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Quantum dots and their chemical adaptation for various photonic applications are presented in this review. The use of quantum dots as photoactive components in many applications requires their combination with other materials playing specific roles for separation and transport of charge carriers. Achieving good interfaces between electronically matched component materials is key to improved performance in photodetectors, photovoltaics, electroluminescence application, etc.
©2012 Optical Society of America
1. Introduction
Quantum dots (QDs) are semiconductor nanocrystals which show extraordinary optical and electrical properties due to quantum confined nature of their energy levels. For a given material, the variation in optical properties of the QD stem from its size. Typically the size ranges from a few nanometers to tens of nanometers. Such small dimensions are usually smaller than the de Broglie wavelength of thermal electrons [1]. When QDs are photo excited, the photogenerated electrons and holes are bound together because of quantum confinement, and they are called excitons. The confined natures of QDs only enables limited pathways for de-excitation of the carrier and give rise to slow cooling dynamics [2]. This allows the extraction of electrical or chemical free energy from QDs, before they can relax to their lowest electronic state. The rate of exciton multiplication becomes comparable to the rate of cooling; this means that the slow cooling dynamics in QDs can give rise to an increased probability of multiple exciton generation. This probability of multiple excitons per excitation has great implications for photo-driven applications employing QDs [3–5]. Though the physics behind this phenomenon has been well defined for quite some time, harnessing them for practical applications on a larger scale has been difficult due to problems in large scale synthesis of QDs and their subsequent processing to achieve good results. During the last decade, many solution based methods have been investigated for the synthesis of QDs. The evolution of new synthetic strategies for monodisperse, size tunable QDs from different materials has given rise to their wide spread applications in imaging, electroluminescent devices, lasing, photonic crystal structures etc [6–12]. The use of QDs in various applications also requires turning their processibility with suitable functionalization strategies. Moreover, the carriers generated by these materials upon electrical or optical stimulation, need to be separated and carried to a suitable interface to complete the working of a device. This would require combining QDs with other materials which would facilitate the delocalization of the excitons and the transport of the separated electrons and/or holes.
In this review, we describe the synthesis of QDs, their functionalization, processing and their combination with suitable materials to match the needs of various applications. Many of our studies aim on improving the interfacial characteristics between QDs or between QDs and other components in the active layers of different devices. The types of devices studied involve photodetectors, photovoltaics, photorefractive and organic light emitting diodes (OLEDs).
2. Materials
2.1. Synthesis of QDs
The PbSe, PbS, CdSe, CdSe@ZnS core-shell and CdTe QDs used for various applications described in this review were synthesized by methods well established in literature. These procedures are briefly summarized below.
(a) PbSe QDs: Lead oxide (PbO, 1.1 g, 5 mmol) and oleic acid (7.1 g, 25 mmol) were added to trioctylamine (20 ml) under inert conditions and heated for 30 min at 155 °C. A clear solution of soluble lead oleate was formed at the end of 30 min. The mixture was further heated up to 180 °C and selenium (1 mol) in trioctylphosphine (10 ml) was added. The solution was allowed to cool to 150 °C at which nucleation growth of PbSe nanocrystals commences. The size of the quantum dots would depend on their time of withdrawal from the reaction mixture. The excess surfactants on the QDs were removed by repeated washing with acetone [13,14].
(b) PbS: The synthesis of PbS quantum dots utilized the same principles followed in the synthesis of PbSe QDs. Lead oxide (0.89g, 4 mmol) and oleic acid (3.56 g, 12.5 mmol) were added to octadecene (20 ml) under inert argon atmosphere and heated (~155 °C) until PbO was completely dissolved by the formation of soluble lead oleate. A previously prepared 1 M solution of hexamethyl disilathiane (HMD) in octadecene was added to the reaction mixture under inert conditions. The temperature was decreased to 120 °C, followed by further injection of HMD giving a dark colored solution. The heating mantle was removed and the reaction vessel was allowed to cool in order to facilitate the growth of quantum dots. The size of the QDs was dependent on the growth time. The quantum dots were precipitated in polar solvent, centrifuged and re-dispersed in chloroform. This process was repeated a few times to remove the excess ligands [15,16].
(c) CdTe: For the synthesis of monodisperse CdTe quantum dots, cadmium oxide (0.4 mmol), trioctylphosphine oxide (22 g) and tetradecylphosphonic acid (TDPA) ligand (0.8 mmol) were mixed and heated to 300 °C to get a clear solution under inert argon atmosphere. The solution was cooled to 270 °C and tellurium (0.5 mmol) dissolved in trioctylphosphine oxide (2 g) was injected. The solution was cooled to 250 °C and maintained at that temperature to aid the growth of CdTe QDs [17].
