Varying polymer crystallinity in nanofiber poly(3-alkylthiophene): PCBM solar cells: Influence on charge-transfer state energy and open-circuit voltage

The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (E-CT) and on the open-circuit voltage (V-oc) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between similar to 0.1 and similar to 0.9 by temperature control. E-CT, as obtained by Fourier-transform photocurrent spectroscopy decreased with f as E-CT=E-CT(0)-0.2f eV. Alkyl side-chain length only influences E-CT(0). V-oc relates to E-CT as V-oc=E-CT/q-0.6 V. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3232242]