(553a) Invited Talk: Tail State Distribution of Polymer Donor Dictates Whether Vertical Phase Separation Affects Device Characteristics in Bulk-Heterojunction Solar Cells

With soft-contact lamination and delamination schemes, we have been able to elucidate how the presence of an electron-donor wetting layer at the electron-collecting interface of solar cells affects device performance.  We constructed conventional bulk-heterojunction solar cells comprising three model electron donors:  poly(3-hexyl thiophene), or P3HT; poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3 benzothiadiazole)], abbreviated PCPDTBT; and 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole), abbreviated p-DTS(FBTTh₂)₂, with fullerene derivatives.  To exaggerate the influence of vertical phase separation, we laminated in each case a thin layer of electron donor on the bulk-heterojunction active layer prior to cathode deposition.  While devices with PCPDTBT and p-DTS(FBTTh₂)₂ exhibit drastically reduced short-circuit current densities compared to devices without the additional electron-donor wetting layer, devices with P3HT only exhibit a marginal drop in short-circuit current density compared to devices without a thin P3HT wetting layer at the electron-collecting interface.  We ascribe this difference in the reduction of short-circuit current density to intrinsic differences in the ionization potential and tail state distribution of the electron donors.  Relative to PCPDTBT and p-DTS(FBTTh₂)₂, P3HT has a shallower HOMO energy level (closer to vacuum level) and a substantially broader tail state distribution.  Against the energy levels of fullerene, both of these factors can support electron-hole recombination, followed by electron injection from the cathode into the tail states of P3HT when the solar cell is under operation.  As such, the presence of a P3HT wetting layer does not substantially affect device performance.  In comparison, PCPDTBT and p-DTS(FBTTh₂)_2­ have deeper HOMO energy levels and narrower tail state distributions compared to P3HT, the combination of which cannot support electron-hole recombination and hole injection into the electron donor from the cathode.  The presence of thin wetting layers of PCPDTBT and p-DTS(FBTTh₂)₂ at the electron-collecting interface thus adversely affects device performance.  These observations implicate the relative importance of vertical phase separation in the bulk-heterojunction active layers of these materials pairs in dictating solar cell device performance.