(d) CdSe: The procedure followed for the synthesis of CdSe QDs is similar to that of CdTe [17–19].
(e) CdSe@ZnS core-shell: For the synthesis of CdSe@Zns QDs, cadmium oxide (0.4 mmol), zinc acetate, oleic acid (5.5 mL) and 1-octadecene (20 mL) were heated to 150 °C. The mixture was degassed under 100 mtorr pressure for 20 min and further heated to 310 °C, at which the solution turned optically clear due to the formation of cadmium oleate and zinc oleate. To this, a pre-prepared solution of Se (0.4 mmol) and S (2.3 mmol) was added. The mixture was allowed to grow at 310 °C for 5 min before removing the heating and subsequent cooling to room temperature [20,21].
2.2. QDs with stimuli responsive ligands
(a) QDs with t-BOC terminated ligands: The synthesis of tert-butyl-N-(2-mercaptoethyl)-carbamate (t-BOC) involved a ligand exchange reaction in which the stabilizing ligands of QDs were replaced by t-BOC. Typically 5 ml (15 mg/ml in chloroform) of QDs (CdSe, PbSe or CdTe) and t-BOC (3 ml) in a centrifuge tube were vigorously mixed in a vortex mixer for two hours. The QDs were twice washed in methanol, centrifuged and re-dispersed in chloroform. This simple treatment is adequate to completely replace the stabilizing ligand from the QD synthesis with the functional t-BOC groups [19,22,23].
(b) QDs with methyl methacrylate (MMA) terminated ligands: The acrylate terminated green CdSe@ZnS QD was accomplished by a two-step process in Fig. 1 . The oleic acid ligands from the synthesis of CdSe@ZnS QDs were replaced by 11-mercapto-1-undecanol in the first step. To accomplish this, 5 mg of oleic acid capped and 50 mg of 11-mercapto-1-undecanol were added to a 1:1 mixture of ethanol and chloroform and sonicated for 3 hours. QDs capped with 11-mercapto-1-undecanol were precipitated by the addition of 40 ml chloroform. In the subsequent step, 20 mg of 11-mercapto-1-undecanol capped CdSe@ZnS was dispersed in DMSO, followed by the addition of 100 μL of 3-(trimethoxysilyl)propyl methacrylate. The mixture was stirred at 50 °C for 6 hours and the resulting methacrylate terminated nanocrystals were precipitated with chloroform and centrifuged. The QDs were repeatedly washed with methanol and chloroform to remove excess ligands [21].
2.3. QD nanocomposites
As mentioned earlier, QDs are highly desirable materials for optically driven applications, due to their well-defined energy levels. By changing the material that constitutes the QDs, and by tuning the size of QDs of a particular material, it is possible to access energy from different regions of the solar spectrum. Utilizing the versatility of QDs in practical photo-driven devices requires design and material-strategies for exposing QDs to radiation, and extracting the resulting carriers from them to achieve the desired application. In devices like OLEDs, where quantum dots are employed to generate color, measures should be taken to address QD films with voltages and to optimize their emission properties. Most devices contain an active layer composed of either QDs or composites of QDs with other materials. For optimal performance tuning, the interface between QDs and other active materials is really important.
(a) PbSe-pentacene-PVK nanocomposite: For applications in photodetection and photovoltaics, QDs are combined with semiconductors to ensure better charge separation and conduction dynamics. Pentacene is an organic semiconductor with some of the highest reported hole mobilities in devices. Though this material is highly desirable, it has limitation regarding solubility in common organic solvents. Solution processability is a much desired quality in organic seimconductors, because it aids fabrication of devices without expensive vacuum deposition processes. Solution processable thermally activated pentacene precursor was prepared by the Diels-Alder reaction between pentacene and N-sulfinylamide in the presence of catalytic amounts of methyltrioxorhenium, following a reported method [24]. To make the pentacene precursor-poly(N-vinylcarbazole) (PVK)-PbSe QD nanocomposite, the first two components were dissolved in choloroform. Chloroform dispersions of oleic acid-capped QDs were then added and the composite solution was homogenized by vigorous stirring and ultrasonication. Films of this composite were heated at 200 °C to generate pentacene from the pentacene precursor. The formation of pentacene in the composite was confirmed through TGA and UV measurements of the composites [25].
