Prospects of Colloidal Nanocrystals for Electronic - Computer Science

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430 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al. Figure 44. (a) Solar cells based on hyperbranched nanocrystals blended with conductive polymer (P3HT). The blend morphology is determined by the 3D structure of the nanocrystals, eliminating macroscopic phase separation. The electron transport is enhanced due to efficient bridging of electrodes by large hyperbranched nanocrystals. (b,c) TEM images showing the morphology of CdSe and CdTe hyperbranched nanocrystals, respectively. Scale bars are 100 nm. Reprinted with permission from ref 396. Copyright 2007 American Chemical Society. be properly addressed in future research. Also, the nanoscale morphology of NC-polymer blends and the ways to tailor it have to be studied in great detail. Furthermore, the optimization of device performance will require accurate determination of the absolute positions of NC energy levels. The effects of surface ligands and dielectric environment on the absolute energies of quantum confined states in semiconductor NC have to be taken into account. 364 7.3.3. Colloidal Nanocrystals for All-Inorganic Solar Cells The weakest link of polymer-based solar cells is rather low stability of conductive polymers against oxidation and photodegradation, which should also affect the solar cells utilizing NC-polymer blends. All-inorganic solar cells made of colloidal NCs could avoid unstable organic components while keeping the benefits of solution-based device fabrication. For example, CdSe and CdTe NCs can create the donor-acceptor pairs with a staggered band alignment, similar to that at CdSe/P3HT interface (Figure 45a). Gur et al. fabricated a photovoltaic cell with spin-coated layers of pyridine-treated CdTe and CdSe nanorods (Figure 45b,c). 168 Under illumination, photogenerated electrons transferred to the CdSe phase, while holes found lower energy states in CdTe phase. The difference in work functions of ITO and Al electrodes provided additional driving force for charge separation. Sintering of the nanorods significantly improved carrier mobility and the device performance. With ISC )13.2 mA/cm 2 , Voc ) 0.45 V, and FF ) 0.49, solution-processed solar cells demonstrated power conversion efficiency approaching 3% and impressive stability in air (Figure 45d,e). In similar device configuration with Cu2S NCs and CdS nanorods, Wu et al. demonstrated a solar conversion efficiency of 1.6%. 464 Unlike hybrid solar cells, blending of Figure 45. (A) An energy diagram of the conduction and valence band levels in the CdTe/CdSe system. (B,C) TEM images of CdSe and CdTe nanorods, respectively, used for fabrication of allinorganic nanocrystal solar cell. Scale bars are 40 nm. (D) Incident photon to current efficiency spectrum for a sintered device composed of the layers of CdSe and CdTe nanorods. (E) Photocurrent density-voltage characteristic for the device exhibiting 2.9% power conversion efficiency under AM1.5 illumination. Reprinted with permission from ref 168. Copyright 2005 American Association for Advancement of Science. CdTe and CdSe rods into a single active layer showed much poorer performance, presumably because of shortcuts and insufficient driving force for carrier separation. Bulk I-III-VI2 chalcopyrite compounds are known as efficient photovoltaic materials. 493 Particularly, solar cells based on copper indium-gallium selenide (CIGS) routinely demonstrate >15% solar conversion efficiency and are used for a wide range of commercial applications. 494,495 Importantly, CIGS has good environmental prospects because it is free of highly toxic elements. However, conventional fabrication of CIGS modules involves costly and elaborated techniques for controlling the stoichiometry of quaternary CIGS phase. High-quality CIGS films are typically prepared by evaporation of In, Cu, Ga, followed by selenization process by Se vapors or toxic H2Se gas. Solution processed solar cells using hydrazine-based CIGS precursor demonstrated promising solar conversion efficiencies of 10.3% under AM1.5 illumination. 496 Inspired by the reports on bulk I-III-VI2 compounds, several research groups dedicate significant efforts to explore colloidal synthesis of CuInS2, 497-502 CuInSe2, 465,498,500,502 and CIGS 502-504 NCs. In several recent reports, these NCs were tested in prototype solar cells, typically in a standard device configuration of glass/Mo/ (p-type I-III-VI2 semiconductor)/CdS/ZnO/ITO. Efficiencies of ∼3% were achieved using sintered CuInSe2 NCs (Figure 46). 465 Sargent and Nozik groups recently reported a number of encouraging papers describing the fabrication and operational principles of Schottky photovoltaic devices using PbS 253,270,272,505,506 and PbSe 253,271,507 NC solids. The Schottky cells reported by these groups had a simple and essentially similar configuration: NC solids were solution-deposited on ITO-coated glass followed by evaporation of top metal contact (Mg, Al, Ca/Al). The NC films were deposited using either layer-by-layer process in which NC monolayers were cross-linked by short-chain dithiol molecules or by spin-
Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 431 Figure 46. Solar cells using sintered CoInSe2 NCs. (a) Crosssectional SEM image of ∼1.5 µm thick active layer spin-coated from CoInSe2 NC ink. (b) Same film after annealing under Se vapors forms large CIS crystallites. (c) A photograph and (d) solar cell characteristics for the complete device. Reprinted with permission from ref 465. Copyright 2008 American Chemical Society. casting followed by dipping into a surfactant solution (butylamine, ethanedithiol, benzenedithiol, etc.) to exchange original bulky surface ligands with smaller molecules. Figure 47 illustrates the basics of Schottky solar cell. As described in section 6.2, pinning of the Fermi level at the NC-metal interface creates a built-in field that separates photogenetared charge carriers. Electrons are collected at the metal contact, while holes escape into ITO. The performance of the Schottky cell is affected by the built-in potential, the width of the depletion region, and the transport of electron and holes through the NC solid. Most of these parameters were demonstrated to be tailorable by adjusting the morphology, thickness, and chemical treatments of NC solids. Presently, reported power conversion efficiencies for PbSe-based device are 3.6% (monochromatic 975 nm illumination, 12 mW/ cm 2 ), 253 1.1% (AM1.5 illumination), 253 and 2.1% (AM1.5 illumination). 271 Devices based on PbS NCs showed efficiencies of 4.2% (monochromatic 975 nm illumination, 12 mW/ cm 2 ) and 1.8% (AM1.5 illumination). 272 7.3.4. Nanocrystals in Dye-Sensitized Solar Cells The dye-sensitized solar cells (DSSC), also known as Grätzel cells, include the nanostructured TiO2 electrode covered with molecular sensitizer such as ruthenium(II) bipyridine compexes, the electrolyte redox couple, typically I - /I3 - , dissolved in an organic solvent, and the metal counter electrode (Figure 48). 508 The role of synthesizer is to absorb visible light and inject photogenerated electrons into the conduction band of TiO2. Next, positively charged synthesizer molecules electrochemically oxidize I - ions to I3 - in solution. The latter is reduced to I - at the counter electrode, closing the circle. As a result, the cell does not undergo any permanent chemical changes but photoelectrochemically converts the light energy into electrical power. To increase the area of the TiO2-electrolyte interface, the nanoporous TiO2 electrodes obtained by sintering anataze nanoparticles are used. The highest reported power conversion efficiencies of DSSC are in the impressive range of 10-11%. 509 However, this technology has several limitations. Ruthenium used in sensitizer is a precious metal, and its supply is limited. The use of liquid electrolyte is another serious drawback of DSSC due to the need for elaborate encapsulation techniques, thermal stability problems, electrolyte freezing at low temperatures, thermal expansion and evaporation at higher temperatures, etc. As a possible solution, nonvolatile ionic liquids as electrolytes were tested demonstrating power conversion efficiency of 7%. 510,511 Recently, the higher solar conversion efficiency of 8.2% was obtained using eutectic imidazolium salt melts as the electrolyte. 512 Another approach to solid-state DSSC includes the use of various solid-state hole conducting materials such as CuI (reported 4.5% solar conversion efficiency) 513 or small organic molecules 514 in place of electrolyte. Semiconductor NCs are considered as a possible replacement for ruthenium complexes in DSSC. Low cost, solution processing, and size-tunable energy levels make semiconduc- Figure 47. (a) Sketch of a Schottky solar cell using a close-packed film of PbSe nanocrystals and (b) the energy diagram showing the Schottky barrier formed at the metal/semiconductor interface. The majority carriers are separated in the depletion region and diffuse through the quasi-neutral region to the negative electrode. (c) Device characteristics under AM1.5 illumination. Reprinted with permission from ref 253. Copyright 2008 American Chemical Society.
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