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GETTERING EFFICACY OF SCREEN-PRINTED EMITTERS IN MULTICRYSTALLINE SILICON FORSOLAR CELLS WITH SELECTIVE EMITTERST. M. Pletzer1,*, E. F. R. Stegemann1,2, H. Windgassen1, D. L. Bätzner1,3, R. Bleidiessel4, and H. Kurz11RWTH Aachen University, Institute of Semiconductor Electronics, Sommerfeldstraße 24, D-52074 Aachen, Germany 2now with: Q-Cells SE, OT Thalheim, Sonnenallee 17-19, 06766 Bitterfeld-Wolfen, Germany3now with: Roth & Rau Switzerland AG, Neuchatel, Switzerland4Solland Solar Cells B.V., Bohr 10 (Avantis), 6422 RL Heerlen, The Netherlands*E-mail: pletzer@iht.rwth-aachen.de, Phone: +49 241 80 27892, Fax: +49 241 80 22246ABSTRACT: Phosphorus dopant pastes are an attractive alternative to the conventional phosphorus oxychloride (POCl3) dopant source for emitter processing in solar cells, as they allow the fabrication of selective emitters on an industrial scale. In this paper it is demonstrated that single-sided uniform screen-printed emitters processed with phosphorus dopant pastes can getter multicrystalline silicon (mc-Si) wafers more effectively than conventional double-sided uniform POCl3 emitters. This result is confirmed by minority carrier lifetime measurements with the quasi-steady-state photoconductance (QSSPC) method. Solar cells with selective emitters were processed on mc-Si wafers and subsequently characterised. The current-voltage (I-V) results are improved compared to uniform POCl3 emitter solar cells and an increased internal quantum efficiency (IQE) for selective emitter solar cells is also demonstrated.Keywords: selective emitter, screen printing, multicrystalline silicon1 INTRODUCTIONThe development of new industrial solar cell concepts is accelerated by the selective emitter technology [1]. Selective emitters can increase the efficiency of solar cells by improving the IQE in the ultra-violet (UV) wavelength range due to lower recombination activity in the emitter [2, 3]. Furthermore, selective emitters permit lower contact resistance values at the front-side metallisation resulting in a lower series resistance of selective emitter solar cells compared to the conventional solar cells [3].Processing selective emitters with screen-printing of phosphorus dopant pastes [4, 5, 6] in combination with the conventional POCl3 diffusion is an attractive and emitter doping profiles [9, 10].Therefore, the purpose of this work focuses on the gettering efficacy in mc-Si wafers using uniform screen-printed emitters [6], the measurement of selective emitter profiles and the investigation of solar cells with selective emitters. The gettering efficacy of uniform screen-printed emitters is obtained by minority carrier lifetime measurements with the QSSPC method [11] and compared with results of conventional uniform POCl3 emitters, which are used as references in this work.The best phosphorus dopant paste was used to process selective emitters on mc-Si wafers. These emitters were characterised to determine phosphorus doping profiles and compared to conventional uniform POCl3 emitters.Solar cells with selective emitters were processed and characterised to explore the potential of screen-printed emitters for industrial selective emitter production.Regarding gettering efficacy and cell performance, the screen-printed doping sources for the emitter diffusion are well suited for solar cell processing with selective emitters. 2 EXPERIMENTALAll samples were processed on mc-Si wafers with boron doping (p-type) of around 1Ωcm (N A ~ 1 · 1016 cm-3), an area of 156 cm², and a thickness of 220 µm after saw damage etch for the gettering investigation and 190 µm for selective emitter characterisation and solar cell analysis. For the gettering investigation and solar cell analysis, neighbouring wafers from the same brick were used.2.1 Sample preparation for the gettering investigationThe samples for the gettering investigation are prepared by the following steps: a wet chemical alkaline texturing of mc-Si wafers in sodium hydroxide (NaOH) and a subsequent cleaning in nitric acid (HNO3) were followed by the emitter formation in two different ways: (a)uniform screen-printing of eleven different phosphorus dopant pastes on one wafer side followed by a drive-in step, (b) conventional batch POCl3 diffusion on both wafer sides as reference. The diffusion of all emitters was carried out in a quartz tube furnace. After emitter diffusion, the remaining phosphorus silicate glass (PSG) was removed in diluted hydrofluoric acid (HF) and the emitter was characterised by four-point-probe measurements to determine the emitter sheet resistance (R SH). Finally, the emitters were etched back. All samples were passivated with a-SiN x:H layers [12] and characterised by QSSPC measurements [11] to investigate the effective minority carrier lifetime (τeff) and determine the gettering efficacy. A control sample without emitter diffusion is included in the study to monitor the as-received material quality.2.2 Sample preparation for selective emitter analysisThe samples for the selective emitter analysis