Solution-processed efficient CdTe nanocrystal/CBD-CdS hetero-junction solar cells with ZnO interlayer

  • Yiyao Tian
  • Yijie Zhang
  • Yizhao Lin
  • Kuo Gao
  • Yunpeng Zhang
  • Kaiyi Liu
  • Qianqian Yang
  • Xiao Zhou
  • Donghuan Qin
  • Hongbin Wu
  • Yuxin Xia
  • Lintao Hou
  • Linfeng Lan
  • Junwu Chen
  • Dan Wang
  • Rihui Yao
Research Paper

Abstract

CdTe nanocrystal (NC)/CdS p–n hetero-junction solar cells with an ITO/ZnO-In/CdS/CdTe/MoOx/Ag-inverted structure were prepared by using a layer-by-layer solution process. The CdS thin films were prepared by chemical bath deposition on top of ITO/ZnO-In and were found to be very compact and pin-hole free in a large area, which insured high quality CdTe NCs thin-film formation upon it. The device performance was strongly related to the CdCl2 annealing temperature and annealing time. Devices exhibited power conversion efficiency (PCE) of 3.08 % following 400 °C CdCl2 annealing for 5 min, which was a good efficiency for solution processed CdTe/CdS NC-inverted solar cells. By carefully designing and optimizing the CdCl2-annealing conditions (370 °C CdCl2 annealing for about 15 min), the PCE of such devices showed a 21 % increase, in comparison to 400 °C CdCl2-annealing conditions, and reached a better PCE of 3.73 % while keeping a relatively high VOC of 0.49 V. This PCE value, to the best of our knowledge, is the highest PCE reported for solution processed CdTe–CdS NC solar cells. Moreover, the inverted solar cell device was very stable when kept under ambient conditions, less than 4 % degradation was observed in PCE after 40 days storage.

Keywords

Nanocrystals solar cells CdTe nanocrystals Solution processed Inverted structure Energy conversion 

Introduction

Thin-film solar cells have attracted intense attention in the past several decades due to their high energy conversion efficiency along with lower material consumption and faster deposition rates (Zweibel 1999). Among which, thin-film solar cells based on cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) active layers had exhibited power conversion efficiencies of 16.7 % (Green et al. 2009) and 20.3 % (Jackson et al. 2011), respectively, along with commercial production costs below $1/Watt. However, these inorganic thin-film solar cells are mostly fabricated by vacuum evaporation, magnetron sputtering, or other vacuum-based deposition methods, which still tend to be times consuming and costly. One way to overcome this drawback and further lower the production cost of thin-film solar cells is to use roll-to-roll solution processing of colloidal semiconductor NCs or polymer to deposit the active layers in a device. To date, our research group has reported power conversion efficiency (PCE) of state-of-the art polymer solar cells approaching 10 % in the scientific literature (He et al. 2012). Comparing to polymer solar cells, semiconductor NC solar cells have attracted significant interest due to the benefits of solution processing combined with the potential to harvest the sun’s visible, near-infrared rays. Most important of all, the band gap of NCs can be readily tuned during the time of synthesis, simply by changing the NCs’ size. This merit makes the fabrication of multiple-junction solar cells by a single materials system consisting of different band gaps easier and lower cost (King 2008). NC solar cells, especially PbS or PbSe NC-based solar cells, have seen rapid advances in the past several years. Promising solar power conversion efficiencies up to 5 % (Tang et al. 2012) and ~7 % (Ip et al. 2012; Liu et al. 2011; Tang et al. 2011) have been reported in PbS NC solar cells based on PbS NC homogeneous p–n junction and PbS–TiO2-depleted hetero-junction devices using a post-deposition chemical and organic/inorganic hybrid passivation strategy. On the other hand, in the case of CdTe (Jasieniak et al. 2011), CuInS2 (Weil et al. 2010), and CuInSe2 (Guo et al. 2010) NCs, in order to achieve high efficiency devices, sintering strategies are often used to obtain excellent electronic properties (high mobility, low lattice mismatch, and low defects) of bulk inorganic semiconductors. Gur et al. (2005) first reported all-inorganic CdTe/CdSe hetero-junction NC solar cells with PCEs as high as 2.9 % by employing a solution processing method and thermal treatment steps. More recently, Olson et al. (2010) and our research group (Sun et al. 2012) adapted this approach to fabricate Schottky solar cells based on CdTe nanorods with efficiencies up to 5 %. To decrease the stress developed during the thermal treatment step within a film of NCs, Mulvaney et al. fabricated CdTe–ZnO (Jasieniak et al. 2011) or CdSexTe1−x–ZnO (MacDonald et al. 2012) NC-based solar cells with PCE ~7.0 %, which was believed to be the highest value ever reported for CdTe thin-film solar cells based on solution-processed NC layers. However, CdTe NC devices with a p–n junction or Schottky structure reported to date are seen to be disadvantaged due to a number of factors. For example, as light absorption begins at the ITO ohmic contact side, rather than the junction, many minority carriers (here electrons) must travel the thickness of the entire film before reaching their destination electrode and are therefore more often subjected to unwanted recombination. On the other hand, the open-circuit voltage in a Schottky device is often limited by Fermi-level pinning due to defects at an interface. Investigations have shown that highly efficient polycrystalline CdTe thin-film solar cells are generally fabricated with p–n device configurations for effectively collecting photo-carriers from devices in which the diffusion coefficients are small (Chu et al. 1991). However, n-CdTe layers are still difficult to produce due the self compensation effect during the doping process, so wide-band-gap n-type semiconductors are often used to construct CdTe hetero-junction devices. As CdS has a band gap of 2.42 eV and similar chemical properties as CdTe, it is the most commonly employed n-type semiconductor partner to p-CdTe. In this hetero-junction system, a large fraction of the photo-generated carriers are generated within the depletion layer allowing more efficient collection.

