Jincheol Kim

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

Enhanced performance via partial lead replacement with calcium for a CsPbI3 perovskite solar cell exceeding 13% power conversion efficiency

Cesium metal halides are potential light-harvesting materials for use in the top cells of multi-junction devices due to their suitable bandgaps and good thermal stabilities. In particular, CsPbI3 has a bandgap of 1.7 eV, which is suitable for perovskite/Si tandem cells. However, the desirable black phase for CsPbI3 is not stable because Cs is too small to support the PbI6 octahedra. Also, there is room for improvement in terms of cell performance. Herein, we partially replace Pb2+ with Ca2+ in the CsPbI3 precursor, producing multiple benefits. Firstly, more uniform films with larger grains are produced from CsPbI3 with Ca2+, due to the reduction in the size of the colloids in the precursor solution with Ca2+. This morphology improvement provides better contact at the interface between the perovskite and the hole transport layer. In addition, it is found that the surface of the film is modified by the formation of a Ca rich oxide layer, providing a surface passivation effect. Finally, incorporation of Ca increases the band gap, leading to an increase in output voltage. The best CsPbI3 solar cell using 5% Ca2+ substitution in the precursor achieves a stabilised efficiency of 13.3%, and maintains 85% of its initial efficiency for over 2 months with encapsulation.

Large area efficient interface layer free monolithic perovskite/homo-junction-silicon tandem solar cell with over 20% efficiency

Monolithic perovskite/silicon tandem solar cells show great promise for further efficiency enhancement for current silicon photovoltaic technology. In general, an interface (tunnelling or recombination) layer is usually required for electrical contact between the top and the bottom cells, which incurs higher fabrication costs and parasitic absorption. Most of the monolithic perovskite/Si tandem cells demonstrated use a hetero-junction silicon (Si) solar cell as the bottom cell, on small areas only. This work is the first to successfully integrate a low temperature processed (≤150 °C) planar CH3NH3PbI3 perovskite solar cell on a homo-junction silicon solar cell to achieve a monolithic tandem without the use of an additional interface layer on large areas (4 and 16 cm2). Solution processed SnO2 has been effective in providing dual functions in the monolithic tandem, serving as an ETL for the perovskite cell and as a recombination contact with the n-type silicon homo-junction solar cell that has a boron doped p-type (p++) front emitter. The SnO2/p++ Si interface is characterised in this work and the dominant transport mechanism is simulated using Sentaurus technology computer-aided design (TCAD) modelling. The champion device on 4 cm2 achieves a power conversion efficiency (PCE) of 21.0% under reverse-scanning with a VOC of 1.68 V, a JSC of 16.1 mA cm−2 and a high FF of 78% yielding a steady-state efficiency of 20.5%. As our monolithic tandem device does not rely on the SnO2 for lateral conduction, which is managed by the p++ emitter, up scaling to large areas becomes relatively straightforward. On a large area of 16 cm2, a reverse scan PCE of 17.6% and a steady-state PCE of 17.1% are achieved. To our knowledge, these are the most efficient perovskite/homo-junction-silicon tandem solar cells that are larger than 1 cm2. Most importantly, our results demonstrate for the first time that monolithic perovskite/silicon tandem solar cells can be achieved with excellent performance without the need for an additional interface layer. This work is relevant to the commercialisation of efficient large-area perovskite/homo-junction silicon tandem solar cells.