Lushuai Zhang

Development of Lead Iodide Perovskite Solar Cells Using Three-Dimensional Titanium Dioxide Nanowire Architectures

Three-dimensional (3D) nanowire (NW) architectures are considered as superior electrode design for photovoltaic devices compared to NWs or nanoparticle systems in terms of improved large surface area and charge transport properties. In this paper, we report development of lead iodide perovskite solar cells based on a novel 3D TiO2 NW architectures. The 3D TiO2 nanostructure was synthesized via surface-reaction-limited pulsed chemical vapor deposition (SPCVD) technique that also implemented the Kirkendall effect for complete ZnO NW template conversion. It was found that the film thickness of 3D TiO2 can significantly influence the photovoltaic performance. Short-circuit current increased with the TiO2 length, while open-circuit voltage and fill factor decreased with the length. The highest power conversion efficiency (PCE) of 9.0% was achieved with ∼ 600 nm long 3D TiO2 NW structures. Compared to other 1D nanostructure arrays (TiO2 nanotubes, TiO2-coated ZnO NWs and ZnO NWs), 3D TiO2 NW architecture was able to achieve larger amounts of perovskite loading, enhanced light harvesting efficiency, and increased electron-transport property. Therefore, its PCE is 1.5, 2.3, and 2.8 times higher than those of TiO2 nanotubes, TiO2-coated ZnO NWs, and ZnO NWs, respectively. The unique morphological advantages, together with the largely suppressed hysteresis effect, make 3D hierarchical TiO2 a promising electrode selection in designing high-performance perovskite solar cells.

High open-circuit voltage, high fill factor single-junction organic solar cells

We demonstrate a high open circuit voltage (Voc), high fill factor, single-junction organic photovoltaic (OPV) cell consisting of tetraphenyldibenzoperiflanthene (DBP) as the donor and pyrrolo[3,4-c]pyrrole- 1,4-dione, 3,6-bis(4-chlorophenyl)-2, 5-dihydro (DPP) as the acceptor in a vapor-deposited, fullerene free organic solar cell. Under one sun illumination, the DBP/DPP based planar heterojunction solar cell exhibits a Voc of 1.19u2009V with a fill factor of 0.67, leading to a final power conversion efficiency (PCE) of 2.2%. Our achieved Voc is the highest value reported thus far for a single-junction, high fill factor organic solar cell. A planar-mixed device architecture allows for high exciton generation while maintaining high exciton dissociation, leading to a PCE of 3.2%. The DBP and DPP interface creates a high interface energy gap (ΔEDA) and relatively low saturation current (JS) due to poor coupling across the donor-acceptor interface. These results support earlier theories that high Voc values are dependent on large ΔEDA and low JS for a given donor-acceptor combination in OPVs.

Observing Electron Extraction by Monolayer Graphene Using Time-Resolved Surface Photoresponse Measurements

Graphene is considered a next-generation electrode for indium tin oxide (ITO)-free organic photovoltaic devices (OPVs). However, to date, limited numbers of OPVs containing surface-modified graphene electrodes perform as well as ITO-based counterparts, and no devices containing a bare graphene electrode have been reported to yield satisfactory rectification characteristics. In this report, we provide experimental data to learn why. Time-resolved surface photoresponse measurements on templated pentacene-on-graphene films directly reveal that p-doped monolayer graphene efficiently extracts electrons, not holes, from photoexcited pentacene. Accordingly, a graphene/pentacene/MoO3 heterojunction displays a large surface photoresponse and, by inference, efficient dissociation of photogenerated excitons, with graphene serving as an electron extraction layer and MoO3 as a hole extraction layer. In contrast, a graphene/pentacene/C60 heterojunction yields a comparatively insignificant surface photoresponse because both graphene and C60 act as competing electron extraction layers. The data presented herein provide experimental insight for future endeavors involving bare graphene as an electrode for organic photovoltaic devices and strongly suggest that p-doped graphene is best considered a cathode for OPVs.

Effects of pulsing and interfacial potentials on tellurium-organic heterostructured films.

Polycrystalline thin films of tellurium and organic semiconductor molecules are paired in heterostructured field-effect transistors built on Si/SiO2 substrates. While charge carrier mobilities can exceed 1 cm(2)/(V s), there is only a limited gate voltage range over which the current is modulated. We employ continuous and pulsed measurements on transistors to explore the influence of charge equilibration time on device behavior, finding that pulsed gating improves output characteristics. We also use surface potential measurements to investigate the interfacial vacuum level offset between materials, and we modify the interlayer potential profile by interposing statically charged dielectric layers on the silicon dioxide. We show that interfacial fields determine the gate voltage range over which Te shows a field effect in heterostructures with organic semiconductors and that modification of these fields can extend this range.

Transforming Commercial Textiles and Threads into Sewable and Weavable Electric Heaters

We describe a process to transform commercial textiles and threads into electric heaters that can be cut/sewn or woven to fashion lightweight fabric heaters for local climate control and personal thermal management. Off-the-shelf fabrics are coated with a 1.5 μm thick film of a conducting polymer, poly(3,4-ethylenedioxythiophene), using an improved reactive vapor deposition method. Changes in the hand feel, weight, and breathability of the textiles after the coating process are imperceptible. The resulting fabric electrodes possess competitively low sheet resistances-44 Ω/□ measured for coated bast fiber textiles and 61 Ω/□ measured for coated cotton textiles-and act as low-power-consuming Joule heating elements. The electrothermal response of the textile electrodes remain unaffected after cutting and sewing due to the robustness of the conductive coating. Coated, conductive cotton yarns can also be plain-woven into a monolithic fabric heater. A demonstrative circuit design for a soft, lightweight, and breathable thermal glove is provided.