Performance and Stability of Polymer Solar Cells Based on the Blends of Poly(3-Hexylthiophene) and Indene-C_60 Bis-Adduct

The performance and morphology stability of polymer bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) as the donor and indene-C_60 bisadduct (ICBA) or methanofullerene 6,6-phenyl C_61-butyric acid methyl ester (PCBM) as the acceptor are compared. Effect of the different donor and acceptor weight ratios on photovoltaic performance of the P3HT:ICBA device is studied. The optimal device achieved power conversion efficiency of 5.51% with J_\rm sc of 10.86 mA/cm^2, V_\rm oc of 0.83 V, and fill factor (FF) of 61.1 % under AM 1.5 G (100 mW/cm^2) simulated solar illumination. However, the stability measurement shows that cells based on P3HT:ICBA are less stable than those of the device based on P3HT:PCBM. Atomic force microscope results reveal that the morphology of the P3HT:ICBA film changed considerably during the storage periods due to unstable interpenetrating D-A network. This observation can be explained by the fact that there is lack of intermolecular hydrogen bonds in the P3HT:ICBA system. However, in the P3HT:PCBM system the molecules in the blend film are firmly held together in the solid state by means of intermolecular hydrogen bonds originating from C-H\cdotsOs bonds (where Os comes from the singly-bonded O atom of PCBM), forming a stable three-dimensional network. The measured PL decay lifetimes for P3HT:PCBM and P3HT:ICBA systems are 33.66 ns and 35.34 ns, respectively, indicating that the P3HT:ICBA system has a less efficient exciton separation efficiency than that of P3HT:PCBM, which may result in the interfacial photogenerated charges accumulated on the D: A interface. Such progressive phase segregation between P3HT and ICBA eventually leads to the degradation in performance and deteriorates the stability of the device. We also present an approach to enhance the stability of P3HT:ICBA systems by adding PCBM as the second acceptor. Our results show that by carefully tuning the contents of PCBM as the second acceptor, more stable polymer solar cells can be obtained.