Presented at the 16th European Photovoltaic

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Presented at the 16 th European Photovoltaic Solar Energy Conference Glasgow, May 2000

HIGH RESOLUTION LASER STEPPING MEASUREMENTS ON POLYCRYSTALLINE SOLAR CELLS

ABSTRACT: Thin-film polycrystalline cells (CuInGaSe in selected devices. 2 J. F. Hiltner and J. R. Sites Colorado State University Fort Collins, CO 80523 USA and CdTe absorbers) in general exhibit grain boundary related effects, which act as recombination centers and reduce the short-circuit current. In addition, local defects can degrade cell performance by increasing series resistance and shunting. Despite increasingly high efficiencies in these types of cells, the grain boundaries and localized defects are not well characterized or understood. To help address this problem, a high-resolution and high-sensitivity laser stepping apparatus has been developed which features diffraction limited laser spot size at multiple wavelengths in the red and infrared. The spatial resolution is approximately 1 micron. Reliable data may be taken at this resolution with a laser intensity equivalent in illumination density to 1 sun. In order to separate electrical and optical effects, a map of the photocurrent can be generated with the cell at an arbitrary bias. Finally, return to the same position to within 1 micron is possible when the device is replaced in the apparatus. Since grain sizes are typically 1 to 10 microns in these materials, the apparatus is able to probe single grains Keywords: Characterisation – 1: Defects – 2: Polycrystalline - 3 1. INTRODUCTION Polycrystalline 2 and CdTe continue to make advances in efficiency in both laboratory-based cells and monolithically integrated modules. Designed to meet reasonable efficiency goals with reduced manufacturing costs, these devices can be limited by spatial non uniformity, including effects due to grain boundaries and localized defects, as well as effects due to processing gradations across the cell or module. Such spatial dependences can be examined conveniently and non destructively by focusing a laser beam on the surface of the solar cell and measuring the induced photocurrent as the beam is moved step-wise across the sample. This technique has been employed by other groups to examine grain boundaries in polycrystalline silicon, as well as measurement of the material resistivity and intensity dependence of current collection [1-7]. It is important to note what kind of information this technique can be expected to provide about the samples. In comparison to polycrystalline silicon devices, it is more difficult to extract materials parameters (such as recombination velocity and diffusion length) on CdTe and CIGS polycrystalline cells because of the much smaller grain sizes (1-10 µ However, useful information such as the basic uniformity of the photocurrent response, local resistivity variations, and the impact of defects is shown here to be extracted from laser stepping. m) and diffusion length (~1 µ m). computer-controlled fiber-based attenuator allows continuous adjustment of the laser intensity over 6 orders of magnitude. The fiber output (N.A. = 0.12) is collimated by an achromatic doublet lens with a 20 mm focal length, which produces a 2.4 mm radius beam, chosen to fill the focusing lens completely. After collimation, a 90/10 ratio splitter reflects part of the beam to a photodiode which monitors the laser power continuously. The same splitter directs spectrally reflected light to another photodiode. Beam steering mirrors align the collimated beam to an Olympus 1-UB367 SL C Plan Fluoride 40x/0.55 N.A. microscope objective which is equipped with a variable correction collar. This feature allows focusing through glass without spherical aberrations, necessary for attaining the minimum spot size when using glass-superstrate devices. The objective lens has a long working distance of 7-8 mm, helpful for focusing through glass and also for placing probes on the cell under test. See Fig. 1 for a schematic of the apparatus. Laser Diode Attenuator Single-Mode Fiber Photodiode Beam Splitter Beam-steering mirrors Objective mounted on Z stage 2. EXPERIMENTAL 2.1 Laser source and focusing In the apparatus recently developed at Colorado State University, solid-state laser diodes (630 to 830 nm range) are coupled into single-mode fibers to produce a very clean gaussian-profile beam, which is necessary for diffraction limited focusing of the beam. The intensity is sine-wave modulated, typically at 150-1000 Hz, which allows lock-in detection of the photocurrent. Modulating intensity as opposed to using a mechanical chopper has the advantage of not introducing high frequency components in the laser stimulus, which can complicate the cell response. A Reflected beam

Figure 1.

