Acceptor Polymers Research Articles

Alkyl Chain Length Effects of Polymer Donors on the Morphology and Device Performance of Polymer Solar Cells with Different Acceptors

AbstractThe development of nonfullerene acceptors has brought polymer solar cells into a new era. Maximizing the performance of nonfullerene solar cells needs appropriate polymer donors that match with the acceptors in both electrical and morphological properties. So far, the design rationales for polymer donors are mainly borrowed from fullerene‐based solar cells, which are not necessarily applicable to nonfullerene solar cells. In this work, the influence of side chain length of polymer donors based on a set of random terpolymers PTAZ‐TPD10‐Cn on the device performance of polymer solar cells is investigated with three different acceptor materials, i.e., a fullerene acceptor [70]PCBM, a polymer acceptor N2200, and a fused‐ring molecular acceptor ITIC. Shortening the side chains of polymer donors improves the device performance of [70]PCBM‐based devices, but deteriorates the N2200‐ and ITIC‐based devices. Morphology studies unveil that the miscibility between donor and acceptor in blend films depends on the side chain length of polymer donors. Upon shortening the side chains of the polymer donors, the miscibility between the donor and acceptor increases for the [70]PCBM‐based blends, but decreases for the N2200‐ and ITIC‐based blends. These findings provide new guidelines for the development of polymer donors to match with emerging nonfullerene acceptors.

Ternary organic solar cells based on polymer donor, polymer acceptor and PCBM components

In this work, ternary organic solar cells (OSCs) combining a fullerene derivative PC71BM with a non-fullerene acceptor N2200-F blended with a polymer donor PM6 were reported. Compared with the binary systems, the highest power conversion efficiency (PCE) of 8.11% was achieved in ternary solar cells with 30 wt% N2200-F content, mainly due to the improved short-circuit current density (Jsc) and fill factor (FF). Further studies showed that the improved Jsc could attribute to the complementary absorption of the two acceptors and the enhanced FF was originated from the higher hole mobility and the fine-tuned morphology in the ternary system. These results demonstrate that the combination of fullerene and non-fullerene acceptors in ternary organic solar cells is a promising approach to achieve high-performance OSCs.

Improving the Photovoltaic Performance and Mechanical Stability of Flexible All-Polymer Solar Cells via Tailoring Intermolecular Interactions

Naphthalene diimide (NDI)-based copolymers are promising polymer acceptors in all-polymer solar cells (all-PSCs), but their large crystal domains cause large-scale phase separation in all-polymer blend films. This limits the photovoltaic performance and mechanical stability of all-PSCs. Herein, we control all-polymer blend films by introducing a fluorinated copolymer of NDI and (E)-1,2-bis(3-fluorothiophen-2-yl)ethene (FTVT) (PNDI–FTVT) as a polymer acceptor for flexible all-PSCs. The copolymer PNDI–FTVT has a less crystalline structure and higher electron mobility than its nonfluorinated copolymer counterpart (PNDI–TVT). A blended film incorporating PNDI–FTVT exhibits a well-mixed morphology and improves the chain interconnectivity with a polymer donor, providing better charge transport pathways and enhanced mechanical resilience. The PNDI–FTVT-based flexible all-PSC exhibits enhanced photovoltaic performance in comparison with a PNDI–TVT-based flexible all-PSC (5.11–7.14%) as well as excellent mechanica...

Augmenting n-Type Performance of Ambipolar Top-Contact Organic Thin-Film Transistors by Self-Generated Interlayers

We report n-type performance enhancement of an ambipolar donor–acceptor (D–A) polymer by suppressing its p-type behavior using a simple and versatile methodology of interlayer processing in top-contact organic field-effect transistors. In this approach, a judiciously selected interlayer material, polyethylene glycol (PEG), is codeposited as an additive with the semiconducting polymer DPP-T-TT active layer. During top-contact aluminum deposition, the PEG molecules migrate from within the bulk film to the organic/Al interface because of additive–metal interactions. The formation of the PEG interlayer was confirmed by X-ray photoelectron spectroscopy and its effect on the polymeric-interfacial microstructure was studied using angle-dependent grazing-incidence wide-angle X-ray scattering. We find that the PEG interlayer has two significant contributions to the n-type characteristics of the device: it suppresses hole injection while improving electron injection by tuning the effective work-function, and it enh...

