Beneficial Role of Reduced Graphene Oxide for Electron Extraction in Highly Efficient Perovskite Solar Cells

Abstract

In this work we systematically investigated the role of reduced graphene oxide (rGO) in hybrid perovskite solar cells (PSCs). By mixing rGO within the mesoporous TiO 2 (m-TiO 2 ) matrix, highly efficient solar cells with power conversion efficiency values up to 19.54 % were realized. In addition, the boosted beneficial role of rGO with and without Li-treated m-TiO 2 is highlighted, improving transport and injection of photoexcited electrons. This combined system may pave the way for further development and optimization of electron transport and collection in high efficiency PSCs.Since its isolation in 2004, [1] graphene has had a huge impact on applications in optoelectronic and photon energy conversion, owing to its unique electronic, optical, and mechanical properties. [2][3][4][5] The development of solution-processable graphene, such as the chemical exfoliation of graphite into graphene oxide, allowed the functionalization and processing of graphene, extending its use in the different layers of solutionprocessable solar cells. [6,7] Among different solar cell technologies, organic-inorganic hybrid perovskite solar cells (PSCs) have been dominating the interest of the scientific research community for their impressive technological development with power conversion efficiency (PCE) values beyond 22 % in a short of six years of research. [8] However, further improvements are necessary to optimize the PSC device operation and stability, and enhance the device performance. From this point, mixed graphene-based derivatives have been proposed to further enhance the device properties. [7] Used in various forms and with different functionalities, either incorporated in mesoscopic or in planar device configuration, [9,10] functionalized reduced graphene oxide (rGO) derivatives have been successfully employed for improved charge extraction in the electrontransport material (ETM) [9,11,12] or hole-transport material (HTM) [13] in PSCs. Its ability to effectively reduce the chargerecombination pathways and decrease the leakage currents has been demonstrated, with a similar role as in organic solar cells. [14][15][16] However, all of the PCEs reported are limited to less than 14 % and the perovskite was not formed as a perfect film, which casts the doubt on the real beneficial role of rGO on high-efficiency PSCs. In contrast to many applications of carbon-based materials that are usually introduced at the perovskite/TiO 2 or perovskite/HTM interfaces, or in substitution of the HTM itself, [17,18] here we propose a systematic study by exploring three different configurations as shown in Figure 1.We integrated the rGO: 1) in the 2,2',7,7'-tetrakis(N,N'-di-pmethoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) HTM, 2) in the matrix of the active perovskite layer, and 3) within the mesoporous TiO 2 (m-TiO 2 ) ETM. In this study, mesoscopic PSCs are composed of a fluorine doped tin oxide (FTO)-coated glass substrate, a compact TiO 2 layer followed by a m-TiO 2 layer, a mixed (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 (FA = formamidin...

Reduced Graphene Oxide as a Stabilizing Agent in Perovskite Solar Cells

“…The study reveals the main role of rGO in improving the electron transport when implemented within the mp‐TiO 2 layer, which resulted in the increase of the device efficiencies (from PCE of 17.4 to 17.8%). Moreover, the effect was further enhanced upon lithium treatment of the mesoporous layer, which resulted in a superior performance (19.5%) relative to the untreated devices (18.8%) . In other cases, however, when the rGO was incorporated into the perovskite or HTL layers, an increased resistance led to the reduction of the overall device performance, which was partly in contrast with other reports in the literature (Table ).…”

“…A systematic investigation of the role of reduced graphene oxide in PSCs was performed by Cho et al, who integrated rGO into three different layers of a mp‐PSC based on (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 . The rGO was employed within the mp‐TiO 2 , the active perovskite layer, as well as the HTL ( Figure ).…”

“…Schematic representation of conventional perovskite solar cell architecture (top) with rGO incorporation into a) mesoscopic‐TiO 2 , b) perovskite, and c) HTM layer, with the corresponding energy level diagrams (bottom). Adapted with permission . The calculated HOMO/LUMO levels of rGO are ≈−4.96 and −3.95 eV, respectively …”

Introduction of Graphene Nanofibers into the Perovskite Layer of Perovskite Solar Cells

“…[11] Recently,t he application of graphene-derived nanomaterials in PSCs has been studied. [7,[20][21][22][23] Reduced graphene oxide (RGO), [6,[24][25][26][27][28][29][30] graphene quantum dots (GQDs), [11,31] or fullerene, [28] have been used as an interlayer between the ETL/HTL and the perovskite layer,o r being incorporated within the ETL/HTL scaffold to improvet he charge-transport efficiency for mesoporousP SCs. [12][13][14][15][16][17][18][19] Both the electronic structures of the variousinterfaces and the morphology in PSCs can affect the charge carrier transport properties.…”

“…[12][13][14][15][16][17][18][19] Both the electronic structures of the variousinterfaces and the morphology in PSCs can affect the charge carrier transport properties. [7,[20][21][22][23] Reduced graphene oxide (RGO), [6,[24][25][26][27][28][29][30] graphene quantum dots (GQDs), [11,31] or fullerene, [28] have been used as an interlayer between the ETL/HTL and the perovskite layer,o r being incorporated within the ETL/HTL scaffold to improvet he charge-transport efficiency for mesoporousP SCs. In another case, the graphene-derived nanomaterials have been used as top electrode for the PSC to provide efficient charget rans-fer.…”