Effective Light Harvesting of Tandem Polymer Solar Cell

A novel soluble conjugated polymers, P2, with coplanar thiophene-phenylene-thiophene unit is designed and synthesized as suitable active material used in tandem cells to compensate the poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C71 butyric acid methyl ester $(PC71BM)$ bulk-heterojunction cell in this paper. P2 polymer bears advantages in both low optical bandgap (1.7 eV) and high hole mobility properties (⁠$3.4×10−3cm2/V-s$ from field-effect transistor measurement). Furthermore, the electrochemical studies of P2 indicate desirable highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) band structure that enables a high open circuit voltage when pairing with PCBM acceptor. The best power conversion efficiency of this polymer solar cell thus far based on $P2/PC71BM$ system with a weight ratio of 1:3 reached 4.4% with a short circuit current density $(Jsc)$ of $10.2mA/cm2$⁠, an open circuit voltage $(Voc)$ of 0.81 V, and a fill factor (FF) of 0.53 under air mass (AM) 1.5 G $(100mW/cm2)$⁠. The preliminary data of the tandem cell with indium tin oxide (ITO) $glass/PEDOT:PSS/P2:PC71BM/TiOx/PEDOT:PSS/P3HT:PC71BM/TiOx/Al$ configuration has reached $Jsc$ of $6.2mA/cm2$⁠, $Voc$ of 1.33 V, FF of 0.56 and an overall efficiency of 4.6% under AM 1.5 G $(100mW/cm2)$⁠.