(b) PbSe-pentacene-P3HT nanocomposite: PbSe QDs could also be combined with polymeric semiconductors to boost transport and increase processability. Poly(3-hexyl thiophene) or P3HT is widely employed in polymer-based solar cells, because of its excellent hole transport with high mobility in the regioregular state 10−2~10−1 cm2/V·s. P3HT was blended with PbSe QDs and the soluble pentacene precursor mentioned above, to give photosensitive hybrid materials. The ambipolar nature of pentacene is exploited in this composite where it acts as a conduction booster. Pentacene is generated by a post thermal treatment of the composite films [26].
(c) Nanotube-QD hybrids: To prepare single walled carbon nanotube (SWNT) with surface carboxylation, 80 mg SWNT was stirred in 40 mL of H2SO4/HNO3 (3:1) solution for 24 hours at room temperature (RT). The reaction mixture was centrifuged and washed with water several times to remove the excess acid. This was followed by the addition of N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDAC) to SWNTs dispersed in DMSO to activate the acid group. This is followed by addition of 500 mg of 2-aminoethanethiol (AET) and stirring for 12 hours at RT. The resulting mixture was washed with acetone and the precipitate was sonicated in chloroform to get a stable dispersion of thiol functionalized SWNT (SWNT-SH). PbSe QDs containing oleic acid stabilizing ligands were washed continuously with ethanol, followed by centrifugation, to remove some of the surface ligands. These QDs are then complexed with SWNT-SH by stirring in chloroform for 12 hours. Free PbSe was separated by centrifuging with microcentrifuge filters. The scheme for this reaction is given in Fig. 2(a) [27].
Fig. 2 Scheme for synthesis of (a) SWNT-PbSe and (b) PPyNT-PbSe ((a) was redrawn from [27] and (b) reprinted with permission from [14]).
Polypyrrole nanotubes (PPyNT) were functionalized with thiol groups following the same procedure as above. The nanotubes are synthesized by a template assisted procedure which yields nanotubes with carboxylic functionalization. The thiol functionalized PPyNTs obtained after reaction with EDAC and AET in DMSO, are further complexed with PbSe QDs to get PpyNT-PbSe, Fig. 2(b) [14].
3. Applications
Solution processability and patternability of QDs increase their potential for practical applications. This property is highly favored by the industry because of its low cost. Functionalization of QDs thermal/photo-active ligands enables a control of the interfacial properties between the different active layers in thin film photonic devices by action of heat or light.
3.1. Photonic microstructures containing QDs
(a) QDs with t-BOC functional groups: The t-BOC functional groups in CdSe, CdTe and PbSe QDs can be thermally or photochemically degraded to form microstructured films. For photopatterning, 30 mg of nanocrystals and a photoacid generator (5 wt% versus NCs), di-tert-butylphenyliodonium perfluorobutanesulfonate were dispersed in 1 mL chloroform and spin casted on a glass plate. The spin casted film is exposed to UV light through a mask (254 nm, 390 μW/cm) for ten minutes followed by heating for 90 s at 100 °C [19]. The t-BOC group undergoes a cleavage, when subjected to UV irradiation in the presence of a photo acid generator (PAG) to release isobutene and carbon dioxide. Depending on the need of the application, either the exposed regions (negative pattern) or the non-exposed regions (positive pattern) could be developed from the exposed films by appropriate solvent selection. The photo-exposed regions of the film are rendered hydrophilic due to the degradation of the t-BOC, the un-exposed regions remain hydrophobic. This solubility change in the QDs is the basis of their patternablity. The un-exposed regions can be removed to obtain the negative pattern by washing with hexane, whereas the exposed regions can be selectively removed to obtain positive pattern by washing with acetone based solvents. The negative patterns obtained from patterning t-BOC functionalized CdTe, CdSe and PbSe QDs is presented in Figs. 3(a) -3(c). The physical properties of the QD s were also found to change with thechemical degradation of the outer ligands. The photoconductivity of CdTe was studied as function of its ligands by designing a metal semiconductor metal (MSM) detector Fig. 3(d). An active layer of t-BOC protected QD solutions were drop-casted on the device. The resulting dark current of the device was measured followed by photocurrent measurements before and after UV exposure. It was found that for any given device, the dark current remains very low, but the devices with the UV exposed QDs showed photoconductivity values at least an order of magnitude greater than those that were not exposed, Fig. 3(e). Thus the QDs functionalized with amine terminated ligands, obtained after UV exposure, are better suited for optoelectronic devices because of their better charge transport properties.
