Polymers For Organic Photovoltaics Research Articles

Novel organic photovoltaic polymer blends: A rapid, 3-dimensional morphology analysis using backscattered electron imaging in the scanning electron microscope

Finding the optimal morphology of novel organic photovoltaic (OPV) polymer blends is a major obstacle slowing the development of more efficient OPV devices. With a focus on accelerating the systematic morphology optimisation process, we demonstrate a technique offering rapid high-resolution, 3-dimensional blend morphology analysis in the scanning electron microscope. This backscattered electron imaging technique is used to investigate the morphological features and length-scales defining the promising PffBT4T-2OD:PC70BM blend system and show how its photovoltaic performance is related to the nature of its phase separation. Low-voltage backscattered electron imaging can be used to probe for structure and domain stacking through the thickness of the film, as well as imaging surface morphology with highly competitive spatial resolution. For reference, we compare our results with equivalent images of the widely studied P3HT:PC60BM blend system. Our results also demonstrate that backscattered electron imaging offers significant advantages over conventional cross-sectional imaging techniques, and show that it enables a fast, systematic approach to control 3-dimensional active layer morphology in polymer:fullerene blends.

XPS analysis of the chemical degradation of PTB7 polymers for organic photovoltaics

The chemical degradation of the Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] or ‘ PTB7’ has been studied using X-ray Photoelectron Spectroscopy (XPS). This material system appears to be intrinsically unstable especially when illuminated in air and XPS studies confirm the rapid photo-degradation is related to changes in chemical structure of the polymer. In particular, XPS spectra show an initial reduction in relative CC intensity, suggests loss of the alkoxy side chains. This is followed by a dramatic increase in the level of oxygen-bonded species, especially CO at ∼286.5 eV and C(=O)O at 289.2 eV, indicative of COOH and OH group formation, and oxidation of S. The XPS results support the view that using processing additives reduces the chemical stability of the polymer and provides insight into strategies to improve molecular design to ensure higher chemical stability.

New benzodithiophene‐ and benzooxadiazole/benzothiadiazole‐based donor–acceptor π‐conjugated polymers for organic photovoltaics

ABSTRACTA new set of push‐pull type 2D‐conjugated polymers (P1–P4) were designed and synthesized where A1, A2 (oxygen analogues) and A3, A4 (sulfur analogues) are electron deficient units used as co‐monomers. On introduction of new repeating units into the polymer backbone, significant changes were observed in optoelectronic properties. Furthermore, the heteroatom exchange in new repeating units has also brought notable changes in photophysical properties, in particular P1 and P2 (oxygen analogues) showed bathochromic shift in UV‐vis absorption spectra and deeper HOMO energy levels than P3, P4 (sulfur analogues). Interestingly P1, P3 absorption spectra shows a vibronic shoulder (659, 652 nm) peak in lower energy region, and this might originated from non‐covalent interactions between the electron rich and electron deficient units. In addition, the systematic investigation of these polymers with additive and solvent treatment, yielded in enhanced power conversion efficiency of 4.29% for P3‐based devices in bulk heterojunction organic solar cells. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2668–2679

In Situ Characterization of Polymer–Fullerene Bilayer Stability

A consensus is emerging that mixed phases are present in bulk heterojunction organic photovoltaic (OPV) devices. Significant insights into the mixed phases have come from bilayer stability measurements, in which an initial sample consisting of material pure layers of donor and acceptor is thermally treated, resulting in swelling of one layer by the other. We present a comparative study of the stability of polymer/fullerene bilayers using two common OPV polymer donors poly(3-hexylthiophene), P3HT, and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl ester, phenyl-C71-butyric acid methyl ester, [60]PCBM bis-adduct, and indene C60 bis-adduct. Using in situ spectroscopic ellipsometry to characterize the quasi-steady state behavior of the films, we find that the polymer glass transition temperature (Tg) is critical to the bilayer stability, with no significant changes occurring below Tg of the high Tg PCDTBT. Above the polymer Tg, we find the behavior is irreversible and most consistent with swelling of the polymer by the fullerene, constrained by tie chains in the polymer network and influenced by the rubbery dynamics of the mixed region. The swelling varies significantly with the nature of the fullerene and the polymer. Across the eight systems studied, there is no clear relationship between swelling and OPV device performance. The relationship between the observed swelling and the underlying fullerene–polymer miscibility is explored via Flory–Rehner theory.

Novel conjugated polymer for organic photovoltaics: Synthesis and device optimization

A novel conjugated polymer, TdTB, having an alternating configuration of electron donor and acceptor was synthesized for organic photovoltaic (OPV) application. Intramolecular charge transfer made TdTB exhibit bimodal-shaped absorption in visible region, which secured about 4% power conversion efficiency (PCE) via enhanced absorption efficiency. OPV cells could be further optimized by means of additive-assisted annealing or additional super critical CO2 liquid treatment after additive annealing, and more developed TdTB aggregation was achieved. Consequently, their performances were dramatically improved about 17% and 33%, compared to the non-annealed PV cell, giving 4.65 and 5.27% PCEs, respectively.

