Reduced Graphene Oxide HTM for Perovskite Solar Cells - University of Padova, 2018

Gatti, T. et al. (2018). Interfacial morphology addresses performance of perovskite solar cells based on composite hole transporting materials of functionalized reduced graphene oxide and …. *Solar RRL*. https://doi.org/10.1002/solr.201800013

Department of Chemical Sciences, University of Padova via Marzolo 1 35131 Padova Italy · Solar RRL · 2018

University of Padova researchers used ACS Material reduced graphene oxide to build functionalized RGO/P3HT hole-transport layers for highly reproducible perovskite solar cells.

About this research

Researchers from the Department of Chemical Sciences, University of Padova via Marzolo 1 35131 Padova Italy, working with collaborators at the Istituto Italiano di Tecnologia and the University of Groningen, used reduced graphene oxide (RGO) powder supplied by ACS Material, LLC (product No. GnP1L-0.5g) to develop composite hole-transport materials (HTMs) for perovskite solar cells (PSCs). By covalently grafting five different aryl substituents onto the ACS Material RGO and blending the products with poly(3-hexylthiophene) (P3HT), the team produced HTM films whose interfacial morphology could be tuned at the molecular level. The best composite, RGO-PhOHex@P3HT, yielded perovskite solar cells with an average power conversion efficiency of 9.8% and a four-fold reduction in PCE standard deviation compared to standard Spiro-OMeTAD devices.

This research matters because perovskite solar cells remain limited by the cost, instability, and doping-related variability of Spiro-OMeTAD, the most widely used HTM. For PSC technology to be industrialized, researchers need HTM formulations that combine acceptable hole mobility with high batch-to-batch reproducibility, low cost, and dopant-free operation. Polymer/graphene composites are attractive because the conductive 2D filler can enhance hole transport while the polymer provides film formability. However, graphene-based fillers tend to aggregate inside polymer matrices, ruining the homogeneity of the selective contact. The challenge addressed here is how to design molecular interactions between RGO and P3HT such that the filler disperses uniformly, the perovskite/HTM interface remains smooth, and the resulting devices are reproducible at scales relevant to module fabrication (>50 cm²).

The ACS Material RGO powder was the carbon nanostructure starting point for the entire study. Twenty milligrams of RGO were ultrasonicated in N-cyclohexylpyrrolidone using a Misonix 3000 titanium-tip sonicator, then reacted in situ with one of five aniline derivatives in the presence of isoamyl nitrite at 80 °C for 4 hours via a Tour-type direct arylation reaction. This grafted thienyl-type substituents (PhTh, PhMeTh, PhBiTh) or alkyl-type substituents (PhOHex, PhOEtHex) onto the RGO flakes, with functionalization degrees ranging from 1.2% to 3.2% as measured by thermogravimetric analysis. The functionalized RGO was then dispersed in P3HT solution in chlorobenzene at an initial 10 wt% loading, ultrasonicated, and centrifuged at 3000 rpm to remove aggregates. Final RGO loadings in the P3HT blends ranged from 5.1% to 6.7%, all above the 4% threshold previously reported as optimal for PSC performance. Thin films were spin-coated at 2000 rpm for 40 s for both characterization and device integration as the HTM on top of mixed-cation Cs0.15FA0.85PbI3 perovskite absorbers.

The RGO-PhOHex@P3HT composite, containing 6.3 wt% RGO, produced the best photovoltaic results. Across a statistically large device batch, this HTM gave an average PCE of 9.8%, slightly higher than the 9.4% average obtained with doped Spiro-OMeTAD, and with a standard deviation at least four times smaller. Bare P3HT averaged 8.7%, and the RGO-PhBiTh@P3HT composite averaged only 8.1% despite its higher 6.7 wt% RGO content. Electrochemical impedance spectroscopy revealed a parallel resistance of 5.6 kΩ for RGO-PhOHex@P3HT versus 0.3 kΩ for bare P3HT and 0.17 kΩ for RGO-PhBiTh@P3HT, consistent with a much smoother and more uniform film. AFM mapping confirmed this: the RMS roughness of RGO-PhOHex@P3HT was 28 ± 17 nm over 30 × 30 µm² areas, versus 150 ± 90 nm for RGO-PhBiTh@P3HT, where protrusions reached almost 1 µm. SEM imaging showed that bithienyl-substituted RGO flakes self-aggregate through intramolecular π-stacking and protrude through the polymer film, creating short-circuit pathways and charge-recombination sites at the perovskite/HTM interface that suppress short-circuit current. UV-vis spectra showed enhanced I0–0/I0–1 ratios in all functionalized blends, indicating improved polymer planarization and conjugation length around the dispersed RGO.