Fig. 3 Negative photopatterns of (a)-(c) CdTe, CdSe and PbSe QD respectively obtained by UV photo-exposure through a mask in presence of a photoacid generator, inset of (a) shows 5 μm circles patterened from CdTe QDs (d) shows the structure of the MSM device (e) photo response of MSM detector with increasing electric field, dark current (black line) corresponds to the current when there is no illumination, the red broken lines represent the response of the device containing un-exposed QD films under illumination, the blue dotted line show the enhancement of photocurrent under illumination when the active QD layer is photocured (Reprinted with permission from [19]).
(b) QDs with methacrylate functionalization: Green colored core-shell CdSe@ZnS QDs with methacrylate functional groups can be easily patterned on to different substrates by spin coating and UV exposure. These materials follow a photo-induced radical polymerization of the methacrylate groups as well as the thermal crosslinking of the inner siloxane moieties. The hybrid nature of the groups in the ligands makes them compatible to both organic and inorganic substrates. The methacrylate groups make the material compatible for plastic substrates, whereas the siloxane groups adapt them for inorganic substrates. To make 2-D patterns of the material, a solution containing 148 mg/ml of photopatternable QDs (PPQDs) in methanol was spin coated (the spin-coating step was characterized by 500 rpm for 5 s, 2000 rpm for 20 s, 500 rpm for 5 s) on a cleaned ITO substrate. The film was baked at 90 °C for 1.5 min, followed by exposure through a photomask using broad spectrum UV source 300-400 nm with a power of 33.2 mw/cm2 at 360 nm. A negative pattern of the photo-crosslinked QDs could be obtained on developing the films in ethanol. Using the same method, this material could be patterened on a flexible PET substrate in Figs. 4(a) -4(b).
Fig. 4 (a) and (b) show the patterning of PPQDs on ITO glass and flexible PET substrate respectively, the patterns are excited at 365 nm, the prominent fluorescence of the PET film in this region of the spectrum causes the green fluorescent QD patterns to appear blue. (c) The EL efficiency of an electroluminescent device fabricated with photopatternable QDs stabilized by methacrylate terminated ligands: there is an increase in EL intensity of the device when the QD layer is photocured during device fabrication, the inset shows the structure of the fabricated device. (d) the normalized emission intensity of the device whose QDs layer is photocured and that with QD layer containing no photocuring. There is a clear distinction between the green emissions of the QD and that of ETL material tris(8-hydroxyquinoline) aluminum(III) (Alq3) as shown in inset 1, the second inset shows the picture of a working device. (e) shows the Scanning electron microscopy (SEM) image of a polymeric pattern embedded with PPQDs (f) the confocal microscopy image PPQD embedded polymeric structures (Reprinted with permission from [21]).
The fluorescence properties of the PPQDs were found to increase two orders of magnitude on irradiation. Transmission electron microscopy of the photocured films revealed a densification of the PPQD films on irradiation. Similar trends were seen in the case of electroluminescence properties, which were investigated by fabricating an organic light emitting diode (OLED) with the green PPQD active layer. The device constituted a five layered geometry. In the device, the conducting polymer PEDOT forms the hole injection layer and tris(8-hydroxyquinoline) aluminum(III) (Alq3) forms the electron transport layer (ETL). The device performance is summarized in Figs. 4(c)-4(d). The OLED device, with the photocured QD layer, showed maximum external quantum efficiency (EQE) of 0.62% at 588 cd/m2, whereas the uncured device shows an EQE of 0.53% at 703 cd/m2.
These QDs could be easily incorporated in to a photopatternable resin and used to fabricate three-dimensional polymeric structures embedded with green QDs. Such structure could find use in applications such as photonic crystals and optical filters [10,11]. A scanning electron microscope image of a QD embedded polymeric microstructure, along with a confocal microscope image of the QD embedded microstructure are presented in Figs. 4(e)-4(f) [21].
3.2. Photodetectors, photoconductors and photovoltaics
This section describes recent results on simple methods to produce QD-based photonic devices by means of bottom-up processes. Specific focus is directed towards approaches to produce optoelectronic and electronic devices using various semiconducting nanocrystals/ polymer composites. Details on technological aspects concerning QD-based devices together with practical examples will be discussed in this section.
Semiconductor nanocrystals (QDs) have attracted much attention since the NCs possess unique, often size-dependent properties associated with magnetic, photonic, chemical, and electrical behavior, which are different from the properties in their respective bulk materials, enabling various applications in optoelectronics. For applications in optoelectronic devices discussed here, the electronic properties of their constituents need to be compatible to each other. This particularly applies to the positions of the conduction and valence band energy levels, which determine their redox potentials. In the case of NCs, their principal optoelectrical properties can be tuned over a wide range by changing not only their size, but also by varying the shape, due to the quantum confinement [1].
