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 (V_OC) was higher. However, the short-circuit current density (J_SC) and fill factor (FF) were low. On the other hand, increasing the ratio of carbon black decreased V_OC, but improved J_SC 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.