method in photovoltaics and it has proven to be very valuable for all types of crystalline inorganic solar cells. This is because in these types of devices, the luminescence signal originates from radiative recombination of free electrons and holes and is therefore a direct measure for the product of their concentrations within the photoactive layer. However, in the case of organic solar cells, the interpretation of photoluminescence is much more complex as the absorption of a photon generates a rather strongly bound exciton in the donor or the acceptor phase. Although most photogenerated excitons dissociate into free charge carriers at donor/acceptor interfaces some of them will decay before this happens. In addition, the probability for their decay to be radiative is much larger than for free charge carriers that recombine via charge transfer (CT) states at the donor/acceptor interface. As a consequence, the PL signal of an organic solar cell is dominated by the contribution from photogenerated singlet excitons in the respective material phases and is therefore not directly correlated to the separation of the quasi Fermi levels.

We have developed a new method of how to separate the PL signals of the photogenerated singlet excitons from the one of the free charge carriers in organic solar cells. This paves the way for gathering deeper insight into the working principles of these devices. Moreover, this particular PL method has the advantage of being applicable not only to complete solar cells but also to half cells and even pure absorber films.

Our results from highly efficient organic solar cells show excellent agreement between the quasi Fermi level separation as derived from the PL intensity of free charge carriers and the electrical voltage as measured between the terminals of the devices.

The major fill factor loss in organic solar cells is caused by the transport resistance, which arises due to low effective conductivity in these systems. Until recently, this loss mechanism has received little experimental attention, hence is not yet well understood. Using the temperature and illumination intensity dependent Suns-V_oc and current-voltage measurements, we have determined the effective conductivity in a set of organic donor-acceptor systems, such as solution processed fullerene and non-fullerene acceptor devices, along with thermally evaporated solar cells based on small molecules. We show that the temperature dependence of the conductivity in these solar cells is closely related to the ideality factor. Furthermore, we demonstrate that the transport resistance can be described analytically in the framework of the multiple trapping and release model. Consequently, the shape of the energetic distribution of localised states, determined from the temperature dependence of the ideality factor, plays a key role in understanding the fill factor losses in organic solar cells.

Organic solar cells (OSCs) based exclusively on small molecules have distinct advantages over polymer-based OSCs for commercial applications. Unlike polymers, small molecules have a defined molecular weight and can be purified easily, resulting in a simpler, more reproducible synthesis with controlled quality. Solution-processed All-small-molecule (ASM) OSCs have reached 17% power conversion efficiency, and ASM-OSCs fabricated via vacuum thermal evaporation (VTE) are leading OSC commercialization. However, ASM-OSCs remain less investigated than polymer-based OSCs in terms of their semiconducting and microstructure properties.

Here, we perform extensive optoelectronic characterization on high-performing ASM-OSC deposited from solution and in vacuum to quantify absorption, voltage losses and charge transport – probed via ellipsometry, sensitive EQE and CELIV, respectively. Example systems include VTE DCV5T-Me:C60, and solution-processed BTR-Cl:Y6. We find that certain VTE ASM-OSC achieve strong absorption, decent charge carrier mobility, or voltage losses matching corresponding polymer-based OSCs but no VTE blend combines all these favourable properties. We further investigate the blend microstructure in terms of phase separation. Interdiffusion experiments offer a thermodynamic perspective while the actual phase separation in probed via soft X-rays. This talk gives an overview over prospects and challenges of All-Small-Molecule organic solar cells and provides an outlook on open research questions.