Photodiode Schematic of laser source and delivery. 2.2 Electronic setup When one uses a focused beam with spot sizes the order of 1 µ Sample on X-Y translation stage m, the local laser intensity can easily become so large as to result in local heating effects and to cause complications for devices which show nonlinear collection efficiency. In addition, theoretical treatments usually assume that the generated majority carrier density is much less than the steady state majority carrier density [4]. For these reasons, a very sensitive electronics setup is required

Presented at the 16 th European Photovoltaic Solar Energy Conference Glasgow, May 2000

so that incident power densities of ~100 mW/cm current levels typically 10 4 2 (1 sun equivalent), which generate nA currents at the highest spatial resolutions, can be used. In addition, probing the cell response at different biases is required to separate different types of defects, which in turn generates DC sample. times greater than the light induced current. Thus a variable gain transimpedance amplifier with a bias adjustment and a DC current input offset adjustment (SRS SR570) is used to amplify the cell current. A lock-in amplifier (SRS SR810) completes the electronics for the sample photocurrent. In addition, a separate transimpedance amplifier has been built to monitor the beam power. The output voltage is digitized and a software-based (written in LabView  ) lock-in technique is used to extract the laser power at the monitor photodiode, which is calibrated to give the laser flux at the 2.3 Computer control Integral to the system is LabView  code which controls the translation stages, laser attenuator, and data acquisition through GPIB, PCI-bus, and RS-232 interfaces. Calibration allows display of the spot size and laser intensity at the sample, as well as other test parameters. This allows results to be given in terms of apparent quantum efficiency (AQE) at various spot sizes, intensities, and biases, all of which can affect the response strongly. 2.4 Experimental limitations The resolution of the instrument is determined by the minimum 1/e 2 by diffraction: laser spot size, which is limited by the fact that a far-field technique is being used and thus is limited w min = λ 2 π N.A.

φ w o = 0.38

µ m, where λ = 680 nm (typically), φ is the clear aperture of the microscope objective, and w o is the 1/e 2 radius of the beam incident on the objective. The smallest 1/e 2 radius thus far measured has been 0.55 ± 0.05 µ m (corresponding to a 1.5

1.0

0.5

Min Spot: 0.55 µ m Depth of Focus: ~4 µ m 0.0

-4 -2 0 Z Position [ µ m] 2 4

Figure 2.

Laser spot size at various positions along the focused beam.

FWHM of 1.3 ± 0.1 µ m). The slightly larger value compared to theory is likely due to small misalignment of the beam to the objective, or possible small distortion of the wavefront by imperfect optical components. When focusing the beam through glass, the minimum 1/e 2 spot size attained has been 0.8 ± 0.1 µ m. Since some surface roughness is always present, the depth of focus is important to note, as it determines the maximum allowed roughness while keeping the same spot size. See Fig. 2, which shows the depth of focus to be less than about 4 µ m, which is many times the local variations in the film thickness. Since it is difficult to align the sample to be perpendicular to the incoming beam with this kind of precision, the reflected light from the sample is used to measure a plane of constant spot size, using the fact that only light reflected at the focal plane of the lens is collimated back through the lens and is therefore incident at the reflection monitoring photodiode. This information is used by the software to define the plane of the sample, and the position of the lens is automatically adjusted to keep the spot size constant. The resolution is also in principle limited by the diffusion length of the minority carriers in the material, and hence the divergence of the beam leads to an effective spot size which increases within the material as it is absorbed [6]. This is directly applicable for silicon, which can have a large diffusion length (>20 µ m) and a relatively low absorption coefficient. However, it is less important for the materials under consideration here. CdTe and CIGS absorbers have a diffusion length of around 1 µ m, and absorb the majority of light within less than 1 µ m of the junction [8,10]. The sensitivity of the apparatus is most degraded by devices with low output impedance, i.e. those which are characterized by high shunting. This results in large DC currents which must be offset with a cost in noise performance. The large capacitance of cells with even modest areas also adversely affects the noise performance, and in the worst cases reduces the bandwidth of the instrumentation. However, it has always been possible to reach the goal of 1-sun equivalent illumination at the minimum spot size with good signal-to-noise characteristics. Since the devices under consideration in general exhibit linear response with near solar incident intensities, it is not clear that higher sensitivity is required. However, equipment limitations (due to large input impedance in high sensitivity) result in complications to the measurement when a bias is applied. For this reason, bias-dependent AQE measurements at high resolution are generally made at greater than 30 suns equivalent illumination. 2.5. Cell mounting and repeatable positioning Small area devices are mounted in custom holders which can be replaced in the system with good spatial repeatability. This allows the cell to be mapped, then removed from the system for stress or other measurements. Cell features or edges incorporated into the cell holder can then be used to find the same position to within due to thermal stress. ± 1 µ m. This is useful for investigating local changes in the material 2.5 Reflection Currently, results are given in terms of physics is

