Applications of the Conductive Carbon Paste to the Counter Electrode Layer

Applications of the Conductive Carbon Paste to the Counter Electrode Layer

Zhang reported the use of low-temperature processed carbon paste as a counter electrode in PSCs without a hole transport layer for the first time. They used the scraper method to deposit commercial carbon paste on the perovskite layer to prepare solar cells with a structure of TiO2/CH3NH3PbI3/C. The width of the depletion region of the heterojunction formed between the TiO2/CH3NH3PbI3 interface has a great influence on the performance of the battery, and the width of the depletion region is dominated by the thickness of the TiO2 thin film. By optimizing the thickness of the TiO2 thin film (the best thickness is 630 nm), the battery with commercial carbon paste as the counter electrode obtained the best efficiency of 8.31%. In the same year, Shi and the rest of the team studied the effect of the carbon layer's thickness on the performance of carbon electrodes prepared under low temperatures. The carbon particles were dispersed in chlorobenzene, and the electrode layer was deposited by the scraper method; the thickness was controlled within the range of 2 to 40 um. The results showed that the photoelectric conversion efficiency of the device increased with the increase of the thickness of the carbon layer. When the thickness of the carbon layer was 20.6 um, average efficiency of 6.90% was obtained.
Subsequently, they used the scraper method to deposit the conductive carbon paste counter electrode layer, and prepared a perovskite solar cell with a simple structure of ZnO/CH3NH3 PbI3/C. The entire cell preparation process is completed at low temperatures. The efficiency of the battery reached 8% when the hole transport layer is not available. Good results were also achieved in the case of using a flexible conductive substrate. Qi and others used commercial conductive carbon paste as the counter electrode to prepare a hole transport layer free carbon-based battery structure of FTO/c-TiO2/m-TiO2/CH3NH3PbI3/C. The thickness of the mesoporous titanium dioxide (m-TiO2) layer determines the contact area between the perovskite layer and dense layer (c-TiO) as well as the morphology of the perovskite thin film. In order to improve the performance of the cell, learnt the influence of the thickness of the mesoporous titania layer and the perovskite layer on the cell’s performance and optimized the thickness; the cell’s efficiency reached 11.11%.
Qiang and others used the sol-gel method to prepare the SnO2 precursor solution, and prepared a SnO2 electron transport layer with a suitable thickness by controlling the concentration of the SnO2 precursor solution. The extremely small SnO2 particles completely cover the FTO substrate, avoiding direct contact between the perovskite material and the FTO substrate and reducing the  shunt path of charge. Use the solvent exchange method to remove the poor solvent in the commercial carbon paste so that the solvent in the final carbon paste can be more volatile at low temperatures. The complete cell without the hole transport layer was prepared under low temperatures, and an efficiency of 8.32% was obtained, with excellent environmental stability.  
Later, Han and the rest of the team solved the problem of poor interface contact between the low-temperature prepared carbon electrode and perovskite by inserting a NiO-C (commercial carbon paste) intermediate layer between the perovskite and commercial carbon paste electrode. Under the active area of l cm2, the obtained efficiency is 12.5%, which increased 20.2% compared with the device without an intermediate layer. The significant increase in the values of Jsc FF and Voc is attributable to the fact that the NiO-C intermediate layer accelerates hole transport to the carbon electrode, hinders the reverse electron transport, and plays a role in matching the energy levels of the carbon electrode and perovskite layer. The successful preparation of perovskite crystals has a key impact on the efficiency of photogenerated carrier transmission to the carbon electrode.
Zhou and others added ILPF6 ionic liquid to the perovskite precursor solution composed of CH3NH3Iand PbI2 to improve the loading of perovskite ore crystals on the TiO2 thin film, enhance the light-harvesting capacity of the perovskite, and has an impact on the nucleation and growth of the perovskite crystals to obtain larger grains and fewer grain boundaries, promote the transport rate of photo-generated carriers in the perovskite crystal layer, and accelerate the hole transport to the carbon electrode. Moreover, ILPF6 ionic liquid and commercial carbon paste also have good hydrophobicity, which makes the cell have good environmental stability.