Although high efficiency had been obtained in CdTe/CdS bulk hetero-junction thin-films solar cells fabricated by vacuum deposition techniques, there are still few reports on the solution processed CdTe NC/CdS thin-films solar cells with satisfy PCE (around or up to 3 %) and stability requirements (device could work under ambient conditions for several days/weeks without obvious PCE drop down). Sahoo et al. (Katiyar et al. 2011) reported the solution-processed CdTe–CdS nanocrystal solar cells with ITO/CdS/CdTe structure and very poor PCE was obtained in this case. A key reason for this is the poor interface between the NC and CdS thin film, partially due to a high number of defects between CBD-CdS and CdTe NC layers formed during the sintering process, which will then result in large leakage current to any devices that are subsequently fabricated. To decrease the defects and optimize interfacial properties, we report herein the realization of good efficiency and stability of all-solution processed NC p–n junction solar cells based on CdTe NC active layers and CBD-CdS window layers using ZnO (TiO2 is also a promising candidate) as interlayer. In addition to acting as an electron transport layer, the ZnO layer provides a number of improvements over samples with a CdS layer deposited directly on the ITO; it provides better electrical stability by forming a smooth and pin-hole-free pre-layer on which the smooth CBD-CdS films can be grown, thus eliminating catastrophic shorts from the upper contact directly through CdTe NC layers to the ITO; it improves the Fermi level and blocks the hole transfer to the ITO electrode, thus allowing more efficient photo-generated carrier extraction and transmission, resulting in higher photocurrents and high efficiency. In this paper, CdTe NC–CdS hetero-junction solar cells with an ITO/ZnO-In/CdS/CdTe/MoOx/Ag-inverted structure are fabricated successfully using a layer-by-layer solution process. For comparing, device with ITO/ZnO-In/CdTe/MoOx/Ag and ITO/CdS/CdTe/MoOx/Ag structures were also fabricated. The devices performance versus annealing temperature and annealing time is investigated and will be discussed. It is found that a further CdCl2 annealing under suitable conditions can largely improve device performance, which may be due to the grain size increasing or defect density decreasing in the junction. Under optimized annealing conditions, a PCE as high as 3.73 % coupled with high stability is obtained, which is the highest PCE reported for solution-processed CdTe NC–CdS thin-film solar cells.

CdTe NCs were synthesized according to a previously published method (Sun et al. 2012) and other research group’s report (Peng and Peng 2001; Yu et al. 2003; Nie et al. 2006). The NC product was extracted from the solution by washing three times with methanol/toluene and separated by centrifugation. Then, NC samples were refluxed in pyridine overnight at 110 °C and centrifuged with hexane. The final NC product was then dispersed into a mixture of pyridine and 1-propanol with a volume ratio of 1:1 at 50 mg mL-1.

