Thin-film solar cells incorporating colloidal quantum dot active layers have recently emerged as a notable third-generation photovoltaic (PV) technology, largely due to the strong absorption, tunable infrared bandgap, and ambient-atmosphere stability of lead sulfide quantum dots (PbS QDs). Photoactive PbS QDs can be solution-deposited on a transparent zinc oxide (ZnO) film to form a depleted np-heterojunction device (Figure 1a,b). However, this standard planar architecture incurs a fundamental trade-off between light absorption and carrier collection: to absorb most incident light, we need a ~1-µm-thick QD film ^[1] , but to collect most photocarriers, we need absorption to occur within a minority carrier diffusion length (~100 nm) of the ~150-nm-thick depletion region ^[2] . By introducing 1-D nanostructures (Figure 1c), we can decouple these parallel requirements and optimize for each independently. A vertical, QD-infiltrated array of ZnO nanowires orthogonalizes the mechanistic length scales of absorption and collection. Absorption is maximized as light traverses a thick QD film in the axial direction, while field-driven carrier collection is retained throughout the film as photogenerated electrons drift to nearby PbS/ZnO interfaces in the radial direction.

Our research demonstrates that moving from a planar ZnO film to a nanowire array can significantly improve QDPV performance, increasing short-circuit current density (J_SC) by ~40% and overall power conversion efficiency by ~15% ^[3] . We confirm the near-complete infiltration of PbS QDs into the ZnO nanowire array via cross-sectional scanning electron microscopy (Figure 1d) and elemental mapping with energy-dispersive x-ray spectroscopy. We further demonstrate a fast solution treatment to assist interfacial charge transfer using a bifunctional linker molecule, 3-mercaptopropionic acid (MPA). A simple MPA treatment increases both J_SC and open-circuit voltage (V_OC) of nanowire-QD devices (see Figure 2). Our work on ZnO nanowire-based QD solar cells—along with the recent demonstration of a 5.6%-efficient TiO₂nanopillar-based QDPV ^[4] —suggests that 1-D nanostructures may be the key to enhancing the efficiency and hence the economic viability of quantum dot photovoltaics.

1. A. G. Pattantyus-Abraham,I. J. Kramer, A. R. Barkhouse,X. Wang, G.Konstantatos, R. Debnath,L. Levina,I. Raabe,M. K. Nazeeruddin, M.Grätzel, and E. H. Sargent, “Depleted-heterojunction colloidal quantum dot solar cells,” ACS Nano, vol. 4, no. 6, pp. 3374-3380, May 2010.
2. K. W. Johnston, A. G. Pattantyus-Abraham, J. P. Clifford, S. H. Myrskog, S. Hoogland, S. Sjoerd, H. Shukla, E. J. D. Klem, L. Levina, and E. H. Sargent, “Efficient Schottky-quantum-dot photovoltaics: The roles of depletion, drift, and diffusion,” Applied Physics Letters, vol. 92, no. 12, pp. 122111, Mar. 2008.
3. P. R. Brown, R. R. Lunt, N. Zhao, T. P. Osedach,D. D. Wanger, L.-Y.Chang, M. G.Bawendi, and V. Bulović, “Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using a MoO₃ interfacial layer,” Nano Letters, vol. 11, no. 7, pp. 2955-2961, June 2011.
4. I. J. Kramer, D. Zhitomirsky, J. D. Bass, P. M. Rice, T. Topuria, L. Krupp, S. M. Thon, A. H. Ip, R. Debnath, H.-C. Kim, and E. H. Sargent, “Ordered nanopillar structured electrodes for depleted bulk heterojunction colloidal quantum dot solar cells,” Advanced Materials, vol. 24, no. 17, pp. 2315-2319, Mar. 2012.

Organic/inorganic hybrid solar cells based on the integration of conductive polymers into semiconducting nanowire arrays offer opportunities for the development of hybrid devices with increased power conversion efficiencies due to high charge carrier collection, one-dimensional transport pathways, and large interfacial area ^{[1] [2]} . However, control of the nanowire density and effective infiltration of conductive polymers into nano-sized gaps within semiconducting nanowire arrays have been challenging. Here, an inverted device structure of a hybrid solar cell (Figure 1) has been realized by utilizing ZnO nanowire arrays hydrothermally grown directly on an ITO electrode and by using an effective polymer coating. The hydrothermal growth technique enables realization of highly uniform, vertically-aligned ZnO nanowires over large areas on electrode.

