SPIE Proceedings [SPIE Photonic Devices + Applications - San Diego, CA (Sunday 10 August 2008)] Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device

Preparations for low-cost silica substrate of CIGS solar cell

Ming Seng Hsu*a, Chung Chih Changa, Hsiang Hshi Chenga, Yueh Ouyanga, Shinn Der Sheub

a Dept. of Physics, Chinese Military Academy, Feng shan Kaohsiung 830, Taiwan; b Dept. of Computer Sciences and Information Engineering, Neo-Zeon Institute of Technology of

Taiwan

ABSTRACT

The production of CuInGaSe2 (CIGS) solar cell is based on vacuum processes, which requires a high manufacturing temperature and high cost. Our result show a simple method has been developed to prepare the silica substrates of CIGS solar cell. It’s synthesized by sol-gel process from tetraethylorthosilicate (TEOS), methanol (CH3OH) and pure water (both ion-exchange and distillation) in the presence of ammonia as catalyst. The preparation procedure was elaborated as the flexible sequence to control chemical composition and properties of the particles in sol-gel-derived silica substrate. The morphology, particle size, and size distribution of CIGS substrate were characterized with dynamic light scattering (DLS) and atomic force microscopy (AFM). The results of AFM morphology and statistic evidence we find an easy way, non-vacuum and low temperature processes, to successfully prepare the CIGS solar cell substrates with surface roughness below 3 nm. It is powerful the advance study in low cost solar cell.

Keywords: nanoparticles, CIGS, solar cell, low cost

1. INTRODUCTION

CIGS thin film solar cell is based on a vacuum processes in high temperature it is hard to develop the industrial processing1-3 now. Thus, the sol-gel method is the first choice of low-cost and low-temperature processes to manufactory the substrate of CIGS thin film solar cell. In addition, the importance and advantages of nanoparticles were shown not only in the scientific field, but also in various industrial applications because of their chemical, thermal, electrical and optical properties. The quality of these products is highly dependent on the size and size distribution of the particles4-5. Silica nanoparticles are used to make catalysts, electronic substrates, thin film substrates, electrical insulators, thermal insulators, pigments, pharmaceuticals and sensors because of the particles play a different role in each of these products. Of these particles, depending on the synthesis process parameters, the structure of colloidal particles may vary from isolated spherical particles to agglomerates of complex structures6. The sol-gel processing of inorganic ceramic and glass materials began as early as the 19th century with Ebelman7 and Graham's8 studies on silica gels. Initial a system of chemical reactions which controlled the growth of spherical silica particles were developed by Stöber and Fink9. Over the past two decades the picture has emerged of acidic silica hydrogen (i.e. at pH<2) being formed by the coalescence of nanometer-scale polymer particles containing ~4 or 5 silica tetrahedra to form secondary particles10. Till 1990 Brinker and Scherer have documented the effect of pH and surface charge on gel time and particle growth, through the extension and cross-linking of polymer chains as is the case for organic polymers. At pH~2 the surface charge is neutral, becoming slightly positive at pH<2 (acidic gels), resulting in long gel times and little particle growth other than through aggregation. The surface charge is negative at pH>2, facilitating more rapid gel up to pH~6. At pH>7 particles are highly anionic, mutually repulsive, form stable sols and grow mainly by the dissolution of smaller particles and deposition of silica on larger particles due to solubility differences11. As device surface particle sizes plays an increasingly important role in increasing product yield. Most studies of particle sizes or size deviation control had focused on the processing parameters of fabrication. In 2002 two-stage method, semibatch/batch hydrolysis reaction of Si(OC2H5)4, was presented for preparing highly mono-dispersed silica particles. The slower rate of hydrolysis of the tetraethylorthosilicate (Si(OR)4 or TEOS; R= C2H5) that occurred during the semibatch process resulted in larger silica particles with a higher yield and narrower size distribution12. * [email protected]; phone 886 7 7425024 ext 523; fax 886 7 7194170

