Kyung Taek Cho

We reveal that 2D/3D interfaces are dynamical in nature which is detrimental for long term perovskite solar cells stability.

Surface passivation of the perovskite photo absorber is a key factor to improve the photovoltaic performance. So far robust passivation strategies have not yet been revealed. Here, we demonstrate a successful passivation strategy which controls the Fermi-level of the perovskite surface by improving the surface states. Such Fermi-level control caused band-bending between the surface and bulk of the perovskite, which enhanced the hole-extraction from the absorber bulk to the HTM side. As an added benefit, the inorganic passivation layer improved the device light stability. By depositing a thick protection layer on the complete device, a remarkable waterproofing effect was obtained. As a result, an enhancement of V_OC and the conversion efficiency from 20.5% to 22.1% was achieved. We revealed these passivation mechanisms and used perhydropoly(silazane) (PHPS) derived silica to control the perovskite surface states.

Herein, the optoelectronic properties of interface‐engineered perovskite 2D|3D‐heterojunction structure solar cells are reported. The reciprocity theorem is applied to determine the maximum open‐circuit voltage (V_oc) the device can deliver under solar illumination. A V_oc of 1.295 V is found, analyzing the measured external quantum efficiency and assuming only radiative recombination. For comparison, the experimental open‐circuit voltage found for the studied 2D|3D heterojunctions is 1.15 V. The contribution of nonradiative recombination is explored by measuring the electroluminescence quantum yield. A quantum yield of 0.4% is found at current densities equivalent to 1 sun illumination. This translates into a V_oc loss of ≈140 mV, which is in very good agreement with the experimental findings. In addition, the fundamental correlation between luminescence intensity and the chemical potential predicted by the generalized Planck law is confirmed for the photoluminescence measured at different light intensities when the device is operated under open‐circuit conditions and for the electroluminescence when operated under a forward bias. The investigations in this study suggest that further efficiency improvements can be achieved by reducing the nonradiative recombination in the studied solar cell. At the same time, a high‐performance near IR light emitting diode can be realized.

Organic–inorganic lead halide perovskites with over 23% power conversion efficiency have attracted enormous academic and industrial attention due to their low-cost fabrication and high device performance. Self-passivated tin oxide as an electron transport layer has shown potential mainly due to the enhanced electron transfer, stability and reduced hysteresis device features. Here we report on novel, non-colloidal tin oxide precursors based on acetylacetonate (one halide free and two halogenated with Cl and Br respectively). We explore the unique film morphology acquired from the non-colloidal precursors and the improved device performance they yield. Our results show that the halide residue in the films plays an impactful role in the thermal durability of the fabricated SnO₂ film, as well as providing a passivation layer. Moreover, our optimized tin oxide films achieved an unprecedented power conversion efficiency of 22.19% in planar perovskite solar cells (21.4% certified by Newport), and once upscaled to large-area modules, 16.7% devices based on a 15 cm² area were achieved.

The search for new classes of hole transporting materials (HTMs) is a very important task on the way towards commercialization of perovskite solar cells (PSCs). In this work, the synthesis and performance in PSCs of new enamine-based HTMs are presented. The synthesis scheme is very short, consisting of only one step, starting from commercially available aromatic amines. Moreover, the reaction does not require metal catalysts and is based on condensation chemistry with water as a by-product. It was shown that PSCs with such materials reach high power conversion efficiencies (PCE), with the highest PCE of 19% achieved for the V1021-based device. This work establishes enamines as a very promising class of materials for application in PSCs.

We describe how the use of photo-induced charge extraction technique can be useful to study and understand the relationship between perovskite composition, ion reorganization process and solar cell efficiency.

We achieved improvement in performance and stability of PSCs by growing 2D perovskite layer on top of 3D perovskite films.

The morphological, opto-physical and electrical properties of the SnO₂layer in perovskite solar cells are investigated.

Through judicious molecular engineering, novel dopant-free star-shaped D–π–A type hole transporting materials coded KR355, KR321, and KR353 were systematically designed, synthesized and characterized. KR321 has been revealed to form a particular face-on organization on perovskite films favoring vertical charge carrier transport and for the first time, we show that this particular molecular stacking feature resulted in a power conversion efficiency over 19% in combination with mixed-perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15. The obtained 19% efficiency using a pristine hole transporting layer without any chemical additives or doping is the highest, establishing that the molecular engineering of a planar donor core, π-spacer and periphery acceptor leads to high mobility, and the design provides useful insight into the synthesis of next-generation HTMs for perovskite solar cells and optoelectronic applications.