Alkoxyphenylthiophene Linked Benzodithiophene Based Medium Band Gap Polymers for Organic Photovoltaics: Efficiency Improvement upon Methanol Treatment Depends on the Planarity of Backbone

Two donor–acceptor (D–A) medium band gap polymers, P1 and P2, alkoxyphenylthiophene (APTh) linked benzodithiophene (BDT) as an electron-rich unit and 1,3-di(2′-bromothien-5′-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (TPD) (A1) or [5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (BT) (A2) as an electron-deficient unit, have successfully been synthesized via microwave-assisted Stille polymerization and utilized for bulk heterojunction (BHJ) polymer solar cells (PSCs). P1 shows a well-distinguished absorption shoulder between 590 and 620 nm attributed to the π–π stacking of a polymer backbone; such kind of absorption shoulder is not observed in P2, indicating that the P1 has more planar structure than that of P2. This is due to the fact that the sulfur atom of thiophene spacer and the oxygen atom of carbonyl groups in TPD have more pronounced intramolecular noncovalent interactions (INCI) in P1 than that of the sulfur atom of thiophene spacer and the oxygen atom of alkoxy groups of BT in P2. The bulk heterojunction polymer solar cells (BHJ PSCs) were fabricated with the configuration of ITO/PEDOT:PSS/polymer (P1 or P2):PC71BM/LiF/Al. The P1 device shows better photovoltaic performance with open-circuit voltage (Voc) of 0.91 V and the power conversion efficiency (PCE) of 4.19% than the P2 device (Voc: 0.71 V; PCE: 1.88%) in neat blend films under the illumination of AM 1.5G (100 mW/cm2). Upon treating the active layers containing P1 and P2 with methanol, the PCE of the P1 device is increased from 4.19 to 7.14%. In contrast, the PCE of the P2 device is decreased from 1.88 to 1.82%. Space charge limited current mobility, atomic force microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and impedance spectroscopy studies strongly support the enhanced PCE for the P1 device is attributed to the increased mobility, nanoscale morphology, and reduced resistance upon methanol treatment; these favorable properties for the P1 polymer are highly correlated with the planarity of the backbone.

Molecular orientation in polymer/fullerene blend films and the influence of annealing

The molecular orientation of selected low band gap polymers for organic photovoltaics (Si-PCPDTBT, PCPDTBT, and PCPDTBBT) and the influence of PCBM on the orientation is studied using Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS). The results are compared to the state of the art material P3HT. A clear angular dependence in the S-K NEXAFS spectra points to a preferred orientation of the π-conjugated system and thus to a high degree of ordering for most materials. Generally, the degree of ordering decreases after blending the polymer with the fullerene PCBM. In the case of P3HT and PCPDTBT, the degree of ordering could be improved considerably due to the annealing of the film. The annealing temperatures depend however on the considered polymer.

Quadrites and crossed-chain crystal structures in polymer semiconductors.

Many high-performance conjugated polymers for organic photovoltaics and transistors crystallize such that chains are parallel, resulting in significant anisotropy of the nanoscale charge transport properties. Here we demonstrate an unusual intercrystallite relationship where thin lamellae adopt a preferred epitaxial relationship with crossed-chains at the interface. The crossed-chains may allow either crystal to use the other as an "electronic shunt", creating efficient quasi-three-dimensional transport pathways that reduce the severity of grain boundaries and defects in limiting transport.

A time dependent DFT study of the efficiency of polymers for organic photovoltaics at the interface with PCBM

In this work an intuitive (TD-DFT) approach was developed to explain the experimental efficiencies seen for organic photovoltaic devices.

New low band gap 2-(4-(trifluoromethyl)phenyl)-1H-benzo[d]imidazole and benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole based conjugated polymers for organic photovoltaics

Novel low band gap conjugated polymers (PBDTTBIandPBDTBBT) are designed and synthesized for polymer solar cell applications.

