Prospects of Colloidal Nanocrystals for Electronic - Computer Science

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422 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al. Figure 37. Structure of an all-inorganic QD-LED. (a) Schematics of the device structure showing the ITO anode, NiO hole transport layer, QDs active luminescent layer composed of Cd1-xZnxSe nanocrystals, ZnO:SnO2 electron transport layer, and silver cathode. (b) A band diagram determined from UV photoemission spectroscopy and optical absorption measurements, giving the approximate electron affinities and ionization energies of the QD-LED materials. Reprinted with permission from ref 266. Copyright 2007 Nature Publishing Group. highly efficient GaN LED and converts it to another wavelength of choice. Taking into account the outstanding characteristics of III-V LEDs, the overall performance of such tandem device can be superior to an LED based on direct injection of carrier into the NCs. Lee et al. combined GaN LED with CdSe/ZnS NC-polymer composites. 417 Roither et al. recently reported colloidal CdSe/ZnS NCs operating as color converters for InGaN LEDs with high color stability. The color conversion was further enhanced using dielectric mirrors with high reflectivity at the emission band of NCs. 418 In 2008, Evident Technologies, Inc. released Christmas lights using III-V LEDs and luminescent NCs as the down converters. These Christmas lights were probably the first commercial optoelectronic devices utilizing colloidal quantum dot technology. Klimov et al. studied an optically pumped device with a monolayer of CdSe/ZnS NCs on top of the InGaN quantum well. 419 Direct nonradiative Förster-type energy transfer from InGaN quantum wells to the NCs resulted in an impressive ∼55% light conversion efficiency; the energy transfer rate was sufficiently high to provide noncontact pumping into NCs from the quantum wells. The LEDs with a single monolayer of CdSe NCs deposited on top of the electrically driven InGaN/GaN quantum well structure demonstrated nonradiative energy transfer from quantum wells into the QDs. 420 The color conversion efficiency was 13%, and the overall efficiency of QD-based hybrid LED was higher than that of III-V LEDs with conventional down-converting phosphor or stand-alone QD-LEDs. Mueller et al. also reported LEDs with semiconductor NCs directly integrated into the p-n junction formed by GaN injection layers. 406 The NCs/GaN hybrid structure was fabricated by encapsulation of CdSe/ZnS NCs within the GaN matrix. The electrical pumping led to emission exclusively from the NCs with a turn-on voltage of 3.5 V. Also, the EL spectra could be tuned by varying the NC diameter. Along the same lines, Chen et al. reported white light generation from red emitting CdSe- ZnS QDs deposited on top of a blue/green InGaN/GaN quantum well LED. 421 Nizamoglu et al. recently reported white light generation controlled by multicolor-emitting CdS/ ZnS/CdS NCs combined with InGaN/GaN quantum well LEDs. 418 After about 15 years of research and development, NC LEDs are approaching commercialization stage; they can now compete with organic-LEDs and other emerging display and solid-state lighting technologies. Commercial perspectives of QD-LEDs will depend on their ability to compete with OLEDs in performance, cost, lifetime, manufacturability, etc. One of the serious obstacles for using QD LEDs in consumer products is the toxicity of Cd-based NCs. Serious attempts are made to address this issue by developing highly luminescent NCs not containing toxic elements, for example, InP/ZnS core-shells 422,423 or CuInSe2. 397,424 Another promising avenue for QD LEDs is the near-infrared (near-IR) emitting devices based on narrow gap semiconductor NCs, such as InAs, 425,426 PbSe, 31,427 and HgTe. 428 Development of inexpensive large area near-IR LEDs addresses the needs of optical communications, chemical spectroscopy, and chemical sensing; such devices are considered for on-chip integration for optoelectronic circuits, etc. Several research groups recently reported solution-processed LED with the emission picked at telecommunication wavelengths of 1.3 and 1.55 µm. 425,429 There are several recent review articles covering this interesting topic. 425,430,431 7.2. Photodetectors There is a large variety of sensitive photon detection systems operating in the visible spectral range: photomultiplier tubes, single crystal silicon detectors, and CCD cameras. Unfortunately, in the infrared the situation is not nearly as good; available detection systems, especially arraybased, are either insensitive or very expensive. The reason is simple; silicon, which is a main workhorse for CCD and APD technologies, cannot operate beyond 1.1 µm, whereas other materials show higher noise levels or are difficult to process using standard single-crystal-based microfabrication techniques. At the same time, markets for near-IR and mid- IR detectors span from telecommunication to night-vision systems, bioimaging, environmental sensing, spectroscopy, and chemical analysis. For all of those areas, it is highly desirable to find new materials that enable high detectivity at a reasonable cost. Recent developments provide good expectations for photodetectors based on NC solids. 