The photovoltaic device performance was investigated by controlling the size and density of ZnO nanowires, the thickness of conductive polymer consistent with carrier diffusion lengths, and the organic interfacial layer ^[3] . The size and density of ZnO nanowires were successfully controlled by the concentration of precursor solution (zinc nitrate hexahydrate, hexamethylenetetramine), growth time, and temperature. Effective infiltration of P3HT was achieved using a vacuum annealing technique which eliminated voids and maximized the interfacial area between ZnO and P3HT. Enhanced P3HT crystallinity was observed through UV-Vis absorption and PL quenching showed effective charge separation in vacuum-annealed samples. Hybrid devices with an interfacial layer, Dibenzo{[f,f′]-4,4′,7,7′-tetraphenyl}diindeno[1,2,3-cd:1′,2′,3′-lm]perylene (DBP), exhibit increased short circuit current (J_sc) and open circuit voltage (V_oc) (Figure 2). The DBP interfacial layer provides cascade charge transfer from P3HT to ZnO nanowires. The highest power conversion efficiency (PCE) has been demonstrated using only ZnO nanowire arrays/organic electron donor materials without any inorganic buffer layer such as TiO₂ or dye modification of ZnO nanowires. The scalable, cost-effective approach to synthesize ZnO nanowire arrays and the effective coating of organic electron donor materials over the arrays are promising for applications in hybrid solar cells device fabrication.

1. S. Ren, N. Zhao, S. C. Crawford, M. Tambe, V. Bulovic, and S. Gradečak, “Heterojunction photovoltaics using GaAs nanowires and conjugated polymers,” Nano Letters, vol. 11, pp. 408–413, Feb. 2011.
2. S. Ren, L.-Y. Chang, S. K. Lim, J. Zhao, M. Smith, N. Zhao, V. Bulovic, M. Bawendi, and S. Gradečak, “Inorganic-organic hybrid solar cell: Bridging quantum dots to conjugated polymer nanowires,” Nano Letters, vol. 11, pp. 3998–4002, Aug. 2011.
3. B. Kannan, K. Castelino, and A. Majumdar, “Design of nanostructured heterojunction polymer photovoltaic devices,” Nano Letters, vol. 3, pp. 1729–1733, Nov. 2003.

Nanowires are an exciting architecture for light emitting diodes (LEDs). To address the challenges of solid state lighting, III-V nitride nanowires have demonstrated many advantages over standard thin-film architectures ^[1] . The one-dimensionality of nanowires makes them inherent waveguides ^[2] .

In designing nanowire-based LEDs, it is important to consider waveguiding and nanowire geometry. GaN nanowires grown in the m-direction are triangular in cross-section. Finite difference time domain photonic simulations were used to calculate electric-field intensities in such nanowires, with varying sidewall lengths, d, assuming lossless 1-µm-long dielectric nanowires with a refractive index of 2.6 suspended in air. At the emission wavelength of GaN nanowires (≈380 nm), m-directional nanowires with sides of less than ≈150 nm support no waveguided modes. Figure 1a demonstrates the difference in light confinement within the nanowires with (d = 300 nm) and without (d = 60 nm) waveguided modes. Figure 1b contrasts propagation in the axial direction of the nanowire as compared to just the dipole source. Light extraction was considered for vertically oriented nanowires on a silicon substrate, with varying emission source positions. Figure 2 summarizes these results and compares them to the thin-film case. Emission at the silicon substrate has low extraction efficiency due the higher index silicon absorbing light from the GaN. In the waveguided case, extraction is generally higher, as most of the light is guided within the extraction cone; however, due to differences of coupling into the guided mode with emission distance from the substrate, the extraction efficiency varies substantially with position. With emission near the tip, the waveguided case is less extracted than the non-waveguided case due to more reflection. For both of the nanowires, extraction efficiency is greatly improved over the thin-film case, warranting further investigation into nanowire-based LED design, which is ongoing.

1. Y. Huang, X. Duan, and C. M. Leiber, “Nanowires for integrated multicolor photonics,” Small, vol. 1, pp. 142-147, Jan. 2005.
2. A. L. Henneghien, B. Gayral, Y. Désières, and J. M. Gérard, “Simulation of waveguiding and emitting properties of semiconductor nanowires with hexagonal or circular sections,” Journal of the Optical Society of America B, vol. 26, no. 12, pp. 2396-2403, Dec. 2009.

GaN-based high electron mobility transistors (HEMTs) are an important platform for the realization of high-power, high-frequency devices.  Nanoribbon (NR) HEMT structures represent a novel route towards piezodoping by allowing external stresses to be applied in the plane of the active layer and have been shown to enhance carrier transport properties ^[1] .  This work uses transmission electron microscopy (TEM) and finite element analysis (FEA) to investigate the stress state of InAlN/GaN NR HEMT devices and explore the role of Al₂O₃ in stress generation.

NR structures were fabricated using top-down techniques and passivated with varying thicknesses of Al₂O₃.  TEM samples were obtained from the device structures by using focused ion beam techniques.  Using convergent beam electron diffraction, strain relaxation profiles were obtained by analyzing the splitting of higher order Laue zone lines contained in the [5 4 0] zone axis pattern.  Splitting profiles were also generated from FEA models of the HEMT structure for comparison.  Finally, device-sized structures were simulated to investigate the stress state of the active HEMT layers as a function of the oxide thickness.

