Organic Polymer Solar Cells Research Articles

Synthesis and Characterization of Dialkylthio Side Chains Modified Benzo[1,2‐b:4,5‐b’]dithiophene‐Based Conjugated Polymer for Organic Solar Cells

AbstractIn this paper, two conjugated polymers named PBDT‐OT‐BT and PBDT‐DST‐BT, derived from 4,8‐bis(4,5‐dioctylthiophen‐2‐yl)benzo[1,2‐b : 4,5‐b’]dithiophene (BDT‐OT) or 4,8‐bis(4,5‐bis(octylthio)thiophene‐2‐yl)benzo[1,2‐b : 4,5‐b’]dithiophene (BDT‐DST) as the electron‐donating unit and benzo[c][1,2,5]thiadiazole (BT) as electron‐withdrawing unit were synthesized and characterized. It was found that compared to the alkyl side chain substituted polymer PBDT‐OT‐BT, the two alkylthio side chain modified polymer PBDT‐DST‐BT exhibited lower highest occupied molecular orbital (HOMO) energy level, weaker crystallinity characteristic, and higher dielectric constant (ϵr) at 1 kHz. However, PBDT‐DST‐BT also presented relatively poor miscibility with Y6 in comparison with PBDT‐OT‐BT, which had a negative effect on the bulk heterojunction morphology and hindered the exciton dissociation and carrier collection. Eventually, the PBDT‐DST‐BT : Y6 based polymer solar cells (PSCs) achieved a power conversion efficiency (PCE) of 2.27 % much lower than that of the PBDT‐OT‐BT : Y6 based device (PCE=9.46 %). These results would provide a deep understanding of the influence of alkylthio side groups on the properties of polymer donors in PSCs.

Effect of counter anions and side-chain modification of conjugated polymers for organic solar cells as an interlayer: An in-depth investigation of the diverse modification of ionic functionality

A conjugated polymer based on dimethylaminopropyl fluorene and naphthalene diimide (PFN-NDI) was synthesized to apply for organic solar cells (OSC) as the interlayer. Further, we convert the amine group to quaternary ammonium salt with various types of counter anion (CA) such as bromide and iodide, in addition to the side chain modification with hydroxyl groups. These materials generate a favorable interface dipole by the ionic functionality at the end of the side chain, while the polar hydroxyl groups increase the magnitude of interface dipole. The power conversion efficiency (PCE) was improved by incorporating polyelectrolytes as the interlayer. The PCE of the device based on PFN-NDI with acetic acid additive as the interlayer was enhanced over the device with pristine ZnO from 8.40% to 8.51%. The PCE of the device based on PFN-NDI-Br was enhanced up to 9.02%, due to the significant improvement of FF. The incorporation of extra hydroxyl groups increases the PCE up to 9.15% as a result of the synergy effect in the interface dipole moment. Furthermore, the PCE improvement was obtained by using PFN-NDI-I as the interlayer, up to 9.41%, which is mainly resulted from the Jsc enhancement. This result suggests a correlation between device performance and ionic functionality modification.

Ladder-Type Thienoacenaphthopyrazine-Based Molecules: Synthesis, Properties, and Application to Construct High-Performance Polymer for Organic Solar Cells

Ladder-Type Thienoacenaphthopyrazine-Based Molecules: Synthesis, Properties, and Application to Construct High-Performance Polymer for Organic Solar Cells

Diketopyrrolopyrrole-Based Chlorinated Bithiophene Polymers for Organic Solar Cells: Effect of Thiophene or Pyridine Flank

Diketopyrrolopyrrole-Based Chlorinated Bithiophene Polymers for Organic Solar Cells: Effect of Thiophene or Pyridine Flank

Utilizing Ternary Strategy to Reduce the Influence of Polymer Batch‐to‐Batch Variation in Organic Solar Cells

Despite producing the best device performances, several major issues such as the batch‐to‐batch variation and low molecular weight (M w) optimization greatly hinder the progress for the organic solar cells. Herein, tetraphenylethylene (TPE) as a third component has been added in a series of PM6:Y6 blends with different donor molecular weights (42–129 kDa). Ultimately, ternary devices fabricated by the PM6 with about half the M w (66 and 74 kDa) of the standard donor (129 kDa) produce an excellent power conversion efficiency (PCE) of 15.09% and 15.15% owing to enhanced morphology and charge mobility, as compared to a standard binary PCE of 15.65%, respectively, indicating the possibility of attaining high‐performance attributes from low M w polymers. Furthermore, the TPE inclusion in the 129 kDa binary blend leads to a remarkable PCE of 16.48%. This strategy, therefore, illustrates a relatively simple approach to overcome the batch‐to‐batch variation problem associated with polymers for organic solar cells.

