Jennifer T. Heath

Measurement of semiconductor surface potential using the scanning electron microscope

We calibrate the secondary electron signal from a standard scanning electron microscope to voltage, yielding an image of the surface or near-surface potential. Data on both atomically abrupt heterojunction GaInP/GaAs and diffused homojunction Si solar cell devices clearly show the expected variation in potential with position and applied bias, giving depletion widths and locating metallurgical junctions to an accuracy better than 10 nm. In some images, distortion near the p-n junction is observed, seemingly consistent with the effects of lateral electric fields (patch fields). Reducing the tube bias removes this distortion. This approach results in rapid and straightforward collection of near-surface potential data using a standard scanning electron microscope.

Scanning Capacitance Spectroscopy on N+-P Asymmetrical Junctions in Multicrystalline Si Solar Cells

We report on a scanning capacitance spectroscopy (SCS) study on the n+-p junction of multicrystalline silicon solar cells. We found that the spectra taken at space intervals of ∼10 nm exhibit characteristic features that depend strongly on the location relative to the junction. The capacitance-voltage spectra exhibit a local minimum capacitance value at the electrical junction, which allows the junction to be identified with ∼10-nm resolution. The spectra also show complicated transitions from the junction to the n-region with two local capacitance minima on the capacitance-voltage curves; similar spectra to that have not been previously reported in the literature. These distinctive spectra are due to uneven carrier-flow from both the n- and p-sides. Our results contribute significantly to the SCS study on asymmetrical junctions.

The effect of copper on the sub-bandgap density of states of CdTe solar cells

Two optical sub-bandgap transitions in CdTe thin-film solar cells have been identified using detailed transient photocapacitance and transient photocurrent spectroscopy measurements. A broad response centered at EV + 0.9 eV directly correlates with the quantity of Cu present in the absorber layer, while a second response at EV + 1.2 eV does not depend on Cu or Zn and may be an intrinsic defect. These results demonstrate the influence of Cu on the sub-bandgap density of states of CdTe, and they are critical to understanding, modeling, and improving its optoelectronic properties.

Diffused junctions in multicrystalline silicon solar cells studied by complementary scanning probe microscopy and scanning electron microscopy techniques

The junction location in textured n+-p multicrystalline Si solar cell devices is quantitatively located. A comprehensive comparison is presented between secondary ion mass spectrometry (SIMS) and three microscopy techniques: scanning capacitance microscopy (SCM), scanning Kelvin probe microscopy (SKPM), and secondary electron contrast (SE) in the scanning electron microscope. The comparison includes capabilities of junction allocation, applicability of the measurement to give a two-dimensional analysis of a textured device, apparent imaging of the depletion region, depletion edge and depletion width determination, sample structure requirements for the specific methods, ease of sample preparation, and data acquisition time for high-quality measurements. We find that SE not only allows the most straightforward sample preparation and provides the quickest measurement, but also seems to show the depletion edge and width with a high degree of accuracy. This is verified by a comparison to PC1D simulations of the band bending in the bulk of the device. These data also contribute to our understanding of the origin of SE doping contrast. SKPM provides reliable junction identification but poor capability in determining the depletion width. All three techniques are able to show how well the diffused n+ emitter region tracks the surface topography, with SCM and SE yielding the clearest images.

Role of Bulk Defect States in Limiting CIGS Device Properties

We have used sustained light-soaking in the near-infrared (780 nm wavelength) to modify the properties of the absorber layer in CIGS solar cells, and thus study the relationship between the absorber electronic properties and the device performance. Through this light-soaking treatment we can increase the hole carrier density in the CIGS absorber, as well as the density of the commonly observed 0.3 eV bulk deep acceptor, by up to a factor of 5. The device performance was periodically recorded under the same 780 nm light, and was found to degrade due to a reduction in fill factor and short circuit current. We demonstrate that these changes can be largely attributed to a reduction in the photogenerated carrier collection, due to a decrease in the depletion width. Based on these data, we estimate the minority carrier diffusion length to be about 0.2 mum. At the same time, no change in open circuit voltage was observed, indicating that the recombination rate through the dominant recombination channel remained nearly constant. This means that it cannot be the 0.3 eV bulk defect. Detailed SCAPS modeling was carried out and reinforces this conclusion

