在此研究中，我們利用多元醇法(polyol route)製備出具有單一黃銅礦晶相結構的CuIn1-xBxS2奈米粒子，並且加入Sb，x介於0~0.5之間。此研究之奈米粉體使用氯化銅(CuCl)、氯化銦(InCl3)、氧化硼(B2O3)及硫元素(S) 、氯化銻(SbCl3)藥品所製備，並去討論加入Sb後，在不同反應溫度、反應時間及PVP對CIBS莫耳比對實驗的影響。
在實驗過程中，我們不斷改變各式參數來討論CIBS奈米粒子的成長情況，並發現在這個實驗系統下，在PVP對CIBS的莫耳(mole)比在1：1.5，反應溫度310oC、反應時間6小時等條件下，合成出的奈米粒子其晶粒大小約介於30~40nm之間，輔以XRD及Raman分析鑑定的CIBS具有單一黃銅礦相結構、TEM、SEM影像了解更細微的顯微結構、使用EDS及ESCA光譜來檢驗CIBS的成分及UV-Visible吸收光譜發現製程較好的參數條件下所製備出的Cu(In0.9B0.1)S2薄膜其光能隙值(Band Gap)約為1.65 eV。

SUMMARY

Chalcopyrite of copper indium aluminum sulfur (Cu(In1-xBx)S2; CIBS) nanocrystals for solar cell materials have been successfully synthesized by a relatively simple and cost-effective method, i.e., a modified polyol route. The In/B ratio in the CIBS nanocrystals can be controlled by varying the In/B ratio in the reactants. The use of reactants, CuCl2, InCl3, B2O3 and S powders, in a three-neck flask with tetraethylene glycol as a solvent, spherical CIBS nanoparticles with diameter in the range of 30-40 nm can be obtained at temperatures in the range of 280 - 310oC. The characteristics and average sizes of these nanocrystals are analyzed with sophisticated instruments, such as X-ray diffraction pattern (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The nanocrystals can be readily dispersed in cyclohexanone with an ultrasound sonication, spin coated on a substrate and annealed at 450oC to form a film with grain growth. The composition of the final film and its optoelectronic properties are analyzed with electron spectroscopy for chemical analysis (ESCA), Hall effect and UV-visible measurements.

Keywords: CIBS nanocrystals, cost effective, solution process, compound materials for PV

INTRODUCTION

Chalcopyrite ternary semiconductors have often been employed as absorber layers in high-efficiency thin film solar cells because of their superior optoelectronic properties such as high mobility, long diffusion length, high absorption coefficient, suitable band gap etc. Increasing the efficiency of AIBIIICVI2 chalcopyrite photovoltaic cells from 14.1% (world record for CuInSe2) [4] by substitutions of In or Se with other elements from the same groups of periodic table have attracted much of the research attention. Photovoltaic cells based on an absorber layer of CuIn(S,Se)2, Cu(In,B)Se2, CuBS2, CuGaSe2, Cu(In,Ga)S2 and many other combinations have been made. Currently, the greatest energy conversion efficiency of 20.3% has been achieved for CIGS solar cells. Although the band gap of CuIn1-xGaxSe2 (CIGS) system is tunable in the range from 1.04 eV (CuInSe2) to 1.69 eV (CuGaSe2), further increasing of Eg towards to an optimum value of 1.37 eV leads to losses in fill factor and open circuit voltage, and a decrease in cell performance. Moreover, gallium is a rare and expensive element and selenium is a toxic material, replacing indium with boron and selenium with sulfur may be a first step to reduce the cost and make the solar cells user friendly. In addition, the optical band gap of CuInS2 and CuBS2 was 1.5 and 3.8 eV, respectively. Therefore it can be expected that the partial replacement of In by B can properly control the optical band gap and serves as another candidate of solar cell material, or as a top cell of the tandem solar cell when the bandgap is relatively large. In fact, fabrication of CIASe solar cells using sequential deposition of Cu, In, and Al with a final selenization process has been made that an energy conversion efficiency of 16.9% has been achieved. It appears that the success in fabrication of a good quality CIASe absorber to make a high efficiency of solar cell promotes the interest of study of CIAS and CIBS film made by a low cost solution process.