Synthesis of new acenaphtho[1,2-c]thiophene-based low bandgap polymers for organic photovoltaics

Donor–acceptor conjugated polymers, PDTTPDA and PDTTPDT, composed of new acenaphtho[1,2-c]thiophene or thiophene as electron donors and 1,3-dithien-2-yl-thieno[3,4-c]pyrrole-4,6-dione (DTTPD) as the electron acceptor were synthesized by a Stille cross-coupling reaction. These polymers combine interesting properties such as good solubility and excellent thermal stability. The weight-averaged molecular weights (Mw) of PDTTPDA and PDTTPDT were found to be 71,000 and 79,000 with polydispersity indices of 1.65 and 1.66, respectively. Photophysical studies revealed a low bandgap of 1.77eV for PDTTPDA and 1.72eV for PDTTPDT. The present study indicates that the combination of DTTPD and acenaphtho[1,2-c]thiophene building blocks can be a very effective way to lower the HOMO energy level and ultimately to enhance the Voc of polymer solar cells. The Voc reported here (up to 0.94V) is one of the highest observed in polymer:PCBM bulk heterojunction devices, and a power conversion efficiency (PCE) of up to 3.28% was observed in the PDTTPDA devices.

Molecular Weight Effect on the Absorption, Charge Carrier Mobility, and Photovoltaic Performance of an Indacenodiselenophene-Based Ladder-Type Polymer

Selenium substitution on a ladder-type indacenodithiophene-based polymer (PIDT-DFBT) is investigated in order to reduce band gap, improve charge mobilities, and enhance the photovoltaic performance of the material. The new indacenodiselenophene-based polymer (PIDSe-DFBT) possessed improved absorption over its sulfur analogue in films, as well as substantially higher charge mobilities (0.15 and 0.064 cm2/(V s) hole and electron mobility, respectively, compared to 0.002 and 0.008 cm2/(V s) for PIDT-DFBT). The enhanced material properties led to an improved power conversion efficiency of 6.8% in photovoltaic cells, a 13% improvement over PIDT-DFBT-based devices. Furthermore, we examined the effect of molecular weight on the properties of PIDSe-DFBT and found not only a strong molecular weight dependence on mobilities, but also on the absorptivity of polymer films, with each 15 000 g/mol increase in weight, leading to a 25% increase in the absorptivity of the material. The molecular weight dependence of the material’s properties resulted in a significant difference in photovoltaic performance with the high-molecular-weight PIDSe-DFBT providing a higher photocurrent, fill factor, and efficiency due to its improved absorption and hole mobility. These results demonstrate the importance of achieving high molecular weight and the potential that selenium-containing ladder-type polymers have in the design of high-performance semiconducting polymers for organic photovoltaics (OPVs).

New selenophene-based low-band gap conjugated polymers for organic photovoltaics

Low-band gap selenophene-based polymers were synthesized. Their optoelectronic and photovoltaic properties and space-charge limited currents were compared with those of the related thiophene-based polymers. The band gaps of the Se-based derivatives were approximately 0.05–0.12 eV lower than those of their thiophene counterparts. Organic photovoltaic (OPV) devices based on the blends of these polymers and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C71 (PC71BM) were fabricated, and the maximum power conversion efficiency of the OPV device based on PSPSBT and PC71BM was 3.1%—with a short-circuit current density (Jsc) of 9.3 mA cm−2, an open-circuit voltage (Voc) of 0.79 V, and a fill factor of 0.42—under AM 1.5 G illumination (100 mW cm−2). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4550–4557

Active Layer-Incorporated, Spectrally Tuned Au/SiO2 Core/Shell Nanorod-Based Light Trapping for Organic Photovoltaics

We demonstrate that incorporation of octadecyltrimethoxysilane (OTMS)-functionalized, spectrally tuned, gold/silica (Au/SiO2) core/shell nanospheres and nanorods into the active layer of an organic photovoltaic (OPV) device led to an increase in photoconversion efficiency (PCE). A silica shell layer was added onto Au core nanospheres and nanorods in order to provide an electrically insulating surface that does not interfere with carrier generation and transport inside the active layer. Functionalization of the Au/SiO2 core/shell nanoparticles with the OTMS organic ligand was then necessary to transfer the Au/SiO2 core/shell nanoparticles from an ethanol solution into an OPV polymer-compatible solvent, such as dichlorobenzene. The OTMS-functionalized Au/SiO2 core/shell nanorods and nanospheres were then incorporated into the active layers of two OPV polymer systems: a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCB60M) OPV device and a poly[2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione] (PBDTT-DPP:PC60BM) OPV device. For the P3HT:PC60BM polymer with a band edge of ~700 nm, the addition of the core/shell nanorods with an aspect ratio (AR) of ~2.5 (extinction peak ~670 nm) resulted in a 7.1% improvement in PCE, while for the PBDTT-DPP:PC60BM polymer with a band edge of ~860 nm, the addition of core/shell nanorods with an AR of ~4 (extinction peak ~830 nm) resulted in a 14.4% improvement in PCE. The addition of Au/SiO2 core/shell nanospheres to the P3HT:PC60BM polymer resulted in a 2.7% improvement in PCE, while their addition to a PBDTT-DPP:PC60BM polymer resulted in a 9.1% improvement. The PCE and Jsc enhancements were consistent with external quantum efficiency (EQE) measurements, and the EQE enhancements spectrally matched the extinction spectra of Au/SiO2 nanospheres and nanorods in both OPV polymer systems.

