Experimental and First-Principles Spectroscopy of Cu₂SrSnS₄ and Cu₂BaSnS₄ Photoabsorbers

Abstract

Cu 2 BaSnS 4 (CBTS) and Cu 2 SrSnS 4 (CSTS) semiconductors have been recently proposed as potential wide band gap photovoltaic absorbers. Although several measurements indicate that they are less affected by band tailing than their parent compound Cu 2 ZnSnS 4 , their photovoltaic efficiencies are still low. To identify possible issues, we characterize CBTS and CSTS in parallel by a variety of spectroscopic methods complemented by first-principles calculations. Two main problems are identified in both materials. The first is the existence of deep defect transitions in low-temperature photoluminescence, pointing to a high density of bulk recombination centers. The second is their low electron affinity, which emphasizes the need for an alternative heterojunction partner and electron contact. We also find a tendency for downward band bending at the surface of both materials. In CBTS, this effect is sufficiently large to cause carrier-type inversion, which may enhance carrier separation and mitigate interface recombination. Optical absorption at room temperature is excitonenhanced in both CBTS and CSTS. Deconvolution of excitonic effects yields band gaps that are about 100 meV higher than previous estimates based on Tauc plots. Although the two investigated materials are remarkably similar in an idealized, defect-free picture, the present work points to CBTS as a more promising absorber than CSTS for tandem photovoltaics.

Assessing the defect tolerance of kesterite-inspired solar absorbers

“…They cannot simply be phonon replicas of the TI/FB peak because they would require much higher phonon wavenumbers than the ones found experimentally in CBTS (below 400 cm À1 ). 19,56 Thus, these peaks must be related to other radiative transitions in CBTS involving deeper defects. Small peak shifts versus excitation intensity are difficult to determine reliably for such low-intensity peaks.…”

“…19,20 Compensation of V Cu by the Cu i donor is stronger in CSTS than in CBTS, which is consistent with a higher Fermi level position measured in CSTS with respect to its valence band maximum. 56 Since V Cu and Cu i are the most probable origin of the shallow defects identified by PL, we compare their calculated charge transition levels E t to the measured defect ionization energies E i in Tables 2 and 3. In CBTS, E t = 63 meV for V Cu and E t = 118 meV for Cu i .…”

Assessing the defect tolerance of kesterite-inspired solar absorbers

“…They cannot simply be phonon replicas of the TI/FB peak because they would require much higher phonon wavenumbers than the ones found experimentally in CBTS (below 400 cm −1 ). 19,54 Thus, these peaks must be related to other radiative transitions in CBTS involving deeper defects. Small peak shifts versus excitation intensity are difficult to determine reliably for such low-intensity peaks.…”

“…19,20 Compensation of V Cu by the Cu i donor is stronger in CSTS than in CBTS, which is consistent with a higher Fermi level position measured in CSTS with respect to its valence band maximum. 54 Since V Cu and Cu i are the most probable origin of the shallow defects identified by PL, we compare their calculated charge transition levels E t to the measured defect ionization energies E i in Table II we assign the ∼40 meV level to the V Cu acceptor and we assign the ∼130 meV level to the Cu i donor as shown in Fig. 5.…”

Comprehensive physicochemical and photovoltaic analysis of different Zn substitutes (Mn, Mg, Fe, Ni, Co, Ba, Sr) in CZTS-inspired thin film solar cells

“…However, there are other more dominant broad peaks at lower energy levels which can be related to the secondary phases with lower band gap or deep defects in these structures which is often found to generate high density of bulk recombination centers. 89 The ERE and QFLS loss in CBaTS and CSrTS are more promising than CZTS. The ERE is defined as ratio between emitted photon flux and excitation flux which means that the quality of radiative emission in these two films seems to be better than CZTS in this study.…”