Small Molecule Donors Design Rules for Non-Halogen Solvent Fabricated Organic Solar Cells.

Compared with all-small-molecule (ASM) and other types of organic solar cells (OSCs), the small molecule donor:polymer acceptor (SMD:PA) OSCs develop much slower due to the lack of material matching rules. Herein, by changing the end-cap substituent of the small molecule donor from ethyl (MPhS-C2) to benzyl (MPhS-Ph), the different selection rules of donor properties and thermal annealing (TA) treatment between the ASM and the SMD:PA system under tetrahydrofuran processing are thoroughly investigated. Therefore, MPhS-Ph exhibits more ordered molecular packing, leading to better adaptability in the SMD:PA system without TA; while the inferior molecular packing of MPhS-C2 after spin-coating performs better in the ASM system with TA. Whether spin-coating or TA process dominates morphological optimization also dominates their energy loss. Therefore, the MPhS-Ph:PYF-T-o and MPhS-C2:BTP-eC9 based devices achieve the highest power conversion efficiency (PCE) of 12.1% and 15.7%, respectively, both of which are cutting-edge PCEs in their own type of OSCs fabricated by non-halogen solvent. This result suggests that intrinsic strong crystallization independent of the thermal drive is hoped in SMD:PA-OSCs, while high miscibility after spin-coating and proper assembly under thermal drive is expected in ASM-OSCs, providing deep understanding and guidance in highly efficient materials design rules in both ASM-OSCs and SMD:PA-OSCs.

The Development of Quinoxaline-Based Electron Acceptors for High Performance Organic Solar Cells.

In the recent advances of organic solar cells (OSCs), quinoxaline (Qx)-based nonfullerene acceptors (QxNFAs) have attracted lots of attention and enabled the recorded power conversion efficiency approaching 20%. As an excellent electron-withdrawing unit, Qx possesses advantages of many modifiable sites, wide absorption range, low reorganization energy, and so on. To develop promising QxNFAs to further enhance the photovoltaic performance of OSCs, it is necessary to systematically summarize the QxNFAs reported so far. In this review, all the focused QxNFAs are classified into five categories as following: SM-Qx, YQx, fused-YQx, giant-YQx, and polymer-Qx according to the molecular skeletons. The molecular design concepts, relationships between the molecular structure and optoelectronic properties, intrinsic mechanisms of device performance are discussed in detail. At the end, the advantages of this kind of materials are summed up, the molecular develop direction is prospected, the challenges faced by QxNFAs are given, and constructive solutions to the existing problems are advised. Overall, this review presents unique viewpoints to conquer the challenge of QxNFAs and thus boost OSCs development further toward commercial applications.

Dual Polymerized Y-Acceptors of Distinct-Dimensionality Create Neuron-Like Interpenetrating Hierarchical Network towards Efficient and Stable All-Polymer Solar Cells.

All-polymer solar cells have garnered particular attention thanks to their superior thermal, photo, and mechanical stabilities for large-scale manufacturing, yet the performance enhancement remains largely restrained by inherent morphological challenges of the bulk-heterojunction active layer. Herein, a 3D Y-branched polymerized small-molecule acceptor named PYBF, characteristic of high molecular weight and glass transition temperature, is designed and synthesized by precisely linking C3h-symmetric benzotrifuran with Y6 acceptors. In comparison to the benchmark thiophene-bridged linear PYIT acceptor, an optical blue-shift absorption is observed for PYBF yet a slightly higher power conversion efficiency (PCE) of 15.7% (vs 15.14%) is obtained when paired with polymer donor PM6, which benefit from the more crystalline and face-on-oriented PYBF domains. However, the star-like bulky structure of PYBF results in the nucleation-growth dominant phase-separation in polymeric blends, which generates stumpy droplet-like acceptor fibrils and impairs the continuity of acceptor phases. This issue is however surprisingly resolved by incorporating a small amount of PYIT, which leads to the formation of the more interconnective neuron-like dual-acceptor domains by long-chain entanglements of linear acceptors and alleviates bimolecular recombination. Thus, the champion device realizes a respectable PCE of up to ≈17% and importantly exhibits thermal and storage stabilities superior to the linear counterpart.

Developing Y-Branched Polymer Acceptor with 3D Architecture to Reconcile Between Crystallinity and Miscibility Yielding >15% Efficient All-Polymer Solar Cells.

