**1. Introduction**

Renewable clean energy devices are urgently demanded for the sustainable development of society. Among them, organic–inorganic metal hybrid perovskite solar cells (PSCs) have attracted ever-increasing attention owing to their excellent photovoltaic performance, simple preparation process, and relatively low cost [1]. PSCs are generally composed of FTO glass, an electron transport layer, a light absorption layer, a hole transport layer, and a counter electrode [2]. In the multi-layer structure of PSCs, the hole transport layer (HTL) is designed to promote the separation of electrons and holes, which is key to the performance and stability of the cell. However, certain problems of HTL hinder the development and application of the PSCs technology. Currently, the HTL of PSCs are based on materials such as Spiro-OMeTAD, PTAA [3] and P3HT [4]. The costs of these materials are all prohibitively high for large-scale applications [5]. What is more, the dopants in Spiro-OMeTAD show strong water absorbency, which seriously threatens the service life of PSCs [6]. Therefore, it is necessary to explore a low-cost and stable hole transport materials (HTMs) for the practical stage of PSCs.

PEDOT, usually combined with PSS, is widely used in inverted PSCs [7–9], whose price is much cheaper than the materials mentioned above. However, sulfonic acid groups contained in PSS are extremely harmful to the device life. To avoid the usage of PSS, Jiang et al. [10] synthesized 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) monomer, which was spin-coated on a perovskite layer and in-situ polymerized as PEDOT; a photoelectric conversion efficiency (PCE) of PSCs of about 17% was achieved. Wei et al. [11] used sulfonated acetone-formaldehyde (SAF), instead of PSS, to composite with PEDOT in inverted PSCs, which effectively increased the life of PSCs. Meanwhile, graphene oxide (GO) was also selected as the HTM of PSCs. Wu et al. [12] fabricated 2 nm thickness GO film as HTL, and the PCE of the inverted PSCs reached 12.40%.

**Citation:** Yuan, T.; Li, J.; Wang, S. Composited Film of Poly(3,4-ethylenedioxythiophene) and Graphene Oxide as Hole Transport Layer in Perovskite Solar Cells. *Polymers* **2021**, *13*, 3895. https://doi.org/10.3390/ polym13223895

Academic Editors: Wei Wu, Hao-Yang Mi, Chongxing Huang, Hui Zhao and Tao Liu

Received: 23 September 2021 Accepted: 30 October 2021 Published: 11 November 2021

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In this work, a harmless HTL was obtained by PEDOT composited with GO. The PE-DOT interacted with GO sheet via π–π stacking and hydrogen-bonding interactions, thus a conjugated system can be formed [13]. Moreover, GO functions as an excellent carrier to enable the dispersion of PEDOT in isopropanol solution, which is also harmless to the perovskite layer. Using PEDOT-GO film as HTL, a PSC with a PCE of up to 14.09% with good stability can be realized based on carbon counter electrode.
