Bulk Heterojunction Polymer Research Articles

Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response.

Indacenodithienothiophene (IDTT)-based postfullerene electron acceptors, such as ITIC (2,2'-[[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3- d:2',3'- d']-s-indaceno[1,2- b:5,6- b']dithiophene-2,8-diyl]-bis[methylidyne(3-oxo-1 H-indene-2,1(3 H)-diylidene)]]bis[propanedinitrile]), have become synonymous with high power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) polymer solar cells (PSCs). Here we systematically investigate the influence of end-group fluorination density and positioning on the physicochemical properties, single-crystal packing, end-group redistribution propensity, and BHJ photovoltaic performance of a series of ITIC variants, ITIC- nF ( n = 0, 2, 3, 4, and 6). Increasing n from 0 → 6 contracts the optical bandgap, but only marginally lowers the LUMO for n > 4. This yields enhanced photovoltaic short-circuit current density and good open-circuit voltage, so that ITIC-6F achieves the highest PCE of the series, approaching 12% in blends with the PBDB-TF donor polymer. Single-crystal diffraction reveals that the ITIC- nF molecules cofacially interleave with ITIC-6F having the shortest π-π distance of 3.28 Å. This feature together with ZINDO-level computed intermolecular electronic coupling integrals as high as 57 meV, and B3LYP/DZP-level reorganization energies as low as 147 meV, rival or surpass the corresponding values for fullerenes, ITIC-0F, and ITIC-4F, and track a positive correlation between the ITIC- nF space-charge limited electron mobility and n. Finally, a heretofore unrecognized solution-phase redistribution process between the 2-(3-oxo-indan-1-ylidene)-malononitrile-derived end-groups (EGs) of IDTT-based NFAs, i.e., EG1-IDTT-EG1 + EG2-IDTT-EG2 ⇌ 2 EG1-IDTT-EG2, with implications for the entire ITIC PSC field, is identified and mechanistically characterized, and the effects on PSC performance are assessed.

Förster resonance energy transfer and morphology optimization for high-performance ternary organic photodetectors

Abstract Aiming at extending the photoresponse range of the photodetectors based on Poly(3-hexylthiophene-2,5-diyl): [6,6]-Phenyl C71 butyric acid methyl ester (P3HT: PCBM) host system, Poly [(5,6-difluoro-2,1,3-benzothiadiazole-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2’;5’,2’’;5’’,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) with wide spectral response range, proper energy levels and high crystallinity was integrated to fabricate ternary organic photodetectors. The PffBT4T-2OD introduces energy cascade and Forster resonance energy transfer (FRET) effect from P3HT to PffBT4T-2OD, which contributes to the photocurrent generation. Furthermore, PffBT4T-2OD creates films with desirable crystallinity to produce a favorable morphology resulting in the apparent d-spacing decrement of π-π stacking of P3HT: PCBM, which probably increases the intermolecular charge hopping efficiency. The optimal photodetector shows high on/off ratio of 6.7 × 104 with a broad spectral response from 300 nm to 800 nm via incorporating 15 wt% PffBT4T-2OD into polymer bulk heterojunction, which is an obvious improvement compared with the 2.6 × 104 on/off ratio of P3HT: PCBM. This work indicates that FRET and morphology optimization have a profound effect on photoresponse and electrical properties of organic photodetectors.

Functionalizing benzothiadiazole with non-conjugating ester groups as side chains in a donor–acceptor polymer improves solar cell performance

Side chain ester substitution on donor–acceptor based conjugated polymers used as solar harvesters in a bulk-heterojunction (BHJ) polymer solar cell (PSC) can improve harvesting properties, phase separation in the active layer and PSC performance.

Photophysical and Photovoltaic Characterization of Flourene-Anthracene-Benzothiadiazole Based Donor–Acceptor Type Copolymers for Bulk Heterojunction Polymer Solar Cells

Photophysical and Photovoltaic Characterization of Flourene-Anthracene-Benzothiadiazole Based Donor–Acceptor Type Copolymers for Bulk Heterojunction Polymer Solar Cells

The role of cation and anion dopant incorporated into a ZnO electron transporting layer for polymer bulk heterojunction solar cells.

