Significant Efficiency Boost in Sb₂S₃ Planar Heterojunction Solar Cells via Rapid Selenylation

Abstract

Thermal instability of Sb₂S₃ in vacuum limits the deposition of high‑quality crystalline films, which in turn hampers photovoltaic performance. We employed a short‑time selenylation post‑treatment to enhance the optoelectronic properties of Sb₂S₃ planar heterojunction solar cells. A mere 15‑minute selenylation step raised the power conversion efficiency (PCE) from ~0.01 % to 2.20 %, accompanied by a ten‑fold increase in short‑circuit current density (J_SC) and a marked improvement in open‑circuit voltage (V_OC). Structural, morphological, compositional, and energy‑level analyses reveal that selenylation improves crystallinity, introduces a favorable gradient in band alignment, and facilitates charge transport from absorber to buffer layer.

Background

Inorganic thin‑film solar cells have attracted significant interest due to their low cost, lightweight nature, and superior environmental stability compared with perovskite and organic counterparts [1], [2]. CuInGaSe₂ (CIGS) and CdTe technologies have already achieved efficiencies of 21.7 % and 19.6 % respectively [6], [7]. The earth‑abundant Cu₂ZnSnSₓSe₄₋ₓ (CZTSSe) material has reached 12.6 % via hydrazine‑based solution processing, but suffers from phase control challenges and hydrazine toxicity [10], [11], [12], [13]. Antimony sulfide (Sb₂S₃) offers a low‑to‑moderate bandgap (1.1–1.7 eV) tunable by Se substitution, low toxicity, and abundant raw materials [14], [15]. While Sb₂S₃‑sensitized heterojunctions have achieved 7.5 % PCE, they rely on complex organic components [19]. Planar heterojunctions, on the other hand, benefit from simpler device architectures and smoother interfaces.

Conventional vacuum deposition of Sb₂S₃ suffers from low thermal stability and high vapor pressure, leading to compositional drift and surface oxidation [25]. Rapid thermal processing (RTP) has previously enabled PCEs up to 4.17 % via careful control of evaporation and annealing [23]. However, RTP requires high temperatures that can degrade flexible substrates. Our approach leverages low‑temperature thermal evaporation (~175 °C) followed by a brief RTP selenylation step, providing a scalable route to high‑performance Sb₂S₃ devices.

Methods / Experimental

The devices adopt a superstrate architecture: FTO (SnO₂:F)/CdS (90 nm)/Sb₂S₃/Sb₂S₃(Se)/Au (60 nm). FTO glass (Pilkington, Toledo, USA) with 7 Ω/□ sheet resistance served as the electron‑collecting electrode. CdS was deposited by chemical bath deposition [26]. Sb₂S₃ films were thermally evaporated from 99.9 % Aladdin powder under <5 × 10⁻⁴ Pa at 175 °C, then cooled naturally. Post‑treatment employed a two‑zone RTP furnace at 10³ Pa in N₂: Sb powder at 350 °C (low‑temperature zone) and the sample at 400 °C (high‑temperature zone). After selenylation, 60 nm Au was sputtered by DC magnetron sputtering.

Device performance was measured under AM 1.5 (100 W cm⁻²) xenon illumination (Newport 94043A) using a Keithley 2400 source meter. External quantum efficiency (EQE) was recorded with a Beijing SOFN 7‑SCSpecIII system. Structural analysis employed XRD (Bruker D8), optical absorption was measured by UV‑Vis (Agilent Cary 5000), and energy levels were probed by UPS (Thermo ESCALAB 250Xi). Surface morphology was examined by SEM (FEI Helios Nanolab 600i). Electrochemical impedance spectroscopy (EIS) under open‑circuit bias evaluated charge‑transport characteristics.

Results and Discussion

The fabrication workflow is illustrated in Fig. 1a. Each sample contains eight cells (4 mm² active area). Untreated devices displayed a negligible PCE (<0.01 %) with V_OC = 0.31 V and J_SC = 0.14 mA cm⁻². A 10‑minute vacuum anneal modestly improved J_SC to 0.66 mA cm⁻² (PCE = 0.08 %). In contrast, selenization dramatically enhanced performance: a 15‑minute treatment yielded J_SC = 9.04 mA cm⁻² and PCE = 2.20 %. Extending selenylation beyond 15 min degraded both V_OC and J_SC, with a 20‑minute run reducing PCE to 0.61 %. EQE spectra (Fig. 1c) confirm the superior spectral response of the 15‑minute selenized devices, with a broadening of the response window and a red‑shift of the EQE peaks.

XRD patterns (Fig. 2a) show that untreated Sb₂S₃ films are largely amorphous. Vacuum annealing and selenization sharpen the orthorhombic Sb₂S₃ peaks (JCPDS 15‑0861). The (120) peak shifts from 17.50° (as‑deposited) to 16.95° after 15‑minute selenylation, aligning with the Sb₂Se₃ PDF (JCPDS 73‑0393). The shift indicates incorporation of larger Se atoms (1.98 Å) in place of S (1.84 Å), expanding the lattice. SEM images (Fig. 3) reveal grain growth from sub‑micron to micron scale with selenization, but 20‑minute treatment introduces surface bulges and cracks that impair interface contact and elevate series resistance.

Optical absorption (Fig. 4a) improves with annealing and selenization, with the absorption edge red‑shifting, indicating a reduced bandgap. Tauc analysis gives E_g = 2.03 eV (as‑deposited), decreasing to 1.60 eV after vacuum anneal, and further to 1.44 eV after 20‑minute selenylation. Photoluminescence (PL) spectra (Fig. 4c) support this trend: the 15‑minute selenized film exhibits a dual‑peak PL (1.62 eV and 1.31 eV), suggesting a gradient in composition where the bulk remains Sb₂S₃ while the surface is partially converted to Sb₂Se₃. UPS measurements (Fig. 4d‑f) confirm the favorable band alignment of Sb₂S₃/Sb₂S₃(Se)/Sb₂Se₃ with CdS, reducing the hole‑transport barrier and improving V_OC and J_SC.

EIS data (Fig. 5) fit an R‑CPE equivalent circuit. The series resistance (R₁) drops from 519.8 × 10⁻³ Ω cm² (untreated) to 0.4 × 10⁻³ Ω cm² (15‑minute selenized), indicating superior charge transport. The shunt resistance (R₂) and capacitance (CPE‑T) remain relatively unchanged across samples, suggesting that interface quality is largely preserved until excessive selenylation induces bulges.

Conclusions

Rapid selenylation at 350 °C (Se) and 400 °C (Sb₂S₃) for 15 minutes significantly enhances Sb₂S₃ planar heterojunction solar cells by improving crystallinity, creating a beneficial band‑gradient, and reducing carrier transport barriers. Excessive selenylation (>15 min) induces surface defects that compromise the absorber–buffer interface, reducing device yield and performance. This method offers a scalable, low‑temperature route to high‑efficiency Sb₂S₃ devices, with potential for flexible photovoltaic applications.