DetectionandLocalizationofDefectsin … · 2018. 3. 29. · Figure 4: SNOM experimental setup for...

Hindawi Publishing CorporationAdvances in Optical TechnologiesVolume 2010, Article ID 805325, 5 pagesdoi:10.1155/2010/805325

Research Article

Detection and Localization of Defects inMonocrystalline Silicon Solar Cell

P. Tománek, P. Škarvada, R. Macků, and L. Grmela

Department of Physics, Faculty of Electrical Engineering and Communication, Brno University of Technology,Technická 8, 616 00 Brno, Czech Republic

Correspondence should be addressed to P. Tománek, [email protected]

Received 14 December 2009; Accepted 16 March 2010

Academic Editor: Peter V. Polyanskii

Copyright © 2010 P. Tománek et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Near-surface defects in solar cell wafer have undesirable influence upon device properties, as its efficiency and lifetime. Whenreverse-bias voltage is applied to the wafer, a magnitude of electric signals from defects can be measured electronically, but thelocalization of defects is difficult using classical optical far-field methods. Therefore, the paper introduces a novel combinationof electric and optical methods showing promise of being useful in detection and localization of defects with resolution of250 nm using near-field nondestructive characterization techniques. The results of mapped topography, local surface reflection,and local light to electric energy conversion measurement in areas with small defects strongly support the development and furtherevaluation of the technique.

1. Introduction

Although the concept of photovoltaic (PV) devices descendsfrom the mid-19th century, its modern age began after1950 [1]. Solar cells fulfill two principal functions: photo-generation of charge carriers—electrons and holes—in alight-absorbing material, and separation of the charge carri-ers to a conductive contact that transmits the electric current[2]. Their efficiency is limited by a number of factors, whichinclude fundamental power losses (incomplete absorption oflight or dissipation of a part of the photon energy as heat);losses caused by the reflection of light from the cell surface;and finally, a recombination of the electron-hole pairs in thesubstrate.

The basic methods for the characterization of siliconsolar cells are generally electrical measurements [3–5].Electrical methods represent an integral measurement onthe whole cell. Unfortunately, they do not enable to localizedefects occurring in the structure. Local defects in the p-n junction may be associated with structural imperfections(such as grain boundaries, dislocations, and scratches),impurities, higher concentrations of donors and acceptors,or both [6]. Therefore, it is important not only to find most

harmful defects, but also to understand their nature andidentify the factors which affect adversely their formationand recombination properties. The used PV analytical toolsare generally divided into two groups.

(i) Mapping techniques which allow the access to theareas of interest (usually the areas with high prob-ability of defects). For materials analysis, lifetime-mapping tools such as surface photo voltage (SPV)[7], microwave photo conductance decay (MW PCD)[8] are frequently applied. Other methods mapspatial distribution of photocurrent induced by laserbeam (LBIC) [9], or by electron beam-inducedcurrent (EB-IC) [10] over whole wafer.

(ii) Nonmapping techniques that are applied to providebetter insight into the nature of recombinationcenters [11, 12].

For this reason, and for more precise mapping, the use oflocal characterization methods seems to be very important.Owing to the diffraction, there are Rayleigh limitations ofthe resolution in traditional optical microscopy. A higher

2 Advances in Optical Technologies

VS C RL

SC

OFP

PA

A

A

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ND

DV

DV PC

Figure 1: Scheme of experimental setup for the measurementof effective values of noise current versus reverse-bias voltage ofelectric (lower arm) and optical (upper arm) signals.

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Figure 2: Defect noise current versus reverse voltage of the solar cellwafer. Numbers show a noise signal. By repeating a measurement,the noise current figure always appears at same bias voltages.

resolution can be achieved with confocal microscopy [13] orScanning Near-field Microscopy (SNOM) [14].

The combination of SNOM with LBIC allows creatinga strong characterization method of Near-field OpticalBeam-Induced Photocurrent (NOB-IC) [15]. This methodprovides a measurement of the current locally induced byoptical near-field. The combination of high resolution of themicroscope with locally induced light by sharpened opticalfiber allows obtaining a resolution bellow the wavelength ofused light.

