Low Band Gap Polymers Research Articles

End Group Tuning in Acceptor–Donor–Acceptor Nonfullerene Small Molecules for High Fill Factor Organic Solar Cells

AbstractHigh fill factors have only recently become commonplace in nonfullerene‐based organic solar cells, with the balance of charge carrier mobilities often cited as the contributing factor. Here an end‐group modification to a commonly used nonfullerene acceptor (O‐IDTBR) is reported, in which the rhodanine end groups are replaced with dicyano moieties, resulting in the acceptor O‐IDTBCN. This new acceptor affords significant improvement in the fill factor (73%) and photocurrent (19.8 mA cm−2) in organic solar cells with the low bandgap polymer PTB7‐Th. A narrowing of the bandgap, as a result of greater push–pull hybridization, allows complementary absorption to the donor and thus improved photon harvesting. Additionally, the measurement of charge carrier mobilities and lifetimes in both systems reveal that the PTB7‐Th:O‐IDTBCN blend possesses more balanced charge carrier mobilities, and longer lifetimes. Morphology studies reveal a slightly greater degree of molecular mixing of the O‐IDTBCN when blended with PTB7‐Th, despite the greater and more balanced charge carrier mobilities in this blend.

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

Abstract

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells.

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution-processed bulk-heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA-based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all-small-molecule OSCs, semi-transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA-OSCs for future material design and challenges ahead is provided.

Low-bandgap polymers with quinoid unit as π bridge for high-performance solar cells

Abstract To construct efficient low band gap polymers, increasing the Quinone structure of the polymer backbone could be one desirable strategy. In this work, two D –Q–A–Q polymers P1 and P2 were designed and synthesized with thiophenopyrrole diketone (TPD) and benzothiadiazole (BT) unit as the core and ester linked thieno[3,4-b]thiophene (TT) segment as π-bridging, and the main focus is to make a comparative analysis of different cores in the influence of the optical, electrochemical, photochemical and morphological properties. Compared with the reported PBDTTEH–TBTTHD−i, P1 exhibited the decreased HOMO energy level of −5.38 eV and lower bandgap of 1.48 eV. Furthermore, when replaced with BT core, P2 showed a red-shifted absorption profile of polymer but with up-shifted HOMO energy level. When fabricated the photovoltaic devices in conventional structure, just as expected, the introduction of ester substituent made an obvious increase of VOC from 0.63 to 0.74 V for P1. Besides, due to the deep HOMO energy level, higher hole mobility and excellent phase separation with PC71BM, a superior photovoltaic performance (PCE = 7.13%) was obtained with a short-circuit current density (JSC) of 14.9 mA/cm2, significantly higher than that of P2 (PCE = 2.23%). Generally, this study highlights that the strategy of inserting quinoid moieties into D –A polymers could be optional in LBG-polymers design and presents the importance and comparison of potentially competent core groups.

Preparation of AZO:PDIN hybrid interlayer materials and application in high-gain polymer photodetectors with spectral response from 300 nm to 1700 nm

Abstract Inorganic-organic hybrid interlayer materials were prepared using aluminum doped ZnO nanoparticles (AZO) and amine-containing perylene diimide (PDIN). By employing this hybrid material as cathode interlayer and BCP as anode interlayer, high-gain polymer photodetectors have been realized. The low-bandgap polymer PB-D-T-TPT photodetector exhibited a significant increase in EQE as large as 25982% (−2.0 V, 360 nm), large responsivity of 77.2 A/W (−2.0 V, 830 nm) and high detectivity of >1012 Jones (All the specific detectivity in this paper were calculated from EQE and dark current.) over a broad spectral region (300–1700 nm). Using the same hybrid cathode interlayer, the devices based on P3HT and PTII-TT polymers also showed detectivity of >1015 Jones in 330–640 nm and >1012 Jones in 320–1600 nm, respectively, further demonstrating a unique ability of AZO:PDIN hybrid interlayer to improve the performance of polymer photodetectors.

Ladder-Type Nonacyclic Arene Bis(thieno[3,2-b]thieno)cyclopentafluorene as a Promising Building Block for Non-Fullerene Acceptors.

