Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) of various donor polymers and molecular acceptors are investigated. The impact of polymer-NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer-NFA phases to consisting of both intermixed and pure phase. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. For high crystalline blends, transient absorption data indicate exciton separation leads to the formation of two spectrally distinct species, assigned to interfacial charge transfer (CT) states and separated charges. CT state decay is correlated with the appearance of additional separate charges, indicating relatively efficient CT state dissociation, attributed to the high crystallinity of this blend. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in OSCs, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.

The currently best organic solar cells suffer from relatively large voltage losses due to non-radiative recombination as compared to inorganic or perovskite solar cells. Further enhancement of the power conversion efficiency to values over 20% will require a reduction of these losses, inevitably corresponding to an increase in the electroluminescence quantum efficiency of the devices.^[1] For a large number of donor-acceptor combinations, we have observed that non-radiative voltage losses decrease with increasing charge-transfer-state energies, consistent with non-radiative decay being facilitated by a common high frequency molecular vibrational mode.^[2] We further identify small molecule donor-acceptor blends with an optical gap in the visible spectral range, with strongly reduced non-radiative losses as compared to systems with a gap in the near infrared (NIR).^[3] This highlights the possibility of a simultaneous occurrence of a high photovoltaic quantum efficiency as well as a high electroluminescence quantum efficiency, occurring in a single organic donor-acceptor blend. For photovoltaic blends with strong absorption in the NIR, we show that the lowest non-radiative decay rates correspond to systems with the narrowest emission linewidths and steepest absorption tails.^[4]

Recent advances in organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) come along with reduced non-radiative voltage losses. We show that the non-radiative voltage losses in these state-of-the-art donor:NFA OSCs show no correlation with the energies of charge-transfer electronic states at donor:acceptor interfaces, different from conventional fullerene-based OSCs. Based on a combined temperature-dependent electroluminescence experiments and dynamic vibronic simulations, we have been able to rationalize the low voltage losses in these devices, where we highlight the critical role of the thermal population of local exciton states in decreasing the non-radiative losses. An important finding is that the molecular photoluminescence properties of the pristine materials define the limit of non-radiative voltage losses in OSCs, indicating that it is critical to design high-luminescence-efficiency donor and acceptor materials with complementary optical absorption bands extending into the near-infrared region. We further demonstrate that there is no intrinsic limit for efficient charge separation in OSCs with small non-radiative voltage losses.

We present a variety of ultrafast spectroscopic techniques in the visible and terahertz (THz) range, showing how they can be optimized to study excited states and light-induced processes in organic semiconductors. We discuss case studies, such as the charge dynamics from generation to extraction in highly efficient organic photovoltaic systems containing non-fullerene acceptors, the short-range transport properties evaluated by THz spectroscopy of doped organic thin films, or time-resolved processes in organic electrochemical transistors. We emphasize the depth of understanding that can be gained by exploring such systems from multiple spectroscopic angles, and the relevance to applications such as solar cells or bioelectronics.

Many studies have reported that charge transfer (CT) states are important for the function of organic solar cells, impacting both on charge photogeneration and on the recombination pathways limiting device open circuit voltage. Such states can be most readily observed optically as CT state absorption or emission, allowing direct observation of CT state energetics. Indeed measurement of CT state energies has been central to many analyses of organic solar cell function. However recent several studies of high performance, low energy offset organic solar cells employing non-fullerene solar cells have reported that CT state absorption and emission is no longer clearly observable. This raises the question of how to probe CT states in such devices, and indeed whether CT states are important for function of these devices. In my talk, I will address the charge carrier dynamics which underlie the performance of organic solar cells employing non-fullerene acceptors. I will consider the impact of charge transfer states on these dynamics, in particular for low energy offset, high performance devices, and analyses of the function of such devices when CT state absorption and emission is not easily measurable.

Processes taking place at contacts are of particular importance in organic and perovskite solar cells where selective contacts that can efficiently collect majority carriers, simultaneously blocking minority carriers are desired. The surface recombination velocity S_R is a key-parameter in describing the dynamic processes at interfaces.

