Low-frequency noise, microplasma, and electroluminescence measurements as faster tools to investigate quality of monocrystalline-silicon solar cells

Low-frequency noise, microplasma, andelectroluminescence measurements asfaster tools to investigate quality ofmonocrystalline-silicon solar cells

Zdenek ChobolaMiroslav LunakJiri VanekRadim Barinka

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Low-frequency noise, microplasma, andelectroluminescence measurements asfaster tools to investigate quality ofmonocrystalline-silicon solar cells

Zdenek ChobolaMiroslav LunakBrno University of TechnologyDepartment of PhysicsŽižkova 17602 00 Brno, Czech RepublicE-mail: [email protected]

Jiri VanekBrno University of TechnologyDepartment of ElectrotechnologyTechnicka 10616 00 Brno, Czech Republic

Radim BarinkaSolartec,s.r.o.Televizní 2618765 61 Rožnov pod Radhoštěm, Czech Republic

Abstract. Two sets of c-Si solar cells varying in front side phosphorusdoped emitters were produced by standard screen printing techniques.The first group of samples, 3121, was prepared by a combination ofstandard washing and a bath with a highly dilute HF before diffusion ofnþ-emitter. The second group of samples, 3122, was treated only withstandard washing. A comparison of solar cell conversion efficiency andresults from a noise spectroscopy, microplasma, and electroluminescencepresence are presented. As was already shown in previous publicationsnoise spectral density reflects the quality of solar cells, and thus repre-sents an alternative advanced cell diagnostic tool. Our results confirmthis relationship andmoreover bring clear evidence for the maximum spec-tral noise voltage density being related to the emitter structure. The bestresults were reached for the group of solar cells in sample 3122, whichwas treated only with standard washing. © 2013 Society of Photo-OpticalInstrumentation Engineers (SPIE). [DOI: 10.1117/1.OE.52.5.051203]

Subject terms: solar cell; monocrystalline-silicon; noise; microplasma; electrolumi-nescence; quality; indicator; efficiency.

Paper 120865SSP received Jun. 28, 2012; revised manuscript received Sep. 11,2012; accepted for publication Oct. 23, 2012; published online Mar. 4, 2013.

1 Introduction

1.1 Excess Noise

It is generally accepted that there are some fundamentalsources of noise that generate noise background. This is thecase of thermal noise, shot noise and, as was shown recently,of fundamental quantum 1/f noise.1–3 Besides fundamentalnoise, which cannot be eliminated from any device, thereexists excess noise, which is believed to carry informationon the device’s technological and structural defects whichare either unintentionally introduced during the device’s pro-duction or appear as a result of degradation processes duringthe device’s operation.4–7 The defects are natural sourcesof excess current and excess noise, and they are responsiblefor changes of several measurable quantities. Physical pro-cesses in electronic devices can give useful information ondevice reliability, provided there is a correlation with failuremechanisms. This paper deals with comparisons of noisespectroscopy and detection of electroluminescence noisesources in single-silicon solar cells with diffusion of nþ-emitter with the source of POCl3. The first group of samples,3121, was prepared by combination of standard washing andbath with and highly dilute HF before diffusion of nþ-emit-ter. The second group of samples, 3122, was treated onlywith standard washing.

The noise voltage spectral density was measured in for-ward biased voltage. The noise voltage was being picked upacross a load resistance RL ¼ 100 Ω, at a band mean fre-quency of 1 kHz and a bandwidth of 20 Hz. When high

electrics are applied to a PN junction with some technologi-cal imperfections like dislocation in the PN junction or acrystal-grid defect causing nonhomogeneity of parameters,it produces tiny areas of enhanced impact ionization calledmicroplasma. Microplasma produces noise, which has arandom spectrum in the frequency range.

Microplasma noise is measurable even before the creationof light emissions. Due to the comparisons microplasmadetection with noise characteristics we can full analyzedsolar cell.

1.2 Microplasma

Creation of microplasma takes place at spots with nonhomo-geneity in structure at PN junctions. Light emission isexhibited in the full spectrum range. The whole process isobserved with a special charge coupled device (CCD) cam-era in a dark special cryogenic box. A CCD camera G2-3200with a low noise Kodak chip KAF-3200ME is used formeasuring.

1.3 Electroluminescence

The principle of the method lies in the electroluminescenceexcitation of a solar cell’s own semiconductor luminescenceradiation by the passage of electric current. A solar cell istested in the forward direction by connection to a voltage(current) source. This leads to the radiant recombination ofelectrons and holes in the silicon. Emitted fluorescent light isscanned across the board CCD camera. The output imagesare displayed in grayscale. The principle of fault detectionand current distribution is the resolution bright and dark0091-3286/2013/$25.00 © 2013 SPIE

Optical Engineering 051203-1 May 2013/Vol. 52(5)

Optical Engineering 52(5), 051203 (May 2013)


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areas. An ideal solar cell is characterized by the same currentdensity across the transition area. Areas with lower intensityof light show disorder in the material’s structure. By contrast,light areas represent areas with higher current density anddominant radiant recombination.

