High Band Gap Polymer Research Articles

Contribution of dark current density to the photodetecting properties of thieno[3,4-b]pyrazine-based low bandgap polymers

Recently, near infrared (NIR) organic photodetectors (OPDs) have been extensively studied. Bulk heterojunction NIR OPDs composed of a high-bandgap polymer donor (PD) and a low-bandgap non-fullerene acceptor (NFA) showed the best performance, whereas the low-bandgap PD-based OPDs were relatively unsuccessful due to the high level of dark current density (Jd) under a negative bias. In this study, we synthesized three low-bandgap PDs based on a thieno[3,4-b]pyrazine (TP) moiety and developed red-NIR OPDs by blending them with a low-bandgap NFA. We found that the PD having a shallow HOMO energy level generated the largest ground-state electron transfer at negative bias, which overestimated the responsivity (R) and detectivity (D*) in OPDs. Notably, under weak light irradiation of 0.1 mW/cm2 at -2V, the contribution of Jd on Jph reached 99.6%. Thus, we modified the existing R and D* equations to better understand photodetecting properties at low light intensity, and these modified equations gave more realistic R and D* values in OPDs. On the other hand, a low-bandgap PD showing low Jd in OPDs was highly beneficial to detect a low light signal because the Jd negligibly contributed to Jph in OPDs. The low Jd values of OPDs at negative bias resulted in a high on/off signal ratio and constant R and D* values at different light intensities.

Synthesis, spectroscopic, electrochemical and photophysical properties of high band gap polymers for potential applications in semi-transparent solar cells

BackgroundThe design of new polymers able to filter the electromagnetic spectrum and absorb distinctly in the UV and high-energy part of visible spectrum is crucial for the development of semi-transparent solar cells. Herein, we report on the synthesis and spectroscopic, electrochemical, and photophysical characteristics of three new polymers, namely (i) Poly(triamterene-co-terephthalate), (ii) Poly[triamterene-co- 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonamide], and (iii) Poly(5-hydroxyindole-2-carboxylate) that might show promise as materials for semi-transparent solar cells.ResultsThe energy band gap, refractive index, dielectric constant, and optical conductivity of the electron donor polymer, poly(triamterene-co-terephthalate), were determined to be 2.92 eV, 1.56, 2.44 and 2.43 × 104 S cm−1, respectively. The synthesized electron acceptor polymers showed a relatively high refractive index, dielectric constant, and optical conductivity. The presence of a direct allowed transition was confirmed between intermolecular energy bands of the polymers.ConclusionsThe polymers showed relatively high energy gap and deep HOMO levels, making them strong absorbers of photons in the UV region and high energy part of the visible region. The synthesized donor and acceptors performed well relative to P3HT and fullerenes due to the close match of the HOMO and LUMO levels. With further development, the polymers could be viable for use as the active layers of semi-transparent solar cells.

Optimizing the Charge Carrier and Light Management of Nonfullerene Acceptors for Efficient Organic Solar Cells with Small Nonradiative Energy Losses

The photovoltaic properties and energy losses of organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) are highly dependent on their molecular structures and aggregation morphologies. Charge carrier and light managements are important to optimize NFA molecules. Herein, four NFAs with different alkyl substituents and end groups, named BTP‐C11‐N2F, BTP‐C9‐N2F, BTP‐C9‐IC4F, and BTP‐C9‐N4F, are designed and synthesized by side‐chain shortening, end‐acceptor π‐extension, and fluorination. As a result, a favorable morphology is achieved in BTP‐C9‐N4F‐based OSCs by using typical high bandgap polymer PM6 as a donor, and this system obtains the highest power conversion efficiency of 17.0% with a short circuit current (Jsc) of 26.3 mA cm−2, an open circuit current (Voc) of 0.85 V, and a fill factor (FF) of 75.7%. In addition, its light (Jsc) and charge carrier (Voc × FF) managements relative to the Shockley–Queisser limit are also increased. Extending the conjugation of the end groups increased the energy levels of NFAs and enabled an Eloss of 0.50 eV with a nonradiative recombination loss of as low as 0.20 eV in BTP‐C11‐N2F‐based OSCs. This work provides an efficient strategy to optimize the molecular structures of nonfullerene acceptors and further improve the properties of OSCs.

