Solar cells based on lead sulfide quantum dots (PbS QDs) represent a promising new class of solution-processable photovoltaics. The highly tunable bandgap of PbS QDs (0.5-2.1 eV) makes them ideally suited for incorporation into multijunction architectures, and their resistance to oxidation and photobleaching represent further advantages over typical solution-processed photovoltaic materials such as organic small molecules and polymers ^{[1] [2]} .

The electronic properties of QD films are determined not only by the size and composition of the QDs themselves but also by the properties of the organic ligands used to passivate the QD surfaces. As-synthesized PbS QDs are stabilized in solution by long, insulating oleic acid ligands, which are unsuitable for use in photovoltaics, where efficient electron transport is required. Therefore, a ligand-exchange treatment is typically performed, replacing the oleic acid ligands with short, bidentate molecules such as 1,2-ethanedithiol (EDT). Our research demonstrates that the power conversion efficiency of ZnO/PbS QD heterojunction photovoltaics ^[3] may be enhanced through ligand exchange with 1,3-benzenedithiol (BDT) rather than the more common EDT, despite the fact that BDT-treated films demonstrate a lower charge carrier mobility than EDT-treated films. Treatment with BDT simultaneously increases the fill factor (FF) and open-circuit voltage (V_OC) relative to treatment with EDT, thereby increasing the power conversion efficiency by a factor of 1.6, from 2.1% to 3.4%. Measurement of the transient photovoltage response of the devices indicates that this increase in FF and V_OC results from a decrease in the rate of bimolecular recombination and the concomitant increase in carrier lifetime within the QD film. This increase in carrier lifetime more than compensates for the lower mobility of BDT-treated films, emphasizing the importance of designing ligands that simultaneously decrease the inter-QD spacing and effectively passivate surface recombination sites.

1. J. Gao, J. M. Luther, O. E. Semonin, R. J. Ellingson, A. J. Nozik, and M. C. Beard, “Quantum dot size dependent J-V characteristics in heterojunction ZnO/PbS quantum dot solar cells,” Nano Letters, vol. 11, pp. 1002-1008, Feb. 2011.
2. X. Wang, G. I. Koleilat, J. Tang, H. Liu, I. J. Kramer, R. Debnath, L. Brzozowski, D. A. R. Barkhouse, L. Levina, S. Hoogland, and E. H. Sargent, “Tandem colloidal quantum dot solar cells employing a graded recombination layer,” Nature Photonics, vol. 5, pp. 480-484, Aug. 2011.
3. P. R. Brown, R. R. Lunt, N. Zhao, T. P. Osedach, D. D. Wanger, L.-Y. Chang, M. G. Bawendi, and V. Bulovic, “Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using a MoO3 interfacial layer,” Nano Letters, vol. 11, pp. 2955-2961, June 2011.

Water oxidation is the thermodynamically demanding step of the water-splitting reaction. Efficient sunlight-driven water splitting into molecular oxygen and hydrogen is a challenging prerequisite for solar-fuels production ^[1] . For this prerequisite to be achieved, two major requirements are necessary: (1) efficient photon-to-charge conversion, with efficient utilization of the solar spectrum, and (2) a lowering of the energetic barriers for the four-proton, four-electron proton-coupled electron transfer (PCET) reaction of water splitting. The first requirement can be addressed by using semiconducting materials that absorb well into the visible range, such as cadmium chalcogenides. However, many of these materials including CdSe are unstable and oxidize rapidly under the aqueous conditions for water oxidation, rendering them unusable for water splitting purposes. The second requirement of lowering energetic barriers to water oxidation has been researched extensively in the last several decades to yield different forms of catalysts. One such catalyst is the cobalt-based water oxidation catalyst (CoPi) ^[2] . CoPi can be formed either via electrochemical deposition from Co²⁺ ions in aqueous solutions containing potassium phosphate (KPi) or by processing of supported thin-film (800 nm thick) cobalt metal anodes in KPi at pH 7 ^[3] .

