Analyzing poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C61 bulk-heterojunction solar cells by UV-visible spectroscopy and optical simulations

A nondestructive method for assessing the thickness of the photoactive layer in poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C-61 (P3HT:PCBM) solar cells is reported. In the approach the absorption spectrum of the solar cell as derived by optical simulations is fitted to the corresponding measured spectrum, varying only the P3HT:PCBM layer thickness. Within the 50-250 nm thickness range, a linear correlation between the position of a certain spectral minimum and the P3HT:PCBM layer thickness is shown, based on simulated absorption spectra. As an initial application, absorption spectra for 240 P3HT:PCBM solar cells prepared at four different spin-coating speeds were recorded, and the average P3HT:PCBM layer thickness estimated for each spin-coating speed. The simulated fraction of light absorbed in the P3HT:PCBM layer of the solar cells is compared with the P3HT:PCBM absorption spectra measured for films spin coated on simpler substrate types. The latter spectra cannot account for the light harvested in the photoactive layer of P3HT:PCBM solar cells because of substantial optical interference in the solar cells. The measured short circuit current densities J(sc) for the solar cells vary with the spin-coating speed in a manner confirmed by optical simulations of the maximal short circuit current densities. The measured efficiencies follow the same pattern. On average the measured J(sc) is 1-2 mA/cm(2) below the simulated maximal short circuit current densities. Based on the resemblance of the measured and simulated absorption spectra such difference can be attributed to recombination exclusively. (c) 2007 American Institute of Physics.