Doping is a widely-implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic devices. A facile solution-processed zinc oxide (ZnO) and various cation and anion-doped ZnO layers were synthesized via the sol–gel method and employed as electron transport layers (ETLs) for inverted polymer solar cells (PSCs). The results indicated that all PSCs with doped ZnO ETLs exhibited better photovoltaic performance compared with the PSCs with a pristine ZnO ETL. By exploring the role of various anion and cation dopants (three compounds with the same Al3+ cation: Al(acac)3, Al(NO3)3, AlCl3 and three compounds with the same Cl− anion: NH4Cl, MgCl2, AlCl3), we found that the work function changed to favor electronic extraction only when the Cl anion was involved. In addition, the conductivity of ZnO was enhanced more with the Al3+ cation. Therefore, in inverted solar cells, doping with Al3+ and Cl− delivered the best power conversion efficiency (PCE). The maximum PCE of 10.38% was achieved from the device with ZnO doped with Al+ and Cl−.

Quantitative structure-property relationship modeling of the C60 fullerene derivatives as electron acceptors of polymer solar cells: Elucidating the functional groups critical for device performance

Using the GUSAR 2013 program, we have performed a quantitative analysis of the “structure–power conversion efficiency (PCE)” on the series of 100 methano[60]fullerenes previously tested as acceptor components of bulk-heterojunction polymer organic solar cells (PSCs) utilizing the same donor polymer, viz. poly(3-hexylthiophene). Based on the MNA and QNA descriptors and self-consistent regression implemented in the program, six statistically significant consensus models for predicting the PCE values of the methano[60]fullerene-based PSCs have been constructed. The structural fragments of the fullerene compounds leading to an increase in the device performances are determined. Based on these structural descriptors, we have designed the three methano[60]fullerenes included in the training sets and characterized by poor optoelectrical properties is performed. As a result, two new compounds with potentially moderate efficiency have been proposed. This result opens opportunities of using the GUSAR 2013 program for modeling of the “structure–PCE” relationship for diverse compounds (not only fullerene derivatives).

Design, Electrical, and Optical Modelling of Bulk Heterojunction Polymer Solar Cell

The energy scenario today is focused on the development and usage of solar cells, especially in the paradigm of clean energy. To readily create electron and hole pairs, solar cells utilize either photoactive or photosensitive components. A bulk heterojunction (BHJ) is a nanolayer consisting of donor and acceptor components with a large interpenetrated acceptor and donor contact area. In this context, a mix of P3HT and PCBM offers novelty for its use as an acceptor as well as a donor. In the work presented here, we address the mechanism of modelling and characterization of a BHJ-based polymer solar cell. Here, a new design of BHJ polymer solar cells have been designed, modelled, using Silvaco TCAD in the Organic Solar module, and matched with an already assembled device having similar features. Using this model, we have been able to estimate key parameters for the modelled devices, such as the short-circuit current density, open-circuit voltage, and fill factor with less than 0.25 error index compared to the fabricated counterpart, paving the way for fabless polymer solar cell design and optimization.

Enhanced response of bulk heterojunction polymer photodetectors upon incorporating CsPbBr3 quantum dots

CsPbBr3 quantum dots (QDs) were doped into a blend of poly(3-hexylthiophene) and indene-C60 bisadduct to fabricate bulk heterojunction polymer photodetectors. The addition of the QDs significantly increased the shunt resistance of the device, thereby suppressing the reverse leakage current and improving both the signal-to-noise ratio and the specific detectivity. The photoresponse and recovery time both decreased because of the enhanced built-in electric field and improved charge carrier mobility.

