Che-En Tsai

A new ladder-type benzodi(cyclopentadithiophene)-based donor–acceptor polymer and a modified hole-collecting PEDOT:PSS layer to achieve tandem solar cells with an open-circuit voltage of 1.62 V

We have developed a new ladder-type conjugated polymer and a robust interconnecting layer (ICL) integrating a hole-collecting m-PEDOT:PSS layer with an electron-collecting ZnO layer. The inverted device using exhibited a high power conversion efficiency (PCE) of 5.76% with a Voc of 0.81 V, a Jsc of 12.82 mA cm(-2), and a FF of 55.5%. The inverted tandem device incorporating the and ICL achieves a Voc of 1.62 V leading to a PCE of 7.08%.

Interface Engineering to Enhance the Efficiency of Conventional Polymer Solar Cells by Alcohol-/Water-Soluble C60 Materials Doped with Alkali Carbonates

Two new C60-based n-type materials, EGMC-OH and EGMC-COOH, functionalized with hydrophilic triethylene glycol groups (TEGs), have been synthesized and employed in conventional polymer solar cells. With the assistance of the TEG-based surfactant, EGMC-OH and EGMC-COOH can be dissolved in highly polar solvents to implement the polar/nonpolar orthogonal solvent strategy, forming an electron modification layer (EML) without eroding the underlying active layer. Multilayer conventional solar cells on the basis of ITO/PEDOT:PSS/P3HT:PC61BM/EML/Ca/Al configuration with the insertion of the EGMC-OH and EGMC-COOH EML between the active layer and the electrode have thus been successfully realized by cost-effective solution processing techniques. Moreover, the electron conductivity of the EML can be improved by incorporating alkali carbonates into the EGMC-COOH EML. Compared to the pristine device with a PCE of 3.61%, the devices modified by the Li2CO3-doped EGMC-COOH EML achieved a highest PCE of 4.29%. Furthermore, we demonstrated that the formation of the EGMC-COOH EML can be utilized as a general approach in the fabrication of highly efficient multilayer conventional devices. With the incorporation of the EGMC-COOH doped with 40 wt % Li2CO3, the PCDCTBT-C8:PC71BM-based device exhibited a superior PCE of 4.51%, which outperformed the corresponding nonmodified device with a PCE of 3.63%.

Triarylamine-based crosslinked hole-transporting material with an ionic dopant for high-performance PEDOT:PSS-free polymer solar cells

A triarylamine-based material DVTPD containing two styryl groups has been developed. Upon isothermal heating at 180 °C for 30 min, DVTPD can be thermally cross-linked to form a solvent-resistant layer to realize the fabrication of solution-processed multilayer devices. The crosslinked DVTPD (denoted as X-DVTPD) layer possesses not only hole-collecting ability (HOMO = −5.3 eV) but also electron-blocking capability (LUMO = −2.2 eV). By incorporation of an ionic dopant, 4-isopropyl-4′-methyldiphenyliodonium tetrakis(pentafluorophenylborate) (DPITPFB), into the X-DVTPD material (1 : 10 in wt%), a favourable morphology of the dopant/matrix layer was formed and the hole-mobility is significantly improved by three orders of magnitude compared to its non-doped state. This DPITPFB : X-DVTPD (1 : 10 in wt%) layer was employed as the hole-transporting layer to fabricate polymer solar cell devices (PSCs). The EHOMO of the polymer in the active layer relative to the EHOMO of the X-DVTPD (−5.3 eV) governs the hole transportation highly associated with the device performance. The higher-lying EHOMO (−5.0 eV) of P3HT causes a large energy barrier for the hole transportation at the interface, leading to an unsatisfactory efficiency. The EHOMO level of the PTB7 copolymer (−5.15 eV) is closer to −5.3 eV. As a result, the PTB7-based device can achieve 80% of the efficiency obtained from the corresponding PEDOT:PSS-based device. Furthermore, the PBDCPDTFBT copolymer has the same EHOMO (−5.3 eV) with X-DVTPD. Consequently, the PBDCPDTFBT-based device showed a comparable efficiency of 5.3% to the corresponding PEDOT:PSS-based device. More importantly, PNDTDTFBT having the lowest-lying EHOMO of −5.4 eV exhibits superior performance with a high PCE of 6.64%, outperforming its reference PEDOT:PSS-based device. This simple and useful hole-transporting system integrating the crosslinking and doping strategies to replace PEDOT:PSS can be widely used in solution-processed organic electronic devices.

