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We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C 60 molecules from a C 60 layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C 60 molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C 60 can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C 60 diffusion is thermally activated and not driven by WFL-type collective motion above T g but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and the approach can be adapted to a wide range of donor and acceptor materials.

In this Article, low-bandgap pTTDPP-BT polymers based on electron-accepting pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) and benzothiadiazole (BT) and electron-donating thienothiophene (TT) moieties were synthesized. Phototransistors have been fabricated using ambipolar-behaving pTTDPP-BT polymers as active channel materials. The electrical and photoresponsive properties of the pTTDPP-BT phototransistors were strongly dependent on the film annealing temperature. As-spun pTTDPP-BT phototransistors exhibited a low hole mobility of 0.007 cm 2 /(V·s) and a low electron mobility of 0.005 cm 2 /(V·s), which resulted in low photocurrent detection due to the limited transport of the charge carriers. Thermal treatment of the polymer thin films led to a significant enhancement in the carrier mobilities (hole and electron mobilities of 0.066 and 0.115 cm 2 /(V·s), respectively, for 200 °C annealing) and thus significantly improved photoresponsive properties. The 200 °C-annealed phototransistors showed a wide-range wavelength (405-850 nm) of photoresponse, and a high photocurrent/dark-current ratio of 150 with a fast photoswitching speed of less than 100 ms. This work demonstrates that a dual acceptor-containing low band gap polymer can be an important class of material in broadband photoresponsive transistors, and the crystallinity of the semiconducting polymer layer has a significant effect on the photoresponse characteristics.

The development of thick organic photovoltaics (OPV) could increase absorption in the active layer and ease manufacturing constraints in large-scale solar panel production. However, the efficiencies of most low-bandgap OPVs decrease substantially when the active layers exceed ∼100 nm in thickness (because of low crystallinity and a short exciton diffusion length). Herein, we report the use of solvent additive diphenyl ether (DPE) that facilitates the fabrication of thick (180 nm) active layers and triples the power conversion efficiency (PCE) of conventional thienothiophene-co-benzodithiophene polymer (PTB7)-based OPVs from 1.75 to 6.19%. These results demonstrate a PCE 20% higher than those of conventional (PTB7)-based OPV devices using 1,8-diiodooctane. Morphology studies reveal that DPE promotes the formation of nanofibrillar networks and ordered packing of PTB7 in the active layer that facilitate charge transport over longer distances. We further demonstrate that DPE improves the fill factor and photocurrent collection by enhancing the overall optical absorption, reducing the series resistance, and suppressing bimolecular recombination.