Simple perylene diimide based polymer acceptor with tuned aggregation for efficient all-polymer solar cells

Polymer acceptors for organic solar cells have got increased attention and becoming a strong candidate to replace small molecular electron acceptors. Here, a polymer acceptor (PBDTβNPDI) was constructed with naphthyl substituted asymmetric benzo [1,2-b:4,5-b']dithiophene (BDTβN) as donor unit and simple N,N′-Bis(1-propylbutyl)-1,7-dibromoperylene-3,4:9,10-tetracarbox- diimide (PDI) as acceptor unit. The designed polymer has broad absorption spectrum and tuned aggregation with asymmetric donor unit. Then all-polymer solar cells based on PTBTz-2 electron donor and PBDTβNPDI electron acceptor were prepared. The results exhibit that the device has matched energy levels, complementary absorption spectra, and desirable crystallites and nanoscale phase separation, which result in balanced mobility, low weak bimolecular recombination, and thus obtain a remarkable power conversion efficiency of 6.14% with high short circuit current density of 13.89 mA cm−2 and decent open circuit voltage of 0.88 V. The results indicate that rational regulation of PDI polymers by asymmetric strategies can obtain ideal polymer accepters, and it has a strong potential to get better results with our in-depth research.

Recent Advances, Design Guidelines, and Prospects of All-Polymer Solar Cells.

All-polymer solar cells (all-PSCs) consisting of polymer donors (PDs) and polymer acceptors (PAs) have drawn tremendous research interest in recent years. It is due to not only their tunable optical, electrochemical, and structural properties, but also many superior features that are not readily available in conventional polymer-fullerene solar cells (fullerene-PSCs) including long-term stability, synthetic accessibility, and excellent film-forming properties suitable for large-scale manufacturing. Recent breakthroughs in material design and device engineering have driven the power conversion efficiencies (PCEs) of all-PSCs exceeding 11%, which is comparable to the performance of fullerene-PSCs. Furthermore, outstanding mechanical durability and stretchability have been reported for all-PSCs, which make them stand out from the other small molecule-based PSCs as a promising power supplier for wearable electronic devices. This review provides a comprehensive overview of the important work in all-PSCs, in which pertinent examples are deliberately chosen. First, we describe the key components that enabled the recent progresses of all-PSCs including rational design rules for efficient PDs and PAs, blend morphology control, and light harvesting engineering. We also review the recent work on the understanding of the stability of all-PSCs under various external conditions, which highlights the importance of all-PSCs for future implementation and commercialization. Finally, because all-PSCs have not yet achieved their full potential and are still undergoing rapid development, we offer our views on the current challenges and future prospects.

Photoactive layer crosslinking in all-polymer solar cells: Stabilized morphology and enhanced thermal-stability

In recent years, a strong growth in power conversion efficiency of all-polymer solar cells (up to 11.0%) has been realized through extensive material and device research. For practical application, high intrinsic thermal stability of the photoactive layer is a crucial requirement for long lifetimes. In this study, two new vinyl-functionalized monomers (TPD-V, NDI-V) were synthesized as the third monomer to produce two new series of cross-linkable benzodithiophene (BDT)-thieno[3,4-c]pyrrole-4,6-dione (TPD)-based polymer donors and naphthalenediimide (NDI)-based polymer acceptors by random copolymerization. By synthetically tuning the molar content of vinyl units, the crosslinked properties of the polymers were explored. It is found that these crosslinkable polymers with small molar content of vinyl unit can be mostly crosslinked upon long-term heating. Meanwhile, by employing crosslinking of PA/PD blend, all-PSCs were fabricated and the thermal-stability were studied. As a result, the all-PSCs based both crosslinkable polymer donor PBDT-TPD-8 and acceptor PNDI-T-5 retained 86% of its highest PCE (106% of its initial PCE) without obvious burn-in efficiency loss under 24 h of continuous accelerated heating (150 °C), while the reference devices based non-crosslinkable PBDT-TPD-0/PNDI-T-0 only retained 40% of its highest PCE (51% of its initial PCE). The results demonstrate photoactive layer crosslinking can stabilize morphology under long term thermal stress leading to excellent thermal-stability of the devices.

