Kaiwen Sun

Enhancing the Cu2ZnSnS4 solar cell efficiency by back contact modification: Inserting a thin TiB2 intermediate layer at Cu2ZnSnS4/Mo interface

In this work, TiB2 thin films have been employed as intermediate layer between absorber and back contact in Cu2ZnSnS4 (CZTS) thin film solar cells for interface optimization. It is found that the TiB2 intermediate layer can significantly inhibit the formation of MoS2 layer at absorber/back contact interface region, greatly reduces the series resistance and thereby increases the device efficiency by short current density (Jsc) and fill factor boost. However, introducing TiB2 degrades the crystal quality of absorber, which is detrimental to device performance especially Voc. The careful control of the thickness of TiB2 intermediate layer is required to ensure both MoS2 with minimal thickness and CZTS absorber with large grain microstructure according to the absorber growth process.

Kesterite Cu2ZnSnS4 thin film solar cells by a facile DMF-based solution coating process

Kesterite Cu2ZnSnS4 (CZTS) thin films were fabricated using a low-cost and environmentally friendly route from a dimethylformamide (DMF) solution of a metal–thiourea complex. Thermal gravimetric analysis (TGA) has been performed to reveal the thermal decomposition behavior of the CZTS precursors for drying and sulfurization process design. A facile solution method of in situ introducing sodium dopant by adding NaOH into the precursor solution is presented. The sodium dopant improves the open circuit voltage (Voc) and fill factor (FF) and thereby enhances the power conversion efficiency from 4.47% to 5.68%. The enhanced performance is related to the increased grain size and increased minority carrier lifetime. A large number of large voids observed in the bulk absorber and at the absorber/back contact interface are considered to be the main reason for the low short circuit current density (Jsc).

Exploring the application of metastable wurtzite nanocrystals in pure-sulfide Cu2ZnSnS4 solar cells by forming nearly micron-sized large grains

An innovative approach to overcome the main challenge of solution-based pure-sulfide Cu2ZnSnS4 thin film solar cells by sulfurizing quaternary Cu2ZnSnS4 nanocrystals into nearly micron-sized large grains in a few minutes is presented. We developed an efficient phase-transition-driven grain growth strategy to explore the application of metastable wurtzite Cu2ZnSnS4 nano-materials in the photovoltaic field. The obtained Cu2ZnSnS4 thin film has a typical bilayer microstructure containing large grains on the top and fine grains at the bottom. Clear variations of phase, morphology, and component redistribution of the Cu2ZnSnS4 thin film were identified after the sulfurization process, which is critical to get a dense large-grained Cu2ZnSnS4 layer from quaternary nanocrystals. By tuning the composition of the wurtzite Cu2ZnSnS4 nanocrystals, annealing conditions, and sodium-containing compound, laboratory-scale photovoltaic cells with 4.83% efficiency were demonstrated without anti-reflection coatings. These results suggest the potential application of metastable wurtzite nanocrystals in pure-sulfide Cu2ZnSnS4 solar cells. This unique approach may also open up new opportunities to other optoelectronic devices, such as CuIn(S,Se)2, Cu2(In,Ga)Se4, CdTe, and Cu2ZnGe(S,Se)4 solar cells.

Famatinite Cu3SbS4 nanocrystals as hole transporting material for efficient perovskite solar cells

To overcome the weakness of the organic hole transporting material in perovskite solar cells (PSCs), we developed inorganic copper antimony sulfide (Cu3SbS4) nanocrystals as a hole transporting material (HTM) for PSCs by a hot-injection and spray-deposition technique. We proved that the Cu3SbS4 nanocrystal layer can enhance hole injection inhibiting the charge-carrier recombination within the perovskite layer. A power conversion efficiency (PCE) of 8.7% was obtained for the Cu3SbS4-based devices with improved stability. This work offers a new route which employs copper antimony sulfide compounds as hole transporting materials for efficient and stable perovskite solar cells.

Ionic liquid modified SnO2 nanocrystals as a robust electron transporting layer for efficient planar perovskite solar cells

Control over charge carrier transport in a low-temperature processed device is of key significance to realize high-performance perovskite solar cells (PSCs) and tandem solar cells. For low-temperature processed perovskite devices, a great challenge still remains due to the commonly inferior crystallinity and poor electron mobility of low-temperature processed electron transport materials. Meanwhile, electron transport layers (ETLs) produced at low-temperature show poor capability of managing the quality of overlying perovskite films, leaving abundant defects at grain boundaries, which hinder the efficient charge carrier transport or even result in severe energy loss by trap-assisted recombination. Here we present highly efficient PSCs realized by employing a tetramethylammonium hydroxide (TMAH) modified SnO2 ETL prepared at low-temperature (100–150 °C). TMAH modified SnO2 significantly enhances not only the conductivity of the SnO2 ETL for efficient electron extraction but also the electronic properties of the overlying perovskite film for fast electron transport across the grain boundaries. With this proposed novel ETL, an average efficiency above 20% is achieved for the low-temperature-processed PSCs, with an even higher efficiency exceeding 21% for the champion device. These low-temperature processed PSC devices also show reliable reproducibility and stability.