Hsiu-g Chen

Conjugated random copolymers of benzodithiophene–benzooxadiazole–diketopyrrolopyrrole with full visible light absorption for bulk heterojunction solar cells

We have used Stille coupling polymerization to synthesize a series of new donor–acceptor (D–A) conjugated random copolymers—PBDTT-BO-DPP—that comprise electron-rich alkylthienyl-substituted benzodithiophene (BDTT) units in conjugation with electron-deficient 2,1,3-benzooxadiazole (BO) and diketopyrrolopyrrole (DPP) moieties that have complementary light absorption behavior. These polymers exhibited excellent thermal stability with thermal degrading temperatures higher than 340 °C. Each of these copolymers exhibited (i) broad visible light absorption from 400 to 900 nm and (ii) a low optical band gap that is smaller than 1.4 eV and a low-lying highest occupied molecular orbital that is deeper than −5.22 eV. As a result, bulk heterojunction photovoltaic devices derived from these polymers and fullerenes provided a high short-circuit current density that is larger than 12 mA cm−2. In particular, a photovoltaic device prepared from the PBDTT-BO-DPP (molar ratio, 1 : 0.5 : 0.5)/PC71BM (w/w, 1 : 2) blend system with 1-chloronaphthalene (1 volume%) as an additive exhibited excellent photovoltaic performance, with a value of Voc of 0.73 V, a high short-circuit current density of 17 mA cm−2, a fill factor of 0.55, and a promising power conversion efficiency of 6.8%, indicating that complementary light-absorption random polymer structures have great potential for increasing the photocurrent in bulk heterojunction photovoltaic devices.

Complementary solvent additives tune the orientation of polymer lamellae, reduce the sizes of aggregated fullerene domains, and enhance the performance of bulk heterojunction solar cells

In this study we employed 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) as binary additives exhibiting complementarily preferential solubility for processing the crystalline conjugated polymer poly[bis(dodecyl)thiophene-dodecyl-thieno[3,4-c]pyrrole-4,6-dione] (PBTC12TPD) and the fullerene [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chloroform. Using synchrotron grazing-incidence small-/wide-angle X-ray scattering and transmission electron microscopy to analyse the structure of the PBTC12TPD–PC71BM blend films, we found that the binary additives with different volume ratios in the processing solvent allow us to tune the relative population of face-on to edge-on PBTC12TPD lamellae and the size of PC71BM clusters in the blend films; the sizes of the fractal-like PC71BM clusters and the aggregated domains of PC71BM clusters increased and decreased, respectively, upon increasing the amount of DIO, whereas the relative ratio of face-on to edge-on PBTC12TPD lamellae increased upon increasing the amount of CN. When fabricating the photovoltaic devices, the short-circuit current density of the devices with the PBTC12TPD–PC71BM active layer having been processed with the binary additives is higher than that of the device incorporating an active layer processed without any additive. As a result, the power conversion efficiency of a device incorporating an active layer of PBTC12TPD–PC71BM (1 : 1.5, w/w) processed with binary additives of 0.5% DIO and 1% CN in chloroform increased to 6.8% from a value of 4.9%, a relative increase of 40%, for the corresponding device containing the same active layer but processed without any additive.

Effects of bulk and interfacial charge accumulation on fill factor in organic solar cells

The effects of charge accumulation on fill factor (FF) are studied by using light intensity-dependent current-voltage and capacitance-voltage measurements based on poly-{bi(dodecyl)thiophene-thieno[3,4-c]pyrrole-4,6-dione}:1(3-methyloxycarbony1)propyl-1-phenyl [6,6] (PBTTPD:PCBM) solar cells. We find that the FF of un-annealed device shows a non-monotonic dependence of light intensity: first an increase and then a decrease with increasing light intensity. This suggests that both bulk charge accumulation and recombination determine the FF before thermal annealing. Furthermore, the morphological enhancement of PBTTPD:PCBM film leads to a monotonic decrease of FF as increasing light intensity. This indicates that only surface charge accumulation plays an important role in the determination of the FF after thermal annealing.

A nanostructured micellar diblock copolymer layer affects the memory characteristics and packing of pentacene molecules in non-volatile organic field-effect transistor memory devices

Organic field-effect transistor (OFET) memory devices incorporating the copolymer polystyrene-block-poly(4-vinylpyridine) (PS56k-b-P4VP8k) layer, which features a thickness-dependent micellar nanostructure (P4VP-core, PS-shell), as a charge trapping layer can exhibit tunable memory windows for p-channel applications. For instance, the memory window increased substantially from 7.8 V for the device incorporating a 60 nm thick PS56k-b-P4VP8k layer to 21 V for the device incorporating a 27 nm thick layer, an increase of more than 2.5 times. Using simultaneous synchrotron grazing-incidence small-angle X-ray scattering and wide-angle X-ray scattering to probe the nanostructured micellar PS56k-b-P4VP8k layer and the pentacene layer positioned directly on the top of the copolymer layers, respectively, we were able to elucidate the structural characteristics of the bilayer and to correlate their effects with the memory performances of devices with similar architectures. For the PS56k-b-P4VP8k layers, we found that the inter-micelle distance and their lateral arrangements depended on the layer thickness: the thickness of the PS shells in the lateral direction decreased upon increasing the layer thickness, as did the memory window for the OFET device that incorporated the PS56k-b-P4VP8k layers, showing a strong dependence of the threshold voltage shifts (i.e., memory window) on the distance between the micelles. Additionally, for the molecular packing of the pentacene layer positioned on the copolymer layer, we found that the PS56k-b-P4VP8k layers affected not only the orientation of the pentacene molecules but also their grain sizes, thereby affecting the hole mobility of the memory devices. These results suggest that tuning the micellar nanostructure of the block copolymer thin film that was used as a trapping layer can be a simple and effective way for optimizing the memory window and affecting the hole mobility of OFET memory devices.