Thus, these tunable optical and electrical properties make it possible to apply QDs into various optoelectronic devices, such as light-emitting diodes [28,29], photodetectors [25,30,31], field-effect transistors [32], photovoltaic cells [33–37], and photorefractives [38]. In addition, QD-based devices have a number of advantages including an easy fabrication process (all solution process), lightweight, low costs, and flexible substrate capability. Therefore, semiconductor nanocrystals can hardly be equaled by other systems in terms of design flexibility.
To date, there have been significant improvements in the development of semiconductor NCs to meet increasing demand for potentially low cost and high performance optoelectronic devices [39]. We have recently demonstrated IR sensitization using QD-based photodetectors and improvement of the efficiency of nanocrystal/polymer composite devices by the incorporation of mobility enhancing pentacene [25]. Figure 5(a) shows the photocurrent density of a PVK/PbSe device fabricated on top of an ITO coated glass substrate. The infrared active thin film photodetector device was fabricated from a nanocomposite of poly(N-vinylcarbazole) (PVK), monodispersed PbSe QDs, and soluble precursor of the organic semiconductor pentacene by using solution processing. The measured photocurrent densities as a function of the applied bias for devices with the increasing amount of pentacene are shown in Fig. 5(a). The asymmetry of the I-V curves is due to the difference in the work function of two electrodes which are ITO (~4.8 eV) and aluminum (~4 eV).The photocurrent increases significantly by means of an increase in pentacene/PVK ratio in the composite device. The devices with equal amounts of PVK and pentacene (containing ~25 wt % of PbSe QDs) show the best performance. The enhancement in photocurrent is more than eight times in the given conditions (PVK:pentacen:PbSe QD = 37.5:37.5:25 by wt.%).
Fig. 5 (a) Photocurrent density as a function of pentacene concentration at the operating wavelength of 1340 nm. The brackets indicate the ratio of PVK to pentacene. (b) Current density–voltage curves display the performance of the photovoltaic device with pentacene (triangles) and without pentacene (circles) under AM 1.5G Solar Simulator (60 mW/cm2). Inset I-V curves indicate near infrared photocurrent response of the same devices, when the devices illuminated with IR part of the solar spectrum by placing a 750 nm long pass filter (Reprinted with permission from [25,40]).
We extend the above mentioned mobility booster strategy to solar cell application to demonstrate an improvement in the photovoltaic performance in the IR region [40]. Figure 5(b) shows the current density–voltage characteristics of a poly(3-hexylthiophene) (P3HT):PbSe: pentacene device fabricated on the top of an ITO coated glass substrate. The photovoltaic response studies show that the photovoltaic performance is significantly increased by this mobility boosting strategy.
In a PbSe:P3HT:pentacene (54:23:23 wt%) device, especially, both the short circuit current density (Isc) and the open circuit voltage (Voc) increased by two times, suggesting that the π-stacked ordered geometry of pentacene provides highly conducting domains in the device. In other words, the improved performance of the device is attributed to pentacene that can boost the mobility of the charge carriers in the composite device. The ability to harness the IR part of the solar spectrum is illustrated in the inset of Fig. 5(b). We have placed a long pass filter with cut off at 750 nm to harness only the IR portion of light so that the photosensitization is caused only by the PbSe QDs. The resulting current-voltage curves (Fig. 5 inset) clearly indicated that the IR response of the device is boosted by adding a pentacene. Although the device shows improved performance, the resulting power conversion efficiency is fairly low. One major reason for the lower efficiency is that the as-prepared PbSe QDs are surrounded by long alkyl chain ligands such as oleic acid (OA). These long alkyl chain ligands are known to act as an electrically insulating layer (1–2 nm) that interrupts efficient charge transfer [32–34,41]. Thus, replacing them with shorter ligands would be beneficial for electrical applications. To overcome this issue, several chemical treatment techniques have been developed to improve the electronic properties of QDs [32,34,41]. For example, Alivisatos and associates demonstrated a significant enhancement of the photovoltaic performance using CdSe nanorods/P3HT by means of pyridine treatment of CdSe nanorods [34].
