Abstract
Interfacial layers (interlayers) are one of the emerging approaches in organic solar cells with bulk heterojunction (BHJ) layers because the solar cell efficiency can be additionally improved by their presence. However, less attention is paid to the use of interlayers for polymer:nonfullerene solar cells, which have strong advantages over polymer:fullerene solar cells. In addition, most polymers used for the interlayers possess a low glass transition temperature (T g). Here, it is demonstrated that two types of quarterthiophene‐containing polyimides (PIs) with high T g (>198 °C), which are synthesized using pyromellitic dianhydride (PMDA) and cyclobutane‐1,2,3,4‐tetracarboxylic dianhydride (CTCDA), can act as an interfacial layer in the polymer:nonfullerene solar cells with the BHJ layers of poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T) and 3,9‐bis(2‐methylene(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene) (ITIC), or (3‐(1,1‐dicyanomethylene)‐1‐methyl‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]‐dithiophene) (IT‐M). Interestingly, the efficiency and stability of devices are improved by the PMDA‐based PI interlayers with a stretched chain structure but degraded by the CTCDA‐based PI interlayers with a bended chain structure.
Highlights
Less attention has been paid to applying such interfacial layers for polymer:nonfullerene solar cells, for the inverted-type device structures that benefit from the use of stable top electrodes with high work functions including, e.g., silver
The coated PAA films were converted to corresponding PI films, PMDA–DAQT PI (P-PI in short) and CTCDA–DAQT PI (C-PI in short), via a thermal imidization process at 200 °C for 90 min
The increased RS by the presence of the C-PI interlayer can be ascribed to the highest interfacial resistance, as measured with impedance spectroscopy, which might be caused by the less desirable morphology formed in the bottom region of the bulk heterojunction (BHJ) (PBDB-T:ITIC) layer owing to the presence of the C-PI interlayer
Summary
The increased RS by the presence of the C-PI interlayer can be ascribed to the highest interfacial resistance (see R2 in Figure S9 in the Supporting Information), as measured with impedance spectroscopy, which might be caused by the less desirable morphology formed in the bottom region (toward the ZnO layer) of the BHJ (PBDB-T:ITIC) layer owing to the presence of the C-PI interlayer. The devices with the C-PI interlayers exhibited much quicker degradation even after 3 h, whereas the P-PI interlayers delivered still better J–V curves than the reference devices after 8 h This result reflects that the limited (partial) contact of the C-PI chains with the ZnO surface (see the energy minimized molecular structures in Figure 4c) might generate unstable interfaces leading to the fast degradation in device performances. It is noteworthy that the performance of the inverted-type PTB7-Th:PC71BM solar cells could be improved by inserting the P-PI interlayer even though it was rather lowered by the presence of the C-PI interlayer (see Figure S13 in the Supporting Information)
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