The article compares the results from noise spectroscopy,microplasma presence, defect detection by electrolumines-cence and effectiveness Eff of solar cells. The evidence sug-gests that the best results are reached by sample group 3122.

2 Experimental Results and DiscussionThe I through V characteristics were measured on alldevices. Figure 1 shows I to V characteristics of sample nos.3121-13, 3121-19 and 3121-22 for the forward direction.This second group of samples, 3121, was prepared by com-bination of standard washing and bath with a highly diluteHF before diffusion of nþ-emitter.

The straight-line-like shape at low DC voltage as shownin Fig. 1 in log-log scale makes it evident8–10 that the PNjunctions are shunted by a shunt of a resistance RSH around20 Ω in forward direction.

The deflection of the I to V characteristic from β slopeappearing at higher voltages (Fig. 2) makes it possible todetermine the series resistance RS, whose value is about0.22 Ω.

The total diode current may be expressed in the followingform:11,12

I ¼ IOfexp½qðV − IRsÞ∕kT� − 1g þ ðV − IRSÞ∕RSH.

The whole current is a sum of the current flowing throughthe shunt resistor RSH, the generation-recombination cur-rent and diffusion current. If the density of charge carriersinjected into the quasi-neutral region exceeds the majoritycarrier density, the junction is operated in high-injectionmode.

All samples were also measured under standard test con-dition in a flash solar simulator (spectrum of light 1.5AM,light intensity 1000 W∕m2, temperature 25°C). All param-eters are in Table 1, where ISC refers to the short circuit cur-rent, Voc is the open circuit voltage, Im is the current in themaximum power point, Vm is the voltage in the maximumpower point, Pm indicates maximum power (power in max-imum power point on V-I figure), FF refers to the fill factor(shows the power ratio of measured real PN junction andideal PN junction), EFF stands for efficiency, and Rso isthe serial resistance.

Figures 3 and 4 show I through V characteristics in theforward DC direction for sample nos 3122-1, 3122-3 and

Fig. 1 I-V characteristics of the group of solar cells 3121 in a log-logscale.

Fig. 2 I-V characteristics of the group of solar cells 3121.

Table 1 I-V parameters for set of samples No. 3121 and No. 3122.

Sample no. Isc A V oc V Im A Vm V Pm W FF% EFF% Rso Ω

3121-13 5,329 0,616 4,757 0,444 2,11 63,9 15,7 0,23

3121-19 5,391 0,617 4,771 0,443 2,12 57,6 15,98 0,23

3121-22 5,382 0,616 4,729 0,440 2,08 62,6 13,90 0,24

3122-1 5,392 0,616 4,879 0,469 2,29 69,2 15,71 0,17

3122-3 5,382 0,615 4,874 0,461 2,25 67,9 15,33 0,18

3122-15 5,395 0,613 4,923 0,459 2,26 68,6 15,23 0,18


Chobola et al.: Low-frequency noise, microplasma and electroluminescence measurements . . .


3122-15. This group of samples, 3122, was treated only withstandard washing.

From Fig. 3, we can calculate shunt resistance RSH asabout 15 Ω. The value of series resistance RSO, is about0.17 Ω.

Figure 5 shows the noise voltage power spectral densityversus DC voltage plots for solar cells group 3121, whichwas treated only with standard washing. The noise voltagewas measured across load resistance RL ¼ 100 Ω. The passband central frequency was 1 kHz, and band-width wasequal to 20 Hz. A marked excess noise component can beobserved at voltages about UF ¼ 0.3 V, reaching maximumat the power match point, where the PN junction dynamicresistance equals the load resistance. The maxima value ofspectral density SUMAX we measured for sample 3121-22and was SUMAX ¼ 5 × 10−16 V2 Hz. At forward voltagesexceeding UF ¼ 0.6 V, all samples show an increase inthe excess their noise component, which is characteristic

of imperfect contacts, where resistance ranges from 0.10to 0.25 Ω. At voltages higher than 0.4 V, we can observeunstable impulse noise, which is related to particle disloca-tion as well as heavy-metal impurity deposits.10,13 In thisway, we may employ the occurrence of 1∕f, g-r and bursnoise as a quality indicator.

