(553a) Invited Talk: Tail State Distribution of Polymer Donor Dictates Whether Vertical Phase Separation Affects Device Characteristics in Bulk-Heterojunction Solar Cells
AIChE Annual Meeting
2014
2014 AIChE Annual Meeting
Materials Engineering and Sciences Division
Conjugated Polymers
Wednesday, November 19, 2014 - 3:15pm to 3:45pm
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.