Selvam Subramaniyan

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.

Nonfullerene Polymer Solar Cells with 8.5% Efficiency Enabled by a New Highly Twisted Electron Acceptor Dimer.

Fullerene-free and processing additive-free 8.5% efficient polymer solar cells are achieved by using a new 3,4-ethylenedioxythiophene-linked arylene diimide dimer with a 76° twist angle. The devices combine high (78-83%) external quantum efficiency with high (0.91-0.95 V) photovoltages and thus have relatively low optical bandgap energy loss.

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.

Fine‐Tuning the 3D Structure of Nonfullerene Electron Acceptors Toward High‐Performance Polymer Solar Cells

Arylene linkers in a series of new tetraaza-benzodifluoranthene diimide dimers enable tuning of the 3D molecular structure of nonfullerene electron acceptors, facilitating observation of dramatic variation of the power conversion efficiency from 2.6% to 6.4% as the twist angle between the monomeric building blocks in the dimer is varied.

Charge generation and energy transfer in hybrid polymer/infrared quantum dot solar cells

Conjugated polymers blended with nanocrystal quantum dots are interesting as solution processable active layers for infrared light harvesting in thin film solar cells. We study photocurrent generation processes in hybrid polymer/quantum dot photovoltaics by comparing device performance and photoinduced absorption (PIA) spectra across blends of three different conjugated polymers, poly(2,3-bis(2-(hexyldecyl)-quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)-dithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx-HD), poly[(4,4′-bis(3-(2-hexyl-decyl)dithieno[3,2-b:2′,3′-d]pyrrole)-2,6-diyl-alt-(2,5-bis(3-(2-ethyl-hexyl)thiophen-2yl)thiazolo[5,4-d]thiazole)] (PPEHTT), and poly[(4,4′-bis(2-octyl)dithieno[3,2-b:2′3′-d]silole)-2,6-diyl-alt-(2,5-bis(3-octylthiophen-2yl)thiazolo[5,4-d]thiazole)] (PSOTT) with PbS quantum dots. The PIA spectra and device performance provide evidence for long-lived photoinduced charge separation and bulk heterojunction device operation for blends of both PDTPQx-HD and PPEHTT with PbS. In contrast we find that PSOTT/PbS blends can produce viable solar cells without any evidence for long-lived charge transfer in the PIA spectra. Even so, the external quantum efficiency (EQE) spectra of PSOTT/PbS solar cells indicate that the polymer plays a significant role in light harvesting. We use photoluminescence excitation spectroscopy to confirm that the polymer funnels energy to the PbS quantum dots via energy transfer, and speculate that these blends may operate as PbS Schottky diodes sensitized by energy transfer from the semiconducting polymer host.

Air-stable ambipolar field-effect transistors and complementary logic circuits from solution-processed n/p polymer heterojunctions.

We demonstrate the use of n/p polymer/polymer heterojunctions deposited by sequential solution processing to fabricate ambipolar field-effect transistors and complementary logic circuits. Electron and hole mobilities in the transistors were ∼0.001-0.01 cm(2)/(V s) in air without encapsulation. Complementary circuits integrating multiple ambipolar transistors into NOT, NAND, and NOR gates were fabricated and shown to exhibit sharp signal switching with a high voltage gain.

The effects of Ta2O5–ZnO films as cathodic buffer layers in inverted polymer solar cells

Ta2O5–ZnO composite films with varied composition were fabricated by sol–gel processing and applied as cathodic buffer layers (CBLs) for inverted polymer solar cells, and demonstrated enhanced power conversion efficiency with excellent stability. Physical and surface properties of Ta2O5–ZnO CBL films were examined by XPS, AFM, UV-Vis absorption spectra, and goniometry. It was found that CBLs incorporated with Ta2O5 exert two competing impacts on the solar cell performances. On one hand, the presence of Ta2O5 is likely to induce more positive charges around the Zn atom and form Ta–O–Zn bonding; it can reduce the surface charge recombination between the bulk heterojunction (BHJ) active layer and the cathodic buffer layer (CBL), and result in high power conversion efficiency; however, on the other hand an excessive amount of Ta2O5 would block the pathways of charge transport and lead to a drastic reduction in power conversion efficiency.

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.

Barbiturate end-capped non-fullerene acceptors for organic solar cells: tuning acceptor energetics to suppress geminate recombination losses

We report the synthesis of two barbiturate end-capped non-fullerene acceptors and demonstrate their efficient function in high voltage output organic solar cells. The acceptor with the lower LUMO level is shown to exhibit suppressed geminate recombination losses, resulting in enhanced photocurrent generation and higher overall device efficiency.