Increasing life span of polymer solar cell. Nagilthes Muthu Chem 4101 Fall 2011

Background Demand for energy is increasing. Main energy source for the time being is from fossil fuel and its derivatives. Alternative energy sources are being studied. Green chemistry is emphasized. Solar energy would be able to increase production of energy and at the same time full fill the requirements of green chemistry. Solar cells are needed to harvest solar energy. Polymer solar cells have caught recent interest because they are lightweight, durable (not fragile) and less hazardous to environment compared to inorganic solar cells.

Structure of polymer solar cells Polymer solar cell consists of many layers. The layer of my interest is the layer of P3HT/PCBM blend. P3HT/PCBM has promising efficiency in the production of polymer solar cell. P3HT, Poly(3-hexylthiophene-2,5-diyl) PCBM, [6,6]-Phenyl C61 butyric acid methyl ester

Problem P3HT/PCBM blend tend to aggregate towards its own species over time reducing electron transfer between them. Temperature is one of the factors that makes P3HT and PCBM to clump back to their own species. Temperature is of my interest because, when exposed to sun light, the solar cell would be receiving heat as well. I anticipate that the temperature of the solar cell can be made constant by using the method used in clothing iron.

Hypothesis When P3HT/PCBM blend is maintained at an optimal temperature, the tendency for them to crumple back to their own species would be reduced (their morphology would be stable over time.) UV-Visible spectroscopy is the best method to determine the morphology of the blend.

Sample preparation P3HT and PCBM are mixed in chlorobenzene solvent. This solution is placed into separate vials. All the solutions in separate vials are heated at 70, 80, 90 and 100 o C using hot plate. And another solution would be left at room temperature. This would be the standard. The solution is spin cast on a glass slide.

Methods to measure absorbance.  UV-Vis spectroscopy *Detectable wavelengths: nm *less costly  FTIR *Detectable wavelengths: micrometers. This wavelength range is outside of the wavelengths I would be monitoring.  Fluorescence spectroscopy *Less interference. *But, so far, did not come across this method being used for this type of problem. So, UV-Vis would be the method of choice.

UV-Vis spectrometer

Sample absorbance spectrum found by UV-Vis spectrometer The absorption spectrum of the blend tend to shift towards the band of the regioregular P3HT. This indicates the P3HT in the blend is getting more ordered.

Sample absorbance spectrum found by UV-Vis spectrometer Fig. 2. (a) Optical absorption spectra for a 12% PCBM:88% PS blend (gray line) and pristine PCBM film (black line). The absorbance of PCBM tend to be higher in a pristine condition as what is illustrated by the black bend at about 450 nm. S. Cook et al. / Chemical Physics Letters 445 (2007) 276–280

Conclusion Anticipate that keeping the P3HT/PCBM blend at a particular temperature would reduce the tendency of PCBM/P3HT blend to crumple back to its own species. UV-Visible spectroscopy is the best method to study this problem.

Reference S. Cook et al. / Chemical Physics Letters 445 (2007) 276–280 P. Vanlaeke et al. / Solar Energy Materials & Solar Cells 90 (2006) 2150–2158  h/VirtTxtJml/Spectrpy/UV-Vis/uvspec.htm h/VirtTxtJml/Spectrpy/UV-Vis/uvspec.htm  J. Phys. Chem. C, Vol. 114, No. 34, 2010