A novel method using magnetic doping to form highly-crystalline semiconductor compound in an ambient environment has been explored for the first time. The geometries and lengths of the resulting Zn-doped CIS nanocrystals can be manipulated as a function of the duration in a high-frequency magnetic field. The Zn-CIS nanocrystals exhibited excellent phase pure optical properties and chemically pure crystallinity compared to those prepared via conventional high-temperature methods. The UV-vis absorption spectra of the Zn-CIS colloids has a strong absorption over a relatively wide region from UV to near IR, depending on the dimensional evolution of the Zn-CIS nanocrystals, indicating that the highly-crystalline Zn-CIS colloidal compound prepared in this work can be considered as a potential candidate for solar absorption. Such a magnetically-induced synthesis technology offers great advantages over currently existing methodologies for the growth of these nanocrystals, resulting in better crystalline development and compositional uniformity for such a complex semiconductor compound. Such a novel synthesis method provides a new avenue for the development of ternary chalcopyrite materials via magnetic doping. We also believe that an expansion of such a synthesis technique can be adapted to other important semiconductor compounds, resulting in considerable improvement in purity and optically tunable properties.
Chapter 7
Application of Zn-CuInS2 Nanocrystals Synthesized through Magnetic Field
To employ the Zn doped CuInS2 (Zn-CIS) nanocrystals, as described in chapter 6, for practical application, we prepared a series of Zn-CIS nanocrystal-based solar cell devices using the nanocrystals of various shapes, to measure the solar-cell parameters. The prototype device is schematically represented in Figure 7.1, and Figure 7.2 presents the current-voltage characteristics of the nanocrystal-based devices. The relevant solar-cell parameters for those three samples are given in Table 7.1, which include the current density at short circuit (JSC in mA cm−2), the voltage at open circuit (VOC in V), the fill factor (FF), and the efficiency of power conversion (η = JSC·VOC·FF/Pin with Pin = 100 mW cm−2).
Nanocube-based device displayed following characteristics: Jsc = 3.012 mA/cm2, Voc = 0.61V, FF = 0.38, and an efficiency of 0.70 % in average. The Jsc values increased from 3.012 to 3.317 mA/cm2 at nanopyramid-based device accompanied with increasing efficiency from 0.7 to 0.80%. The maximum value of Jsc reached 4.21 mA/cm2 at nanobar-based device with a power conversion efficiency of 1.01% in this study. This result indicated that the ability of light-absorption of Zn-CIS nanocrystal was enhanced once the crystal size was increased. It is obviously suggested that the increasing efficiency of these devices is related to the broadened absorption wavelength of larger Zn-CIS nanocrystals.
Referring to previous chapter (see chapter 6), the absorption band of Zn-CIS nanocrystals shows a red shift with increasing size, as shown in Figure 7.3. As can be seen in the Table 7.1, compared to other parameters such as Voc and FF, the Jsc is more determinative for this trend, corresponded to the absorbance spectrum.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
However, the highest value (4.21 mA/cm2) of Jsc here is still lower than reported [166] (17 mA cm-2), which might result from thinner absorber layer. Therefore, we deposited the Zn-CIS layers of different thickness for prototype devices, in order to enhance efficiency of the device by increasing Jsc. (The Zn-CIS nanocrystals used here is nanobar-like sample.)
Figure 7.1 Prototype structure of Zn-CIS device.
Figure 7.2 Current-voltage characteristics of Zn-CIS devices with different shape nanocrystal under stimulated AM 1.5 solar illumination (100 mW/cm2) and active area 0.28 cm2.
The thickness of Zn-CIS layer can be controlled by varying the nanocrystal concentration in the suspensions. Figure 7.4 shows the relationship between absorber layer thickness and the efficiency (the thickness here measured by α-step analysis.). After increasing the thickness of Zn-CIS film, the Voc value shows a slightly increase from 0.59 to 0.62 V which is related to the amount of Zn in the Zn-CIS layer. Based on chapter 6, the incorporation of small amounts of Zn led to an increased Voc [2]. The amount of Zn in the nanobar-like nanocrystal is lightly more than that in other two kinds of nanocrystals, which might provide explanation to the variation of the Voc. The FF value shows a decline from 0.41 to 0.39 at 2.132 μm, indicating the increasing charge recombination with thicker Zn-CIS absorber film.
The source of charge recombination might have been a result of film cracking, which became more significant for thicker films than for those two thinner ones. The results display a notably systematic trend for JSC, such that the current density increases significantly from JSC = 4.21 mA/ cm2 at 1.012 μm to JSC = 5.871 mA/cm2 at 2.132 μm because thicker absorber offers a promotion of light absorption and photo carrier collection. Because the extent of the increase in JSC was much greater than the extent of the decrease in FF, the overall efficiency of conversion of photons to current exhibits a systematic increase from η = 1.01 % at 1.012 μm to η = 1.44 % at 2.132 μm.
These devices provide a baseline performance and demonstrate as a proof-of-concept that these nanocrystals can be used in PVs. Practical devices, however, require higher efficiencies. There are many ways to try to promote PV efficiency, including using nanocrystals with shorter chain capping ligands, incorporation Ga into the films, and using various chemical or thermal treatment of nanocrystal layers to increase their conductivity. New device architectures that are more favorable to using nanocrystal absorber layers and low-temperature
400 500 600 700 800 900
Normalized Intensity (a.u.)
