Abstract Understanding how donor–acceptor stoichiometry governs charge carrier separation in non-fullerene bulk heterojunction active layers remains incomplete. By combining transient absorption and time-resolved photoluminescence spectroscopy with selective donor excitation, we elucidate the composition-dependent charge carrier generation pathways in PM6:Y6 blends. Donor excitations evolve via two competing parallel channels: direct charge transfer (CT) and energy transfer to the acceptor followed by subsequent hole transfer. We find that direct ultrafast CT state formation dominates in near-optimal blends, resulting in efficient carrier generation. Under moderate compositional imbalance, CT state formation and subsequent charge separation are only weakly perturbed, and the overall dynamics remain largely unchanged. In contrast, strong compositional imbalance introduces distinct limiting mechanisms. In strongly donor-rich blends, a significant fraction of donor excitons decay within the donor phase before reaching the donor-acceptor heterojunction. In strongly acceptor-rich blends, the energy transfer pathway is favored, accompanied by relaxation losses of acceptor excitons within the acceptor phase. Furthermore, both non-optimal compositions hinder efficient CT state splitting into separated charges, likely resulting from trapping of one of the charges within less favorable donor or acceptor domains. This work clarifies how efficient charge separation emerges in low-offset non-fullerene PM6:Y6 blends and identifies stoichiometry as a key parameter governing charge generation efficiency.

Over a relatively short period of time, the power conversion efficiency of perovskite solar cells (PSCs) has increased from 3.8 to 27%. Despite this rapid progress, stability issues still remain a major challenge for widespread commercial application, thus motivating extensive research on perovskite surface defect passivation that would improve the longevity of PSCs. Although many reported passivation strategies are demonstrated under specific deposition conditions and device architectures, the reasons behind their effectiveness are not fully understood. In this study, we systematically examine quaternary ammonium and thiouronium organic salts as surface-passivating agents for triple-cation PSCs and compare their behavior with that of the commercially available sulfonium-based salt DMPESI. By combining photoluminescence (PL), time-resolved photoluminescence (trPL), X-ray photoelectron spectroscopy (XPS), liquid-state nuclear magnetic resonance (NMR), and device measurements, we show that different characterization techniques probe different features of organic-perovskite interactions. Although some treatments improve radiative recombination, as indicated by PL and trPL data, these results do not directly relate to the photovoltaic performance. Only the iodide-containing ammonium salt CzEAI shows device improvement consistent with XPS analysis. This work reveals the complex nature of defect passivation in PSCs.

Although organic solar cells have surpassed 20% power conversion efficiency, a persistent trade-off between open-circuit voltage and fill factor (FF) prevents them from closing the gap with inorganic technologies. Here we investigate this trade-off across a wide range of devices and identify an FF limit arising from field-dependent free-charge generation. This limit becomes more severe as voltage losses are minimized, thereby imposing an open-circuit voltage-FF trade-off. To quantitatively describe this limit, we develop an analytical model for field-dependent charge generation, revealing that the underlying cause is the field-sensitive charge-transfer process between excitons and charge-transfer states. This sensitivity originates from the field-induced charge-transfer state energy variations, mainly caused by the Stark effect. Guided by this physics-based model, we highlight that a long exciton lifetime is one of the practical and effective methods to overcome the FF limit.

Although the power conversion efficiencies of organic solar cells (OSCs) have surpassed 20

Ytterbium ions are important in the development of lanthanoid-based optical and optoelectronic materials due to their stable near-infrared emission and minimal nonradiative losses. In lead halide perovskites, Yb3+ ions can perform quantum cutting, converting a single high-energy photon into a pair of near-infrared photons, offering a route to enhance the efficiency of silicon solar cells. This process relies on the 2F5/2 -> 2F7/2 transition, which is parity-forbidden in free Yb3+ ions but becomes partly allowed due to crystal field interactions in the perovskite lattice. Electron-phonon coupling further enhances the transition probability, resulting in a temperaturedependent increase in its oscillator strength. Previous studies have identified Yb3+ species that are both active and inactive in photon cutting within the CsPb(ClxBr1-x)3 perovskite lattice. Here, we investigate photoluminescence (PL) dynamics and the oscillator strength of the 2F5/2 -> 2F7/2 radiative transition of different Yb3+ species in CsPb(ClxBr1-x)3 polycrystalline powders. Using time-resolved PL measurements under selective excitation of the perovskite host across its band gap and direct intra-4f excitation of Yb3+ ions, we demonstrate how the PL decay rate varies with temperature across different perovskite matrices. Increasing temperature leads to faster PL decay and enhanced oscillator strength of Yb3+ ions independently of the anion content in the lead halide perovskites. On the other hand, the temperature dependence of the PL decay rate differs between Yb3+ species that are active and inactive in the quantum cutting process. In CsPbBr3 powders, we show that temperature-dependent energy transfer from the perovskite host to Yb3+ ions, together with their nonradiative relaxation, governs the luminescence intensity and decay rate of the dopant.

