Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO22)
Publication date: 11th November 2021
Perovskites're currently in the spotlight of research, due to their exceptional optoelectronic properties, and their suitability as a component in tandem PV technologies. In this context, tandem solar cells based on the combination of perovskite top cell and Cu(In,Ga)Se2 (CIGS) bottom cell configuration is a promising next-generation, commercially viable, PV technology. Such tandem cells have demonstrated record conversion efficiencies higher than 24%.[1] However, the success of this approach relies strongly on the employed recombination layer and transparent conductive electrode technologies. To optimize the efficiency of the tandem solar cell, the energy levels of the recombination layer have to perfectly match those of the other layers of the device. In this context, reduced graphene oxide (RGO) is a promising material to use in CIGS/Perovskite tandem solar cells.
RGO is the form of graphene oxide (GO) that is commonly processed by chemical or thermal methods in order to reduce the oxygen content. Optical transmittance and conductivity of the RGO film can be tailored by adjusting the thickness of the film and the degree of reduction.[2] So, the modulation of the degree of reduction of RGO is an important variable in its application in tandem solar cells. In this work, spray-coated films of GO are reduced (RGO) by ultraviolet nanosecond laser irradiation. All samples were 80 nm thick GO films on top of silicon substrates. The degree of reduction was obtained by varying the number of laser pulses (LP) used for the reduction (0, 10, 50, 100, 500, and 1000 LP), with a laser fluence fixed at 20 mJ/cm2. This allows a fine control over the degree of reduction as well as patterning and fine-tuning of the electronic properties.[3] We investigated the C 1s and O 1s XPS core level spectra and also the NEXAFS spectra of RGO films at the C and O edges to identify the changes in the RGO composition at different stages of reduction.[4] Moreover, by angle-resolved NEXAFS at both absorption edges, we investigated the orientation of the functional groups with respect to the RGO plane. The obtained NEXAFS results together with UPS, XPS and Kelvin Probe measurements contribute to a better understanding of the role of oxidation functional groups on the electronic structure and properties of the RGO films. The outcome of these results allows to optimize the laser-induced reduction process to achieve the desired properties of the RGO for the application in tandem solar cells.
The authors acknowledge Vinnova within the Flag-ERA program and the Swedish Energy Council for the LASERGRAPH project (contract FLAG–ERA JTC 2019).
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