High Mobility Thiazole–Diketopyrrolopyrrole Copolymer Semiconductors for High Performance Field-Effect Transistors and Photovoltaic Devices

New donor–acceptor copolymers incorporating both a strong electron-accepting diketopyrrolopyrrole unit and a weak electron-deficient thiazolothiazole or benzobisthiazole moiety were synthesized, characterized, and found to exhibit very high charge carrier mobility. Stille coupling copolymerization gave copolymers having moderate number-average molecular weights of 17.0–18.5 kDa with polydispersities of 3.3–4.0 and optical band gaps of 1.22–1.38 eV. High performance p-channel field-effect transistors were obtained using the thiazolothiazole-linked copolymers, PDPTT and PDPTTOx, giving hole mobilities of 0.5 and 1.2 cm²/(V s), respectively, with on/off current ratios of 10⁵ to 10⁶. In contrast, the benzobisthiazole-linked copolymer PDPBT had a substantially lower field-effect mobility of holes (0.005 cm²/(V s)) due to its amorphous solid state morphology. Bulk heterojunction solar cells fabricated by using one of the thiazolothiazole-linked copolymer, PDPTT, as electron donor and PC₇₁BM acceptor show a power conversion efficiency of 3.4% under 100 mW/cm² AM1.5 irradiation in air

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

Photoinduced Charge Transfer and Polaron Dynamics in Polymer and Hybrid Photovoltaic Thin Films: Organic vs Inorganic Acceptors

We use photoinduced absorption (PIA) spectroscopy to study charge generation and recombination in a series of bulk heterojunction blends relevant to organic and hybrid solar cells. We compare both organic and inorganic electron acceptors, including the fullerene, phenyl-C₆₁-butyric acid methyl ester (PCBM), oxides such as ZnO and TiO₂, and colloidal quantum dots, including CdSe and PbS nanocrystals. We use a variety of donor host polymers, including poly(3-hexylthiophene) (P3HT), poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-<i>p</i>-phenylenevinylene] (MDMO-PPV), and poly(2,3-bis(2-hexyldecyl)quinoxaline-5,8-diyl-<i>alt</i>-<i>N</i>-(2-hexyldecyl)-dithieno[3,2-<i>b</i>:2′,3′-<i>d</i>]pyrrole) (PDTPQx-HD). In every case, we measure longer average carrier lifetimes in blends with the inorganic acceptors as compared to blends with PCBM. The PIA data also suggest that the internal electric fields are attenuated in the inorganic blends, consistent with increased screening between the photogenerated carriers due to the higher dielectric constants of the inorganic nanoparticles. Using ligand exchange experiments, we further demonstrate that surface electron trapping on the inorganic colloids contributes to at least part of the increased lifetime in the PDTPQx-HD/PbS blends and that ligand exchange to remove traps can be used to improve the performance of these polymer/low-band-gap quantum dot hybrid photovoltaics

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/cm²) 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

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