Evaluation of Recombination Velocity at Grain Boundaries in Poly-Si Solar Cells with Laser Beam 指導老師：林克默 博士 學 生：楊顯奕 報告日期：2010.11.15

Outline 1 Introduction 2 Experimental procedure 3 Results and discussion 4 Conclusion

Introduction Laser beam induced current (LBIC), monochromatic beam induced current (MBIC), and electron beam induced current (EBIC) techniques are used to probe the localized properties [1,2]. The minority-earner diffusion length in the vicinity of grain boundaries and the grain boundary recombination velocity can be evaluated using SPV (surface photo voltage) technique [3] and an equation brought forward by Zook [4]. It is well known that metallic impurities in a substrate degrade the characteristics of solar cell, and that the metal impurities can be removed by boron and phosphorus gettering [5,6].

In this work, we evaluated the change of the diffusion length and the boundary recombination velocity in the main operating temperature range (2O-60°c)[7] by boron and phosphorus gettering using laser beam induced current (LBIC) technique.

Experimental Methods Figure 1 shows a schematic illustration of the LB1C system. An He-Ne laser (632.8nm) and an IR laser (830nm) with 5pm spot in diameter were used as light sources. Samples were mounted on an X-Y translation stage with 0.5-pm minimum step motors that was supplied with a heater and a thermoregulator. The samples were n+pp+ polycrystalline silicon diodes fabricated by diffusion of phosphorous and boron in p-type materials at 900°C and 1000°C.

Figure 1: Schematic diagram of scanning LBIC system

Results and Discussion Figure2 : (a) LBIC mapping at 2O°C (20 μm step, 100 X 100 points, Laser : He-Ne (632.8nm) )and (b) Surface image by Nomarski microscope

Figure 3 : Temperature dependence of the diffusion length Le before and after gettering

Figure 4: LBIC current I at 20°C using He-Ne laser (normalized by its bulk value far from the grain boundary interface)

Where A = αLe­ and S = sLe / D Where A = αLe­ and S = sLe / D. Here, α, Le, s, and D are the absorption coefficient the minority-carner diffusion length, the interface recombination velocity and the minority-carrier diffusion coefficient, respectively.

Figure 5: Temperature dependence of the boundary interface recombination velocity spg before and after gettering

結論 在多晶矽太陽能電池已被評估由激光束掃描技術在感應電流的溫度範圍從20°C至100°C。 隨溫度的升高擴散長度也增高。隨溫度升高邊界界面複合速率而下降。 擴散長度和邊界界面複合速度可以改善在吸收的時候。 吸收處理是有效的和實際太陽能電池的工作溫度範圍（20-60℃）在日本。

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