Photonic Fiber and Crystal Devices: Advances in Materials andInnovations in Device Applications II, edited by Shizhuo Yin, Ruyan Guo,

Proc. of SPIE Vol. 7056, 70561V, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.795005

Proc. of SPIE Vol. 7056 70561V-1

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Recent results concerning structural and optical features of the sol-gel silica glasses with semiconductor (copper selenide) and metal (copper) nanoparticles are presented13-15. The manufacturing procedure was elaborated as the flexible sequence to control chemical composition and properties of the particles in sol-gel-derived silica matrix16. In addition, K+ and Na+-doped silica glasses have been prepared through sol–gel processes employing Si(OC2H5)4, K2S and Na2S as precursor, with HCl as catalyst. The photoluminescence spectra of all the samples show that the fluorescent intensity of K- and Na-doped samples is about 2–3 times of that of un-doped sol–gel silica glass because of the K+ and Na+ ions can act as a good sensitizer and activator in the sol–gel silica glasses17. Furthermore. the surface roughness enhances the lighe scattering18-19. Thus, the purpose of the present study is to investigate different reaction temperatures, catalyst and their effects, nuclear and growth, in preparing the silica substrate with nanoparticles. Different catalyst such as ammonia was considered. Furthermore, different reaction temperature was controlled in 328 K.

2. EXPERIMENTAL

Silica substrates of the thin film solar cell were prepared using a typical sol-gel technique, Fig. 1, which proceeds through the steps of hydrolysis and condensation of TEOS in a methanol solution containing an ammonia catalyst, gelling of the sol, followed by molding and drying of the gel in normal laboratory conditions (298 K and 80 % RH). TEOS, NH4OH aqueous solution and methanol were used (Merck Co.) and the water used for the sample preparation was purified by both ion-exchange and distillation.

Fig. 1. The proceeding steps of substrate for CIGS thin film solar cell.



Reagents were mixed into the two starting time solutions of methanol: (І) TEOS (6.8ml)/ CH3OH (3.3ml); and (ІІ) NH4OH (2ml)/ H2O (24.9ml)/ CH3OH (3.3ml). The contents of the solutions (І) and (ІІ) were adjusted so that the concentrations of TEOS, H2O, and NH4OH would be at the prescribed concentrations. The solutions were prepared in a beaker at temperatures 328 K. The solutions (Ι) and (ΙΙ) were mixed with each other at the prescribed temperature, and the mixtures were stirred vigorously for approximately 20 minutes. Depending on different temperature, the particle size was based on the development of nucleation and growth. In this study the morphology, particle size and size distribution were carried out using dynamic light scattering (DLS, Zetasizer nano90) and atomic force microscopy (AFM).

3. RESULTS AND DISCUSSION

The sol-gel technique is the most common method of synthesize nanometer silica particles. In general, it involves the simultaneous hydrolysis and condensation reaction of the metal alkoxide. The hydrolysis and initial condensation reaction of main group or transition metal alkoxides (Si(OR)n) are given below Hydrolysis Reactions:

Si(OR)n + m H2O → Si(OH)m(OR)n-m + m ROH Initial Condensation Reactions: 2 Si(OH)m(OR)n-m → (RO)n-m(HO)m-1 Si -O- Si (OH)m(OR)n-m-1 + ROH 2 Si(OH)m(OR)n-m → (RO)n-m(HO)m-1 Si -O- Si (OH)m-1(OR)n-m + H2O In this study we choice ammonia as catalyst to separate the nuclear and growth action during the hydrolysis and condensation process because Brinker and Scherer have documented the effects of pH value and surface charge on particle growth. The DLS statistics graph of the ammonia catalyst at 313 K supported on silica gel is presented in fig. 2. Two groups of particle distribution show a few primary particles, ~100nm, had grown into large particles, ~1000 nm, by self-assembly, surface charge are mutually repulsive anionic8, in condensation reaction stage. Our result shows the synthesis is a term more usually applied to gels, but at the molecular level it involves, in the simplest form, shrinkage via conversion of monosilicic acid, Si(OH)4, to siloxane, –Si–O–Si–, bridging bonds 4 through poly-condensation reactions, (i.e., ≡Si–OH + OH–Si≡ → ≡Si–O–Si≡ + H2O). It implies that aggregation and the aggregation rate for silica primary particles would be expected to slow down.

