1. Meeting
指導教授:李明倫
學生:劉書巖

2. 報告內容
Solar cell 相關paper一篇

3. Ultrathin crystalline silicon solar cells on glass substrates
Appl. Phys. Lett. 70 (3), 20 January 1997
Rolf Brendel,a) Ralf B. Bergmann, Peter Lo¨ lgen, Michael Wolf, and Ju¨rgen H. Werner
Max-Planck-Institut fu¨r Festko¨rperforschung, D70569 Stuttgart, Germany

4. Outlines Abstract Introduction Experiment Result & discussion
Conclusion

5. Abstract
We fabricate thin crystalline silicon solar cells with a minority carrier diffusion length of 0.6 ± 0.2 μm by direct high-temperature chemical vapor deposition on glass substrates.
This small diffusion length does not allow high cell efficiencies with conventional cell designs.
We propose a new cell design that utilizes submicron thin silicon layers to compensate for low minority carrier diffusion lengths.

6. Abstract
According to theoretical modeling, our design exhibits excellent light trapping properties and allows for 10% efficiency at an optimum cell thickness of 0.4 μm only.
This submicron range of cell thicknesses was formerly thought to require direct band gap semiconductors.

7. Introduction
Commercial crystalline c-Si solar cells are W=200~500μm thick.
Thinner cells are intensively investigated, because of the reduced material consumption and a high efficiency potential.
Thin cells require an optical design that traps the light and thus compensates for the otherwise insufficient light absorption.

8. Introduction
Such cells require a thickness W similar to L for high minority carrier collection efficiencies.
Since the diffusion length L is largely controlled by the deposition technique, the optimum thickness is also strongly dependent on the deposition technique applied.

9. Introduction
Direct deposition of c-Si films onto glass substrates by chemical vapor deposition (CVD) is a promising technique, since glass technology and CVD are both well established in industry, and therefore have a high potential for large scale production.

10. Experiment
We use direct c-Si deposition by CVD at 1000 °C onto highly temperature resistant glass substrates to produce Si solar cells.
The diffusion length is only L= 0.6 μm in our cells.
We show by theoretical modeling that a sufficiently thin (W=0.4 μm) film of this diffusion length yields 10% efficient cells, if our newly introduced light trapping design is applied.

11. Experiment
With this design, a cell thickness of W=1 μm may yield efficiencies of 15% for material with a diffusion length L=5 μm.
A silicon film of thickness W=3.5 μm is deposited byatmospheric pressure CVD from SiHCl3 at 1000 °C directly onto a flat 1-mm-thick glass.
The average grain size ga is 1.5 ± 0.2 μm as determined from transmission electron microscope images.

12. Experiment
The Hall mobility of the holes is 52 cm2/V s at room temperature after hydrogen passivation. This value is 1/4 of that for monocrystalline Si.
Ti/Pd/Ag front grid contacts & Al back contact
Hydrogen plasma treatment at 350 °C for 150 min increases the shunt resistance across the junction to 250 V cm2.

13. Experiment The open circuit voltage is 361 mV.
The short circuit current density is 6.8 mA/cm2.

14. Result & discussion
The IQE is small in the ultraviolet spectral region due to the non-optimized emitter with a high donor concentration of 1020 cm-3 that creates a dead layer.
The quantum efficiency peaks at 470 nm.

15. Result & discussion
Figure 2 shows our proposal for an advanced light trapping scheme:
The pyramidal-film texture consists of a two-side-structured, 2-mm-thick glass superstrate.
The structured front side of the glass superstrate reduces the reflectance at the air/glass interface.
The period s and angle β of the front side texture may be different from period p and angle α of the backside.
The pyramidally structured backside is covered by a thin Si film.
The pyramidal tips may be used to define point contacts from the p-type base and the n-type emitter to metal stripes of Al and thus avoid front side shadowing.

16. Result & discussion
Absorption enhancement by light trapping bases on the randomization of the direction of light propagation in the Si layer.
This randomization takes place at the edges between the facets. Light trapping is better for small than for large periods since small scale periods have more such edges per area.
Here, we investigate the texture shown in Fig. 2 for a small period p=s=15 μm.

17. Result & discussion
Our Monte Carlo-Ray tracing program SUNRAYS is used to calculate the maximum short circuit current densities jsc* under AM1.5 G illumination.
jsc*= 40.2±0.1 mA/cm2. This value is larger than the previously reported ones, and demonstrates the potential of thin films deposited onto two-sides-structured glasses.
However, for very sharp angles α= 75°, a large thickness-toperiod- ratio W/p = 4 mm/15 mm = 0.27 is extremely challenging to manufacture.
Therefore, a more practical ratio of W/p = 0.1 is investigated below.

18. Result & discussion We determine jsc* at constant ratio W/p = 0.1.
For Si material with diffusion length L and (SRV) S, it is important to optimize the cell thickness W.
Therefore, the simulation varies the film thickness W for optimum cell efficiency

19. Conclusion
For a 1-mm-thick cell with L= 5 μm the efficiency reaches 15.3% (Voc= 606 mV, jsc= 30.9 mA/cm2).
This high efficiency potential is primarily due to the excellent light trapping properties of the pyramidal-film texture shown in Fig. 2.