Lead halide perovskite has shown amazing optoelectronic performance, while the issues of its toxicity and the thermal instability remain to be intractable. Recently, lead-free halide perovskite Cs₂PdBr₆ as a narrow bandgap semiconductor with long-term stability has attracted great attention. Herein, through performing first-principles calculations, we find that (i) Cs₂PdBr₆ has a quasi-direct bandgap with an indirect bandgap of 1.71 eV and a 20 meV smaller direct bandgap. The bandgap increases when the lattice constant enlarges, which is opposite to that of the traditional zincblende semiconductors. (ii) Band offsets between Cs₂PdBr₆ and popular perovskites indicate that Cs₂PdBr₆ is more difficult to be doped p-type according to the doping limit rule. The commonly used electron transport materials such as TiO₂, SnO₂, ZnO, and C₆₀ can also be suitable for Cs₂PdBr₆ solar cell devices, but the commonly used hole transport materials with a lower highest occupied molecular orbital is an alternative. (iii) Cs₂PdBr₆ is shown to be difficult to be intrinsic p-type. The electric conductivity would not be too high due to the charge compensation of the defects if there is no suitable external doping. Since the Pd poor and Br rich condition can suppress the formation of the defects with deep levels, this condition is proposed to synthesize Cs₂PdBr₆ as a solar cell absorber.

Due to the absence of high-performance ambipolar wide-bandgap (WBG) semiconductors, the realization of active transparent photoelectronic devices is precluded. Herein, based on the hybrid functional calculations, we predict that, in a wide-bandgap semiconductor strontium sulfide (SrS), the Br (Rb) substituting S (Sr) is an ideal n (p)-type defect. SrBr₂ and Rb₂S are promising dopant sources for introducing Br and Rb, respectively. Moreover, the Sr-rich (Sr-poor) condition is the optimum growth environment to fabricate the Br_S (Rb_Sr) defects. Thermodynamic equilibrium simulations indicate that the concentration of Br_S and Rb_Sr can exceed 4 × 10¹⁹ cm⁻³ at high growth temperatures. After rapid quenching from the growth temperature to room temperature, the free carrier densities can reach 1.56 × 10¹⁹ cm⁻³ for electrons and 1.02 × 10¹⁸ cm⁻³ for holes. These results show SrS is a promising ambipolar WBG semiconductor that has huge potential applications in future optoelectronic devices.