Taeshik Earmme

All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor

The lack of suitable acceptor (n-type) polymers has limited the photocurrent and efficiency of polymer/polymer bulk heterojunction (BHJ) solar cells. Here, we report an evaluation of three naphthalene diimide (NDI) copolymers as electron acceptors in BHJ solar cells which finds that all-polymer solar cells based on an NDI-selenophene copolymer (PNDIS-HD) acceptor and a thiazolothiazole copolymer (PSEHTT) donor exhibit a record 3.3% power conversion efficiency. The observed short circuit current density of 7.78 mA/cm(2) and external quantum efficiency of 47% are also the best such photovoltaic parameters seen in all-polymer solar cells so far. This efficiency is comparable to the performance of similarly evaluated [6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM)/PSEHTT devices. The lamellar crystalline morphology of PNDIS-HD, leading to balanced electron and hole transport in the polymer/polymer blend solar cells accounts for its good photovoltaic properties.

n-Type Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling Crystallinity, Blend Morphology, and Compatibility Toward High-Performance All-Polymer Solar Cells

Knowledge of the critical factors that determine compatibility, blend morphology, and performance of bulk heterojunction (BHJ) solar cells composed of an electron-accepting polymer and an electron-donating polymer remains limited. To test the idea that bulk crystallinity is such a critical factor, we have designed a series of new semiconducting naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene random copolymers, xPDI (10PDI, 30PDI, 50PDI), whose crystallinity varies with composition, and investigated them as electron acceptors in BHJ solar cells. Pairing of the reference crystalline (crystalline domain size Lc = 10.22 nm) NDI-selenophene copolymer (PNDIS-HD) with crystalline (Lc = 9.15 nm) benzodithiophene-thieno[3,4-b]thiophene copolymer (PBDTTT-CT) donor yields incompatible blends, whose BHJ solar cells have a power conversion efficiency (PCE) of 1.4%. However, pairing of the new 30PDI with optimal crystallinity (Lc = 5.11 nm) as acceptor with the same PBDTTT-CT donor yields compatible blends and all-polymer solar cells with enhanced performance (PCE = 6.3%, Jsc = 18.6 mA/cm(2), external quantum efficiency = 91%). These photovoltaic parameters observed in 30PDI:PBDTTT-CT devices are the best so far for all-polymer solar cells, while the short-circuit current (Jsc) and external quantum efficiency are even higher than reported values for [70]-fullerene:PBDTTT-CT solar cells. The morphology and bulk carrier mobilities of the polymer/polymer blends varied substantially with crystallinity of the acceptor polymer component and thus with the NDI/PDI copolymer composition. These results demonstrate that the crystallinity of a polymer component and thus compatibility, blend morphology, and efficiency of polymer/polymer blend solar cells can be controlled by molecular design.

Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics

New electron-acceptor materials are long sought to overcome the small photovoltage, high-cost, poor photochemical stability, and other limitations of fullerene-based organic photovoltaics. However, all known nonfullerene acceptors have so far shown inferior photovoltaic properties compared to fullerene benchmark [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), and there are as yet no established design principles for realizing improved materials. Herein we report a design strategy that has produced a novel multichromophoric, large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T, whose π-conjugated framework occupies space comparable to an aggregate of 9 [C60]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC60BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a high short-circuit current (10.14 mA/cm(2)) with a high open-circuit voltage (0.86 V) to give a power conversion efficiency of 5.0%. The external quantum efficiency spectrum of PSEHTT:DBFI-T devices had peaks of 60-65% in the 380-620 nm range, demonstrating that both hole transfer from photoexcited DBFI-T to PSEHTT and electron transfer from photoexcited PSEHTT to DBFI-T contribute substantially to charge photogeneration. The superior charge photogeneration and electron-accepting properties of DBFI-T were further confirmed by independent Xenon-flash time-resolved microwave conductivity measurements, which correctly predict the relative magnitudes of the conversion efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC60BM > PSEHTT:BFI-P2. The results demonstrate that the large size, multichromophoric, nonplanar 3D molecular design is a promising approach to more efficient organic photovoltaic materials.

All‐Polymer Bulk Heterojuction Solar Cells with 4.8% Efficiency Achieved by Solution Processing from a Co‐Solvent

All-polymer solar cells with 4.8% power conversion efficiency are achieved via solution processing from a co-solvent. The observed short-circuit current density of 10.5 mA cm(-2) and external quantum efficiency of 61.3% are also the best reported in all-polymer solar cells so far. The results demonstrate that processing the active layer from a co-solvent is an important strategy in achieving highly efficient all-polymer solar cells.

High-performance multilayered phosphorescent OLEDs by solution-processed commercial electron-transport materials

Highly efficient multilayered polymer-based phosphorescent organic light-emitting diodes (PhOLEDs) are realized by orthogonal sequential solution-processing of commercial small-molecule electron-transport materials (ETMs), including 1,3,5-tri(3-pyrid-3-yl-phenyl)benzene (TmPyPB), 4,7-diphenyl-1,10-phenanthroline (BPhen) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). The performance of PhOLEDs with solution-deposited ETMs is found to be far superior compared to devices with vacuum-deposited ETMs. PhOLEDs with a solution-deposited BPhen electron-transport layer gave a luminous efficiency of 53.8 cd A−1 with an external quantum efficiency of 16.1%. The solution-processed electron-transport layer exhibits a unique rough surface morphology which facilitates charge-injection and transport from the cathode metal. The nanostructured surface morphology and charge carrier mobility of the solution-processed electron-transport layers could be tuned and controlled by the solution concentration. The orthogonal solution-processing demonstrated here is a promising strategy for applications in various solution-processed multilayered organic electronic devices.