Due to the fact that solar cells are optoelectronic devicesbased on photoelectric effect, it is natural and desirabletesting them by using optical and optoelectronic methods.Nevertheless, almost all scientific groups studied one kindof these characteristics only—electrical or optical ones. Ourprevious effort has been focused on the investigation ofsolar cells [16, 17], because their local properties are notwell described yet. To elucidate slightly more this problem,elaborated combination of electric and localized opticalmeasurement, which allows the detection and localization ofdefects in the solar cell wafer, and to compare experimentalresults and obtain higher resolution, is presented.

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Figure 3: Localization of defects imperfection areas of solar cellwafer using reverse-bias light emission from different wafer sites(Ur = 18.5 V).

2. Experimental Methods and Material

The sample of monocrystalline silicon solar cell wafer witharea of 10 cm × 10 cm has been tested. Most important partof the solar cell is its p-n junction. When reverse-bias voltageis applied, lower voltage breakdown of p-n junction occurs indefect sites. reversing the current shift in the homogeneousbreakdown may be primarily formed by the current flux inlocal defects. The emission from defect could be consideredas noise current. In areas of increased concentration of freecharge carriers due to the small cross sections, there is a largecurrent density which can lead to strong local heating andthen to the local diffusion and heat breakdown.

2.1. Photoelectric Measurement. To set a suitable reversevoltage, which leads to emission of radiation from defects,computer-controlled voltage source (VS), filter capacitor(C), and parallel load resistance (RL) were used (Figure 1).The circuit was connected to the reverse state monocrys-talline solar cell wafer (SC) with area of 10 cm × 10 cm. Theelectrical signal was detected at the load resistor RL = 5.17Ω.The obtained signal was amplified by preamplifier (PA) andamplifier (A). Plastic optical fiber (OF) with aperture of200 μm has scanned over solar cell and collected a weakoptical signal emitted from this place and sent it to thephotomultiplier (P) and amplifier (A) (Figure 1).

To measure noise voltage characteristics and photo-electric signals in the sample, two noise detectors (NDS)(selective Nanovoltmeters Unipan 237) have been used. Theupper arm of the setup was tuned to the frequency of10 kHz for the optical signal, and the lower arm to 4.2 kHz

Advances in Optical Technologies 3

PMT

Laser

Coupling system

Optical fiber

Tuning fork

Figure 4: SNOM experimental setup for the measurement of effective values of electric response, optical properties—reflectance, andtopography of solar cell in the near-field.

for the electric one. The voltage was measured by digitalvoltmeter (DV) and values were stored on PC. With thissetup, the effective values of noise current versus reverse-biasvoltage of electric and optical signals were measured. Thebias voltage was continuously set from 0 to 25 V. Figure 2represents the relation between defect noise current andreverse-bias voltage over whole solar cell wafer. By repeatingthis measurement, the noise signals appeared for the samevalues of bias voltages.

The corresponding optical application in upper armprovides localization of defects or imperfections. When thereverse-bias voltage was applied, any noise signal from defectsite has not been observed up to first important current peakat Ur = 6.8 V (peak 1 in Figure 2). With further increasing ofthe voltage, other near-surface defects appeared in differentsites on the sample. When bias voltage reached a value ofUr = 18.5 V, several very intensive spots, originated mainlyin ill-cutting edges of solar cell, defects in p-n junction,or imperfections of structure, have been clearly localized(Figure 3), and a corresponding current signal was quitestrong (peak 5 in Figure 2). For other values, the locationof these sites could vary in function of defects nature. Above23 V, the electric noise signals inside silicon wafer dominatedover defect signals and interpretation of results was no moremeaningful.

2.2. Near-Field Measurements. In the second method (near-field NOBIC experiment) [18, 19], a very small area of siliconsolar cell surface (approx. 150 nm in diameter) has beenexcited by green laser diode (λ = 532 nm) light transmittedthrough a nanometer-sized (70 nm) aperture in the Ag-coated sharpened single mode fiber probe (Figure 4).

The excitation light was amplitude modulated by thelight chopper at frequency of 300 Hz. The input powercoupled into the optical fiber probe was 3 mW, and output

power from the fiber probe varied between 10 nW and100 nW, when detected by remote detector. Consequently,the detected photo-induced current varied in the range 100–300 PA. The localized photo-induced current across the layerof solar cell was then detected as a function of the tip positionabove the sample surface mounted on an x-y-z piezo and wasscanned (the scanning step of 50 nm) related to the probetip. During the scan the tip-sample distance is kept constantat (5 ± 1) nm using an optical shear force feedback control.Thanks to this setup, the xy current distribution map of solarcell has been obtained. The photo-induced current signal hasbeen detected by a lock-in nanovoltmeter while the solar cellwas reverse-biased or unbiased [20].

