Wang Lilong, Wang Haiyan, Li Junhui, Zhang Haitao, Shen Pengchao, Wang Runzhe
International Laboratory for Quantum Functional Materials of Henan, School of Physics, Zhengzhou University, Zhengzhou 450001, People's Republic of China.
J Phys Condens Matter. 2021 Nov 4;34(4). doi: 10.1088/1361-648X/ac31fa.
In this study, the electronic properties of J50:N2200 (benzodithiophene-alt-benzotriazole: NDI-bithiophene) interface before and after fluorination/chlorination were investigated based on the first-principles density functional theory (DFT). The results reveal that the donor (D) and acceptor (A) molecules exhibit direct band gap whether to be fluorinated/chlorinated or not, and the six D:A pairs constructed all display indirect band gap. Next, for the fluorinated/chlorinated D molecule J50, the slope of total density of states (TDOS) curve edge at the highest occupied molecular orbital (HOMO) energy level enlarges, indicating high electron locality; the fluorination/chlorination of the A molecule N2200 reduces the slope of the TDOS at the HOMO level, and the electron delocalization strengthens. Then, the difference Δ1 of the lowest unoccupied molecular orbital (LUMO) levels between D and A, the difference Δ2 of HOMO levels between D and A, and the difference Δ3 between the HOMO level of the D and the LUMO level of the A were calculated about the D:A complexes. The consequences present that by using fluorine/chlorine (F/Cl) substitution at J50, Δ1 and Δ2 both decrease, and Δ3 increases; for N2200, both Δ1 and Δ2 increase, and Δ3 decreases. Since the higher open circuit voltage () is directly proportional to Δ3, again Δ1 and Δ2 afford the driving force for charge transport, these expose that the fluorination/chlorination of J50 is beneficial to obtain the higher, meanwhile, the F/Cl replacement in N2200 facilitates the separation of excitons. In addition, by the Bader charge analysis, the F/Cl substitution at D in D:A blends will promote the intramolecular charge transfer and enhance the molecular polarity; moreover, the substitution at A will improve the intermolecular charge transfer and the dipole electric field may be enhanced. Finally, the details also depend on the type of element and the position of substitution.
在本研究中，基于第一性原理密度泛函理论（DFT）研究了氟化/氯化前后J50:N2200（苯并二噻吩-alt-苯并三唑：NDI-联噻吩）界面的电子性质。结果表明，供体（D）和受体（A）分子无论是否氟化/氯化均表现出直接带隙，且构建的六对D:A均显示间接带隙。接下来，对于氟化/氯化的D分子J50，最高占据分子轨道（HOMO）能级处的总态密度（TDOS）曲线边缘斜率增大，表明电子局域性高；A分子N2200的氟化/氯化降低了HOMO能级处的TDOS斜率，电子离域增强。然后，计算了关于D:A复合物的最低未占据分子轨道（LUMO）能级之间的差值Δ1、D和A之间HOMO能级的差值Δ2以及D的HOMO能级与A的LUMO能级之间的差值Δ3。结果表明，通过在J50处使用氟/氯（F/Cl）取代，Δ1和Δ2均减小，而Δ3增大；对于N2200，Δ1和Δ2均增大，而Δ3减小。由于较高的开路电压（）与Δ3成正比，且Δ1和Δ2又为电荷传输提供驱动力，则这些结果表明J50的氟化/氯化有利于获得更高的，同时，N2200中的F/Cl取代促进了激子的分离。此外，通过Bader电荷分析，D:A共混物中D处的F/Cl取代将促进分子内电荷转移并增强分子极性；而且，A处的取代将改善分子间电荷转移且偶极电场可能增强。最后，具体细节还取决于元素类型和取代位置。
