Professional Education

  • Doctor of Philosophy, National Taiwan University (2015)

Stanford Advisors

All Publications

  • Inducing Molecular Aggregation of Polymer Semiconductors in a Secondary Insulating Polymer Matrix to Enhance Charge Transport CHEMISTRY OF MATERIALS Nikzad, S., Wu, H., Kim, J., Mahoney, C. M., Matthews, J. R., Niu, W., Li, Y., Wang, H., Chen, W., Toney, M. F., He, M., Bao, Z. 2020; 32 (2): 897–905
  • A Highly Stretchable and Self-Healing Supramolecular Elastomer Based on Sliding Crosslinks and Hydrogen Bonds ADVANCED FUNCTIONAL MATERIALS Du, R., Xu, Z., Zhu, C., Jiang, Y., Yan, H., Wu, H., Vardoulis, O., Cai, Y., Zhu, X., Bao, Z., Zhang, Q., Jia, X. 2019
  • Fine-Tuning Semiconducting Polymer Self-Aggregation and Crystallinity Enables Optimal Morphology and High-Performance Printed All-Polymer Solar Cells. Journal of the American Chemical Society Wu, Y., Schneider, S., Walter, C., Chowdhury, A. H., Bahrami, B., Wu, H., Qiao, Q., Toney, M. F., Bao, Z. 2019


    Polymer aggregation and crystallization behavior play a crucial role in the performance of all-polymer solar cells (all-PSCs). Gaining control over polymer self-assembly via molecular design to influence bulk-heterojunction active-layer morphology, however, remains challenging. Herein, we show a simple yet effective way to modulate the self-aggregation of the commonly used naphthalene diimide (NDI)-based acceptor polymer (N2200), by systematically replacing a certain amount of alkyl side-chains with compact bulky side-chains (CBS). Specifically, we have synthesized a series of random copolymer (PNDI-CBSx) with different molar fractions (x = 0-1) of the CBS units and have found that both solution-phase aggregation and solid-state crystallinity of these acceptor polymers are progressively suppressed with increasing x as evidenced by UV-vis absorption, photoluminescence (PL) spectroscopies, thermal analysis, and grazing incidence X-ray scattering (GIWAXS) techniques. Importantly, as compared to the highly self-aggregating N2200, photovoltaic results show that blending of more amorphous acceptor polymers with donor polymer (PBDB-T) can enable all-PSCs with significantly increased PCE (up to 8.5%). The higher short-circuit current density (Jsc) results from the smaller polymer phase-separation domain sizes as evidenced by PL quenching and resonant soft X-ray scattering (R-SoXS) analyses. Additionally, we show that the lower crystallinity of the active layer is less sensitive to the film deposition methods. Thus, the transition from spin-coating to solution coating can be easily achieved with no performance losses. On the other hand, decreasing aggregation and crystallinity of the acceptor polymer too much reduces the photovoltaic performance as the donor phase-separation domain sizes increases. The highly amorphous acceptor polymers appear to induce formation of larger donor polymer crystallites. These results highlight the importance of a balanced aggregation strength between the donor and acceptor polymers to achieve high-performance all-PSCs with optimal active layer film morphology.

    View details for DOI 10.1021/jacs.9b10935

    View details for PubMedID 31793773

  • Stretchable self-healable semiconducting polymer film for active-matrix strain-sensing array. Science advances Oh, J. Y., Son, D., Katsumata, T., Lee, Y., Kim, Y., Lopez, J., Wu, H., Kang, J., Park, J., Gu, X., Mun, J., Wang, N. G., Yin, Y., Cai, W., Yun, Y., Tok, J. B., Bao, Z. 2019; 5 (11): eaav3097


    Skin-like sensory devices should be stretchable and self-healable to meet the demands for future electronic skin applications. Despite recent notable advances in skin-inspired electronic materials, it remains challenging to confer these desired functionalities to an active semiconductor. Here, we report a strain-sensitive, stretchable, and autonomously self-healable semiconducting film achieved through blending of a polymer semiconductor and a self-healable elastomer, both of which are dynamically cross-linked by metal coordination. We observed that by controlling the percolation threshold of the polymer semiconductor, the blend film became strain sensitive, with a gauge factor of 5.75 * 105 at 100% strain in a stretchable transistor. The blend film is also highly stretchable (fracture strain, >1300%) and autonomously self-healable at room temperature. We proceed to demonstrate a fully integrated 5 * 5 stretchable active-matrix transistor sensor array capable of detecting strain distribution through surface deformation.

