Unlocking the potential of transport layers in solar cells: a universal design principle for high efficiency despite high extraction barriers

In this study, a long-term misunderstanding of the role of extraction barriers in transport layer (TL) optimization, is clarified. It is generally believed that high extraction barriers (>0.026 eV) result in low power conversion efficiency (PCE). However, the present research revealed that efficient charge separation could be achieved even with barriers as high as 0.3 eV. A comprehensive computational analysis of over 10 000 simulated solar cells was then conducted, and a universal design principle was identified: minimizing the voltage drop across the TL is key for maintaining high PCE, regardless of the extraction barrier. Experimental validation of this principle was followed in perovskite solar cells, and exceptional efficiency was achieved for a 0.3 eV barrier, which was widely considered impossible before. To make these results practical, an evaluation factor (θ) was developed, which integrates various TL parameters, including extraction barrier, thickness, carrier mobility, and dielectric constant into a single, easy-to-calculate metric. This tool enables rapid and overall assessment of TL effectiveness, significantly accelerating the design and optimization process.