In bulk heterojunction organic solar cells, the energetic landscape at the donor-acceptor interface provides the driving force for charge separation. The mechanism leading to efficient charge separation in fullerene-based blends has been intensively investigated, however with the recent advent of high-efficiency non-fullerene acceptors (NFAs) now surpassing 19% power conversion efficiency, the previous findings have to be revisited for NFA-based systems. In this presentation, I will discuss our latest insights into the photophysical processes governing charge separation, recombination, and energetic (voltage) losses in novel NFA-based systems studied by steady-state and advanced transient spectroscopy techniques. I will address the question, how the interfacial energy offsets control exciton dissociation and charge separation in binary and ternary blends of polymer or small molecular donors with novel NFAs, including photoactive layers using state-of-the-art Y-type acceptors. Generally, it appears that it is primarily the ionization energy (IE) offset that limits the exciton-to-charge transfer (CT) state conversion in many low-bandgap NFA-based systems, while the subsequent separation of the CT state into free charges is barrier-less. Sizeable IE offsets of 0.4-0.5 eV are required to ensure quantitative exciton-to-CT state conversion. The underlying reasons of this limitation, their implications for future donor and acceptor material design strategies, and novel computational (in-silico) approaches to material design will be discussed.

In recent years, the power conversion efficiency of organic bulk heterojunction solar cells has increased rapidly to values exceeding 18% and can largely be attributed to the development of non-fullerene acceptors, especially the ones with A-D-A structure. Many reports have highlighted the critical role of the photoactive layer nanoscale morphology in determining the power conversion efficiency. Therefore, this work takes a holistic approach to correlate the intrinsic molecular structure of each component in a ternary blend of an organic polymer donor with two non-fullerene acceptors (NFAs), to the nanoscale morphology, and together to the phototophysical processes in these blend systems, in order to gain deeper insights as to the requirements for a more efficient device.  We employed resonance Raman (RR) spectroscopy as a sensitive probe of molecular structure to a) identify the effect of blending on the conformation of each component, b) to recognize any interactions that evolve between them, c) to probe the effect of NFA side chain substitution, and d) to assess the effect of thermal annealing treatment.  Grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments revealed the extent of crystallinity of each material and the impact of blending and thermal annealing on the film macromolecular structure, complemented by atomic force microscopy (AFM) imaging. We find that blending leads to loss of crystallinity of all the components in the films, which is, however, recovered with thermal annealing, but only for the polymer, leading to phase separation, and affecting the photoexcited species formed as probed by ultrafast transient absorption spectroscopy.  

Organic solar cells utilize an energy-level offset between the electron donor and acceptor material to generate free charge carriers. The offset provides the driving force to overcome the Coulomb attraction of the initially formed bound electron-hole pair (exciton). However, this driving force constitutes an internal energy loss. To maximize the open circuit voltage and the short circuit current at the same time, the necessary driving force must be pushed to a minimum without penalizing the free carrier yield. We have shown that in donor-acceptor systems with small driving force for charge separation, a Boltzmann stationary state equilibrium causes a residual population of excitons of the lower bandgap material throughout their lifetime [1]. This highlights the importance of minimizing nonradiative recombination in the pristine non-fullerene acceptor materials.

Recently, several groups, including us, have demonstrated barrierless separation of the coulombically bound intermediate charge transfer state into free charge carriers. We show that if neutral and charged excitations are in a Boltzmann equilibrium, there is an optimum energy of the charge transfer state to maximize both open circuit voltage and short circuit currents [2].

Finally, we will report on recent progress to understand and control electrical performance loss in organic solar cells, and to fine control and interrogate the energetics and dynamics at the interface between donor materials and non-fullerene acceptors.

Single-junction organic solar cells (OSC) nowadays have reached promising power conversion efficiencies around 19% [1]. Besides new materials, going beyond the current efficiencies could, in principle, be achieved by multi-junction devices, which promise a reduction in thermalization losses. Nevertheless, state-of-the-art multi-junction OSC, leaded by the tandem approach in which two single junction devices are stacked on top of each other, exhibit, thus far, similar efficiency values.[3], [4] This is attributed to the challenges that arise when depositing subsequent layers via solution processing, as well as the need for either a current matching or an extra transparent electrode.[4]

In this talk, we will present a new multi-junction in-plane spectral splitting geometry that we call Rainbow solar cells. 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 any current matching nor processing challenges. After presenting the Rainbow OSCs concept, we use device simulations to provide design rules for increased efficiency in a Rainbow configuration. Then, we will show experimental results for PM6:IO-4Cl and PTB7-Th:COTIC-4F blends, as high and low band-gap sub-cells, respectively. In agreement with simulations, we demonstrate an efficiency increase of around 30% of the Rainbow geometry with respect to our best single junction device.

