Recently, Prof. Hairen Tan’s research group, working with Prof. Jia Zhu, Prof. Aidong Li of our department, and Prof. Chunfeng Zhang of School of Physics, realized high-efficiency monolithic all-perovskite tandem solar cells using perovskites with different band gaps. The power conversion efficiencies ( PCEs ) certified by a third-party testing agency are as high as 24.8%, which is currently the world record.The manuscript titled as Monolithic All-perovskite Tandem Solar Cells with 24.8% Efficiency Exploiting Comproportionation to Suppress Sn(II) Oxidation in Precursor Ink is published on the national top energy journal Nature Energy(https://www.nature.com/articles/s41560-019-0466-3).
Tandem solar cells are the most effective and feasible way to break through the efficiency limit of single junction cells and realize higher photoelectric conversion efficiency. By far, the highest conversion efficiency of multi-junction tandem batteries has exceeded 38%, surpassing the best single-junction batteries; however, the materials for realizing such high-efficiency tandem batteries include expensive III-V semiconductors. Due to the high cost, this type of tandem battery fail to realizea large-scale applications in ground power generation
The perovskite solar cell is a solar cell that utilizes a perovskite-type organic-inorganic hybrid metal halide semiconductor as a light-absorbing material. Organic-inorganic hybrid perovskite solar cells have attracted international attention and developed rapidly due to their outstanding advantages such as low cost, easy preparation, and excellent photoelectric performance. The power conversion efficiency has increased from 3.8% in 2009 to above 24% in 2019.Perovskite solar cells are also considered to be the low-cost, high-efficiency photovoltaic technique with the most promising applications in the next generation.
In the perovskite / perovskite tandem solar cell, the wide-bandgap perovskite is used as the top cell to absorb the short-wavelength sunlight, and the narrow-bandgap perovskite as the bottom cell to absorb the long-wavelength sunlight and the utilization of the solar spectrum is effectively improved. The thermal relaxation loss of carriers in the single-junction cell is also reduced, thereby improving the photoelectric conversion efficiency. The perovskite / perovskite tandem solar cell can be processed by full-solution method, with low energy consumption and simplicity.
In the perovskite / perovskite tandem solar cell, when the second layer of perovskite is prepared by the method of solution, the prepared first layer of perovskite is likely to be damaged. Therefore a compact solvent barrier layer is required between them to realize tandem series interconnection. In the tunnel junction used in most of the perovskite / perovskite double sided tandem batteries that have been reported so far, the compact layer and the tunneling recombination layer are made of tin-doped indium oxide (ITO) prepared by sputtering. Generally, the ITO layer with a thickness up to 100 nanometerscan serve as a solvent barrier. But as the thickness of ITO increases, in addition to theincrease of its parasitic absorption, the expense also rises. Moreover, ITO has good electrical conductivity, and it is easy to cause short circuit of adjacent batteries in the preparation of battery. In addition, although the narrow band gap perovskite based on lead-tin ion blending is an ideal material for the preparation of the light absorbing layer of all-perovskite tandem batteries, the divalent tin ion in the narrow band gap perovskite containing tin can be oxidized, resulting in the high density of surface and bulk defects, and short carrier diffusion length. The efficiency of the narrow band gap perovskite solar cell is still low, and the short circuit current density is too small, which limit the development of the perovskite tandem solar cell.
To address the above two key scientific and technical problems, on the one hand, this work proposes a new tunnel junction structure with a compact layer and a metal layer (as shown in Fig. a), using atomic layer deposition technology to prepare a compact SnO2 layer (about 20 nanometers thick), which well solves the orthogonal problem of the solvent in the perovskite / perovskite tandem battery prepared by the existing solution method; at the same time, the introduction of a thin metal layer (about 1 nanometer thick) realizes efficient tunnel recombination of carriers and effectively reduces the loss of the open circuit voltage and fill factor of the tandem battery in the tunnel junction (shown in Fig.. b), which simplify the preparation of the perovskite / perovskite tandem solar cell and is generally suitable for large-scaleindustrial production of perovskite / perovskite tandem batteries. On the other hand, by introducing metal tin powder into the lead-tin blended perovskite precursor solution, the comproportionation reaction between different valence states of tin is utilized to effectively suppress the oxidation of divalent tin ions and convert the tetravalent state tin ions to divalent tin ions. Then a highquality narrowbandgap perovskite thin film was successfully prepared. The maximum photoelectric conversion efficiency of single-junction narrowbandgap perovskite solar cells was 21.1%, and the efficiency was certified by Newport, a third-party testing agency. It is the highest value reported so far.
Combining high-efficiency narrowbandgap perovskite batteries and the new tunnel recombination junction, Prof. Hairen Tan’s research group successfully achieved high-performance all-perovskite tandem solar cells (shown in Fig. c and Fig. d), which was certified by a third-party testing agency. According to the tests, the conversion efficiency of smallarea batteries is as high as 24.8%, and the efficiency of largearea tandem batteries is up to 22.1%, both of which are currently reported world records for perovskite tandem batteries; the certified efficiency of largearea tandem batteries is much higher than the previously reported maximum efficiency of 20.9% (created by Korea KRICT).
Grade 16 bachelor-straight-to-doctorate Renxing Lin ( co-supervised by Jia Zhu and Hairen Tan) and research assistant Ke Xiao (now a doctor student of School of Electronic Science and Engineering ) of our department are the joint first authors of the paper. Prof. Hairen Tan, Prof. Jia Zhu, and Prof. Chunfeng Zhang of School of Physics are the co-corresponding authors. This work receives close support from Prof. Aidong Li and Prof. Min Xiao, as well as Prof. Jun Xu of School of Electronic Science and Engineering and Prof. Edward Sargent of University of Toronto. This work is also supported by the National Key R & D Program of China, the Thousand Talent Programme for Young Outstanding Scientists in China, the National Natural Science Foundation of China, National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, and Jiangsu Key Laboratory of Artificial Functional Materials.
Device structure and corresponding cross-sectional SEM image of a all-perovskite tandem solar cell;(b) J–V curves of tandem solar cells without and with an ultrathin Au layer in the tunnel recombination junction;(c) J–V curves of the small area tandem solar cell;(d) J–V curves of a large area tandem solar cell.
