Sb 2 Se 3 emerges as a promising solar cell absorber material recently. The merits, such as a suitable bandgap around 1.2 eV, [1,2] a high optical absorption coefficient over 10 5 cm À1 , and [3,4] the cheap and nontoxic elements Sb and Se, are intriguing to the researchers in the photovoltaic (PV) field. [5] In the past years, diverse growth methods have been used for growing Sb 2 Se 3 thin films with high qualities, such as thermal evaporation, [6] vapor transport deposition (VTD), [7] magnetron sputtering, [8] and solution processing. [3] Furthermore, the postselenization process and the orientation control of the atomic chain are also combined with the growth methods, due to the natural Se-poor conditions and its unique quasi-1D structure. [9][10][11][12][13][14][15][16] These techniques significantly improve the thin-film morphology and achieve high power conversion efficiency (PCE). However, the reported open-circuit voltage (V OC ) of Sb 2 Se 3 solar cell is limited in the range of 0.3-0.5 V even though these processes are performed, [7,11,12,[17][18][19] hindering the further pursuit of higher PCE.The complicated intrinsic defects are believed to be one of the critical factors that can severely limit the V OC . [20,21] The interaction and compensation of the charged defects, such as the anionÀcation antisites Sb Se and Se Sb , the defect complexes 2Se Sb (Se dimer at Sb site), and the vacancies V Se , pin the Fermi level far from the valence band maximum (VBM). [20,22] As a result, the experimentally reported hole carrier density is only about 10 13 cm À3 , [7,23] much lower than that of CuSbSe 2 [24,25] and Cu 2 ZnSnSe 4 . [26] To gain a larger V OC in pÀn heterojunction solar cells, a larger quasi-Fermi-level splitting is necessary, which requires an efficiently p-type-doped Sb 2 Se 3 thin film. However, to the best of our knowledge, p-type doping in Sb 2 Se 3 is currently unachievable. On the contrary, it is easy to dope Sb 2 Se 3 into n-type. [27][28][29][30][31] To realize effectively p-type doping in such a low-symmetry quasi-1D material, one way is to choose anions with smaller atomic radii as dopants so that they can form interstitial defects between atomic chains, but both iodine-(I) and chlorine (Cl)doped samples were reported to be n-type rather than p-type. [30,31] The underlying mechanism was explained by the easy formation of the substitutional donor defect Cl Se , and the charge-state transition level of Cl i is close to the midgap. [31] This is quite similar to anions in low-symmetry halide perovskite, as it was shown that I i and Br i are amphoteric with its (þ/-) charge-state transition level located near the middle of the bandgap. [32,33] As a result, the contribution of anion doping to p-type conductivity is limited. Cation doping is another possible way to realize p-type. However, as discussed earlier, atoms with small radii tend to stay at the interchain space-forming interstitial donor defects in quasi-1D Sb 2 Se 3 , for instance, the reported Cu,-Cd-, Mg-, and Fe-doped samples only...