High‑Performance PEDOT:PSS/n‑Si Solar Cells with Textured Surfaces and Silver Nanowire Electrodes

Background

Crystalline silicon dominates the global photovoltaic market, accounting for roughly 90% of sales due to its balance of performance and cost. Hybrid heterojunction solar cells that pair n‑crystalline silicon with PEDOT:PSS have emerged as a compelling alternative, thanks to their dopant‑free, vacuum‑free, low‑temperature, solution‑processed fabrication. The current record for a PEDOT:PSS/n‑Si HHSC is 16.2% PCE, and the efficiency gap with conventional silicon cells is steadily closing.

In a PEDOT:PSS/n‑Si cell, crystalline silicon acts as the high‑mobility absorber, while the PEDOT:PSS layer serves as a transparent, conductive hole‑transporting window. However, the overall efficiency is often limited by the quality of the PEDOT:PSS/n‑Si interface. Interface engineering—through surface texturing, passivation layers, or back‑surface fields—has proven effective in reducing recombination and improving charge collection.

While many strategies focus on planar substrates, the contact quality between PEDOT:PSS and textured silicon has been less explored. Texturing, typically achieved with alkaline etchants, creates random micro‑pyramids that enhance light trapping but also pose challenges for uniform film deposition. Moreover, traditional silver electrodes can suffer from limited optical transparency on textured surfaces. Silver nanowires offer a solution, providing high transmittance and conductivity while being compatible with drop‑casting onto textured polymers.

Methods

Preparation of Textured Si Substrates for HHSCs

We used n‑Si(100) Czochralski wafers (210 µm thick, 1–3 Ω·cm) as substrates. After standard SC1/SC2 cleaning, the wafers were polished in a high‑concentration KOH solution at 75 °C for 2–3 min to remove the damaged layer. The surfaces were then etched in a 2 wt.% KOH/2 wt.% isopropanol bath at 75 °C for 15–20 min to form a double‑sided random pyramid structure with an average height of ~1 µm. Final RCA cleaning and a brief HF dip (0.5–1 min) produced an oxide‑free, hydrophobic silicon surface ready for PEDOT:PSS deposition.

Fabrication of Si/PEDOT:PSS Hybrid Solar Cells

Figure 1 illustrates the fabrication sequence. A 200 nm aluminum back contact was sputtered onto the wafer backside. The PEDOT:PSS (Clevios PH1000) solution was doped with 5 wt.% DMSO and 0.1 wt.% fluoride surfactant (Capstone FS31) to improve conductivity and wetting. The mixture was spin‑coated at various rates (1 000–8 000 rpm) and annealed at 130 °C for 15 min. For conventional electrodes, a 200 nm silver grid was thermally evaporated through a shadow mask. For the AgNWs variant, a 5 mg/mL dispersion of 50 nm diameter, 100–200 µm long silver nanowires (XFNANO) in isopropanol was drop‑cast onto the wafer and dried at 150 °C for 5 min.

Schematic of preparing the n‑Si/PEDOT:PSS solar cells with (a‑f) silver grid electrodes or (a‑e, g) silver nanowire electrodes.

Device Characterization

Reflectance was measured with an integrating sphere; SEM images were captured with a Hitachi S4800. Current–voltage (J‑V) characteristics were recorded under AM 1.5 (100 mW/cm²) illumination using an Oriel solar simulator (94063A) and a Keithley 2400 source meter. Absorption spectra were obtained with a UV‑8000 spectrophotometer, and transmittance of the PEDOT:PSS film was measured with a QEX10 spectrophotometer. Sheet resistance was determined by a four‑probe tester (SDY‑4).

Results and Discussion

Enhancing the PEDOT:PSS film’s optical and electrical properties via additives is a proven strategy. Adding 5 wt.% DMSO to the PEDOT:PSS solution increases its conductivity by an order of magnitude, while 0.1 wt.% FS31 reduces the contact angle on hydrophobic silicon from 104.3° to a much lower value, enabling more uniform coating.

The contact angle between the wafer and PEDOT:PSS solution without (a) and with (b) FS31.

Figure 3 shows that the PEDOT:PSS film transmits ~85% of visible light when coated at 5 000 rpm. With DMSO and FS31, transmittance increases slightly across 600–1 000 nm, and the film’s absorption in the 400–600 nm range improves, making it an excellent optical window.

Red: absorbance of PEDOT:PSS with additives (400–1 000 nm); blue: transmittance of PEDOT:PSS with/without additives and reference glass.

Micro‑pyramidal texturing enhances light trapping, reducing baseline reflectance to ~10–20%. Adding a PEDOT:PSS layer lowers reflectance by ~5% across the spectrum, but the exact value depends on spin‑coating speed. Higher spin speeds increase surface tension, enabling the polymer to penetrate the pyramid valleys and improve contact. Figures 4 and 5 illustrate that at 4 000–5 000 rpm the film conforms closely to the texture, eliminating voids and maximizing junction area. However, at 8 000 rpm the film becomes too thin, leaving gaps that reduce the open‑circuit voltage.

SEM top‑view images of textured Si with PEDOT:PSS film at spin speeds 1 000–5 000 rpm (a–e). (f) shows the uncoated surface.

Cross‑sectional views of PEDOT:PSS on textured Si at 4 000 (a) and 5 000 rpm (b).

Reflectance measurements (Figure 6) confirm that the PEDOT:PSS layer improves antireflection, particularly for wavelengths 600–1 000 nm. The optimal spin speed balances film thickness (to minimize parasitic absorption) and junction coverage (to maximize charge collection). The best device performance—J_sc = 26.88 mA/cm², FF = 62.1%, V_oc = 0.56 V, PCE = 11.07%—was achieved with a 4 000 rpm coating and silver nanowire electrodes.

Reflectance curves for PEDOT:PSS-coated textured Si at various spin speeds and for the uncoated substrate.

The J‑V curves (Figure 7) illustrate the clear trend: as spin speed increases from 1 000 to 5 000 rpm, J_sc rises due to improved junction area and reduced recombination. Beyond 5 000 rpm, performance drops because the film becomes too thin.

J‑V curves for cells with different PEDOT:PSS spin speeds.

Silver nanowire electrodes further improve device metrics by lowering series resistance from 0.84 to 0.38 Ω·cm², increasing FF from 62.1% to 72.2%, and boosting V_oc from 0.51 to 0.56 V. The AgNWs also provide plasmonic light scattering, enhancing absorption in the 400–700 nm range and contributing to a higher short‑circuit current density.

J‑V curves for PEDOT:PSS/n‑Si cells with silver nanowire electrodes.

a) Cross‑sectional view of a cell with AgNWs; b) close‑up of the contact area.

Conclusions

By adding DMSO and FS31 to PEDOT:PSS, we achieved a highly conductive, low‑contact‑angle film that conforms well to micro‑textured silicon. Optimizing the spin‑coating rate to 4 000 rpm maximizes junction area while maintaining an appropriate film thickness, yielding a PCE of 11.07% with silver nanowire electrodes. This approach offers a simple, scalable route to high‑efficiency, low‑cost hybrid solar cells.