Research

More energy in the form of sunlight strikes the Earth’s surface every hour than the entire human race uses in six months. This amazingly abundant, clean, and renewable energy source provides the key to meeting our future energy needs, as we only need a small fraction of this resource to supply all the energy requirements necessary to sustain our global civilization. However, the utilization of this energy supply is still hindered by the higher cost solar generated electricity (relative to natural gas). While we are broadly interested in the physics, chemistry, and engineering of materials for energy conversion, we are currently focused on accelerating the utilization of solar energy by developing innovative low-cost and high-efficiency solar cells. Current and past thrusts in the group include:

Below is a brief summary of each effort and a few selected references or research highlights.


CIGS Materials and Solar Cells

One of the most promising thin film solar cell materials is CuInGaSe2 (commonly known as CIGS). Given the scale-up challenges and processing costs associated with vacuum deposition techniques we have focused on developing solution phase routes to CIGS. We developed and patented the first nanocrystal synthesis that yields correct stoichiometry CIGS nanocrystals and a method to form a nanocrystal-ink composed of the nanocrystals and a dense CIGS layer by selenization. In 2008 we reported the very first nanocrystal-ink based CIGS solar cells in Nano Letters [1]. The article was foundational to our CIGS effort, and it now has over 350 citations. While our first devices were only 3% efficient in converting energy in the solar spectrum into electricity, by understanding the growth mechanism of the nanocrystals [2] and improving the processing steps and sodium incorporation we have shown the ability to reach 12% power conversion efficiency [3]. In 2013, we published a brief review nanocrystal-ink based solar cells in Current Opinion in Chemical Engineering [4]. This article provides a good overview of both our research in this area and other important contributions to nanocrystal growth and device development.

  1. Guo, Q.J., Kim, S.J., Kar, M., Shafarman, W.N., Birkmire, R.W., Stach, E.A., Agrawal, R., Hillhouse, H.W., “Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar Cells,” Nano Letters 8, 9, 2982-2987 (2008). Link to article

  2. Kar, M., Agrawal, R. & Hillhouse, H.W., “On the Formation Pathway of CuInSe2 Nanocrystals for Solar Cells,” J. Am. Chem. Soc. 133 (43), 17239–17247 (2011). Link to article

  3. Guo, Q., Ford, G. M., Agrawal, R., Hillhouse, H. W., “Ink formulation and low-temperature incorporation of sodium to yield 12% efficient Cu(In,Ga)(S,Se)2 solar cells from sulfide nanocrystal inks.” Progress in Photovoltaics: Research and Applications 21 (1), 64–71 (2013). Link to article

  4. Bucherl, C.N., Oleson, K. R., and Hillhouse, H.W., “Thin film solar cells from sintered nanocrystals” Current Opinion in Chemical Engineering 2 (2), 168-177 (2013), Link to article

CZTS Materials and Solar Cells

There is one Achilles heel to CIGS solar cells, and that is the production of and price volatility of indium. As touch screen devices become more ubiquitous, the price of indium is expected to rise since it the key ingredient in ITO which is used in responsive touch screens. While the abundance of indium will not limit CIGS solar cell production in the near term, price volatility introduces substantial financial risk. In addition, in the long term, abundance may become an issue. As a result, we have developed nanocrystal syntheses and solar cell fabrication routes that utilize only Earth abundant elements such as copper, zinc, tin, and sulfur. In 2009, we reported the first synthesis of stoichiometric Cu2ZnSnS4 (CZTS) nanocrystals and showed the ability to use a CZTS nanocrystal-ink to fabricate a solar cells [1]. The first cells were less than 1% efficient, but again, further understanding of the materials chemistry and mastery of techniques to control the nanocrystal stoichiometry have resulted in CZTS-based devices that are 7.2% efficient [2]. One of the important aspects of efficient thin film photovoltaics is the ability to control the bandgap and band edge positions to create electric field gradients that direct minority carriers away from high recombination interfaces. In 2011, we reported the ability to use germanium to tune the bandgap in CZTS related materials [3]. This is the first report of cation control of the bandgap in this material system and is a route to create a back surface field in CZTS-based solar cells.

Nanocrystal-ink based photovoltaics have a lot of promise. However, they do have some drawbacks. Importantly, one must synthesize the nanocrystals. This can range from routine to tricky and is usually a source of process variability and a decrease in material utilization. As a result, we have developed new solution chemistry that yields stable inks composed of molecular complexes that can be formed by simply dissolving inexpensive salt precursors in a DMSO/thiourea solution. In 2011, we published our first report on this method in Advanced Energy Materials [4]. The devices were only about 4% efficient, but since then, we have developed a better understanding of the complexation in solution and its importance. In 2014, we reported the details of important redox reactions in solution and the ability to make 8% efficiency CZTS devices [5]. In particular, we reported the importance of the use of Sn+2 precursors despite the fact that the tin is in a +4 oxidation state in CZTS. At the 40th IEEE PVSC, we reported devices with greater than 11% efficiency. In addition, we have recently published a combinatorial study of the effects of native point defects in CZTS [6]. One important take-away from this last paper is that the ratio of quasi-Fermi level splitting to the maximum theoretical quasi-Fermi level splitting can reach 69% in the spray coated films we analyzed. This is important since the measured Voc is only 57% of the theoretical in the record CZTS devices made from hydrazine processing or sputtering. This highlights that fact that further improvements in the device Voc are possible.

