Local open-circuit voltage (Voc) distributions on amorphous and nanocrystalline mixed-phase silicon solar cells were measured using a scanning Kelvin probe microscope (SKPM) on the p layer of an n-i-p structure without the top ITO contact. During the measurement, the sample was illuminated with a laser beam that was used for the atomic force microscopy (AFM). Therefore, the surface potential measured by SKPM is the sum of the local Voc and the difference in workfunction between the p layer and the AFM tip. Comparing the SKPM and AFM images, we find that nanocrystallites aggregate in the amorphous matrix with an aggregation size of {approx}0.5 ..mu..m in diameter, where many nanometer-size grains are clustered. The Voc distribution shows valleys in the nanocrystalline aggregation area. The transition from low to high Voc regions is a gradual change within a distance of about 1 ..mu..m. The minimum Voc value in the nanocrystalline clusters in the mixed-phase region is larger than the Voc of a nc-Si:H single-phase solar cell. These results could be due to lateral photo-charge redistribution between the two phases. We have also carried out local Voc measurements on mixed-phase SiGe:H alloy solar cells. The magnitudes of Voc in the amorphous and nanocrystalline regions are consistent with the J-V measurements

Al-Jassim, M. M.

Guha, S.

Jiang, C.-S.

Kazmerski, L. L.

Moutinho, H. R.

Owens, J. M.

Yan, B.

Yang, J.

