The structural phases and optoelectronic properties of coevaporated CsPbI3 thin films with a wide range of [CsI]/[PbI2] compositional ratios are investigated using high throughput experimentation and gradient samples. It is found that for CsI‐rich growth conditions, CsPbI3 can be synthesized directly at low temperature into the distorted perovskite γ‐CsPbI3 phase without detectable secondary phases. In contrast, PbI2‐rich growth conditions are found to lead to the non‐perovskite δ‐phase. Photoluminescence spectroscopy and optical‐pump THz‐probe mapping show carrier lifetimes larger than 75 ns and charge carrier (sum) mobilities larger than 60 cm2 V−1 s−1 for the γ‐phase, indicating their suitability for high efficiency solar cells. The dependence of the carrier mobilities and luminescence peak energy on the Cs‐content in the films indicates the presence of Schottky defect pairs, which may cause the stabilization of the γ‐phase. Building on these results, p–i–n type solar cells with a maximum efficiency exceeding 12% and high shelf stability of more than 1200 h are demonstrated, which in the future could still be significantly improved, judging on their bulk optoelectronic properties.

Optical in situ monitoring tracks crystallization and optoelectronic properties of halide perovskites during growth in a glovebox environment.

The three-dimensional carrier confinement in GaN nanodiscs embedded in GaN/Al x Ga 1−x N nanowires and its effect on their photoluminescence properties is analyzed for Al concentrations between x = 0.08 and 1. Structural analysis by high-resolution transmission electron microscopy reveals the presence of a lateral Al x Ga 1−x N shell due to a composition-dependent lateral growth rate of the barrier material. The structural properties are used as input parameters for three-dimensional numerical simulations of the confinement that show that the presence of the Al x Ga 1−x N shell has to be considered to explain the observed dependence of the emission energy on the Al concentration in the barrier. The simulations reveal that the maximum in the emission energy for x ≈ 30% is assigned to the smallest lateral strain gradient and, consequently, the lowest radial internal electric fields in the nanodiscs. Higher Al concentrations in the barrier cause high radial electric fields that can overcome the exciton binding energy and result in substantially reduced emission intensities. Effects of polarization-induced axial internal electric fields on the photoluminescence characteristics have been investigated using nanowire samples with nanodisc heights ranging between 1.2 and 3.5 nm at different Al concentrations. The influence of the quantum confined Stark effect is significantly reduced compared to GaN/Al x Ga 1−x N quantum-well structures, which is attributed to the formation of misfit dislocations at the heterointerfaces, which weakens the internal electric polarization fields.

No abstract

The incorporation of even small amounts of strontium (Sr) into lead-based quadruple cation hybrid perovskite solar cells results in a systematic increase of the open circuit voltage (Voc) in pin-type perovskite solar cells. We demonstrate via transient and absolute photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer and specifically at the perovskite/C60 interface. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C60 electron-transporting layer. The resulting solar cells exhibited a Voc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, bringing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high Voc values and efficiencies in Srcontaining quadruple cation perovskite pin solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer. IntroductionOrganic-inorganic halide perovskites are considered one of the most promising materials for photovoltaic applications due to their rather easy and low-cost fabrication, as well as outstanding optoelectronic properties. Notably, these semiconductors combine a high absorption coefficient with panchromatic absorption of light 1 with a long carrier diffusion length 2,3 , allowing efficient photon absorption and charge extraction for a typical active layer thickness of only 500 nm. Another peculiarity of hybrid perovskites is that defects create mostly shallow energy levels, allowing high open circuit voltage (Voc) and long carrier lifetime 4 . Moreover, perovskite materials can be obtained from in-nature abundant precursors, which potentially reduce further the costs of future large scale production. Despite the fact that the first full solid state perovskite solar cell was reported only in 2012, with a power conversion efficiency (PCE) of 9.7% 5 , this technology has experienced a tremendous improvement 6-8 , currently reaching a record PCE of 22.7% 9 . Regardless of the state of the art of perovskite solar cells and their astonishing performances, the metrics fill factor (FF) andVoc are currently still limiting their PCE. Thus, in order to achiev...