  1. Guo, Q.J., Hillhouse, H.W., Agrawal, R., “Synthesis of Cu2ZnSnS4 Nanocrystal Ink and Its Use for Solar Cells,” Journal of the American Chemical Society 131 (33), 11672–11673 (2009). Link to article

  2. Guo, Q.J., Ford, G.M., Yang, W.C., Walker, B.C., Stach, E.A., Hillhouse, H.W., Agrawal, R., "Fabrication of 7.2% Efficient CZTSSe Solar Cells using CZTS Nanocrystals," Journal of the American Chemical Society 132, 17384–17386 (2010). Link to article

  3. Ford, G.M., Guo, Q., Agrawal, R. & Hillhouse, H.W., “Earth Abundant Element Cu2Zn(Sn1-xGex)S4 Nanocrystals for Tunable Band Gap Solar Cells: 6.8% Efficient Device Fabrication,” Chemistry of Materials 23 (10), 2626–2629 (2011). Link to article

  4. Ki, W. & Hillhouse, H.W., “Earth Abundant Element Photovoltaics Directly from Soluble Precursors using a Non-Toxic Solvent,” Advanced Energy Materials 1 (5), 732–735 (2011). Link to article

  5. Xin, H., Katahara, J.K., Braly, I.L., and Hillhouse, H.W., “8% Efficient Cu2ZnSn(S,Se)4 Solar Cells from Redox Equilibrated Simple Precursors in DMSO” Advanced Energy Materials 4 (11), 1301823, (2014), Link to article

  6. Collord, A.D., Xin, H., and Hillhouse, H.W., “Combinatorial Exploration of the Effects of Intrinsic and Extrinsic Defects in Cu2ZnSn(S,Se)4 IEEE Journal of Photovoltaics (2014), DOI: 10.1109/JPHOTOV.2014.23610536. Link to article

Hybrid Perovskite Solar Cells

While the term “perovskite” (named after the Russian mineralogist Lev Perovski) refers to a large class of ionic materials with ABX3 stoichiometry, the hybrid perovskites we are investigating are quite different from conventional perovskites. These are hybrid organic-inorganic materials that are composed of organic cations that are regularly interspersed in an inorganic network of corner-sharing octahedra. The material may be created from from inks formed from inexpensive compounds. The materials are polycrystalline, but their radiative efficiency is already better than silicon and almost as good as the best single crystalline gallium arsenide, which currently holds the record for the best single junction solar cell. While hybrid perovskites shared runner-up honors for Science Magazine's "Breakthrough-of-the-Year" in 2013, new chemistry and new materials processing strategies need to be developed to make the perovskites more stable and increase their bandgap so that they can be used as the top cell in tandem architecture. With support from the UW Clean Energy Institute and the U.S. Department of Energy SunShot Initiative, We are currently exploring the materials chemistry and processing of these hybrid perovskites.

Our first reports about hybrid perovskites will be presented that the Fall 2014 MRS meeting in Boston (and published in two forthcoming articles). Ian Braly will present results from combinatorial experiments that yield high quality high-bandgap perovskites, and Dr. Selin Tosun will present a new growth method that yields hybrid perovskites with significantly enhanced carrier lifetimes. Additional results will be presented at the Spring 2015 MRS Meeting in San Francisco.


Tandem Solar Cells

In order to harness the benefits from a tandem solar cell architecture, the bandgaps of the top and bottom absorbers must fall into a narrow range and be tuned to each other. Fortunately, CIGS, CZTS, and silicon are each well suited as the bottom cell material if the bandgap of the hybrid perovskite can in increased slightly. We are currently modeling and developing novel tandem cell architectures for these devices.


Photovoltaic Material Characterization

A good photovoltaic material will also be a good radiative emitter. This may seem counter-intuitive at first since radiative recombination of excited carriers is a loss mechanism. However, the mechanism is always present and it tends to be the slowest recombination pathway. Thus, if you turn off all the faster recombination pathways, you observe the photoluminescence. As a result, the magnitude and spectral character of the photoluminescence can reveal a lot about the inherent material quality and potential device performance. We are currently developing optical characterization methods that can be used to screen materials for their photovoltaic potential or monitor material and interface properties in-situ during manufacturing. Our first report on one of our methods was recently published in the Journal of Applied Physics [1]. In many PV materials, sub bandgap states alter the spectral distribution of luminescence. This has hindered previous efforts to extract the quasi-Fermi level splitting or reveal the character of the sub-bandgap states. In our recent article we develop a new and general model of direct bandgap semiconductor absorption coefficients. We then use this model with a corrected Lasher-Stern-Wurfel equation (referred to as a non-equilibrium Planck emission law by some) to model photoluminescence. We validate the model and show a tremendous range of applicability to model PL data from GaAs, CIGS, and CZTS. We use the model to extract the quasi-Fermi level splitting and characterize the sub-bandgap "tail" states. We have used this method in other papers including the combinatorial investigation of CZTS [2].