Rational Design of a Narrow-Bandgap Conjugated Polymer Using the Quinoidal Thieno[3,2-b]thiophene-Based Building Block for Organic Field-Effect Transistor Applications

A new quinoidal acceptor building block (namely IQTT) is designed with directed diradical character to control and narrow the bandgap while good chemical stability is maintained with the assistance of quantum mechanics simulations. Then IQTT-based monomer and donor–acceptor (D–A) polymer (PIQTT) are synthesized as well as the isoindigo-based (IID) monomer and D–A polymer (PITT). It is found that the IQTT building block displays stronger electron-withdrawing ability and more planar backbone conformation than IID in the copolymers. Both IQTT-based monomer and PIQTT exhibit significantly red-shifted absorption compared to their IID-based counterparts. In the solid state, PIQTT exhibits the superior thin film order and a decent field-effect transistor mobility. This work not only shows that IQTT is an excellent building block for organic electronics but also indicates that the quantum chemical simulation tool that developed in diradicaloid studies is a powerful approach to design novel quinoidal narrow-bandgap D–A conjugated polymers for organic electronics.

Efficient photovoltaic performances and enhanced dielectric constants enabled by a fluorinated quinoxaline-based polymer with non-fullerene acceptors

Non-fullerene acceptors (NFAs) are widely applied in polymer solar cells (PSCs) to solve the inherent drawbacks like unadjustable energy levels and limited absorptive region of fullerene-based acceptors. The suitable bandgaps with low-lying highest occupied molecular orbital (HOMO) energy levels of the donor materials are two preconditions to achieve highly efficient non-fullerene PSCs. A mono-fluorinated quinoxaline-based alternating polymer ( PBDTTFTQ-DO ) possesses deep HOMO energy level (−5.37 eV) and wide bandgap (1.80 eV), which ensure its potential application as donor material with low bandgap NFAs. To investigate its performances in PSCs with different NFAs, three structurally similar NFAs ( ITIC , ITIC-Th and ITIC-Th1 ) with tunable energy levels and adjustable absorption spectra were selected to match with PBDTTFTQ-DO . It was found that the performances of all three NFAs-based devices were improved to varying degrees after thermal annealing, especially for the PBDTTFTQ-DO : ITIC-Th -based device, a high power conversion efficiency of 7.81% was achieved due to the intensely enhanced film absorption and largely increased dielectric constant after thermal annealing treatment. These results illustrated that the promising performances of quinoxaline-based polymers in NF–PSCs, and what's more, the significant effects of suitable donor-acceptor combination and dielectric constants on the performances of NF–PSCs. • Quinoxaline-based polymer and non-fullerene acceptors were used in solar cells. • High PCE (7.81%) was achieved in polymer:ITIC-Th blend after thermal annealing. • Thermal annealing improved the absorption and dielectric constant of the films.

Vertical Distribution to Optimize Active Layer Morphology for Efficient All-Polymer Solar Cells by J71 as a Compatibilizer

From the miscibility viewpoint, the mechanism of how the third polymer influences the morphology evolution of an active layer composed of a polymer donor and a polymer acceptor remains to be unknow...