Indium-doped ZnO thin films of ~60 nm in thickness were deposited on the ITO/glass substrate by magnetron sputtering, as reported before (Lan et al. 2011). As low hole-carrier density and low mobility (Jasieniak et al. 2011) of CdTe NCs, the used of indium-doped ZnO (high electron density and high mobility) is preferred in order to make full depletion of the CdTe NCs/CdS film and increased the carrier-collecting efficiency. The ITO/ZnO-In was then subjected sonication in acetone and isopropyl alcohol in that order and dried in a hotplate to remove any remaining solvent and other impurities. Before the deposition of CdS thin film, the ITO/ZnO-In substrate was dipped into de-ionized water at 60 °C for 1 min in order to eliminate bubble on the surface of ZnO-In film, which will insure pin-hole free of CdS and good adhesion of the CdS film on ZnO. The CdS thin film was formed on the ITO/ZnO-In substrate by a simple CBD method in an aqueous solution-containing cadmium acetate, ammonium acetate, ammonium hydroxide, and thiourea at concentrations of 5 × 10−4, 1 × 10−2, 1 × 10−3, and 0.1 M, respectively, with bath temperature maintained at 90 °C for 30 min (Britt and Ferekides 1993). The ITO/ZnO-In/CdS products were subjected to sonication in deionized water twice and then dried in an oven before use.

CdTe NCs films were deposited using a layer-by-layer spin-coating process under ambient conditions. For each layer, the CdTe NC ink (50 mg mL−1 in pyridine and 1-propanol) was deposited on the ITO/ZnO-In/CdS substrate and spin-cast at 1,000 rpm for 30 s. The substrate was placed on a hot plate at 150 °C for 3 min to remove any solvent and then dipped in a saturated CdCl2 methanol solution for ~3 s, taken out, and rinsed with 1-PrOH then dried under a nitrogen stream. Finally, the sample was sintered on a hot plate at 350 °C for 40 s. This process was repeated several times until a final CdTe NC thin film with thickness of ~500 nm was obtained. To activate the CdTe–CdS junction, a further CdCl2 treatment was carried out. Several drops of saturated CdCl2 methanol solution were deposited on top of the CdTe NC thin film and then spin-cast at 1,000 rpm for 10 s. The substrate was immediately placed on a hot plate at 360–400 °C for different time periods (0–20 min). The ITO/ZnO-In/CdS/CdTe products were subjected to sonication in methanol twice more to remove any remaining CdCl2 and blown dry under a nitrogen stream. The MoOx (~30 nm) and silver (~100 nm) back contact were deposited in sequence via thermal evaporation through a shadow mask. The active area of the solar cells was 0.16 cm2.

PCE of CdTe NC solar cells was measured under an illumination of 1,000 W m−2 with an AM1.5 solar simulator (Oriel model 91192) while the current density–voltage (JV) curves were measured with a Keithley 240 source measure unit. The external quantum efficiencies (EQE) of the inverted PVCs were measured with a commercial photomodulation spectroscopic setup, and a calibrated Si photodiode was used as a standard.

As shown in Fig. 1a and b, a hetero-junction device based on CdTe NCs was composed of an indium doped ~60-nm ZnO film made by using rf magnetron sputtering; a ~120-nm CdS film made by CBD method; a ~500-nm CdTe NCs film (Fig. 1b) deposited from solution in a layer-by-layer-sintering process (five layers); an ohmic MoOx/Ag contact deposited via thermal evaporation. MoOx had been found to be very useful as an Ohmic contact for hole extraction in organic photovoltaic cells and also for use as a CdS/CdTe solar cell back contact material, as was reported recently (He et al. 2012; Lin et al. 2010). The use of MoOx/Ag back contacts can avoid the shunting problems associated with CdCl2 solution pretreatment and low functional metal doped in the interface. As shown in the inset of Fig. 1a, the electrons were injected from the CdTe NC layer into to the conducting band of CdS and then to ZnO, while MoOx/Ag acted to collect holes from the CdTe.
Fig. 1

a Schematic of the CdTe NCs–CdS hetero-junction solar cells device used in this study. Inset schematic band diagram of ITO, In-ZnO, CdS, CdTe, MoOx, and Ag. b Cross-sectional SEM image of the same device. Scale bar 2000 nm

The morphology of the as-prepared CBD-CdS with and without ZnO buffer layer was shown in Fig 2. The particle size of CdS was about 100 nm and showed very well compaction between NCs which was pin-hole free over a large area (~1 cm2). On the contrary, without a ZnO buffer layer (Fig. 2b), the morphology of ITO–CdS was much rougher and a lot of pin-holes were found throughout the whole substrate. The pin-hole will result in large leakage current in the device without ZnO buffer layer and low PCE was expected in that case. Therefore, we have chosen ZnO thin films that were prepared by sputtering as a buffer layer for fabrication of CdTe–CdS hetero-junction solar cells with an inverted device structure.
Fig. 2