Naphthobisthiazole diimide-based n-type polymer semiconductors: synthesis, π-stacking, field-effect charge transport, and all-polymer solar cells

New n-type conjugated polymer semiconductors bearing an electron-deficient naphthobisthiazole diimide (NBTDI) moiety have been synthesized and their electronic energy levels, solid-state morphology, field-effect charge transport and photovoltaic properties were investigated. Stille coupling polymerization of the new monomer 5,11-bis(4-bromophenyl)-2,8-bis(2-decyltetradecyl)-benzothiazolo[4,5,6,7-lmn]thiazolo[5,4-f][3,8]phenanthroline-1,3,7,9(2H, 8H)-tetrone with distannyl derivatives of the arylene moiety (1,4-phenylene/2,5-thienylene/vinylene) yielded poly(naphthobisthiazole diimide)s (PNBTDIs) with number-average molecular weights of 73–89 kDa and polydispersity indexes of 2.46–3.08. Thin films of the PNBTDIs have strong absorption bands in the visible region, resulting in an absorption edge optical band gap of 1.73–2.02 eV. X-ray diffraction analysis of neat films of PNBTDIs revealed lamellar crystalline materials with a rather short intermolecular π–π stacking distance of 0.34–0.35 nm. In thin film transistors, the PNBTDIs showed unipolar n-channel transport with electron mobility as high as 1.5 × 10−2 cm2 V−1 s−1. All-polymer solar cells incorporating the PNBTDIs as an electron acceptor and thiazolothiazole copolymer (PSEHTT) as a donor had power conversion efficiencies of 1.1–1.5%. The results demonstrate that poly(naphthobisthiazole diimide)s are promising n-type materials for field-effect transistors and all-polymer solar cells.

Annealing temperature dependence of the efficiency and vertical phase segregation of polymer/polymer bulk heterojunction photovoltaic cells

We report observation of annealing temperature-induced simultaneous vertical phase segregation and large enhancement of power conversion efficiency (PCE) of all-polymer bulk heterojunction (BHJ) solar cells composed of a poly(3-hexylthiophene) (P3HT) donor and a naphthalene diimide-selenolo[3,2-b]selenophene copolymer (PNDISS) acceptor. The PCE of P3HT:PNDISS BHJ devices increased over 50-fold from 0.04% to 2.03% when the annealing temperature was increased from 50 to 150 °C. Absorption spectroscopy and photoluminescence quenching experiments provide evidence of increasing phase segregation of the polymer/polymer blend films with increasing annealing temperature. Field-effect charge transport, contact angle, surface energy, and variable angle ellipsometry measurements on the P3HT:PNDISS blend films showed that thermal annealing induced vertical phase segregation, whereby the low surface energy polymer (P3HT) migrated to the bulk, while the high surface energy polymer (PNDISS) enriches at the substrate/ble...

New sulfone-based electron-transport materials with high triplet energy for highly efficient blue phosphorescent organic light-emitting diodes

A series of new electron transport materials, which combines a diphenylsulfone core with different electron withdrawing end groups, has been synthesized, characterized, and found to exhibit high triplet energy (ET > 2.8 eV) for use in phosphorescent organic light emitting diodes (PhOLEDs). The new materials, including 3,3′-(4,4′-sulfonylbis(4,1-phenylene))dipyridine (SPDP), 5,5′-(4,4′-sulfonylbis(4,1-phenylene))bis(3-phenylpyridine) (SPPP), and 3,3′-(4,4′-sulfonylbis(4,1-phenylene))diquinoline (SPDQ) had wide band gaps (3.6–3.8 eV) and LUMO levels of −2.4 to −2.7 eV. The triplet energy measured from phosphorescence spectra at 77 K varied from 2.53 eV for SPDQ and 2.81 eV for SPPP to 2.90 eV for SPDP, which are in good agreement with density functional theory calculated values. High performance blue PhOLEDs using the sulfone-based materials are exemplified by devices containing a poly(N-vinylcarbazole) host and SPDP electron transport layer, which had a high quantum efficiency (19.6%) and a high current efficiency (33.6 cd A−1) even at very high luminances (4500 cd m−2). These results demonstrate that sulfone-based molecules are promising electron transport materials for application in developing highly efficient phosphorescent OLEDs.

Improved electron injection and transport by use of baking soda as a low-cost, air-stable, n-dopant for solution-processed phosphorescent organic light-emitting diodes

Sodium bicarbonate (baking soda, NaHCO3) is found to be an efficient low-cost, air-stable, and environmentally friendly n-dopant for electron-transport layer (ETL) in solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). A 2.0-fold enhancement in power efficiency of blue PhOLEDs is observed by use of NaHCO3-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) ETL. The bulk conductivity of NaHCO3-doped BPhen film is increased by 5 orders of magnitude. Enhanced performance of PhOLEDs is similarly observed by use of NaHCO3-doped 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene ETL. These results demonstrate that sodium bicarbonate is an effective n-dopant in organic electronics.