After several meetings with professor Iván Mora-Seró sharing our works, he suggested to estimate the ideality factor for perovskite minimodules under indoor conditions in order to track the evolution of this parameter over time. In this way, we explored different methodologies to estimate the nID, observing an excellent agreement between the nID values estimated from Voc-light intensity data in accordance with the equation 1, and the values estimated from the impedance/frequency-response (IFR) analysis involving the recombination resistance (Rrec, which is extracted fitting the impedance to an equivalent circuit), in accordance with the equation 2. This agreement between both methodologies for the estimation of the nID values, motivated us to characterize under indoor conditions not only the minimodules, but also, perovskite solar cells fabricated from different ways as was shown in previous work (Yoo et al., 2020). To do that, a procedure was developed to record the Voc, impedance frequency response and I-V curve at different light intensities in a potensiostat by Autolab.
During the developing of this procedure, we recognized that the illumination conditions setup in indoor test, could be provided naturally from outdoor test, resulting in a broad range of illumination conditions. Therefore, as we have monitored encapsulated perovskite minimodules under natural sunlight in the facilities of the Solar Cell Outdoor Performance Laboratory of the University of Antioquia (OPSUA) in Medellín, Colombia, recording the Voc, I-V curves, irradiance levels, ambient temperature and devices temperature every minute during the sun-hours, the obligatory next step, was to try to estimate the nID values for exposed devices over the time based on the relationship between the Voc and irradiance levels in accordance with the equation 1, using the outdoor data. The initial results caught out attention because of the well-defined shape that this parameter exhibited over time. Therefore, various methods and procedures to process the outdoor data were evaluated and analyzed in order to validate the values and parameter evolution.
Once, this procedure to estimate nID from the outdoor data was established, we wished to compare the nID behavior with the performance behavior obtained from the I-V curve analysis, that is the Pmax evolution. Thus, because no international standards have been established for perovskite, we decided to involve the IEC 61853-1, to estimate the performance evolution based on the power rating defined by the standard as the standard test condition (STC) or Nominal Operative cell temperature (NOCT). For that, the outdoor data were processed in accordance with these power rating conditions to estimate the Pmax over time for each evaluated sample, as was performed for commercial photovoltaic modules in previous work (Velilla et al., 2019).
Once, the nID and the Pmax patterns were compared, the result was astonished, and a lot of ideas and questions started to come out. How to correlate both patterns in a properly way? . Is it possible to define an equivalent of T80 for nID?. What does mean the shape of the patterns? Etc. Nevertheless, because this technology is in its infancy, no international standards have been fully established and most published works have focused on laboratory-scale cells; consequently, there are few statistical data available for large devices operated, hence, in the first draft version of the paper, we only remark the advantages of use the nID to track the performance evolution of perovskite devices.
However, as the reviewers of Nature Energy suggested, combining both methodologies one based on the nID another on the Pmax analysis, could be more useful. Therefore, we agree with the reviewers and for that, we highlighted the complementary between both methodologies to provide physical insight into the recombination mechanism dominating the performance, improving the understanding of the degradation processes and device characterization.
You can find more details about this work in a paper published in Nature Energy: Velilla E., Jaramillo F., Mora-Seró, I., High-throughput analysis of the ideality factor to evaluate the outdoor performance of perovskite solar minimodules. Nature Energy, (2020). DOI: https://doi.org/10.1038/s41560-020-00747-9
In addition, you can check our previous works related to outdoor evaluation:
Velilla, E., Ramirez, D., Uribe, J.-I., Montoya, J. F. & Jaramillo, F. Outdoor performance of perovskite solar technology: Silicon comparison and competitive advantages at different irradiances. Sol. Energy Mater. Sol. Cells 191, 15–20 (2019).
Velilla, E., Cano, J. B. & Jaramillo, F. Monitoring system to evaluate the outdoor performance of solar devices considering the power rating conditions. Sol. Energy 194, 79–85 (2019).
Ramirez, D., Velilla, E., Montoya, J. F. & Jaramillo, F. Mitigating scalability issues of perovskite photovoltaic technology through a p-i-n meso-superstructured solar cell architecture. Sol. Energy Mater. Sol. Cells 195, 191–197 (2019).