Surface morphology of the CBD-CdS thin film a with ZnO buffer layer, scale bar 1 μm and b without ZnO buffer layer, scale bar 2 μm

Figure 3 shows the absorbance and transmission of the CBD-CdS thin-film structure. The measurement was taken out by using ITO/ZnO-In as standard sample (which means that the absorbance of ITO/ZnO-In has been eliminated). The band gap of CBD-CdS was about 2.4 eV calculated from the absorbance edge. From the transmission spectra (inset of Fig. 3) we find that the ITO/ZnO-In/CdS thin-films blocked the light with a wavelength shorter than 500 nm and showed almost transparent behavior for wavelengths from 500 to 900 nm (the transmission of bare ITO is about 90 % for wavelength from 400 to 900 nm).
Fig. 3

Absorption and transmission spectrum of the CBD-CdS

CdTe NC active layers with ~500-nm thickness were prepared on top of ITO/ZnO-In/CdS by using a layer-by-layer sintering process described before (Sun et al. 2012). The substrate was then subjected to CdCl2 treatment at 350–400 °C for different time periods to activate the CdTe–CdS junction. It is well known that the CdCl2 treatment at around 400 °C offers many benefits to the solar cell devices, such as increased grain size, active the CdTe/CdS junction, stress release and reduced lattice mismatch between the CdS and CdTe layers, which improves the CdS/CdTe junction quality by forming an alloyed CdTexSl−x interface between CdS and CdTe (Paulson and Dutta 2000; Moutinho et al. 1998). Due to this merit, the CdCl2 treatment could improve the electrical properties of the CdTe solar cells and eventually result in a significant increase in PCE. Following this successful treatment on CdTe/CdS hetero-junction solar cells, 400 °C-annealing temperature is first choice temperature to treat our CdTe NCs solar cells. Figure 4a presents the current density versus voltage (JV) characteristics of the CdTe NC–CdS hetero-junction solar cells with CdCl2 treatment at 400 °C for different time periods, under 1,000 W m−2 (AM 1.5G) illumination. Device parameters such as JSC, VOC, FF, and PCE are deduced from the JV characteristics (summarized in Table 1). We can see from Table 1 that low JSC of 9.14 mA/cm2 is obtained when no further CdCl2 treatment is adopted, while the FF below 30 % resulted in a low PCE of 1.34 %, which implies a large defect density and large recombination exists at the interface of CdTe/CdS, thus resulting in large series resistance. The PCE of devices is exclusively below 2 % under limited annealing time (less than 3 min). However, the JSC is almost double that without further CdCl2 treatment when CdCl2-annealing time is up to 5 min and the FF also increases, resulting in PCE values up to 2 %. Device with highest efficiency of 3.08 % is obtained in the case of 5-min annealing time. The enhanced JSC and FF in the devices may origin from NC grain size increase, reduced stress and increasing absorption of photons due to the formation of a CdTexSl−x layer and reduced recombination in the interface, or a combination of these factors. It should be noted that the Voc is around 0.5 V for the case of limited annealing time (less than 3 min) produced samples, while only 0.4 V for long-annealing time (up to 5 min) samples, almost 20 % off in Voc. The dropdown in Voc implied that samples are over heat-treated and the junction is somewhat deteriorated, although this rarely occurs in the case of CdTe–CdS solar cells fabricated by the CSS method. We speculate that this may be due to the fact that our devices are fabricated in ambient conditions and so oxidation of CdTe NCs is inevitable. Furthermore, CdTe NCs with small size (below 100 nm) is easily broken down during over treatment and result in large leakage current, which also confirmed by Carter et al. (Olson et al. 2010). Low CdCl2-annealing temperature may result in good device performance by increasing the JSC but keeping Voc at a relatively high scale, our results confirm this. To investigate the effect of the annealing temperature, we fabricated samples with CdCl2 treatment at 370–390 °C for around 15 min, respectively. As shown in Fig. 3b and Table 1, evidently, the Voc of all the samples is up to 0.44 V, and FF up to 40 % coupled with high JSC, resulting in PCE up to 2.4 %. It should be pointed out that our best performing device is obtained in the case of CdCl2 treatment at 370 °C for about 15 min. The open-circuit voltage (Voc), short-circuit current density (JSC), and fill factor (FF) of such a device are 0.49 V, 17.50 mA/cm2, and 43.5 %, respectively, corresponding to a high PCE of 3.73 %. This is, to the best of our knowledge, the highest PCE reported for solution-processed CdTe NC–CdS hetero-junction solar cells. We should point out that the PCE of most device (at least ten piece devices are measurement) treated at this temperature is in the scale of 3.4 ± 0.3 %. The device performance shows negligible changing between 15 and 30 min annealing time.
Fig. 4

