Effect of Graphite and Carbon Black in Carbon Back Contacts of Carbon-based Multi-Porous-Layered-Electrode Perovskite Solar Cells
Ryuki Tsuji a, Kenichirou Tanaka a, Kota Oishi a, Takaya Shioki a, Seigo Ito a
a University of Hyogo, Department of Materials and Synchrotron Radiation Engineering, Graduate School of Engineering, 2167 Shosha, Himeji, Hyogo, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP23)
Kobe, Japan, 2023 January 22nd - 24th
Organizers: Seigo Ito, Hideo Ohkita and Atsushi Wakamiya
Oral, Ryuki Tsuji, presentation 062
DOI: https://doi.org/10.29363/nanoge.iperop.2023.062
Publication date: 21st November 2022

Perovskite solar cells (PSCs) have a photoelectric conversion efficiency of over 20 % and they can be fabricated only by printing and coating processes, so it is expected as next-generation solar cells. However, the back-contact electrode (e.g. Au, Ag) and hole transport materials (e.g. Spiro-OMeTAD) used for PSCs are unstable against water and oxygen, and there is a problem with long-term stability. Therefore, we focused on fully printable carbon-based multi-porous-layered-electrode PSCs (MPLE-PSCs) which have an electron transport layer (mesoporous TiO2), an insulation layer (mesoporous ZrO2), and hole transport/back contact electrode layer (carbon) [1-5]. MPLE-PSCs have long-term stability because the thick carbon layer (~15 μm) can be protected the light absorption layer from ambient water and oxygen. However, MPLE-PSCs have a low efficiency of less than ~18%, so it is necessary to aim for higher efficiency for commercialization. In this work, we focus on carbon electrodes, which have roles of hole transport and back contact. Typically, carbon electrodes are made from a mixture of large-sized graphite particles and nano-sized carbon black. However, the role of each material remains unclear. Therefore, carbon electrodes with different mixing ratios of graphite and carbon black are fabricated and compared. This fundamental comparison reveals the role of carbon materials used in MPLE-PSCs.
         A TiO2 compact layer was deposited by a spray pyrolysis method on patterned FTO glass. Then, porous TiO2, ZrO2, and carbon layers were deposited by a screen-printing method, and each layer was sintered at 400 to 500 ºC. Six mixing ratios of graphite and carbon black for carbon electrodes were prepared: 100-0, 80-20, 65-35, 50-50, 20-80, and 0-100. Finally, (5-AVA)0.05(MA)0.95PbI3 perovskite precursor solution was drop-casted and permeated through the carbon layer, and the MPLE-PSCs were completed by removing the solvent and crystallizing the perovskite material by heating and drying. Various measurements were performed on the obtained devices.
         The results show that the mixing ratio of graphite to carbon black has a significant effect on the performance of MPLE-PSC devices. In the graphite-rich, the open-circuit voltage (VOC) was higher. However, the short-circuit current density (JSC) and fill factor (FF) were low. On the other hand, increasing the ratio of carbon black decreased VOC, but improved JSC and FF. To understand these changes, electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) analysis were performed. The results show that carbon black has the effect of promoting hole extraction and graphite has the effect of efficiently transporting the generated charge. In summary, the MPLE-PSC device achieved maximum performance and a champion efficiency of 13% when graphite and carbon black were in a 50-50 or 20-80 ratio. This study is important for realizing inexpensive and sustainable carbon electrodes not only for PSCs but also for various electronic devices.

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