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112/04/21 STUT 太陽能材料與模組實驗室

Spatially resolved carrier lifetime calibrated via quasi-steadystate photoluminescence

報告者 : 楊顯奕指導教授 : 林克默博士日期 :2011/10/31J.A. Giesecke*, B. Michl, F. Schindler, M.C. Schubert, W. Warta



Outilne

• Introduction

• Theory

• Tackling the Microsecond Range with QSSPL

• Experimental Results

• Conclusuons

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Introduction• Two roads for photovoltaics to enhance

competitiveness are to be pursued: the first aims at boosting solar cell efficiencies, and the second attempts to drive down material cost while maintaining the electronic material quality.

• This is because the smaller minority carrier lifetime of silicon solar cell precursors is, the higher is the relative error when predicting solar cell current and voltage from inaccurate lifetime measurements.

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Theory

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Photoluminescence imaging via a CCD camera provides a spatially resolved steady-state distribution of luminescence intensities of an optically excited silicon wafer.


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Figure 1. Depiction of the experimental setup of QSSPL. A time modulated laser irradiates a silicon wafer or solar cell. The irradiation intensity is recorded as a function of time in an irradiation detector, while the photoluminescence intensity is recorded in another – encapsulated – detector which can be mounted either beneath the sample (I: in the case of unmetallized cell precursors) or toward the irradiated sample side (II: for metallized wafers or solar cells). Both the relative irradiation and photoluminescence intensity are read into a computer and evaluated in terms of injection dependent carrier lifetime.



2.2. Injection dependent effective carrier lifetime from QSSPL

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while N denotes net dopant concentration, and the factor A incorporates the coefficient of radiative recombination as well as all relevant measurement setup and optical sample properties. Under quasisteady-state conditions, it can be shown with Eqs. (1) and (2) that effective minority carrier lifetime can be extracted from the time shift between maxima of irradiation and luminescence intensities [4].



Figure 2. (left) Example of a QSSPL measurement on a silicon wafer (modulation frequency: 80Hz); (right) Interpretation of the QSSPL measurement at the left in terms of injection dependent minority carrier lifetime (self-consistent evaluation).

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3. Tackling the Microsecond Range with QSSPL

• Several measures have to be taken in order to ensure a sufficient sensitivity limit of QSSPL in terms of carrier lifetime. As QSSPL is most sensitive to the phase shift of two modulated light intensities with respect to each other, particular attention must be paid to any phase changes throughout acquisition and processing of the measured signals.

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3.1. Requirements for microsecond lifetime measurements

• Sufficient suppression or filtering of radiation within the relevant band to band luminescence spectrum, either generated directly in the light source, or originating from optical activity along the light path (fibers and optics), must be ensured.

• The phase delays imposed upon the photocurrents during the amplification process, either through RC interaction between diode capacitance and resistive elements of the amplifier, or caused by the finite response time of the operational amplifiers in use, must be considered.

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4. Experimental Results

Table 1. Comparison of arithmetically averaged effective carrier lifetimes ‹τ› as obtained from a QSSPC and a QSSPL calibration. Representative plasma cleaned and upgraded metallurgical grade multicrystalline wafers were investigated. Each value shown here represents an average value of five calibrations carried out on different wafer spots respectively. The specified uncertainty refers to the variance of these five calibrations.



Figure 3. (a) Effective carrier lifetime image of a multicrystalline upgraded metallurgical grade silicon wafer as calibrated viaQSSPL; (b) Effective carrier lifetime image of a multicrystalline upgraded metallurgical grade silicon solar cell as calibrated viaQSSPL. Feedstock from near to the brick bottom causes a relatively low effective lifetime of about 2.7μs.



Conclusion•隨著壽命的技術在此呈現的，定量的載流子的壽命測量範圍從準穩態光致發光的基礎上擴大到矽太陽能電池和金屬或非金屬的任意一個電子材料上。

•這篇文章的重點是，可以利用 QSSPL來判斷產品的生命週期，最重要材料參數是電荷的載子遷移率和摻雜濃度

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112/04/21 STUT 太陽能材料與模組實驗室 13

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