Effects of interlayer properties on the performance of tandem organic solar cells with low and high band gap polymers

Abstract

Orange to green switching anthraquinone‐based electrochromic material

ABSTRACTAn easily accessible anthraquinone‐benzodithiophene‐based high bandgap polymer (PTAq) was synthesized by Stille coupling reactions in remarkably high yield (96.5%). The highest occupied molecular orbital energy level of the polymer was estimated from the onset of oxidation in a cyclic voltammetry study to be −5.7 eV. PTAq showed an orange‐to‐green color switching with the application of a 1.0‐V external potential to the polymer film, which was visible to the naked eye. The optical behavior change was also monitored using ultraviolet–visible absorption spectroscopy and revealed a respectable 75% transmittance change when the polymer film was subjected to a 1.0‐V external potential. The high color contrast observed makes PTAq one of the most promising materials for electrochromic device applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47729.

Understanding the Effects of a High Surface Area Nanostructured Indium Tin Oxide Electrode on Organic Solar Cell Performance.

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor-acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C60-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC71BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

Charge carrier transport properties in ternary Si-PCPDTBT:P3HT:PCBM solar cells

In this work we investigate ternary blends of an active layer in bulk heterojunction solar cells and estimate the influence of their composition on solar cell parameters such as efficiency, mobility and recombination. The studied ternary blends are composed of low bandgap polymer poly[2,6-(4,4-bis(2-ethylhexyl)dithieno[3,2-b:2,3-d]silole)-alt-4,7-(2,1,3 benzothiadiazole)] (Si-PCPDTBT), high bandgap polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The Langevin recombination reduction factor as well as charge carrier mobilities show an increasing trend with increasing Si-PCPDTBT content in the blends. The highest efficiencies have been achieved for the optimized blends of Si-PCPDTBT:P3HT:PCBM with ratios of 0.4:0.6:1, respectively.

Surpassing the Exciton Diffusion Limit in Single-Walled Carbon Nanotube Sensitized Solar Cells.

Semiconducting single-walled carbon nanotube (s-SWNT) light sensitized devices, such as infrared photodetectors and solar cells, have recently been widely reported. Despite their excellent individual electrical properties, efficient carrier transport from one carbon nanotube to another remains a fundamental challenge. Specifically, photovoltaic devices with active layers made from s-SWNTs have suffered from low efficiencies caused by three main challenges: the overwhelming presence of high-bandgap polymers in the films, the weak bandgap offset between the LUMO of the s-SWNTs and the acceptor C60, and the limited exciton diffusion length from one SWNT to another of around 5 nm that limits the carrier extraction efficiency. Herein, we employ a combination of processing and device architecture design strategies to address each of these transport challenges and fabricate photovoltaic devices with s-SWNT films well beyond the exciton diffusion limit of 5 nm. While our solution processing method minimizes the presence of undesired polymers in our active films, our interfacial designs led to a significant increase in current generation with the addition of a highly doped C60 layer (n-doped C60), resulting in increased carrier separation efficiency from the s-SWNTs films. We create a dense interconnected nanoporous mesh of s-SWNTs using solution shearing and infiltrate it with the acceptor C60. Thus, our final engineered bulk heterojunction allows carriers from deep within to be extracted by the C60 registering a 10-fold improvement in performance from our preliminary structures.

Solar Cells: High Bandgap (1.9 eV) Polymer with Over 8% Efficiency in Bulk Heterojunction Solar Cells (Adv. Electron. Mater. 7/2016)

In article number 1600084, Wang and co-workers report a high bandgap (>1.9 eV) polymer based on benzodithiophene with octylthio side chains and fluorinated benzotriazole. Due to deep HOMO energy level and high hole mobility, bulk-heterojunction solar cells of this polymer exhibit a power conversion efficiency of 8.3%. This demonstrates the advantages of alkylthio side chains for the design of conjugated polymers.