Wavelength (nm)
nanocube nanopyramid nanobar
manufacturing steps may also provide ways to increase device efficiency and eliminate the need for high temperature processing. These are all important topics for further study.
Table 7.1 Photovoltaic performance of the different shape-Zn-CIS nanocrystal-based devices under AM 1.5 solar illumination (100 mW/cm2) and active area 0.28 cm2.
nanocube nanopyramid nanobar
Jsc/mAcm-2 3.012 3.317 4.21
Voc/V 0.61 0.62 0.59
FF 0.38 0.39 0.41
η(%) 0.7 0.80 1.01
Figure 7.3 UV-vis absorption spectra of different shape Zn-CIS nanocrystals.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0
2 4 6
Current Density (mA/cm2 )
Voltage (V)
1 um 1.5 um 2 um
Figure 7.4 Current-voltage characteristics of Zn-CIS devices with different Zn-CIS film thickness under stimulated AM 1.5 solar illumination (100 mW/cm2)
and active area 0.28 cm2.
Table 7.2 Photovoltaic performance of the different thickness of Zn-CIS nanocrystal film devices under AM 1.5 solar illumination (100 mW/cm2) and active area 0.28 cm2.
Thickness (μm) 1.012 1.594 2.132
Jsc/mAcm-2 4.21 4.959 5.871
Voc/V 0.59 0.61 0.62
FF 0.41 0.41 0.39
η(%) 1.01 1.25 1.44
Chapter 8
Conclusion
In this thesis, we study the synthesis of nano-scale chalcopyrite CuInS2 (CIS) semiconductors and their application to energy issue. Firstly, we have synthesized CIS nanocrystals smaller than the Wannier-Mott size (＜8.1 nm) through a colloidal solvent process. The nanocrystals produced by this method are 2.4±0.5 nm in size.
We have observed the emission peak around 450 nm of these CIS nanoparticles, which is significantly shorter than that of bulk CIS (810 nm) due to quantum confinement effect. After deposition of ZnS layer onto these bare CIS nanocrystals, the photoluminescence intensity was dramatically enhanced by elimination of surface defects. However, new defects were generated with the thickness of ZnS layer over 2 monolayers and resulted in a decreasing PL intensity. (Chapter 4)
In order to extend the CIS nanocrystals onto solar energy application, we dope the Zn into CIS nanocrystal to broaden the absorbance spectrum. The average particle sizes of synthesized Zn-CIS nanocrystals with different Cu/Zn ratio (n) from 1~3 are 3.5, 4.5, and 4.4 nm respectively. The excitonic transition peaks are shifted to 530, 570, and 660 nm for n = 1, 2, and 3, respectively. It is noticeable that the absorption and PL properties of Zn-CIS nanocrystal are dominated by the concentration of Zn doping. The Cu site in the lattice is preferentially substituted by Zn and the substitution is found to be more energetically favorable to prevent anti-site defects, which will affect the absorption spectrum. We have deposited these Zn-CIS nanocrystals onto 5 μm long ZnO nanowires to fabricate the solar device.
The best power conversion efficiency of these devices we obtained with different n values is 0.28 % at n = 2 (Jsc = 1.71 mA/cm2, Voc = 0.34 V, FF = 0.48). The reasons for such low efficiency of this kind of quantum dot-based solar device might
be the QD corrosion from electrolyte and fast recombination. We have demonstrated ZnS deposition as a protecting layer to prevent the corrosion and reduce the recombination by prolonging the exciton lifetime. The efficiency is 0.66 % for the initial ZnS deposition with 0.6 monolayers and promotes to 0.71 % by 1.3 monolayers ZnS deposition due to efficient enhancement of Jsc from 1.71 to 3.21 mA/cm2. (Chapter 5)
Based on Zn-CIS nanocrystals, we continue to investigate the effect of Zn doapant and new synthesis method of this series material. We have synthesized Zn-CIS nanocrystals with different geometries in the presence of high frequency magnetic field (HFMF) at ambient condition. Crystal shape deviates from spherical, cubic, pyramidal, and finally bar with duration time of HFMF. The Zn-CIS nanocrystals obtained within the first 60 seconds exhibited quantum confinement effect, showing emission peaks at 590 (30 sec) ad 630 (45 sec) nm. After longer (120, 300, and 420 seconds, respectively) duration, the UV-vis spectra show the typical absorption curve of a semiconductor material: a band-edge peak was visible at around 710, 780, and 800 nm respectively. All these nanocrystals with variety shapes are monodisperse and highly-crystalline, indicating convenience, rapidity, and novelty of our method. This result demonstrates the fast crystal growth under magnetc-induced heating of such magnetic dopant chalcopyrite semiconductors have potential for optical and energy application. (Chapter 6)
For expansion of magnetic-induced synthesized Zn-CIS nanocrystals, we have fabricated a series of solar cell devices using Zn-CIS nanocrystals of various shapes.
The best efficiency we obtained is 1.01 % for the nanobar-based device, with Jsc = 4.12 mA/cm2, Voc = 0.59V, FF = 0.41. The thickness of absorber layer in these devices is around 0.983~1.012 μm. The efficiency of performance is promoted to 1.41 % after deposition of thicker absorber layer (2.132 μm). There are many ways
to try to promote PV efficiency, but our results show the great potential for application in optical and energy device. (Chapter 7)
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