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We investigated the quality of the n+-p diffused junction beneath the front-side Ag contact of multicrystalline Si solar cells by characterizing the uniformities of electrostatic potential and doping concentration across the junction using the atomic force microscopy-based electrical imaging techniques of scanning Kelvin probe force microscopy and scanning capacitance microscopy. We found that Ag screen-printing metallization fired at the over-fire temperature significantly degrades the junction uniformity beneath the Ag contact grid, whereas metallization at the optimal- and under-fire temperatures does not cause degradation. Ag crystallites with widely distributed sizes were found at the Ag-grid/emitter-Si interface of the over-fired cell, which is associated with the junction damage beneath the Ag grid. Large crystallites protrude into Si deeper than the junction depth. However, the junction was not broken down; instead, it was reformed on the entire front of the crystallite/Si interface. We propose a mechanism of junction-quality degradation, based on emitter Si melting at the temperature around the Ag-Si eutectic point during firing, and subsequent re-crystallization with incorporation of Ag and other impurities and with formation of crystallographic defects during quenching. The effect of this junction damage on solar cell performance is discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We investigated cross sections of working CdTe/CdS solar cells using scanning Kelvin probe microscopy (SKPM). The cross sections were prepared by polishing to avoid steps between the glass substrate and film that generally make the analysis difficult. However, this process resulted in strong pinning of the Fermi level. During the measurements, the cells were biased under different conditions, revealing the distribution of the electrical potential inside the device. We were able to identify different regions inside the device: in the region away from the CdTe/CdS junction, there was only a small variation in the potential; closer to the junction, the potential increased, due to the increase in the depletion regions with the reverse bias; at the junction, there was a sudden increase in the potential, which was attributed to interdiffusion between CdTe and CdS. By taking the first derivative of the potential, we were able to calculate the electric field inside the device. The maximum of the electric field, which locates the p-n junction, occurred at the interface between CdTe and CdS. However, the electric field at this location had a strong peak, in agreement with the existence of the interdiffusion layer, with higher doping, at the junction. The presence of this layer was confirmed by transmission electron microscopy. We also investigated the distribution of the potential and electrical field inside a CdTe/SnO2 device, without the CdS layer, and showed that the interdiffusion does not happen in this case. Finally, we used Poisson's equation to estimate the doping inside the CdTe film in both devices. [ABSTRACT FROM AUTHOR]

Using the first-principle density-functional theory, we study the structure and effects of vacancies in Σ3 (112) grain boundary with the coincident-site lattice structure in Si. We find that the formation energy for a Si vacancy in the grain boundary is significantly lower than that in Si perfect region, indicating strong segregation of Si vacancy in grain boundary regions. The formation of Si vacancies in grain boundaries either cleans up the deep levels or facilitates complete passivation by H atoms. Our results suggest that vacancies in grain boundaries may play important role in determining grain boundary physics and passivation behavior. [ABSTRACT FROM AUTHOR]

We report on a two-dimensional investigation of the p-n junction in multicrystalline silicon solar cells using scanning Kelvin probe force microscopy (SKPFM). The junction location and depth were identified by SKPFM potential measurement and subsequent data analysis, where a procedure taking bias-voltage-induced changes in the potential and electric field was developed to avoid the effects of surface Fermi level pinning. Device simulation supported the junction identification procedure and showed a possible deviation of ∼40 nm in the junction identification. The two-dimensional electric-field images show that the shape of the junction follows the surface topography of the device, or, in other words, the junction depth is identical over the device. [ABSTRACT FROM AUTHOR]

The effects of P doping on the nanocrystalline formation in mixed-phase Si:H thin films were investigated using secondary-ion mass spectrometry, Raman spectroscopy, atomic force microscopy, cross-sectional transmission electron microscopy, and scanning Kelvin probe microscopy. We found that Si nanocrystallites in the intrinsic and weakly P-doped materials aggregate to form cone-shaped structures. The local workfunction of the nanocrystalline aggregation areas is larger than the surrounding amorphous areas. Increasing the P-doping level requires an increased hydrogen dilution to reach the similar Raman crystallinity. The nanocrystalline aggregation disappears in the heavily P-doped materials, but isolated nancrystallites appear. The effect of P-doping on the nanostructure is explained with the coverage of P-related radicals on the existing nanocrystalline surface during the deposition and the P segregation in grain boundaries, which prevent new nucleation on the surface of existing nanocrystallites. [ABSTRACT FROM AUTHOR]

Both the origins of the high open circuit voltages (VOC) in amorphous silicon solar cells having p layers prepared with very high hydrogen dilution and the physical structure of these optimum p layers remain poorly understood topics, with several studies offering conflicting views. This work attempts to overcome the limitations of previous studies by combining insights available from electronic measurements, real time spectroscopic ellipsometry, atomic force microscopy, and both high-resolution transmission electron microscopy (TEM) and dark field TEM of cross sections of entire solar cells. It is found that solar cells fabricated with p layers having a low volume fraction of nanocrystals embedded in a protocrystalline Si:H matrix possess lower recombination at the i/p interface than standard cells and deliver a higher VOC. The growth of the p layers follows a thickness evolution in which pure protocrystalline character is observed at the interface to the i layer. However, a low density of nanocrystallites nucleates with increasing thickness. The advantages offered by the protocrystalline character associated with the amorphous phase of the mixed-phase (amorphous+nanocrystalline) p layers prepared with excess H2 dilution account for the improved VOC of the optimum p layers. In this model, the appearance of a low volume fraction of nanocrystals near the top transparent conductor interface is proposed to be incidental to the high VOC. [ABSTRACT FROM AUTHOR]

Local current flow in amorphous and nanocrystalline mixed-phase n-i-p silicon solar cells is measured using conductive atomic force microscopy (C-AFM) and correlated to the material structure from Raman measurement. The C-AFM images show that the current is very low over the entire surface of the fully amorphous region. High current spikes are observed in the mixed-phase region, where the current spike corresponds to aggregations of nanocrystallites. The size of the nanocrystalline aggregations is on the order of a half micrometer in diameter. The density of the current spike increases from the mixed phase to the heavily nanocrystalline regions. A thick amorphous silicon buffer layer inserted between the p and i layers significantly reduced the magnitude of the current spike. The C-AFM measurements suggest that the mixed-phase cells can be considered as a two-phase parallel-connected diode structure, consistent with our previously proposed model. [ABSTRACT FROM AUTHOR]

We have measured the two-dimensional electrical potential distribution on the surface of photovoltaic Cu(In,Ga)Se2 (CIGS) thin films using the nanoscale electrical characterization of scanning Kelvin probe microscopy. The potential peak on the grain boundaries becomes sharper after the sample is rinsed in high-purity water, and the height of the potential peak becomes smaller after chemical treatments in a solution similar to that used in the chemical bath deposition of CdS films. This demonstrates an effect of surface Na removal by the water rinsing and downward band bending on the CIGS film surface induced by the chemical treatment. This electrical modification is expected to benefit the properties of the electrical junction and, hence, CIGS/CdS device performance. [ABSTRACT FROM AUTHOR]