Origin of Red Shift in the Photoabsorption Peak in MEH-PPV Polymer

Development of efficient and cost-effective solar cells capable of absorbing solar radiation over a wide range of wavelengths is essential to meet the current need for clean and renewable energy. This has led to considerable interest in understanding ways in which the band gap of materials can be altered. Recently, electrospinning of MEH-PPV polymer has been found to not only cause a red shift but also an enhancement of the absorption spectrum. However, the origin of this shift has remained a mystery. Using time dependent density functional theory (TDDFT) we have examined the origin of this red shift by studying the various effects that can influence the optical absorption. These include length of the polymer chain, stretching of the linkage C=C double bonds, and the solvent. The UV-vis spectra were calculated by modeling the MEH-PPV polymer with different chain lengths, systematically stretching the C=C double bonds, and immersing it in chloroform solution. While all these effects lead to a red shift, the dominant feature for the redshift in the electrospun polymers is traced to be due to the stretching of the C=C bonds.