Duhan, ParulBartholazzi, GabrielHaggren, TuomasGupta, BikeshAdhikari, SonachandBlack, Lachlan E.Zhang, DoudouJagadish, ChennupatiKaruturi, SivaTan, Hark Hoe2026-07-032026-07-03ORCID:/0000-0003-2119-0256/work/219172859ORCID:/0000-0002-5399-3312/work/219174801ORCID:/0000-0002-7816-537X/work/219179173ORCID:/0000-0003-1528-9479/work/219179355ORCID:/0000-0001-6033-7391/work/219179989https://hdl.handle.net/1885/733812867GaAs heterojunction solar cells with carrier-selective contacts are emerging as a promising alternative to traditional homojunction GaAs solar cells, offering low-cost fabrication, elimination of complex semiconductor doping processes, and high efficiency potential. Despite significant progress, the performance of GaAs heterojunction solar cells remains constrained by nonradiative recombination and processing issues, particularly for devices with a hole-selective contact. In this work, using SCAPS-1D simulations of the GaAs/CuO/PEDOT:PSS structure, we show that undoped GaAs with a thickness of ∼1.5 μm, combined with a 2 nm CuO layer, maximizes Jsc, Voc, fill factor, and overall efficiency. Guided by these simulations, we experimentally show that an ultrathin (2 nm) CuO layer deposited by plasma-enhanced atomic layer deposition is an effective hole-selective contact for GaAs heterojunction solar cells. X-ray and ultraviolet photoelectron spectroscopy show favorable band alignment at the CuO/GaAs interface, with a low valence band offset (0.11 eV) and large conduction band offset (0.52 eV), confirming its electron-blocking and hole-selective behavior. The CuO layer increases Voc, Jsc, fill factor, and overall efficiency from 8.6% to 13.5%. Power loss analysis indicates that optical losses are the primary contributors, suggesting a clear approach to achieving higher efficiencies.We acknowledge the funding support from the Australian Research Council through the Discovery Project. Access to the epitaxial growth and fabrication facilities was made possible through the ACT node of the Australian National Fabrication Facility (ANFF-ACT), and access to microscopy was provided through the ANU Centre for Advanced Microscopy (CAM). We also acknowledge Dr. Bill Bin Gong from the Solid State & Elemental Analysis Unit at the Mark Wainwright Analytical Centre, UNSW Sydney, for his technical support. Finally, we thank Dr. Marc Burgelman (University of Gent) for providing the simulation software SCAPS.9en©2026 The authorsheterojunctionhole-selective contactIII−V solar cellinterface engineeringPE-ALDSCAPS-1D2026-03-2310.1021/acsaem.5c04065105033729465