The ladder-type nonacyclic arene (bis(thieno[3,2-b]thieno)cyclopentafluorene (BTTF)) has been designed and synthesized through fusing thienothiophenes with the fluorene core from the synthon of dimethyl 9,9-dioctyl-2,7-bis(thieno[3,2-b]thiophen-2-yl)fluorene-3,6-dicarboxylate. With BTTF as the central donor unit, a novel acceptor-donor-acceptor (A-D-A) type non-fullerene small-molecule acceptor (BTTFIC) was prepared with 1,1-dicyanomethylene-3-indanones (IC) as the peripheral acceptor units. The energy level of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of BTTFIC locate at -5.56 and -3.95 eV, respectively, presenting a low optical band gap of 1.58 eV. Encouragingly, polymer solar cells based on the blends of BTTFIC with both the representative wide- and low-bandgap polymer donors (PBDB-T, 1.82 eV. PTB7-Th, 1.58 eV) offer power conversion efficiencies over 8 % (8.78±0.18 % for PBDB-T:BTTFIC and 8.18±0.29 % for PTB7-Th:BTTFIC). These results highlight the advantage of ladder-type BTTF on the preparation of nonfullerene acceptors with extended conjugated backbones.

Easily Accessible Low Band Gap Polymer for Efficient Nonfullerene Polymer Solar Cells with a Low Eloss of 0.55 eV.

Polymer solar cells (PSCs) have been attracting attention as a promising photovoltaic technology, and power conversion efficiencies (PCEs) of over 14% have been demonstrated. However, the state-of-the-art conjugated polymers for PSCs are based on the complex molecular structure that requires several steps for synthesis and purification, which impedes low materials cost for the PSCs. Thus, it is a key challenge to develop high-performance conjugated polymers with easily accessible building blocks for applications in practical PSCs. Here, a novel low band gap polymer (PFBTBOX) which comprises readily available building blocks was designed and synthesized to be used as an electron donor for PSCs. Affirmative optoelectronic properties of PFBTBOX for photovoltaic applications resulted in high PCEs up to 10.20 and 8.11% with nonfullerene and fullerene acceptors, respectively. Moreover, PFBTBOX exhibited minimal energy loss as low as 0.55 eV with a nonfullerene acceptor, indicating that PFBTBOX is a promising electron donor polymer for practical PSCs with high efficiency exceeding 10%.

Highly efficient near-infrared and semitransparent polymer solar cells based on an ultra-narrow bandgap nonfullerene acceptor

Non-fullerene polymer solar cells based on a low bandgap polymer PTB7-Th and an ultra-narrow bandgap acceptor ACS8 exhibited an optimal PCE of 13.2%, indicating that the blend of PTB7-Th/ACS8 has potential for the practical applications of PSCs.

Composition-Morphology Correlation in PTB7-Th/PC71BM Blend Films for Organic Solar Cells.

From a morphological perspective, the understanding of the influence of the [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) content on the morphology of the active layer is not complete in organic solar cells (OSCs) with bulk heterojunction (BHJ) configuration based on the low-bandgap polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b;4,5- b']dithiophene-2,6-diyl- alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4- b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th). In this work, we obtain the highest power conversion efficiency (PCE) of 10.5% for BHJ organic solar cells (OSCs) with a PTB7-Th/PC71BM weight ratio of 1:1.5. To understand the differences in PCEs caused by the PC71BM content, we investigate the morphology of PTB7-Th/PC71BM blend films in detail by determining the domain sizes, the polymer crystal structure, optical properties, and vertical composition as a function of the PC71BM concentration. The surface morphology is examined with atomic force microscopy, and the inner film morphology is probed with grazing incidence small-angle X-ray scattering. The PTB7-Th crystal structure is characterized with grazing incidence wide-angle X-ray scattering and UV/vis spectroscopy. X-ray reflectivity is employed to yield information about the film vertical composition. The results show that in PTB7-Th/PC71BM blend films, the increase of PC71BM content leads to an enhanced microphase separation and a decreased polymer crystallinity. Moreover, a high PC71BM concentration is found to decrease the polymer domain sizes and crystal sizes and to promote polymer conjugation length and formation of fullerene-rich and/or polymer-rich layers. The differences in photovoltaic performance are well explained by these findings.