Layered semiconductors are promising for new optoelectronic applications due to their unique optical and electronic properties. This study focuses on the synthesis of thin germanium monosulfide (GeS) films and the characterization of their structural and optoelectrical properties. We show that rapid thermal evaporation (RTE) enables effective synthesis of large-area polycrystalline GeS thin films on glass, fluorine-doped tin oxide and interdigitated array (IDA) electrode substrates. However, the one-step deposition process leads to uncontrolled, randomly oriented crystal growth. We have developed a two-step process that involves the initial deposition of an amorphous film at a temperature below 350 degrees C and subsequent annealing at 370 degrees C, ensuring complete GeS crystallization and the formation of high-quality polycrystalline GeS films with controlled thickness and layer orientation. Photoelectric characterization using current-voltage (JV) characteristics, transient photocurrent and time-delayed collection field (TDCF) measurements revealed that polycrystalline GeS is a photosensitive material with long carrier lifetimes but with rapidly decreasing mobility. GeS is potentially promising for optoelectrical applications.

In recent years, organic solar cells (OSCs) have made significant progress, with power conversion efficiencies over 20 %, mainly due to advances in non-fullerene acceptors. Nevertheless, high voltage losses- especially those caused by non-radiative recombination- remain a major obstacle to further efficiency improvements. In contrast to traditional fullerene-based OSCs, state-of-the-art non-fullerene OSCs enable efficient charge generation and extraction even at small energy offsets (<0.3 eV) between the HOMO levels of the donor and the acceptor materials (Delta HOMO). However, achieving low voltage loss while maintaining high fill factor and shortcircuit current (Jsc) is a major challenge due to inherent trade-offs. In this study, we systematically investigate the relationship between voltage losses and the other two parameters in nine non-fullerene OSC devices with Delta HOMO in the range of about 0-0.5 eV. Both radiative and non-radiative voltage losses show a clear increase with Delta HOMO. However, no general correlation was found between the voltage loss, Jsc and FF. Only poorly performing IDTBR-based devices show clear increase of Jsc with Delta HOMO, which, according to the transient absorption data, is caused by inefficient charge generation at low Delta HOMO attributable to high exciton binding energy or inferior molecule packing. At the same time, the opposite trend is observed for well-performing blends. These features suggest that variations of other blend parameters rather Delta HOMO cause variations of Jsc and FF overwhelming dependence on Delta HOMO.

In some applications, perovskite light‐emitting diodes (PeLEDs) shall operate in pulsed mode. The generation of high‐intensity light pulses requires PeLED driving by high‐power electrical pulses, which can lead to deterioration of PeLED performance and their degradation. Contrarily, PeLEDs operating in a nonconventional regime, based on the so‐called overshoot effect, enable the generation of short, high‐intensity optical pulses at relatively low driving pulse power. Here, the generation of overshoot pulses (OSPs) by FAPI PeLEDs is analyzed. The intensity and shape of the OSPs are determined not only by the driving (injection) pulse parameters but also by the offset voltage applied between the injection pulses and the afterpulse applied after the injection pulse. The offset voltage determines the distribution of the mobile ions, which strongly affect the internal electric field during the pulse action and after its termination, thus strongly affecting the evolution of the conventional electroluminescence (EL) and generation of the OSPs. Meanwhile, the afterpulse voltage controls the intensity and duration of the OSPs. The intensity of the OSPs increases strongly at temperatures below ≈200 K. Mathematical modeling reproduces the EL dynamics and reveals two distinct PeLED operation modes: one that facilitates OSP generation and another that prevents it.