  1. Katahara, J.K. and Hillhouse, H.W., “Quasi-Fermi level splitting and Sub-bandgap Absorptivity from Semiconductor Photoluminescence” Journal of Applied Physics (2014), Link to article

  2. Collord, A.D., Xin, H., and Hillhouse, H.W., “Combinatorial Exploration of the Effects of Intrinsic and Extrinsic Defects in Cu2ZnSn(S,Se)4 IEEE Journal of Photovoltaics (2014), DOI: 10.1109/JPHOTOV.2014.23610536. Link to article

Quantum Dot and Nanowire Solar Cells

Description coming...

  • Urade, V.N., Wei, T.C., Tate, M.P., Kowalski, J.D., & Hillhouse, H.W., "Nanofabrication of double- gyroid thin films," Chemistry of Materials, 19 (4) 768-777 (2007). Link to article

  • Khlebnikov, S. & Hillhouse, H.W. “Electronic Structure of Double-Gyroid Nanostructured Semiconductors: Perspectives for Carrier Multiplication Solar Cells,” Phys. Rev. B 80, 115316 (2009). Link to article

  • McCarthy, R.F., & Hillhouse, H.W., “The Shockley-Queisser Limit and Practical Limits of Nanostructured Photovoltaics,” 38th IEEE Photovoltaic Specialists Conference (2012).

  • Hillhouse H.W. & Beard M.C., “Solar Cells from Colloidal Nanocrystals: Fundamentals, Materials, Devices, and Economics,” Current Opinion in Colloid & Interface Science, 14, 245-259 (2009). Link to article

  • Midgett, A.G., Hillhouse, H.W., Huges, B.K., Nozik, A.J., Beard, M.C., “Flowing versus Static Conditions for Measuring Multiple Exciton Generation in PbSe Quantum Dots,” J. Phys. Chem. C 114 (41), 17486-17500 (2010). Link to article

Nanostructured Thermoelectrics

Descriptions coming...

  • Hillhouse, H.W. & Tuominen, M., “Modeling the Thermoelectric Transport Properties of Nanowires Embedded in Oriented Microporous and Mesoporous Films,”Microporous and Mesoporous Mater. 47, 39-50 (2001). Link to article

Self-Assembly of Nanostructured Films

Description coming...

  • Wei, T.C. & Hillhouse, H.W., “Mass Transport and Electrode Accessibility through Periodic Self- Assembled Nanoporous Silica Thin Films” Langmuir 23, 5689-5699 (2007). Link to article

  • Urade, V.N., Wei, T.C., Tate, M.P., Kowalski, J.D., & Hillhouse, H.W., "Nanofabrication of double- gyroid thin films," Chemistry of Materials, 19 (4) 768-777 (2007). Link to article

  • Urade, V.N., Bollmann, L., Kowalski, J.D., Tate, M.P., & Hillhouse, H.W., "Controlling Interfacial Curvature in Nanoporous Silica Films Formed by Evaporation-Induced Self-Assembly from Nonionic Surfactants. I. Effect of Processing Parameters on Film Structure," Langmuir, 23 (8) 4268-4278 (2007). Link to article

  • Bollmann, L., Urade, V.N., & Hillhouse, H.W., "Controlling Interfacial Curvature in Nanoporous Silica Films Formed by Evaporation-Induced Self-Assembly from Nonionic Surfactants. I. Evolution of Nanoscale Structures in Coating Solutions," Langmuir, 23 (8) 4257-4267 (2007). Link to article

  • Tate, M.P., Urade, V.N., Gaik, S.J., Muzzillo, C.P., Hillhouse, H.W., “How to Dip-Coat or Spin-Coat Nanoporous Double-Gyroid Silica Films with EO19-PO43-EO19 Surfactant (Pluronic P84) and Know it Using a Powder X-ray Diffractometer,” Langmuir26 (6), 4357-4367 (2010). Link to article

X-ray Scattering

Description coming...

  • Tate, M.P., Urade, V.N., Kowalski, J.D., Wei, T.C., Hamilton, B.D., Eggiman, B.W., & Hillhouse, H.W., "Simulation and Interpretation of 2D Diffraction Patterns from Self-Assembled Nanostructured Films at Arbitrary Angles of Incidence: from Grazing Incidence (above the critical angle) to Transmission Perpendicular to the Substrate," Journal of Physical Chemistry B, 110 (20) 9882-9892 (2006). Link to article

  • Tate, M.P., & Hillhouse, H.W., "General Method for Simulation of 2D GISAXS Intensities for Any Nanostructured Film Using Discrete Fourier Transforms," Journal of Physical Chemistry C, 111 (21) 7645-7654 (2007). Link to article