MATERIALS AND METHODS

For synthesis of the CIBS nanocrystals in the polyol process, a typical reaction can be made with 1 mmol of CuCl2, 0.9 mmol of InCl3, 0.1 mmol of B2O3, 2 mmol of elemental S and 0.1 mmol of SbCl3 in a 100-mL three-neck flask with attached condenser. the solution in the flask is then heated to 280-310 oC for 6h under vigorous stirring. Then, PVP dissolved in 20-mL of tetraethylene glycol was poured into the three-necked flask after reaction is complete. The use of PVP is to cap the nanoparticles so that not only it avoids aggregation of nanoparitcles, but also it can restrict diffusion of reactants into nanoparticles and significantly reduce the growth rate and size of the nanoparticles. After cooled to the room temperature, isopropyl alcohol is added into the solution to precipitate the CIBS nanocrystals. Purification is made initially by a centrifugation with a speed at 13,000 rpm for 5 min. Then, the brown-colored supernatant is decanted and the remaining solids are washed with isopropyl alcohol. The compositions and morphologies of the nanocrystals were determined by energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM) using an acceleration voltage of 160 kV, respectively. Whereas the structural properties were determined by the X-ray diffraction (XRD) technique using CuK radiation (λ = 0.15418 nm). Finally the precipitates were dispersed in cyclohexanone with an ultrasound sonication to form a stable ink with a concentration of ~100 mg/ml. The cyclohexanone dispersed with CIBS nanocrystals can be spin coated on a quartz glass and annealed at 450 oC for 30 min in a graphite box to enhance grain growth. The thickness of the spin coated CIBS film is approximately 1.2 μm. Measurements of the optoelectronic properties for the CIBS thin film obtained can be made and provided for solar cell applications.

RESULTS AND DISCUSSION

Typical result of the nanoparticles from the XRD measurements is shown in Fig. 1, for B at different content and the diffraction peaks shift to higher 2θs with increasing the B content, due to the decreased lattice spacing with the smaller B atoms substituting for the larger In atoms. The TEM images of the CIBS nanoparticles formed is shown in Fig. 2. The size and morphology of the CIBS nanoparticles synthesized at 310oC for 6 h were further examined, as shown in Fig. 2 for both the TEM image of the CIBS nanoparticles. Figure 2 shows that the CIBS nanoparticles are poly-dispersed in both size and shape with particle sizes in the range of 30~40 nm. After spin coating the CIBS nanoparticles and formation into film by annealing, the grain size of the film with addition of Sb is much greater than the grain size of the film without Sb, as shown in Fig. 3. It appears that the Sb in the compound acts only as catalyst to promote grain growth during the annealing process.

The concentration and mobility of charge carries in the CIBS film are in the range from 3.5132 x 1013 to 4.1215 x 1012cm-3, and from 28.86 to 86.31 cm2/(V s), respectively in the Hall effect. The concentration and mobility of charge carriers in the CuInS2 film are 3.1521 x 1013 cm-3 and 31.58 cm2/(V s), respectively. It appears that slight addition of B into the CIS material, e.g. CI0.9B0.1S2, can increase carrier concentration but decrease the mobility of charge carriers. The variations of both carrier concentration and mobility with the molar ratio of B/(In+B) are shown in Fig. 4. When the molar ratio of B/(In+B) is greater than 0.1, the carrier concentration decreases with increasing the B contents whereas mobility increases with the B contents. The bandgap of the materials can be estimated from linear extrapolation of the curve for versus photo energy, as shown in Fig. 5 for the CIBS film with x=0.1 and the bandgap of 1.65 eV.

CONCLUSION

Chalcopyrite CIBS nanopcrystals for solar cell materials are successfully synthesized by using a relatively simple and convenient modified polyol route in three-neck flask with tetraethylene glycol as a solvent. The CIBS nanocrystals with diameters in the range of from 30 to 40 nm are obtained with increasing the B content from the reduction reactions of the chlorides of Cu, In, B, and elemental S powders under the temperatures at 310oC for 6h. The characteristics of these nanoparticles produced were analyzed with different analytical techniques. It appears that the reaction processes presented have advantages of short period, without sealing the container and requiring only cheap and simple precursors. The polyol solution acts as both a solvent and a reducing agent and is nontoxic ,and Sb is added with a modified surface structure. Formation of the nanocrystals synthesized into CIBS thin films with different contents of B can have good optoelectronic properties that are suitable for solar cell fabrication. The method proposed is user friendly and suitable for mass production.

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