JV characteristics of the ITO/ZnO-In/CdS/CdTe/MoOx/Ag solar cells a with different annealing time, and b difference annealing temperature

Table 1

Photovoltaic performances of solar cells fabricated under different conditions (under irradiation of AM1.5G at 100 mW/cm2)

Annealing temperature (°C)

Annealing time (min)

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

400

1

8.36

0.54

30.57

1.38

400

3

10.98

0.47

29.07

1.50

400

5

17.22

0.40

44.72

3.08

400

8

16.63

0.40

32.02

2.13

400

10

14.76

0.40

34.89

2.06

400

15

17.10

0.41

37.23

2.61

390

15

14.04

0.44

43.71

2.70

380

15

13.20

0.45

40.40

2.40

370

15

17.50

0.49

43.50

3.73

No

0

9.14

0.51

28.75

1.34

Figure 5 shows the external quantum efficiency (EQE) spectrum of devices fabricated with a ITO–CdTe–Al Schottky structure (the Jsc, Voc, FF, and PCE are 15.20 mA/cm2, 0.51 V, 53 and 4.1 %, respectively) and ITO/ZnO-In/CdS/CdTe/MoOx/Ag (the Jsc, Voc, FF, and PCE are 17.14 mA/cm2, 0.50 V, 42, and 3.6 %, respectively) p–n junction structure using the same CdTe NCs solution. The conventional ITO/CdTe/Al Schottky device shows good spectral response at short wavelength (below 500 nm) and a maximum EQE of ~60 % at 550 nm, consistent with our previous report (Sun et al. 2012). For the p–n junction device with ZnO and CdS window layers, the EQE decreases in short-wavelength response (below 500 nm) due to the absorption of CdS while substantial enhancement in EQE over 550 nm and the photo-response, with average EQEs exceeding 50 % in the range 500–830 nm. It is noted that the spectrum response is extended to ~860 nm while this value is ~830 nm for the Schottky device. We attribute this enhancement to increased absorption in the bulk hetero-junction layer due to grain size growth and the formation of a CdTexSl−x transitional layer.
Fig. 5

EQE spectra for ITO/ZnO-In/CdS/CdTe/MoOx/Ag (black symbols) and ITO/CdTe/Al (red symbols) devices

To compare the CdTe NCs device performance with inverted structure, device with ITO/CdS/CdTe/MoOx/Ag and ITO/ZnO-In/CdTe/MoOx/Ag are also fabricated (device with normal structure of ITO/CdTe/CdS/Al is also fabricated but exclusively fail due to the peel off CdTe NCs layers during the deposition of CdS layer in alkaline solution). The device preparing process is similar to those of ITO/ZnO-In/CdS/CdTe/MoOx/Ag devices with high efficiency (PCE of 3.73 %). As shown in Fig. 6, the Jsc, Voc, FF, and PCE of device with ITO/CdS/CdTe/MoOx/Ag structure are 4.75 mA/cm2, 0.3 V, 40, and 0.57 %, respectively, while these values are 5.09 mA/cm2, 0.3 V, 25.54, and 0.39 %, respectively, for ITO/ZnO-In/CdTe/MoOx/Ag device structure. The low PCE obtained in the case of ITO/CdS/CdTe/MoOx/Ag device is due to the pin-hole in CBD-CdS. On the contrary, we speculate that large defect emerge during the CdTe NCs sintering process as large lattice mismatch between CdTe and ZnO. Further investigation is still going on.
Fig. 6

JV curves of CdTe NCs solar cells with different invert structure

Figure 7 shows the dependence of device performance on light intensity, while the device parameters such as JSC, VOC, FF, and PCE are summarized in Table 2. It is clearly that the PCE (corrected to one Sun) of all devices measured under different light intensity contains up to 90 % of its one Sun PCE, only a little drop down is found in the case of 2 Sun light density. The short-circuit current (JSC) shows almost linear behavior with light intensity from 0.2 Sun to 2 Sun (Fig. 7b). For example, the JSC is 17.76 mA/cm2 at one Sun, while this value is 35.52 mA/cm2 at 2 Suns, twofold the value before, which indicates no build-up of net space charges in either device even at very high illumination intensity. We speculate that this is due to the high electric field as well as high and balanced charge carrier mobility.
Fig. 7

