Xinxiu Cao

Constructing the nanointerpenetrating structure of PCDTBT:PC70BM bulk heterojunction solar cells induced by aggregation of PC70BM via mixed-solvent vapor annealing

In this paper, mixed-solvent vapor annealing (M-SVA) was proposed to improve polymer crystallinity and inhibit fullerene intercalation into polymer side chains in a poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) blend system. Experiments showed the optimal volume ratio of a mixed solvent, i.e., for tetrahydrofuran (THF) and carbon disulfide (CS2) vapor it is 1 : 1, and the appropriate annealing time is 5 min. After the M-SVA process, out-of-plane grazing incidence X-ray diffraction (GIXD) and spectroscopic ellipsometry (SE) data show that PCDTBT crystals with larger size and higher crystallinity were formed due to enhanced π–π interaction of polymer chains; the aggregation of PC70BM was promoted as well, which inhibited the intercalation behavior, resulting in a continuous electron path and improved phase purity. The morphology transition improved the carrier mobility, in particular electron mobility. Also, the recombination of a charge transfer (CT) state was reduced and the lifetime of mobile carriers was increased, which could be verified by transient-absorption (TA) spectra. As a result, the power conversion efficiency (PCE) of the BHJ reached 6.60%, an increase of more than 40% compared with the reference device (PCE = 4.54%), even though the thickness of the active layer is up to 220 nm and the PC70BM content is as low as 71.4 wt% by weight.

Restricting the liquid–liquid phase separation of PTB7-Th:PF12TBT:PC71BM by enhanced PTB7-Th solution aggregation to optimize the interpenetrating network

The current understanding of the active layer morphology in ternary organic solar cells (OSCs) is superficial owing to more variables and complexity compared to that of binary OSCs. The PTB7-Th:PF12TBT:PC71BM ternary system with complementary polymer absorption spectra and efficient energy transfer from PF12TBT to PTB7-Th was anticipated to have an outstanding performance. However, only a limited improvement in the power conversion efficiency (PCE) was achieved when the ternary devices were processed from CB/DIO. This is because large PC71BM domains formed with the addition of amorphous PF12TBT and the PF12TBT molecules embedded in the large PC71BM domains served as trap sites. Constraining the formation of large PC71BM domains and avoiding PF12TBT molecules being fully embedded in PC71BM domains are needed for a better performance. Polymer pre-aggregation before liquid–liquid phase separation is beneficial to construct bicontinuous interpenetrating networks with proper phase-separated domains. Therefore, para-xylene was introduced into CB/DIO to cause a weak polymer–solvent interaction for enhanced PTB7-Th solution-phase aggregation. The enhanced PTB7-Th aggregation in the CB/PX/DIO solution restricted the extent of the liquid–liquid phase separation, and a well-developed polymer network formed with improved PTB7-Th crystallinity, which prevented large PC71BM domains from forming via fluid-phase Ostwald ripening in the liquid–solid phase separation stage. The appearance of PF12TBT emission and increased PTB7-Th emission in the ternary film processed from CB/PX/DIO suggested that fewer PF12TBT molecules were embedded in the PC71BM domains, and the energy transfer from PF12TBT to PTB7-Th was more efficient. Thus, PCE increased from 8.09% for binary blends processed from CB/DIO to 9.28% for ternary blends processed from CB/PX/DIO.

UV-curable ladder-like diphenylsiloxane-bridged methacryl-phenyl-siloxane for high power LED encapsulation

A UV curable ladder-like diphenylsiloxane-bridged methacryl-phenyl-siloxane (L-MPS) was synthesized from phenyltrichlorosilane, diphenylsilanediol and methacryloxypropyldimethylmethoxysilane via dehydrochlorination precoupling, supramolecular architecture-directed hydrolysis-condensation and end-capping reactions. The L-MPS has a condensation degree of ∼100%, and can be complete crosslinked by UV curing. XRD, TEM and molecular simulation suggest that the ladder-like molecules are close packed with a periodic distance of ca. 1.2 nm. The L-MPS shows transmittance of 98% and a refractive index of ca. 1.61 at 450 nm. The cured L-MPS with a Td5% value of 465.5 °C showed excellent anti-yellowing and anti-sulfidation properties. The cured L-MPS film and the encapsulated LED samples were compared with those of Dow Corning OE-6630 and OE-7662. It is believed that the dense nano-ladder unit also contributes to the thermal, gas barrier and even optical properties. L-MPS shows promising potential as a high power LED encapsulant and optical coating for use in harsh environments. This work provides an approach to integrate this novel ladder structure with advanced properties.