UNT Digital Library

 National Renewable Energy Laboratory Innovation for Our Energy Future A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNREL is operated by Midwest Research Institute ● Battelle     Contract No. DE-AC36-99-GO10337 Distribution of Local Open-Circuit Voltage on Amorphous and Nanocrystalline Mixed-Phase Si:H and SiGe:H Solar Cells  Preprint C.-S. Jiang, H.R. Moutinho, M.M. Al-Jassim, and L.L. Kazmerski National Renewable Energy Laboratory B. Yan, J.M. Owens, J. Yang, and S. Guha United Solar Ovonic Corporation Presented at the 2006 IEEE 4th World Conference on  Photovoltaic Energy Conversion (WCPEC-4) Waikoloa, Hawaii May 7–12, 2006 Conference Paper NREL/CP-520-39882 May 2006  NOTICE The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridgeAvailable for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone:  865.576.8401 fax: 865.576.5728 email:  mailto:reports@adonis.osti.govAvailable for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone:  800.553.6847 fax:  703.605.6900 email: orders@ntis.fedworld.gov online ordering:  http://www.ntis.gov/ordering.htmPrinted on paper containing at least 50% wastepaper, including 20% postconsumer waste DISTRIBUTION OF LOCAL OPEN-CIRCUIT VOLTAGE ON AMORPHOUS AND NANOCRYSTALLINE MIXED-PHASE Si:H AND SiGe:H SOLAR CELLS*   C.-S. Jiang, H. R. Moutinho, M.M. Al-Jassim, L. L. Kazmerski  National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 B. Yan, J. M. Owens, J. Yang, and S. Guha United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI 48084    ABSTRACT Local open-circuit voltage (Voc) distributions on amorphous and nanocrystalline mixed-phase silicon solar cells were measured using a scanning Kelvin probe microscope (SKPM) on the p layer of an n-i-p structure without the top ITO contact.  During the measurement, the sample was illuminated with a laser beam that was used for the atomic force microscopy (AFM).  Therefore, the surface potential measured by SKPM is the sum of the local Voc and the difference in workfunction between the p layer and the AFM tip.  Comparing the SKPM and AFM images, we find that nanocrystallites aggregate in the amorphous matrix with an aggregation size of ~0.5 µm in diameter, where many nanometer-size grains are clustered.  The Voc distribution shows valleys in the nanocrystalline aggregation area.  The transition from low to high Voc regions is a gradual change within a distance of about 1 µm. The minimum Voc value in the nanocrystalline clusters in the mixed-phase region is larger than the Voc of a nc-Si:H single-phase solar cell.  These results could be due to lateral photo-charge redistribution between the two phases.  We have also carried out local Voc measurements on mixed-phase SiGe:H alloy solar cells.  The magnitudes of Voc in the amorphous and nanocrystalline regions are consistent with the J-V measurements.   INTRODUCTION It has been shown that the best hydrogenated amorphous silicon (a-Si:H) solar cells are deposited under the condition close to the amorphous/nanocrystalline transition, but still in the amorphous regime [1], whereas the best hydrogenated nanocrystalline silicon (nc-Si:H) solar cells are deposited under the condition close to the transition, but in the nanocrystalline regime [2].  Solar cells made under the transition condition are called mixed-phase solar cells and show a distribution of open-circuit voltage (Voc) ranging from 0.5 to 1.0 V [3,4].  One interesting phenomenon in the mixed-phase solar cells is the light-soaking-induced Voc increase [3,4], which is opposite to the light-soaking-induced Voc decrease in conventional a-Si:H cells caused by the Staebler-Wronski effect [5]. Our original explanation for the light-induced Voc increase was light-induced structural changes from the crystalline to amorphous phase [3,4].  Subsequently, we proposed a complementary model with two parallel-connected diodes (two-diode) for explaining the Voc changes with crystalline volume fraction and light-induced Voc increase in the mixed-phase solar cells [6]. The key element in the two-diode model is that the amorphous phase and crystalline phase can be considered as two separate diodes with significantly different characteristics of a-Si:H and nc-Si:H solar cells.  However, the challenge is that the nanocrystallites observed by X-ray diffraction and Raman are very small with a size of a few nm to 30 nm.  It is difficult to believe that such small grains can form complete diodes through the intrinsic layer.  Recently, we used a conductive atomic force microscope (c-AFM) to measure the local current flow and found that the nanocrystallites aggregated and the nanocrystalline aggregation area showed a significantly higher forward current than the amorphous area [7]. In this paper, we report our new results of local Voc distribution in the mixed-phase solar cells measured using a scanning Kelvin probe microscope (SKPM).  We have established this nanometer-scale measurement technique [9,10] and applied this method for mapping the two-dimensional electrical potential distributions in III-V, Cu(In,Ga)Se2, and a-Si:H solar cells [10-12]. The measurement results have improved our understanding of device physics in the solar cells and provided us a direct method to assess the quality of the p-n junctions. EXPERIMENTAL Mixed-phase Si:H and SiGe:H n-i-p structures were deposited onto a 4×4 cm2 stainless-steel substrate. The n layer is an a-Si:H layer doped with phosphorus and the p is a nc-Si:H layer doped with boron. The i layer was deposited under the condition of the amorphous/nanocrystalline transition regime. Due to the high sensitivity of the transition to the plasma properties, the mixed-phase solar cell shows different characteristics at different locations even on the same substrate. Figure 1 (a) shows the solar cell distribution on the 4×4 cm2 stainless-steel substrate, where the large circles are indium tin oxide (ITO) dots with a area of 0.25 cm2 and the small ones of 0.05 cm2. Figure 1 (b) plots the Voc of the 16 large solar cells. One can see that the center area shows an amorphous signature, the outer region a mixed-phase, and the corners substantially nanocrystalline characteristics. *This work at NREL has been authored by Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy.  