A study about lifetime of photovoltaic fibers

In this study, the stability of polymer-based photovoltaic fibers was investigated in ambient and inert atmosphere. Photovoltaic (PV) fibers, similarly produced by using both polymer blend of wide and low bandgap polymers (P3HT: PTB7: PCDTBT) and only wide bandgap (P3HT) polymer with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in active layer were measured in ambient and in inert environment after dark storage, and then, their photovoltaic performances and stabilities were compared. Maximum power conversion efficiency of 2.01% was obtained from photovoltaic fiber with P3HT: PTB7: PCDTBT polymer blend. When stability results were considered, at end of 7th day, photovoltaic fibers having P3HT: PTB7: PCDTBT:PCBM active layer showed better performance than P3HT: PCBM-based PV fibers.

Structural, morphological, optical and dielectric properties of M3+/PVA/PEG SPE Films (M = La, Y, Fe or Ir)

Low band gap polymer complexes are promising due to its flexibility, and exhibiting electronic and optical properties of inorganic semiconductors. The effect of PEG on the physical properties of PVA was evaluated. Then, blend (PVA: PEG = 50:50) doped with rare earth (La or Y) and transition metal (Fe or Ir) chlorides to obtain solid polymer electrolyte films. XRD shows that adding PEG to PVA results in a new peak, 2θ = 23owith increased intensity as PEG ratio increases. However, doping with La3+, Fe3+or Ir3+eliminate this peak and decrease the crystallinity. SEM exhibits significant changes in the morphology of films. FTIR confirms miscibility between PVA & PEG and the complexation of the salts. The optical band gap (Eg) of PVA ~ 5.37 eV, decreased slightly by blending with PEG. While it decreased significantly to 2.64 eV and 2.78 eV after doping with Fe3+or Ir3+. There are a consistency betweenEgvalues obtained by Tauc's model and that obtained from the optical dielectric loss. The dielectric constant and loss, in temperature range 303–405 K & frequency range 1.0 kHz ‐ 5.0 MHz, indicate one or two relaxation peak(s) depending on the film composition. Accordingly, conduction mechanism varied between correlated barrier hopping and large polaron tunneling. The DC conductivity was strongly depend on the dielectric loss. The transition metal salts appear to be more effective than the rare earth ones in increasingσacof films to higher values that candidates them in semiconductors industry.

Overcoming efficiency and stability limits in water-processing nanoparticular organic photovoltaics by minimizing microstructure defects

There is a strong market driven need for processing organic photovoltaics from eco-friendly solvents. Water-dispersed organic semiconducting nanoparticles (NPs) satisfy these premises convincingly. However, the necessity of surfactants, which are inevitable for stabilizing NPs, is a major obstacle towards realizing competitive power conversion efficiencies for water-processed devices. Here, we report on a concept for minimizing the adverse impact of surfactants on solar cell performance. A poloxamer facilitates the purification of organic semiconducting NPs through stripping excess surfactants from aqueous dispersion. The use of surfactant-stripped NPs based on poly(3-hexylthiophene) / non-fullerene acceptor leads to a device efficiency and stability comparable to the one from devices processed by halogenated solvents. A record efficiency of 7.5% is achieved for NP devices based on a low-band gap polymer system. This elegant approach opens an avenue that future organic photovoltaics processing may be indeed based on non-toxic water-based nanoparticle inks.

Negative Correlation between Intermolecular vs Intramolecular Disorder in Bulk-Heterojunction Organic Solar Cells.

By varying the concentration of a solvent additive, we demonstrate the modulation of intermolecular (donor/acceptor (D/A) interface) and intramolecular (bulk) disorder in blends of the low-band gap polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4 H-cyclopental[2,1- b;3,4- b']-dithiophene)- alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Using the solvent additive concentration of 1,8-diiodooctane (DIO) in the host-processing solvent, the disorder in the bulk and at the interface is studied in terms of Urbach energy, electroluminescence (EL) broadening, and EL quantum efficiency (ELQE). The Urbach energy varies from 80 to 39 meV for bulk and 39 to 51 meV for D/A interface. An interesting feature is that changes in the Urbach energy of the D/A interfaceare opposite to those of the Urbach energy of bulk; i.e., the disorder at the D/A interface increases as the disorder in the bulk decreases with increase in DIO concentration. Our study evidently suggested a negative correlation between intermolecular and intramolecular property in a bulk-heterojunction solar cell. Furthermore, scanning photocurrent microscopy measurements show that the effective hole transport length is double in magnitude for cells processed from 3 vol % DIO in comparison to that in cells processed from 0 vol %. This increase in effective holetransport length is explained by an increase in the delocalization of the electronic states involved in charge transport, as confirmed by dark J- V knee voltage, JSC and EU-bulk measurements. Henceforth, we provide a functional relationship between the additive-induced bulk-heterojunction morphology and the optoelectronic properties of PCPDTBT-based solar cells.

