Critical assessment of carbon pastes for carbon electrode-based perovskite solar cells

Perovskite solar cells using carbon electrodes (C–PSCs) possess the advantageous features of low cost, high stability and a simple fabrication process. They are considered a promising technology for producing cost-effective solar electricity. However, the power conversion efficiency of C–PSCs lags far behind that of the conventional gold-based counterparts due to the inefficient charge transfer and charge collection associated with the carbon electrodes and poor perovskite/carbon interfacial contact. A carbon electrode is generally deposited from a carbon paste (CP) composed of various carbon allotropes, additives, and solvents. The properties and compositions of the CPs directly influence the properties of the resultant carbon electrodes, their interfacial contact with layers underneath, and the device performance. In this review, we summarise the research on the development of CPs for carbon electrodes to achieve desirable properties, including electrical conductivity, processibility, chemical compatibility, and cost-effectiveness for the fabrication of C–PSCs. We also discuss the various structures of C–PSCs and the fabrication techniques for carbon electrodes. Finally, we present future perspectives on C–PSCs, focusing on the current challenges and possible solutions for making efficient, stable, and scalable C–PSCs.