The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes.1The SKPM measurements were made at the three areas labeled with A, B, and C, as shown in Fig. 1(a), where no ITO was deposited.  The SKPM technique is based on the non-contact mode of AFM [8-10].  Similar to the classical Kelvin probe, SKPM measures the contact potential difference (CPD) between the sample surface and the probe. However, unlike the classical Kelvin probe using a small ac current signal, SKPM measures the Coulomb force between the sample and the AFM tip.  The extremely small distance between the tip and sample surface (~nm) and the extremely force-sensitive cantilever (~nN/m) ensure high resolutions in both spatial (~30 nm) and energy (~10 mV).  Details of the technique are described elsewhere [9,10]. RESULTS AND DISCUSSIONS  During the SKPM measurements, a built-in laser (used for the AFM operation) with 1.85-eV photon energy is used to illuminate the sample.  In the thermal equilibrium state in the dark, the CPD measured by SKPM is the workfunction difference (WD) between the sample and the tip, as illustrated by Fig. 2(a).  However, when the solar cell sample is illuminated, the CPD is the sum of WD and the local Voc, as shown in Fig. 2(b).  To obtain the Voc distribution, the WD of the p layer must be measured and subtracted from the measured CPD of the n-i-p structure.  We deposited a p-type nc-Si:H layer directly on stainless-steel substrates and measured the CPD.  Because the p layer is heavily doped and there is no p-n junction in the sample, the measured CPD should be close to the WD, even though the sample is illuminated by the laser light during the measurement. Figure 3 shows the SKPM CPD and the corresponding AFM topographic images taken on the amorphous, substantially nanocrystalline, and mixed-phase regions, as well as on the p-type nc-Si:H single layer. The potential distributions are relatively uniform on the amorphous and nanocrystalline single-phase regions, as shown in Fig. 3 (a) and (c), as well as on the p layer in Fig. 3 (g).  The microstructures on the morphology of the amorphous region [Fig. 3 (b)] show a small surface roughness in this region, while the cauliflower-like structures in the nanocrystalline region [Fig. 3 (d)] are aggregates of nanometer-size grains, which results in a large surface roughness.  On the mixed-phase region, a few large cauliflower-like structures appear on the  (a)11 12 13 1421 22 23 2431 32 33 3441 42 43 4451 52 53 54 5561 62 63 64 6571 72 73 74 7581 82 83 84 8592 93 94ABC1 2 3 4S1S2S3S40.40.50.60.70.80.91.01.1V oc (V)X Y (b) FIG. 1.  (a) Solar cell distribution on a 4 cm × 4 cm substrate.  The squares labeled A, B, and C are the three areas where SKPM electrical potential and AFM morphology images are taken.  (b) Voc distribution of the sixteen 0.25-cm2 cells on the same substrate. WDWD+VocKP=CPD=WD+VocKP=CPD=WDVocEvacEcEvEFEFEvacEcEvEFn EFpEFpin n ip(a) dark(b) illuminated Fig. 2.  Schematic band diagrams in the SKPM measurements.  In (a), the SKPM measures the WD between the tip and the sample surface in the thermal equilibrium state; and in (b), it measures the sum of WD and Voc when the sample is illuminated. 2topographic image [Fig. 3 (f)], which correspond to the valleys in the Voc distribution image [Fig. 3 (e)].  Note that the sizes of the Voc valleys are larger than the corresponding nanocrystalline aggregates that appeared in the AFM image. Figure 3 (i) shows line profiles of the SKPM images of the amorphous, nanocrystalline, and mixed-phase regions along the lines indicated by the arrows in Fig. 3 (a), (c), and (e), respectively.  The three dotted straight lines are the averaged values through several SKPM images of the amorphous and nanocrystalline regions, and the p layer.  The difference between the amorphous and nanocrystalline regions is 0.45 V, which is consistent with the Voc difference between a-Si:H and nc-Si:H solar cells measured by J-V characteristics.  Figure 3 (j) show the Voc line profile of the mixed-phase region obtained by subtracting the p-layer potential from the potential profile in Fig. 3 (i).  Two important features are observed from this plot.  First, the minimum Voc value in the nanocrystalline valleys is much larger than the average value in the fully nanocrystalline region, as indicated by the bottom straight line.  Second, the transition from the low to high Voc regions is a gradual change within a distance of about 1 µm.  We think these two phenomena are related and caused by the lateral charge redistribution.  The electrical interaction between the two phases by the relatively conductive p layer and the charge redistribution near the amorphous/nanocrystalline boundary in the bulk of the i layer could be a mechanism.  