Harvesting near- and far-field plasmonic enhancements from large size gold nanoparticles for improved performance in organic bulk heterojunction solar cells

Abstract The high stability and strong coupling nature of gold nanoparticles (Au-NPs) than other metal counter parts have attracted the solar cell industry to pursue enhanced performances. Herein, we report on the improved performance of polymer bulk hetero-junction (BHJ) solar cells by the incorporation of large-size Au-NPs in the hole transport layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). To examine the reproducibility of the enhancement parameters, two different donor photoactive materials have been adapted and the role of larger-size (>70 nm, i.e. 71, 80, 87, 103 nm) Au-NPs in BHJ solar cells have been studied extensively. Significantly, when employing Au-NPs smaller than 80 nm, near-field coupling (localized surface plasmon resonance; LSPR) was prevalent, while the infusion of Au-NPs with sizes greater than 87 nm resulted in far-field scattering enhancement as the dominant effect, which was clearly determined using time resolved photo luminescence studies. The superior power conversion efficiency of 5.35% and 8.58% was achieved with PBDTTT-C: PC61BM and PTB7: PC71BM BHJs respectively, by employing 87 nm Au-NPs due to the balanced contribution of near- and far-field plasmonic effects, improved vertical coverage and better interfacial properties. This study illustrates that 87 nm Au-NPs is the maximum size to attain the improved efficiency, above which the rate of enhancement reduces dramatically.

Effect of Aryl Substituents and Fluorine Addition on the Optoelectronic Properties and Organic Solar Cell Performance of a High Efficiency Indacenodithienothiophene‐alt‐Quinoxaline π‐Conjugated Polymer

AbstractA series of donor‐acceptor (D‐A) π‐conjugated polymers, based on indacenodithienothiophene (IDTT) as an electron‐donating unit and quinoxaline as an electron‐deficient moiety, are synthesized via a Pd‐catalyzed Stille cross‐coupling polymerization. Molecular characteristics, photovoltaic parameters, and optoelectronic properties are examined through structural differences corresponding to thienyl versus phenyl side group substitutions on the IDTT and the non‐fluorinated versus the monofluoro quinoxaline derivatives. One of the most important outcome is that the power conversion efficiency (PCE) in the studied polymers is more device architecture dependent (conventional vs inverted) rather than chemical structure dependent. From single junction solar cells based on bulk heterojunction polymer:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) systems as the active layer, a maximum PCE of 5.33% has been achieved from the polymer containing the thienyl substituent on the IDTT and one fluorine atom on the quinoxaline. This demonstrates that finding the optimum molecular weight of ThIDTT‐QF or introducing the monofluoro‐quinoxaline in a regioregular motif in the polymer backbone significantly higher PCE can be expected versus the fully optimized high performance PhIDTT‐Q conjugated polymer.

Alkanedihalides additives for morphology control of PTB7:PC71BM-based polymer solar cells

Solvent additive can effectively regulate the phase-separated morphology of the active layer and improve the power conversion efficiency of polymer solar cells. In this study, the influence of solvent additives 1,8-dichlorooctane (DClO), 1,8-dibromooctane (DBrO) and 1,8-diiodooctane (DIO) on the morphology of PTB7:PC71BM active layer films is investigated systematically, and the photovoltaic performance of the prepared bulk heterojunction polymer solar cells is analyzed detailedly. The DIO and DBrO additives facilitate PC71BM diffusing into the PTB7 polymer network and forming interpenetrated bulk heterojunction junction morphology, while the DClO additive deteriorates the active layer morphology. The optimized morphology of DIO and DBrO additives promotes exciton dissociation efficiency, improves charge transport and collection efficiency. The optimum performances were obtained in the devices with DIO additive.

Nanomorphology in A–D–A type small molecular acceptors-based bulk heterojunction polymer solar cells

Abstract Recent developments in acceptor–donor–acceptor (A– D –A) type non-fullerene acceptors have led to substantial improvements in bulk-heterojunction polymer solar cells efficiency. The device performance strongly depends on photoactive layer morphology, as the molecular packing, donor–acceptor interface and phase separation significantly affect the charge-transfer states and charge carrier dynamics. In this review, we start with a brief introduction of the techniques most effectively utilized to characterize multiphase morphology. Then, we summarize recent progress in A– D –A type acceptors, with the emphasis on understanding the molecular structure–morphology–performance relationships. Finally, an outlook on correlating morphological characteristics with photovoltage losses is presented for further improving device performance.

Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells.

The progress of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is closely related to the innovation of photoactive materials (donor and acceptor materials), interface engineering, and device optimization. Especially, the development of the photoactive materials dominates the research filed in the past decades. Photoactive materials are basically classified as p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). In the past two decades, fullerene derivatives are the dominant acceptors for high efficiency PSCs. Nevertheless, the limited absorption and challenging structural tunability of fullerenes hinder further improve the efficiency of PSCs. Encouragingly, the recent progresses of fused-ring based A-D-A type nonfullerene acceptors exhibit great potential in enhancing the photovoltaic performance of devices, driving the power conversion efficiency to over 13%. Such kind of nonfullerene acceptors is usually based on indacenodithiophene (IDT) or its extending backbone core and end-caped with strong electron-withdrawing group. Owing to the strong push-pulling effects, the acceptors possess strong absorption in the visible-NIR region and low-lying HOMO (highest occupied molecular orbital) level, which can realize both high open-circuit voltage and short-circuit current density of the devices. Moreover, the photo-electronic and aggregative properties of the acceptors can be flexibly manipulated via structural design. Many strategies have been successfully employed to tune the energy levels, absorption features, and aggregation properties of the fused-ring based acceptors. In this review, we will summarize the recent progress in developing highly efficient fused-ring based nonfullerene acceptors. We will mainly focus our discussion on the correlating factors of molecular structures to their absorption, molecular energy levels, and photovoltaic performance. It is envisioned that an analysis of the relationship between molecular structures and photovoltaic properties would contribute to a better understanding of this kind of acceptors for high-efficiency PSCs.

Effects of solvent vapour annealing on the performances of benzo[1,2-b:4,5-b′]dithiophene and 4,7-di(4-hexyl-thiophen-2-yl)-5,6-difluorine-2,1,3-benzothiadiazole-based alternating polymer solar cells with different configurations

Solvent vapour annealing (SVA) treatments have been proven to be an efficient approach to optimize the morphology of the active layer in bulk heterojunction polymer solar cells. For an alternating polymer based on dioctylthiophene substituted benzo[1,2-b:4,5-b′]dithiophene and 4,7-di(4-hexyl-thiophen-2-yl)-5,6-difluorine-2,1,3-benzothiadiazole (PBDTT-DFDTBT), enhanced power conversion efficiency (PCE) from 7.65% to 8.06% was achieved in regular device configuration as ITO/PEDOT:PSS/PBDTT-DFDTBT:phenyl-C71-butyric acid methyl ester (PC71BM)/poly(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluoren)(PFN)/Al after tetrahydrofuran (THF) SVA treatment. As for the inverted device configuration of ITO/ZnO/PFN/PBDTT-DFDTBT:PC71BM/MoO3/Al with THF SVA post-treatment, simultaneously decreased open-circuit voltage (VOC) and short-circuit current density (JSC) were obtained, giving rise to a reduced PCE of 6.60%. We observed increased absorption intensity, improved hole/electron mobility and phase purity for the THF treated blend films, implying the improvement of the photovoltaic performances. However, the THF SVA-induced upper migration of the PC71BM in the active layer and the resulting trap-induced recombination are ascribed to the suppressed photovoltaic parameters of inverted device relative to the regular one.

Benzodithiophene-Dithienylbenzothiadiazole Copolymers for Efficient Polymer Solar Cells: Side-Chain Effect on Photovoltaic Performance.

