Reproducibility issues of perovskite materials and devices stem from their extreme sensitivity to fractional, quite possibly unintentional, deviations in the precursor solution stoichiometry.

Solution-processed quantum dots (QDs) have a high potential for fabricating low cost, flexible and large-scale solar energy harvesting devices. It has recently been demonstrated that hybrid devices employing a single monovalent cation perovskite solution for PbS QD surface passivation exhibit enhanced photovoltaic performance when compared to standard ligand passivation. Herein we demonstrate that the use of a triple cation Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3 perovskite composition for surface passivation of the quantum dots results in highly efficient solar cells, which maintain 96 % of their initial performance after 1200h shelf storage. We confirm perovskite shell formation around the PbS nanocrystals by a range of spectroscopic techniques as well as high-resolution transmission electron microscopy. We find that the triple cation shell results in a favorable energetic alignment to the core of the dot, resulting in reduced recombination due to charge confinement without limiting transport in the active layer. Consequently, photovoltaic devices fabricated via a single-step film deposition reached a maximum AM1.5G power conversion efficiency of 11.3 % surpassing most previous reports of PbS solar cells employing perovskite passivation.

Understanding the photophysics of charge generation in organic semiconductors is a critical step toward the further optimization of organic solar cells. The separation of electron–hole pairs in systems with large energy offsets is relatively well-understood; however, the photophysics in blends with low driving energy remains unclear. Herein, we use the material system PffBT4T-2OD:PC71BM as an example to show that the built-in electric field plays a critical role toward long-range charge separation in high-performance devices. By using steady-state and time-resolved spectroscopic techniques, we show that in neat films an energetic barrier impedes polymer exciton dissociation, preventing charge transfer to the fullerene acceptor. In complete devices, this barrier is diminished due to the built-in electric field provided by the interlayers/contacts and accompanying space-charge distribution. The observed behavior could also be relevant to other systems with low driving energy and emphasizes the importance of using complete devices, rather than solely films, for photophysical studies.

The stability of lead sulfide (PbS) quantum dots (QD) under continuous illumination in oxygenated environments depends on the choice of ligands, determining the evolution of photovoltaic performance of high efficiency PbS QD solar cells.

The internal luminescence quantum efficiency (Q lum i ) provides an excellent assessment of the optoelectronic quality of semiconductors. To determine Q lum i from the experimentally accessible external luminescence quantum efficiency (Q lum e ), it is essential to account for photon recycling, and this requires knowledge of the photon escape probability (p e ). Here, we establish an analysis procedure based on a curve-fitting model that accurately determines p e of perovskite films from photoluminescence (PL) spectra measured with a confocal microscope and an integrating sphere setup. We show that scattering-induced outcoupling of initially trapped PL explains commonly observed red-shifted and broadened PL spectral shapes and leads to p e being more than a factor of two higher compared with earlier assumptions. Applying our model to CH 3 NH 3 PbI 3 films with exceptionally high Q lum e up to 47.4% corrects previous estimates for Q lum i of $90% to a real benchmark of 78.0% G 0.5%. Thereby, our study reveals there is beyond a factor of two more scope for reducing non-radiative recombination in perovskite films than previously thought.

Bulk-heterojunction (BHJ) non-fullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq-BHJ) have recently been promising to be highly efficient, environmentally friendly, and compatible with large area and roll-to-roll fabrication. However, the related photophysics at donor-acceptor interface and the vertical heterogeneity of donor-acceptor distribution, critical for exciton dissociation and device 2 performance, are largely unexplored. Herein, steady-state and time-resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlating with the luminescent efficiency of interfacial states and its non-radiative recombination, interfacial trap states are characterized to be about 50% more populated in the sq-BHJ devices than the ascast BHJ (c-BHJ), which probably limits the device voltage output. Cross-sectional energydispersive X-ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly visualize the donor-acceptor vertical stratification with a precision of 1-2 nm. From the proposed "needle" model, the high exciton dissociation efficiency is rationalized. Our study highlights the promise of sequential deposition to fabricate efficient solar cells, and points towards improving the voltage output and overall device performance via eliminating interfacial trap states.

Despite tremendous advances in improving the efficiency of organic solar cells above 14%, the environmental stability of such devices remains an essential and widely inadequately addressed challenge. Understanding the underlying principles of device degradation is a critical step toward further development and commercialization of organic photovoltaics. Herein, we report on the effect of oxygen exposure on the operation and degradation of highly efficient PffBT4T-2OD:PC 71 BM photovoltaic devices. Ultrafast pump−probe transient absorption (TA) measurements and ultrasensitive photothermal deflection spectroscopy (PDS) in combination with field-effect transistors suggest that oxygen-induced doping of the active layer is responsible for the severe degradation of the photovoltaic performance. We find that light exposure further accelerates this effect without causing photo-oxidation of the materials.

In this work, we examine the effect of microstructure on ion-migrationinduced photoluminescence (PL) quenching in methylammonium lead iodide perovskite films. Thin films were fabricated by two methods: spin-coating, which results in randomly oriented perovskite grains, and zone-casting, which results in aligned grains. As an external bias is applied to these films, migration of ions causes a quenching of the PL signal in the vicinity of the anode. The evolution of this PLquenched zone is less uniform in the spin-coated devices than in the zone-cast ones, suggesting that the relative orientation of the crystal grains plays a significant role in the migration of ions within polycrystalline perovskite. We simulate this effect via a simple Ising model of ionic motion across grains in the perovskite thin film. The results of this simulation align closely with the observed experimental results, further solidifying the correlation between crystal grain orientation and the rate of ionic transport.