are prepared by the following steps: a wet chemical acid texturing and a subsequent cleaning in HNO3 of mc-Si wafers were followed by the emitter formation with the subsequent steps: screen-printing of a phosphorus dopant paste for the highly doped area of the selective emitter on2039the wafer front side, followed by a uniform POCl 3 diffusion to form the lowly doped area of the selective emitter and drive the phosphorus pastes into the wafer. Further, reference emitters were processed with the conventional batch POCl 3 diffusion on both wafer sides. The diffusion of all emitters was carried out in the same quartz tube furnace. After emitter diffusion, the PSG was removed in HF and the emitter was characterised byfour-point-probe measurements to determine the R SH . Furthermore, electrochemical capacitance-voltage (ECV) measurements [13] were used to determine the phosphorus doping profiles of the selective emitter structure. R SH values were also calculated using the corresponding doping profiles and results were compared with values from the four-point-probe measurements.2.3 Sample preparation of selective emitter solar cellsAll solar cells in this work were processed in a conventional industrial process line with wet chemical acid texturing, emitter formation, a-SiN x :H depositon for the anti reflection coating (ARC) [12] and metallisation by screen-printing, which also creates an aluminium back surface field (BSF). Solar cells with selective emitters were processed in the way described in chapter 2.2 with the combination of screen-printing and POCl 3 diffusion. The reference solar cells were processed with uniform emitters using the conventional POCl 3 diffusion. The selective emitter solar cells were studied to demonstrate the potential of screen-printed emitters for the realisation of selective emitters on an industrial scale. For this purpose, the solar cells were characterised by I-V , Suns-V OC [14], spectral response (SR) and reflection measurements. The I-V data were fitted by the two diode model and the IQE was calculated from the SR and reflection data [15]. The effective diffusion length (L eff ) was calculated graphically from the abscissa intersection of the inverse IQE over absorption length plot [16, 17, 18].3 RESULTS AND DISCUSSION3.1 Gettering investigationThe R SH of all processed samples were determined by averaging 36 measurements taken across the emitter surface. The eleven different screen-printed emitters (sample 1 to 11) were realised with the different phosphorus dopant pastes. The emitter formations with these pastes result in normally doped (50 to 65 Ω/sq.), weakly doped (65 to 110 Ω/sq.) and critically low doped emitters (above 110 Ω/sq.). It is predominantly observed that an increase in R SH is mostly correlated to the value of the standard deviation (σ). The reference POCl 3 emitter (sample 12) is normally doped, leading to R SH = 59 Ω/sq..The measured τeff values of all samples show no obvious trend. Plotting the relative differences between τeff of the gettered samples and the non-gettered control samples as shown in figure 1 clears up the data structure. The test samples 1 to 5 and 10 exhibit even more effective gettering than the POCl 3 reference sample 12. Therefore, phosphorus dopant pastes are well suited for processing selective emitters. Samples 6, 8, 9 and 11 were less effectively gettered than the POCl 3 reference.A low R SH associated with a high phosphorus concentration [9] yields higher gettering efficacy while low phosphorus concentrations reduce the getteringefficacy. A similar correlation between gettering efficacy and the phosphorus concentration in POCl 3 emitters has been found in earlier studies [8].Figure 1: Gettering efficacy based on material control sample vs. sheet resistance (R SH ). A low R SH allows a more effective gettering. Samples processed with dopant pastes of the same manufacturer are displayed with the same shape and connected by a line as a guide to the eye.3.2 Selective emitter analysisThe analysis of selective emitter structures revealed two well distinguished areas featuring different doping. This is observed by four-point-probe and ECV measurements [13]. The four-point-measurements certify the uniform POCl 3 reference emitter a typical R SH of 59 Ω/sq.. For the selective emitter R SH in the highly doped area is around 29 Ω/sq. and for the lowly doped area around 103 Ω/sq.. These measurements demonstrate the existence of the selective emitter structure.Figure 2: Electrically active phosphorus concentration (C P ) versus emitter depth (x ). C P was determined with ECV measurements. The selective emitter shows highly and lowly doped areas in contrast to the conventional uniform POCl 3 emitter.In Figure 2, the electrically active phosphorus concentrations (C P ) of the two selective emitter regions and the uniform reference POCl 3 emitter are shown. These measured C P profiles of selective emitter structures enable the identification of highly and lowly doped areas and prove the existence of selective emitter structures. The C P profiles were measured