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The subtle link between photogenerated charge generation yield (CGY) and bimolecular recombination in organic semiconductor-based photovoltaics is relatively well established as a concept but has proven extremely challenging to demonstrate and probe especially under operational conditions. Received wisdom also teaches that charge generation in excitonic systems will always be lower than non-excitonic semiconductors such as GaAs – but this view is being challenged with the advent of organic semiconductor blends based upon non-fullerene acceptors (NFAs) with power conversion efficiencies exceeding 18%. Using a newly developed approach based upon temperature dependent ultra-sensitive external quantum efficiency measurements, we observe near unity CGY in several model NFA-based systems measured with unprecedented accuracy. We find that a relatively small increase in yield from 0.984 to 0.993 leads to a reduction in bimolecular recombination from 400 times to 1000 times relative to the Langevin limit. In turn, this dramatic reduction delivers the best thick junction performance to date in any binary organic solar cell – notably 16.2% at 300 nm. The combination of high efficiency and thick junction is the key for industrial fabrication of these devices via high-throughput deposition processing such as roll-to-roll, and thus central to a viable solar cell technology. These results also clearly reveal and elucidate the relationship between photo-generation and recombination in excitonic semiconductor photovoltaics thus providing an important bridge between basic device physics and practical cell engineering.

The spectral shape near the energy gap determines the radiative limit of the open-circuit voltage in organic photovoltaic devices. In this work, we employ ultrasensitive photocurrent measurements and detect sub-gap states with energies far below gap in a large number of different donor-acceptor blends. We provide evidence that these low-energy sub-gap states are associated with radiative mid-gap trap states, generating photocurrent via an optical release process. To account for the radiative mid-gap states, we implement a two-diode model which accurately describes both the dark current and the open-circuit voltage in organic solar cells. These findings provide important insights for our current understanding of organic photovoltaic devices.

Non-radiative losses to the open-circuit voltage are a primary factor in limiting the power conversion efficiency of organic photovoltaic solar cells. The dominate non-radiative loss is intrinsic to the active layer which, along with the thermodynamic limit to the open-circuit voltage, define the quasi-Fermi level splitting (QFLS). Quantification of the QFLS in organic photovoltaic devices is challenging due to the excitonic nature of photoexcitation and device-related non-radiative losses. In this presentation I will outline an experimental approach based on electro-modulated photoluminescence to quantify the QFLS in organic solar cells. Drift-diffusion simulations are used to verify the accuracy of the method, while state-of-the art PM6:Y6 solar cells are created with varying non-radiative losses. This method quantifies the QFLS in organic photovoltaics, fully characterizing the magnitude of different contributions to the non-radiative losses of the open-circuit voltage.

Recent advances in organic solar cell material development based around non-fullerene electron acceptors in bulk heterojunctions have propelled power conversion efficiencies to >18%, with 20% on the horizon and 25% predicted. These efficiencies are close to traditional inorganic semiconductor photovoltaics and thus focus is now turning to manufacturability and creating a viable solar cell technology. In this presentation we report the highest efficiency to date (16% with a Fill Factor >70%) in a thick junction binary organic solar cell based upon PM6:BTP-eC9. Using a very accurate approach based upon temperature dependent ultra-sensitive EQE measurements we find that this system (and a similar one based upon PM6:Y6) have near unity charge generation yields (CGY > 99%). In this regime, we observe that a small increase in CGY of only 0.5% leads to a 2.5 times more reduction in bimolecular recombination relative to the Langevin limit enabling high efficiency thick junction solar cells.