N‐Acylisoindigo Derivatives as Polymer Acceptors for “All‐Polymer” Bulk‐Heterojunction Solar Cells

AbstractPolymer acceptors are a class of “non‐fullerene” electron‐transporting materials that can replace [6,6]‐phenyl C61/71 butyric acid (PC61/71BM) in bulk‐heterojunction (BHJ) solar cells. In this contribution is reported the use of N‐acyl‐substituted isoindigo (IID) motifs (IID(CO)) as electron‐deficient units in the design of polymer acceptors for “all‐polymer” BHJ solar cells. While IID motifs are commonly used in the design of polymer donors for efficient BHJ solar cells with the fullerene PC61/71BM acceptors, here it is shown that IID(CO) building units used in conjunction with suitable co‐monomers represent a practical avenue for polymer acceptors with electron affinity values comparable to that of PC61/71BM (i.e., 4.2 eV).

A Protocol for Fast Prediction of Electronic and Optical Properties of Donor–Acceptor Polymers Using Density Functional Theory and the Tight-Binding Method

The ability of donor-acceptor (D-A) type polymers to control the positions of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals makes them a popular choice for organic solar cell applications. The alternating D-A pattern in a monomer leads to a weak electronic coupling between the constituent monomers within the polymer chain. Exploiting the weak electronic coupling characteristics, we developed a method to efficiently calculate (1) the electronic properties and (2) the optical gap of such polymer chains. The electronic properties (HOMO and LUMO energies, ionization potential, electron affinity, and quasiparticle gap of an oligomer of any length up to an infinitely long polymer) of the D-A polymers are predicted by combining density functional theory calculation results and a tight-binding model. The weak electronic coupling implies that the optical gap of the polymer is size-independent, and thus, it can be calculated using a monomer. We validated the methods using a set of 104 polymers by checking the consistency where the electronic gap of a polymer is larger than the optical gap. Furthermore, we establish relationships between the results obtained from more accurate, yet slower methods (i.e., B3LYP functional, singlet-ΔSCF) with those obtained from the faster counterparts (i.e., BLYP functional, triplet-ΔSCF). Leveraging the found relationships, we propose a way in which the electronic and optical properties of the polymers can be calculated efficiently while retaining high accuracy. The use of the tight-binding model combined with the approach to estimate more accurate results based on less expensive simulations is crucial in the applications where a large volume of computations needs to be carried out efficiently with sufficiently high accuracy, such as high-throughput computational screening or training a machine-learning model.

Triimide‐Functionalized n‐Type Polymer Semiconductors Enabling All‐Polymer Solar Cells with Power Conversion Efficiencies Approaching 9%

Two triimide‐functionalized n‐type acceptor polymers are designed and synthesized, which show narrower bandgap, lower‐lying frontier molecular orbital energy levels, and improved film morphology than the diimide‐functionalized analogue polymers. When blended with a p‐type donor polymer semiconductor PTB7‐Th, an outstanding power conversion efficiency of 8.98% with a remarkable open‐circuit voltage of 1.03 V is attained. This efficiency is among the highest values in all‐polymer solar cells (all‐PSCs) reported till today, surpassing that (6.85%) of the diimide‐functionalized analogue polymers by a big margin and even higher than that (8.69%) of the fullerene‐based solar cells. The results demonstrate that the triimide‐functionalized f‐BTI3 is an excellent building block for developing n‐type polymer semiconductors, and the polymer f‐BTI3‐T is among the best‐performing n‐type polymers for applications in all‐PSCs. The structure–property correlations of these imide‐functionalized polymer semiconductors offer important guides for developing high‐performance n‐type polymer semiconductors.

