Polyethylene Glycol Enhances NiO Photocathodes for Quantum‑Dot Tandem Solar Cells

Abstract

In this study, a uniform nanoporous NiO film up to 2.6 µm thick was fabricated by incorporating polyethylene glycol (PEG) into the precursor solution. PEG markedly suppressed crack formation and prevented delamination from fluorine‑doped tin oxide (FTO) substrates. The NiO cathode, sensitized with CdSeS quantum dots (QDs), achieved a record photo‑conversion efficiency of 0.80 %. When assembled with a TiO₂ anode, the resulting QD‑sensitized p–n‑type tandem solar cell displayed an open‑circuit voltage (OCV) higher than the individual electrodes, delivering 0.43 % efficiency (OCV = 0.594 V, J_SC = 2.0 mA cm^-2, FF = 0.36).

Background

Solar energy is poised to become a primary power source due to its clean, abundant, and rapidly deployable nature. Over the past three decades, dye‑sensitized solar cells (DSSCs) have evolved into efficient devices, yet most progress has focused on n‑type architectures using TiO₂, ZnO, or SnO₂ photoanodes, achieving short‑circuit current densities (J_SC) above 15 mA cm^-2 and efficiencies near 13 %. p–n‑type tandem DSSCs, combining a p‑type photocathode with an n‑type anode, promise higher OCVs and overall efficiencies. Notable advances include a 0.918 V OCV from an NK‑2684‑sensitized NiO/TiO₂ pair and a 1079 mV OCV with donor–acceptor dye optimization, the highest reported for p–n tandems to date.

Despite these gains, p‑type photocathodes typically deliver photocurrents an order of magnitude lower than their n‑type counterparts, partly due to mechanical instability in thick films. Recent work has explored high‑surface‑area NiO nanostructures, improved crystallinity, and layered architectures to boost performance. However, the photo‑conversion efficiencies of NiO electrodes remain in the 0.02–0.30 % range when paired with various dyes.

This work builds on the PEO–PPO–PEO triblock copolymer templating strategy to synthesize thick NiO films, adding PEG as a structural modifier to enhance film integrity and porosity, ultimately enabling high‑efficiency QD‑sensitized tandem devices.

Experimental

NiO precursor solutions were prepared by dissolving anhydrous NiCl₂ (1 g) and F108 (1 g) in 3 g deionized water and 6 g ethanol. After a 3‑day resting period, PEG (MW ≈ 20 000) was added at 0.03, 0.075, 0.15, or 0.30 g. The mixture was stirred for 4 h and centrifuged at 8000 rpm. The resulting gel was doctor‑bladed onto FTO glass and dried at room temperature, then sintered at 400 °C for 30 min in air. CdSeS QDs were synthesized via hot‑injection, and NiO films were sensitized by electrophoretic deposition in a 1:2.5 (v/v) acetonitrile/toluene solution under 50 V DC. TiO₂ anodes were co‑sensitized with CdS/CdSe using SILAR, replacing CuS as the counter electrode for tandem cell assembly.

Film morphology was examined by JSM‑7001F FE‑SEM. Photocurrent–voltage (J–V) characteristics were recorded under AM 1.5G illumination (1 Sun) with a Keithley 2440 source meter.

Results and Discussion

SEM imaging revealed that NiO films fabricated without PEG exhibited pronounced micro‑ravines and curled nanosheets that delaminated after multiple doctor‑blading passes. In contrast, PEG‑containing films, after seven blading cycles, displayed a continuous, crack‑free surface with a thickness of ~2.6 µm. PEG likely acts as a binder, enhancing inter‑particle cohesion, and as a structure‑directing agent that increases specific surface area and pore volume.

SEM micrographs: a, c, e – films without PEG; b, d, f – films with PEG.

J–V measurements showed that adding PEG up to 0.15 g improved the photo‑conversion efficiency from 0.08 % (PEG = 0) to 0.32 %. The optimal NiO cathode achieved OCV = 0.158 V, J_SC = 4.40 mA cm^-2, and FF = 0.46. Excessive PEG (0.30 g) caused performance decay, indicating an optimal concentration range.

Current‑density vs. voltage for NiO photocathodes with different PEG contents.

Varying film thickness (fixed PEG = 0.15 g) revealed that increasing thickness from 0.6 to 2.1 µm raised OCV and J_SC, but beyond ~1.5 µm the benefits plateaued due to limited hole transport and shortened lifetimes.

Influence of NiO thickness on photovoltaic performance.

When paired with a TiO₂ anode, the TiO₂(down)/NiO(up) tandem configuration achieved an OCV of 0.594 V and 0.43 % overall efficiency, surpassing the individual electrodes. Although J_SC and efficiency were lower than the standalone NiO cathode or TiO₂ anode, this represents the first demonstration of a QD‑sensitized p–n‑type tandem device, underscoring the potential for further optimization.

J–V characteristics of p–n‑type QD‑sensitized tandem solar cells.

Conclusion

PEG incorporation yields a crack‑free, 2.6‑µm‑thick nanoporous NiO film with superior mechanical stability and surface area. The optimized QD‑sensitized NiO cathode delivers 0.80 % photo‑conversion efficiency. As part of a TiO₂/NiO tandem architecture, the device achieves an OCV of 0.594 V and a 0.43 % efficiency, outperforming the isolated electrodes. Future work will focus on enhancing charge extraction and reducing recombination to unlock the full potential of QD‑sensitized p–n‑type tandem solar cells.