In situ Polymerization of PEDOT: π-Conjugated Polyelectrolyte and Application as Hole Transport Layer in Polymer Solar Cells

Poly( 3,4-ethylenedioxythiophene) ( PEDOT) is usually dispersed in water by incorporating polystyrene sulfonate ( PSS) as a charge compensator and a template for polymerization. The polymerization behavior of 3,4-ethylenedioxythiophene (EDOT) without assistance of insulating PSS was explored. Due to their semiconducting and hydrophilic characteristics, pi-conjugated polyelectrolytes with sulfonic acid groups, namely PCPDT-T and PCPDT-2T, were used as template for in situ oxidative polymerization of PEDOT. The obtained PEDOT: polyelectrolyte composites showed good dispersibility in aqueous solution, which was used as hole transport layer (HTL) in polymer solar cells (PSCs). The morphologies and photoelectric properties of PEDOT:PSS and PEDOT:polyelectrolyte composites were characterized by Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, ultraviolet photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy and contact angle test, respectively. PEDOT:polyelectrolyte composites displayed suitable energy level, transmittance up to 95% (30 nm), hydrophobic surface morphology, as well as superior hole mobility, which were in favour of forming Omega contact with the active layer and increasing the injection and collection efficiency of the holes. Thus, improving the device photovoltaic performance. The current density-voltage (J-V) characteristics of the PSCs with the structure of Glass/ITO/HTL/PTB7-Th:PC71BM/PFN/Al were measured under one standard sun by solar simulator with an Air Mass 1.5 global (AM 1.5G) and an irradiation intensity of 100 mW/cm(2). Compared to the photovoltaic characteristics for PEDOT:PSS based device, PSCs with PEDOT:T (1:2) as HTL yielded the highest power conversion efficiency (PCE) of 8.6% with a short current density (J(SC)) of 18.07 mA/cm(2), open circuit voltage (V-OC) of 0.77 and fill factor (FF) of 61.31%, respectively. Compared to that of PEDOT:PSS, the obvious increased J(SC) for PEDOT:polyelectrolyte composites based PSCs was ascribed to the high hole mobility and transmittance, which were consistent with the results from the space-charge limited current (SCLC) method and the external quantum efficiency (EQE) measurement.