Density Functional Study on A-Units Based on Thieno[3,4-c]pyrrole-4,6-dione for Organic Solar Cells

Abstract

The use of polymer donor materials has allowed great progress in organic solar cells. To search for potential donor materials, we have designed a series of donor-acceptor (D-A)-type alternating polymers composed of dithieno[3,2-b:2¢,3¢-d]pyrrole (DTP) electron-rich units and thieno [3,4-c]pyrrole-4,6-dione (TPD) electron-deficient units. Their electronic and optical properties have been investigated using density functional theory and Marcus theory. The calculation results demonstrate that introduction of cyclic compounds (furyl, thienyl, and phenyl) into electron-deficient units of the molecules can result in lower highest occupied molecular orbital (HOMO) levels and reorganization energies compared with the experimental molecule (X 0 ). To investigate the effects of electron-withdrawing units, three electron-withdrawing substituents (-OCH 3 , -F, and -CN) were introduced into the thienyl. The results indicated that the polymer X 2-3 will show the best performance among the designed polymers, offering low-lying HOMO energy level (À5.47 eV), narrow energy gap (1.97 eV), and high hole mobility (7.45 9 10 À2 cm 2 V À1 s À1 ). This work may provide a guideline for the design of efficient D-A polymers for organic solar cells with enhanced performance. Graphical AbstractDonor-acceptor (D-A) unit polymers have been extensively investigated by researchers as donor materials. However, most such work has focused on donor units, while the work presented herein outlines an effective way to modulate the properties of acceptor units based on thieno [3,4-c]pyrrole-4,6-dione in D-A polymers. The results show that incorporation of a thienyl substituent and bonding -CN (X 2-3 ) as acceptor moiety in D-A-type polymers is an efficient strategy to improve the absorption and mobility (l h = 7.45 9 10 À2 cm 2 V À1 s À1 ) and thus enhance the photovoltaic performance (PCE % 5.82%) of organic solar cells.