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ABSTRACT: 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.
Journal of Applied Physics 07/2011; 110(1):014514-014514-5. · 2.17 Impact Factor
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ABSTRACT: 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.
Journal of Applied Physics 10/2010; 108(7):074503-074503-7. · 2.17 Impact Factor
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ABSTRACT: In this contribution, the authors report on near-field scanning optical microscopy measurements of the luminescence emitted from localized junction breakdown in epitaxial silicon solar cells. Our measurements suggest that the observed local reduction in breakdown voltage results from avalanche multiplication assisted by the reinforcing combination of (i) the local enhancement of the electrostatic field at the apex of inverted pyramid pits and (ii) the participation of defect states in the avalanche breakdown. Transmission electron microscopy reveals the microstructure of the defect responsible for the local junction breakdown.
Applied Physics Letters 08/2010; 97(9):092107-092107-3. · 3.84 Impact Factor
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ABSTRACT: 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.
Journal of Applied Physics 01/2010; · 2.17 Impact Factor
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ABSTRACT: Following our previous study on the material-quality limiting factors of evaporated solid-phase crystallized (SPC) poly-Si thin films fabricated on planar glass for photovoltaic applications, we extend our study to investigate the impurity levels, optical properties, transport properties, and device performance of so-called ¿EVA¿ (EVAporated Si) solar cells. These potentially cost-effective cells are systematically characterized with electron microscopy-based techniques, external quantum efficiency, and standard current-voltage measurements. We conclude that 5% efficient cells are now attainable, and much room remains for further improving device performance. We are confident that 10%-efficient EVA solar cells will be realized in the future, with proper metallization schemes, good light trapping, and improved poly-Si film quality.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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ABSTRACT: Scanning Kelvin probe force microscopy was applied to the microelectrical characterizations of junctions in solar cell devices. Surface Fermi-level pinning effects on the surface potential measurement were avoided by applying a bias voltage (V(b)) to the device and taking the V(b)-induced potential and electric field changes. Two characterizations are presented: the first is a direct measurement of Bi-induced junction shift in GaInNAs(Bi) cells; the second is a junction-uniformity measurement in a-Si:H devices. In the first characterization, using Bi as a surfactant during the molecular beam epitaxy growth of GaInNAs(Bi) makes the epitaxial layer smoother. However, the electrical potential measurement exhibits a clear Bi-induced junction shift to the back side of the absorber layer, which results in significant device degradation. In the second characterization, the potential measurement reveals highly non-uniform electric field distributions across the n-i-p junction of a-Si:H devices; the electric field concentrates much more at both n/i and i/p interfaces than in the middle of the i-layer. This non-uniform electric field is due possibly to high defect concentrations at the interfaces. The potential measurements further showed a significant improvement in the electric field uniformity by depositing buffer layers at the interfaces, and this indeed improved the device performance.
Ultramicroscopy 05/2009; 109(8):952-7. · 2.47 Impact Factor
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ABSTRACT: 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.
Journal of Applied Physics 12/2008; · 2.17 Impact Factor
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ABSTRACT: The spinel cobalt aluminate has gained interest as a potential photoelectrochemical catalyst for the renewable production of hydrogen. Using band structure theory, we determine the energetics of possible intrinsic point defects in spinel CoAl2O4 and analyze their effect on its electronic and chemical properties. Extrinsic Fe-doping is also examined. Cation vacancies are found to be shallow acceptors, but their formation energy is sensitive to the growth conditions; an oxygen rich environment is required to enhance the p-type conductivity. Fe is an isovalent substituent on the Co (Al) site, exhibiting a preference for octahedral coordination, and forms a deep donor (acceptor) level near the center of the band gap, corresponding to a Fe(II) to Fe(III) transition.
