Name: Guogen Liu Advisor: Prof. Chin Co-advisor: Prof. Barat Date:06/20/2010 The chemical dynamics of the CSS growth of CdTe

Outline: 1. Introduction 2. Preparation of CdTe solar cell 3. Model of CSS growth of CdTe 4. Effect factor of CSS growth of CdTe 5. Effect factor of device performance 6. Conclusion

1. Introduction

Fig 1: Band structure of doped semiconductors Fig. 2: Heterojunction band-bending Fig. 3: Principle of photovoltaic device Principle of p-n junction solar cell Fig. 4: CdS/CdTe solar cell

The important development of CdTe Solar cell

The emission of Cd Advantage: easy to deposite and very cheap Disadvantage: Te is scarce and Cd is toxic, but it’s emission is least First solar produces1.1 GW, revenue $2.1B. Cost: $0.93/W (0.84/W in 2010) 1 MW of First Solar need 300~340Kg CdTe, 1MW need 130~140Kg Te. World’s current Te can only support 1000GW Appolo hold 3000tons Te, which can afford 30GW, revenue $21B(conservative estimate). With the increase of efficiency, the reduce of Te use and the recycle of CdTe solar cell, the shortage of Te will be solve in the future. The evaluation of CdTe solar cell Potential for using less Te

CdTe solar cell companies and papers

First Solar processes its modules using vapour transfer deposition (VTD), it is very similar to CSS (closed-space sublimation). The key is that the deposition rate of VTD is very high. First Solar

2. Preparation of CdTe solar cell

The process of CdTe solar cell

CSS process CSS deposition chamber. The CSS growth of CdTe

The physical process of the CSS is: First, CdTe source decomposes and sublimates; Second, the diffusion process of Cd and Te 2 from source to substrate; Third, Cd and Te 2 combine into CdTe and CdTe resublimates if T sub is very high. CdTe begins sublimating at 470 ℃ under 1KPa, at ΔT = 120 ℃, the vapor P sub =1/100 P sou, so the resublimation is neglected. Keeping the increasing- temperature speed at l0 ℃ /min can guarantee the surface T sub and get high quality CdTe thin films 1. Sublimation 2. Depositing The process analysis of CSS

3.1. Diffusion and sublimation model 3. Model of CSS growth of CdTe 3.2. Diffusion and reaction model The model was validated ： 400 < T < 600 ℃, 102 < P < 760 Torr, 0.87 < d < 1mm, but could have greater range of applicability. In general, faster growth rates are expected at higher source temperatures, higher difference in temperatures (Tsource-Tsubstrate), lower pressures and lower substrate source separation until the sublimation limit is reached. The presence of oxygen shifts CSS from diffusion-limited to reaction-limited growth, primarily through source oxidation. Oxygen enhances CdTe nucleation, reducing pinhole density and grain size. An important beneficial effect of oxygen is that it lessens the harmful effects of decomposition of the front contact. Another primary benefits of oxygen may be its ability to passivate donors and defects if they are present and its effect as a reactive gas that ensures uniform growth initiation.

Diffusion-limited case 3.1. Diffusion and sublimation model

Determination of diffusion- vs. sublimation-limited case Sublimation-limited case Adjustable parameters α and β are empirical constants that adjust the model’s output to match experimental data. α is associated with losses during evaporation and diffusion. β is associated with losses during condensation including the sticking coefficient and molecular transport. α =0.36 and β =0.035 In the diffusion-limited transport region, the growth rates have a positive exponential dependence on T source and an inverse proportional behavior to pressure. The transition between the diffusion and sublimation regimes will occur at pressures below 30–40 Torr.

0 and 5mm, the growth rate is sublimation- limited; Greater than 5mm, it is diffusion-limited. The growth rate is constant (sublimation- limited) at P<30 Torr. Above 30 Torr, the growth rate diminishes as the pressure increases (diffusion-limited ).

Effect of P O2 and T sou on deposition rate Effect of P O2 and d on deposition rate Effect of P O2 and P total on deposition rateEffect of P O2 and T sub on deposition rate 3.2. Diffusion and reaction model with oxygen

4. Effect factor of CSS growth of CdTe

Sublimation study of CdTe single crystal on glass substrates (ΔT = 100 ℃ ) Four growth regions

Fig. 4. SEM pictures of CdTe films as formed at different Tsub. In the first regime up to about 320 °C the grains grow as column, crystalline orientation and small grains. In the second growth regime (above 370 °C), the grains are larger and form pinholes and voids. at the transition regime (340 °C), the films are very compact without pin-holes and voids. which is usually used for the manufacturing of solar cells.

