“…The earth-abundant Cu(I)-based semiconductors, mostly known as cation-deficient hole conductors, hold the greatest promise in photovoltaic devices that require p-type doping technology for tuning the key device parameters. p-Type doping in the photoabsorbers can enhance the photovoltaic cell performance by reducing nonohmic contacts and series resistance and increasing the open-circuit voltages related to the built-in potential of the junction. , The prominent examples are Cu(In,Ga)Se 2 (CIGS) and Cu 2 O photoabsorbers that improve solar cell efficiency by the substantial increase in hole concentration using alkali metal dopants, although the mechanism behind the improvement is still under debate. − Similarly, p-type doping is pivotal for the high-performance hole transport materials (HTM) for perovskite/organic solar cells to improve charge extraction efficiency from the absorbers to the external electrodes, ensuring high-workfunction ohmic contacts and adjusting the Fermi level position to the corresponding absorbers. − Wide-gap p-type Cu(I)-based semiconductors are investigated for the great potential of the long-term stability to replace conventional organic HTMs. , Copper iodide (γ-CuI) crystallized in a zinc-blende-type cubic is one of the promising inorganic HTMs available by low-temperature processing due to the wide direct band gap of 2.95 eV and high hole mobility of 12–44 cm 2 /(V s) for single crystals, although the intrinsic hole concentration remains at (4–9) × 10 16 cm –3 . − So far, little enhancement of device performance has been achieved yet for perovskite solar cells with γ-CuI HTMs; therefore, extrinsic carrier doping would be an important step toward the development of Cu(I)-based HTMs. , …”