The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C61 (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.

A distinct thermal annealing temperature exhibiting a maximum efficiency was observed for polymer solar cells with a ternary blend film that consists of one soluble fullerene and two kinds of conjugated polymer (electron-donating polymer and electron-accepting polymer). The distinct sharp shape trend of device performance with annealing temperature has been assigned primarily to the influence of the electron-accepting polymer [poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)], since the glass transition temperature of F8BT was exactly the same as with the best annealing temperature. The performance of the best ternary blend solar cell made in this work was poorer than the well-optimized binary blend with the higher content (donor: acceptor ) 1:1) of electron-donating polymer. However, it was slightly better than the comparative binary blend solar cell with the same content of electron-donating polymer (donor:acceptor ) 1:2), indicative of further improvement in the performance of ternary blend solar cells through the control of blend composition and/or thickness.

Here we report the influence of electron-donating polymer addition on the performance of poly(3-hexylthiophene) (P3HT) : 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) solar cells. Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) was chosen as the electron-donating polymer to improve the open circuit voltage (VOC) due to its higher level of the highest occupied molecular orbital energy compared with P3HT. Results showed that the MDMO-PPV addition led to an improved VOC for ternary blend (P3HT : MDMO-PPV : PCBM) solar cells. In particular, after thermal annealing at 110 °C, the short circuit current density of ternary blend solar cells was greatly improved, close to that of comparative binary blend (P3HT : PCBM) solar cells.

We found that 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C(61) (PCBM) molecules make a distorted asymmetric body-centered cubic crystal nanostructure in the bulk heterojunction films of reigoregular poly(3-hexylthiophene) and PCBM. The wider angle of distortion in the PCBM nanocrystals was approximately 96 degrees , which can be assigned to the influence of the attached side group to the fullerene ball of PCBM to bestow solubility. Atom concentration analysis showed that after thermal annealing the PCBM nanocrystals do preferentially distribute above the layer of P3HT nanocrystals inside devices.