    View details for DOI 10.1126/sciadv.aav3097

    View details for PubMedID 31723597

  • Effect of Extensional Flow on the Evaporative Assembly of a Donor-Acceptor Semiconducting Polymer ACS APPLIED ELECTRONIC MATERIALS Nikzad, S., Wu, H., Wang, G., Yan, H., Schneider, S. A., Toney, M. F., Bao, Z. 2019; 1 (11): 2445–54
  • Intrinsically Stretchable Temperature Sensor Based on Organic Thin-Film Transistors IEEE ELECTRON DEVICE LETTERS Zhu, C., Wu, H., Nyikayaramba, G., Bao, Z., Murmann, B. 2019; 40 (10): 1630–33
  • An Intrinsically Stretchable High-Performance Polymer Semiconductor with Low Crystallinity ADVANCED FUNCTIONAL MATERIALS Zheng, Y., Wang, G., Kang, J., Nikolka, M., Wu, H., Tran, H., Zhang, S., Yan, H., Chen, H., Yuen, P., Mun, J., Dauskardt, R. H., McCulloch, I., Tok, J., Gu, X., Bao, Z. 2019
  • Tuning the Cross-Linker Crystallinity of a Stretchable Polymer Semiconductor CHEMISTRY OF MATERIALS Wang, G., Zheng, Y., Zhang, S., Kang, J., Wu, H., Gasperini, A., Zhang, H., Gu, X., Bao, Z. 2019; 31 (17): 6465–75
  • High-Transconductance Stretchable Transistors Achieved by Controlled Gold Microcrack Morphology ADVANCED ELECTRONIC MATERIALS Matsuhisa, N., Jiang, Y., Liu, Z., Chen, G., Wan, C., Kim, Y., Kang, J., Tran, H., Wu, H., You, I., Bao, Z., Chen, X. 2019; 5 (8)
  • Multi-scale ordering in highly stretchable polymer semiconducting films NATURE MATERIALS Xu, J., Wu, H., Zhu, C., Ehrlich, A., Shaw, L., Nikolka, M., Wang, S., Molina-Lopez, F., Gu, X., Luo, S., Zhou, D., Kim, Y., Wang, G., Gu, K., Feig, V., Chen, S., Kim, Y., Katsumata, T., Zheng, Y., Yan, H., Chung, J., Lopez, J., Murmann, B., Bao, Z. 2019; 18 (6): 594-+
  • Characterization of Hydrogen Bonding Formation and Breaking in Semiconducting Polymers under Mechanical Strain MACROMOLECULES Gasperini, A., Wang, G., Molina-Lopez, F., Wu, H., Lopez, J., Xu, J., Luo, S., Zhou, D., Xue, G., Tok, J., Bao, Z. 2019; 52 (6): 2476–86
  • Conjugated Carbon Cyclic Nanorings as Additives for Intrinsically Stretchable Semiconducting Polymers. Advanced materials (Deerfield Beach, Fla.) Mun, J., Kang, J., Zheng, Y., Luo, S., Wu, H. C., Matsuhisa, N., Xu, J., Wang, G. N., Yun, Y., Xue, G., Tok, J. B., Bao, Z. 2019: e1903912


    Molecular additives are often used to enhance dynamic motion of polymeric chains, which subsequently alter the functional and physical properties of polymers. However, controlling the chain dynamics of semiconducting polymer thin films and understanding the fundamental mechanisms of such changes is a new area of research. Here, cycloparaphenylenes (CPPs) are used as conjugated molecular additives to tune the dynamic behaviors of diketopyrrolopyrrole-based (DPP-based) semiconducting polymers. It is observed that the addition of CPPs results in significant improvement in the stretchability of the DPP-based polymers without adversely affecting their mobility, which arises from the enhanced polymer dynamic motion and reduced long-range crystalline order. The polymer films retain their fiber-like morphology and short-range ordered aggregates, which leads to high mobility. Fully stretchable transistors are subsequently fabricated using CPP/semiconductor composites as active layers. These composites are observed to maintain high mobilities when strained and after repeated applied strains. Interestingly, CPPs are also observed to improve the contact resistance and charge transport of the fully stretchable transistors. ln summary, these results collectively indicate that controlling the dynamic motion of polymer semiconductors is proved to be an effective way to improve their stretchability.

    View details for DOI 10.1002/adma.201903912

    View details for PubMedID 31489716

  • Stretchable and Fully Degradable Semiconductors for Transient Electronics. ACS central science Tran, H., Feig, V. R., Liu, K., Wu, H. C., Chen, R., Xu, J., Deisseroth, K., Bao, Z. 2019; 5 (11): 1884–91


    The next materials challenge in organic stretchable electronics is the development of a fully degradable semiconductor that maintains stable electrical performance under strain. Herein, we decouple the design of stretchability and transience by harmonizing polymer physics principles and molecular design in order to demonstrate for the first time a material that simultaneously possesses three disparate attributes: semiconductivity, intrinsic stretchability, and full degradability. We show that we can design acid-labile semiconducting polymers to appropriately phase segregate within a biodegradable elastomer, yielding semiconducting nanofibers that concurrently enable controlled transience and strain-independent transistor mobilities. Along with the future development of suitable conductors and device integration advances, we anticipate that these materials could be used to build fully biodegradable diagnostic or therapeutic devices that reside inside the body temporarily, or environmental monitors that are placed in the field and break down when they are no longer needed. This fully degradable semiconductor represents a promising advance toward developing multifunctional materials for skin-inspired electronic devices that can address previously inaccessible challenges and in turn create new technologies.