0

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Size (d.nm)

Statistics Graph (1 measurements)

Mean with Max-Min error bar

Fig. 2. DLS statistics graph of particle volume fraction versus log diameter plot for the initial condensation reaction stage formation

and growth of silica in the presence of ammonia obtained from temperature 313 K. The DLS statistics graph of the ammonia catalyst at 328 K supported on silica gel is present in figure 3. Figure 3 DLS statistics graph shows the smallest particle size of nuclei reduce to 3nm. Furthermore, the particle size distribution down



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to a sharp region which is 2 to 20nm. Thus. the improvement in particle size and particle size distribution show temperature raise is a useful method to have the small particle size and narrow particle size distribution in procedure of silica substrate preparation.

Fig. 3. DLS statistics graph of particle volume fraction versus log diameter plot for the initial condensation reaction stage formation

and growth of silica in the presence of ammonia obtained from temperature 328 K.



Line Profile: Red

nm

0StatisticsC

PSD F1(1/m) F2(lIijm) PSD1(pm4) PSD2(nm P12(nm Rq12(nm) P(nmr) Rq(nm)

•2D 0.000 256.000 OEO 1 .926E-2 7.714 2.777 7.714 2.777

PSD F1(lIm) F2(l4im) PSD1(pm PSD2(nm) P12(nm) Rq12(nm) P(nm) Rq(nm)- Xds 0.000 256.000 OEO 2.533E-1 6.925 2.632 6.925 2.632

•Yis 0.000 256.000 OEO 3.229E-1 7.218 2.687 7.218 2.687

Fig. 4. AFM micrograph and line profile of the silica substrate after modeling and drying of sample in fig. 3.

AFM micrograph and line profile after modeling and drying supported on the sample of fig. 3 is presented in fig. 4. The AFM micrograph shows all of those silica primary particles were grow slowly. Furthermore, the line profile shows these particles were controlled below 15 nm. It was observed that the reaction temperature increase will decrease the particle size and sharp the particle size distribution. In addition, the AFM 3D-image and statistics table of the silica substrate after modeling and drying of sample in fig. 3 is present in fig. 5. The data of statistics table in figure 5 indicated that we get a silica substrates with nanometer particle size, <15 nm, and surface roughness, below 2.8 nm, through the temperature controlled successfully. Our smooth product is a utile material to be a substrate of CIGS thin film solar cell.

Fig. 5. AFM 3D-image and statistics table of the silica substrate after modeling and drying of sample in fig. 3.



4. CONCLUSIONS

The CIGS solar cell is based on vacuum processes, which requires a high manufacturing temperature and high cost. A need therefore exists to have generated considerable academic and commercial interest for an improved method of preparing low manufacturing temperature and low cost silica substrate of CIGS solar cell. Unlike conventional Stöber’s processes, the nuclear growth of silica was hardly control in synthesis process of acidic solution. In this study, aggregation and aggregation rate for silica primary particles were expect to reduce by increasing the pH value of solutions. Furthermore, the primary particles and particle size distribution of silica can decrease by raising the temperature of reaction. It is clear from the DLS statistics graph and AFM line profile of morphology and statistic table studies we find an easy way, non-vacuum and low-temperature processes, to successfully prepare the substrate of CIGS thin film solar cell with particle sizes less 15 nm and the surface roughness less than 3 nm. Our methods are significant not only for the potential low-temperature production of substrate materials, but also for low-cost processes to the advance studies of CIGS thin film solar cell.

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SPIE Proceedings [SPIE Photonic Devices + Applications - San Diego, CA (Sunday 10 August 2008)] Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device