Assessing the role of hydrogen in Fermi-level pinning in chalcopyrite and kesterite solar absorbers from first-principles calculations

Abstract

Understanding the impact of impurities in solar absorbers is critical to engineering high-performance in devices, particularly over extended periods of time. Here, we use hybrid functional calculations to explore the role of hydrogen interstitial (Hi) defects in the electronic properties of a number of attractive solar absorbers within the chalcopyrite and kesterite families to identify how this common impurity may influence device performance. Our results identify that Hi can inhibit the highly p-type conditions desirable for several higher-band gap absorbers and that H incorporation could detrimentally affect the open-circuit voltage (Voc) and limit device efficiencies. Additionally, we find that Hi can drive the Fermi level away from the valence band edge enough to lead to n-type conductivity in a number of chalcopyrite and kesterite absorbers, particularly those containing Ag rather than Cu. We find that these effects can lead to interfacial Fermi-level pinning that can qualitatively explain the observed performance in high-Ga content CIGSe solar cells that exhibit saturation in the Voc with increasing band gap. Our results suggest that compositional grading rather than bulk alloying, such as by creating In-rich surfaces, may be a better strategy to favorably engineering improved thin-film photovoltaics with larger-band gap absorbers.

Upper limit to the photovoltaic efficiency of imperfect crystals from first principles

“…At high temperature during the thin-film growth or thermal annealing, the incorporation of the hydrogen or alkali metals at the interstitial sites will increase the Fermi level as they act as donors in p-type semiconductors. 60 The high Fermi level decreases the hole concentration and the formation of donor type defects as well (see Fig. 8c).…”

“…Alkali-metal elements may be less effective dopants due to their low solubility. 61 On the other hand, the previous calculations 60 have shown that the formation energies of H i in kesterites are low at p-type Fermilevel, suggesting high solubility of H in kesterites. We also noted that Son et al formed a S-Se grading in the current champion device 6 using H 2 S gas, which may introduce the H-doping unintentionally and be responsible for the high efficiency.…”

Hydrogen states in mixed-cation CuIn(1−x)GaxSe2 chalcopyrite alloys: a combined study by first-principles density-functional calculations and muon-spin spectroscopy

“…Hydrogen implantation in single-crystal CuInSe 2 (CIS) was responsible for a conductivity type conversion in the near-surface regions from an initially p-type material to n-type [5]. Fermi-level pinning in CIGS-based heterojunction devices was attributed to hydrogen and other interfacial defects, and led to a rollover of the open-circuit voltage limiting device efficiencies [6]. The introduction of hydrogen in CIS single crystals was also shown to improve the lattice structural stability by avoiding the formation of a radiation damage layer [7].…”

“…The electrical activity of hydrogen in the ternary CIS and CuGaSe 2 (CGS) chalcopyrite compounds has been thoroughly studied in the past by first-principles approaches [6,10,11] based on density-functional theory [12,13]. For CIS the most recent study by Varley and coworkers [6] made use of the HSE06 hybrid functional [14][15][16], a choice that enabled a trusted reproduction of the experimental electronic gap.…”

“…The electrical activity of hydrogen in the ternary CIS and CuGaSe 2 (CGS) chalcopyrite compounds has been thoroughly studied in the past by first-principles approaches [6,10,11] based on density-functional theory [12,13]. For CIS the most recent study by Varley and coworkers [6] made use of the HSE06 hybrid functional [14][15][16], a choice that enabled a trusted reproduction of the experimental electronic gap. The calculated defect formation energies suggested that interstitial hydrogen can be the source of n-type conductivity, with its E(+/-) charge-transition level (also known as pinning level) lying inside the conduction band.…”