Weijun Ke

Employing Lead Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells.

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

Efficient fully-vacuum-processed perovskite solar cells using copper phthalocyanine as hole selective layers

The state-of-the-art organic–inorganic lead halide perovskite solar cells use organic materials as hole selective layers that are unstable, thermally and chemically, and expensive. Here we demonstrate efficient room-temperature fully-vacuum-processed planar perovskite solar cells using small-molecule organic copper phthalocyanine (CuPc), which has chemical and thermal stability while also being relatively inexpensive, as the hole selective material. The best-performing planar perovskite solar cell using a CuPc hole selective layer achieved a power conversion efficiency of 15.42% with an open circuit voltage of 1.04 V and a fill factor of 77.47%, measured under reverse voltage scanning, and a steady-state efficiency of 14.5%. The cell showed improved stability as compared to reference cells using conventional organic hole selective layers. Our results suggest that thermally evaporated CuPc is an excellent hole selective material for fabricating large-area and low-cost flexible perovskite solar cells with good long-term stability.

Photovoltaic Properties of Two-Dimensional (CH3NH3)2Pb(SCN)2I2 Perovskite: A Combined Experimental and Density Functional Theory Study

We explore the photovoltaic-relevant properties of the 2D MA2Pb(SCN)2I2 (where MA = CH3NH3(+)) perovskite using a combination of materials synthesis, characterization and density functional theory calculation, and determine electronic properties of MA2Pb(SCN)2I2 that are significantly different from those previously reported in literature. The layered perovskite with mixed-anions exhibits an indirect bandgap of ∼2.04 eV, with a slightly larger direct bandgap of ∼2.11 eV. The carriers (both electrons and holes) are also found to be confined within the 2D layers. Our results suggest that the 2D MA2Pb(SCN)2I2 perovskite may not be among the most promising absorbers for efficient single-junction solar cell applications; however, use as an absorber for the top cell of a tandem solar cell may still be a possibility if films are grown with the 2D layers aligned perpendicular to the substrates.

Effects of annealing temperature of tin oxide electron selective layers on the performance of perovskite solar cells

Efficient lead halide perovskite solar cells have been realized using SnO2 as electron selective layers (ESLs). Here, we report on the effects of the annealing temperature of solution-processed SnO2 ESLs on the performance of perovskite solar cells. We find that the cells using low-temperature annealed SnO2 (LT-SnO2) ESLs outperform the cells using high-temperature annealed SnO2 (HT-SnO2) ESLs, exhibiting higher open circuit voltages and fill factors. Structural, electrical, optical, and electrochemical characterizations reveal the origin of the performance differences: LT-SnO2 produces better film coverage, wider band gap, and lower electron density than that of HT-SnO2. The confluence of these properties results in more effective transportation of electrons and blocking of holes, leading to lower interface recombination. Therefore, LT-SnO2 ESLs are preferred for manufacturing perovskite solar cells on flexible substrates.

Efficient planar perovskite solar cells using room-temperature vacuum-processed C60 electron selective layers

Efficient organic–inorganic lead halide perovskite solar cells with regular structure typically use solution-processed electron selective layers. Here, we demonstrated efficient planar perovskite solar cells using vacuum-processed C60 compact electron selective layers at room temperature. The best-performing planar perovskite solar cell has shown a power conversion efficiency of 15.14% with an open circuit voltage of 1.08 V and a fill factor of 74.51% measured under reverse voltage scanning. Our results suggest that vacuum-processed fullerene electron selective layer is a good candidate for fabricating all perovskite tandem solar cells.

Cooperative tin oxide fullerene electron selective layers for high-performance planar perovskite solar cells

Both tin oxide (SnO2) and fullerenes have been reported as electron selective layers (ESLs) for producing efficient lead halide perovskite solar cells. Here, we report that SnO2 and fullerenes can work cooperatively to further boost the performance of perovskite solar cells. We find that fullerenes can be redissolved during perovskite deposition, allowing ultra-thin fullerenes to be retained at the interface and some dissolved fullerenes infiltrate into perovskite grain boundaries. The SnO2 layer blocks holes effectively; whereas, the fullerenes promote electron transfer and passivate both the SnO2/perovskite interface and perovskite grain boundaries. With careful device optimization, the best-performing planar perovskite solar cell using a fullerene passivated SnO2 ESL has achieved a steady-state efficiency of 17.75% and a power conversion efficiency of 19.12% with an open circuit voltage of 1.12 V, a short-circuit current density of 22.61 mA cm−2, and a fill factor of 75.8% when measured under reverse voltage scanning. We find that the partial dissolving of fullerenes during perovskite deposition is the key for fabricating high-performance perovskite solar cells based on metal oxide/fullerene ESLs.