The demonstrated ability to control HTM morphology through covalent surface chemistry is broadly relevant to perovskite photovoltaics, large-area solar modules, and any application where 2D-material/polymer composites must form smooth, defect-free interfaces. The paper itself points toward stability studies under environmental stress and the fabrication of large-area (>50 cm²) PSCs as immediate follow-up work. The contrasting morphology of RGO-PhBiTh@P3HT, with its wrinkled, high-surface-area filler distribution, is suggested as potentially useful for photocatalysis and supercapacitor electrodes, where high active area is more important than smooth interfaces. The general design principle—matching grafted chemistry to the host polymer side chains—extends to organic field-effect transistors, thermoelectric composites, and flexible electronics where RGO/conjugated-polymer blends are already established.

The success of this approach hinges on starting from a consistent, well-characterized RGO source. The reduced graphene oxide used in this work was supplied by ACS Material, and researchers pursuing similar HTM, composite, or 2D-material/polymer dispersion projects can source the same RGO powder grade and other reduced graphene oxide products from the ACS Material graphene series catalog. The paper demonstrates that with appropriate covalent functionalization, commercial RGO can be transformed into a competitive, doping-free hole-transport layer that outperforms Spiro-OMeTAD in device-to-device uniformity—a property critical for commercial PSC manufacturing.

How ACS Material products were used

• Reduced Graphene Oxide (RGO) Powder (GnP1L-0.5g) (Graphene Series)  — “RGO powder was purchased from ACS Material, LLC (product No.: GnP1L-0.5g).”

Product Performance in this Study

The RGO powder served as the carbon nanostructure backbone for covalent functionalization with five different aryl groups. After functionalization and sedimentation-based separation, the resulting RGO derivatives were homogeneously dispersed in P3HT and used as composite hole-transporting layers in perovskite solar cells, with one blend (RGO-PhOHex@P3HT) outperforming standard Spiro-OMeTAD in reproducibility.

Related product categories

• Graphene Series

Frequently asked questions

How does reduced graphene oxide improve perovskite solar cell hole transport layers?

Reduced graphene oxide added to a P3HT hole-transport layer increases hole mobility through its conductive 2D network and improves polymer planarization. In this work, covalently functionalized RGO dispersed at 6.3 wt% in P3HT produced perovskite solar cells with 9.8% average efficiency and four-fold tighter reproducibility than Spiro-OMeTAD. The RGO also eliminates the need for hygroscopic dopants like Li-TFSI that compromise long-term stability.

Why does functionalization of RGO matter for polymer composite hole transport materials?

Pristine RGO self-aggregates in polymer matrices due to π-π stacking, ruining film homogeneity. Covalent functionalization with substituents matched to the host polymer—such as hexyloxy groups for P3HT—stabilizes individual flakes in solution and yields smooth films. In this study, hexyl-decorated RGO gave 28 nm RMS roughness while bithienyl-decorated RGO gave 150 nm roughness with 1 µm protrusions that caused short circuits and lower photocurrent.

What is the role of P3HT in graphene-based perovskite solar cell hole transport layers?

P3HT (poly-3-hexylthiophene) provides a solution-processable, dopant-free polymer matrix with reasonable hole mobility and good film-forming properties. When blended with functionalized reduced graphene oxide, P3HT hosts the 2D filler through π-π stacking and side-chain interactions, allowing the composite to act as a uniform selective contact above the perovskite absorber while avoiding the doping instability of Spiro-OMeTAD.