Fig. 2: Ultraviolet photoemission spectroscopy of valence band.

Legend: red, PBDTTPD:PCBM (1:1.5 w/w) film; blue, P3HT:PCBM (1:0.8 w/w) film. Ionization energy (I_E) is marked by standard extrapolation of inflection point. Films were measured consecutively in identical configuration with same He-I radiation intensity.