07/2008;
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ABSTRACT: We use the combination of high-resolution electron microscopy and density-functional theory to study the atomic structure and electronic effects of grain boundaries in polycrystalline photovoltaic materials such as Si, CdTe, CuInSe 2 , and CuGaSe 2 . We find that grain boundaries containing dislocation cores create deep levels in Si, CdTe, and CuGaSe 2 . Surprisingly, however, they do not create deep levels in CuInSe 2 . We further find that the presence of Ga in grain boundaries in CuInSe 2 generates deep levels. These results may explain the fact that Si and CdTe solar cells usually require special passivation, whereas CuInSe 2 solar cells do not. The passivation of grain boundaries in Si and CdTe is also studied. We find that grain boundaries in CdTe can be passivated very well by Cl, Br, and I.
Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 06/2008
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ABSTRACT: 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.
Journal of Applied Physics 03/2008; 103(6):063515-063515-6. · 2.17 Impact Factor
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ABSTRACT: The classic grain-boundary (GB) model concludes that GBs in polycrystalline semiconductors create deep levels that are extremely harmful to optoelectronic applications. However, our first-principles density-functional theory calculations reveal that, surprisingly, GBs in CuInSe2 (CIS) do not follow the classic GB model: GBs in CIS do not create deep levels due to the large atomic relaxation in GB regions. Thus, unlike the classic GB model, GBs in CIS are electrically benign, which explains the long-standing puzzling fact that polycrystalline CIS solar cells with remarkable efficiency can be achieved without deliberate GB passivation. This benign electrical character of GBs in CIS is confirmed by our scanning Kelvin probe microscopy measurements on Cu(In,Ga)Se2 chalcopyrite films.
Physical Review Letters 01/2008; 99(23):235504. · 7.37 Impact Factor
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ABSTRACT: Both the origins of the high open circuit voltages (V<sub> OC </sub>) 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 V<sub> OC </sub> . 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 H <sub>2</sub> dilution account for the improved V<sub> OC </sub> 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 V<sub> OC </sub> .
Journal of Applied Physics 07/2007; · 2.17 Impact Factor
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ABSTRACT: The asymmetry doping problem has severely hindered the potential applications of many wideband gap (WBG) materials. Here, we propose a possible approach to overcome this long-standing doping asymmetry problem for WBG semiconductors. Our approach is based on the reduction of the ionization energies of dopants through introduction and effective doping of mutually passivated impurity bands, which can be realized by doping the host with passive donor-acceptor complexes or isovalent impurities. Our density-functional theory calculations demonstrate that this approach provides excellent explanations for the n-type doping of diamond and p-type doping of ZnO, which could not be understood by previous theories. In principle, this approach can be applied to any WBG semiconductors and therefore will open a broad vista for the application of WBG materials.
Physical Review Letters 04/2007; 98(13):135506. · 7.37 Impact Factor
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ABSTRACT: Microcharacterization of the luminescent, structural, electrical and topographical properties of thin films of close-spaced sublimation (CSS)-fabricated CdTe was performed. the film morphology was found to be dependent on the film thickness, deposition conditions and post-deposition treatment. the complementary use of electron beam-induced current (EBIC) and cathodoluminescence (CL) analysis in the scanning electron microscope demonstrated large inter- and intragrain inhomogeneities in the luminescent and electrical properties of the films. Follow-on plan-view examinations with the transmission electron microscope revealed varying densities of structural dejects, such as stacking faults and threading dislocations, which could explain the variations observed in the CL and EBIC images. Consisting primarily of experimental observations and comments, this report is phenomenological in nature.
Progress in Photovoltaics Research and Applications 01/2007; 2(3):203 - 209. · 5.79 Impact Factor
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ABSTRACT: We report on the measurement of local current flow in hydrogenated amorphous and nanocrystalline mixed-phase n-i-p silicon solar cells in the initial, light-soaked, and annealed states using conductive atomic force microscopy (C-AFM). The C-AFM measurement shows that the nanometer-size grains aggregate, and the local current densities in the nanocrystalline aggregation areas decreased significantly after light soaking and recovered to values similar to the initial state after annealing at a high temperature in a vacuum. This result supports the model of two parallel-connected diodes for explaining the light-induced open-circuit voltage increase in the mixed-phase solar cells.
MRS Proceedings. 12/2006; 989.