12001250mV0 nm50800850mV50100150200(a)(b)(c)(d)Distance (µm) 0 1 2 3 40200400600800100012001400Potential (mV)p-nc layerMixed phasea-phasenc-phase(i)Distance (µm) 0 1 2 3 4Mixed phasea-phasenc-phase500700600800100012001100900V oc (mV)(j)100011001200mV0 nm50100150150200mV0 nm2040(e)(f)(g)(h)2 µm0 nmPotential (mV)Potential (mV)Potential (mV) Fig. 3. SKPM potential and the corresponding AFM topographic images taken on (a) and (b) the amorphous phase, (c) and (d) the nanocrystalline phase, (e) and (f) the mixed-phase regions of an n-i-p solar cell. (g) and (h) were taken on a p-type nc-Si:H layer directly deposited on the stainless-steel substrate.  Solid-line profiles in (i) show the potentials along arrows in (a), (c), and (e). Dotted straight lines in (i) show averaged potential values over several SKPM images. (j) shows the Voc line profile in the mixed-phase deduced from the potentials in (i).  Although there is a lateral interaction between the amorphous phase and nanocrystalline aggregation areas, the Voc characteristic shows a clear distinction between the two phases. This is consistent with our recent C-AFM measurement on the mixed-phase cells [7]. The two-dimensional electrical current images show much larger local current flow through the nanocrystalline aggregates than through the surrounding a-Si:H matrix. However, unlike the gradual potential transition that is related to the charge redistribution around the amorphous/nanocrystalline boundary, the transition of local current flow between the two phases is sharp at the boundary, because of the much smaller bandgap and higher conductivity in the nanocrystalline aggregates than in the surrounding amorphous regions. We also measured the local Voc distributions in mixed-phase SiGe:H alloy solar cells (Fig. 4).  In general, the 3results are similar to the mixed-phase Si:H cells.  However, some differences are noticed.  First, the Voc values are lower than the corresponding values in the mixed-phase Si:H cells.  Second, the size of the nanocrystalline aggregates is also smaller than that in the mixed-phase Si:H cells.  Third, the Voc of the nanocrystalline aggregate is more heavily influenced by the surrounding amorphous matrix, showing smaller depth in the valley than the mixed-phase Si:H cells. Fourth, the transition from the low to high Voc regions becomes even broader. The phenomena above may result from the small size of nanocrystalline aggregates and/or heavy electrical interaction between the amorphous phase and nanocrystalline aggregates. The electrical interaction may relate to the electronic structure in the i layer, such as band alignment at the amorphous/nanocrystalline boundary. In addition, the conductivity of the SiGe:H alloy materials is higher than that of the Si:H materials.  The heavier Voc smooth-out in the mixed-phase SiGe:H cell could be partially caused by the high conductivity in the SiGe:H materials. SUMMARY By combining SKPM and AFM, we have developed a method to measure the local Voc distribution in mixed-phase solar cells.  The results clearly show the nanocrystalline aggregation. The Voc is smaller in the nanocrystalline aggregates than in the surrounding amorphous matrix, and the transition from the low to high Voc is a gradual change.   Although there are some lateral charge redistributions, a clear distinction between the amorphous and nanocrystalline regions has been observed.  The current SKPM results and previous C-AFM results provide extra support for the two-diode model for explaining the carrier transport in mixed-phase solar cells. ACKNOWLEDGEMENT This work at NREL is supported or funded under DOE Contract No. DE-AC36-99GO10337. The work at United Solar was supported by NREL under the Thin Film Partnership Program Subcontract No. ZXL-6-44205-14. 50070060080010004001100900V oc (mV )Distance (µm) 0 1 2 3 4950mV10000 nm50100(a) (b) (c)2 µm Fig. 4. (a) A SKPM electrical potential and (b) the corresponding topographic images taken on a mixed-phase a-SiGe:H solar cell. The solid curve in (c) shows a potential line profile along the arrows in (a), and the two dotted straight lines show averaged potential values take on the amorphous and nanocrystalline regions.  REFERENCES [1] J. Yang, A. Banerjee, K. Lord, and S. Guha, Proc. of 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK, September 15-22, 2000 (IEEE, 2000), p. 742.  [2] T. Roschek, T. Repmann, J. Müller, B. Rech, and H. Wagner, Proc. of 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK, September 15-22, 2000, (IEEE, 2000), p. 150.  [3] K. Lord, B. Yan, J. Yang, and S. Guha, Appl. Phys. Lett. 79, 3800 (2001). [4] J. Yang, K. Lord, B. Yan, A. Banerjee, S. Guha, D. Han, and K. Wang, Mat. Res. Soc. Symp. Proc. 715, 601 (2002). [5] D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31, 292 (1977). [6] B. Yan, J. Yang, and S. Guha, Proc. of 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, May 11-18, 2003, p. 1627. [7] B. Yan, J. Yang, S. Guha, C.-S. Jiang, H. R. Moutinho, and M. M. Al-Jassim, Mat. Res. Soc. Symp. Proc., A23.6 (2006). [8] J. M. R. Weaver and D. W. Abraham, J. Vac. Sci. Technol. B9, 1559 (1991). [9] A. Kikukawa, S. Hosaka, and R. Imura, Appl. Phys. Lett. 66, 3510 (1995). [10] C.-S. Jiang, H. R. Moutinho, D. J. Friedman, J. F. Geisz, and M. M. Al-Jassim, J. Appl. Phys. 93, 10035 (2003). [11] C.-S. Jiang, H. R. Moutinho, M. J. Romero, M. M. Al-Jassim, Y. Q. Xu, and Q. Wang, Thin Solid Films 472, 203 (2005). [12] C.-S. Jiang, H. R. Moutinho, Q. Wang, M. M. Al-Jassim, B. Yan, J. Yang, and S. Guha, Mat. Res. Soc. Symp. Proc.  808, 587 (2004).  4

Distribution of Local Open-Circuit Voltage on Amorphous and Nanocrystalline Mixed-Phase Si:H and SiGe:H Solar Cells: Preprint

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Distribution of Local Open-Circuit Voltage on Amorphous and Nanocrystalline Mixed-Phase Si:H and SiGe:H Solar Cells: Preprint

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