Photoluminescence and efficiency in perovskite solar cells

10 October 2022 by Publications

The paper "Re‐evaluation of Photoluminescence Intensity as an Indicator of Efficiency in Perovskite Solar Cells" has been published in the Solar RRL journal. It is the outcome of a collaboration between EFSL, CNR-IMM (Istituto per la Microelettronica e i Microsistemi) and CHOSE (Centre for Hybrid and Organic Solar Energy – Regione Lazio) researchers: Valerio Campanari, Faustino Martelli, Antonio Agresti, Sara Pescetelli, Narges Yaghoobi Nia, Francesco Di Giacomo, Daniele Catone, Patrick O'Keeffe, Stefano Turchini, Bowen Yang, Jiajia Suo, Anders Hagfeldt and Aldo Di Carlo.

The photoluminescence (PL) intensity is often used as an indicator of the performance of perovskite solar cells (PSCs) and indeed steady-state PL is therefore used for the characterization of these devices and their constituent materials. However, some features of perovskite materials and devices, if not taken adequately into account, could lead to misleading results. In this work we show that the value of the photoluminescence quenching occurring when the PSCs are switched from open- to short-circuit is an excellent figure of merit (FOM) for the evaluation of the PSC efficiency. In order to take into account the large inhomogeneity of the performance in the PSC yield and their temporal instability, we have used a systematic approach to comparison the PSC conversion efficiency and their PL intensity using a large group of PSCs with efficiency varying from 5.63% to 21.5%. The PSCs were fabricated using different materials both as regards the absorbing materials and the transport layers, different structures, and relatively different fabrication processes. We found that the use of the bare PL intensity in open-circuit (OC) is not a general and reliable indicator of the performances when a large heterogeneous ensemble of devices is used, Instead, considering the values of the PL intensity both at open OC and short circuit (SC) , it is possible to correlate the PL intensity to the device power conversion efficiency (PCE) through a figure of merit based on the quenching of the PL observed in SC and OC conditions (PLQoc-sc=(PLoc-PLsc)/PLoc). We explain the observed correlation between PLQoc-sc and the PCE of the device by means of a simplified model based on the carrier density rate equation, in which the extraction time of the photoexcited carriers in the PSC is duly considered. We have also shown that the analysis of PLQoc-sc using our model can provide a tool to estimate the carrier extraction time in PSCs through a simple cw-PL measurement, provided the carrier lifetime in OC conditions is known. Without suggesting that a PL measurement can substitute a J-V characterization, we point out that our study is an important contribution to the understanding the physical mechanisms underlying carrier recombination and extraction. Moreover, we have found a clear correlation between the increase over time of the PL intensity in SC conditions and the contemporary decrease of the short-circuit current, Isc, a feature that can be easily understood within the framework of our model and FOM.

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