Mixed tin-lead halide perovskites are considered promising materials for narrow-bandgap photovoltaic applications, particularly in tandem solar cells. However, their practical implementation is hindered by stability issues, especially due to tin oxidation and trap-state formation. In this study, we investigate the impact of argon, nitrogen, and oxygen storage environments on the structural, optical, and electronic properties of mixed tin-lead halide CsFAPb0.5Sn0.5I3 perovskites. Optical absorption, transient photoluminescence (PL), transient photocurrent, and time-delayed collection field (TDCF) measurements reveal the significant role of environmental conditions on carrier dynamics. Carrier trapping over tens of nanoseconds is observed in samples prepared and stored in argon, with a trapping rate increasing several times after exposure to nitrogen (with less than 0.1 ppm of oxygen) and further increasing upon exposure to O2. Photocurrent transients also show a fast photocurrent decay component occurring within tens of nanoseconds, independent of the oxygen-created traps. Based on the TDCF measurements, we attribute this fast photocurrent decay component to the spatial traps created by the perovskite boundaries, which reduce the carrier mobility to values below 0.05 cm2/V·s, as estimated from transient photocurrent measurements. Our findings highlight the importance of carefully controlling fabrication and storage conditions, often overlooked due to their initially minor impact on device performance, as these conditions critically affect material stability and charge carrier dynamics.

Understanding the dynamics of injected charge carriers is crucial for the analysis of the perovskite light-emitting diode (PeLED) operation. The behavior of the injected carriers largely dictates the external quantum efficiency (EQE) roll-off at high current densities and the temperature dependence of the EQE in PeLEDs. However, limitations such as sample capacitance and external circuitry hinder precise control of carrier injection rates, making it challenging to directly track the dynamics of individual carriers. Here, we explore the recombination dynamics of injected charge carriers in a small-grain methylammonium lead iodide (MAPI) PeLED pumped at high current densities by investigating the dynamics of additional carriers photogenerated by ultrashort optical pulses. We show that photogenerated charge carriers predominantly recombine in a geminate fashion within a single perovskite grain. Conversely, recombination between photogenerated and injected carriers is rare, even at current densities up to 100 A/cm2, due to the spatial separation caused by the internal electric field, which confines injected carriers near opposite electrodes. This spatial separation is a key mechanism behind the EQE roll-off in PeLEDs, with reduced carrier mobility at lower temperatures, mitigating this effect by weakening carrier localization and electron-hole separation.

We provide the first direct evidence of singlet fission occurring with water-soluble compounds. We show that perylene-3,4,9,10-tetracarboxylate forms dynamic dimers in aqueous solution, with lifetimes long enough to allow intermolecular processes such as singlet fission. As these are transient dimers rather than stable aggregates, they retain a significant degree of disorder. We performed a comprehensive analysis of such dynamic assemblies using time-resolved absorption and fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, and theoretical modelling, allowing us to observe the characteristic signatures of singlet fission and develop a model to characterize the different species observed. Our findings reveal that structure fluctuations within perylene associations are key in favoring either singlet fission or charge separation. The efficiency of triplet formation is higher than 100%, and the disordered system leads to triplets living in the nanosecond time range.

We provide direct evidence of singlet fission occurring with water-soluble compounds. We show that perylene-3,4,9,10-tetracarboxylate forms dynamic dimers in aqueous solution, with lifetimes long enough to allow intermolecular processes such as singlet fission. As these are transient dimers rather than stable aggregates, they retain a significant degree of disorder. We performed a comprehensive analysis of such dynamic assemblies using time-resolved absorption and fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, and theoretical modelling, allowing us to observe the characteristic signatures of singlet fission and develop a model to characterize the different species observed. Our findings reveal that structure fluctuations within perylene-3,4,9,10-tetracarboxylate associations are key in favoring either singlet fission or charge separation. The efficiency of triplet formation is higher than 100%, and the disordered system leads to triplets living in the nanosecond time range.

