Electrostatic nanoassembly of contact interfacial layer for enhanced photovoltaic performance in polymer solar cells

Improved performance in the bulk heterojunction (BHJ) polymer solar cells (PSCs) can be achieved through multiple mechanisms by tuning the structure of the components and design architecture. Therefore, in this study, we designed and fabricated an electrostatically assembled contact interfacial layer utilizing the electrostatic interactions between positively charged poly(diallyldimethylammonium chloride) (PDDA) and negatively charged sulfonate ions in a polymer, i.e. toluene sulfonic acid incorporated self-doped polyaniline (SPAN(TSA^-)) (designated as SPAN(TSA^-)/EA^-CIL). We investigated the influence of SPAN(TSA^-)/EA^-CIL on the photovoltaic performance of BHJ PSCs. We adopted a simple "dip-dry" technique to fabricate the SPAN(TSA^-)/EA^-CIL. Cyclic Voltammetry, X-ray photoelectron spectroscopy, Ultraviolet-visible spectroscopy, water contact angle measurements, electrochemical impedance spectroscopy and atomic force microscopy were employed to characterize the new SPAN(TSA^-)/EA^-CIL and to elucidate the associated enhancements in the photovoltaic performance. The fabricated PSCs with the SPAN(TSA^-)/EA-CILs exhibited an improved photoconversion efficiency (PCE) through enhancement of both the short-circuit current density (J_SC) and fill factor compared to the device consisting of only SPAN(TSA^-) or PEDOT:PSS. A plausible mechanism is presented here that explains the enhanced J_SC and PCE in terms of transient dipole induction at the SPAN(TSA^-)/EA-CIL surface. Interestingly, the PCE of devices based on ITO/SPAN(TSA^-)/EA-CIL(1:1)/P3HT:PC₆₀BM/Al and ITO/SPAN(TSA^-)/EA-CIL(1:1)/PBDTTT-C-T:PC₇₀BM/Al was enhanced by 13% (3.19-3.67%) and 10% (5.11-5.60%) compared to those of devices based on ITO/PEDOT:PSS/P3HT:PC₆₀BM/Al and ITO/PEDOT:PSS/PBDTTT-C-T:PC₇₀BM/Al, respectively. The present work thus identifies a novel class of low-temperature processable interface layer with a potential to fabricate highly efficient photovoltaic devices.