Methanol‑Enhanced PEDOT:PSS Hole Transport Layer Boosts Silicon/Organic Hybrid Solar Cell Efficiency to 12.2%

Abstract

In a recent experimental investigation, efficient Si/organic hybrid solar cells were fabricated using dimethyl sulfoxide (DMSO) and a surfactant‑doped PEDOT:PSS blend. By subjecting the PEDOT:PSS films to a post‑treatment with polar solvents, the device performance could be further increased. We found that higher solvent polarity leads to higher efficiencies. Methanol treatment produced a PEDOT:PSS conductivity of 1105 S cm⁻¹ and the corresponding hybrid cells achieved a record efficiency of 12.22 %. X‑ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed that the removal of insulating PSS and conformational changes in the polymer are key to the performance boost. Electrochemical impedance spectroscopy (EIS) demonstrated that methanol‑treated devices exhibit larger recombination resistance and capacitance, underscoring the effectiveness of this simple, low‑cost approach.

Background

Silicon‑organic hybrid solar cells combine the high absorption of crystalline silicon with the lightweight, solution‑processable nature of organic layers, offering a path toward low‑temperature, cost‑effective photovoltaics. The hole‑transporting layer (HTL) is crucial for charge extraction and series resistance; PEDOT:PSS, a conductive polymer, is widely used as an HTL in organic electronics due to its high conductivity and optical transparency. However, the insulating polystyrene sulfonate (PSS) component limits its performance. Previous studies have employed co‑solvents (e.g., DMSO, ethylene glycol) or acid treatments to enhance PEDOT:PSS conductivity, yet these approaches can compromise stability or require harsh chemicals. This work explores a gentle, post‑treatment with polar alcohols—particularly methanol—to selectively remove PSS and reorient PEDOT chains, thereby improving the Si/PEDOT:PSS interface without compromising device stability.

Methods

Double‑side‑polished n‑type CZ Si(100) wafers (2.6–3.5 Ω cm, 450 µm thick) were cleaned sequentially in acetone, ethanol, and DI water (ultrasonic 20 min each). The wafers were then oxidized in 80 °C piranha solution (3:1 H₂SO₄/H₂O₂) for 30 min, rinsed, and etched in 5 % HF for 5 min to expose a H‑Si surface. A thin SiO₂ passivation layer was formed by immersing the wafers in 10 % HNO₃ (5 min). The PEDOT:PSS (Clevios PH1000) dispersion was mixed with 5 wt % DMSO and 1 wt % Triton X‑100, then spin‑coated on the SiO₂/Si substrate (1500 rpm, 60 s) and annealed at 140 °C for 10 min under N₂. A 60 µL aliquot of methanol (or other alcohols) was then deposited on the dried PEDOT:PSS film and spin‑coated again (2000 rpm, 60 s) before a final anneal at 120 °C for 10 min. Silver (200 nm) grids were thermally evaporated as the top electrode, and aluminum (200 nm) was deposited on the rear side under high vacuum (~10⁻⁷ Pa). Devices had an active area of 0.3 cm².

Device performance (J‑V, EQE) was measured under AM 1.5 (100 mW cm⁻²) illumination from a xenon lamp, calibrated with a reference silicon cell. PEDOT:PSS film conductivity was determined by a 4‑point probe (RST‑9). XPS spectra were collected on a Thermo ESCALAB 250 (Al Kα). Raman spectra were acquired with a 532 nm laser. Electrochemical impedance spectroscopy (EIS) was performed with a CHI660E workstation (10⁻¹–10⁶ Hz). Transmittance and AFM were used to assess optical and surface properties.

Results and Discussion

PEDOT:PSS/Si Hybrid Cell Performance

Figure 1 presents the J‑V and EQE characteristics of cells treated with different alcohols. The control (DMSO only) exhibited V_OC = 0.552 V, J_SC = 27.09 mA cm⁻², FF = 63.6 %, yielding PCE = 9.51 %. IPA treatment increased PCE to 9.98 % (J_SC = 27.71 mA cm⁻², FF = 64.7 %). Ethanol treatment further raised PCE to 10.69 % (V_OC = 0.556 V, J_SC = 28.16 mA cm⁻², FF = 68.3 %). Methanol treatment produced the highest PCE of 12.22 % (J_SC = 30.58 mA cm⁻², FF = 72.0 %)—a 28 % improvement over the control.

Conductivity and Optical Properties

Four‑point probe measurements revealed that PEDOT:PSS conductivity increased from 0.3 S cm⁻¹ (without DMSO) to 650 S cm⁻¹ with DMSO. Post‑treatment with IPA, ethanol, and methanol further raised conductivities to 826, 908, and 1105 S cm⁻¹, respectively. Sheet resistance dropped from 200 Ω cm² (control) to 105 Ω cm² (methanol). Transmittance at 550 nm remained >95 % for all films, indicating that the improvement originates from electronic rather than optical changes.

Structural Insights from Raman and XPS

Raman spectra showed a systematic shift of the PEDOT C_α–C_β peak from 1429 cm⁻¹ (control) to 1425.8 cm⁻¹ (IPA), 1422.7 cm⁻¹ (methanol), reflecting a transition from benzoid to quinoid conformation, which enhances charge mobility. XPS S 2p analysis indicated a reduction of the PSS/PEDOT ratio from 2.48 (control) to 1.33 (methanol), confirming selective removal of insulating PSS. AFM confirmed that surface roughness remained low (≤2.4 nm) across all treatments.

Impedance Spectroscopy

EIS revealed a larger recombination resistance (R_PN) and longer minority‑carrier lifetime (τ ≈ 751 µs) for methanol‑treated devices compared to 622 µs for controls, indicating reduced interfacial recombination. Mott‑Schottky analysis showed negligible change in built‑in potential, confirming that the enhanced performance stems from improved charge transport rather than band‑alignment shifts.

Conclusions

This study demonstrates that a simple, post‑treatment of PEDOT:PSS films with methanol can markedly enhance the conductivity (up to 1105 S cm⁻¹) and consequently the efficiency of Si/PEDOT:PSS hybrid solar cells (12.22 %). The improvement is attributed to selective PSS removal, conformational reorganization of PEDOT chains, and reduced interfacial recombination. Because the process involves only mild solvent spin‑coating and low‑temperature annealing, it offers a scalable, cost‑effective route to high‑performance hybrid photovoltaics.