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Issue 27, 2017
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Tuning molecule diffusion to control the phase separation of the p-DTS(FBTTh2)2/EP-PDI blend system via thermal annealing

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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 < Tm EP-PDI + Ta or T > Tc p-DTS(FBTTh2)2Tb (T is the thermal annealing temperature, Ta and Tb 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 Tm EP-PDI + Ta and Tc p-DTS(FBTTh2)2Tb, 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.

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

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Publication details

The article was received on 22 Apr 2017, accepted on 09 Jun 2017 and first published on 09 Jun 2017


Article type: Paper
DOI: 10.1039/C7TC01763D
Citation: J. Mater. Chem. C, 2017,5, 6842-6851
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    Tuning molecule diffusion to control the phase separation of the p-DTS(FBTTh2)2/EP-PDI blend system via thermal annealing

    Q. Liang, J. Han, C. Song, Z. Wang, J. Xin, X. Yu, Z. Xie, W. Ma, J. Liu and Y. Han, J. Mater. Chem. C, 2017, 5, 6842
    DOI: 10.1039/C7TC01763D

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