In the present article, we try to illustrate the behavior of the most common organic solar cell under the influence of polymer
molecular weight (in the range of 21,100-128,800 g/mol) on light harvesting and device performance parameters. The present
work demonstrates that, the molecular weight of donor polymer plays an important role in the light harvesting and performance
of a common-bulk heterojunction organic solar cell based on poly(3-hexylthiophene) (P3HT), as a donor polymer, and
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as an acceptor. Through this study it is found that, the optical absorption
of P3HT:PCBM solar cell active layer is increased by increasing P3HT molecular weight. This may be as a result of increasing
active layer thickness and/or increasing backbone chain length of the polymer. In the same time, the absorption wavelength
range is narrowed and the absorption peak position is shifted towards lower wavelengths due to the entanglement in polymer
chains and reduction in polymer crystallinity at higher P3HT molecular weights. The external quantum efficiency (EQE) of the
investigated solar cells is increased by increasing P3HT molecular weight until the molecular weight that entangles the P3HT
chins and, therefore, restricts the charge carrier transfer and, then, reduces the EQE of the P3HT:PCBM solar cell. The performance
parameters of the P3HT:PCBM solar cell are improved by increasing P3HT molecular weights around 25,300 g/mol
and, after that, deteriorated at higher P3HT molecular weights up to 128,800 g/mol as a result of polymer chain entanglement
and reduction of the P3HT crystallinity. The present study shows that the best P3HT molecular weight is 25,300 g/mol, which
produces power conversion efficiency around 2.5 %.