The Renaissance of Oligothiophene‐Based Donor–Acceptor Polymers in Organic Solar Cells

AbstractThe power conversion efficiencies (PCEs) of organic solar cells (OSCs) have increased rapidly owing to the development of non‐fullerene acceptors (NFAs). However, the development of polymer donors lags behind significantly. Currently, the polymer donors are dominated by a handful of thiophene‐substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTT) polymers, which suffer from lengthy synthesis and high production cost. Compared with BDTT‐based polymers, oligothiophene‐based donor‐acceptor polymers feature much easier synthesis, which were the prevailing polymer donors in fullerene‐based OSCs, but almost disappeared in non‐fullerene OSCs. Herein, two oligothiophene‐based donor‐acceptor polymers (PTTz‐3HD and PTTz‐4HD) are reported to re‐evaluate this kind of polymer in non‐fullerene OSCs. Benefiting from the exquisite alkyl chain design, the polymer PTTz‐3HD exhibits more planar conformation, stronger aggregation, and higher crystallinity, which in turn contributes to the formation of an optimal active layer morphology when blended with NFA. As a result, a PCE of 16.1% and 16.7% is achieved by PTTz‐3HD in binary and ternary OSCs, respectively. Of particular note, the product of short‐circuit current density and fill factor of PTTz‐3HD is fully comparable to those of BDTT‐based polymers. These results suggest the renaissance of oligothiophene‐based donor‐acceptor polymers in OSCs and demonstrate a promising avenue to access high‐efficiency OSCs from low‐cost materials.

Progress review of asymmetric polymers for organic solar cells

This review summarizes the recent advances of asymmetric donor polymers developed using a symmetry-breaking strategy, including polymers with asymmetric side chains and polymers with asymmetric backbones.

Triphenyleno[1,2-c:7,8-c']bis([1,2,5]thiadiazole) as a V-Shaped Electron-Deficient Unit to Construct Wide-Bandgap Amorphous Polymers for Efficient Organic Solar Cells.

The backbone shape of semiconducting polymers strongly affects their electronic properties and morphologies in films, yet the conventional design principle for building blocks focuses on using linear main chains to maintain high crystallinity. Here, we developed a V-shaped unit, triphenyleno[1,2-c:7,8-c']bis([1,2,5]thiadiazole) (TPTz), featuring two 1,2,5-thiadiazole rings fused to a triphenylene core with strong electron-withdrawing properties and an extended conjugation plane. We used TPTz to prepare a highly soluble copolymer, PTPTz-indacenodithiophene (IDT), which exhibited a wide bandgap of 1.94 eV and energy levels suitable for the donor polymer in organic solar cells (OSCs) in combination with non-fullerene acceptors. Despite the amorphous nature of the polymer film, single-junction OSCs with PTPTz-IDT:Y6 as the active layer achieved a power conversion efficiency of 10.4% (JSC = 19.8 mA cm-2; VOC = 0.80 V; fill factor = 0.66), which is the highest value reported for a single-junction OSC with IDT-based donor polymers. This work demonstrates that TPTz is a promising electron-acceptor unit for developing functional polymers with zigzag structures.

Flexible Polymer-Organic Solar Cells Based on P3HT:PCBM Bulk Heterojunction Active Layer Constructed under Environmental Conditions.

