n‑Type Naphthalene Diimide–Biselenophene Copolymer for All-Polymer Bulk Heterojunction Solar Cells

A new solution processable n-type polymer semiconductor is synthesized and characterized for use as an electron acceptor material in all-polymer bulk heterojunction solar cells. The new crystalline copolymer, poly­(naphthalene diimide-<i>alt</i>-biselenophene) (PNDIBS), has a high field-effect electron mobility (0.07 cm²/(V s)) and broad visible-near-infrared absorption band with an optical band gap of 1.4 eV. All-polymer bulk heterojunction solar cells comprised of PNDIBS acceptor and poly­(3-hexylthiophene) donor have a photovoltaic power conversion efficiency of 0.9%. The external quantum efficiency spectrum of the all-polymer solar cells shows that about 19% of the photocurrent comes from the near-infrared (700–900 nm) light harvesting by the new n-type polymer semiconductor

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/cm² 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, <i>x</i>PDI (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 <i>L</i>_c = 10.22 nm) NDI-selenophene copolymer (PNDIS-HD) with crystalline (<i>L</i>_c = 9.15 nm) benzodithiophene-thieno­[3,4-<i>b</i>]­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 (<i>L</i>_c = 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%, <i>J</i>_sc = 18.6 mA/cm², 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 (<i>J</i>_sc) 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

Polymer/Polymer Blend Solar Cells Using Tetraazabenzodifluoranthene Diimide Conjugated Polymers as Electron Acceptors

Two n-type semiconducting polymers with alternating arylene (thiophene or selenophene)–tetraazabenzodifluoranthene diimide (BFI) donor–acceptor architecture have been investigated as new electron acceptors in polymer/polymer blend solar cells. The new selenophene-linked polymer, PBFI-S, has a significantly smaller optical band gap (1.13 eV) than the thiophene-linked PBFI-T (1.38 eV); however, both polymers have similar HOMO/LUMO energy levels determined from cyclic voltammetry. Blends of PBFI-T with the thiazolothiazole–dithienylsilole donor polymer (PSEHTT) gave a 2.60% power conversion efficiency (PCE) with a 7.34 mA/cm² short-circuit current. In contrast, PBFI-S:PSEHTT blends had a 0.75% PCE with similarly reduced photocurrent and external quantum efficiency. Reduced free energy for charge transfer and reduced bulk electron mobility in PBFI-S:PSEHTT blends compared to PBFI-T:PSEHTT blends as well as significant differences in bulk film morphology are among the reasons for the large loss in efficiency in PBFI-S:PSEHTT blend solar cells

Low-Vapor-Pressure Solvent Additives Function as Polymer Swelling Agents in Bulk Heterojunction Organic Photovoltaics

Bulk heterojunction (BHJ) photovoltaics based on blends of conjugated polymers and fullerenes require an optimized nanoscale morphology. Casting BHJ films using solvent additives such as 1,8-diiodooctane (DIO), 1,8-octanedithiol (ODT), chloronapthalene (CN), or diphenyl ether (DPE) often helps achieve this proper morphology: adding just a few volume percent of additive to the casting solution can improve polymer/fullerene mixing or phase separation, so that solvent additives have become staples in producing high-efficiency BHJ solar cells. The mechanism by which these additives improve BHJ morphology, however, is poorly understood. Here, we investigate how these additives control polymer/fullerene mixing by taking advantage of sequential processing (SqP), in which the polymer is deposited first and then the fullerene is intercalated into the polymer underlayer in a second processing step using a quasi-orthogonal solvent. In this way, SqP isolates the role of the additives’ interactions with the polymer and the fullerene. We find using ellipsometry-based swelling measurements that when adding small amounts of low-vapor-pressure solvent additives such as DIO and ODT to solutions of poly­(3-hexylthiophene-2,5-diyl) (P3HT), poly­[(4,4′-bis­(3-(2-ethyl-hexyl)­dithieno­[3,2-b:″,3′-d]­silole)-2,6-diyl-<i>alt</i>-(2,5-bis­(3-(2-ethyl-hexyl)­thiophen-2yl)­thiazolo­[5,4-<i>d</i>]­thiazole)] (PSEHTT), or poly­[4,8-bis­(2-ethylhexyloxy)-benzol­[1,2-b:4,5-b′]­dithiophene-2,6-diyl-<i>alt</i>-4-(2-ethylhexyloxy-1-one)­thieno­[3,4-<i>b</i>]­thiophene-2,6-diyl] (PBDTTT-C), the additives remain in the polymer film, leading to significant swelling. Two-dimensional grazing-incidence wide-angle X-ray scattering measurements show that the swelling is extensive, directly affecting the polymer crystallinity. When we then use SqP and cast phenyl-C₆₁-butyric acid methyl ester (PCBM) onto DIO-swollen polymer films, X-ray photoelectron spectroscopy and neutron reflectometry measurements demonstrate that vertical mixing of the PCBM in additive-swollen polymer films is significantly improved compared with films cast without the additive. Thus, low-vapor-pressure solvent additives function as cosolvent swelling agents or secondary plasticizers, allowing fullerene to mix better into the swollen polymer and enhancing the performance of devices produced by SqP, even when the additive is present only in the polymer layer. DIO and ODT have significantly different fullerene solubilities but swell polymers to a similar extent, demonstrating that swelling, not fullerene solubility, is the key to how such additives improve BHJ morphology. In contrast, higher-vapor-pressure additives such as CN and DPE, which have generally high polymer solubilities, function by a different mechanism, improving polymer crystallinity