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

π-conjugated spacer: Two A-π-D-π-A type novel small molecule donors are prepared by a one-pot-two-step procedure. The effect of the π-conjugated spacer is explored in optoelectronic, thermal, electrochemical, photovoltaic and computational. Both donors have broad optical absorption, low optical bandgap, and good thermal stability with a high glass transition temperature. The power conversion efficiencies reached up to 2.78% with an inverted device structure using PC₇₁BM as an acceptor.

Abstract

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 π-conjugated spacer, the A-π-D-π-A architecture was constructed with an efficient dithienopyrrole donor (D) and rhodanine acceptor (A) units. Two types of π-conjugated spacers, thiophene in TRh and phenyl in PhRh, are used to fine-tune the various properties. A detailed investigation of the effect of π-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 °C and a 5% decomposition temperature at 402 °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.

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