High Bandgap (1.9 eV) Polymer with Over 8% Efficiency in Bulk Heterojunction Solar Cells

Two high bandgap donor-acceptor polymers based on benzodithiophene with different alkyl side chains and fluorinated benzotriazole are designed and synthesised. Due to the deep HOMO energy level and high hole mobility of the octylthio substituted polymer PBDTTS-FTAZ, it exhibits higher efficiency of 8.3% in bulk-heterojunction solar cells. This indicates the advantages of alkylthio over alkyl for the design of conjugated polymers. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Low Bandgap Conjugated Polymers Based on a Nature-Inspired Bay-Annulated Indigo (BAI) Acceptor as Stable Electrochromic Materials

The donor–acceptor (D–A) approach, which is to incorporate alternating electron-rich (donor) and electron-deficient (acceptor) units along the conjugated polymer mainchain, has become an effective method to provide an informed search for high performance electrochromic low bandgap polymers. Herein a potent electron acceptor, namely, a much more soluble version of the nature-inspired bay-annulated indigo (BAI), was employed in the synthesis of two solution-processable donor–acceptor polymers for efficient electrochromic devices (ECDs). The devices fabricated from spin-coated polymer thin films can switch reversibly between deep blue and transmissive light green hues, with high optical contrasts in the visible and near-infrared (NIR) regions, good coloration efficiency and promising ambient stability. In particular, electrochromic devices based on the copolymer containing a carbazole donor unit exhibit optical contrasts of 41% and 59% in the visible and NIR regions, respectively, and a long-term stability o...

Effects of the incorporation of an additional pyrrolo[3,4-c]pyrrole-1,3-dione unit on the repeating unit of highly efficient large band gap polymers containing benzodithiophene and pyrrolo[3,4-c]pyrrole-1,3-dione derivatives

Property modulation of 2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (DPPD)-based high energy converting wide band gap polymers, P(BDT-TDPPDT) and P(BDTT-TDPPDT), was studied via the incorporation of an additional DPPD unit on their repeating units. A new electron acceptor (BDPPD) unit containing two DPPD units was prepared and copolymerized with the distannyl derivatives of benzodithiophene (BDT) or 2D-conjugated benzodithiophene (BDTT) to provide two new polymers, P(BDT-BDPPD) and P(BDTT-BDPPD). The optical band gaps and highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of P(BDT-BDPPD) and P(BDTT-BDPPD) were 2.16 eV, 2.08 eV and −5.37 eV/‒3.21 eV, −5.44 eV/‒3.36 eV, respectively. The hole mobilities of P(BDT-BDPPD) and P(BDTT-BDPPD) were in the order of 10−4 cm2V−1 s−1. The polymer solar cells (PSCs) prepared with the configuration of ITO/PEDOT:PSS/P(BDT-BDPPD) or P(BDTT-BDPPD):PC70BM/Al gave maximum power conversion efficiencies (PCEs) of 2.74% and 3.63%, respectively. The insertion of a BDPPD unit instead of a TDPPDT unit on the DPPD-based polymer backbone did not affect the optical and electrochemical properties considerably. On the other hand, the new polymers, P(BDT-BDPPD) and P(BDTT-BDPPD), resulted in improved photovoltaic performances compared to the reported polymers, P(BDT-TDPPDT) and P(BDTT-TDPPDT), for the devices prepared without additives.

Dielectric Antireflection Fiber Arrays for Absorption Enhancement in Thin-Film Organic Tandem Solar Cells

Effort to improve the bulk heterojunction organic solar cells' absorption efficiency utilizes tandem structures combining low- and high-bandgap absorbing polymers so that a broader spectrum of sunlight's energy can be used for the photon-to-electron conversion and the thermalization loss of photon energy can be reduced. However, at the intersection of the upper energy level of the high-bandgap polymer and the lower energy level of the low-bandgap polymer, photon absorption efficiency remains low due to the destructive interference of these energy states at the band edges. In order to circumvent this issue, we theoretically propose using dielectric fiber arrays to generate lensing effects atop of a thin-film polymer/organic tandem solar cell (OTSC), aiming to enhance its optical absorption efficiency. Dielectric fibers have been calibrated to generate the optimal antireflection coating resulting in an integrated absorption enhancement up to 11% compared to its flat-OTSC counterpart. The overall integrated AM1.5 G (air mass 1.5 global simulated solar spectra) absorption yields absorption efficiencies of 83% in an optimal configuration, with a cell size of 600 nm and fiber radius of 150 nm. By using fibers as focusing lenses/antireflectors, we demonstrate that there exists zero reflection at multiple visible frequencies, leading to a relative broadband absorption of an ultrathin substrate.