Organic semiconductor-based photovoltaic (OPV) devices have many advantageous properties including tailorable light absorption, low embodied energy manufacturing, structural conformality, and low material toxicity. Apart from outdoor solar energy harvesting, these properties also make OPVs attractive for indoor applications which operate at considerably lower light intensities. However, owing to their low charge carrier mobilities, the competition between charge carrier extraction and recombination is an important factor limiting the performance of OPV devices [1]. This competition is known to be strongly dependent on both the applied voltage and the prevailing light intensity, however, the role of the contacts has remained elusive [2],[3]. In this work, we investigate processes limiting the collection of photogenerated charge carriers in OPV devices. We derive analytical expressions describing the current-voltage characteristics of thin-film devices based on low-mobility semiconductors at different light intensity conditions. The theoretical framework is further substantiated by numerical drift-diffusion simulations. Based on these findings, the light intensity and voltage dependence of the photogenerated current, and the impact of different loss mechanisms and contact-related effects, in OPV devices is clarified. This work provides intriguing insights into the differences between OPV devices operating under indoor and outdoor conditions. 

Nanostructure plays an important role in organic active layers for solar cell applications. The way in which individual material components are arranged with respect to each other is decisive for electronic transport properties and hence for device performance.

In many of these material systems, we have nanostructures that are arrested out-of-equilibrium during a fast processing quench. Therefore the route for structure formation, the provision of energy to the system in form of light and heat or other external stimuli like electric fields [1] can induce structural changes during processing and in the final solar cell [2]. Systematically examining these structural changes as a function of the environmental parameters helps us to identify options for the improvement of structural quality necessary for optimized electronic properties. Using time-resolved optical in-situ investigations as well as x-ray scattering experiments we investigate the quality of nanostructures as well as the effect of accelerating and slowing down such nanostructural changes. This approach allows us to learn more about how to systematically control nanostructure formation and stability.

High efficiency and mechanical robustness are both crucial for the practical applications of all-polymer solar cells (all-PSCs) in stretchable and wearable electronics.[1] In this regard, a series of new polymer acceptors (PAs) is reported by incorporating a flexible conjugation-break spacer (FCBS) to achieve highly efficient and mechanically robust all-PSCs. Incorporation of FCBS affords the effective modulation of the crystallinity and pre-aggregation of the PAs, and achieves the optimal blend morphology with polymer donor (PD), increasing both the photovoltaic and mechanical properties of all-PSCs. [2] In particular, an all-PSC based on PYTS-0.3 PA incorporated with 30% FCBS and PBDB-T PD demonstrates a high power conversion efficiency (PCE) of 14.68% and excellent mechanical stretchability with a crack onset strain (COS) of 21.64% and toughness of 3.86 MJ m-3, which is significantly superior to those of devices with the PA without the FCBS (PYTS-0.0, PCE = 13.01%, and toughness = 2.70 MJ m-3). In a follow up work, we have investigated the influence of the length of flexible spacer. It was found that the polymer acceptor with shorter spacer achieved the highest performance. These results reveal that the introduction of FCBS into the conjugated backbone is a highly feasible strategy to simultaneously improve the PCE and stretchability of PSCs.[3]

Over the past few decades, organic solar cells (OSCs) have made a significant progress, showing their great potential for low-cost, flexible, lightweight, portable and large-area energy-harvesting devices. Although PC₆₁BM and/or PC₇₁BM structures have been exploited successfully in OSC devices, efforts to modify the fullerene structures for further improving the device performance have been tried recently because fullerene derivatives have the inflexibility in molecular design, difficult purification, poor morphological stability, and limited light absorption in the visible region, etc. In recent years, nonfullerene acceptors have emerged as an alternative candidate of n-type materials to overcome the difficulties of fullerene derivatives in tuning optical and electronic properties. The strong and easily adjustable absorption characteristics of nonfullerene acceptors have been considered as a strong point compared to fullerene-type structures, showing a photovoltaic efficiency over ~18%. To further optimize the OSCs for next generation green energy sources, several important points need to be considered carefully. Here we discuss the fundamental correlations between molecular structure, blend morphology and device performance in new nonfullerene acceptor-based OSCs. The photovoltaic properties of semi-transparent OSCs and sequential layer-by-layer OSCs will be also discussed in detail.  