In this study, some crucial parameters were determined of flexible polymer–organic solar cells prepared from an active layer blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) mixed in 1:1 mass ratio and deposited from chlorobenzene solution by spin-coating on poly(ethylene terephthalate) (PET)/ITO substrates. Additionally, the positive effect of an electron transport layer (ETL) prepared from zinc oxide nanoparticles (ZnO np) on flexible photovoltaic elements’ performance and stability was investigated. Test devices with above normal architecture and silver back electrodes deposed by magnetron sputtering were constructed under environmental conditions. They were characterized by current-voltage (I–V) measurements, quantum efficiency, impedance spectroscopy, surface morphology, and time–degradation experiments. The control over morphology of active layer thin film was achieved by post-deposition thermal treatment at temperatures of 110–120 °C, which led to optimization of device morphology and electrical parameters. The impedance spectroscopy results of flexible photovoltaic elements were fitted using two R||CPE circuits in series. Polymer–organic solar cells prepared on plastic substrates showed comparable current–voltage characteristics and structural properties but need further device stability improvement according to traditionally constructed cells on glass substrates.

Numerical Study of Plasmonic Effects of Ag Nanoparticles Embedded in the Active Layer on Performance Polymer Organic Solar Cells

In this paper, the light absorption in the active layer of polymer solar cells (OPV) by using plasmonic nanocrystals with a hexagonal lattice structure is investigated. To study the relationship between the performance of the OPV solar cell and its active layer, a three-dimensional model of its morphology is utilized. Therefore, the three-dimensional (3D) finite-difference time-domain method and Lumerical software were used to measure the field distribution and light absorption in the active layer in terms of wavelength. OPV solar cells with bilayer and bulk heterojunction structured cells were designed using hexagonal lattice crystals with plasmonic nanoparticles, as well as core–shell geometry to govern a design to optimize light trapping in the active layer. The parameters of shape, material, periodicity, size, and the thickness of the active layer as a function of wavelength in OPV solar cells have been investigated. A very thin active layer and an ultra-thin shell were used to achieve the highest increase in optical absorption. The strong alternating electromagnetic field around the core–shell plasmonic nanoparticles resulting from the localized surface plasmon resonance (LSPR) suggested by the Ag plasmonic nanocrystals increased the intrinsic optical absorption in the active layer poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Based on the photovoltaic results, the short circuit current ranged from 19.7 to 26.7 mA/cm2.

A Chlorinated Donor Polymer Achieving High‐Performance Organic Solar Cells with a Wide Range of Polymer Molecular Weight

AbstractIn the field of non‐fullerene organic solar cells (OSCs), compared to the rapid development of non‐fullerene acceptors, the progress of high‐performance donor polymers is relatively slow. The property and performance of donor polymers in OSCs are often sensitive to the molecular weight of the polymers. In this study, a chlorinated donor polymer named D18‐Cl is reported, which can achieve high performance with a wide range of polymer molecular weight. The devices based on D18‐Cl show a higher open‐circuit voltage (VOC) due to the slightly deeper energy levels and an outstanding short‐circuit current density (JSC) owing to the appropriate long periods of blend films and less ([6,6]‐phenyl‐C71‐butyric acid methyl ester) (PC71BM) in mixed domains, leading to the higher efficiency of 17.97% than those of the D18‐based devices (17.21%). Meanwhile, D18‐Cl can achieve high efficiencies (17.30–17.97%) when its number‐averaged molecular weight (Mn) is ranged from 45 to 72 kDa. In contrast, the D18‐based devices only exhibit relatively high efficiencies in a narrow Mn range of ≈70 kDa. Such property and performance make D18‐Cl a promising donor polymer for scale‐up and low‐cost production.

Transfer Learned Designer Polymers For Organic Solar Cells.

Organic photovoltaic (OPV) materials have been examined extensively over the past two decades for solar cell applications because of the potential for device flexibility, low-temperature solution processability, and negligible environmental impact. However, discovery of new candidate OPV materials, especially polymer-based electron donors, that demonstrate notable power conversion efficiencies (PCEs), is nontrivial and time-intensive exercise given the extensive set of possible chemistries. Recent progress in machine learning accelerated materials discovery has facilitated to address this challenge, with molecular line representations, such as Simplified Molecular-Input Line-Entry Systems (SMILES), gaining popularity as molecular fingerprints describing the donor chemical structures. Here, we employ a transfer learning based recurrent neural (LSTM) model, which harnesses the SMILES molecular fingerprints as an input to generate novel designer chemistries for OPV devices. The generative model, perfected on a small focused OPV data set, predicts new polymer repeat units with potentially high PCE. Calculations of the similarity coefficient between the known and the generated polymers corroborate the accuracy of the model predictability as a function of the underlying chemical specificity. The data-enabled framework is sufficiently generic for use in accelerated machine learned materials discovery for various chemistries and applications, mining the hitherto available experimental and computational data.

