Chemically and Physically Modified Reduced Graphene Oxides and Application to Printable Hole Transport Layer for Polymer and Perovskite Solar Cells

Abstract

The chemically converted graphenes (CCGs), such as graphene oxide (GO) and reduced graphene oxide (rGO), have been applied in various electronic fields as interfacial layer due to their beneficial advantages of low cost, solution processability, and tunable electronic properties. The focus of this thesis is investigation of characteristics of chemically and physically modified reduced graphene oxide as orthogonal printable hole transport layer in polymer solar cells with inverted structure and perovskite solar cells with conventional structure.

Chapter 1 provide a general introduction of carbon materials, graphene derivatives, requirements of charge transport materials, and general reported hole transport materials (HTMs) used in polymer and perovskite solar cells.

In Chapter 2, we prepared orthogonal-processable fluorine-functionalized reduced graphene oxide (FrGO) series with various 2-dimensional sheet sizes such as large-sized FrGO (1.1 μm), medium-sized FrGO, (0.7 μm), and small-sized FrGO (0.3 μm) and systematically investigate the size effect of FrGOs on the hole transport properties of PSCs. The FrGOs exhibit highly stable dispersion without change over 90 days in 2-propanol solvent, indicating very high dispersion stability. Decreasing the sheet size of FrGOs enhanced hole-transporting properties, resulting in power conversion efficiencies (PCEs) of 9.21% for PTB7-Th:EHIDTBR and 9.02% for PTB7-Th:PC71BM-based PSCs. Compared to devices with solution processed PEDOT:PSS, 14% enhancement of PCEs was achieved. Interestingly, the PCEs of devices with the smallest FrGO sheet is higher than the PCE of 8.77% of a device with vacuum deposited MoO3. The enhancement in the performance of PSCs is attributed to the enhanced charge collection efficiency, decreased leakage current, internal resistance, and minimized charge recombination. Finally, Small-sized FrGO HTLs were successfully coated on the photoactive layer using spray coating method, and they also exhibited PCEs of 9.22% for PTB7-Th:EH-IDTBR and 13.26% for PM6:Y6-based inverted PSCs.

In Chapter 3, It has been very limited that reduced graphene oxide (rGO) derivatives use as solution-processed HTL in perovskite solar cells (PeSCs) with n-i-p structure due to their low dispersity in non-polar solvents such as chlorobenzene (CB) and toluene, which are orthogonal solvent of perovskite. The limited concentration of rGO derivatives dispersion in non-polar solvents precludes forming the full-covered thick films on top of the perovskite layer using solution-process. In this chapter, we synthesized alkylated rGO using dodecyl amine (AF-rGO), that can be highly dispersed in non-polar solvent, for application as dopant-free and solutionprocessed hole transport Material (HTM) on top of the perovskite layer. The AF-rGO can be dispersed in CB with high concentration (~5 mg ml-1), and the full-covered thick AF-rGO film was successfully introduced on top of the perovskite layer as HTM in the PeSC through simple solution-process. AF-rGO HTL in PeSCs exhibited minimized series resistance and charge recombination efficient, and also showing efficient hole extraction and transport ability, resulting in achieving maximum power conversion efficiency (PCE) of 15.88%, which is the highest PCE among the PeSCs with n-i-p structure employing graphene-based materials as the sole HTM. Moreover, devices with AF-rGO HTM exhibited good ambient stability (30% ~ 40% relative humidity), remaining 89% of their original performance after 10 days of storage without encapsulation.