We demonstrate the dual benefit gained by using thin-film cobalt metal as the precursor in the preparation of CoPi on CdSe photoanodes. First, the cobalt layer protects the underlying semiconductor from oxidation and degradation in the aqueous solution. This process is followed by the advantageous incorporation of the CdSe layer into the CoPi layer during continued processing of the electrode. The resulting hybrid material forms a stable photoactive anode for light-assisted water oxidation. Figure 1 shows the high stability and reproducibility of currents through the hybrid material under demanding conditions. The benefits of the photoactive CdSe component are demonstrated in Figure 2, which presents the shift in catalytic onset under illumination.

1. N. S. Lewis and D. G. Nocera, “Powering the planet: Chemical challenges in solar energy utilization,” Proc. Nat’l. Acad. Sci. USA, , vol. 103, pp. 15729-15735, Oct. 2006.
2. M. W. Kanan and D. G. Nocera, “In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+,” Science, vol. 321, pp. 1072-1075, Aug. 2008.
3. E. R. Young, D. G. Nocera and V. Bulović, “Direct formation of a water oxidation catalyst from thin-film cobalt,” Energy Environ. Sci., vol. 3, pp. 1726-1728, Nov. 2010.

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.

Disordered semiconductors exhibit poor electronic transport properties due to their amorphous nature. Low carrier mobility and lifetime limits the diffusion length to 100 ~ 300 nm. Thus, conventional p-n junction photovoltaic design that is composed of a large quasi-neutral region and a small depletion region (Figure 1a) leads to poor charge extraction. To overcome this material limit, a p-i-n junction has been adopted in amorphous silicon solar cells. The internal electric field extends throughout the intrinsic absorber layer and assists carriers to be efficiently extracted to their respective electrodes (Figure 1b). Films composed of quantum dots (QD) show electronic transport properties similar to disordered semiconductors and may also benefit from the p-i-n “drift” device architecture, which has not been applied to QD solar cells to date.

This project aims to implement two major advantages of the p-i-n heterojunction using the QD as an intrinsic absorber layer. The first goal is to augment the width of the depletion region. This widening enhances the light absorbance of the device, allowing more photogenerated carriers and thereby increasing short-circuit current (J_sc). The second advantage to exploit is the ability to dope n- and p-type layers without having to affect the intrinsic absorber layer. Increasing the Fermi level difference by doping both window layers would increase the build-in electric field, enhancing open-circuit voltage (V_oc). Figure 1c and d show schematic illustrations of the device structure and energy diagram, respectively. Figure 2a and b show the device characteristic studied for each separate junction. Rectifying behavior is observed in both p-i, i-n and p-i-n junctions. Figure 2d and e show the JV curve under illumination with and without the intrinsic QD absorber layer. Future studies will focus on V_oc, J_sc, and fill factor (FF) as a function of p-type and n-type layer doping as well as intrinsic layer thickness.

1. M. Zeman, “Solar Cells,” TU Delft OpenCourseWare, Delft University of Technology, 2011. [Online]. Available: http://ocw.tudelft.nl/courses/microelectronics/solar-cells/readings/

Colloidal lead sulfide quantum dot (PbS QD) solar cells have recently shown attractive improvements in efficiency [1]. PbS QDs have been investigated in various device architectures—including Schottky junction, depleted planar heterojunction, and ordered bulk heterojunction photovoltaic (PV) architectures—with the aim of maximizing light absorption while maintaining efficient charge transport ^[1] . However, the thickness of the PbS QD layer in the aforementioned architectures remains limited by the relatively short exciton diffusion length characteristic of all QD films.  We developed a novel architecture in which PbS QDs and zinc oxide (ZnO) nanoparticles are co-deposited from solution to form a bulk heterojunction (BHJ). Much like a typical polymer BHJ solar cell, these PbS QD PV devices can decouple the light absorption, which is proportional to device thickness, from the exciton diffusion length ^[2] . We fabricated these BHJ PbS QD:ZnO solar cells in solution by layer-by-layer deposition in which each step is followed by ligand exchange as shown in Figure 1a. By conducting preliminary optimization of the processing condition, we obtained the current density as a function of the applied voltage graph shown in Figure 1b. Further studies aim at improving device performance while studying the morphological conformation of PbS and ZnO domains.

