Influential effects of pi-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties of alkylselenyl substituted benzodithiophene based polymers

Abstract

pi-spacers and alkyl side chains play a key role in the optical, electrochemical, and photovoltaic properties of pi-conjugated polymers. To investigate the collective effects of pi-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties, an array of four new low bandgap (LBG) pi-conjugated polymers (P1-P4) was designed and synthesized for their application as donor materials in bulk heterojunction polymer solar cells (BHJ PSCs). These pi-conjugated polymers contain a benzodithiophene (BDT) donor unit substituted with 2-ethylhexylselenyl or 2-hexyldecylselenyl as pi-conjugated side chains and a dialkoxybenzothiadiazole (dialkoxyBT) electron deficient unit connected with thiophene or selenophene as pi-spacers. Among the four polymers, the absorption spectra of P3 with the thiophene pi-spacer showed a well enhanced vibronic shoulder peak between 620 and 650 nm, indicating that P3 possesses a strong interchain aggregation tendency and attains a planar backbone structure due to the non-covalent interactions arising between the sulfur atom in thiophene and the oxygen atom in dialkoxyBT. Under suitable device processing conditions optimized pristine PSCs of P3 showed a maximum power conversion efficiency (PCE) of 4.09%. After employing 1,8-diiodooctane as an additive, one of the PSC devices based on P2 displayed a PCE of 5.34%. The active layers of P1-P4 showed a positive response towards methanol treatment, especially the P3-based devices delivered an improved PCE of 5.63%, which was further assessed by electrical impedance spectroscopy. These findings in the current article provide a good specimen for efficiently fine tuning the optical and photovoltaic properties of p-conjugated polymers via varying the size of alkyl chains, p-spacer groups and device processing conditions for the imminent growth of LBG p-conjugated polymers.