As an alternative to this approach, we have investigated an in situ ligand exchange method to replace the original ligand on PbS in the fabricated composite films [16]. To demonstrate in situ ligand exchange of the device, the PbS:P3HT film was immersed into acetic acid solution (0.01 M in acetonitrile) and incubated for 30 min, and then the film was washed by excess acetonitrile and was dried at 150 °C for 10 min. We demonstrated a considerable improvement of the photovoltaic performance by the in situ acetic acid treatment of the composite device to end up with acetic acid capped PbS nanocrystals. Resulting I-V curves are displayed in Fig. 6(a) . Specifically, the acetic acid treated devices show an increased performance in the short circuit current (Isc) (approximately two-orders of magnitude) compared to that of the untreated device; as a result, the power conversion efficiency (PCE) is increased by more than 100 times. This result clearly demonstrated that replacing long alkyl chain ligands with shorter ligands by in situ chemical treatment plays a key role in improving the photovoltaic performance. In addition, transient decay of photoluminescence study indicated that in situ chemical treatment provide better charge transfer by improving the interfacial contact between P3HT and PbS QDs which results in a decrease in photoluminescence of P3HT.
Fig. 6 (a) Current-Voltage curve displays improved photovoltaic response of the photovoltaic devices consisting of ITO/PEDOT:PSS/P3HT:PbS (10:90 wt %)/Al due to post chemical treatment and (b) time-resolved photoluminescence showing improved charge transfer (Reprinted with permission from [16]).
In addition to the post-chemical treatment approach, we have developed a method to fabricate QD-based photovoltaic cells with improved optoelectronic properties using quantum nanocrystals possessing thermally cleavable t-BOC ligands which can be eliminated by gentle heat treatment [23]. This thermal deprotection concept can lead to a change in the structure and, in consequence, to a change in the physical properties of the ligand which fully protects and covers the NCs. We have demonstrated a relative improvement in composite photovoltaic cells (P3HT:CdSe NCs) by simple heat treatment of the device at 200 °C in order to eliminate the thermally cleavable t-BOC ligands from the CdSe nanocrystals. In a P3HT:CdSe (10: 90 wt%) device, the power conversion efficiency (PCE) was enhanced by almost two orders of magnitude as a consequence of enhancement in the short circuit-current density (Jsc) which is about 61 times (Fig. 7(a) ). Several conditions were tested by varying the P3HT:CdSe ratio in order to optimize the device performance, and a ratio of 10:90 by wt% revealed the best result. Subsequently, the heat treatment temperature was varied systematically to obtain the best performance and 240 °C was found to produce the optimum result (Fig. 7(b)). On the other hand, we observed gradual changes in Voc when the devices annealed at high temperature. This is probably due to the phase transition of P3HT because the melting point of P3HT waspreviously reported to be in the range of 230–240 °C [42]. This result clearly shows that the simple and straightforward ligand deprotection strategy can dramatically enhance the photovoltaic performance as a result of better charge transport between the closer packed nanocrystals. Since the thermal deprotection could endow totally different solubility of nanocrystals, the modified NCs can be used in multilayered device fabrication for further photovoltaic device applications.
Fig. 7 (a) Current density-voltage curves display enhanced photovoltaic response by heat treatment of the photovoltaic devices consisting of ITO/PEDOT:PSS/P3HT:CdSe-t-BOC (10:90 wt %)/Al. The inset shows the schematic diagram of the thermal deprotection process. (b) Summary of device performance in the same device structures as a function of different heating temperatures for t-BOC deprotection (Reprinted with permission from [23]).
QDs have been shown to generate multiple excitons from a single high energy photon (> 3 times of bandgap of the nanocrystal). There have been numerous reports of carrier multiplication from the QDs such as PbSe, PbS, InAs, and Si by using transient absorption spectroscopy [43,44]. We have recently demonstrated that multiple carrier extraction is possible from a photoconductor fabricated using PbSe QDs, as shown in Fig. 8(a) [45]. In that work a thin film of PbSe nanocrystals is deposited on top of an ITO contact on a glass substrate. The PbSe nanocrystal layer is treated with hydrazine to improve the interparticle charge transfer (resulting in drastic increases in film conductivity) [32]. Subsequently, aluminum electrodes are deposited onto the PbSe film to produce a photoconductor. As shown in Fig. 8(b), we observed a multiplication of 200% at a photon energy of 4.5 times the bandgap energy of the PbSe QDs. Most importantly for this research, theoretical calculations show that QDs can increase the power conversion efficiency (PCE) by more than 32% when compared to the conventional materials [46,47].
Fig. 8 (a) Representative absorption spectra of synthesized PbSe nanocrystals. (b) Device structure of a photoconductor and SEM image of the PbSe thin films. (c) Normalized extracted electrons/photon for the photoconductor reported recently by our group, plotted with the normalized generated excitons/photon reported by the Nozik and Klimov groups (Reprinted with permission from [45]).