by ECV and were used to calculate the R SH values (see labels in figure 2) in a second way. These calculated R SH values are similar to the measured R SH values above and support2040the existence of a selective emitter structure.3.3 Comparison between selective and uniform emitter cellsSolar cells with screen-printed selective emitters and uniform POCl3 reference emitters were processed and characterised. The I-V measurements (see table I) certify the selective emitter solar cells the highest average solar cell efficiencies (η) with values of up to 15.3 %. The ηgain of up to 0.2 % absolute is already realised with lower fill factors (FF) than the reference solar cells with uniform POCl3 emitters. The slightly lower FF of the selective emitter solar cells are due to the solar cell process not being optimised for this emitter type. Nevertheless, values of the open-circuit voltage (V OC) and short-circuit current density (J SC) certify an increased performance for the selective emitter solar cells.Table I: Parameters of the I-V measurements on mc-Si solar cells with screen-printed selective emitters and uniform POCl3 emitters as reference. Values listed are averaged over five cells and errors given are σ.emitter type V OC J SC FF η[mV] [mA/cm²] [%] selective 615.6±1.234.0±0.1 0.73±0.01 15.3±0.2 POCl3 613.7±1.533.3±0.00.75±0.01 15.1±0.1In the following, the differences in the I-V plots are discussed with the two-diode-model parameters [19] (see table II) and further solar cell characterisation. From the I-V data, the parameters diffusion current density (J01), recombination current density (J02), series resistance (R S) and parallel resistance (R P) are derived according to the two-diode-model [19] and listed in table II. The R P of all solar cells is sufficiently high to exclude shunting loses, which would result in a low FF. The solar cells with selective emitters have lower R S values due to the lower contact resistance using selective emitter structures as well as differences in individual co-firing parameters.Table II: Parameters of the I-V data fit by the two-diode model of mc-Si solar cells with screen-printed selective emitters and uniform POCl3 emitters as reference. Values listed are averaged over five cells and errors given are σ.emitter type J01J02R S R P[pA/cm²] [nA/cm²] [Ωcm²] [kΩcm²] selective 0.7±0.087.6±8.2 0.53±0.08 1.4±0.1 POCl3 0.9±0.078.2±6.80.56±0.10 1.1±0.3J02 is traditionally considered a measure for recombination in the space charge region and can be related to the density of recombination centres in solar cells. A more effective gettering should reduce the Shockley-Read-Hall (SRH) recombination, yielding a low J02. The lowest J02 values of 78.2 nA/cm² are shown by the reference solar cells due to the more effective gettering. This is caused by the doping of the reference POCl3 emitter with R SH of around 59 Ω/sq. compared to the doping of the selective emitters with R SH of around 103 Ω/sq. for ~ 92 % of the emitter area.J01 mainly provides information about the recombination in the emitter and at the surface. The lower doping in the largest area of the selective emitter structure results in lower J01 values of 0.7 pA/cm² for these solar cells compared to the reference solar cellswith J01 values of 0.9 pA/cm². The difference in J01 isdirectly responsible for the difference in V OC, as confirmed by a simulation using the two-diode-model.Further, the differences of J01 are also observed in the determined IQE curves (see figure 3). The selectiveemitter solar cells result in a higher IQE than the uniformPOCl3 reference solar cells in the wavelength (λ) rangefrom 300 to 600 nm. This IQE gain is caused by less recombination in the selective emitter structure due to thelower emitter doping. Furthermore, the demonstratedIQE gain of the selective emitter solar cells increases theJ SC of these cells (see table I). The IQE spectra of bothsolar cell types are nearly identical above 600 nm.Figure 3: Internal quantum efficiencies (IQE) versuswavelength (λ) for selective emitter solar cells (redcurve) and reference cells with uniform POCl3 emitters(blue curve). All IQE data are averaged over five solarcells, error bars indicate σ.The IQE curves were used to calculate the L eff of thesolar cells [16, 17, 18]. These values obtained in theinfra-red (IR) region are listed in table III. The largest distance the minority carrier travel before they recombineis observed in the reference solar cells with the uniformPOCl3 emitter (L eff = 330.3 µm). The L eff value of the selective emitter solar cells is slightly lower(L eff = 327.9 µm). The differences between these twovalues can be explained by the different gettering efficacy of the processed emitters. This assumption canbe confirmed with the observed differences in the J O2values (see table II), which correlate very well with theL eff values and are also an indicator for the gettering efficacy.Table III: Parameters of the L eff calculation andSuns-V OC measurements of mc-Si solar cells withscreen-printed selective emitters and uniform POCl3emitters as reference. Values