Investigation of Ag(Ga,In)Se2 as thin-film solar cell absorbers: A first-principles study

Using first-principles calculations, the structural, electronic, and defect properties of AgInSe2 (AIS), AgGaSe2 (AGS), and their alloys (AIGS) are systematically studied and compared with their Cu counterparts as potential candidates for thin-film solar cell absorbers. The bandgap energies of AIS (1.24 eV) and AGS (1.84 eV) are larger than their Cu counterparts, despite their larger lattice parameters. According to the Shockley-Queisser theory, AIS or AIGS could be more suitable for solar-cell-absorber materials than their Cu counterparts. However, after investigating the band structures and intrinsic defect properties of AIS and AGS, we find that, (i) AIS and AGS have large negative crystal field splitting, thus low density of states near the valence band maximum (VBM); (ii) similar to the Cu counterparts, Ag vacancy (V-Ag) is the main hole-carrier provider, while In-Ag (or Ga-Ag) serves as the hole-carrier killer in p-type AIS (or AGS). However, because the positions of the VBM and conduction band minimum of AIS (or AGS) are lower than those of CuInSe2 (CIS) [or CuGaSe2 (CGS)], the compensation of the p-type doping in AIS (or AGS) is more severe. Thus, the p-type doping of AIS (or AIGS) is more difficult than that of CIS (or CIGS), which is consistent with the doping limit rule. To improve the p-type doping of the AIS (or AIGS) as the solar-cell absorber, thus, improve the power conversion efficiency (PCE), the Ag-rich/(In,Ga)-poor/Se-rich growth condition is preferred. Alloy engineering of AIS with AGS can enhance the PCE because it can tune the bandgap energy of the absorber and band alignment at the absorber/buffer interface. More importantly, we suggest that for AIS (or AIGS) solar cell, the traditional buffer material of CdS is not suitable anymore due to the large conduction band offset between AIS and CdS. A new buffer layer material with a lower conduction band edge is necessary for better electron transport in AIS (or AIGS) solar cell.

Automated summary

In this study, the structural, electronic, and defect properties of AgInSe2, AgGaSe2, and their alloys were investigated using first-principles calculations. The results suggest that alloy engineering can enhance the power conversion efficiency of the solar cell absorber, and a new buffer layer material is needed for better electron transport in the solar cell.