Recent years have witnessed considerable progress in the development of solar cells based on lead halide perovskite materials. However, their intrinsic instability remains a limitation. In this context, the interplay between the thermal degradation and the hydrophobicity of perovskite materials is investigated. To this end, the salt 1-(4-ethenylbenzyl)-3-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylimidazolium iodide (ETI), is employed as an additive in hybrid perovskites, endowing the photoactive materials with high thermal stability and hydrophobicity. The ETI additive inhibits methylammonium (MA) permeation in methylammonium lead triiodide (MAPbI 3 ) occurring due to intrinsic thermal degradation, by inhibiting out-diffusion of the MA + cation, preserving the pristine material and preventing decomposition. With this simple approach, high efficiency solar cells based on the unstable MAPbI 3 perovskite are markedly stabilized under maximum power point tracking, leading to greater than twice the preserved efficiency after 700 h of continuous light illumination and heating (60 °C). These results suggest a strategy to tackle the intrinsic thermal decomposition of MAI, an essential component in all state-of-the-art perovskite compositions.

Understanding of charge trapping processes in halide perovskites is vital to further improve performance of perovskite optoelectronic devices such as solar cells, photodetectors, and LEDs. In this work, transient photocurrent, time‐delayed collection field and transient fluorescence techniques along with numerical simulations are combined to address charge carrier trapping processes during their lateral motion in prototypical methylammonium lead iodide perovskite films formed on interdigitated electrodes. Carrier mobility decreases on hundreds of ns timescale, and its rate depends on the motion character—it is faster when charge carriers drift in the electric field and slower when the motion is caused by diffusion only. This difference becomes particularly evident at low temperatures. Based on the time‐delayed collection field data and carrier motion modelling results, it is demonstrated that the rapid mobility decay at low temperatures is mainly caused by the energy barriers, most likely formed at crystallite boundaries. Even though these barriers are surmountable at room temperature, they still play a major role in determining carrier mobility and diffusion rates. Suggested concept of the potential barriers moves beyond the conventional understanding of carrier mobility, diffusion, and recombination processes in hybrid perovskites.

accepting material in one phase and electron donating material in the other. [2] However, such strongly phase-separated morphologies are prone to cause extraction problems due to the formation of dead ends and/or isolated domains. [3] Fortunately, in state-of-the-art polymer:fullerene and polymer:small molecule organic photovoltaic (OPV) devices performance is not limited by recombination losses during extraction (but mostly by voltage losses) as witnessed by large fill factors and relatively high internal quantum efficiency (IQE) values. [4] As a consequence, extraction of photocreated charge carriers can often be well described using models that consider the BHJ as an effective medium with the energy levels of the medium reflecting the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. [5,6] In such models, morphological effects are only implicitly accounted for by the values of the (effective) transport and disorder parameters. One may wonder why this evident simplification works so well. It has recently become clear that one important reason might be that charge carriers in organic semiconductors may actually be able to move over relatively large distances, up to several nm, by longrange tunneling or molecular superexchange. [7][8][9][10] This enables transport to non-nearest neighbor sites, and thereby greatly relaxes the need to have connected phases of pure material for efficient charge transport.OPV devices using non-fullerene acceptors are currently receiving increased attention and start to outperform fullerenebased devices. [11][12][13] While having evident benefits over fullerenes in terms of improved absorption and larger energy level tunability, charge transport and extraction in non-fullerene BHJ is still less understood. In particular all-polymer BHJ, in which both donor and acceptor materials are polymers, are in many respects in their infancy. While the entanglement of polymer chains in such BHJ is expected to lead to a much needed increased morphological stability at elevated temperatures inherent to solar cell operation, [14] the same entanglement can also lead to extraction problems. Such problems are less pronounced in BHJ where at least one of the constituents is a small molecule and dispersion of a minor fraction of the small molecule in the other compound is sufficient to enable reasonably efficient charge transport between these molecules. [7,9] However, the required "molecular" dispersion, in which sites of Extraction of photocreated charge carriers from a prototypical all-polymer organic solar cell is investigated by combining transient photocurrent and time-delayed collection field experiments with numerical simulations. It is found that extraction is significantly hampered by charges getting trapped in spatial traps that are tentatively attributed to dead ends in the intermixed polymer network-in photovoltaic devices based on the same donor polymer and a fullerene acceptor this effect is much weaker. The slow-dow...

Non-fullerene organic solar cells (NFOSCs) demonstrate record efficiencies exceeding 16%. In comparison with cells with the fullerene-based acceptor, the NFOSCs benefit from a longer wavelength absorption, which leads to a small highest occupied molecular orbital (HOMO) level offset. Here, we use several advanced transient investigation techniques, covering timescale from sub-ps to μs, to address all sequence of processes starting from photoexcitation of donors or acceptors to carrier extraction in several NFOSCs and cells with phenyl-C71-butyric acid methyl ester (PCBM). Though small offsets result in higher open-circuit voltage, we show that at the same time, it limits cell performance because of inefficient hole transfer from excited acceptors to donors and enhanced geminate recombination. We demonstrate that 100 meV HOMO level offset and proper acceptor molecule structures are sufficient for efficient hole transfer (>70%) and for reduction of the geminate recombination losses down to about 20%. Subsequent extraction of unbound charge carriers in all NFOSCs is slightly faster than in cells with PCBM, while non-geminate carrier recombination causing additional losses is slightly slower in the best performing NFOSCs than in cells with PCBM.

Transient absorption and time‐resolved fluorescence measurements in a wide temperature range are used to investigate the mechanism of charge carrier generation in efficient organic solar cells based on a PM6:Y6 donor–acceptor blend. The generation mechanisms differ significantly under excitation of a donor or acceptor. The investigations reveal a temperature‐dependent interplay between the formation of interfacial charge transfer (CT) states and intra‐moiety CT states of the acceptor, their separation into free charge carriers and carrier recombination. The efficient charge carrier generation is ensured by the carrier separation over a small energy barrier, which is easily surmountable at room temperature. However, the overall yield of charge carrier generation at room temperature is reduced by the recombination of charge carriers due to the thermally activated back transfer of electrons from the acceptor to the donor via the highest occupied molecular orbit (HOMO) levels, which is enabled by the small energy offset between HOMO levels of the donor and the acceptor.