Facile synthesis of A-pi-D-pi-A architecture organic small molecules. Experimental and theoretical investigation of the effect of a pi-conjugated spacer

Understanding the structure-property relationship is one of the critical factors in the practical design of organic small molecules for bulk heterojunction organic solar cells (BHJ-OSCs). In this study, we designed and synthesized two low-band-gap organic small molecule donors, TRh and PhRh. To study the effect of the pi-conjugated spacer, the A-pi-D-pi-A architecture was constructed with an efficient dithienopyrrole donor (D) and rhodanine acceptor (A) units. Two types of pi-conjugated spacers, thiophene in TRh and phenyl in PhRh, are used to fine-tune the various properties. A detailed investigation of the effect of pi-conjugated spacers on optoelectronic, thermal, and photovoltaic properties have carried out. TRh is planar, more conjugated, has a more prolonged absorption wavelength and leads to a low band gap of 1.48 eV. DFT studies demonstrate that TRh has lower BLA (Bond length alternation) and has more quinoid structure than PhRh. PhRh is thermally more stable than TRh. PhRh displayed a high glass transition temperature (T-g) at 193 degrees C and a 5% decomposition temperature at 402 degrees C. The inverted device architecture of BHJ-OSCs blended with PC71BM displayed the PCE of 2.78% for TRh and 1.10% for PhRh. TRh : PC71BM blend exhibits higher hole and electron mobilities than PhRh : PC71BM blend.