In all-polymer solar cells (all-PSCs), there remains such a dilemma that obtains good miscibility and crystallinity simultaneously. Herein a new family of Y-shape polymer acceptor, namely PYTT is developed, which is copolymerized from Y6 and benzotrithiophene units in three-way directions. Benefiting from its high-density end-chains and extended π-conjugation thanks to highly-branched 3D architecture, PYTT displays better organic solubility despite much higher molecular weights, larger crystallinity, and tighter π-stacking than the linear counterpart-PYT comprising Y6 and thiophene moieties, while showing identical optical absorption yet threefold higher photoluminescence intensity. In PYTT blend film with PM6 polymer donor, the interpenetrating nano-fibrillar structures are formed with well-intermixed polymeric domain sizes close to the exciton diffusion length, which is greatly conducive to exciton dissociation and charge transport in device. Consequently, PYTT-based all-PSCs exhibit all increased photovoltaic parameters, yielding a decent power conversion efficiency of 15.60%, which is ≈20% enhancement over PYT-based device, along with low nonradiative loss of 0.221 meV.

Unfused Electronic Acceptor-Based Polymers as Interfacial Materials for Efficient Inverted Perovskite Solar Cells.

The interfacial issue resulting from surface trap states has become the crucial factor that limits the development of inverted perovskite solar cells (PSCs). Here, three unfused electronic acceptor-based polymers (PC-1-PC-3) with tailored alkyl groups were designed as interfacial materials to modify the interface contact between the perovskite active layer and electron transporting layer (ETL). Among them, PC-2 was found to extract interfacial charge faster and passivate trap states more efficiently. This leads to a remarkable increase in the short-circuit current density (22.49 mA cm-2) and fill factor (0.821), as well as a maximum power conversion efficiency of 20.50% with a negligible hysteresis, which is superior to the PC-1/PC-3 based devices and reference device with only the ETL. This study provides an insight for future molecular design of efficient interfacial materials for inverted PSCs.

Improved Stability of All-Polymer Solar Cells Using Crosslinkable Donor and Acceptor Polymers Bearing Vinyl Moieties in the Side-Chains.

Crosslinkable polymers have attracted tremendous attention in various fields of science and technology, owing to their potential utilization in applications requiring dimensional and morphological stability under thermal and mechanical stress. In this study, random terpolymers were successfully synthesized by introducing thiophene-based monomers bearing vinyl functional groups in the side-chain of the polymer donor (PBDBT-BV20) and polymer acceptor (N2200-TV10) structures. The physical properties of the blend films of PBDBT-BV20 and N2200-TV10 before and after thermal crosslinking were extensively investigated and compared to those of the homogeneous individual polymer films. The results revealed that a network polymer with donor and acceptor polymer chains, which can lock the internal morphology, could be achieved by inducing crosslinking between the vinyl groups in the mixed state of PBDBT-BV20 and N2200-TV10. In addition, the power conversion efficiency (PCE) of the polymer solar cells (PSCs) containing the blend films that were crosslinked by a two-step thermal annealing process was improved. The enhanced PCE could be attributed to the individual crystallization of PBDBT-BV20 and N2200-TV10 in the blend phase at 120 °C and then thermal crosslinking at 140 °C. In addition, the PSCs with the crosslinked blend film exhibited an excellent shelf-life of over 1200 h and a thermally stable PCE. Furthermore, the crosslinked blend film exhibited excellent mechanical stability under bending stress in flexible PSCs using plastic substrates.

Nonconjugated Terpolymer Acceptors with Two Different Fused-Ring Electron-Deficient Building Blocks for Efficient All-Polymer Solar Cells.

