Organic solar cells (OSCs) exhibit tremendous potential in the world's energy strategy due to their predominant advantages such as low cost, light weight, and large-area fabrication on fl exible substrates. [1][2][3][4] Small molecule semiconductors for bulk heterojunction (BHJ) OSCs are attractive because of their advantages over their polymer counterparts, which include welldefi ned molecular structure, defi nite molecular weight, and high purity without batch to batch variations. [5][6][7] In recent years, a great amount of effort has been dedicated to develop this class of materials, including dendritic oligothiophenes, [8][9][10] star-or X-shaped molecules, [11][12][13][14][15] linear analogs with donor-acceptordonor (D-A-D) structures, [16][17][18][19][20][21] fused polycyclic arene, [ 22 ] and other organic dyes. [23][24][25] However, the highest power conversion effi ciencies (PCEs) reported for small molecule/fullerene derivative BHJ OSCs were only β 3-4.4%, [5][6][7] lower than that ( β 6-8%) of polymer/fullerene derivative BHJ solar cells. [1][2][3][4] The lower effi ciency of small molecul BHJ OSCs became the biggest hindrance to their application.
Triphenylamine (TPA) has been regarded as a promising unit for effi cient photovoltaic materials due to its good electrondonating and high hole-transporting capabilities. [ 26 ] Benefi ting from its special propeller starburst molecular structure, amorphous materials with isotropic optical and charge-transporting properties could be expected when combining TPA with linear Ο -conjugated systems. Solution-processable TPA-based small molecules have been widely investigated for application in OSCs with PCEs below 3%. [ 12-15 , 27-29 ] On the other hand, solutionprocessed BHJ OSCs based on oligothiophenes exhibited PCEs as high as 3.7%. [ 8-11 , 30 ] Here, we report the synthesis and characterization of a new 3D, star-shaped, D-A-D organic small molecule with TPA as the core and donor unit, benzothiadiazole as the bridge and acceptor unit, and oligothiophene as the arm