    View details for DOI 10.1021/acscentsci.9b00850

    View details for PubMedID 31807690

    View details for PubMedCentralID PMC6891860

  • Synthesis and Properties of Soluble Fused Thiophene Diketopyrrolopyrrole-Based Polymers with Tunable Molecular Weight MACROMOLECULES Niu, W., Wu, H., Matthews, J. R., Tandia, A., Li, Y., Wallace, A. L., Kim, J., Wang, H., Li, X., Mehrotra, K., Bao, Z., He, M. 2018; 51 (23): 9422–29
  • Effect of Nonconjugated Spacers on Mechanical Properties of Semiconducting Polymers for Stretchable Transistors ADVANCED FUNCTIONAL MATERIALS Mun, J., Wang, G., Oh, J., Katsumata, T., Lee, F. L., Kang, J., Wu, H., Lissel, F., Rondeau-Gagne, S., Tok, J., Bao, Z. 2018; 28 (43)
  • Enhanced Charge Transport and Stability Conferred by Iron(III)-Coordination in a Conjugated Polymer Thin-Film Transistors ADVANCED ELECTRONIC MATERIALS Wu, H., Rondeau-Gagne, S., Chiu, Y., Lissel, F., To, J. F., Tsao, Y., Oh, J., Tang, B., Chen, W., Tok, J., Bao, Z. 2018; 4 (9)
  • Efficient and UV-stable perovskite solar cells enabled by side chain-engineered polymeric hole-transporting layers JOURNAL OF MATERIALS CHEMISTRY A Tsai, C., Li, N., Lee, C., Wu, H., Zhu, Z., Wang, L., Chen, W., Yan, H., Chueh, C. 2018; 6 (27): 12999–3004

    View details for DOI 10.1039/c8ta03608j

    View details for Web of Science ID 000438548800013

  • Nonhalogenated Solvent Processable and Printable High-Performance Polymer Semiconductor Enabled by Isomeric Nonconjugated Flexible Linkers MACROMOLECULES Wang, G., Molina-Lopez, F., Zhang, H., Xu, J., Wu, H., Lopez, J., Shaw, L., Mun, J., Zhang, Q., Wang, S., Ehrlich, A., Bao, Z. 2018; 51 (13): 4976–85
  • Enhancing Molecular Alignment and Charge Transport of Solution-Sheared Semiconducting Polymer Films by the Electrical-Blade Effect ADVANCED ELECTRONIC MATERIALS Molina-Lopez, F., Wu, H., Wang, G., Yan, H., Shaw, L., Xu, J., Toney, M. F., Bao, Z. 2018; 4 (7)
  • Quadruple H-Bonding Cross-Linked Supramolecular Polymeric Materials as Substrates for Stretchable, Antitearing, and Self-Healable Thin Film Electrodes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yan, X., Liu, Z., Zhang, Q., Lopez, J., Wang, H., Wu, H., Niu, S., Yan, H., Wang, S., Lei, T., Li, J., Qi, D., Huang, P., Huang, J., Zhang, Y., Wang, Y., Li, G., Tok, J., Chen, X., Bao, Z. 2018; 140 (15): 5280–89


    Herein, we report a de novo chemical design of supramolecular polymer materials (SPMs-1-3) by condensation polymerization, consisting of (i) soft polymeric chains (polytetramethylene glycol and tetraethylene glycol) and (ii) strong and reversible quadruple H-bonding cross-linkers (from 0 to 30 mol %). The former contributes to the formation of the soft domain of the SPMs, and the latter furnishes the SPMs with desirable mechanical properties, thereby producing soft, stretchable, yet tough elastomers. The resulting SPM-2 was observed to be highly stretchable (up to 17 000% strain), tough (fracture energy ∼30 000 J/m2), and self-healing, which are highly desirable properties and are superior to previously reported elastomers and tough hydrogels. Furthermore, a gold, thin film electrode deposited on this SPM substrate retains its conductivity and combines high stretchability (∼400%), fracture/notch insensitivity, self-healing, and good interfacial adhesion with the gold film. Again, these properties are all highly complementary to commonly used polydimethylsiloxane-based thin film metal electrodes. Last, we proceed to demonstrate the practical utility of our fabricated electrode via both in vivo and in vitro measurements of electromyography signals. This fundamental understanding obtained from the investigation of these SPMs will facilitate the progress of intelligent soft materials and flexible electronics.