The accuracy of this method depends primarily on thelight spot size and on the scanning step of the piezo driver,which are inversely proportional. By long step the accuracyof the method is low, but whole measurement processaccelerates because of reduced number of measured points.Therefore, it was very important to choose an optimal ratioscanning step/spot size.

The topography of the sample with a pyramidal textureis shown in Figure 5(a). The electrical response signals,corresponding reflectance, and topography are demonstratedby dependence on spatial coordinate in Figure 5(b). Blackcurve corresponds to the profile of the sample surface. Blueone corresponds to the electrical response signal and purpleone represents a local surface reflectance (in one scanningline).

Relative electrical response mapped by color scale ontooriginal topography of the sample is shown in Figure 6. Thepyramidal structure form Figure 5 has been etched so todecrease the electrical response on the tops of texture, whichallow obtaining higher electric efficiency of the cell. Thisnew mesa structure of the samples is presently the object ofintensive study.

4 Advances in Optical Technologies

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Figure 5: Local topography of solar cell sample with pyramidalstructure (a), and corresponding scan of electrical response,reflectivity and topography (b).

3. Conclusions

The novel combination of methods for samples local electricdetection and optical localization with micro- and nano-scale resolution for the study of monocrystalline silicon solarcell wafer is presented. applying the reverse-bias voltage,several intensity spots, originated mainly in ill-cutting edgesof solar cell, defects in p-n junction, or imperfections ofstructure, have been clearly localized (Figure 3), and noisecurrent signal peak was quite strong (peak 5 in Figure 2).Above 23 V, the electric noise signals inside silicon waferdominated over defect signals and interpretation of resultswas no more meaningful.

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Figure 6: Constant local light to current conversion distributionmapped at sample surface topography. Measurement parameters:scanning velocity v = 18.5μm/s, set poin U = 8.9 V, feedback gain0.5, modulation frequency f = 850 Hz, used light wavelength λ =532 nm, temperature T = 298 K, load resistance R = 3.3Ω.

A combination of NOBIC and reflectivity measurementwith the resolution of about 250 nm has also been estab-lished. After calibration of the setup, the accuracy of thecombined reflection measurements is better than 5%. Atshort circuit condition, the NOBIC photocurrent of this celldominates over the variation of the reflectivity. Based oncorrelations with aperture-SNOM, the sites correspondingto largest and smallest reflections have been assigned. Thephotocurrent is the smallest on top of protruding peakswhich have a greater local reflectivity, and is the largest in thevalleys with the smallest reflectivity. Using this correlationwe have found, that the photocurrent for applying a forwardvoltage decreases inhomogeneously at different locations(Figures 5 and 6). The measurement has shown smallerrelative fall of the photocurrent for the illumination of valleysin comparison with the peaks in the structure.

Proposed characterization method based on scanningprobe microscopy technique SNOM allows nondestructiveand noncontact sample study (defects in p-n junction, struc-ture imperfections, and local photoelectric measurements).A maximum of optically excited photocurrent is indicatorof local conversion efficiency due to local light constantenergy excitation, and number of imperfections is a qualityindicator for solar cell lifetime and only precise testing canhelp to determine a nature of defects. At present time, itis quite difficult find a correlation between defect natureand its appearance. Therefore, for further improvement ofmonocrystalline silicon solar cells efficiency, more intensivemapping and nonmapping measurements of optical andelectric properties are challenged.

Acknowledgments

This work has been supported by the Czech Ministry of Edu-cation in the frame of MSM 0021630503 Research IntentionMIKROSYN “New Trends in Microelectronic System andNanotechnologies” and by GACR Grant 102/08/1474 “Localoptical and electric characterization of optoelectronic deviceswith nanometer resolution”.

Advances in Optical Technologies 5

References

[1] D. M. Chapin, C. S. Fuller, and G. L. Pearson, “A new siliconp-n junction photocell for converting solar radiation intoelectrical power,” Journal of Applied Physics, vol. 25, no. 5, pp.676–677, 1954.

[2] European Renewable Energy Council, “Renewable EnergyTarget for Europe—20% by 2020,” January 2004, http://www.erec.org/fileadmin/erec docs/Documents/Publications/Renewable Energy Technology Roadmap.pdf.