Hybrid tandem solar cells with depleted-heterojunction quantum dot and polymer bulk heterojunction subcells

We investigate hybrid tandem solar cells that rely on the combination of solution-processed depleted-heterojunction colloidal quantum dot (CQD) and bulk heterojunction polymer:fullerene subcells. The hybrid tandem solar cell is monolithically integrated and electrically connected in series with a suitable p–n recombination layer that includes metal oxides and a conjugated polyelectrolyte. We discuss the monolithic integration of the subcells, taking into account solvent interactions with underlayers and associated constraints on the tandem architecture, and show that an adequate device configuration consists of a low bandgap CQD bottom cell and a high bandgap polymer:fullerene top cell. Once we optimize the recombination layer and individual subcells, the hybrid tandem device reaches a VOC of 1.3V, approaching the sum of the individual subcell voltages. An impressive fill factor of 70% is achieved, further confirming that the subcells are efficiently connected via an appropriate recombination layer.

Stability study of low and high band gap polymer and air stability of PTB7:PC71BM bulk heterojunction organic photovoltaic cells with encapsulation technique

This study describes the air stability of organic photovoltaic (OPV) cells based on low band gap polymer, thieno(3,4-b)-thiophene/benzodithiophene copolymer and [6,6]-phenyl C71 butyric acid methyl ester (PTB7:PC71BM) bulk heterojunction (BHJ) with titanium oxide (TiOx) layer. The ITO/PEDOT:PSS/PTB7:PC71BM/TiOx/Al device was fabricated and protected via an encapsulation technique. An optical sealant glass was used for encapsulating the device. We succeeded to keep the device in air for 20 days without a significant loss of its initial power conversion efficiency (PCE). After 20 days, the PTB7:PC71BM/TiOx device without encapsulation in air showed a rapid decrease in power conversion efficiency (PCE), losing approximately more than 98.0% of its initial value. The efficiency decays of the encapsulated devices in the nitrogen filled glove box and in air were about 32.3% and 41.7%, respectively. The organic photovoltaic cells based on high band gap polymer, poly(3-hexylthiophene) and [6,6]-phenyl C71 butyric acid methyl ester (P3HT:PC71BM) BHJ with TiOx layer was fabricated. We compared the stability of P3HT:PC71BM and PTB7:PC71BM OPV devices for 10 days in a glove box with and without TiOx layer. We found that the device based on PTB7:PC71BM/TiOx organic photovoltaic cell has better stability than the P3HT:PC71BM/TiOx organic photovoltaic cell.

Revealing the effect of donor/acceptor intermolecular arrangement on organic solar cells performance based on two-dimensional conjugated small molecule as electron donor

A series of solution processed organic solar cells (OSCs) were fabricated with a two-dimensional conjugated small molecule SMPV1 as electron donor and fullerene derivatives PC71BM or ICBA as electron acceptor. The champion power conversion efficiency (PCE) of OSCs arrives to 7.05% for the cells with PC71BM as electron acceptor. A relatively large open circuit voltage (VOC) of 1.15V is obtained from cells using ICBA as electron acceptor with an acceptable PCE of 2.54%. The fill factor (FF) of OSCs is 72% or 61% for the cells with PC71BM or ICBA as electron acceptor, which is relatively high value for small molecule OSCs. The relatively low performance of OSCs with ICBA as electron acceptor indicates that ICBA cannot play positive role in photoelectric conversion processes, which is very similar to the phenomenon observed from the OSCs with high efficient narrow band gap polymers other than P3HT as electron donor, the underlying reason is still in debate. The SMPV1 has strong self-assemble ability to form an ordered two dimensional lamellar structure, which provides an effective platform to investigate the effect of electron acceptor chemical structure on the performance of OSCs. Experimental results exhibit that ICBA molecules may prefer to vertical cross-intercalation among side chains of SMPV1, PC71BM molecules may have better miscibility with SMPV1 in the active layer. The different donor/acceptor (D/A) intermolecular arrangement strongly influences photon harvesting, exciton dissociation and charge carrier transport, which may provide a new sight on performance improvement of OSCs by adjusting D/A intermolecular arrangements.

Tuning the physical properties of pyrrolo[3,4-c]pyrrole-1,3-dione-based highly efficient large band gap polymers via the chemical modification on the polymer backbone for polymer solar cells

The property modulation of DPPD-based high energy converting large band gap polymer (PBDT–DPPD) was studied via the incorporation of strong electron accepting TPD, TT or DPP units on the polymer backbone.