The ternary polymerization strategy of incorporating different donor and acceptor units forming terpolymers as photovoltaic materials has been proven advantageous in improving power conversion efficiencies (PCEs) of polymer solar cells (PSCs). Herein, a series of low band gap nonconjugated terpolymer acceptors based on two different fused-ring electron-deficient building blocks (IDIC16 and ITIC) with adjustable photoelectric properties were developed. As the third component, ITIC building blocks with a larger π-conjugation structure, shorter solubilizing side chains, and red-shifted absorption spectrum were incorporated into an IDIC16-based nonconjugated copolymer acceptor PF1-TS4, which built up the terpolymers with two conjugated building blocks linked by flexible thioalkyl chain-thiophene segments. With the increasing ITIC content, terpolymers show gradually broadened absorption spectra and slightly down-shifted lowest unoccupied molecular orbital levels. The active layer based on terpolymer PF1-TS4-60 with a 60% ITIC unit presents more balanced hole and electron mobilities, higher photoluminescence quenching efficiency, and improved morphology compared to those based on PF1-TS4. In all-polymer solar cells (all-PSCs), PF1-TS4-60, matched with a wide band gap polymer donor PM6, achieved a similar open-circuit voltage (Voc) of 0.99 V, a dramatically increased short-circuit current density (Jsc) of 15.30 mA cm-2, and fill factor (FF) of 61.4% compared to PF1-TS4 (Voc = 0.99 V, Jsc = 11.21 mA cm-2, and FF = 55.6%). As a result, the PF1-TS4-60-based all-PSCs achieved a PCE of 9.31%, which is ∼50% higher than the PF1-TS4-based ones (6.17%). The results demonstrate a promising approach to develop high-performance nonconjugated terpolymer acceptors for efficient all-PSCs by means of ternary polymerization using two different A-D-A-structured fused-ring electron-deficient building blocks.

Cyano-Functionalized Quinoxaline-Based Polymer Acceptors for All-Polymer Solar Cells and Organic Transistors.

Quinoxaline (Qx) derivatives are promising building units for efficient photovoltaic polymers owing to their strong light absorption and high charge-transport abilities, but they have been used exclusively in the construction of polymer donors. Herein, for the first time, Qx-based polymer acceptors (PA s) were developed by introducing electron-withdrawing cyano (CN) groups into the Qx moiety (QxCN). A series of QxCN-based PA s, P(QxCN-T2), P(QxCN-TVT), and P(QxCN-T3), were synthesized by copolymerizing the QxCN unit with bithiophene, (E)-1,2-di(thiophene-2-yl)ethene, and terthiophene, respectively. All of the PA s exhibited unipolar n-type characteristics with organic field-effect transistor (OFET) mobilities of around 10-2  cm2 V-1 s-1 . In space-charge-limited current devices, P(QxCN-T2) and P(QxCN-TVT) exhibited electron mobilities greater than 1.0×10-4  cm2 V-1 s-1 , due to the well-ordered structure with tight π-π stacking. When the PA s were applied in all-polymer solar cells (all-PSCs), the highest performance of 5.32 % was achieved in the P(QxCN-T2)-based device. These results demonstrate the significant potential of Qx-based PA s for high-performance all-PSCs and OFETs.

Effects of the Selective Alkoxy Side Chain Position in Quinoxaline-Based Polymer Acceptors on the Performance of All-Polymer Solar Cells.

The effects of the position of alkoxy side chains in quinoxaline (Qx)-based polymer acceptors (PAs) on the characteristics of materials and the device parameters of all-polymer solar cells (all-PSCs) are investigated. The alkoxy side chains are selectively located at the meta, para, and both positions in pendant benzenes of Qx units, constructing PAs denoted as P(QxCN-T2)-m, P(QxCN-T2)-p, and P(QxCN-T2), respectively. Among them, P(QxCN-T2)-m exhibits the deepest energy levels owing to the enhanced electron-withdrawing effect of meta-positioned alkoxy chains, which is in contrast to P(QxCN-T2)-p where para-positioned alkoxy chains have an electron-donating property. In addition, the meta-positioned alkoxy chains induce good electron-conducting pathways, while the para-positioned ones significantly interrupt crystallization and intermolecular interactions between the conjugated backbones. Thus, when the PAs are applied to all-PSCs, a power conversion efficiency (PCE) of 5.07% is attained in the device using P(QxCN-T2)-m with efficient exciton dissociation and good electron-transporting ability. On the contrary, the P(QxCN-T2)-p-based counterpart has a PCE of only 1.62%. These results demonstrate that introducing alkoxy side chains at a proper location in the Qx-based PAs is crucial for their application to all-PSCs.

Impact of Chlorination Patterns of Naphthalenediimide-Based Polymers on Aggregated Structure, Crystallinity, and Device Performance of All-Polymer Solar Cells and Organic Transistors.