Figure 1: a) Schematic of the solution process for fabrication of bulk heterojunction lead sulfide quantum dots (PbS QD) and zinc oxide (ZnO) solar cells. The devices are fabricated by layer-by-layer deposition in which each step is followed by ligand exchange. b) JV characteristics under dark (squares) and AM 1.5 illumination (circles) for planar (control – red) and bulk heterojunction (blue) PbS QD:ZnO devices.

1. J. Tang and E. H. Sargent, “Infrared colloidal quantum dots for photovoltaics: Fundamentals and recent progress,” Advanced Materials, vol. 23, no. 1, pp. 12–29, Jan. 2011.
2. C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. J. Jia, and S. P. Williams, “Polymer-fullerene bulk-heterojunction solar cells,” Advanced Materials, vol. 22, pp. 3839–3856, Aug. 2010.

Nanostructured solar cells are attracting increasing attention as a promising photovoltaic (PV) technology ^[1] . Generation of free charge carriers in nanostructured PV devices occurs at the electron donor-acceptor interface, analogous to the pn-junction interface in traditional crystalline silicon solar cells. However, recombination at this interface constitutes one of the major charge carrier loss pathways. Thus characterizing and controlling recombination dynamics is critical for informing the design of novel device architectures. Recombination parameters also enable comparisons between different device architectures.

In this work, we employ the transient photovoltage (TPV) technique ^[2] to probe recombination mechanisms under standard operating conditions in three different solar cells, as shown in Figure 1: a poly(3-hexylthiophene) and phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction; a chloroaluminium phthalocyanine and fullerene (ClAlPc:C₆₀) planar mixed heterojunction; and a lead sulfide quantum dot and zinc oxide (QD PbS:ZnO) pn-heterojunction. The normalized TPV data acquired at 0.5-sun illumination intensity are shown in Figure 2a, which compares the recombination lifetimes of charge carriers in these devices. The observed differences in carrier lifetimes may arise from variations in the respective interface morphologies: for example, the slower recombination transients observed in the ClAlPc:C₆₀ device may be attributed to the intrinsic planarity of this particular architecture.  We can also measure the charge carrier lifetime as a function of the light intensity, as shown in Figure 2b; this result confirms that recombination dynamics are faster in P3HT:PCBM and QD PbS:ZnO than in ClAlPc:C₆₀ PV devices.

1. Anonymous, “A sunny outlook,” Nature Photonics, vol. 6, no. 3, p. 129, Mar. 2012.
2. C. G. Shuttle, B. O’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, J. de Mello, and J. R. Durrant, “Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell,” Applied Physics Letters, vol. 92, p. 3, 2008.

It is desirable to extend the functionality of MEMS to different form factors including large-area arrays of sensors and actuators, and to various substrate materials, by developing a means to fabricate large-area suspended thin films. Conventional photolithography-based MEMS fabrication methods limit the device array size and are incompatible with flexible polymeric substrates.

A new method for additive fabrication of thin (125±15-nm-thick) gold membranes on cavity-patterned silicon dioxide substrates using contact-transfer printing is presented for MEMS applications. The deflection of these membranes, suspended over cavities in a silicon dioxide dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. The fabrication process employs a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto the substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactive-ion etching, which in turn removes the need for etch-stops and wafer bonding. Membranes up to 0.78 mm² in area are fabricated, and their deflection is measured using optical interferometry. The membranes have a maximum deflection of about 150 nm across 28-μm-diameter cavities, as shown in Figure 1. Using the membrane deflection data, Young’s modulus of these gold films is extracted (74±17 GPa), and it is comparable to that of bulk gold. Additionally, a 15 Hz sinusoidally varying voltage of 15 V peak-to-peak amplitude is applied to the MEMS device to demonstrate that the large membrane deflection is a repeatable deflection (Figure 2).

These films can be utilized in microspeakers, pressure sensors, microphones, deformable mirrors, tunable optical cavities, and  large-area arrays of these devices.

