Cho Fai Jonathan Lau

Electric field induced reversible and irreversible photoluminescence responses in methylammonium lead iodide perovskite

Electric field induced effects have attracted considerable interest in the field of perovskite materials and solar cells because they are closely related to the performance and stability. In this work, we visualize and characterize the electric field induced effects in laterally structured Au/FTO/CH3NH3PbI3/FTO/Au samples via photoluminescence optical microscopy, in situ time-correlated single photon counting measurements and scanning electron microscopy. Both irreversible and reversible responses are observed under different electric fields and humidity conditions. Firstly, the irreversible response near both electrodes includes permanent photoluminescence quenching and morphology changes. Such changes are observed when the applied field is larger than a nominal value, which depends on the humidity conditions. The irreversible change is a result of perovskite decomposition, which is indicated by the appearance of a PbI2 peak in the localized photoluminescence spectrum. We show that this moisture-assisted electric field induced decomposition can be minimized by encapsulation. Secondly, a reversible response near the anode observed under a weak electric field, which is characterized by photoluminescence quenching and a reduced lifetime with negligible morphology change, is attributed to the migration and accumulation of mobile ions. The dominant mobile species is ascribed to be iodide ions by mobility calculations. Thirdly, a slowdown of the irreversible response, i.e., decomposition within the bulk of the perovskite and away from the electrodes, is observed. This is because of the negative feedback between perovskite decomposition and ion accumulation, which offsets the field induced effect in the perovskite bulk. This work demonstrates the effective use of photoluminescence microscopy revealing different mechanisms behind the observed instability of perovskite devices under different bias and moisture conditions that cause either reversible or irreversible changes.

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.