To date, the reported power conversion efficiencies of hybrid solar cells using nanocrystal QDs are relatively low. Since the QDs show multiple exciton generation (MEG), however, there is still great opportunity in using QDs as a light absorber for photovoltaic devices. We have demonstrated hybrid inorganic/organic tandem cell structure that consist of a PbSe photoconductor layer and a polymer (P3HT:PCBM) bulk hetero-junction solar cell as shown in Fig. 9(a) [26]. The lower polymer cell supplies an electric field to extract carriers from the PbSe photoconductor when the device is illuminated. By using light bias spectral response measurements, the extracted carriers were measured as a function of wavelength and enhanced extraction due to carrier multiplication which was demonstrated in the UV. This device structure has several advantages: i) all solution-based processing; ii) UV protection of the polymer cell by the PbSe layer; and iii) the multifunctional PbSe nanocrystal layer also provides MEG in the UV. Figure 9(b) shows the resulting differences in device lifetime under intense UV illumination. The normalized short circuit current is plotted as a function of time under continuous illumination from AM 1.5G radiation. After 13 hours, a mercury UV lamp was added to the system to further accentuate the degradation due to the UV illumination. The tandem cell with the upper protective layer has a noticeably longer device lifetime [48].
Fig. 9 (a) Hybrid tandem solar cell consisting of a PbSe nanocrystal thin film and an organic (P3HT:PCBM) solar cell. Band energy alignment is shown on the top. (b) Comparison of a polymer solar cell to the hybrid tandem solar cell under intense UV illumination. Optimization of the PbSe layer will result in higher efficiencies and longer lifetimes (Reprinted with permission from [26,48]).
Semiconducting nanotubes such as carbon nanotubes (CNTs) and polymeric nanotubes are promising candidates for developing next-generation photovoltaics. There have been significant improvements in the development of nanotube-based photovoltaic devices to enhance the device performance. We have also demonstrated IR sensitization using quantum dots/polymer nanocomposite and improvement of the efficiency of the nanocomposite devices by the incorporation of QD-tethered carbon nanotubes as a conduction booster [14,27]. Quantum dots (PbSe) were tethered to nanotubes with a view or improving the charge separation dynamics in the devices. The high electrical conductivity of single walled CNTs (SWNT) are used to achieve efficient extraction of carriers from PbSe QDs, the conduction boosting polymer PVK mixed in to the composites aids further conductivity in the nanocomposites. The photocurrent densities (at an excitation wavelength 1340 nm) from the studied photodetector devices containing different nanocomposites as active layers are shown in Fig. 10(a) .
Fig. 10 (a)The I–V characteristics of PbSe-QD/PVK as well as SWNT–PbSe/PVK devices in the dark and under illumination, illustrating the enhancement of photocurrent in the IR region with increasing percentage content of SWNT–PbSe. Inset: structure of the IR photodetector device employed in this study, (b) I–V characteristics of PPyNT-PbSe (Reprinted with permission from [14,27]).
The structure of the photodetector device is shown in the inset of Fig. 10(a). It is seen that the photocurrent density increases with the applied bias for the (PbSe-SWNT)-PVK nanocomposites with increasing amounts of PbSe-SWNT. This trend was found to peak for nanocomposites containing 20 wt% CNT-QD. Pure PVK has a negligible absorption at wavelengths longer than 370 nm, and no photoresponse was obtained on 1340 nm excitation with only PVK. The efficiency of light harvesting, charge separation and conduction in (20 wt% PbSe-SWNT)-PVK nanocomposites compared to a nanocomposite containing 20 wt% PbSe-PVK could be seen in Fig. 10(a). The external quantum efficiency of the (20 wt% PbSe-SWNT)-PVK c.a. 2.6% which was a 100% enhancement on the value showed by devices containing 20 wt% PbSe-PVK (c.a. 1.2%).
In a similar way to QD-coupled carbon nanotube, an alternative method for the fabrication of QDs-conjugated polypyrrole nanotubes (PPyNTs) was developed and the ability to use them in photovoltaic was demonstrated [14]. The PbSe NCs to be used as a p-type semiconductor were tethered onto polypyrrole nanotubes by covalent bonding and successfully fabricated in photovoltaic device geometry. We experimentally demonstrated the photovoltaic effect of the P3HT:PbSe-PPyNTs structure, with the open-circuit voltage (Voc) of around 0.45 V and the short-circuit current (Isc) of 2.4 A/cm2. Note that this is a preliminary result and we are currently optimizing the device. Figure 10(b) shows the current density–voltage characteristics of a P3HT:PbSe-PPynTs device fabricated on the top of an ITO coated glass substrate.
