German Researchers Fabricate Black Perovskite Cells with 15% Efficiency

In a recent paper entitled “Efficient Thermally Evaporated γ-CsPbI3 Perovskite Solar Cells,” published this month in the journal Advanced Energy Materials, scientists report on the fabrication of a ‘black perovskite‘ PV device — a solar cell based on all-organic cesium-lead iodide (CsPbI3) — by thermal evaporation.

Researchers at the Dresden University of Technology (TUD) note that the physical vacuum deposition process, which is commonly used for the fabrication of organic light-emitting diodes for display and lighting applications, is challenging to use for the deposition of perovskite materials, complicated by the need to co-deposit several precursors from multiple sources while precisely controlling their deposition rates in order to obtain a high-quality perovskite film of the desired stoichiometric composition.

However, the scientists argue, thermal evaporation of perovskites offers many advantages over alternative methods based on solution-processing, in particular for industrial-scale production. Some of these advantages include the high purity provided by the sublimed materials, accurate and simple control over the layer thickness and the ease of uniform deposition over large areas. Moreover, the solvent-free nature of this deposition method offers further advantages, such as freedom of substrate selection, low temperature annealing, or even no annealing treatment needed for perovskite formation and finally noteworthy benefits in terms of sustainability of PSCs.

According to them, despite some progress made in the field of thermally evaporated perovskite solar cells, with 20.28% efficiency being the highest recorded so far, their performance still lags far behind that of their solution-processed counterparts. It is imperative to develop strategies to improve the perovskite film quality, which are suitable for the thermal evaporation process of perovskite materials.

In their work, the German researchers precisely control the crystallization process of black perovskite by co-evaporating small amounts of the ammonium salt phenethylammonium iodide (PEAI) alongside CsI and PbI2. Consequently, a stable γ-phase is formed at a low temperature of 100 °C, which indicates that the formation energy of γ-CsPbI3 is reduced with the addition of PEAI. The PEAI containing thermally evaporated γ-CsPbI3 perovskite films exhibit a far better film microstructure and a preferential crystal orientation.

The addition of PEAI prompts crystallization of uniform and ordered grains that protrude throughout the entire film’s thickness. Consequently, the defects in the PEAI containing γ-CsPbI3 layers are substantially reduced, leading to a cell efficiency of 15% for the optimized devices. Moreover, this performance surpasses that reported for solution-processed PEAI containing γ-CsPbI3 PSCs based on the same composition, illustrating that thermally evaporated PSCs can compete and even outperform most PSCs deposited from solution, say the researchers.

Finally, they demonstrate the enhancement in the stability of the PEAI containing γ-CsPbI3 PCSs, which maintain ≈90% of their initial performance after 215 days of exposure to dim light at room temperature.

The scientists conclude that the incorporation of PEAI leads to a significantly improved microstructure with columnar grains and a preferred crystalline orientation. The PEAI containing perovskite layers exhibit suppressed nonradiative recombination losses due to a reduced density of traps. Consequently, highly efficient devices with a maximum PCE of 15% with an enhanced stability are demonstrated, opening the route for mass production of γ-CsPbI3 perovskite solar cells by thermal evaporation.

This research was financially supported by the China Scholarship Council, the European Regional Development Fund (ERDF), the Free State of Saxony, and the European Research Council (ERC). The paper was authored by the following researchers: Zongbao Zhang, Ran Ji, Martin Kroll, Yvonne J. Hofstetter, Xiangkun Jia, David Becker-Koch, Fabian Paulus, Markus Löffler, Frederik Nehm, Karl Leo, and Yana Vaynzof.