Imaging the solar cell P-N junction and depletion region using secondary electron contrast

We report on secondary electron (SE) images of cross-sectioned multicrystalline Si and GaAs/GaInP solar cell devices, focusing on quantifying the relationship between the apparent n+-p contrast and characteristic electronic features of the device. These samples allow us to compare the SE signal from devices which have very different physical characteristics: differing materials, diffused junction versus abrupt junction, heterojunction versus homojunction. Despite these differences, we find that the SE image contrast for both types of sample, and as a function of reverse bias across the diode, closely agrees with PC1D simulations of the bulk electrostatic potential in the device, accurately yielding the depletion edge and width. A spatial derivative of the SE data shows a local maximum at the metallurgical junction. Such data are valuable, for example, in studying the conformity of a diffused junction to the textured surface topography. These data also extend our understanding of the origin of the SE contrast.

Two-dimensional measurement of n + -p asymmetrical junctions in multicrystalline silicon solar cells using AFM-based electrical techniques with nanometer resolution

Lateral inhomogeneities of modern solar cells demand direct electrical imaging with nanometer resolution. We show that atomic force microscopy (AFM)-based electrical techniques provide unique junction characterizations, giving a two-dimensional determination of junction locations. Two AFM-based techniques, scanning capacitance microscopy/spectroscopy (SCM/SCS) and scanning Kelvin probe force microscopy (SKPFM), were significantly improved and applied to the junction characterizations of multicrystalline silicon (mc-Si) cells. The SCS spectra were taken pixel by pixel by precisely controlling the tip positions in the junction area. The spectra reveal distinctive features that depend closely on the position relative to the electrical junction, which allows us to indentify the electrical junction location. In addition, SKPFM directly probes the built-in potential over the junction area modified by the surface band bending, which allows us to deduce the metallurgical junction location by identifying a peak of the electric field. Our results demonstrate resolutions of 10–40 nm, depending on the techniques (SCS or SKPFM). These direct electrical measurements with nanometer resolution and intrinsic two-dimensional capability are well suited for investigating the junction distribution of solar cells with lateral inhomogeneities.

Characterization of bulk defect response in Cu(In, Ga)Se 2 thin-film solar cell using DLTS

Deep level transient spectroscopy has been used to study the spatial properties of the N1-admittance feature in Cu(In, Ga)Se2 devices. By comparison to admittance spectra, the N1-admittance feature has been identified in deep level transient spectra. By varying the filling pulse voltage used to generate the deep level transient spectra, the spatial properties of the N1-admittance feature are determined. In particular, the N1-admittance feature is shown to arise from a uniformly distributed bulk defect in the sample studied.

Investigation of Charge Trapping at Grain Boundaries in Polycrystalline and Multicrystalline Silicon Solar Cells

Scanning capacitance microscopy (SCM) often shows a change in contrast at grain boundaries [1-3]. The origins of this contrast and the efficacy of SCM as a tool to identify band bending at grain boundaries in pc-Si and mc-Si are discussed. Contrast at these grain boundaries could be influenced by different oxide growth rates or by defect states at the oxide interface. In order to determine the influence of such mechanisms on the SCM signal, such effects must be modeled; we show that a simple one-dimensional model agrees well with more detailed models of SCM signal strength and indicates, for example, that very small changes in oxide thickness measurably affect the SCM signal. In our experimental data, the uniformity and quality of the oxide layer are confirmed, and increased contrast consistent with depletion regions is still observed at higher order grain boundaries as identified by electron backscattering diffraction, including 9 and 27a. Scans of the SCM signal as a function of dc probe voltage allow such regions to be more quantitatively investigated.