Azadipyrromethene-based near-IR dyes with styryl substituents at the pyrrolic positions for organic photovoltaic applications

Azadipyrromethene dyes are attractive building blocks for the development of near-IR dyes, with applications ranging from biosensing, photodynamic therapy and solar energy conversion. In particular, Zn(II) bis(2,8-diphenylethynyl 1,3,7,9 tetraphenylazadipyrromethene) complexes are promising n-type non-fullerene electron acceptors for organic photovoltaics. Here, the triple bond in phenylethynyl pyrrolic substituents is replaced with a double bond. The UV–Vis absorption spectra were not significantly different, indicating that both the double and triple bond similarly extend the conjugation of ADP. Interestingly, the Zn(II) complex with styryl substituents (Zn(WS6)2) showed emission in the near IR, with a λmax of 773 nm. The BF2+ chelate also emitted in the near IR with a λmax of 815 nm, but the emission was very weak. Cyclic voltammetry revealed that double bond compounds were easier to oxidize than un-substituted ADP analogues, and unlike with triple bond, were not easier to reduce. In organic photovoltaics, the Zn(WS6)2 did not perform as well as the Zn(WS3)2 analogue, showing unfavorable film morphology when blended with P3HT and low blend electron mobility of 2 × 10−8 cm2V−1s−1 by SCLC, compared to 1 × 10−7 cm2V−1s−1 in neat film. The hole mobility of Zn(WS6)2 in neat films is 6 × 10−5 cm2V−1s−1, higher than its electron mobility, suggesting its potential as p-type material as well. Further work in finding an appropriate n-type polymer acceptor is required to confirm this hypothesis. Using a triple bond at the pyrrolic positions is therefore better than a double bond for the development of n-type materials based on ADP dyes.

Molecular Orientation of Polymer Acceptor Dominates Open-Circuit Voltage Losses in All-Polymer Solar Cells

Low open-circuit voltage (Voc) induced by energy loss in organic solar cells is considered to be one of the most influencing factors limiting device performance, in which morphology of the active layer plays a crucial role in determining energy loss. By employing a bilayer structure of the P3HT:N2200 all-polymer system, we have identified the isolated impact of a molecular packing structure on device Voc with analysis of energy loss processes. Thermal annealing and various solvents were used to control molecular orientation in P3HT:N2200 bilayer devices, in which different Voc spanning from 0.45 to 0.54 V could be obtained. It was found that energy of charge-transfer state (Ect) differed in these bilayer devices. Besides, increased charge recombination could be observed in bilayer devices when N2200 layers exhibited face-on orientation, which caused an additional energy loss and decreased Voc. Our results suggest that rational control of polymer molecular orientation is essential to reduce the energy loss...

Effect of Backbone Regiochemistry on Conductivity, Charge Density, and Polaron Structure of n-Doped Donor-Acceptor Polymers.

We investigated the influence of backbone regiochemistry on the conductivity, charge density, and polaron structure in the widely studied n-doped donor–acceptor polymer poly[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) [P(NDI2OD-T2)]. In contrast to classic semicrystalline polymers such as poly(3-hexylthiophene) (P3HT), the regioirregular (RI) structure of the naphthalenediimide (NDI)-bithiophene (T2) backbone does not alter the intramolecular steric demand of the chain versus the regioregular (RR) polymer, yielding RI-P(NDI2OD-T2) with similar energetics and optical features as its RR counterpart. By combining the electrical, UV–vis/infrared, X-ray diffraction, and electron paramagnetic resonance data and density functional theory calculations, we quantitatively characterized the conductivity, aggregation, crystallinity, and charge density, and simulated the polaron structures, molecular vibrations, and spin density distribution of RR-/RI-P(NDI2OD-T2). Importantly, we observed that RI-P(NDI2OD-T2) can be doped to a greater extent compared to its RR counterpart. This finding is remarkable and contrasts benchmark P3HT, allowing us to uniquely study the role of regiochemistry on the charge-transport properties of n-doped donor–acceptor polymers.