The photoluminescence (PL) properties of four types of blue fluorescent semi-aliphatic polyimides (PIs) derived from aromatic dianhydrides (ODPA, BPDA, HQDEA, and BPADA) and an alicyclic diamine (DCHM) were investigated at temperatures ranging from room temperature (RT, 298 K) to 30 K to analyse the origins of their non-radiative relaxation (NR) processes. These PIs exhibited significant increases in fluorescence (FL) intensity and lifetimes when lowering the temperature, stabilising below 100 K. The PIs containing ether (-O-) linkages showed a shoulder peak at around 500 nm below 150 K, which is attributable to phosphorescence (PH). These results show that the NR deactivation at RT includes three processes: intersystem crossing (ISC) from the excited singlet (S1) to the triplet (T1) state, temperature-dependent NR from the S1 state, which becomes suppressed below around 100 K, and temperature-independent NR. Based on the analyses of the temperature dependences, polymer structures, and quantum chemical analysis of molecular orbitals, we contemplate that the temperature-dependent NR is attributable to the excitation quenching by defect states mediated by excitation migration, and the temperature-independent NR may be caused by the deactivation of the excited state induced by molecular vibrations. The photoluminescence properties of four types of blue fluorescent semi-aliphatic polyimides (PIs) derived from aromatic dianhydrides and an alicyclic diamine were investigated between 298 K and 30 K to analyse the origins of their non-radiative relaxation (NR) processes.

Self-assembled monolayers (SAMs) deposited on the hole-collecting electrodes of p-i-n perovskite solar cells effectively replace bulky hole transporting layers. However, the mechanism by which monolayers control the electronic processes and how they depend on the properties of the monolayer molecules remain poorly understood. In this study, we developed a simplified perovskite solar cell imitator with blocked electron extraction to investigate the photocurrent dynamics between the perovskite and the hole-collecting ITO electrode. We investigated the photoluminescence and photovoltage dynamics under short laser pulse excitation and addressed the influence of bulky and monomolecular hole transport layers. Our findings reveal that the photovoltage dynamics is significantly affected by the properties of the transport and perovskite layers, which in turn depend on the methods of sample preparation and exploration. Photocurrent dynamics is determined by several processes, including charge carrier displacement in the local electric field, hole transport to ITO, trapping of holes in interface trap states, and electron-hole recombination at the interface. We propose a model that takes into account molecular dipole moments and their ionization potentials to partially explain the different influences of different monolayers on the hole extraction and interfacial recombination rates. Additionally, the photovoltage dynamics also strongly depends on the illumination of the sample and shows memory effects that persist over minutes and hours and are attributed to the redistribution of ions.

Correction for ‘Perylene-derivative singlet exciton fission in water solution’ by Chloe Magne et al., Chem. Sci., 2024, 15, 17831–17842, https://doi.org/10.1039/D4SC04732J.

In recent years, organic solar cells (OSCs) have shown high power efficiencies approaching 20%. However, the fundamental mechanisms of charge separation in these highly efficient devices have been a subject of intensive debates. Here, the charge separation efficiency (CSE) is extensively investigated across a wide range of blend systems with different energetic offsets. The findings unveil the temperature-dependent nature of charge separation in low-offset systems, emphasizing its significant contribution to the overall CSE. An intriguing inverse correlation between CSE and charge separation activation energy in relation to the offset is also observed. These results shed new light on the factors underlying the high CSE observed in the state-of-the-art devices.

Ytterbium-doped cesium lead halide perovskites exhibit unusual down-conversion with quantum efficiencies exceeding 100%. This phenomenon has been attributed to quantum cutting (QC) when a perovskite exciton is converted into two Yb3+ excitations. However, the mechanism of energy transfer from the perovskite to the Yb3+ ions is still debated, and it is still unclear what processes limit the conversion efficiency. Here, Yb3+-doped CsPbX3 (X: Cl-, Br-, or both) perovskite samples prepared by different techniques were investigated. Electron paramagnetic resonance (EPR) measurements were applied to assess ytterbium incorporation into the perovskite matrix and ultrafast time-resolved fluorescence, together with microscopic studies, to evaluate the QC efficiency limiting factors. It is demonstrated that two types of ytterbium species are formed, depending on the concentration of Yb3+. The photoluminescence (PL) of ytterbium shows a biexponential decay at low concentrations, attributed to the presence of monomeric and dimeric ytterbium species. The fast decay component is attributed to monomeric Yb3+ ions, which do not perform QC and have a low PL yield. This component disappears when the nominal concentration of ytterbium increases to about 5% and the PL of dimeric species dominates. The PL decay becomes faster again when the ytterbium concentration exceeds about 10%, due to migration limited excitation quenching. These two processes are important QC efficiency limiting factors. Monomeric and dimeric Yb species form in Yb-doped cesium lead halide perovskites. At low concentration monomers create a quantum-cutting efficiency loss channel, while diffusion-limited quenching of Yb excited states dominates at high concentration.