Conjugated polymers: A systematic investigation of their electronic and geometric properties using density functional theory and semi-empirical methods

Ten conjugated polymers (polypyrrole, polyfuran, polythiophene, polyborole, polycyclopentadiene, polysilole, polyphosphole, polycyclopentadienethione, polycyclopentadieneone, poly(p-phenylene)) have been investigated using density functional theory (DFT) and semi-empirical method. BP86-30% HF hybrid functional with CEP-31G* basis set combined with the quadratic regression provides bandgaps of polymers that are consistent with their optical experiment bandgaps. The higher contribution of quinoid character in polymer results in lower bandgap, and this observation is supported by shorter carbon – carbon bondlength of inter-rings of polymer with prefered quinoid character. The averaged crystal orbital overlap population (COOP) between carbon – carbon of inter-rings shows that polymer with higher orbital overlap population leads to lower bandgap. The resonance energies of polymers calculated using natural bond orbital method (NBO) show two opposite observations: very low bandgap polymer (≤0.19 eV) has highest resonance energy; however, with polymer having bandgap larger than 1 eV, higher resonance energy results in higher bandgap. This observation is due to the fact that resonance energy may be associated with electron delocalization over entire polymer backbone or within the rings. Band structures of polymers typically present direct bandgap at gamma point, and lower bandgap polymers do not essentially show higher carrier mobility.

Ternary Blend Solar Cells Based on a Conjugated Polymer With Diketopyrrolopyrrole and Carbazole Units

Herein, we have synthesized a low-bandgap polymer (PCDPP4T) consisting of diketopyrrolopyrrole (DPP) and carbazole (Cz) units in the main chain. The absorption coefficient is as high as 1.3 ×10^5 cm−1 for PCDPP4T, which is slightly higher than 1.0 × 10^5 cm−1 for the monomer model compound (DPP4T-Cz). For ternary blend solar cells based on poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and only 3.4 wt% of PCDPP4T, the photocurrent increased and thus the power conversion efficiency (PCE) was improved by 30% compared to those of P3HT/PCBM binary reference cells. The improved photocurrent was ascribed partly to complementary absorption of PCDPP4T in the near-infrared (near-IR) region, and partly to efficient long-range energy transfer from P3HT to PCDPP4T, which can collect P3HT excitons that would be lost in large P3HT domains. We also discuss the origin for the improved photocurrent and requirements for further improvements.

Development of n-Type Porphyrin Acceptors for Panchromatic Light-Harvesting Fullerene-Free Organic Solar Cells.

The development of n-type porphyrin acceptors is challenging in organic solar cells. In this work, we synthesized a novel n-type porphyrin acceptor, PZn-TNI, via the introduction of the electron withdrawing naphthalene imide (NI) moiety at the meso position of zinc porphyrin (PZn). PZn-TNI has excellent thermal stability and unique bimodal absorption with a strong Soret band (300–600 nm) and weak Q-band (600–800 nm). The weak long-wavelength absorption of PZn-TNI was completely covered by combining the low bandgap polymer donor, PTB7-Th, which realized the well-balanced panchromatic photon-to-current conversion in the range of 300–800 nm. Notably, the one-step reaction of the NI moiety from a commercially available source leads to the cheap and simple n-type porphyrin synthesis. The substitution of four NIs in PZn ring induced sufficient n-type characteristics with proper HOMO and LUMO energy levels for efficient charge transport with PTB7-Th. Fullerene-free organic solar cells based-on PTB7-Th:PZn-TNI were investigated and showed a promising PCE of 5.07% without any additive treatment. To the best of our knowledge, this is the highest PCE in the porphyrin-based acceptors without utilization of the perylene diimide accepting unit.

Poly(thieno[3,4-b]pyrazine-alt-2,1,3-benzothiadiazole)s: A New Design Paradigm in Low Band Gap Polymers.