Figure 6 illustrated the noise voltage power spectraldensity versus DC voltage plots for solar cells group 3122treated only with standard washing. The noise voltage wasmeasured across load resistance RL ¼ 100 Ω. A markedexcess noise component can be observed at voltages aboutUF ¼ 0.3 V, reaching maximum at the power match point,where the PN junction dynamic resistance equals the loadresistance. The maxima value of spectral density SUMAX wemeasured for sample 3122-15 and was SUMAX ¼ 1.5 ×10−15 V2 Hz.

Figure 7 shows a voltage noise spectral density SU versusfrequency at a temperature T ¼ 300 K, the noise voltagebeing picked up from a load resistor RL ¼ 100 Ω forthe cells from sample No. 3121-22. The curve labelled UF ¼0 V indicates the measuring setup background noise.The shapes of the noise curves for the applied DC forwardvoltage UF ¼ 0.3 V show the excess noise component to be

Fig. 3 2 I-V characteristics of the group of solar cells 3122 in a log-logscale.

Fig. 4 I-V characteristics of the group of solar cells 3122.

Fig. 5 The noise spectral density as a fiction of forward voltage forgroup of samples 3121.

Fig. 6 The noise spectral density as a fiction of forward voltage forgroup of samples 3122.




of the type or 1∕f2 type, which is typical of generation-recombination (g-r) noise. Excess noise parameters that cor-relate very well with the transport characteristic measure-ment are shown in Figs. 1 and 2.

In Fig. 8, the correlation between spectral density SUMAX

and solar cell efficiency is shown. We can see a high correla-tion factor in both groups of samples.

The generation of microplasma is influenced by severalfactors. The first of them is defected silicon crystal-grid caus-ing nonhomogeneity of parameters that, in turn, creates visi-ble defects. The second is dislocation and impurities in thePN junction. At places where the junction is thinner ormechanically damaged, microplasma discharge and emissionof light is present.

Another sign of observed microplasma is noise, which hasa random spectrum in the frequency range. Microplasmanoise is measurable even before the creation of light emis-sions. That provides a way to obtain information aboutmicroplasma creation with exiguous reverse voltage.

The microplasma light intensity highly depends onreversed biased voltage. The results of our noise diagnosticshow strong correlation between type of technology andnoise level. The output from observed microplasma of each

cell shows that a correlation between these two methodscan exist.

For sample 3121-22, we have observed at a voltage of10 V, a test current 0.27 A, and at a scanning time 20 s, thepresence of about 120 microplasma sources (Fig. 9), whichwere located mainly in a circle at the bottom of the sample.The noise spectral density reaches a value over SUMAX ¼4.10−5 V2 Hz for voltage UF1 ¼ 0.3 V, and its efficiencywas only Eff ¼ 14,0.

Fig. 7 The noise spectral density versus frequency for sampleno. 3121-22.

Fig. 8 The correlation between spectral voltage density SUMAX andsolar effectiveness Eff for all Gross of samples.

Fig. 9 Microplasma method—scanning time 20 s, voltage 10 V,current 0.27 A, sample no. 3121-22.

Fig. 10 Microplasma method—scanning time 20 s, reverse voltage4.0 V, current 0.15 A, sample no. 3122-3.




Figure 10 shows results of a microplasma-emitted lightfor the sample no. 3122-3. At the reverse voltage 4.0 V andcurrent 0.15 A, we saw only about 30 microplasma sources.This sample generated a very small noise voltage spectraldensity of (for only SUMAX ¼ 8.10−16 V2 Hz UF1 ¼ 0.3 V)and its efficiency was Eff ¼ 16.04.

Figure 11 shows results of a electroluminescence for thesample no. 3121-22 at the reverse voltage 1.5 V, current 2 A,and scanning time 20 s. Most important is the darker area

extending from the cells downwards. We know from experi-ence that such a defect is most likely caused when creating adiffusion layer N, where some places wrongly diffusethrough so a poor PN junction function forms, resulting in,a lower current density excitation current. Figure 12 showselectroluminescence at sample no. 3122-3, a test currentof 2 A, voltage 1.4 V, time 20 s, temperature 25°C, withoutusing an optical filter. We can see that the solar surface ofthis sample is much more homogeneous.

In Fig. 13, we can see the correlation between micro-plasma spot counts and cell efficiency Eff for sampleno. 3121.

3 ConclusionThe article compares the results from noise spectroscopy,microplasma presence, and solar cell efficiency.

From the measured results, it follows that the noise spec-tral density related to defects consists of 1/f and generation-recombination types. A group of samples, 3122, which wastreated only with standard washing produced lower noisevoltage spectral density of only SUMAX ¼ 8.10−16 V2 Hzfor UF1 ¼ 0.3 V and has a smaller number of microplasmacounts—around 30 for the voltage of 4.0 V and higher effi-ciency of 16.04%. The electroluminescence measurementsshows that the solar surface of this sample is much morehomogenous.

