Perovskite solar cells have seen a rapid rise in recent years due to their potential for low-cost processing and high power conversion efficiencies. Passivation of perovskite cells, especially at the perovskite-charge transport layer interfaces, plays a key role in reducing nonradiative recombination and to achieve high open-circuit voltages. Polymer passivation layers are typically used for this purpose; however, such layers are poor conductors, leading to a trade-off between passivation quality (voltage) and series resistance (fill factor).

A nanopatterned electron transport layer consisting of a sparse array of TiO₂ nanorods was developed that overcomes this trade-off by forming nanoscale localized charge transport pathways through a passivated interface, thereby providing both effective passivation and excellent charge extraction.

A certified power conversion efficiency of 21.6% was achieved for a 1-square-centimetre cell with a FF of 0.839. Encapsulated cells retained >91% of their initial efficiency after 1000 hours of damp heat exposure.

[1]. J. Peng, D. Walter, Y. Ren, M. Tebyetekerwa, Y. Wu, T. Duong, Q. Lin, J. Li, T. Lu, M. A. Mahmud, O. L. C. Lem, S. Zhao, W. Liu, Y. Liu, H. Shen, L. Li, F. Kremer, H. T. Nguyen, D.-Y. Choi, K. J. Weber, K. R. Catchpole and T. P. White, “Nanoscale localized contacts for high fill factors in polymer-passivated perovskite solar cells”, Science, 2021, 371, 390-395.

Nanoscale localized contacts for high fill factors in polymer-passivated perovskite solar cells – Report
New world record in perovskite solar cell efficiency

Australia achieved another impressive milestone in the transition to a low carbon future when South Australia was fully powered by Photovoltaics for 1 hour on Sunday, 11 October 2020. This is a unique achievement globally as South Australia has an area of close to 1 million km²  which is one and a half times bigger than Texas and five times the United Kingdom! The photovoltaic energy was mostly supplied by rooftop solar at 77%, whereas the remainder was provided by large-scale PV. The actual renewable energy production was well above 100%, and the remainder was either stored in batteries or exported to Victoria.

The long-term stability of solar cells under illumination is of key importance for their application. Generally, it assumed that solar cells using n-type silicon do not suffer from light-induced degradation due to the absence of significant amounts of boron in the bulk of the material, i.e., making these solar cells immune to the boron-oxygen defect. However, Madumelu and co-workers [1] recently reported that they observed a significant amount of degradation after light soaking. These measurements were done at temperatures of 160 ^oC to accelerate the testing.

As can be seen in the figure below, a significant reduction in solar cell efficiency up to 1 % absolute was found when the solar cells were illuminated, whereas an improvement was observed when the solar cells were kept in the dark. The authors related the change in solar cell performance to changes in the amorphous silicon heterojunction contact. It should be noted that amorphous silicon is stable at the temperatures used in this work and that similar illuminated anneals do not seem to affect other types of solar cells such as PERC.

Further work is needed to identify the exact nature of this defect and to investigate if this can also affect photovoltaic modules with silicon heterojunction cells in the field which would typically operate at significantly lower temperatures. 

Solar conversion efficiency of individual cells as a function of time after either dark-annealing (DA) or light-soaking at 1 kWm^-2 illumination intensity at a temperature of 160 ◦C. A reference cell that was stored at room temperature was included to test the repeatability of the measurement. All I-V measurements were conducted ex-situ under standard testing conditions.

[1]           C. Madumelu, B. Wright, A. Soeriyadi, M. Wright, D. Chen, B. Hoex, and B. Hallam, “Investigation of light-induced degradation in N-Type silicon heterojunction solar cells during illuminated annealing at elevated temperatures,” Solar Energy Materials and Solar Cells, vol. 218, p. 110752, 2020

The photovoltaic field is currently working hard to identify the ideal candidate for a silicon-based tandem solar cell. From a material perspective, an ideal top cell has a bandgap of ~1.6 eV, high absorption coefficient, and consists of only earth abundant and ecofriendly materials. In addition, it should also be easy to manufacture, compatible with silicon, and have a long-term stable performance.

Solar cells based on antimony sulphide/selenide have all these desired properties, however, their solar cell performance was still too low with record efficiencies around the 7% range. On 20 July 2020, a team of researchers from China and Australia have reported the first antinomy sulphide/selenide with an efficiency above 10% in the journal Nature Energy [1].

They achieved this landmark result by optimising the deposition and post-processing step of the absorber material resulting in a significant improvement in its morphology, grain size, and a reduction in the defect density in the film. This result illustrates the potential for this material as a candidate for future silicon-based tandem solar cells.

[1]  R. Tang, X. Wang, W. Lian, J. Huang, Q. Wei, M. Huang, Y. Yin, C. Jiang, S. Yang, G. Xing, S. Chen, C. Zhu, X. Hao, M. A. Green, and T. Chen, “Hydrothermal deposition of antimony selenosulfide thin films enables solar cells with 10% efficiency,” Nature Energy, 5, pages587–595(2020).