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ABSTRACT: The authors present their first-principles calculations of doping effects in ZnO with group-IB elements such as Cu, Ag, and Au. The calculated transition energies ε(0/−) for substitutional Cu, Ag, and Au are 0.7, 0.4, and 0.5 eV, respectively. The calculated formation energies are very low for these group-IB elements on the substitutional sites, but rather high at the interstitial sites under oxygen-rich growth conditions. Under the conditions, the formation of major hole-killer defects, such as oxygen vacancies and Zn interstitial, are suppressed. Thus, Ag may be a good candidate for producing p-type ZnO.
Applied Physics Letters 10/2006; 89(18):181912-181912-3. · 3.84 Impact Factor
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ABSTRACT: The authors report on the observation of electroluminescence (EL) in CuGaSe2 solar cells using tapping-mode atomic force microscopy based on tuning-fork sensors. Individually injected current pulses are seen during intermittent contact driven by an external bias applied to the conducting tip. It follows that EL can be stimulated when the solar cell is forward biased during the contact cycle. Local L-V characteristics show evidence for EL, with a threshold voltage of 3.0–3.7 V. Mapping of the photon emission suggests that grain boundaries effectively isolate grain interiors, which behave as individual light-emitting diodes.
Applied Physics Letters 10/2006; 89(14):143120-143120-3. · 3.84 Impact Factor
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ABSTRACT: We investigated the effects of the etching processes using bromine and nitric-phosphoric acid solutions, as well as of Cu, in the bulk electrical conductivity of CdTe/CdS solar cells using conductive atomic force microscopy (C-AFM). Although the etching process can create a conductive layer on the surface of the CdTe, the layer is very shallow. In contrast, the addition of a thin layer of Cu to the surface creates a conductive layer inside the CdTe that is not uniform in depth, is concentrated at grains boundaries, and may short circuit the device if the CdTe is too thin. The etching process facilitates the Cu diffusion and results in thicker conductive layers. The existence of this inhomogeneous conductive layer directly affects the current transport and is probably the reason for needing thick CdTe in these devices
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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ABSTRACT: We report on a direct measurement of the electrical potential on cross-sections of GaInP<sub>2</sub>/GaAs multiple-junction solar cells by using an ultrahigh-vacuum scanning Kelvin probe microscope (UHV-SKPM). The UHV-SKPM allows us to measure the potential without air molecules being adsorbed on the cross-sectional surface. Moreover, it uses a GaAs laser with photon energy of 1.4 eV for the atomic force microscope (AFM) operation. This eliminated the light-absorption-induced bottom-junction flattening and top-junction enhancement, which happened in our previous potential measurement using a 1.85-eV laser for the AFM operation. Three potentials were measured at the top, tunneling, and bottom junctions. Values of the potentials are smaller than the potentials in the bulk. This indicates that the Fermi level on the UHV-cleaved (110) surface was pinned, presumably due to defects upon cleaving. We also observed higher potentials at atomic steps than on the terraces for both GaInP<sub>2</sub> epitaxial layer and GaAs substrate. Combining scanning tunneling microscopy (STM) and SKPM measurements, we found that the potential height at steps of the GaAs substrate depends on the step direction, which is probably a direct result of unbalanced cations and anions at the steps
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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ABSTRACT: In this paper we report on the design, fabrication and modeling of 49 cm<sup>2</sup>, 200-mum thick, 1-5 Omega-cm, n- and p-type lang111rang and lang100rang screen-printed silicon solar cells. A simple process involving RTP front surface phosphorus diffusion, low frequency PECVD silicon nitride deposition, screen-printing of Al metal and Ag front grid followed by co-firing of front and back contacts produced cell efficiencies of 15.4% on n-type lang111rang Si, 15.1% on n-type lang100rang Si, 15.8% on p-type lang111rang Si and 16.1% on p-type lang100rang Si. Open circuit voltage was comparable for n and p type cells and was also independent of wafer orientation. High fill factor values (0.771-0.783) for all the devices ruled out appreciable shunting which has been a problem for the development of co-fired n-type lang100rang silicon solar cells with Al back junction. Model calculations were performed using PC1D to support the experimental results and provide guidelines for achieving >17% n-type silicon solar cells by rapid firing of Al back junction
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