A new design paradigm for the production of low band gap polymers is reported, in which an ambipolar unit exhibiting both donor and acceptor properties is combined with a conventional acceptor. As initial examples of this approach, the synthesis of two alternating copolymers of thieno[3,4-b]pyrazine and 2,1,3-benzothiadiazole via direct arylation polymerization is reported to give soluble, processable materials with band gaps of 0.97 and 1.05 eV. Although direct arylation polymerization has been previously used to synthesize donor-acceptor materials with band gaps below 1.5 eV, this represents only the second material generated by this polymerization method with a band gap below 1.0 eV.

Influence of the alkyl chain lengths in perylenetetracarboxylic diimide (PTCDI) derivatives on the photovoltaic properties of planar organic solar cells

Abstract Control of the lowest unoccupied molecular orbital (LUMO) energy offset between an electron-donating polymer (D) and electron-accepting molecule (A) is important for the efficient dissociation of photo-induced excitons into free carriers and for improving the power conversion efficiencies (PCEs) of organic photovoltaics (OPVs). The distance between D and A also affects the dissociation of charge transfer (CT) excitons into free carriers, because the exciton binding energy is inversely proportional to the spatial separation in electron-hole coupling. In addition, the distance between D and A influences the probability that photo-induced excitons in the D (or A) domain become CT excitons through the D/A interface. In this study, the influence of D-A separation on OPV characteristics was investigated by fabricating planar OPVs composed of perylenetetracarboxylic diimide derivatives with different alkyl chain lengths (PTCDI-Cn) as A and a low band-gap polymer (PTB7) as D. The D-A separation distance was controlled by the alkyl chain length of the PTCDI-Cn, which functions as a spacer separating D and A. The open-circuit voltage (Voc) improved at the expense of short-circuit current densitie (Jsc) with increasing alkyl chain length in PTCDI-Cn. To examine the electron-hole coupling between A and D, the electroluminescence (EL) from CT states was measured. A flat CT EL was observed for the OPVs with PTCDI-C3 (perylenetetracarboxylic diimide with propyl groups), while the maxima of CT EL bands were clearly seen for other PTCDI-Cn-based OPVs. This result indicates the existence of CT excitons with different electron-hole coupling distances at the PTB7/PTCDI-C3 interface, suggesting the existence of a variety of molecular orientations of PTCDI-C3 on the surface of PTB7. This may be the origin of the increased PCE for the PTCDI-C3-based OPVs compared to other PTCDI-Cn-based OPVs.

Synthesis and characterization of a low band gap polymer based on cyclopentadithiophene and thieno[3,4-b]thiophene

ABSTRACTA low band gap polymer based on thieno[3,4-b]thiophene (TT) and cyclopentadithiophene (CPDT), named as P-TT, was synthesized through palladium-catalyzed Stille coupling reaction. Due to the strong electron-donating property of CPDT and quinoidal structural TT, P-TT exhibited narrow band gap (1.30 eV). Optical and thermal properties of P-TT had been investigated.

Molecular organization relationship of low-bandgap polymers at the air-water interface and in solid films

Low-bandgap organic polymers, poly[(4,4‑bis(2‑ethylhexyl)cyclopenta‑[2,1‑b:3,4‑b′]dithiophene)2,6‑diyl‑al‑(2,1,3‑benzothiadiazole)‑4,7‑diyl](PCPDTBT), and poly [(4,4′‑dioctyldithieno[3,2‑b:2′,3′d]silol‑2,6‑diyl)‑alt‑(2,1,3‑benzothiadiazole)‑4,7‑diyl)], (Si-PCPDTBT) were analyzed at the air-water interface forming a Langmuir monolayer. In order to form stable monolayers and to transfer to solid supports, amphiphilic molecules of stearic acid (SA) were mixed with them. For the pristine polymers, the floating monolayers were transferred onto solid substrates via the Langmuir-Schaefer (LS) technique. Surface pressure-area isotherms and compressibility modulus curves demonstrated that the SA incorporation to the polymers at the air-water interface modified the rheological properties of the Langmuir films, since the films became less compressible at higher pressures and there is clear conformational reorganization taking place at intermediary pressures. The UV–Vis absorption also depicted the changes on the overall film morphology by the shift on the maximum absorption bands, and along with cyclic voltammetry curves the absorption spectra made it possible to estimate the energy diagrams for the polymers. Photoconductivity effects were observed for all the sample, among which the pristine polymers fabricated by LS showed better results, suggesting that the organization provided by the Langmuir-Blodgett (LB) technique was not enough to overcome the insulating characteristic of the SA molecules in this specific configuration.