Prospects of Colloidal Nanocrystals for Electronic - Computer Science

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418 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al. Among the benefits provided by this class of electronic materials, there are properties inherent to nanoscale materials such as size-dependent electronic structure, charging energy, melting temperature, and other parameters that can be finetuned by varying NC size. For example, superior luminescent properties of semiconductor NCs (size-tunable color, narrow emission band, almost 100% luminescence quantum efficiency, high stability, etc.) have been utilized in lightemitting devices (LEDs). In addition, colloidal NCs offer opportunities for inexpensive device fabrication by solution-based techniques like spin-coating, dip-coating, inkjet printing, etc., and can be used in roll-to-roll processing. As a result, colloidal NCs are condidered as a very promising class of materials for large area applications, such as thin film solar cells and for the applications where low fabrication cost is very important. Along these lines, solution-cast NC field-effect transistors (FETs) show respectable carrier mobilities, which compare favorably to the best devices made from organic electronic materials. 23,24,87,386,393 Several interesting concepts have been proposed for NC-based solar cells. 271,394-397 The progress in developments of commercially successful NC devices requires collaborative efforts of chemists synthesizing new materials, scientists studying materials properties, engineers working on device architecture, and technologists involved in the process of device fabrication. In this section, we provide an overview of state-of-the-art for several classes of applications utilizing colloidal NCs. 7.1. Light-Emitting Devices (LEDs) Colloidal NCs have been explored as the emitters for thin film light-emitting diodes (LEDs). 398-407 In a typical LED, a thin layer of light-emitting NCs, for example, CdSe/ZnS core-shells or Cd1-xZnxSe (recombination layer), is sandwiched between the hole transport layer (HTL) and electron transport layer (ETL), which provide injection of carriers into the NCs (Figure 34). The performance of NCs-based LEDs has remarkably improved over the past decade. 404,408,409 Among the strong points of NC-based LEDs are their high color purity (i.e., narrow emission band) and tunability of the emission color from UV to near-IR by simply varying the NC size. 410 Because size-dependent emission of semiconductor NCs is well described by the quantum dot (QD) formalism, the NC-based LEDs are often named “QD- LEDs”. To be fully competitive with other emerging Figure 34. Schematic diagram and a typical structure of a thin film LED utilizing semiconductor NCs. technologies such as organic LEDs, both brightness and especially lifetime of QD-LEDs have to be considerably improved. 411 The room for such improvements is associated with the higher stability of core-shell NCs and electronand hole-transport layers, better understanding and controlling chemical and physical phenomena at the NC-organics interfaces, optimization of the energy transfer, and carrier injection from organic molecules into the NCs. In this section, we introduce the basic principles of QD-LED operation and review recent progress in NC-based LEDs. 7.1.1. LED Performance Characteristics Internal Quantum Efficiency (ηint). Not every electron– hole pair injected into LED recombines radiatively. Nonequilibrium carriers can recombine radiatively or nonradiatively depending on the available local recombination pathways. The internal quantum efficiency (ηint) can be expressed as ηint ) (number of photons generated per second) (number of electrons injected into LED per second) ) (Pint /hν) (26) (I/e) where Pint is the optical power emitted from the active region, and I is the injection current. Light Extraction Efficiency (ηextraction). Photons emitted by the active region should escape from the LED. In an ideal LED, all photons emitted by the active region are emitted into free space. However, in a real device, not all emitted photons can escape due to reabsorption or internal reflections. The light extraction efficiency is defined as η extraction ) (number of photons emitted into free space per second) (number of photons generated per second) where P is the optical power emitted into free space. External Quantum Efficiency (ηext). A very important metric of an LED is the external quantum efficiency, which is the ratio of the number of photons emitted into free space to the number of injected carriers. ηext can be defined as The power efficiency is defined as (P/hν) (P int /hν) where I is the current flowing through the LED and V is the applied voltage. The power efficiency is comparable with ηexternal only if the device operational voltage is approximately equal to the energy gap of semiconductor. 411 Eye Sensitivity and Brightness. The physical properties such as the number of photons, photon energy, and optical ) (27) η ext ) (number of emitted photons per second) (number of electrons injected into LED per second) ) (P/hν) (I/e) ) η int × η extraction (28) η power ) P IV ) number of photons emitted externally × hν IV (29)
Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 419 Table 3. Typical Values for Luminance of Displays, Organic LEDs, and Inorganic LEDs411 device luminance (cd/m2 ) display 100 (operation) display 250-750 (max. value) III-V LED 106-107 organic LED 100-10 000 power of electromagnetic radiation are characterized by radiometric units. However, response of human eye (luminous efficiency) is limited to the wavelengths between 400 and 700 nm. Because eye sensitivity varies significantly within this spectral range, practical LED applications (e.g., displays) should take into account relative response of the eye at the wavelength of interest. For human brightness and color perception, different types of units are required. These units are called photometric units. The luminous flux (lumen, lm) is the light power of a source as sensed by human eye. One lumen (lm) is defined as a monochromatic light source emitting an optical power of (1/683) watt at 555 nm. For all other wavelengths, the luminous flux can be calculated by multiplying optical power by the luminosity function V(λ), which describes the average sensitivity of the human eye to light of different wavelengths. The luminous intensity unit, candela (cd), is defined as a monochromatic light source emitting an optical power of (1/683) watt at 555 nm into the solid angle of 1 sr. The brightness of an LED output is measured by the luminous flux (F) as F ) L o∫V(λ)P op (λ) dλ (30) where Lo is a constant (683 lm/W), and Pop(λ) is the light power (watts per unit wavelength). V(λ) is normalized to unity for the peak at λ ) 555 nm where the human eye has its maximum sensitivity. The difference between lm and cd reveals one candela equals one lumen per steradian (sr), cd ) lm/sr. The luminance of an LED (Lv) is measured in units of cd/m 2 as L v ) d 2 F dA dΩ cos θ (31) where θ is the angle between the surface normal and the specified direction, A is the area of device surface (m2 ), and Ω is the solid angle (sr). Table 3 compares typical luminance of displays, organic LEDs, and inorganic LEDs based on III-V semiconductors. 7.1.2. Nanocrystal-Based QD-LEDs First LEDs utilizing CdSe colloidal NCs have been reported by Colvin et al. in 1994. 398 Since then, significant progress has been achieved in optimization of all components of the QD-LEDs. Using core-shell NCs as the emitters allowed one to significantly increase the efficiency of radiative recombination and device internal quantum efficiency. 399 To achieve higher injection currents, heavily doped electron transporting layers were introduced, whereas the thickness of the emitting layer was reduced down to one or two monolayers of highly luminescent semiconductor NC. 408 The important breakthrough in understanding operation of efficient QD-LEDs is associated with the works of the Bulovic and Bawendi groups, who realized that excitons Figure 35. Electroluminescence spectra and structures of QD- LEDs (a) without and (b) with a hole-blocking (TAZ) layer. Alq3 is tris (8-hydroxyquinoline) aluminum; TPD is N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine; TAZ is 3-(4-biphenylyl)-4-phenyl- 5-t-butylphenyl-1,2,4-triazole. (c) Proposed energy level diagram of the device. Reprinted with permission from ref 408. Copyright 2002 Nature Publishing Group. (d) Optical photographs of green, yellow, and red light emitting devices using CdSe/ZnS core-shell nanocrystals with different sizes of CdSe cores. Reprinted with permission from ref 405. Copyright 2007 Nature Publishing Group. formed in the polymer can be nonradiatively transferred to and recombine in the semiconductor NCs. 408 Nowadays, the most widely accepted structure of QD-LED is outlined in Figures 34 and 35. Typically, a thin layer of luminescent NCs is sandwiched between the electron- and hole-transporting layers. When forward bias is applied, the electrons and holes are injected into the NC layers from ETL and HTL layers, respectively. The recombination of electron– hole pairs in the NCs generates photons with the energy corresponding to the gap between highest occupied (1Sh) and lowest unoccupied (1Se) states in the NC. Optimization of ETL and HTL layers, development of highly luminescent, stable, and, ideally, environmentally benign semiconductor NCs are the active areas of ongoing research. Generally speaking, core-shell NCs have an advantage for solid-state QD-LEDs due to their enhanced photoluminescence and electroluminescence (EL) quantum efficiencies and their greater tolerance to the processing conditions during device fabrication. Table 4 provides a summary of reported QD LEDs structures, Lv, emission colors and ηext. In the first QD LEDs, the Alivisatos group used TOPO- TOP capped CdSe NCs as the emitting and electron transporting layer and poly(p-phenylenevinylene) (PPV) as HTL. 398 The spectrum of electroluminescence (EL) was dominated by the NC emission, and the emission color of the QD-LED could be changed from red to yellow by tuning the NC size. The luminance of ∼100 cd m -2 was obtained for a device with the structure glass/ITO/PPV/CdSe QDs/ Mg. 398 However, ηext of those first QD-LEDs was fairly low (0.001-0.01%) due to imbalanced carrier injection into the NCs. Moreover, the EL spectra often combined light emission from the QDs and organic layers, negatively affecting the color purity. Mattoussi et al. also reported LED devices
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