Monolithic All-Perovskite Tandem Solar Cells with Minimized Optical and Energetic Losses

Solution-processed all-perovskite tandem solar cells present an effective device platform to maximize power-conversion efficiencies by combining complementary wide-bandgap mixed-halide perovskites and narrow-bandgap lead-tin perovskites in monolithic architectures to minimize the loss in chemical potential observed in single-junction solar cells due to thermalization and transmission losses.¹ Currently, the open-circuit voltage of such devices is limited by the low radiative yield in the wide- and narrow-bandgap sub-cells. Furthermore, optical losses reduce short-circuit current density contribution of the narrow-bandgap sub-cell, which increases the current-mismatch between sub-cells, thereby decreasing the effective short-circuit current-density of the tandem device.<br/><br/>This work, firstly, identifies sites of non-radiative recombination in the wide-bandgap (<i>E</i>_g ~ 1.77 eV) top-cell using absolute photoluminescence spectroscopy and pinpoints the interface between the perovskite and the electron transport layer (C₆₀) as a dominant surface for energetic losses.² Using surface treatment strategies based on the use of quaternary ammonium salts, the non-radiative loss is recovered, leading to high open-circuit voltages in single-junction, wide-bandgap solar cells. Compositional engineering, such as potassium substitution, is further used to improve the photostability of devices against light-induced halide segregation. Similarly, in narrow-bandgap (<i>E</i>_g ~ 1.23 eV) single-junction solar cells, the interface with the C₆₀ layer is improved, as confirmed by electroluminescence spectroscopy, by compensating for sublimed halide ions at the surface, leading to an improvement in the open-circuit voltage. At the same time, high short-circuit current density is ensured by using thin, optically benign, hole-transport layers, such as self-assembled monolayers,³ that minimize parasitic absorption, and by reducing external quantum efficiency losses due to optical interference by optimizing the perovskite layer thickness.<br/><br/>The monolithic tandem combining the wide-bandgap and narrow-bandgap sub-cells is integrated using a dense, atomic layer deposition-based SnO_x recombination junction that improves stability against solvent ingress during processing. A thin gold monolayer is used to maintain high conductivity across this interface.⁴<br/><br/>Free-carrier absorption in the front electrode of the tandem device is reduced by opting for hydrogenated indium oxide (IOH) instead of commonly used indium tin oxide (ITO),⁵ which increases the near infrared response of the narrow-bandgap sub-cell, reducing the current-mismatch between sub-cells. Further decreases in parasitic absorption by using thin hole-transport layers and highly transparent interfacial layers additionally ensures improved light absorption in the narrow-bandgap sub-cell, which leads to current-matched tandem devices. A key highlight of this work is the effective light management strategy that ensures a high NIR response of the narrow-bandgap cell, despite using a low active layer thickness (600 nm). Collectively, these pathways to limit non-radiative recombination and reduce optical losses ensure 23% – 24% efficient tandem solar cells in small (0.09 cm²) and large (1 cm²) device areas which mark a &gt; 7% (abs.) and &gt; 6% (abs.) gain in the power conversion efficiency compared to the performance of single-junction wide- and narrow-bandgap solar cells.<br/><br/>References<br/>1. <i>ACS Energy Lett.</i> <b>2017</b>, <i>2</i> (10), 2506–2513.<br/>2. <i>Adv. Energy Mater.</i> <b>2022</b>, <i>12</i> (12), 2103567.<br/>3. <i>Energy Environ. Sci.</i> <b>2019</b>, <i>12</i> (11), 3356–3369.<br/>4. <i>Nat. Commun.</i> <b>2020</b>, <i>11</i> (1), 5254.<br/>5. <i>ACS Appl. Energy Mater.</i> <b>2019</b>, <i>2</i> (11), 7823–7831.