New small organic molecules based on thieno[2,3-b]indole for efficient bulk heterojunction organic solar cells: a computational study

This study aims to evaluate the characteristics of novel organic D-π-A-π-D class small-molecules by using carefully the density functional theory, and time-dependent density functional theory calculations. Thedesigned sequence of (D-A) BHJ-1a to BHJ-4a in organic Bulk Heterojunction (BHJ) solar cells has been comprehensively analysed. Thiéno[2,3-b]indole (TI) has been used as donor, and Diketopyrrolopyrrole (DPP) as acceptor for all compounds. In order to improve the electronic, photovoltaic, and opticalproperties, we have substituted thiophene unit with furan, thieno[2,3-b]thiophene, thiazole and thiazolothiazole as π-bridge moieties. Thus, the result shows that the wise choice of the π-bridge units plays a significant role in improving Egap, producing a high bathochromic shift, and increasing VOC as well as a theoretical power conversion efficiency (PCE) over 7%. Interestingly, BHJ-4a with suitable π-bridge presents the optimal electronic properties with low band gap (1.870 eV) and high VOC (1.534 eV). Furthermore, we have modelled a Bulk heterojunction organic photovoltaic cells based on donor-PCBM complex in order to achieve the optimum Egap and VOC. Consequently, the obtained results provide a new way to design BHJ small molecule donors with higher power conversion efficiency.