3.3. Photorefractive devices
Photorefractive materials have applications in reversible memories and volume phase holograms [49,50]. Polymer QD nanocomposites have been tried out as photorefractive materials before [38]. Although QDs have excellent optical properties such composites have limited applicability due to their poor conductivity. The poor electrical property of QD composite films is partly due to a limited contact between QDs arising from the long alkyl ligands used for stabilizing the nanoparticles during synthesis. The ligand exchange strategies during device fabrication were found to show positive effect on the eventual device performances. We fabricated a photorefractive device constituting three charge transport polymers poly (N-vinylcarbazole) (PVK), N-ethylcarbazole (ECZ), and 4-(N,N-diethylamino)-b-nitrostyrene (DEANST), together with t-BOC protected PbS QDs [22]. The solution processability of the QDs aids the formation of good films and the thermal cleavable nature of t-BOC groups are utilized to create an in situ degradation of the ligands on the QDs leading to an improvement in properties. Heat treatment at 200 °C leads to the degradation of the t-BOC groups to form shorter amine terminated ligands. Photorefractivity was studied using a two-beam coupling (TBC) experiment shown in Fig. 11(a) .
Fig. 11 (a) Two beam coupling (TBC) experiments for studying the photorefractive effect of t-BOC functionalized PbS QD polymer nanocomposites (b) shows the variation of two beam coupling gain coefficient Γ with increasing electric field. Diamond markers represent the data for oleic acid stabilized PbS QDs, circular markers show that of t-BOC functionalized QDs treated at 100 °C, and the triangular markers represent the values obtained for t-BOC functionalized QDs treated at 200 °C. There is marked improvement in the gain coefficient with the thermal degradation of the QDs at 200 °C. (Reprinted with permission from [22].)
The measurement of TBC inside a photorefractive sample, verifying an effective energy transfer between the two beams inside the photorefractive sample, is give in Fig. 11(b). The extent of energy transfer is defined by the TBC gain coefficient, Γ. From the Fig. 11(b), Γ of the composite sample with the t-BOC deprotected PbS QD annealed at 200 °C, is much higher than that of the sample with the t-BOC PbS annealed at 100 °C, and the sample with the OA PbS. This should be due to the higher charge transport efficiency, similar to the trend showed in Fig. 3(e). A TBC measurement conducted on a polymer only sample as a control experiment, showed negligible energy exchange under 1310 nm excitation, verifying that the PbS QDs were responsible for photoinduced charge generation in the composite device at 1310 nm. For practical applications Γ should exceed the measured optical loss α. The device containing QDs thermally cured at 200 °C has a net gain coefficient of Γ-α = 85.2 cm−1. This is one of the best values reported for net gain at the operation wavelength 1310 nm for PR composite devices.
4. Conclusions
The results from different studies summarized in the article would serve to demonstrate that QD based photonic devices can be improved by enhancing the charge separation and carrier dynamics in these devices. This is optimally achieved by ensuring good electronic match between the interacting components and turning the interactions between various components constituting the devices. The influence of nanotubes, small molecule organic semiconductors, as well as polymers in improvement of charge separation and carrier dynamics has been investigated. The effect of surface functionalization of QDs on the performance of devices was also explored. Generally, shorter ligands led to better interfacial characteristics and transport. The improved performance is also due to a better contact between the QDs functionalized by shorter ligands in the case of applications like photorefractivity, where conductivity of QDs is important. This is also true for photodetectors fabricated from QDs functionalized with photocleavable ligands. Photopolymerization and the resultant densification led to increased fluorescence and electroluminescent properties in the case of photocrosslinkable QDs. The principles evolved and materials synthesized during the study of various applications present a tool box for the design of new devices, these studies also frames the problems to be addressed to obtain devices with excellent performance.
Acknowledgments
This work was supported by the National Science Foundation (Grant No. DMR0702372), the Mid-career Researcher Program (No. 2010-0000499), and the Active Polymer Center for Patterned Integration (ERC R 11-2007-050-01002-0) of the National Research Foundation of Korea. One of us, K.-S. Lee, would like to thank the Asian Office of Aerospace Research and Development (AOARD), Air Force Office of Scientific Research, USA, for their support. Both authors Prem Prabhakaran and Won Jin Kim contributed equally to this work.
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