Mild wetting poor solvent induced hydrogen bonding interactions for improved performance in bulk heterojunction solar cells

Abstract

In this paper, we demonstrate a facile, mild wetting (short residence time) poor solvent [isopropyl alcohol (IPA)] treatment, carried out on the top surface of a spin-cast poly (3-hexylthiopene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PCBM) blend film, in order to fabricate high-performance polymer solar cells (PSCs). This method utilizes the hydrophilic and polar/hydrogen bonding interactions of IPA with the blend components (P3HT and PCBM). The photovoltaic (PV) performance of the fabricated PSCs was optimized by utilizing a preheated IPA wetting treatment and devices fabricated with the configuration: ITO/PEDOT:PSS/P3HT:PCBM/M-IPA-T°/Al (T°: 25 °C/45 °C/65 °C/85 °C) (where “M” stands for modified IPA and “T” signifies the temperature used for the IPA wetting). Our investigation encompasses electrical, optical, crystalline, and morphological studies on the P3HT:PCBM blend films ,modified by preheated IPA, to elucidate the associated enhancements in the PV characteristics and performance. The device fabricated with IPA-85 °C (ITO/PEDOT:PSS/P3HT:PCBM/M-IPA-85 °C/Al) exhibited the best power conversion efficiency (PCE) of 3.51%, with an open circuit voltage of 0.65 V, a fill factor of 0.52, and a short-circuit current density of 10.20 mA cm⁻². In contrast, the non-modified blend film device showed a PCE of only ∼3.04%. Ultraviolet-visible absorption studies and X-ray diffraction results suggest that the use of the pre-heated mild-wetting IPA treatment improves the crystallinity and self-organization of the blend layer. We rationalize our findings based on the interactions between IPA and the blend components, due to its high polar and hydrogen bonding Hansen solubility parameters to impart supramolecular assembly of P3HT chains during the blend film formation. This is the first report demonstrating that the poor solvent (IPA) can induce an optimal phase separation in a P3HT:PCBM blend through our proposed mild wetting preheated treatment, toward achieving high-performance PSCs.