Reducing Hysteresis and Enhancing Performance of Perovskite Solar Cells Using Low-Temperature Processed Y-Doped SnO2 Nanosheets as Electron Selective Layers

Despite the rapid increase of efficiency, perovskite solar cells (PSCs) still face some challenges, one of which is the current-voltage hysteresis. Herein, it is reported that yttrium-doped tin dioxide (Y-SnO2 ) electron selective layer (ESL) synthesized by an in situ hydrothermal growth process at 95 °C can significantly reduce the hysteresis and improve the performance of PSCs. Comparison studies reveal two main effects of Y doping of SnO2 ESLs: (1) it promotes the formation of well-aligned and more homogeneous distribution of SnO2 nanosheet arrays (NSAs), which allows better perovskite infiltration, better contacts of perovskite with SnO2 nanosheets, and improves electron transfer from perovskite to ESL; (2) it enlarges the band gap and upshifts the band energy levels, resulting in better energy level alignment with perovskite and reduced charge recombination at NSA/perovskite interfaces. As a result, PSCs using Y-SnO2 NSA ESLs exhibit much less hysteresis and better performance compared with the cells using pristine SnO2 NSA ESLs. The champion cell using Y-SnO2 NSA ESL achieves a photovoltaic conversion efficiency of 17.29% (16.97%) when measured under reverse (forward) voltage scanning and a steady-state efficiency of 16.25%. The results suggest that low-temperature hydrothermal-synthesized Y-SnO2 NSA is a promising ESL for fabricating efficient and hysteresis-less PSC.

Enhanced photovoltaic performance and stability with a new type of hollow 3D perovskite {en}FASnI3

High-performance and good-stability hollow Sn-based perovskite solar cells using ethylenediammonium and formamidinium cations. Perovskite solar cells have revolutionized the fabrication of solution-processable solar cells. The presence of lead in the devices makes this technology less attractive, and alternative metals in perovskites are being researched as suitable alternatives. We demonstrate a new type of tin-based perovskite absorber that incorporates both ethylenediammonium (en) and formamidinium (FA), forming new materials of the type {en}FASnI3. The three-dimensional ASnI3 structure is stable only with methylammonium, FA, and Cs cations, and the bandgap can be tuned with solid solutions, such as ASnI3−xBrx. We show that en can serve as a new A cation capable of achieving marked increases in the bandgap without the need for solid solutions. The en introduces a new bandgap tuning mechanism that arises from massive Schottky style defects. In addition, incorporation of the en cation in the structure markedly increases the air stability and improves the photoelectric properties of the tin-based perovskite absorbers. Our best-performing {en}FASnI3 solar cell has the highest efficiency of 7.14%, which is achieved for a lead-free perovskite cell, and retains 96% of its initial efficiency after aging over 1000 hours with encapsulation. Our results introduce a new approach for improving the performance and stability of tin-based, lead-free perovskite solar cells.

Dopant-Free Tetrakis-Triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells

Developing dopant-free hole transporting layers (HTLs) is critical in achieving high-performance and robust state-of-the-art perovskite photovoltaics, especially for the air-sensitive tin-based perovskite systems. The commonly used HTLs require hygroscopic dopants and additives for optimal performance, which adds extra cost to manufacturing and limits long-term device stability. Here we demonstrate the use of a novel tetrakis-triphenylamine (TPE) small molecule prepared by a facile synthetic route as a superior dopant-free HTL for lead-free tin-based perovskite solar cells. The best-performing tin iodide perovskite cells employing the novel mixed-cation ethylenediammonium/formamidinium with the dopant-free TPE HTL achieve a power conversion efficiency as high as 7.23%, ascribed to the HTLs suitable band alignment and excellent hole extraction/collection properties. This efficiency is one of the highest reported so far for tin halide perovskite systems, highlighting potential application of TPE HTL material in low-cost high-performance tin-based perovskite solar cells.

Improved Performance of Electroplated CZTS Thin-Film Solar Cells with Bifacial Configuration

Annealing in S vapor greatly improves the performance of electroplated Cu2 ZnSnS4 (CZTS) solar cells based on the bifacial configuration of Al-doped ZnO (AZO, front contact)/ZnO/CdS/CZTS/indium tin oxide (ITO, back contact), as compared to H2 S annealing in our previous works. S-vapor annealing does not cause severe damage to the conductivity of the ITO back contact. The highest device efficiency of 5.8 % was reached under 1 sun illumination from the AZO side. The well-preformed devices based on the ITO back contact demonstrate smaller series resistances and better fill factors, as compared to our substrate-type devices using Mo back contacts. An interfacial reaction at the ITO back contact has been revealed in experiments, which contributes to the formation of SnO2 -enriched interfacial layer and diffusion of In from ITO into CZTS through the Sn sites. Incorporation of In does not significantly change the optical and structural properties or the grain size of CZTS absorbers.