1. A. Murarka, C. Packard, F. Yaul, J. Lang, and V. Bulovic, “Micro-contact printed MEMS,” IEEE 24th International Conference on Micro Electro Mechanical Systems (MEMS), 2011, pp. 292-295.
2. C. Packard, A. Murarka, E. W. Lam, M. A. Schmidt, and V. Bulovic, “Contact-printed microelectromechanical systems,” Advanced Materials, vol. 22, pp. 1840–1844, 2010.
3. A. Murarka, S. Paydavosi, T. L. Andrew, A. I. Wang, J. H. Lang, and V. Bulovic, “Printed MEMS membranes on silicon,” IEEE 25th International Conference on Micro Electro Mechanical Systems (MEMS), 2012, pp. 309-312.

Due to the remarkable physical properties of graphene, applications in various areas such as transistors ^[1] , chemical sensors ^[2] , and logic devices ^[3] have been explored; a variety of proof-of-concept devices have been demonstrated.  In this work, organic photovoltaics (OPV) with graphene electrodes are constructed so that the effects of graphene morphology (Figure 1), hole transporting layers (HTL) (Figure 2), and counter electrodes are presented.  One of the challenges in the integration of graphene in OPV is the incompatibility between the graphene electrode and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layer (HTL) which significantly increases the device failure rate ^[4] .  When hydrophilic PEDOT:PSS is spin-coated onto graphene, it is difficult to achieve uniform and conformal coating due to the hydrophobic nature of the graphene surface, i.e., lower surface free energy.  Instead of the conventional PEDOT:PSS HTL, an alternative transition metal oxide HTL (molybdenum oxide (MoO₃)) is investigated to address the issue of surface immiscibility between graphene and PEDOT:PSS.  The graphene films considered here are synthesized via low-pressure chemical vapor deposition (LPCVD) using a copper catalyst, and experimental issues concerning the transfer of synthesized graphene onto the substrates of OPV are discussed.  The morphology of the graphene electrode and HTL wettability on the graphene surface are shown to play important roles in the successful integration of graphene films into the OPV devices.  The effect of various cathodes on the device performance is also studied.  These factors (i.e., suitable HTL, graphene surface morphology and residues, and the choice of well-matching counter electrodes) will provide better understanding for utilizing graphene films as transparent conducting electrodes in future solar cell applications.

1. Y. M. Lin, K. A. Jenkins, A. Valdes-Garcia, J. P. Small, D. B. Farmer, and P. Avouris, “Operation of graphene transistors at gigahertz Frequencies,” Nano Letters, vol. 9, no. 1, pp. 422-426, 2009.
2. P. K. Ang, W. Chen, A. T. S. Wee, and K. P. Loh “Solution-gated epitaxial graphene as pH sensor,” Journal of the American Chemical Society, vol. 130, no. 44, pp. 14392-14393, 2008.
3. R. Sordan, F. Traversi, and V. Russo, “Logic gates with a single graphene transistor,” Applied Physics Letters, vol. 94, no. 7, p. 073305, 2009.
4. H. Park, J. A. Rowehl, K. K. Kim, V. Bulović, and J. Kong, “Doped graphene electrodes for organic solar cells,” Nanotechnology, vol. 21, no. 50, p. 505204, 2010.

Polymer materials doped with conductive particles exhibit piezoresistive properties. These materials are fabricated such that their conductivity changes with an applied compressive force. When compressed, the formation of percolation pathways allows increased electrical conduction through tunneling between the particles. This work explores and utilizes this property of composites to fabricate various devices with the ultimate goal of developing integrated flexible systems resembling sensory skins.