270 Application of NCs for photon detection is receiving steadily growing attention. The progress in this direction is driven by the unique opportunities given by the size-tunable NC electronic structure, NC surface chemistry, and surface trap engineering, compositional flexibility, and the possibility to manufacture devices using inexpensive solution processing. Relatively wide band gaps of conductive organic polymers and small molecules limit their absorption to visible spectral range. For applications requiring light absorption/ emission in the near-IR region, inorganic NCs, especially those made of narrow-gap PbS, PbSe, PbTe, HgTe, InAs, and InSb semiconductors, are in strong position to compete with other technologies, because their band gap can be precisely tuned from the visible spectral region upto the wavelengths of 3500 nm. For more detail on the synthesis and optical properties of IR-active NCs, we refer the readers to a recent review by Rogach et al. 425 7.2.1. Figures of Merit for Photoconductive Detectors Photoconductive detectors change their electrical conductivity under illumination due to the generation of additional mobile charge carriers. Other types of IR photodetectors (bolometers and pyrometers) exploit thermal effects and will not be discussed here. The photoconductivity can be observed
Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 423 in virtually any semiconductor; however, development of a photoconductor with characteristics suitable for a particular application is a big scientific and technological challenge. There are several figures of merits used to characterize photoconductive photodetectors. 432,433 These are responsivity, spectral response, noise-equivalent power (NEP), detectivity, response time, and frequency response. Responsivity (Ri). This parameter is also often called sensitivity. The responsivity provides a quantitative measure for the output signal such as photocurrent iph per watt of the input optical power Pin. Ri is the function of both modulation frequency (f) and photon wavelength (λ): R i (f, λ) ) i ph P in Spectral response describes the spectral dependence of Ri, that is, the dependence of Ri versus λ. Typically, responsivity of IR detectors rises monotonically with increasing wavelength, then peaks and drops to zero upon reaching semiconductor band gap energy (so-called cutoff wavelength of intrinsic photodetector). In NC photodetectors, the spectral response generally follows the shape of the NC absorption spectrum. Noise-Equivalent-Power. The internal noise current in is the main factor limiting the ability to detect small optical signals because the signal produced by the input power must be above the noise level. The noise-equivalent-power (NEP) is determined as a light power that yields a signal-to-noise ratio equal to 1 and can be expressed as: NEP ) i n R i Detectivity. For comparison of different devices, it is important to provide an area independent figure of merit D* (detectivity): D*(λ, f) ) R i √A∆f i n where A is the detector area. The detectivity D* is the rootmean-square signal-to-noise output when 1 W of monochromatic radiant flux modulated at frequency f is incident on 1 cm 2 detector area, with a noise-equivalent bandwidth of 1 Hz. D* is typically reported in Jones (1 Jones ) 1cmHz 1/2 W -1 ). Response time (t). Response time, also known as the time constant, is calculated as follows: t ) 1 2πf 3db (32) (33) where f3db is the frequency at which the signal power is 3 dB lower than the value at zero frequency; that is, the photocurrent is 1/�2 ≈ 0.707 of that at zero frequency. Frequency Response. At low modulation frequencies, the photocurrent can follow the rise/decay of the illumination intensity. However, at higher frequencies, responsivity becomes a strong function of the response time: R 0 R(f) ) √1 + (2πft) 2 (34) (35) (36) where R0 is the responsivity at zero frequency, and t is the response time. Other important characteristics of photodetectors are the linear dynamic range (the range over which detector responds linearly to the incident optical power) and the noise spectrum (i.e., the dependence of in vs f). 7.2.2. Photoconductivity in Nanocrystal Solids A basic structure of the photoconductive detector is depicted in Figure 38. Ohmic contacts are attached to a slab or a thin film of a semiconductor. 434 For the latter case, smaller electrode spacings are accessible, favorable for efficient collection of photogenerated charges. For NC-based photodetectors, thin film geometry is preferred because homogeneous submicrometer-thick NC films can be readily prepared from colloidal solutions. Furthermore, it is a common practice to use a substrate with lithographically defined electrodes and deposit a layer of NCs on top of the electrodes (Figure 38b). The photocurrent in a semiconductor can be generally described as: i ph ) ηeN λ G i (37) where η is the quantum efficiency (i.e., the number of excess carriers produced per absorbed photon), e is the elemental charge, Nλ is the number of photons of wavelength λ absorbed in the sample per unit time, and Gi is the internal (photoconductive) gain. Figure 38. (a) A sketch of typical thin film photoconductive photodetector. Interdigitate electrode structure is deposited onto the surface of active semiconductor. (b) A photodetector geometry that is typically used for NC-based devices: thin NC layer is deposited on top of the prepatterned electrode structure. Au is shown as an example of metal contact material.
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