The aggregation properties of conjugated polymers can play a crucial role in their thin film structures and performance of electronic devices. Control of these aggregated structures is particularly important in producing efficient all-polymer solar cells (all-PSCs), considering that strong demixing of the polymer donor and polymer acceptor typically occurs during film formation because of the low entropic contribution to the thermodynamics of the system. Here, three naphthalenediimide (NDI)-based polymer acceptors with different backbone chlorination patterns are developed to investigate the effect of the chlorination patterns on the aggregation tendencies of the polymer acceptors, which greatly influence their crystalline structures, electrical properties, and device performances of the resultant all-PSCs and organic field-effect transistors (OFETs). The counterparts of NDI units, dichlorinated bithiophene (Cl2T2), monochlorinated bithiophene (ClT2), and dichlorinated thienylene-vinylene-thienylene (Cl2TVT), are employed to synthesize a series of P(NDIOD-Cl2T2), P(NDIOD-ClT2), and P(NDIOD-Cl2TVT) polymers. The P(NDIOD-Cl2T2) polymer takes advantage of strong noncovalent bonding induced by its chlorine substituents, resulting in the formation of optimal face-on oriented crystalline structures which are suitable for efficient all-PSC devices. In comparison, the P(NDIOD-Cl2TVT) polymer forms bimodal crystalline structures in thin films to yield optimal performances in the resultant OFETs. When the three chlorinated polymers are applied to all-PSCs with the PBDTTTPD polymer donor, P(NDIOD-Cl2T2) achieves a maximum power conversion efficiency (PCE) of 7.22% with an appropriate blend morphology and high fill factor, outperforming P(NDIOD-ClT2) (PCE = 4.80%) and P(NDIOD-Cl2TVT) (PCE = 5.78%). Our observations highlight the effectiveness of the chlorination strategy for developing efficient polymer acceptors and demonstrate the important role of polymer aggregation in modulating the blend morphology and all-PSC performance.

Impact of Incorporating Nitrogen Atoms in Naphthalenediimide-Based Polymer Acceptors on the Charge Generation, Device Performance, and Stability of All-Polymer Solar Cells.

Substitution of C atoms in a polymer backbone by N atoms allows for the facile tuning of the energy levels as well as the backbone conformation and packing structures of conjugated polymers. Herein, we report a series of three polymer acceptors (PAs) with N atoms introduced at different positions of the backbone and investigate how these N atoms affect the device performances of all-polymer solar cells (all-PSCs). The three PAs, namely, P(NDI2DT-BTT), P(NDI2DT-PTT), and P(NDI2DT-BTTz), are composed of naphthalenediimide (NDI)-based and benzothiadiazole (BT)-based derivatives (dithiophene-BT (BTT), dithiophene-thiadiazolepyridine (PTT), and dithiazole-BT (BTTz)). The PTT and BTTz units are synthesized by replacing the C atoms in BT and thiophene, respectively, with N atoms, which effectively tune the optical, electrochemical, and charge-transporting properties of the corresponding PAs. The all-PSCs using poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl))benzo[1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione)] (PBDB-T) as a polymer donor and P(NDI2DT-PTT) as PA exhibit a significantly enhanced power conversion efficiency (PCE) of 6.95%, whereas the all-PSCs based on the other PAs show relatively lower PCEs (6.02% for PBDB-T:P(NDI2DT-BTT) and 1.43% for PBDB-T:P(NDI2DT-BTTz)). The high PCE of the PBDB-T:P(NDI2DT-PTT) device is due to the superior charge transfer and charge dissociation, resulting from the closely matched energy levels between PBDB-T and P(NDI2DT-PTT), as well as a more favorable bulk heterojunction morphology with improved miscibility. Importantly, the P(NDI2DT-PTT)-based all-PSC device shows improved air stability compared to the P(NDI2DT-BTT)-based device, which is most likely due to a decreased lowest unoccupied molecular orbital level of the PA. Our findings suggest that the incorporation of N atoms into the PAs is an effective strategy for improving the efficiency and stability of all-PSCs.

8.78% Efficient All-Polymer Solar Cells Enabled by Polymer Acceptors Based on a B←N Embedded Electron-Deficient Unit.

In the field of all-polymer solar cells (all-PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron-deficient units to build high-performance polymer acceptors is critical. A novel electron-deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single-crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4-difluorothiophene, two novel conjugated polymers named BN-T and BN-2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350-800 nm, low-lying energy levels, and ambipolar film-transistor characteristics. Using PBDB-T as the donor and BN-2fT as the acceptor, all-PSCs afford an encouraging efficiency of 8.78%, which is the highest for all-PSCs excluding the devices based on imide and dicyanoethylene-type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron-deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.