Chemical Vapor Deposition of Cuprous Iodide as a Hole Transport Material for Perovskite Photovoltaics

A typical perovskite solar cell (PSC) requires both an electron and a hole transport material (ETM and HTM, respectively). The commonly employed organic HTMs are a significant source of instability in PSCs: the organic HTMs themselves have high resistivities, on the order of 10⁵ Ohm cm, and require dopants, which can migrate and wreak havoc throughout the cell [1, 2]. Inorganic HTMs have the potential to improve both device stability and performance due to their own inherent stability and higher conductivity, which eliminates the need for dopants.<br/><br/>Inorganic HTMs, which include NiO_x, VO_x, copper delafossites, and cuprous iodide, are relatively underexplored, which motivates the development of new inorganic HTMs and techniques for depositing device-quality films of those HTMs. In considering deposition techniques for inorganic HTM candidates, vapor methods such as chemical vapor deposition (CVD) and atomic layer deposition (ALD) could enable rapid scaling and conformal coatings even on textured substrates [3, 4]. This makes them especially attractive for application in tandem silicon/perovskite devices where a perovskite cell is deposited directly atop a textured crystalline silicon cell [4, 5].<br/><br/>The optically transparent p-type semiconductor cuprous iodide (CuI) is a promising HTM candidate, with a band gap ~3.1 eV, low resistivity (~10^-2 Ohm cm), and high hole concentration and mobility (~10¹⁹ cm^-3 and ~1-10 cm²V^-1s^-1, respectively) [6]. PSCs employing CuI as the HTM have achieved up to 17.6% photoconversion efficiency [7], but while CuI crystallite arrays have been obtained by CVD [8], CVD of continuous CuI thin films has only been accessed through a two-step vapor conversion process, requiring initial deposition of a copper chalcogen followed by vapor-phase conversion to the halide [6].<br/><br/>We have recently developed a CVD technique capable of direct deposition of continuous films of CuI at temperatures as low as 50 °C [9]. X-ray diffraction and Rutherford backscatter spectrometry confirm deposition of stoichiometric zincblende CuI. Progress in depositing continuous CuI films on substrates relevant to both n-i-p and p-i-n PSC device configurations will be discussed.<br/><br/>[1] W. Zhang et al., <i>Adv. Sci.</i> <b>5</b>, 1800159 (2018).<br/>[2] B. Gil et al., <i>Electron. Mater. Lett.</i> <b>15</b>, 505 (2019).<br/>[3] R. Gordon, <i>J. Non-Cryst. Solids</i> <b>218</b>, 81 (1997).<br/>[4] J. A. Raiford et al., <i>Energy Environ. Sci.</i> <b>13</b>, 1997 (2020).<br/>[5] F. Sahli et al., <i>Nature Mater.</i> <b>17</b>, 820 (2018).<br/>[6] R. Heasley et al., <i>ACS Appl. Energy Mater.</i> <b>1</b>, 6953 (2018).<br/>[7] X. Li et al., <i>ACS Appl. Mater. Interfaces</i><b> 9</b>, 41354 (2017).<br/>[8] V. Gottschalch et al., <i>J. Cryst. Growth</i>, <b>471</b>, 21 (2017).<br/>[9] L. M. Davis, E. K. Spear, R. G. Gordon, U.S. Provisional Application No. 63/347,325, filed May 31, 2022.