Benzothiadiazole-based Conjugated Polymers for Organic Solar Cells

Benzothiadiazole (BT) is an electron-deficient unit with fused aromatic core, which can be used to construct conjugated polymers for application in organic solar cells (OSCs). In the past twenty years, huge numbers of conjugated polymers based on BT unit have been developed, focusing on the backbone engineering (such as by using different copolymerized building blocks), side chain engineering (such as by using linear or branch side units), using heteroatoms (such as F, O and S atoms, and CN group), etc. These modifications enable BT-polymers to exhibit distinct absorption spectra (with onset varied from 600 nm to 1000 nm), different frontier energy levels and crystallinities. As a consequence, BT-polymers have gained much attention in recent years, and can be simultaneously used as electron donor and electron acceptor in OSCs, providing the power conversion efficiencies (PCEs) over 18% and 14% in non-fullerene and all-polymer OSCs. In this article, we provide an overview of BT-polymers for OSCs, from donor to acceptor, via selecting some typical BT-polymers in different periods. We hope that the summary in this article can invoke the interest to study the BT-polymers toward high performance OSCs, especially with thick active layers that can be potentially used in large-area devices.

Conducting Polymers for Optoelectronic Devices and Organic Solar Cells: A Review.

In this review paper, we present a comprehensive summary of the different organic solar cell (OSC) families. Pure and doped conjugated polymers are described. The band structure, electronic properties, and charge separation process in conjugated polymers are briefly described. Various techniques for the preparation of conjugated polymers are presented in detail. The applications of conductive polymers for organic light emitting diodes (OLEDs), organic field effect transistors (OFETs), and organic photovoltaics (OPVs) are explained thoroughly. The architecture of organic polymer solar cells including single layer, bilayer planar heterojunction, and bulk heterojunction (BHJ) are described. Moreover, designing conjugated polymers for photovoltaic applications and optimizations of highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy levels are discussed. Principles of bulk heterojunction polymer solar cells are addressed. Finally, strategies for band gap tuning and characteristics of solar cell are presented. In this article, several processing parameters such as the choice of solvent(s) for spin casting film, thermal and solvent annealing, solvent additive, and blend composition that affect the nano-morphology of the photoactive layer are reviewed.

Recent Progress of Organic Solar Cells with Insulating Polymers

Organic solar cells (OSCs), as a promising photovoltaic technology, have made great progress recently due to the various optimization methods and designs of new materials. However, the general strategies show some restrictions on photovoltaic systems and commercialization of OSCs. Insulating polymers with low costs exhibit rich functionality, enhance device performance, and stabilize film morphology in air or at a high temperature. Herein, the strategies of the ternary, interface layer, and block copolymer in insulator‐modified OSCs (i‐OSCs) reported in recent years are highlighted. In addition, the corresponding underlying mechanisms, such as crystallinity, self‐assembly, dipole interaction, and charge transfer, are also discussed. The applications of insulating polymers in OSCs open a novel avenue for optimizing device performance and bestow a potential pathway to achieve large‐scale industrial production in the future.

Structural design of asymmetric diketopyrrolopyrrole polymers for organic solar cells processed from a non-halogenated solvent

Diketopyrrolopyrrole (DPP) polymers possess narrow optical bandgaps and high charge carrier mobilities which make them attractive for solar cell applications. DPP polymers are generally only soluble in chlorinated solvents, which is a drawback for commercial application. Solubility in non-halogenated solvents can be achieved by reducing the translational symmetry of the chain by employing two different aromatic moieties on either side of the DPP units. Here a series of polymers in which thiophene (T) and pyridine (Py) sandwich the DPP units in the main chain is reported. These asymmetric T-DPP-Py units increase the solubility dramatically. The side chain length, nature of the co-monomer, and regioregularity of the main chain are varied to investigate their effect on the solubility in toluene, the active layer morphology and the performance of organic solar cells. We demonstrate that polymers processed from both chloroform and toluene reach very similar power conversion efficiencies and blend morphologies. In general, a small co-monomer, short side chains, a regioregular main chain, and a high molecular weight give the best performance for solar cells processed from toluene. • Asymmetric substitution enhances solubility of diketopyrrolopyrrole polymers. • The asymmetry enables processing from non-halogenated solvents. • In solar cells, power conversion efficiencies are similar for both types of solvents. • Regioregular derivatives provide higher efficiencies than regiorandom polymers.