Energy transfer within small molecule/conjugated polymer blends enhances photovoltaic efficiency

In this study, we employed ternary blends capable of energy transfer—a synthesized high-band-gap small molecule (SM-4OMe) comprising benzodithiophene (BDT) and rhodanine units (a molecular structure that was designed for energy transfer), a low-band-gap polymer (PTB7-TH) comprising BDT and thienothiophene units with desired packing orientation, and a fullerene—as active layers for single-junction photovoltaic devices. The light absorption of the small molecule and the polymer was partially complementary, owing to their band gap difference, thereby broadening the absorption spectrum of solar light while maintaining the energy band structures that facilitated energy and charge transfer. The synthesized small molecule SM-4OMe and the PTB7-TH had somewhat similar chemical structures—with the same planar BDT donor units—and thus allowed sufficient mixing between them for energy transfer to take place. The power conversion efficiency of a device incorporating a ternary blend of PTB7-TH:SM-4OMe:PC71BM (0.9 : 0.1 : 1.5, w/w/w) as the active layer, processed with diiodooctane (2 vol%) in chlorobenzene, was 10.4%, which is higher than the value of 8% of the corresponding device incorporating PTB7-TH:PC71BM (1 : 1.5, w/w)—an increase of 30%. We attribute this enhancement to the energy transfer from the high-band-gap small molecule SM-4OMe to the low-band-gap polymer PTB7-TH and to the optimal phase-separated bulk heterojunction morphology that comprises a mean PC71BM cluster size of 6 nm, which is lower than 12 nm for the PTB7-TH and PC71BM binary blends, and slightly better in-plane packing, arising from the inducements of the presence of SM-4OMe. This approach provides a facile and effective way to enhance the power conversion efficiency of single junction organic photovoltaics.

Block Copolymer-Tuned Fullerene Electron Transport Layer Enhances the Efficiency of Perovskite Photovoltaics

In this study, we enhanced the power conversion efficiency (PCE) of perovskite solar cells by employing an electron transfer layer (ETL) comprising [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) and, to optimize its morphology, a small amount of the block copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO), positioned on the perovskite active layer. When incorporating 0.375 wt % PS-b-PEO into PC61BM, the PCE of the perovskite photovoltaic device increased from 9.4% to 13.4%, a relative increase of 43%, because of a large enhancement in the fill factor of the device. To decipher the intricate morphology of the ETL, we used synchrotron grazing-incidence small-angle X-ray scattering for determining the PC61BM cluster size, atomic force microscopy and scanning electron microscopy for probing the surface, and transmission electron microscopy for observing the aggregation of PC61BM in the ETL. We found that the interaction between PS-b-PEO and PC61BM resulted in smaller PC61BM clusters that further aggregated into dendritic structures in some domains, a result of the similar polarities of the PS block and PC61BM; this behavior could be used to tune the morphology of the ETL. The optimal PS-b-PEO-mediated PC61BM cluster size in the ETL was 17 nm, a large reduction from 59 nm for the pristine PC61BM layer. This approach of incorporating a small amount of nanostructured block copolymer into a fullerene allowed us to effectively tune the morphology of the ETL on the perovskite active layer and resulted in enhanced fill factors of the devices and thus their device efficiency.

Photovoltaic performance of ladder-type indacenodithieno[3,2-b]thiophene-based polymers with alkoxyphenyl side chains

Two new ladder-type conjugated polymers, PIDTT-DFBT-EH and PIDTT-F-PhanQ-EH, are prepared through the copolymerization of heptacyclic IDTT with F-PhanQ and DFBT electron deficient moieties. The introduction of 4-(2-ethylhexyloxy)-phenyl (EHOPh) side-chains onto the polymer is beneficial for achieving high molecular weight and good solution-processability of materials. The derived polymer solar cells yielded PCEs of 5.48% (PIDTT-DFBT-EH) and 5.14% (PIDTT-F-PhanQ-EH), without engaging post-solvent or solvent additive treatments.

Doping ZnO Electron Transport Layers with MoS2 Nanosheets Enhances the Efficiency of Polymer Solar Cells

In this study, we incorporated molybdenum disulfide (MoS2) nanosheets into sol-gel processing of zinc oxide (ZnO) to form ZnO:MoS2 composites for use as electron transport layers (ETLs) in inverted polymer solar cells featuring a binary bulk heterojunction active layer. We could effectively tune the energy band of the ZnO:MoS2 composite film from 4.45 to 4.22 eV by varying the content of MoS2 up to 0.5 wt %, such that the composite was suitable for use in bulk heterojunction photovoltaic devices based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene- alt-(4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl] (PTB7-TH)/phenyl-C71-butryric acid methyl ester (PC71BM). In particular, the power conversion efficiency (PCE) of the PTB7-TH/PC71BM (1:1.5, w/w) device incorporating the ZnO:MoS2 (0.5 wt %) composite layer as the ETL was 10.1%, up from 8.8% for the corresponding device featuring ZnO alone as the ETL, a relative increase of 15%. Incorporating a small amount of MoS2 nanosheets into the ETL altered the morphology of the ETL and resulted in enhanced current densities, fill factors, and PCEs for the devices. We used ultraviolet photoelectron spectroscopy, synchrotron grazing incidence wide-/small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy to characterize the energy band structures, internal structures, surface roughness, and morphologies, respectively, of the ZnO:MoS2 composite films.