Tuning molecule diffusion to control the phase separation of the p-DTS(FBTTh2)2/EP-PDI blend system via thermal annealing

Abstract

An interpenetrating bulk-heterojunction structure with a domain size of 10–20 nm is the ideal morphology for carrier generation, separation and transportation in organic solar cells. However, depending on the blend composition, the phase-separation behavior of the crystalline small molecule blend system of donor 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) (p-DTS(FBTTh₂)₂) and acceptor N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) is quite different. When the weight ratio of p-DTS(FBTTh₂)₂/EP-PDI is greater than 8 : 2 or smaller than 4 : 6, a large phase separation structure is observed induced by p-DTS(FBTTh₂)₂ or EP-PDI crystallization. When the ratio of p-DTS(FBTTh₂)₂ : EP-PDI changes from 7 : 3 to 5 : 5, no obvious phase separation can be detected due to the interaction between p-DTS(FBTTh₂)₂ and EP-PDI. In order to obtain the interpenetrating bulk-heterojunction structure with a domain size of 10–20 nm, we proposed to tune the molecule diffusion of p-DTS(FBTTh₂)₂ and EP-PDI by different thermal annealing temperature to control the phase separation domain size and phase purity. When the annealing temperature is T < T_m EP-PDI + T_a or T > T_{c p-DTS(FBTTh2)2} − T_b (T is the thermal annealing temperature, T_a and T_b are constants), the molecule diffusion rate of both p-DTS(FBTTh₂)₂ and EP-PDI is too slow or too fast, resulting in no phase separation or large phase separation morphology. When T is between T_m EP-PDI + T_a and T_{c p-DTS(FBTTh2)2} − T_b, EP-PDI is in a melting state, but p-DTS(FBTTh₂)₂ self-assembles into crystals. As a result, p-DTS(FBTTh₂)₂ crystallized and formed a framework, which inhibited the massive crystallization of EP-PDI due to the spatial confinement of the p-DTS(FBTTh₂)₂ crystallization framework, leading to the formation of a bi-continuous phase separation structure with suitable domain size and phase purity. Based on the above phase separation structure we got, a power conversion efficiency of 4.25% was obtained, which is relatively high in this system without any additives.