Comparison of the experimental and simulated splitting profiles in Figure 1 shows not only that the FEA model correctly replicates overall splitting behavior and the dependence on sample thickness, but that it also consistently under-estimates the experimental results, suggesting an additional source of stress not present in the current model.  Models of device structures showed a compressive stress generated in the active HEMT layer upon the creation of a NR structure that becomes tensile when a layer of Al₂O₃ is applied, as shown in Figure 2.  The magnitude of the tensile stress approaches that of the planar structure as the thickness of the oxide increases.  This data correlates well with earlier published ^[1] electrical characterization of these structures considering the decrease in carrier concentration observed for a compressive strain ^[2] .

1. M. Azize, A. L. Hsu, O. I. Saadat, M. Smith, X. Gao, S. P. Guo, S. Gradečak, and T. Palacios, “High-electron-mobility transistors based on InAlN/GaN nanoribbons,” IEEE Electron Device Letters, vol. 32, pp. 1680-1682, Dec 2011.
2. J. Kuzmik, “InAlN/(In)GaN high electron mobility transistors: Some aspects of the quantum well heterostructure proposal,” Semiconductor Science and Technology, vol. 17, pp. 540-544, June 2002.

Collaborators

• V. Bulović, EECS
• M. Bawendi, Chemistry
• J. Kong, EECS

Graduate Students

• J. W. J. Cheng, Research Assistant, MSE
• J. Chesin, Research Assistant, MSE
• S. Crawford, Research Assistant, MSE
• S. Ermez, Research Assistant, MSE
• J. Hanson, Research Assistant, NSE
• E. Jones, Research Assistant, MSE
• P.l Rekemeyer, Research Assistant, MSE
• M. Smith, Research Assistant, MSE
• X. Zhou, Research Assistant, MSE

Support Staff

• B. MacEachern, Administrative Assistant II

Publications

S. Ren, N. Zhao, S. Crawford, V. Bulovic, S. Gradečak, “Heterojunction photovoltaics using GaAs nanowires and conjugated polymers,” Nano Letters, vol. 11, pp 408-413, 2011.

M. Brewster, X. Zhou, S. K. Lim, S. Gradečak, “Role of Au in the growth and nanoscale optical properties of ZnO nanowires,” Journal of Physical Chemistry Letters, vol. 2, pp. 586-591, 2011.

M. Brewster, M.-Y. Lu, S. K. Lim, M. Smith, X. Zhou, and S. Gradečak, “The growth and optical properties of ZnO nanowalls,” Journal of Physical Chemistry Letters, vol. 2, pp. 1940-1945, 2011.

S. Ren, L.-Y. Chang, S. K. Lim, N. Zhao, M. Smith, V. Bulović, M. Bawendi, S. Gradečak, “Inorganic-organic nanohybrid solar cell: bridging quantum dots and conjugated polymer nanowires,” Nano Letters, vol. 11, pp. 3998-4002, 2011.

M. Smith, Y.-T. Lin, M.-J. Sher, M. Winkler , E. Mazur, S. Gradečak, “Pressure-induced phase transformations during femtosecond-laser doping of silicon,” vol. 110, pp. 053524(1-8), 2011.

M. Azize, O. Saadat, A. Hsu, M. Smith, S. Guo, S. Gradečak, T. Palacios, “High electron mobility transistors based on InAlN/GaN nano-ribbons,” IEEE Electron Device Letters, vol. 32, pp. 1680-1682, 2011.

M. Smith, M. Winkler, M.-J. Sher, Y.-T. Lin, E. Mazur, S. Gradečak, “Effects of a thin film dopant precursor on the structure and properties of femtosecond-laser doped silicon,” Applied Physics A, vol. 105, pp. 795-800, 2011.

S. Ren, M. Bernardi, R. R. Lunt, V. Bulovic, J. C. Grossman, S. Gradečak, “Towards efficient carbon nanotube / P3HT solar cells: active layer morphology, electrical and optical properties,” Nano Letters, vol. 11, pp. 5316-5321, 2011.

M. Brewster, X. Zhou, M. Y. Lu, and S. Gradečak, “The interplay of structure and optical properties in individual ZnO nanostructures”, Nanoscale, vol. 4, pp. 1455-1462, 2012.

S. K. Lim, S. Crawford, G. Haberfehlner, S. Gradečak, “Controlled modulation of diameter and composition along individual III-V nitride nanowires”, Nano Letters, pp. 1-6, published online February 7th, 2012.

M. T. Winkler, M.-J. Sher, Y.-T. Lin, M. Smith, H. Zhang, S. Gradečak, E. Mazur, “Studying femtosecond-laser hyperdoping by controlling surface morphology”, Journal of Applied Physics, vol. 111, pp. 093511(1-7), 2012.

F. Qian, M. Brewster, S. K. Lim, C. Greene, O. Laboutin, J. W. Johnson, S. Gradečak, Y. Cao, Y. Li, “Synthesis and optical studies of AlN/GaN multiple quantum well nanowire structures”, Nano Letters, vol. 12, pp. 3344–3350, 2012.

X. Zhou, J. Chesin, S. Crawford, S. Gradečak, “Using seed particle composition to control structural and optical properties of GaN nanowires”, accepted for publication in Nanotechnology, 2012.