Synthesis, photophysics and pyrolytic ceramization of a platinum(II)-containing poly(germylacetylene) polymer

A synthetic route to a new polymeric platinum(II)-containing germylacetylene has been developed. Soluble and thermally stable platinum(II) polyyne polymer trans-[–Pt(PBu3)2CCGePh2CC–]n (P1) was prepared in good yield by CuI-catalyzed polymerization of trans-[Pt(PBu3)2Cl2] and Ph2Ge(CCH)2. We report the optical absorption and photoluminescence spectra of P1 and the results are compared with platinum(II) polyynes with purely acetylenic and other (hetero)aromatic conjugated units. Harvesting of organic triplet emission harnessed through the strong heavy-atom effect of Pt and Ge was examined. Polymer P1 was found to have a high optical bandgap (ca. 3.84 eV). The influence of metal and sp3-hybridized germyl-based conjugation interrupter on the intersystem crossing rate and the spatial extent of the lowest singlet and triplet excitons were fully elucidated. Our investigations indicate that a high-bandgap polymer P intrinsically gives rise to a very efficient phosphorescence with fast radiative decay. Remarkably, a simple single-step method was also established for the synthesis of platinum–germanium (PtGe) alloy nanoparticles using P1 as the polymer precursor and the resulting ceramic metal alloys have been characterized by powder X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscopy.

N-Octyl-2,7-dithia-5-azacyclopenta[a]pentalene-4,6-dione-Based Low Band Gap Polymers for Efficient Solar Cells

We report the synthesis, characterization, and solar cell properties of new donor–acceptor-type low band gap polymers (POBDTPD and PEBDTPD) that incorporate dialkoxybenzodithiophene (BDT) as the donor and N-octyl-2,7-dithia-5-azacyclopenta[a]pentalene-4,6-dione (DTPD) as the acceptor. The newly developed DTPD moiety was carefully designed to lower a band gap via strong interaction between donor–acceptor moieties and keep polymer energy levels deep. Remarkably, the DTPD acceptor moiety effectively widens the light absorption range of the polymers up to ∼900 nm while positioning their HOMO and LUMO levels in the optimal range, i.e., −5.3 and −4.0 eV, respectively, for high power conversion efficiencies (PCEs) as we intended. Solar cell devices were fabricated according to the structure ITO/PEDOT:PSS/photoactive (polymer:PC70BM)/TiO2/Al. The POBDTPD devices exhibited a PCE of 4.7% with a Voc of 0.70 V, a Jsc of 10.6 mA/cm2, and a FF of 0.64. The PEBDTPD devices yielded a higher PCE of 5.3% with a Voc of 0.72 V, a Jsc of 13.5 mA/cm2, and a FF of 0.54. AFM, TEM, and PL quenching measurements revealed that the high Jscs are a result of the appropriate morphology and efficient charge separation. In comparing the performances of the two polymer devices, the higher Jsc for the PEBDTPD device was attributed to its better nanoscale phase separation, smoother surface, and higher carrier mobility in the polymer:PC70BM blend films. The higher FF for the POBDTPD device was ascribed to a good balance between the hole and electron mobilities. Overall, we demonstrate that the DTPD unit is a promising electron-accepting moiety to develop high performance low band gap polymers.

High performance low band gap polymer solar cells with a non-conventional acceptor

A novel C70 fullerene derivative was designed and synthesized by [4+2] cyclic addition reaction between indene derivative (methyl 1H-indene-3-carboxylate) and C70. The absorption and photoluminescence of H120 and its mixed films with different polymer donor materials were investigated, as well as its electrochemical property and electron mobility. It was found that H120 has 0.05 eV higher LUMO level than that of PC(70)BM. Its electron mobility reached 6.32 × 10(-4) cm(2) V(-1) s(-1), which is slightly lower than 9.55 × 10(-4) cm(2) V(-1) s(-1) of PC(70)BM. The photovoltaic devices based on P3HT, and two high efficiency low band gap polymers, PBDTTT-C and PBDTTDPP as donors, with H120 as an acceptor gave power conversion efficiencies of 4.2%, 6.0% and 6.2%, respectively.