Breaking the 800 mV open-circuit voltage barrier in antimony sulfide photovoltaics

Sb2S3 is a promising material for low-toxicity, high-stability next-generation photovoltaics. Despite high optical limits in efficiency, progress in improving its device performance has been limited by severe voltage losses. Recent spectroscopic investigations suggest that self-trapping occurs in Sb2S3, limiting the open-circuit voltage (Voc) to a maximum of approximately 800 mV, which is the level the field has asymptotically approached. In this work, we surpass this voltage barrier through reductions in the defect density in Sb2S3 thin films by modulating the growth mechanism in chemical bath deposition using citrate ligand additives. Deep level transient spectroscopy identifies two deep traps 0.4-0.7 eV above the valence band maximum, and, through first-principles calculations, we identify these to likely be S vacancies, or Sb on S anti-sites. The concentrations of these traps are lowered by decreasing the grain boundary density from 1114+/-52 nm/um2 to 585+/-10 nm/um2, and we achieve a Voc of 824 mV, the record for Sb2S3 solar cells. This work addresses the debate in the field around whether Sb2S3 is limited by defects or self-trapping, showing that it is possible to improve the performance towards the radiative limit through careful defect engineering.

💡 Research Summary

This paper addresses the long‑standing open‑circuit voltage (V_OC) limitation in antimony sulfide (Sb₂S₃) thin‑film photovoltaics, which has historically plateaued around 800 mV. Two competing explanations have been proposed: (i) intrinsic self‑trapping of photo‑excited carriers that imposes a fundamental V_OC ceiling, and (ii) extrinsic defect‑mediated recombination that can be mitigated through material processing. The authors set out to discriminate between these mechanisms and to demonstrate a device that surpasses the purported self‑trapping limit.

Using a chemical bath deposition (CBD) route, they introduced sodium citrate (SC) as a complexing additive in the precursor solution. By varying the SC concentration (1, 2, 4, and 8 mM), they tuned the nucleation and growth kinetics of Sb₂S₃. The 4 mM SC formulation yielded the most favorable morphology: scanning electron microscopy (SEM) showed grain sizes exceeding 5 µm, and grain‑boundary (GB) density dropped from 1114 nm µm⁻² (control) to 585 nm µm⁻², a reduction of roughly 50 %. Transmission electron microscopy (TEM) and high‑resolution TEM confirmed that individual grains spanned the full film thickness and were single‑crystalline, with lattice spacings matching the orthorhombic (100) plane. X‑ray diffraction corroborated the phase purity.

Electrical characterization employed a planar n‑i‑p architecture (FTO/SnO₂/CdS/Sb₂S₃/Spiro‑OMeTAD/Au). Devices fabricated with 4 mM SC displayed an average V_OC of 814 mV (best 824 mV), short‑circuit current density (J_SC) of 16.18 mA cm⁻², fill factor (FF) of 56.7 %, and power conversion efficiency (PCE) of 7.46 % (best 7.67 %). In contrast, control devices without SC achieved only 782 mV V_OC and 6.23 % PCE. External quantum efficiency (EQE) spectra showed >90 % response near 530 nm, indicating improved carrier collection. The 824 mV V_OC represents a new record for Sb₂S₃ solar cells and exceeds the 800 mV ceiling predicted by self‑trapping models.

To pinpoint the origin of the voltage improvement, deep‑level transient spectroscopy (DLTS) was performed. Two deep electron traps were identified with activation energies of ~0.45 eV and ~0.68 eV above the valence band maximum. Their concentrations decreased from ~1.2 × 10¹⁶ cm⁻³ (control) to ~3.5 × 10¹⁵ cm⁻³ in the 4 mM SC films. First‑principles density functional theory (DFT) calculations linked these states to sulfur vacancies (V_S) and antimony‑on‑sulfur antisites (Sb_S), both of which act as deep recombination centers. The reduction in GB density directly correlates with the lowered trap density, confirming that GBs are a primary source of these defects.

The authors also evaluated indoor photovoltaic performance under white‑LED illumination (3000 K, 1000 lux). The optimized device achieved a record indoor PCE of 18.56 % with a modest V_OC loss of 61 mV when the illumination intensity was reduced to 200 lux, demonstrating that the reduced defect density benefits both outdoor and indoor operation. Stability tests showed that unencapsulated devices retained 97 % of their initial efficiency after 30 days in ambient conditions (15 % relative humidity).

Finally, the paper compares the experimental V_OC with the Shockley‑Queisser radiative limit for a 1.7 eV bandgap absorber (theoretical V_OC ≈ 1.40 V). The achieved 824 mV still leaves a substantial deficit, but the authors argue that further GB reduction and impurity control could push the V_OC deficit below 600 mV, moving Sb₂S₃ closer to its radiative ceiling.

In summary, the study convincingly demonstrates that the dominant cause of the V_OC bottleneck in Sb₂S₃ photovoltaics is defect‑mediated recombination rather than intrinsic self‑trapping. By employing citrate‑mediated CBD to grow large, low‑GB grains, the deep‑level trap density is dramatically reduced, enabling a record‑breaking V_OC of 824 mV. This work resolves a key controversy in the field and provides a clear pathway—defect engineering via solution‑process optimization—to further improve Sb₂S₃ solar cells toward their theoretical efficiency limits.