Achieving high-efficient, cost effective, easy to process and versatile solar cells has always been a challenge for the scientific community. Perovskite solar cells (PSC) is a very attractive candidate to fulfill all these requirements and is marching ahead in the emerging photovoltaic energy conversion efficiency race. However, there are still limitations hindering its commercialization, as the expensive and highly thermally unstable back-contact made of gold. [1] Carbon-based materials, mainly carbon pastes made of carbon black and graphite, have already proved to be an excellent candidate to be used as back-contact due to their features as low cost, high conductivity and high stability. [2]

A very interesting carbon material that has never been reported in previous studies to the best of the authors' knowledge is carbon paper. It is commonly used in proton-exchange membrane fuel cells and electrolysers applications as gas diffusion layer (GDL) and consists in carbon fibers (with ≈7 µm diameter) held together by a carbon matrix that forms the microporous coating (MPC). Some GDLs are coated with a microporous layer (MPL), made of an amorphous mixture of carbon black and PTFE (polytetrafluoroethylene), enhancing the electrical surface contact area. [3]

In this study, the main goal is to replace the gold back-contact by a carbon paper. A layer of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT:PSS) was deposited on the MPC side of the carbon paper, to enhance the electrical contact between the carbon paper and the spiroOMeTAD underlying layer. [3] PEDOP:PSS presents high electrical conductivity, excellent thermal stability and proper energy band alignment (between the -5.2 eV from the spiroOMeTAD HOMO level and the -4.7 eV work function of the carbon paper). [4] Moreover, remarkable results have been presented by Pen You et al. [6] by using this material as adhesion layer between graphene and spiroOMeTAD HTM in a PSC device.

Different carbon papers were tested and the best performing one presented lower trough-plane area-specific resistance (related with the lower porosity and thickness). Before being deposited, PEDOT:PSS dispersion was treated with two additives: D-sorbitol to confer adhesive properties and Zonyl FS300^® to decrease the dispersion surface tension. D-sorbitol concentration was defined according to previous studies, while an optimal Zonyl FS300^® concentration value was found to be 4.5% wt. The treated PEDOT:PSS dispersion was then annealed at 120 ºC during 60 min to dry its dispersant agent (water) and to induce the D-sorbitol plasticizing effect. [6]

A PCE of 9.22 % was achieved for the best performing cell with a back-contact made of carbon paper, representing 62 % of the power conversion efficiency (PCE) obtained for the same cell but with a gold back-contact. The parameter that most contributed to the 38 % of PCE loss was the J_sc (loss of J_sc 79 %, 97 % for V_oc and 90 % for FF). This loss was assigned to the defective contact at interface SpiroOMeTAD /carbon paper. Future work will focus on establishing a better electrical contact at the mentioned interface, in order to equal the PCE obtained with gold.