All-polymer solar cells based on photostable bis(perylene diimide) acceptor polymers

Fullerene-free organic photovoltaics have recently reached impressive power conversion efficiencies above 14% for single junctions, increasing their competitiveness with respect to alternative thin-film technologies. In most record devices, electron-donating conjugated polymers are combined with novel generation small molecule acceptors. All-polymer organic solar cells, on the other hand, still lag behind in efficiency, although they have specific advantages in terms of ink formulation and long-term operational stability. Another point of attention is the synthetic complexity of the active layer materials, notably on the side of the new acceptor molecules. Therefore, the present study focuses on the implementation of the stable and cost-effective perylene diimide structure as the key component of high-performance electron-accepting polymers. The synthesis, structural and optoelectronic characterization of four push-pull type copolymers containing the electron-deficient bis(perylene diimide) (bis-PDI) unit is reported, as well as the photovoltaic analysis of these acceptor materials in combination with a well-known donor polymer (PTB7-Th). The acceptor polymers differ in the electron-rich part of the alternating push-pull structure and their solar cell power conversion efficiencies range from 3.2 to 4.7%. The maximum efficiency - the best performance achieved with a bis-PDI polymer so far - is obtained for the structurally most simple polymer, containing merely thiophene as the electron-rich building block. Controlled degradation under blue light in air is monitored by the bleaching of the relevant UV–Vis absorption bands, demonstrating high stability for the bis-PDI-thiophene containing polymers as compared to some prototype small molecule acceptors (FBR and ITIC).

The impact of fluorination on both donor polymer and non-fullerene acceptor: The more fluorine, the merrier

Fluorination of the donor polymer or non-fullerene acceptor (NFA) in an organic photovoltaic device is an effective method to improve device efficiency. Although there have been many studies on donor polymer fluorination, blends containing both a fluorinated donor and fluorinated NFA have rarely been reported. In this study, we use two donor polymers (4′-FT-HTAZ and 4′-FT-FTAZ) and two NFAs (ITIC-Th and ITIC-Th1) with different amounts of fluorine (from 2F to 6F) to investigate how the degree of fluorination in a blend impacts device performance. We find that fluorinating the NFA leads to a higher short-circuit current density (Jsc) and fill factor (FF), however, the open-circuit voltage (Voc) is decreased due to a depressed lowest unoccupied molecular orbital (LUMO) level. Adding additional fluorine to the donor polymer does not have a large effect on the Jsc or FF, but it does lead to an improved Voc. By fluorinating the NFA and having more fluorine on the donor polymer, we obtain both a high Jsc and Voc simultaneously, leading to a power conversion efficiency over 10% in the case of 4′-FT-FTAZ:ITIC-Th1, which has the most amount of fluorine (6F).

The Critical Role of Electron‐Donating Thiophene Groups on the Mechanical and Thermal Properties of Donor–Acceptor Semiconducting Polymers

AbstractOrganic semiconducting donor–acceptor polymers are promising candidates for stretchable electronics owing to their mechanical compliance. However, the effect of the electron‐donating thiophene group on the thermomechanical properties of conjugated polymers has not been carefully studied. Here, thin‐film mechanical properties are investigated for diketopyrrolopyrrole (DPP)‐based conjugated polymers with varying numbers of isolated thiophene moieties and sizes of fused thiophene rings in the polymer backbone. Interestingly, it is found that these thiophene units act as an antiplasticizer, where more isolated thiophene rings or bigger fused rings result in an increased glass transition temperature (Tg) of the polymer backbone, and consequently elastic modulus of the respective DPP polymers. Detailed morphological studies suggests that all samples show similar semicrystalline morphology. This antiplasticization effect also exists in para‐azaquinodimethane‐based conjugated polymers, indicating that this can be a general trend for various conjugated polymer systems. Using the knowledge gained above, a new DPP‐based polymer with increased alkyl side chain density through attaching alky chains to the thiophene unit is engineered. The new DPP polymer demonstrates a record low Tg, and 50% lower elastic modulus than a reference polymer without side‐chain decorated on the thiophene unit. This work provides a general design rule for making low‐Tg conjugated polymers for stretchable electronics.

Improving Active Layer Morphology of All-Polymer Solar Cells by Dissolving the Two Polymers Individually

All-polymer solar cells (all-PSCs) use a blend of polymer donor and polymer acceptor as the active layer. The active layer morphology in all-PSCs cannot be effectively tuned by conventional methods...