We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory taking the effect of built-in voltage, diffusion and band-bending into account [1-4]. By doing so we have been able to extend the CELIV into obtaining new regimes, namely for metal-insulator-metal structures, doped semiconductors and for metal-insulator-semiconductor structures. We have used the new regimes of CELIV as in-device characterization techniques to clarify important device physical parameters. We have derived analytical expressions describing the effective reduction of the built-in voltage, the (effective) open-circuit voltage providing means to quantify and distinguish various loss-mechanisms occurring at contacts in thin-film solar cells.

We show how to use CELIV to directly determine surface recombination velocities at selective and/or blocking contacts in thin-film devices, allowing us to directly estimating the dynamics at selective contacts.

Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells, however, this morphology compromises charge collection. Here, we reveal rapid exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells. Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 ×10^-2 cm² / s – substantially exceeding typical organic semiconductors, and consistent with the 20-50 nm domain sizes in optimized blends.[1] Enhanced 3-dimensional diffusion accounted for computationally [2] and is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, high chromophore density, and low disorder – all enhancing long-range resonant energy transfer. Relieving exciton diffusion constraints has important implications for OPVs; large, ordered, and pure domains enhance charge separation and transport, and suppress recombination, thereby boosting fill factors. Further enhancements to diffusion lengths may even obviate the need for the bulk heterojunction morphology [3].

The transport of charges and excitons is well understood in two extremes: in highly ordered materials, transport is by band conduction, while in highly disordered ones, it is by hopping. Many organic semiconductors fall in the intermediate regime between band transport and hopping, making either set of approximations inaccurate. In particular, intermolecular couplings mean that there is usually some delocalisation across multiple molecules (or segments of polymers), while disorder ensures that this effect is spatially limited. Theoretically describing the movement of partially delocalised carriers and excitons is difficult, because it depends on a complicated interplay of energetic disorder, quantum-mechanical couplings, and polaron formation.

We report delocalised kinetic Monte Carlo (dKMC), a new computational method that is able to describe the motion of partially delocalised charges and excitons in all regimes of disorder [1]. We implement numerical innovations that allow us to work in three dimensions, a regime that had proven too complicated for all comparable approaches. dKMC reveals new, basic physics of transport in organic semiconductors and explain why mobilities predicted by traditional kinetic Monte Carlo are usually too low. In particular, delocalisation over just a few molecules can increase mobilities by orders of magnitude.

We also extend dKMC to describe charge separation at a heterojunction [2], the first approach to do so that includes all the necessary ingredients: delocalisation, disorder, and polaron formation. We show that delocalisation can play a decisive role in charge separation, with even modest delocalisation able to double internal quantum efficiencies.

[1] Balzer, Smolders, Blyth, Hood, and Kassal, Chem. Sci. 12, 2276 (2021).
[2] Balzer and Kassal, arXiv:2108.05032 (2021).

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a donor / orthogonal acceptor motif, referred to as PCPDT-sFCN [1]. The role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO, allowing for negligible exchange energy between electrons in these orbitals and minimising the energy gap between singlet and triplet charge transfer states.  We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation.

Furthermore, clear evidence of enhanced triplet populations, which form in less than 1 ps, is observed in PCPDT-sFCN. Using group theory, we show that this ultrafast triplet formation is due to highly efficient, quantum mechanically allowed intersystem crossing between the bright, initially photoexcited local singlet state and the triplet charge transfer state. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, due to the stabilisation imparted by the spatial separation of PCPDT-sFCN’s donor and orthogonal acceptor motifs.  The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors.

The photogeneration of free charges in light-harvesting devices is a multi-step process, which can be challenging to probe due to the complexity of the involved inter-and-intramolecular energetic states and the competitive character of various driving mechanisms. Here, we present a technique which allows the measurement of photogenerated charge carrier densities sensitively and to probe charge generation processes in thin-film solar cells. Our technique is based on the integral time-of-flight method of sandwich-type thin-film devices but extended to the low intensity regime (LIITOF) and combined with voltage dependent device capacitance measurements. The theoretical framework of LIITOF is verified by drift-diffusion simulations and the applicability of our method is demonstrated on thin-film solar cells based on organic and perovskite semiconductors by examining the voltage dependence of charge generation efficiency. Our experimental results are compared to those obtained via Time Delayed Collection Field (TDCF) measurements conducted on the same devices and found to be in excellent agreement.