The dependence of device performance on light intensity. aJV curves and bJSC as a function of light intensity

Table 2

Summarize devices performance at different light intensity (under irradiation of AM1.5G)

Number of Sun

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (one sun) (%)

0.2

3.34

0.42

41.92

2.94

0.4

6.85

0.44

40.35

3.04

0.6

10.58

0.45

44.72

3.17

0.8

13.37

0.45

39.62

2.98

1.0

17.76

0.45

39.79

3.18

1.2

20.57

0.45

37.23

3.04

1.5

26.36

0.45

37.05

2.93

2.0

35.52

0.43

33.00

2.52

To investigate the stability of our devices, a freshly made and packed device with 3.59 % efficiency (Voc = 0.47 V, Jsc = 17.97 mA/cm2, FF = 42.51 %) is kept under ambient conditions (exposure to air and room light) for different time periods and taken out for measurement. As can be seen from Fig. 8a,b, the cell with such an inverted structure is very stable and experienced only minor changes over 40-day period. A slight increase in the JSC and decay in FF is observed following 20 days storage, while VOC is kept unchanged during the same time, leading to less than 3 % drop down in device efficiency over the test period. We speculate that the increase in the JSC is likely due to an increase in the conductivity of CdTe NC film or MoOx/Ag interface. A small drop down in VOC (0.01 V) is found after 40 days storage which may be due to the diffusion of Ag from MoOx to the CdTe NC active layer, which results in less than 4 % device efficiency during the subsequent test. The devices stability is mainly due to the avoidance of using low work function metals for direct electrode contact. The result also compares favorably to other solution-processed solar cell technologies such as organic photovoltaic devices (He et al. 2012), CdTe–CdSe all-inorganic hetero-junction (Gur et al. 2005).
Fig. 8

Stability of ITO/ZnO-In/CdS/CdTe/MoOx/Ag solar cells kept under ambient conditions with different storage time aVOC and FF, b PCE and JSC

In conclusion, sputtered ZnO thin film was used as a buffer layer to prepare pin-hole-free CdS thin film and CdTe NC/CdS p–n hetero-junction solar cells with a ITO/ZnO-In/CdS/CdTe/MoOx/Ag-inverted structure were demonstrated. Annealing temperature and annealing time were found to have great effect on the performance of solar cell devices. A PCE of 3.73 % is obtained in the case of CdCl2 treatment at 370 °C, which is the best result ever reported for solution-processed CdTe NC/CdS hetero-junction solar cells. The JSC of devices showed linear behavior upon exposure to light intensity in the range of 0–2 Sun, which indicates no build-up of net space charges even at very high illumination intensity. We also found that solar cell devices have very good stability. When the device was kept under ambient conditions, there was no obvious degradation observed in the Jsc, Voc, and FF, resulting in less than 4 % dropdown in PCE after 40 days. Our results confirm that, if junctions of CdS and CdTe undergo better optimized processing and treatment, the device performance can be definitely improved.

Notes

Acknowledgments

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51073056, 50990065, 51010003, 61274062, and 11204106), National Science Foundation for Distinguished Young Scholars of China (Grant No. 51225301) and SCUT Grant (No. 2013ZZ0016).

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Copyright information

© Springer Science+Business Media Dordrecht 2013

Authors and Affiliations

  • Yiyao Tian
    • 1
  • Yijie Zhang
    • 1
  • Yizhao Lin
    • 1
  • Kuo Gao
    • 1
  • Yunpeng Zhang
    • 1
  • Kaiyi Liu
    • 1
  • Qianqian Yang
    • 1
  • Xiao Zhou
    • 2
  • Donghuan Qin
    • 2
  • Hongbin Wu
    • 2
  • Yuxin Xia
    • 3
  • Lintao Hou
    • 3
  • Linfeng Lan
    • 2
  • Junwu Chen
    • 2
  • Dan Wang
    • 2
  • Rihui Yao
    • 2
  1. 1.School of Materials Science and EngineeringSouth China University of TechnologyGuangzhouChina
  2. 2.Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & DevicesSouth China University of TechnologyGuangzhouChina
  3. 3.College of Science and EngineeringJinan UniversityGuangzhouChina

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