A new series of low band gap D-A alternating polymers based on 4,5-bis((2-ethylhexyl)oxy)benzo[2,1- b:3,4- b']dithiophene (BDT) and 5-fluoro-4,7-bis(4-alkylthien-2-yl)benzo[ c][1,2,5]thiadiazole bearing different size of lateral alkyl substituents, namely, PfBB- n, n = 8, 10, 12, 14, and 16, was designed and synthesized for high-performance bulk heterojunction (BHJ) polymer solar cells (PSCs). PfBB- n-bearing linear alkyl side chains exhibited strong and controllable aggregation in both solution and solid states, which gives rise to a significant bathochromic shift of the absorption cut-off down to ∼780 nm in thin film. In addition, the strong and wide absorption (350-800 nm) of PfBB- n polymers can compensate for the relatively weak absorption of PC71BM, particularly in the 300-400 range nm to enhance light harvesting of such an active blend. BHJ solar cells based on PfBB- n:PC71BM blends as an active layer showed power conversion efficiency (PCE) in the range 7.8-9.7%. Because of the strong stacking interchain interactions, PfBB-12-based PSC exhibited aggregation-induced spectral broadening, superior structural order, higher exciton dissociation, higher and more balanced charge carrier mobilities, as well as reduced recombination losses. As a result, PfBB-12-based device afforded the best PCE of 9.7%, with the highest short-circuit current density ( Jsc) of 16.6 mA cm-2 and open-circuit voltage ( Voc) of 0.92 V among devices fabricated. These results demonstrate that the alkyl side chain of the polymer significantly affects the absorption, morphology, and electronic properties of the active blend of PfBB- n/PC71BM, which would provide an alternative useful tool to fine-tune the device performance. Our results also highlight that the electron-rich benzo[2,1- b:3,4- b']dithiophene building block, BDT, is highly useful for the construction of low band gap D-A polymer for highly efficient PSCs.

60]Fullerene‐quinoxaline, benzothiadiazole and benzoselenadiazole based dyads for thermally stable polymer solar cells: anchoring of substituent on fullerene with a poly(3‐hexylthiophene) polymer chain

ABSTRACT[6,6]‐Phenyl‐C61‐butyric acid methyl ester (PC61BM and PC71BM) is an archetypal electron acceptor in organic photovoltaic devices. However, it nucleates and grows to form aggregates or crystallites on a length scale of tens of nanometres to micrometres under thermal aging, which often results in a significant decrease in device efficiency and stability. To overcome this thermally induced performance degradation, many methods have been reported to date such as the introduction of hydrogen, halogen bonding, thermally or photochemically crosslinkable groups onto the fullerene, and the suppression of nucleation and growth of fullerene crystallites under thermal aging has been successfully demonstrated. Even though those methods are highly useful for the suppression of aggregation, we successfully demonstrated another one simple method for the same: Introduction of bulkier groups onto the fullerene, which can act as anchoring group to suppress the aggregation. In an extension of our previous work, quinoxaline (TQT), benzothiadiazole (TBTT) and benzoselenadiazole (TBST) based bulkier groups are linked to the fullerene, denoted as TQT‐C60, TBTT‐C60 and TBST‐C60, respectively, through the 1,3‐dipolar cycloaddition of corresponding azomethine ylides with fullerene. Single junction bulk heterojunction polymer solar cells were fabricated with the configuration ITO/PEDOT:PSS/P3HT:dyad/Ca/Al. The morphological stability of the active layer was monitored by transmission electron microscopy and optical microscopy. Independent of heteroatoms, all the dyads show excellent morphological stability under thermal aging compared to the archetypal acceptor PCBM due to the anchoring of substituent groups. © 2018 Society of Chemical Industry

Exploring photophysical processes in a ternary-blended polymer solar cell

Ternary bulk-heterojunction polymer solar cells have been fabricated using a polymer blend (PBDB-T: PBDTTPD) as a donor in combination with a small molecule ITIC as a non-fullerene acceptor. In addition to the similarity in their energy levels, both PBDB-T and PBDTTPD polymers demonstrated rather similar crystalline or lamellar packing structure to support an alloy-type blend model, which well-explained the progressively monotonic change in photovoltaic VOC against the compositions. In addition, photoinduced electron-transfer (PET) between the PBDB-T/PBDTTPD polymers forming an intermediate charge separation state between the donors. A synergistic improvement in exciton generation rate, enhancement in charge carrier transport with good balance, and charge separation rate due to PET all contribute to the achieve a photovoltaic device with high efficiency.

