The mixed caesium and formamidinium lead triiodide perovskite system (Cs1-xFAxPbI3) in the form of quantum dots (QDs) offers a new pathway towards stable perovskite-based photovoltaics and optoelectronics. However, it remains challenging to synthesize such multinary QDs with desirable properties for high-performance QD solar cells (QDSCs). Here we report an effective ligand-assisted cation exchange strategy that enables controllable synthesis of Cs1-xFAxPbI3 QDs across the whole composition range (x: 0-1), which is inaccessible in large-grain polycrystalline thin films. The surface ligands play a key role in driving the cross-exchange of cations for the rapid formation of Cs1-xFAxPbI3 QDs with suppressed defect density. The hero Cs0.5FA0.5PbI3 QDSC achieves a certified record power conversion efficiency (PCE) of 16.6% with negligible hysteresis. We further demonstrate that QD devices exhibit substantially enhanced photostability compared to their thin film counterparts because of the suppressed phase segregation, retaining 94% of the original PCE under continuous 1-sun illumination for 600 hours.

Detailed balance is a cornerstone of our understanding of artificial light-harvesting systems. For next generation organic solar cells, this involves intermolecular charge-transfer (CT) states whose energies set the maximum open circuit voltage VOC. We have directly observed sub-gap states significantly lower in energy than the CT states in the external quantum efficiency spectra of a significant number of organic semiconductor blends. Taking these states into account and using the principle of reciprocity between emission and absorption results in non-physical radiative limits for the VOC. We propose and provide compelling evidence for these states being non-equilibrium mid-gap traps which contribute to photocurrent by a non-linear process of optical release, upconverting them to the CT state. This motivates the implementation of a two-diode model which is often used in emissive inorganic semiconductors. The model accurately describes the dark current, VOC and the long-debated ideality factor in organic solar cells. Additionally, the charge-generating mid-gap traps have important consequences for our current understanding of both solar cells and photodiodes – in the latter case defining a detectivity limit several orders of magnitude lower than previously thought.

In crystalline semiconductors, absorption onset sharpness is characterized by temperature-dependent Urbach energies. These energies quantify the static, structural disorder causing localized exponential-tail states, and dynamic disorder from electron-phonon scattering. Applicability of this exponential-tail model to disordered solids has been long debated. Nonetheless, exponential fittings are routinely applied to sub-gap absorption analysis of organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes of organic semiconductors and their blends by temperature-dependent quantum efficiency measurements. We find that sub-gap absorption due to singlet excitons is universally dominated by thermal broadening at low photon energies and the associated Urbach energy equals the thermal energy, regardless of static disorder. This is consistent with absorptions obtained from a convolution of Gaussian density of excitonic states weighted by Boltzmann-like thermally activated optical transitions. A simple model is presented that explains absorption line-shapes of disordered systems, and we also provide a strategy to determine the excitonic disorder energy. Our findings elaborate the meaning of the Urbach energy in molecular solids and relate the photo-physics to static disorder, crucial for optimizing organic solar cells for which we present a revisited radiative open-circuit voltage limit.

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.

acceptor in the so-called bulk-heterojunction (BHJ) organic solar cell results in new hybrid intermolecular charge transfer (CT) states [2] which determine the effective band gap and thereby the open circuit voltage. [3] These CT states mediate the charge photogeneration between the excitons and relaxed free charges, and thus their kinetics determine most of the recombination processes [4,5] and the thermodynamic limit of the solar cell. [3,6] The open circuit voltage of any solar cell is determined primarily by the band gap energy (i.e., the energy of the CT states in the case of BHJs), band-to-band recombination, and non-radiative recombination. [7,8] These all add up to different contributions in the losses in the open circuit voltage (the loss being the difference between the energy gap and the observed open circuit voltage). [9] The radiative limit of the open circuit voltage is governed by the solar photon absorption by the singlet excitons and thermal photon absorption by the CT states, while nonradiative losses are related to how efficiently these states emit. [8] Through the so-called reciprocity principle, the emission and absorption spectra are interrelated intrinsically, although for low mobility organic semiconductors with substantial second order recombination there may be deviations from this principle. [10] Energetic disorder and non-equilibrium states can also play a role in deviations from this principle as shown recently. [11] As such, if the CT states are fully characterized, the open circuit voltage can be explained or predicted to within some accuracy. [8] The energy and dynamics of the CT states also contain quite profound information from a structure-property-design consideration. Thus, from multiple perspectives, CT state analysis has become a central theme in not just organic solar cells, but organic optoelectronics more broadly. [12] Inspired from models of partially radiative ion pairs, nonadiabatic Marcus theory has been employed to explain the thermal broadening of the CT states, their emission, and absorption. [13,14] This parameterization includes an energy of the CT state (E CT), reorganization energy (λ CT), and oscillator strength (f σ) related to the electronic coupling, as well as emission probability f I. Ideally, the absorption cross-section (σ CT) and emission (I) are related through reciprocity so that