As a first generation of piezoresistive devices, a squeezable switch (squitch) is fabricated with a three-terminal configuration shown in Figure 1 ^[1] . In this study, the squitch is fabricated from a composite of polydimethylsiloxane doped with 60 wt% Ni microparticles that shows more than 5 orders of magnitude change in conductivity over a 20% strain (Figure 2). In the absence of an applied gate bias, the composite is a poor conductor. An applied gate voltage generates an electrostatic force between the source and the gate that compresses the composite, causing the squitch to conduct. To allow fabrication of reliable and reproducible devices, the composite needs to be engineered such that its mechanical properties are more stable. To achieve this goal, current research explores the effects of the type of polymer and conductive particles and the method of fabrication on the properties of the nanocomposite and performance of the squitch. The surfaces of the metal particles are chemically treated to allow better distribution in the polymer matrix while also chemically binding the particles to the polymer preventing particle migration over repeated use of the device. After the composite is optimized, future work will involve extending the squitch design to fabricate devices such as analog amplifiers, digital inverters, and various sensors and developing processes to allow large-area fabrication. The devices will then be integrated to develop artificial skins.

1. S. Paydavosi, F. Yaul, A. Wang, F. Niroui, V. Bulovic, J. H. Lang, “MEMS switches employing active metal-polymer nanocomposites,” in IEEE MEMS International Conference, 2012, pp. 180-183.

Nano-electromechanical systems (NEMS) are an emerging area of research with potential applications as low-power switches for electronic circuits. The proliferation of electronics in both stationary and portable applications demands the development of more energy-efficient devices than are currently available. While solid-state silicon MOS-based transistor circuits, the dominant technology in today’s electronics, have greatly reduced their power requirements by aggressive scaling, the concurrent increase in off-state leakage current limits their energy efficiency. In contrast, microelectromechanical relays have been demonstrated with zero off-state currents and abrupt switching characteristics ^{[1] [2]} . As these and other electromechanical devices are shrunk to the nanoscale, their actuation voltages, and hence power requirements, are expected to be reduced significantly.

Our group recently presented a three-terminal electromechanical switch based on a piezoresistive polymer nanocomposite as the active material ^[3] . The metal-polymer composite consisted of a polydimethylsiloxane polymer matrix doped with 60 wt% nickel particles. A schematic diagram of this squeezable switch, or “squitch,” is shown in Figure 1. In its initial state, the conductive metal particles are separated by the insulating polymer matrix. Thus, the active material is highly resistive, and little current flows through the device (in the “off” state). When compressed, the metal-metal distances decrease until the onset of tunneling allows current to flow from source to drain (“on” state). The first-generation squitch demonstrated transistor-like behavior with drain-source conduction modulation over 4 orders of magnitude when electromechanical force was applied. However, the large mechanical dimensions of this concept demonstration necessitated higher supply voltages than desired. Our current work focuses on incorporating the squitch concept into nanoscale devices by (a) developing improved device structures and fabrication methods and (b) exploring new materials such as ligand-coated nanoparticles and self-assembled monolayers as active materials.

Figure 1: Schematic diagram of squitch. Applying a voltage bias between gate and source generates an electrostatic force that compresses the active material and allows carriers to tunnel from source to drain. When the voltage bias is removed, the active material acts as a mechanical spring and recovers from its compressed state to cut off current flow through the device.

1. M. Spencer, F. Chen, C. C. Wang, R. Nathanael, H. Fariborzi, A. Gupta, K. Hei, V. Pott, J. Jaeseok, L. T.-J. King, D. Markovic, E. Alon, and V. Stojanovic, “Demonstration of integrated micro-electro-mechanical relay circuits for VLSI applications,” IEEE Journal of Solid-State Circuits, vol. 46, pp. 308-320, January 2011.
2. R. Parsa, M. Shavezipur, W. S. Lee, S. Chong, D. Lee, H. S. P. Wong, R. Maboudian, and R. T. Howe, “Nanoelectromechanical relays with decoupled electrode and suspension,” in IEEE 24th International Conference on Micro Electro Mechanical Systems, 2011, pp. 1361-1364.
3. S. Paydavosi, F. M. Yaul, A. I. Wang, F. Niroui, T. L. Andrew, V. Bulovic, and J. H. Lang, “MEMS switches employing active metal-polymer nanocomposites,” in IEEE 25th International Conference on Micro Electro Mechanical Systems, 2012, pp. 180-183.