SEM images of CdTe films: a) 0% O 2 ; and b) 10% O 2. Composition of CdTe films deposited at Tso = 750 °C, Tsub = 600 °C, Pmixt = 10 Torr, using different amounts of oxygen in the gaseous atmosphere. Pressure of oxygen on grain size and composition

Average grain diameter for CdTe layers deposited under different nitrogen pressures. Series resistance values determined from dark J–V curves for cells with CdTe layers deposited under various pressures of nitrogen

Peak height versus the separation between the substrate and baffle. Schematic diagram of flow patterns in CSS: (a) much of the mass is lost before reaching the substrate; (b) with reduced distance, mass distribution is more nearly two dimensional.

912 K < T < 1324 K The mean free path h the deposition at 7.5 × 10–5 mbar is by free molecular transport because the mean free path is longer than the space. The growth rate is independent of pressure. The deposition at 6 and 2 mbar is probably diffusion limited because the mean free path is short compared to the space between substrate and baffle and the Cd and Te 2 vapour molecules will collide several times with nitrogen molecules before they condense on the substrate. In the diffusion-limited transport model, the deposition rate is an inverse function of pressure,

AFM images of islands after various growth times Tsub=500 °C and Tsou= 600 °C, P=200Torr): a) t=1 min, b) t=5 mins. c) t=10 mins d) t=30 mins, e) t=60 mins. f) shows coalescence of growth islands (t=30 mins). Nucleation of CdTe with growth time

Variation of a) island area, and b) island density with growth time. Variation of the nearest neighbour distribution parameter, Rn, with growth time.

Thickness of CdTe films obtained from different sources as a function of the deposition time. It was shown that the deposition rate depends on the way the CdTe source is prepared. source-plates led to lower rates (poor heat transmission of plate substrate); The deposition rate of compacting powder increases due to the better thermal contact between powder particles.

Fig. 7. Average device performance parameters extracted from J– V curves as a function of the background gas pressure present during deposition. 5. Effect factor of device performance

Cell parameters as a function of T Cell parameters as a function of CdTe thickness.

6. Conclusion 1. CdTe thin film solar cell will continue very competitive in future 30 years. It will drop behind because of the increase of Te cost and the lack of Te. 2. CSS is a very effective way to produce CdTe, because it is simplicity and cost- effectiveness, well-suited to large-scale and high efficiency. It has undergone some modifications for industry, such as CSVT, VTD. 3.Two models has been used to explain the effect factors on CSS growth of CdTe. 4. The following is an example of CdTe solar cell produced by NREL with an AM1.5 efficiency of 15.4%, The cell has a Voc of 830 mV, Jsc of 24.7 mA/ cm2, and an FF of 74.8%. Much low temperature has also been used in lab and industry to produce high efficiency CdTe solar cell.

References: 1. High efficiency CSS CdTe solar cells 2. Preparation and Properties of CdTe Polycrystalline Films for Solar Cells 3. The Effect of oxygen on CdTe-absorber solar cells Deposited by close-spaced sublimation 4. Growth of thick CdTe films by close-space sublimation techniqe 5. SEM characterization of CdTe growth on CdTe(111)by close-spaced sublimation 6. CdTe thin film solar cells: Interrelation of nucleation, structure, and performance 7. Influence of Deposition Parameters on the Properties of CdTe Films Deposited by CSS 8. Control of grain size in sublimation-grown CdTe, and the improvement in performance of devices 9. The growth of CdTe thin film by close space sublimation system 10.Nucleation of CdTe thin films deposited by close-space sublimation under a nitrogen ambient 11. Close-spaced sublimation growth of homo- and hetero-epitaxial CdTe thick films 12. Comparative Study of CdTe Sources Used for Deposition of CdTe 13. Thin Films by Close Spaced Sublimation Technique 14. PHOTOVOLTAIC PROPERTIES OF CLOSE-SPACE SUBLIMATED CdTe SOLAR CELLS 15. Fabrication Procedures and Process Sensitivities for CdS/CdTe Solar Cells NREL, DOE, University of Delaware, Apollo, CSU, CSM

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