• Energy Research

Hybrid lead halide perovskites have reached comparable efficiencies to state-of-the-art silicon solar cell technologies. However, a remaining key challenge toward commercialization is the resolution of the perovskite device instability. In this work, we identify for the first time the mobile nature of bis(trifluoromethanesulfonyl)imide (TFSI-), a typical anion extensively employed in p-type dopants for 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'spirofluorene (spiro-OMeTAD). We demonstrate that TFSI- can migrate through the perovskite layer via the grain boundaries and accumulate at the perovskite/electron-transporting layer (ETL) interface. Our findings reveal that the migration of TFSI- enhances the device performance and stability, resulting in highly stable p-i-n cells that retain 90% of their initial performance after 1600 h of continuous testing. Our systematic study, which targeted the effect of the nature of the dopant and its concentration, also shows that TFSI- acts as a dynamic defect-healing agent, which self-passivates the perovskite crystal defects during the migration process and thereby decreases nonradiative recombination pathways.

Methylammonium lead halide perovskites have emerged as high performance photovoltaic materials. Most of these solar cells are prepared via solution-processing and record efficiencies (>20%) have been obtained employing perovskites with mixed halides and organic cations on (mesoscopic) metal oxides. Here, we demonstrate fully vacuum deposited planar perovskite solar cells by depositing methylammonium lead iodide in between intrinsic and doped organic charge transport molecules. Two configurations, one inverted with respect to the other, p-i-n and n-i-p, are prepared and optimized leading to planar solar cells without hysteresis and very high efficiencies, 16.5% and 20%, respectively. It is the first time that a direct comparison between these two opposite device configurations has been reported. These fully vacuum deposited solar cells, employing doped organic charge transport layers, validate for the first time vacuum based processing as a real alternative for perovskite solar cell preparation.

Hybrid lead halide perovskites typically form polycrystalline films that have multiple grain sizes and surface defects. A key engineering challenge toward commercialization is therefore the production of homogeneous, defect‐free large‐area devices achieving high efficiency. New market opportunities may arise from vacuum‐deposited perovskites if detailed understanding and control of crystal formation are available. Of the many factors that make reproducibility of device performance difficult, two variables are identified that have not yet been considered in detail: deposition speed and underlayer material selection. Herein, it is demonstrated that small changes in the perovskite growth rate (0.18–0.72 Å·s−1) substantially affect the preferred crystal orientation. Further, varying underlayer interfaces greatly influence the composition of the final perovskite and thus its energetic profile. The research aids control in fine‐tuning the perovskite film at the nanometer scale, which enables the reproducible fabrication of vertically aligned and micrometer‐sized grain features, highly demanded for in high‐quality semiconductors.

AbstractThe synthesis of three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold are reported. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps. The best performing material, HTM3, demonstrated 18.62 % PCE in PSCs, rivaling spiro‐OMeTAD in efficiency, and showing markedly superior long‐term stability in non‐encapsulated devices. In dopant‐free PSCs, HTM3 outperformed spiro‐OMeTAD by a factror of 1.6. The high glass‐transition temperature (Tg=176 °C) of HTM3 also suggests promising perspectives in device applications.

New Spiro‐OMeTAD analogues and the simpler “half” structures with the terminated methoxyphenyl and/or carbazolyl chromophores are successfully synthesized under Hartwig–Buchwald amination conditions using commercially available starting materials. New fluorene‐based hole‐transporting materials combined with suitable ionization energies properly align with the valence band of the perovskite absorber. Additionally, these compounds are amorphous, which is an advantage for the formation of homogenous films, as well as eliminate the possibility for films to crystallize during operation of the devices. The most efficient perovskite solar cells devices contain carbazolyl‐terminated Spiro‐OMeTAD analogue V1267 and reach a power conversion efficiency of 18.3%, along with a short‐circuit current density, open‐circuit voltage, and fill factor of 23.41 mA cm−2, 1.06 V, and 74.0%, respectively. Moreover, “half” structures with methoxyphenyl/carbazolyl fragments show excellent long‐term stability and outperform Spiro‐OMeTAD and, therefore, hold a great prospect for practical wide‐scale applications in optoelectronic devices.

In this work, we identify the importance of C60 and compact-TiO2 (cTiO2) as electron transport layers on the device performance of coevaporated n–i–p perovskite solar cells. We found (1) a synergetic effect between both layers when extracting the charges and (2) that optimization of the C60 layer is essential for obtaining devices with enhanced device performance. In particular, we found that a C60 layer of an optimum thickness (20 nm), an additional charge transport resistance is observed by impedance analysis, indicating that devices with nonoptimized C60 thickness could limit the fabrication of highly efficient perovskite solar cells. N.K., C.M., A.O.A., and F.F.-S. thank the European Union′s Horizon 2020 research nos. 764787 and 763977. H.K. acknowledges the support of the H2020 program for Solar-ERANET funding of the BOBTANDEM (2019-2022) A.A.S. and C.I. acknowledge the Swiss National Science Foundation (SNSF) funding through Synergia Grant EPISODE (grant no. CRSII5_171000). The authors thank the project German Research Foundation (DFG) (Projekt number 424101351)–Swiss National Foundation (SNF) (200021E_186390). A.O.A. and F.F.-S. acknowledge Ministerio de Economía y Competitividad (MINECO) from Spain under the project ENE2017-85087-C3-1-R and Generalitat Valenciana under the project PROMETEO/2020/028 for financial support.

Defects of metal-halide perovskites detrimentally influence the optoelectronic properties of the thin film and, ultimately, the photovoltaic performance of perovskite solar cells (PSCs). Especially, defect-mediated nonradiative recombination that occurs at the perovskite interface significantly limits the power conversion efficiency (PCE) of PSCs. In this regard, interfacial engineering or surface treatment of perovskites has become a viable strategy for reducing the density of surface defects, thereby improving the PCE of PSCs. Here, an organic molecule, tris(5-((tetrahydro-2H-pyran-2-yl)oxy)pentyl)phosphine oxide (THPPO), is synthesized and introduced as a defect passivation agent in PSCs. The P=O terminal group of THPPO, a Lewis base, can passivate perovskite surface defects such as undercoordinated Pb2+. Consequently, improvement of PCEs from 19.87 to 20.70% and from 5.84 to 13.31% are achieved in n−i−p PSCs and hole-transporting layer (HTL)-free PSCs, respectively.