Wide-Bandgap Perovskite Solar Cells for Multi-Junction Photovoltaics

The ability to easily tune the bandgap (<i>E</i>_g) of perovskite semiconductors makes them excellent candidates for multi-junction applications. By combining narrow-bandgap lead-tin perovskites (<i>E</i>_g ~ 1.2 eV), iodide-based mid-bandgap perovskites (<i>E</i>_g = 1.4 – 1.5 eV), and mixed-halide wide-bandgap perovskites (<i>E</i>_g ~ 2.0 eV), high efficiency triple-junction devices can be developed¹. High quality wide-bandgap perovskites are a primary bottleneck in the development of these devices. Specifically, the deposition of smooth, mixed-halide perovskite layers is a key challenge. High open-circuit voltage (<i>V</i>_oc) losses in such devices are also detrimental to their efficacy in multi-junction solar cells². Another important hurdle in developing multi-junction devices is the poor photostability of wide-bandgap perovskites as a result of light-induced halide segregation, which impairs their operational stability³.<br/><br/>This work investigates the organic-inorganic FA_xMA_1-xPb(Br_yI_1-y)₃ (FA: formamidinium, MA: methylammonium, Br: bromide, I: iodide) and fully-inorganic CsPb(I_1-yBr_y)₃ perovskites for triple-junction photovoltaics. Several aspects such as phase purity, morphology, photostability and photovoltaic performance are examined through <i>in-situ</i> and <i>ex-situ</i> characterization tools.<br/><br/>First, FA_1-xMA_xPb(I_1-yBr_y)₃ perovskite films with different MA and Br contents are prepared using a one-step antisolvent-based process to approach <i>E</i>_g ~ 2.0 eV. We observe that the composition strongly affects surface morphology; the films with high Br content demonstrate surface wrinkling, with features as high as 1 – 2 µm and widths around 5 – 10 µm. The wrinkling correlates to a preferential vertical crystal orientation, as observed by grazing incidence wide-angle X-ray scattering. However, the presence of wrinkles increases surface roughness that impedes the use of such layers in solution-processed multi-junction solar cells.<br/><br/>In contrast, a sequential interdiffusion method to prepare FA_xMA_1-xPb(Br_yI_1-y)₃ is found to yield unwrinkled films with <i>E</i>_g ~ 2.0 eV⁴. However, we find that such films show the presence of de-mixed I-rich and Br-rich phases that lead to low-energy photoluminescence (PL) emissions. Under continuous irradiation, using <i>in-situ</i> PL spectroscopy, we find that ion migration further exacerbates the formation of I-rich and Br-rich domains. Upon subsequent storage in the dark, entropy-driven halide remixing causes the PL emission to blue-shift to ~ 2.0 eV, indicating the formation of a homogeneous mixed-halide phase. Thereafter, the process of halide segregation and remixing can be cycled by light-dark exposure in these materials. When used in <i>p-i-n</i> solar cells, the efficiency of 2.0 eV organic-inorganic mixed-halide cells decreases as a result of light-induced halide segregation, dominated by a loss in <i>J</i>_sc, while the <i>V</i>_oc is hardly affected. Nevertheless, the high efficiency (&gt; 10%) of the solar cell is undermined by its limited photostability, making it less ideal for multi-junction use⁵.<br/><br/>Finally, fully-inorganic <i>p-i-n</i> CsPb(I_0.67Br_0.33)₃ (<i>E</i>_g = 1.9 eV) solar cells are prepared, yielding an efficiency of over 9%. A high-energy PL peak (maximum at ~ 1.9 eV) confirms the I-Br homogeneity of the film. Furthermore, under prolonged illumination the PL peak displays only a marginal red-shift, indicating superior stability compared to organic-inorganic counterparts. Similarly, CsPb(I_0.67Br_0.33)₃ solar cells demonstrate negligible performance degradation under operational conditions, emphasizing their photostability.<br/><br/>This work demonstrates key elements in the processing and compositional engineering of wide-bandgap perovskites to develop triple-junction-compatible materials. The results highlight the high performance and stability of inorganic mixed-halide perovskite solar cells.<br/><br/>1.<i> ACS Energy Lett. </i><b>2017</b>, <i>2</i>, 10, 2506-2513<br/>2.<i> J. Mater. Chem. A</i> <b>2017</b>, <i>5</i>, 11401<br/>3.<i> Chem. Sci.</i> <b>2015</b>, <i>6</i>, 613<br/>4.<i> Adv. Funct. Mater</i>. <b>2017</b>, <i>27</i>, 1700920<br/>5.<i> Nature Energy </i><b>2022, </b><i>7</i>, 107–115