Improving the efficiency of solution processable organic photovoltaic devices by a star-shaped molecular geometry

A new solution processable star-shaped organic molecule S(TPA-BT) has been synthesized for application in organic solar cells (OSCs). The properties and structures of S(TPA-BT) and a related linear molecule L(TPA-BT) were studied, including UV-visible spectroscopy, hole charge mobility, and theoretical calculated geometry and electronic properties. S(TPA-BT) film shows a broader and stronger absorption band in the range of 440-670 nm, lower band gap of 1.86 eV, higher hole mobility of 4.71 x 10(-5) cm(2)V(-1) s(-1) and better film-forming properties compared with those of L(TPA-BT) film. ITO/PEDOT:PSS/S(TPA-BT) or L(TPA-BT): PCBM/Ba/Al bulk-heterojunction OSCs were fabricated with S(TPA-BT) or L(TPA-BT) as donor material. The power conversion efficiency (PCE) of an OSC based on a blend of S(TPA-BT) and PCBM (1 : 3, w/w) reached 1.33% under A. M. 1.5 illumination, 100 mW cm(-2), with a short-circuit current density (J(SC)) of 4.18 mA cm(-2), an open circuit voltage of 0.81 V, and a fill factor of 39%. The PCE of 1.33% and J(sc) of 4.18 mA cm(-2) are among the highest values reported so far for solution processable OSCs.