Investigation of the effects of thermal annealing to PEDOT:PSS on the photovoltaic response of hybrid solar cells

This work presents research results concerning to the fabrication of hybrid solar cells in a superstrate configuration with the following structure: glass/SnO₂:F/ZnO + CdS/CdTe + CdCl₂-TT/PEDOT:PSS-TT/Cu-Mo. After Cadmium Telluride (CdTe) absorber layer processing, the organic conjugated polymer Poly(3,4-ethylenedioxythiophene): poly(styrene sulphonate) (PEDOT:PSS) was deposited with a thickness around 50 nm, then a thermal annealing (TA) was carried out varying annealing time (20-40 min) and temperature (80-120 °C). The physical properties and output electrical parameters of the devices were measured and compared with a reference solar cells without TA. A decrease of the resistivity values was reached as a result of the incorporation of PEDOT:PSS on CdTe as a hole transport layer. CdTe/PEDOT:PSS structure was characterized by profilometry, four-probe method, UV–Vis spectroscopy, Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and Raman spectroscopy. The electrical performance of the fabricated hybrid solar cells was analysed through the current density vs. voltage (J vs. V) characteristic, External Quantum Efficiency (EQE) measurements and the values spread distribution for each electrical parameter was also discussed. A highest conversion efficiency around 15.2% was obtained for a device in which the TA was performed at 100 °C during 30 min with output electrical parameters values of V_oc ~ 0.778 V, J_sc ~ 34.0 mA/cm², FF ~ 0.55 and EQE values above 55%, resulting this in an improvement of the use of PEDOT:PSS in hybrid solar cells. A monitoring of the degradation effect of the output electrical values was carried out after a period of 24 months and an average degradation rate around 20% was found, however for devices processed at higher temperatures of TA, degradation rate of the conversion efficiency was at least 3%.