Theoretical study of phenyl-substituted indacenodithiophene copolymers for high performance organic photovoltaics

The theoretical estimation of energy levels and energy gaps of conjugated polymers for organic photovoltaics (OPVs) represents in principle a useful tool for the prescreening of new donor systems as a suitable pair for the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). In this study, ten tetraphenyl-substituted indacenodithiophene (IDT) copolymers (eight in the form of donor-acceptor), whose energy gaps vary in the range of 1.48-2.11 eV have been selected and their highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), and gap energies have been calculated by applying density functional theory (DFT) and/or time-dependent density functional theory (TD-DFT) methods. In spite of the examined molecular structure variety, nice correlations (theoretical models) between experimental and theoretical electronic parameters were found. It is shown that the theoretical band gap estimated by the TD-DFT using dimer model compounds and DFT using tetramer model compounds provide in good agreement the optical band gap of these polymers. Finally, the optimum theoretical limits of the LUMO offset between the fullerene and the IDT tetramer model compounds, for which high performance OPVs (efficiency > 6%) are obtained, is presented for the first time.

Effect of PCBM on the Photodegradation Kinetics of Polymers for Organic Photovoltaics

The photo-oxidation behavior of three different polymers—namely, poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT), and 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)—is investigated in neat polymer films and in blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for different polymer:PCBM ratios. PCBM is shown to have both stabilizing and destabilizing effects, the extent of which is dependent on the type of polymer with which it is blended. Screening of the polymer from incident light by PCBM turns out to play an only minor role in the stabilization of P3HT. Quenching of the polymer excited states is also not a significant stabilization mechanism, as demonstrated by the comparison of the reduction of photo-oxidation rates to the extent of photoluminescence quenching by PCBM and 2,7-dinitrofluorenone (DNF). Photoinduced absorption spectroscopy reveals that the enhanced degradation of C-PCPDTBT in blend films with PCBM correlates with the population of the polymer triplet state via the polymer:PCBM charge-transfer state.

Mediating Solar Cell Performance by Controlling the Internal Dipole Change in Organic Photovoltaic Polymers

We report synthesis and characterizations of two novel series of polymers, namely the PBTZ and PBIT series. The PBTZ1 polymer was synthesized as a copolymer of 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b′]dithiophene (BDT) along with 2,5-bis(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer of TzDPP and 2-(1-butylheptyl)thieno[3,4-d]thiazole (TTz). The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT), and BDT was also synthesized. The PBIT series of polymers showed enhanced ground and excited state dipole moments (μg and μe) when compared to the previously reported PBB3 polymer, while PBTZ1 showed the largest dipole change (1.52 D) from ground to excited state (Δμge) in respective single polymer units. It was found that the power conversion efficiencies of the polymer series were strongly correlated to Δμge. The results reported demonstrate the utility of the calculated parameter Δμge of single units of the polymers to predict the perfo...

Improving the Ordering and Photovoltaic Properties by Extending π–Conjugated Area of Electron‐Donating Units in Polymers with D‐A Structure

A systematic molecular design process from PBDTTT-S to PBDTDTTT-S-T, a high-performance semiconducting polymer for organic photovoltaics, has been achieved by enhancing structural order, self-assembly and carrier mobility. Solar cells made from PBDTDTTT-S-T blended with PC(70) BM show a power conversion efficiency (PCE) of 7.81%, which is 25% higher than that of the parent PBDTTT-S.

A Systematic Approach to the Design Optimization of Light‐Absorbing Indenofluorene Polymers for Organic Photovoltaics

AbstractThe synthesis and optimization of new photovoltaic donor polymers is a time‐consuming process. Computer‐based molecular simulations can narrow the scope of materials choice to the most promising ones, by identifying materials with desirable energy levels and absorption energies. In this paper, such a retrospective analysis is presented of a series of fused aromatic push–pull copolymers. It is demonstrated that molecular calculations do indeed provide good estimates of the absorption energies measured by UV–vis spectroscopy and of the ionization potentials measured by photoelectron spectroscopy in air. Comparing measured photovoltaic performance of the polymer series to the trend in efficiencies predicted by computation confirms the validity of this approach.

Synthesis of novel thieno[3,2-b]thienobis(silolothiophene) based low bandgap polymers for organic photovoltaics

Thieno[3,2-b]thienobis(silolothiophene), a new electron rich hexacyclic monomer has been synthesized and incorporated into three novel donor-acceptor low-bandgap polymers. By carefully choosing the acceptor co-monomer, the energy levels of the polymers could be modulated and high power conversion efficiencies of 5.52% were reached in OPV devices.