    View details for PubMedID 29595956

  • Deformable Organic Nanowire Field-Effect Transistors ADVANCED MATERIALS Lee, Y., Oh, J., Kim, T., Gu, X., Kim, Y., Wang, G., Wu, H., Pfattner, R., To, J. F., Katsumata, T., Son, D., Kang, J., Matthews, J. R., Niu, W., He, M., Sinclair, R., Cui, Y., Tok, J., Lee, T., Bao, Z. 2018; 30 (7)
  • Soft Poly(butyl acrylate) Side Chains toward Intrinsically Stretchable Polymeric Semiconductors for Field-Effect Transistor Applications MACROMOLECULES Wen, H., Wu, H., Aimi, J., Hung, C., Chiang, Y., Kuo, C., Chen, W. 2017; 50 (13): 4982–92
  • n-Type Doped Conjugated Polymer for Nonvolatile Memory. Advanced materials Lee, W., Wu, H., Lu, C., Naab, B. D., Chen, W., Bao, Z. 2017


    This study demonstrates a facile way to efficiently induce strong memory behavior from common p-type conjugated polymers by adding n-type dopant 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole. The n-type doped p-channel conjugated polymers not only enhance n-type charge transport characteristics of the polymers, but also facilitate to storage charges and cause reversible bistable (ON and OFF states) switching upon application of gate bias. The n-type doped memory shows a large memory window of up to 47 V with an on/off current ratio larger than 10 000. The charge retention time can maintain over 100 000 s. Similar memory behaviors are also observed in other common semiconducting polymers such as poly(3-hexyl thiophene) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene], and a high mobility donor-acceptor polymer, poly(isoindigo-bithiophene). In summary, these observations suggest that this approach is a general method to induce memory behavior in conjugated polymers. To the best of the knowledge, this is the first report for p-type polymer memory achieved using n-type charge-transfer doping.

    View details for DOI 10.1002/adma.201605166

    View details for PubMedID 28234405

  • Isoindigo-Based Semiconducting Polymers Using Carbosilane Side Chains for High Performance Stretchable Field-Effect Transistors MACROMOLECULES Wu, H., Hung, C., Hong, C., Sun, H., Wang, J., Yamashita, G., Higashihara, T., Chen, W. 2016; 49 (22): 8540–48
  • Significance of the double-layer capacitor effect in polar rubbery dielectrics and exceptionally stable low-voltage high transconductance organic transistors SCIENTIFIC REPORTS Wang, C., Lee, W., Kong, D., Pfattner, R., Schweicher, G., Nakajima, R., Lu, C., Mei, J., Lee, T. H., Wu, H., Lopez, J., Diao, Y., Gu, X., Himmelberger, S., Niu, W., Matthews, J. R., He, M., Salleo, A., Nishi, Y., Bao, Z. 2015; 5


    Both high gain and transconductance at low operating voltages are essential for practical applications of organic field-effect transistors (OFETs). Here, we describe the significance of the double-layer capacitance effect in polar rubbery dielectrics, even when present in a very low ion concentration and conductivity. We observed that this effect can greatly enhance the OFET transconductance when driven at low voltages. Specifically, when the polar elastomer poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVDF-HFP) was used as the dielectric layer, despite a thickness of several micrometers, we obtained a transconductance per channel width 30 times higher than that measured for the same organic semiconductors fabricated on a semicrystalline PVDF-HFP with a similar thickness. After a series of detailed experimental investigations, we attribute the above observation to the double-layer capacitance effect, even though the ionic conductivity is as low as 10(-10) S/cm. Different from previously reported OFETs with double-layer capacitance effects, our devices showed unprecedented high bias-stress stability in air and even in water.

    View details for DOI 10.1038/srep17849

    View details for PubMedID 26658331

  • Effect of Spacer Length of Siloxane-Terminated Side Chains on Charge Transport in Isoindigo-Based Polymer Semiconductor Thin Films ADVANCED FUNCTIONAL MATERIALS Mei, J., Wu, H., Diao, Y., Appleton, A., Wang, H., Zhou, Y., Lee, W., Kurosawa, T., Chen, W., Bao, Z. 2015; 25 (23): 3455-3462
  • A Rapid and Facile Soft Contact Lamination Method: Evaluation of Polymer Semiconductors for Stretchable Transistors CHEMISTRY OF MATERIALS Wu, H., Benight, S. J., Chortos, A., Lee, W., Mei, J., To, J. W., Lu, C., He, M., Tok, J. B., Chen, W., Bao, Z. 2014; 26 (15): 4544-4551

    View details for DOI 10.1021/cm502271j

    View details for Web of Science ID 000340346300029