Significance
Solar energy is becoming an essential component in our efforts to address the global energy crisis with intensive research conducted worldwide by leading institutions to push the power conversion efficiency (PCE) of solar cells closer to their theoretical radiative limit, defined by the Shockley-Queisser (S-Q) theory. Despite advances in improving short-circuit current density and fill factor, the open-circuit voltage (VOC) still lags behind due to non-radiative losses during bulk radiation and light outcoupling. These losses limit the potential of solar cells, such as perovskite, silicon (Si) and gallium arsenide (GaAs) to achieve their full efficiency potential. Photon recycling which is defined as the process where photons emitted from within the cell are reabsorbed has been traditionally viewed as a method to enhance VOC by improving light outcoupling. However, recent theoretical studies have raised concerns about the effectiveness of enhancing photon recycling especially in textured solar cells where it may increase non-radiative losses instead of reducing them. This discrepancy has created a need for a deeper understanding of the true impact of photon recycling on solar cell performance. To this account, new study published in the Journal Advanced Materials and conducted by PhD Student Haofeng Zheng, Qi Liu, Yanlong Wang, Jing Hu, Dechun Zou and led by Professor Shaocong Hou from the School of Electrical Engineering and Automation at Wuhan University, the researchers investigated the complex relationship between photon recycling and radiation processes for the bulk and surface radiation. They studied both the enhancement and weakening of photon recycling to determine the most effective strategies for minimizing non-radiative losses and pushing solar cells closer to their radiative efficiency limits. Their findings were interesting because it challenged the conventional approach of enhancing photon recycling and demonstrated that weakening it could be a more effective pathway to higher device performance and stability.
Initially the researchers analyzed the role of photon recycling in reducing non-radiative losses which traditionally considered the mechanism to enhance device performance. They modeled photon recycling as the reabsorption of emitted photons, which could then be re-emitted to contribute to light outcoupling. They used the detailed balance model to evaluate how bulk radiation and surface radiation interacted with photon recycling in each type of solar cell and observed that enhancing photon recycling did not always reduce non-radiative voltage losses (ΔVOC, nr) as expected. Instead, the experimental models showed that increasing photon recycling could sometimes lead to higher non-radiative losses especially in textured cells which was thought to be due to the enhancement of bulk and surface radiation and resulting in suboptimal performance. The authors then shifted their focus to weakening photon recycling as a strategy to improve efficiency. They performed their studies to reduce the photon recycling probabilities (P̄r) in the perovskite, Si, and GaAs cells and observed the corresponding changes in ΔVOC, nr and overall efficiency. They found that by reducing photon recycling, the non-radiative voltage losses decreased dramatically across all three materials. For example, in GaAs solar cells, weakening photon recycling reduced ΔVOC, nr to as low as 0.08 mV, a near-complete reduction compared to the higher losses observed when photon recycling was enhanced. Similarly, perovskite and Si solar cells saw significant improvements with VOC values increasing by 5.2% and 8.9%, respectively, when photon recycling was reduced.
Professor Shaocong Hou and his research team further analyzed the current density-voltage (J–V) characteristics of the cells to quantify how these changes impacted the practical performance of the devices and found that weakening photon recycling led to an increase in both VOC and PCE. Specifically, the perovskite solar cells achieved a power conversion efficiency of 27.9%—98.5% of their theoretical radiative limit while Si cells reached 27.6%, and GaAs cells approached nearly 100% of their efficiency limit. According to the authors, these findings demonstrated that weakening photon recycling allowed the cells to perform closer to their radiative limit than enhancing photon recycling, particularly in terms of voltage and efficiency. Another key finding of the study published in Advanced Materials emerged when the researchers evaluated the impact of internal radiative efficiency (ηint) on device stability. It is well known that solar cells are prone to degradation over time which can reduce ηint and lead to increased non-radiative losses. However, by simulating how devices with weakened and enhanced photon recycling behaved under reduced ηint, the researchers found that weakening photon recycling provided greater resilience. In GaAs solar cells, for example, when ηint dropped, the increase in ΔVOC, nr was much smaller in devices with weakened photon recycling compared to those where photon recycling was enhanced. This finding implied that weakening photon recycling not only improved performance but also helped maintain efficiency under conditions of device degradation, suggesting better long-term stability.
In conclusion, Professor Shaocong Hou and colleagues re-evaluated the photon recycling in solar cells and successfully challenged the traditional assumption that enhancing photon recycling always improves performance. They demonstrated that weakening photon recycling can more effectively reduce non-radiative losses and push solar cells closer to their theoretical radiative limits and indeed provided a paradigm shift in solar cell design and optimization. The results are truly valuable for the development of highly efficient devices such as perovskite, silicon, and GaAs cells, which are considered the leading next generation of photovoltaic technologies. This new strategy for improving the PCE of solar cells by reducing photon recycling rather than enhancing it can be adopted in both current and future solar cell designs to provide a pathway to maximize efficiency without relying solely on the internal radiative efficiency of materials. Moreover, the results suggest that with weakening photon recycling we can achieve better device stability because it helps maintaining higher performance even when internal radiative efficiency decreases due to degradation which can be used to extend the operational lifetime of solar cells and make them more viable for long-term renewable energy solutions. Finally, the study also provides valuable knowledge into new fabrication strategies, such as altering material complex refractive indices and designing curved cells with larger relative surface areas which could further optimize light management and efficiency.
Reference
Zheng H, Liu Q, Wang Y, Hu J, Zou D, Hou S. Understanding and Weakening Photon Recycling in Solar Cells to Approach the Radiative Limit. Adv Mater. 2024 Jul;36(28):e2405063. doi: 10.1002/adma.202405063.