Hole Transport Materials Based on PEDOT Derivatives for Perovskite Solar Cells

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used as a hole conducting polymer in many optoelectronic devices, such as light-emitting diodes (LEDs), photodetectors, and solar cells. As a benchmark hole transport material, PEDOT:PSS has the advantages such as solution processability, high transparency, and good flexibility. However, relatively low electrical conductivity and poor electron-blocking capacity of PEDOT:PSS are detrimental to the operational parameters of perovskite solar cells, including open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF). We proposed a new strategy to modify the electronic, structural, and surface morphological properties of PEDOT:PSS by introducing a hydroxymethyl (−MeOH) or chloromethyl (-MeCl) functional group to ethylenedioxythiophene (EDOT) to form hydroxymethylated-3,4-ethylenedioxythiophene (EDOT-MeOH) or chloromethylated-3,4-ethylenedioxythiophene (EDOT-MeCl) monomers and to further polymerize them to water-soluble poly(hydroxymethylated-3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT-MeOH:PSS) or poly(chloromethylated-3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT-MeCl:PSS) using the oxidative chemical polymerization method. The introduced functional groups can either modify the work function or the conductivity of PEDOT-MeOH:PSS and PEDOT-MeCl:PSS. We systematically varied the amount of the ferric oxidizing and basic agents and reaction temperature and time in polymerization to control the reaction kinetics, the chain length, and the oxidation state of polymers. We also varied the amount of PSS used in polymerization to control the doping level and work function of synthesized PEDOT-MeOH:PSS and PEDOT-MeCl:PSS. Furthermore, we added ethylene glycol (EG) in the synthesized PEDOT-MeOH:PSS and varied annealing conditions such as in polar solvent environment and EG washing for PEDOT-MeCl:PSS to further tune the microstructures, electrical conductivities and work function of PEDOT-MeOH:PSS and PEDOT-MeCl:PSS thin films. We used Raman scattering spectroscopy to study the backbone structural changes among the benzoid and quinoid conformations and their impacts on the surface morphologies using atomic force microscope (AFM), electrical conductivity using four-probe measurement, and work function using ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV). The surface chemical compositions were obtained by x-ray photoelectron spectroscopy (XPS) to confirm the presence of PEDOT-MeCl on the thin film surface. The inverted structured MAPbI₃ perovskite solar cells with PEDOT-MeOH:PSS treated with EG exhibited the enhanced Voc and Jsc because of the enlarged work function and electrical conductivity. Elongated photoluminescence lifetime of MAPbI₃on PEDOT-MeCl:PSS indicated the strong interaction between Cl- and perovskites. Additionally, the inverted structured MAPbI₃ perovskite solar cells with PEDOT-MeCl:PSS annealed in polar solvent vapor exhibited the enhanced Voc and Jsc because of significantly reduced trap states at the HTL/perovskite interfaces. This work opens the way to develop new hole transport materials for highly efficient inverted perovskite solar cells and other optoelectronic devices with low-cost and solution processability.