High Performance Flexible Organic Phototransistors with Ultrashort Channel Length

Organic phototransistors with high responsivity and sensitivity to light irradiance have great potential applications in environmental monitoring, space exploration, security, image sensors and healthcare systems. In this manuscript, a novel polymer bulk heterojunction field effect phototransistor with ultrashort channel length (tens of nanometers) and ultrahigh sensitivity to visible light was proposed. Due to the nanoscale channel and bulk heterojunction structure, a high-performance phototransistor with high responsivity of 750 A/W, photosensitivity of 1.0 × 106, and detectivity as high as 4.54 × 1015 Jones was demonstrated under 720 nm light illumination with 0.1 mW/cm2 intensity, which was even better that those lateral organic phototransistors. Moreover, organic phototransistors with ultrashort channel length were investigated for the first time on a flexible substrate, which exhibited outstanding mechanical flexibility due to their unique designs. Further investigation of the correlation between th...

Investigation of the atmosphere influence on device characteristics and NO2 sensing performance of organic field-effect transistors consisting of polymer bulk heterojunction

Abstract Organic field-effect transistors (OFETs) using polymer as the active layer are being intensively developed for flexible electronics including gas sensor. The strategy of polymer blends comprising semiconducting polymer mixed into an insulating polymer matrix has shown great potential in improving the electrical and sensing performance of OFETs. Herein two semiconducting polymers of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(9-vinylcarbazole) (PVK) are used to construct a polymer bulk heterojunction, through exploring the atmosphere influence on device characteristics and NO2 sensing performance of the transistors, an in-depth illumination of the underlying sensing mechanism is carried out. Firstly, by using nitrogen (N2) and dry air as carrier gases, the individual device characteristics and sensing performance under certain NO2 concentrations are investigated. It can be observed that O2 has a greater effect on the electrical performance than other atmospheric components, which is different from the previous report of neat P3HT-based device. Specifically, at the high gate voltage, the response of drain current under N2 is more than 2 times higher than that under dry air, and the mobility variation exhibits a 2-fold enhancement. In contrast, at a low gate voltage (0 V) under dry air, the response is 2-fold greater than under N2. Then, the underlying sensing mechanism of these gas sensors consisting of various P3HT fractions in different operating environment is discussed. Finally, the OFET gas sensor with a high sensitivity to NO2 under dry air is obtained, i.e., the highest current sensitivity is ∼12381% for 15 ppm with 50% P3HT, which is more than 30 times higher than that of pure one. This study on gas sensing mechanism of OFET consisting of semiconducting polymer bulk heterojunction can offer novel inspiration to realize high performance OFET and the corresponding gas sensor.

Surfactant-Enriched ZnO Surface via Sol-Gel Process for the Efficient Inverted Polymer Solar Cell.

In this study, we demonstrate that the top surface is enriched by surfactants, tetraoctylammonium bromide, and cetylpyridinium bromide (CPB), in the sol-gel ZnO, being evidenced by the Br depth profile of electron spectroscopy for chemical analysis data. X-ray photoelectron spectroscopy results showed the formation of Zn-Br bonding due to the oxygen defects occupied by Br at the surfactant-enriched ZnO surface. The surfactant-enriched ZnO surface possessed a smoother surface and more hydrophobicity than the pristine ZnO from the experimental results of atomic force microscopy and contact angle, respectively. On the basis of ultraviolet photoelectron spectroscopy data, the work function slightly reduced due to the dipole built-up by the electrostatic force between Br- and N+ to enhance the electron extraction ability. The improved properties benefited the power conversion efficiency (PCE) of bulk-heterojunction polymer solar cells (PSCs) by spin-coating the active layer on the surfactant-enriched ZnO surface. The inverted PSCs with the surfactant-enriched ZnO surface showed the highest PCE of 9.55% for the CPB case, in comparison with the pristine ZnO surface (8.08% PCE). This study discloses that turning the ZnO surface is easily achieved by the addition of surfactants with different molecular structures in the sol-gel ZnO for high performance polymer solar cells.