Bispentafluorophenyl-Containing Additive: Enhancing Efficiency and Morphological Stability of Polymer Solar Cells via Hand-Grabbing-Like Supramolecular Pentafluorophenyl:Fullerene Interactions

A new class of additive materials bis(pentafluorophenyl) diesters (BFEs) where the two pentafluorophenyl (C6F5) moieties are attached at the both ends of a linear aliphatic chain with tunable tether lengths (BF5, BF7, and BF13) were designed and synthesized. In the presence of BF7 to restrict the migration of fullerene by hand-grabbing-like supramolecular interactions induced between the C6F5 groups and the surface of fullerene, the P3HT:PC61BM:BF7 device showed stable device characteristics after thermal heating at 150 °C for 25 h. The morphologies of the active layers were systematically investigated by optical microscopy, grazing-incidence small-angle X-ray scattering (GISAXS), and atomic force microscopy. The tether length between the two C6F5 groups plays a pivotal role in controlling the intermolecular attractions. BF13 with a long and flexible tether might form a BF13-fullerene sandwich complex that fails to prevent fullerenes movement and aggregation, while BF5 with too short tether length decreases the possibility of interactions between the C6F5 groups and the fullerenes. BF7 with the optimal tether length has the best ability to stabilize the morphology. In sharp contrast, the nonfluorinated BP7 analogue without C6F5-C60 physical interactions does not have the capability of morphological stabilization, unambiguously revealing the necessity of the C6F5 group. Most importantly, the function of BF7 can be also applied to the high-performance PffBT4BT-2OD:PC71BM system, which exhibited an original PCE of 8.80%. After thermal heating at 85 °C for 200 h, the efficiency of the PffBT4BT-2OD:PC71BM:BF7 device only decreased slightly to 7.73%, maintaining 88% of its original efficiency. To the best of our knowledge, this is the first time that the thermal-driven morphological evolution of the high-performance PffBT4BT-2OD polymer has been investigated, and its morphological stability in the inverted device can be successfully preserved by the incorporation of BF7. This research also demonstrates that BF7 is not only effective with PC61BM but also to PC71BM.

Synthesis and side-chain isomeric effect of 4,9-/5,10-dialkylated-β-angular-shaped naphthodithiophenes-based donor–acceptor copolymers for polymer solar cells and field-effect transistors

A systematic methodology is developed to construct the angular-shaped β-form naphthodithiophene (β-aNDT) core with regiospecific substitution of two alkyl groups at its 4,9- or 5,10-positions via the base-induced double 6π-cyclization of dithienyldieneyne precursors, leading to the two isomeric 4,9-β-aNDT and 5,10-β-aNDT monomers. It is found that a more curved geometry of the β-aNDT units intrinsically increases the solubility and thus the solution-processability of the resultant polymers. Therefore, β-aNDT units are ideal for polymerization with an acceptor-containing monomer without the need for any solubilizing aliphatic side chains, which are considered the insulating portion that jeopardizes charge transport. Based on this consideration, the 4,9- and 5,10-dialkylated β-aNDT monomers are polymerized with the non-alkylated DTFBT acceptor to afford two P4,9-βNDTDTFBT and P5,10-βNDTDTFBT copolymers for head-to-head comparison of the 4,9-inner/5,10-outer isomeric alkylation effect. It is found that 4,9-β-aNDT adopts a twisted conjugated structure due to the intramolecular steric repulsion between the inner branched side chains and the β-hydrogens on the thiophene rings. The slightly twisted 4,9-β-aNDT moiety allows P4,9-βNDTDTFBT to have higher solubility upon polymerization and thus a higher molecular weight, which eventually induces a higher ordered packing structure in the thin film compared to P5,10-βNDTDTFBT. As a result, P4,9-βNDTDTFBT exhibits a higher OFET mobility of 0.18 cm2 V−1 s−1, and the P4,9-βNDTDTFBT:PC71BM-based solar cell device also achieves a higher PCE of 7.23%, which is even better than the corresponding P4,9-αNDTDTFBT-based device.

Cross-linked Triarylamine-Based Hole-Transporting Layer for Solution-Processed PEDOT:PSS-Free Inverted Perovskite Solar Cells

The device performance of inverted organic metallohalide perovskite solar cells (OMPSCs) is optimized via tailoring the electrode surfaces with electron- and hole-transporting materials. This work demonstrates the fabrication of PEDOT:PSS-free OMPSCs using a hole-transporting composite material consisting of bilayered vanadium oxide (VO x) and a thermally cross-linked triarylamine-based material X-DVTPD, which contributes to higher Voc and Jsc values. The hydrophobicity of X-DVTPD resulted in the formation of large perovskite crystals and enhanced the stability of OMPSCs. Integration of ionic fullerene derivative, fulleropyrrolidinium iodide, in OMPSCs as a hole-blocking interfacial layer at the interface with Ag proves effective to further boost the device efficiency to 18.08%.