Organic photovoltaics have been developed in the concept of bulk heterojunctions (BHJs) by blending donor and acceptor materials because of the short exciton diffusion length of classic organic semiconductors. While non-fullerene acceptors (NFAs) have recently demonstrated long-range exciton diffusion, most of studies focus yet on blended polymer:NFA systems. Here, we calculate the long exciton diffusion length of a NFA (~40 nm) by ultrafast spectroscopy measurements and fabricate NFA/polymer planar heterojunctions (PHJs) as a function of the thickness of the NFA. From thick to thin NFA layer, close to the exciton diffusion length of the NFA, additional hole transfer and photocurrent was observed, indicating that more excitons created away from the NFA/polymer interface can diffuse to the interface. Field-dependent charge generation is observed by exciton diffusion from the neat NFA. Further, due to smaller acceptor/donor interfacial area, the PHJ devices exhibit less bimolecular recombination losses and suppressed dark leakage current than the corresponding BHJ device. By employing the NFA/polymer PHJ to photodetectors, we demonstrate 3.6 times higher detectivity at -2 V bias than that of the BHJ photodetector.

The device performance of organic bulk heterojunction (BHJ) photovoltaic cells is ultimately derived from processes taking place between excitons, charge-transfer states, and free charge carriers. In fullerene-acceptor-based BHJ systems, characterized by large donor-acceptor energy level offsets, the photovoltage is critically limited by the energetics and kinetics of CT states [1]. However, this picture has been challenged with the emergence of nonfullerene-acceptor-based systems with low energy offsets, where the role of excitons becomes important [2]. In this work, factors limiting key device parameters in organic BHJ solar cells based on low-offset systems are investigated. We find that photovoltaic parameters are not only limited by the energetics but also critically determined by the relative kinetics between different species [3,4]. Finally, based on these considerations, the corresponding power conversion efficiency limits are predicted. These findings provide insights into the operation of state-of-art non-fullerene organic photovoltaic cells with low offsets, paving the way for organic solar cells with power conversion efficiencies exceeding 20%.

Organic solar cells (OSCs) are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs) [1]. One particularly successful type of NFAs are Y-type moleucles such as Y6 [2]. In blends with PM6 and other donor polymers, high efficiency (>18 %) have been recently achieved in single junction devices, with the perspective to approach the commercially relevant 20 %. Here, we summarizes our recent understanding of the processes governing the performance of OSCS based on Y6 and related compounds. For PM6:Y6, we find that free charge generation is essentially barrierless [3] and that the fill factor of the device is essentially limited by the diffusion length of the charges, which is smaller than the active layer thickness [4]. This puts the understanding of the recombination processes at the focus of the further understanding. By studying the emission properties of such cells, we conclude that the radiative recombination of free charges in Y6-based cells proceeds almost entirely through the re-occupation and decay of the Y6 singlet, but that this pathway contributes to less than 1 % of the total recombination current [5]. As such, the open-circuit voltage of the PM6:Y6 blend is almost entirely determined by the energetics and kinetics of the charge transfer (CT) state, irrespective of the position and emission properties of the singlet state. Most recently, we addressed the role of energetic disorder in the competition between charge extraction and recombination [6] By performing temperature dependent charge transport and recombination studies, we come to a consistent picture of the the density of state distributions for free charges, which allows us to analytically describe the dependence of the open-circuit voltage on temperature and illumination intensity. We conclude that energetic disorder of charge separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend.

With the advent of non-fullerene acceptors, organic solar cells have made impressive improvements in terms of power conversion efficiency, so that breaking the 20 % limit is within close reach. Understanding the efficiency-limiting processes remains important to optimize the solar cells effectively.

The impact of the most important loss mechanisms for state-of-the-art organic solar cells is schematically shown in the Figure. We will present our recent findings on two of them, nongeminate recombination of charge carriers, and a loss in fill factor due to the transport resistance. 

Nongeminate recombination takes place via charge transfer complexes, and we discuss different models for describing them, in particular their absorption and emission, and how they relate to the open circuit voltage. Concerning the transport resistance, it is a major loss mechanism in non-fullerene based solar cells. We will discuss our findings on how the related voltage drop is related to the active layer conductivity.