[3] A. G. Chynoweth and K. G. McKay, “Light emission and noisestudies of individual microplasmas in silicon p-n junctions,”Journal of Applied Physics, vol. 30, no. 11, pp. 1811–1813, 1959.

[4] A. R. Haitz, “Model for the electrical behaviour ofmicroplasma,” Journal of Applied Physics, vol. 35, no. 5, pp.1370–1376, 1964.

[5] J. Nelson, The Physics of Solar Cells, Imperial College Press,London, UK, 2007.

[6] P. Škarvada and P. Tománek, “Local light to electric energyconversion measurement of silicon solar cells,” in Proceedingsof the Reliability and Life-Time Prediction, pp. 101–104, ZsoltIllyefalvi-Vitéz, Bálint Balogh, Budapest Hungary, 2008.

[7] L. Kronik and Y. Shapira, “Surface photovoltage spectroscopyof semiconductor structures: at the crossroads of physics,chemistry and electrical engineering,” Surface and InterfaceAnalysis, vol. 31, no. 10, pp. 954–965, 2001.

[8] P. A. Basore and B. R. Hansen, “Microwave-detected photo-conductance decay,” in Proceedings of the Conference Recordof the 21st IEEE Photovoltaic Specialists Conference, vol. 1, pp.374–379, 1990.

[9] C. Donolato, “Theory of beam induced current characteriza-tion of grain boundaries in polycrystalline solar cells,” Journalof Applied Physics, vol. 54, no. 3, pp. 1314–1322, 1983.

[10] P. Koktavy, J. Vanek, Z. Chobola, K. Kubickova, and J.Kazelle, “Solar cell noise diagnostic and LBIC comparison,”in Proceedings of the International Conference on Noise andFluctuations, vol. 922, pp. 306–309, 2007.

[11] D. K. Schroder, Semiconductor Material and Device Character-ization, Wiley-IEEE Press, 3rd edition, 2006.

[12] S. Rein, Lifetime Spectroscopy: A Method of Defect Characteri-zation in Silicon for Photovoltaic Applications, vol. 85, Spinger,Berlin, Germany, 2005.

[13] E. Esposito, F.-J. Kao, and G. McConnell, “Confocal opticalbeam induced current microscopy of light-emitting diodeswith a white-light supercontinuum source,” Applied Physics B,vol. 88, no. 4, pp. 551–555, 2007.

[14] P. Tománek, J. Brüstlová, P. Dobis, and L. Grmela, “HybridSTM/R-SNOM with novel probe,” Ultramicroscopy, vol. 71,no. 1–4, pp. 199–203, 1998.

[15] M. S. Unlü, B. B. Goldberg, W. D. Herzog, D. Sun, and E. Towe,“Near-field optical beam induced current measurements onheterostructures,” Applied Physics Letters, vol. 67, no. 13, pp.1862–1864, 1995.

[16] P. Škarvada, P. Tománek, and R. Macků, “Study of localproperties of silicon solar cells,” in Proceedings of the 23rdEuropean Photovoltaic Solar Energy Conference, pp. 1644–1647,WIP-Renewable Energies, Valencia, Spain, 2008.

[17] P. Tománek, P. Škarvada, and L. Grmela, “Local optical andelectron characteristics of solar cells,” in 9th InternatiuonalConference on Correlation Optics, vol. 7388 of Proceedings ofSPIE, Chernivtsi, Ukraine, September 2009.

[18] D. C. Coffey, O. G. Reid, D. B. Rodovsky, G. P. Bartholomew,and D. S. Ginger, “Mapping local photocurrents in poly-mer/fullerene solar cells with photoconductive atomic forcemicroscopy,” Nano Letters, vol. 7, no. 3, pp. 738–744, 2007.

[19] M. Benešová, P. Dobis, P. Tománek, and N. Uhdeová, “Localmeasurement of optically induced photocurrent in semicon-ductor structures,” in Photonics, Devices, and Systems II, vol.5036 of Proceedings of SPIE, pp. 635–639, 2002.

[20] P. Škarvada, L. Grmela, I. F. Abuetwirat, and P. Tománek,“Nanooptics of locally induced photocurrent in monocrys-talline Si solar cells,” in Photonics, Devices, and Systems IV, vol.7138 of Proceedings of SPIE, 2008.

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