internal apparent

adjacent crystals in polycrystalline silicon cells [6]. While quantum efficiency (AQE). More pertinent to device quantum efficiency, which corrects for local reflection losses at the cell surface(s). Some authors have found a large difference in the reflection from this apparatus can only measure the spectral portion of the

Presented at the 16 th European Photovoltaic Solar Energy Conference Glasgow, May 2000

reflection while the sample is at the focal plane of the objective, the materials under study have showed very spatially uniform reflection, with the exception of obvious surface defects and grid lines. With these exceptions, the results given in terms of AQE to a good approximation differ from the internal QE only by a constant. 2.6 Method Most similar data in the literature have relied on scanning to generate maps of the photocurrent. The method used here is distinct in that each point in the map is independent of the others, i.e. each point is taken by moving the translation stages to it and stopping, waiting 2 3 time constants (determined by the modulation frequency), and finally reading the output of the lock-in. This results in an increase in the effective resolution compared to high speed scanning, since there is no effect of ‘blurring’. Several ‘standard’ types of measurements have emerged, which are useful for comparing different cells. To cover large areas, a 100 µ m spot is used, and data points are taken every 50 spot sizes of 10 µ µ m. A 5 x 5 mm area can be mapped in this way in less than one hour. The translation stages’ travel time is the limiting factor in the data acquisition rate for large area mapping. Subsequent higher resolutions use m and 1 µ m and map out areas of 500 x 500 µ m and 50 x 50 µ m, respectively. The latter high resolution mappings take about 15 minutes each, since the time for the stages to move between each point is reduced. One dimensional stepping scans are also a useful way of comparing parameters on and off of the location of a feature of interest. For example, measuring the AQE at various biases is a very useful tool for determining the electrical nature of a feature. A set of one dimensional stepping scans, each taken with the cell held at a different bias, can be used to generate AQE vs. cell bias curves at various locations. 3. ANALYSIS AND RESULTS 3.1 Photocurrent mapping examples Fig. 3 shows an example of AQE mapping of a 500 x 500 µ m area with a 10 µ m spot size. The top portion of the IEC CdTe 1.0

0.8

0.6

0.4

0.2

0 100 X Po 200 sition [ µ m] 300 400 500 400 100 200 300 sit io n [ µ m] Y Po 500 0

Figure 3:

Example of 500 x 500 µ m photocurrent mapping using a 10 µ m spot at 1 sun equivalent intensity.

graph is a contour plot of the data. Shown on the x-y plane is the projection of the data. The set of lines intersecting to form a square indicate the region where high resolution mapping was done, as in Fig. 4. The sample is a CdTe cell fabricated at the Institute of Energy Conversion (IEC). In this case the collection is quite uniform over the region mapped. Note, however, that the structure seen is not noise, but are local regions of slightly lower collection efficiency, most likely due to local variations in the series resistance of the material [1,9]. The higher resolution mapping (Fig. 4), using a spot size of approximately 1 µ m, is consistent with the lower 1.0

0.8

0.6

IEC CdTe 0.4

Appa 0.2

120 130 X Posit ion [ 140 µ m] 150 160 260 250 220 230 Y P 240 osit io n [ µ m]

Figure 4:

Example of 50 x 50 µ m photocurrent mapping using a 1 µ m spot at 3 suns equivalent intensity.