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-methyltetrahydrofuran (2-MeTHF).  2-MeTHF is a biomass-derived (furfural or levulinic acid) and environmentally friendly solvent that is widely used in organic synthesis, which can be produced from low-cost and renewable agriculture feedstock.  A combination of characterization methods were employed to gain insight into the film morphology and solar cell performance.

Non-radiative recombination is currently still limiting the efficiency of organic solar cells. A reduction of these losses, inevitably leads to an increase in the electroluminescence quantum efficiency of the devices.[1] We have recently identified small molecule donor-acceptor blends with an optical gap in the visible spectral range, with strongly reduced non-radiative losses and high electroluminescence quantum efficiencies (> 1%) as compared to high efficiency systems with a gap in the near infrared (NIR).[2] As a significant fraction of free carrier recombination in these devices results in the emission of a photon, we have identified time resolved emission spectroscopy at nanosecond to millisecond timescale as an excellent probe for the recombination dynamics and interplay between free carriers, charge transfer states and triplet states. In this talk I will discuss the molecular and morphological factors which are responsible for the efficient charge generation and strongly reduced non-radiative recombination in these high gap systems, and will provide guidelines for future high efficiency devices with reduced voltage losses.

Green hydrogen, produced from water using renewable energy, is expected to become a prominent renewable fuel of the future, providing clean, carbon neutral energy for a wide range of industrial applications. It can also provide complementary energy storage in combination with intermittent solar energy. However, competitive economic solar generated hydrogen production on a large scale remains challenging. One promising approach is photochemical water splitting, using lght absorbing nanoparticle semiconductors that can drive redox reactions on their surface. The more light the photocatalytic nanoparticle absorbs, the more efficiently it can split water into hydrogen and oxygen. Traditionally, wide bandgap inorganic semiconductors have been used for photocatalytic applications. However, these materials almost exclusively absorb UV light which only carries a small fraction (<5%) of solar energy, limiting their efficiency. In this presentation, the development of photocatalysts fabricated from organic semiconductors, chemically tuned to absorb strongly throughout the UV-visible spectrum will be discussed. We demonstrate a larger solar to hydrogen efficiency than traditional inorganic photocatalysts with the potential to achieve solar hydrogen production at a lower levelized cost. We have developed organic semiconductor nanoparticles that contain an internal donor/acceptor heterojunction between two organic semiconductors with a type II energy level offset. The donor/acceptor heterojunction greatly improves charge generation within the nanoparticles, which in turn greatly improves their hydrogen production efficiency. We demonstrate a substantial increase in the H2 production efficiency by tuning the nanoparticle composition. We also observe that the high efficiency of these nanoparticles originates from their ability to generate exceptionally long-lived reactive charges upon illumination, increasing their likelihood to participate in a photocatalytic reaction.

In the recent years, the development of new non-fullerene-based organic acceptor (NFA) materials have strongly contributed to increasing the efficiency of organic photovoltaics (OPV), going from an average efficiency of about 12% to over 18%. More often than not, these excellent and fundamental developments, in terms of efficiency, are not compatible with potential production applications as well as still having severe limitations in lifetime and stability in a practical context.

It is therefore essential to improve the intrinsic characteristics of the devices, such as organic semiconductor, adapting formulations or manipulation of the interlayers for a fully compatibility with a productive process like slot-die by roll-to-roll (R2R). In the other side it is equally fundamental to have a clear vision of their behavior with the extrinsic stability of the panels themselves, such as both physical and chemistry compatibility with adhesives and barrier properties maintaining reasonable compromise between costs, final application, and context.

In this talk we will analyze some fundamental points required by a real scale-up, practical problems in their applications and give an analysis on the motivation of why these materials still take time to be used commercially.