A futuristic strategy to influence the solar cell performance using fixed and mobile dopants incorporated sulfonated polyaniline based buffer layer

In this work, we hypothesized and demonstrated a new strategy to tune/modulate the electrochemical, microstructural and opto-electronic properties based on the manipulation of the intentionally included external dopant ion (X^-) within the sulfonated polyaniline (SPANs). Through our new strategy, we developed a different type of SPANs comprising of internal (fixed) and external (mobile) dopant. The X^- included SPANs were prepared through a sequential doping, dedoping and redoping processes and designated as SPAN-R (X^-) (where X^- is the anion of toluene sulfonic acid (TSA) or camphor sulfonic acid (CSA) or napthalene sulfonic acid (NSA)) by modifying the structure of 4-aminodiphenylamine-2-sulfonic acid with additional polyaniline chains to accommodate X^-. SPAN-R(X^-) polymers were characterized by cyclic voltammetry, UV-visible spectroscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Atomic force microscopy to elucidate the influence of X^- on the electrical, optoelectronic, microstructural properties and surface properties on the performance characteristics of polymer solar cells (PSCs) fabricated with SPAN-R(X^-) as a buffer layer. The electrochemical band gap, degree of doping (DD), electrical conductivity and degree of crystallinity (CD) were evaluated and correlated to understand the influence of X^- on them. The power conversion efficiency (PCE) of PSCs featuring SPAN-R(TSA^-) as a buffer layer showed a ~3.2 times improvement in the overall PCE, compared with the PSCs having pristine SPAN (not containing X^-) as a buffer layer and is higher than that of SPAN-R(CSA^-) and SPAN-R(NSA^-) based devices. The superior photovoltaic (PV) characteristics observed for SPAN-R(TSA^-) is due to the synergistic contributions from appropriate energy-level/work function alignment, higher conductivity, higher DD and induced molecular order with the photoactive layer. Importantly, PSCs with SPAN-R(X^-) buffer layer processed at low temperature (30 °C) (without thermal treatment) exhibited improved PV characteristics and better air-stability as compared to the device having poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (thermally annealed at 150 °C) as buffer layer. As buffer layers, SPAN-R(X^-) polymers, containing fixed and mobile dopants, are most attractive because of low temperature processability and improved solar cell performance.