IEC CdTe 1.0

0.8

0.6

0.4

0.2

120 130 X Po sition [ 140 µ m] A 150 C B 160 260 250 220 230 240 sit io n [ µ m] Y Po

Figure 5:

High Intensity photocurrent map of the same area as Fig. 4. Labels A-C correspond to AQE vs. bias curves in Fig. 6.

resolution case in overall AQE, but shows the feature in greater detail. In this case the feature is due to a scratch in the TCO intentionally introduced. That there is very little contrast in the photocurrent even at this resolution indicates that the grain boundaries are well passivated and the material is quite uniform on this level. Finally, Fig. 5 shows the same area as Fig. 4, but using very high laser

Presented at the 16 th European Photovoltaic Solar Energy Conference Glasgow, May 2000

intensity, about 300 suns. In this case the structure is quite different – the mapping shows that a large portion of the area has a lowered AQE, and a much larger number of local variations. 3.2 Intensity Dependence CdTe-based solar cells examined in general exhibit a much larger non-linear response with intensity compared to CIGS-based solar cells, consistent with results by other researchers [2,3]. This difference is likely due to a higher material resistivity in the case of CdTe-based devices. Other researchers have attributed the intensity dependence to some effect of high carrier injection, but the variation with intensity generally supports the resistivity-variation thesis. By examining the dependence of the AQE on cell bias, as well as intensity, wavelength and spot size, it is hoped that the effects can be separated. It should be noted that the effect is clearly an effect of current or power

density

3). , as evidenced by the uniformity of the response with a larger power flux and a larger beam area (as in Fig. 3.3 AQE bias dependence Fig. 6 shows AQE vs. bias curves for several points on the IEC cell. The high intensity curves show much more dependence on voltage in reverse bias than the curves IEC CdTe 1.0

A C 76 suns 0.8

A B C 3000 suns 0.6

B 0.4

0.2

Spot Size = 1 µ m λ = 680 nm 0.0

-5 -4 -3 -2 -1 Voltage [V] 0 1 2

Figure 6:

Bias dependence of the AQE at different positions and intensities. Labels A - C correspond to positions on Fig. 4. taken at lower intensity, suggesting that the effect is resistive in nature. The softening of the curve in forward bias also strongly suggests a resistive effect. Additionally, Fig. 6 shows that regions giving the same photocurrent at zero bias may be distinct electrically. Positions A and B appear identical on a zero-bias mapping at 3000 suns, but the bias dependence clearly shows an electrical difference. The exact interpretation of these curves is difficult due to the complexity of the material and of the equivalent circuit, which must take into account the rest of the diode in the dark, shunting and series resistances, and light-induced changes in the material [9]. resistance with decreasing spot size, due to the relation R = ρ One important consideration is the increase in L/A, where ρ is the resistivity of the material, L is the length over which the photocurrent runs, and A is the beam area incident on the sample (in the simplest approximation) [1]. This introduces a complication in separating high injection effects from resistive effects, since both depend on the carrier generation area. However, high injection should be reached at higher beam intensity with longer wavelength laser light (due to the smaller absorption coefficient), providing a direct method of separating the effects. Note that both the series resistance and the impedance of the measurement circuit can have an impact on the response of the device even at zero bias. CONCLUSIONS High resolution measurements with extremely low laser intensities are a valuable way of determining the uniformity of solar cell devices. By using conditions which are directly comparable to field conditions, the photocurrent response gives direct feedback about the whole-cell performance, allowing quantitative evaluation of spatial variations in the collection efficiency, material resistivity and defect concentrations. Also, with the higher intensities available, the ability of particular devices to perform in concentrator applications can be evaluated. ACKNOWLEDGMENTS This work is supported by the National Renewable Energy Laboratory. In addition, the authors would like to thank the Institute of Energy Conversion and Brian McCandless for providing the samples used in this work. REFERENCES [1] [2] A.E. Delahoy and A.M. Payne, Proceedings of the 25 th IEEE PVSC (1996) 841. T. J. McMahon and B. von Roeder, Proceedings of the 26 th IEEE PVSC (1997) 375. [3] [4] [5] [6] [7] [8] [9] [10] S.A. Galloway, P.R. Edwards, K. Durose, Sol. Energy Mat. & Sol. Cells

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