Effects of π-bridge units on the properties of donor-π-acceptor type benzodithiophene-thienothiophene based polymers for organic solar cells

Conjugated donor (D)-π-acceptor(A) cooligomers based on donor benzodithiophene(BDT) and acceptors thieno-thiophene(TT) are designed with furan, fused-thiophene, thiophene and thiazole π bridges. Effects of incorporation of the π-bridges on structures and properties of oligomers and oligomer-fullerene composites are calculated and discussed. Maximum absorption wavelengths of newly designed oligomers are red shifted compared to that of the parent oligomer P1. The newly designed P2-P5 oligomers have good light absorption efficiencies compared to that of P1. P4/PCBM and P5/PCBM blends have smaller intermolecular charge recombination rates than P1 and these two oligomers are expected to be more suitable for efficient solar cells.

Chlorination and thiophene strategies to adjust molecular energy levels for high-performance organic polymer solar cells

Three novel polymers with D-A (π)-BT backbone, PBDBT-0, PBDBT-1, and PBDBT-2, were built through effective side-chain chlorination strategies and thiophene-induced approaches. Meanwhile, the effect of different thiophene numbers was studied. As expected, all of the chlorinated polymers showed low HOMO levels. The introduction of the thiophene unit improved the solubility of the polymer and slightly adjusted the energy level of the molecule. Furthermore, polymers as donors and PC71BM as acceptors are used to construct organic solar cell devices. The devices based on polymer PBDBT-1 with one thiophene unit delivered the best efficiency of 5.73 % with an open-circuit voltage of 0.85 V, a short-circuit current density of 13.07 mA cm−2 and a fill factor of 51.6 %, which is superior to that of PBDBT-0 and PBDBT-2.

DFT Study of Donor-Acceptor-Donor [1,2,5]Thiadiazolo[3,4-c]pyridine Conjugated Polymer for Solar Cell Application

Using the density functional theory (DFT) method, the electronic and optical properties of the most stable conformers of the di(selenophen-2-yl) thieno[3,4-c]pyridine (Se-PT-Se) monomer and its oligomers were studied. Optical properties in the ultraviolet and visible regions are determined using the non-stationary theory of the density functional (TD-DFT). By extrapolating the results of DFT obtained for oligomers, it was shown that the band gap for the donor-acceptor-donor conjugated polymer based on [1,2,5]thiadiazole[3,4-c]pyridine is in the range 1.1-1.2 eV, which is consistent with experimental data. The study allowed us to propose a new donor-acceptor-donor conjugated polymer for organic solar cells.

Design of novel thiazolothiazole-containing conjugated polymers for organic solar cells and modules

One of the major challenges in the field of organic photovoltaics is associated with high-throughput manufacturing of efficient and stable organic solar cells. Practical realization of technologies for production of large-area organic solar cells requires the development of novel materials with a defined combination of properties ensuring sufficient reliability and scalability of the process in addition to good efficiency and operation stability of the devices. In this work, we designed two novel polymers comprising thiazolothiazole units and investigated their performance as absorber materials for organic solar cells and modules. Optimized small-area solar cells based on P1/[70]PCBM ([6,6]-phenyl-C71-butyric acid methyl ester) blends exhibited promising power conversion efficiency (PCE) of 7.5%, while larger area modules fabricated using slot die coating showed encouraging PCE of 4.2%. Additionally, the fabricated devices showed promising outdoor stability maintaining 60–70% of the initial efficiency after 20 sun days being exposed to natural sunlight at the Negev desert. The obtained results feature the designed polymer P1 as a promising absorber material for a large-scale production of organic solar cells under ambient conditions.