The second group of samples, 3121, was prepared bycombination of standard washing and bath with highly diluteHF before diffusion of nþ-emitter. In this group of samples,we observed great noise voltage spectral density overSUMAX ¼ 4.10−15 V2 Hz for UF1 ¼ 0.3 V and the numberof microplasma counts greater than 120 for voltage 4.0 Vand these samples showed smaller efficiency of 14.0%.

We observed a significant correlation between the spectralvoltage density and conversion efficiency and number ofmicroplasma counts, and also dark areas by electrolumines-cence study.

From the measurements, we can conclude that of theobserved changes in the two groups of samples, the one thatachieves better performance was 3122, the group of samplestreated only with standard washing.

Fig. 11 Electroluminescence at the sample no. 3121-22, a test cur-rent of 2 A, voltage 1.5 V, time 20 s, temperature 25°C, without usingan optical filter.

Fig. 12 Electroluminescence at the sample no. 3122-3, a test currentof 2 A, voltage 1.4 V, time 20 s, temperature 25°C, without using anoptical filter.

Fig. 13 The correlation between number of observed points and solareffectiveness Eff for sample no. 3121.




AcknowledgmentsThis paper is also based on the research supported by OPVKCZ.1.07/2.3.00/20.0103 and the project FEKT-S-11-7.

References

1. L. K. J. Vandamme, R. Alabedra, and M. Zommiti, “Noise as a diag-nostic tool for quality and reliability of electronics devices,” in Proc.of IEEE Transactions on Electron Devices, Vol. 41, pp. 2176–2187(1994).

2. J. Sikula et al., “1/f noise in Schottky Barier diodes,” in Proc. of 6th Int.Conf. on Noise in Physical Systems, Washington, pp. 100–106 (1981).

3. A. Van der Ziel and H. Tong, “Low frequency noise predicts when atransistor will fail,” Electronics 39(24), 95–97 (1966).

4. A. Konczakowska, “Quality and 1/f noise of electronic components,”Qual. Reliab. Eng. Int. 11(3), 165–169 (1995).

5. B. K. Jones, “Electrical noise as a measure of reliability in electronicdevices,” Adv. Electron. Electron. Phys. 67, 201–257 (1994).

6. M. Jevtic, “Noise as a diagnostic and prediction tool in reliability phy-sics,” Microelectron. Reliab. 38(3), 331–334 (1998).

7. L. K. J. Vandamme, “Opportunities and limitations to use low-frequency noise as a diagnostic tool for device quality,” in Proc. ofthe 17th International Conf. ICNF 2003, Prague, pp. 735–748(2003).

8. Z. Chobola, “Noise as a tool for non-destructive testing of single-crystalsilicon solar-cells,” Microelectron. Reliab. 41(12), 1947–1952 (2001).

9. P. Hruska, Z. Chobola, and L. Grmela, “Diode I-U curve fitting withLambert W function,” in Proc. of 25th International Conf. on Micro-electronics, Serbia and Montenegro IEEE Section, pp. 501–504(2006).

10. Z. Chobola, “Impulse noise in silicon solar cells,” Microelectron. J.32(9), 707–711 (2001).

11. R. Macků, P. Koktavý, and P. Škarvada, “Non-destructive characteriza-tion of micro-sized defects in the solar cell structure,” Key Eng. Mat.465(1), 314–317 (2011).

12. P. Macků and P. Koktavý, “Analysis of fluctuation processes in forward-biased solar cells using noise spectroscopy,” Phys. Stat. Sol. A 207(10),2387–2394 (2010).

13. J. Nelson, The Physics of Solar Cells, p. 363, Imperial College Press,London (2003).

Zdenek Chobola graduated from the Facultyof Science, J. E. Purkyně University of Brno,with a specialization in mathematics—physics. For the past four decades, he hasbeen working at the Physics Department,Faculty of Civil Engineering, Brno Universityof Technology. Here he was granted the CSc(PhD) degree with a thesis topic of “Studyof stochastic phenomena in photodiodes.”Since 1990, he has held the post of associateprofessor at Brno University of Technology

as a professor in Physical and Civil Materials Engineering from2002. He became head of the Institute of Physics at the Faculty ofCivil Engineering Brno University of Technology in1994 and servedas vice-dean for science and research from 2005 to 2009. Hisareas of interest include applied physics, noise and stochastic phe-nomena in solid, degradation and reliability of electronic materialsand devices, and noise spectroscopy. He is the author or co-authorof more than 280 scientific publications.

Biographies and photographs of the other authors are not available.




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Low-frequency noise, microplasma, and electroluminescence measurements as faster tools to investigate quality of monocrystalline-silicon solar cells