Probing the photovoltaic external quantum efficiency (EQE) at photon energies well below the semiconductor bandgap is an important tool for achieving a better understanding of the contribution of trap and tail states involved in charge generation processes in photovoltaic devices, notably solar cells. In this work, we present an electrical and optical noise-reduced EQE apparatus achieving 100 dB dynamic range. We carefully identify and study several device- and EQE apparatus-related factors limiting the EQE measurement sensitivity. Minimizing these factors allows us to detect photocurrents smaller than a fA, corresponding to EQE signals as small as -100 dB. We use these ultra-sensitive EQE measurements to probe weak sub-bandgap absorption features in organic, inorganic and perovskite semiconductors. In this regard, we directly observe photocurrent-contributing sub-gab trap states in organic solar cells significantly lower in energy than the corresponding charge-transfer state.

Trap-assisted recombination caused by localised sub-gap states is one of the most important first-order loss mechanism limiting the power-conversion efficiency of all solar cells. The presence and relevance of trap-assisted recombination in organic photovoltaic devices is still a matter of some considerable ambiguity and debate, hindering the field as it seeks to deliver ever higher efficiencies and ultimately a viable new solar photovoltaic technology. In this work, we show that trap-assisted recombination loss of photocurrent is universally present under operational conditions in a wide variety of organic solar cell materials including the new non-fullerene electron acceptor systems currently breaking all efficiency records. The trap-assisted recombination is found to be induced by states lying 0.35-0.6 eV below the transport edge, acting as deep trap states at light intensities equivalent to 1 sun. Apart from limiting the photocurrent, we show that the associated trap-assisted recombination via these comparatively deep traps is also responsible for ideality factors between 1 and 2, shedding further light on another open and important question as to the fundamental working principles of organic solar cells. Our results also provide insights for avoiding trap-induced losses in related indoor photovoltaic and photodetector applications., Trap-assisted recombination caused by localised sub-gap states is one of the factors limiting power-conversion efficiency in solar cells, yet the presence and relevance is still under debate in organic solar cells. Here, the authors reveal that this recombination loss is universally present under operational conditions in these devices.

Nonradiative losses to the open-circuit voltage are a primary factor in limiting the power-conversion efficiency of organic photovoltaic devices. The dominate nonradiative loss in the bulk is intrinsic to the active layer and can be determined from the quasi-Fermi-level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin-film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electromodulated photoluminescence, which can be used to directly measure the intrinsic nonradiative loss to the open-circuit voltage, thereby quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related nonradiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the nonradiative losses of the open-circuit voltage. The reported method will be useful not only in characterizing and understanding losses in organic solar cells but also in other device platforms such as light-emitting diodes and photodetectors.

The impact of intermixed donor-acceptor domains in organic bulk heterojunction (BHJ) solar cells, using low-donor-content devices as model systems, is clarified. At low donor contents, the devices are found to exhibit anomalously high open-circuit voltages independent of the donor-acceptor energetics. These observations can be consistently explained by a theoretical model based on optical release of trapped holes, assuming the donors behave as trap sites in the gap of the acceptor. Our findings provide guidelines for reducing the large open-circuit voltage losses in organic solar cells and avoiding morphology-induced losses in state-of-the-art BHJ solar cells and photodetectors.

Energy states below the bandgap of a semiconductor, such as trap states or charge transfer states in organic donor acceptor blends, can contribute to light absorption. Due to their low number density or ultrasmall absorption cross-section, the absorption coefficient of these states is challenging to measure using conventional transmission reflection spectrophotometry. As an alternative, the external quantum efficiency (EQE) of photovoltaic devices is often used as a representative of the absorption coefficient, where the spectral line shape of the EQE is considered to follow the absorption coefficient of the active layer material. In this work, it is shown that the subbandgap EQE is subject to thickness dependent low finesse cavity interference effects within the device, making this assumption questionable. A better estimate for the absorption coefficient is obtained when EQE spectra corresponding to different active layer thicknesses are fitted simultaneously for one attenuation coefficient using an iterative transfer matrix method. The principle is demonstrated for two model acceptor-donor systems (PCE12ITIC and PBTTTPC71BM) and accurate subgap absorption coefficients are determined. This approach has particular relevance for both understanding sub-gap states and their utilization in organic optoelectronic devices.