listed are averaged overfive cells and errors given are σ.emitter type L eff pseudoFFpseudoη[µm] [%]selective 327.9±3.10.80±0.01 16.3±0.2POCl3 330.3±4.30.79±0.0115.8±0.2Finally, the processed solar cells were characterisedby Suns-V OC measurements [14] to evaluate the potentialof the developed selective emitter cells. These measurements neglect the influence of R S in the solarcells and allow the calculation of pseudo FF and pseudo η. The values determined are listed in table III.2041The pseudo FF of selective emitter solar cells shows a gain of 0.01 absolute over the reference solar cells. Considering the higher real FF (see table I) of the reference solar cells, it is obvious that selective emitter solar cells can benefit greatly from further optimisations. The pseudo η of selective emitter solar cells show a gainof 0.5 % absolute over the reference solar cells. Taking into account that R S is neglected in this measurement, the largest optimisation potential is in the geometrical designof the selective emitter grid lines and in the individually adjusted metallisation using modified silver pastes.4 CONCLUSIONThe developed selective emitter concept realises an ηgain of 0.2 % absolute in the processed solar cells and shows a theoretical η gain of 0.5 % absolute. Further, an IQE gain in the UV region is demonstrated with selective emitter solar cells and an increase of J SC and V OC.The ECV measurements prove the existence of selective emitter structures manufactured by the highly and lowly doped areas, which were processed in single diffusion step. This selective emitter formation in one diffusion step is realised by combining screen-printing of phosphorus dopant pastes and POCl3 diffusion.The used phosphorus dopant pastes are suitable for the emitter formation and allow a high gettering efficacy. This is shown with single-sided uniform screen-printed emitters, which getter mc-Si wafers more effectively than the reference double-sided uniform POCl3 emitter.5 ACKNOWLEDGEMENTThe authors would like to thank B. Mayer and M. Thore from the Institute of Semiconductor Electronics, RWTH Aachen University for assistance with sample preparation and measurements. This work is part of the project “Admitter–Innovative Emitterstrukturen für Solarzellen” and has been supported by the Ministry of Economic Affairs and Energy of the State of North Rhine-Westphalia, Germany, and the European Commission.6 REFERENCES[1] S. K. Chunduri: Be selective!. Photon International,Volume 11 (2009) pp. 108-116[2] J. Horzel et al.: High efficiency industrial screenprinted selective emitter solar cells. Proceedings ofthe 16th EU PVSEC, Glasgow (2000),pp. 1112-1115[3] T. M. Pletzer: Die multikristalline Silizium-Solarzelle: Die Entwicklung zum selektivenEmitter. PhD-Thesis of the RWTH AachenUniversity, Aachen (2010)[4] J. Salami et al.: Characterization of screen printedphosphorous diffusion paste for silicon solar cells.Proceedings of the 14th International PhotovoltaicScience and Engineering Conference, Bangkok(2004), pp. 263-264[5] M. Edwards et al.: Screen-print selective diffusionsfor high-efficiency industrial silicon solar cells.Progress in Photovaltaics, Volume 16, Issue 1(2008), pp. 31-45 [6] T. M. Pletzer et al.: Extensive investigation andcharacterisation of solar cells with screen-printedemitters using phosphorus dopant pastes.Proceedings of the 24th EU PVSEC, Hamburg(2009), pp. 2080-2083[7] L. Janßen et al.: Phosphor diffusion gettering andemitter optimization of multi-crystalline siliconsolar cells. Proceedings of the 19th EU PVSEC,Paris (2004), pp. 628-631[8] D. L. Bätzner et al.: Dependence of phosphorousgettering of multicrystalline silicon on diffusionsheet resistance and ingot position. Proceedings ofthe 20th EU PVSEC, Barcelona (2005), pp. 655-658[9] J. J. C. Tsai: Shallow phosphorus diffusion profilesin silicon. Proceedings of the IEEE, Volume 57,Number 9 (1969), pp. 1499-1506[10] T. Brammer et al.: Analysis of phosphorus dopedemitter profiles of multicrystalline Si solar cells.Proceedings of the 17th EU PVSEC, Munich(2001), pp. 1842-1845[11] R. Sinton et al.: Contactless determination ofcurrent-voltage characteristics and minority-carrier lifetimes in semiconductors fromquasi-steady-state photoconductance data. AppliedPhysics Letters, Volume 69, Issue 17 (1996)pp. 2510-2512[12] L. Janßen et al.: Passivating thin bifacial siliconsolar cells for industrial production. Progress inphotovoltaics, Volume15, Issue6 (2007),pp. 469-475[13] R. Bock et al.: Accurate extraction of dopingprofiles from electrochemical capacitancemeasurements. Proceedings of the 23rd EU PVSEC,Valencia (2008), pp. 1540-1543[14] R. A. Sinton et al.: A quasi-steady-state open-circuit voltage method for solar cellcharacterization. Proceedings of the16th EU PVSEC, Glasgow (2000), pp. 1152-1155[15] W. J. Yang et al.: Internal quantum efficiency forsolar cells. Solar Energy, Volume 82, Issue 2(2008) pp. 106-110[16] P. A. Basore: Extended spectral analysis of internalquantum efficiency. Conference Record of the23th IEEE PVSC, Louisville (1993), pp. 147-152[17] M. Hirsch et al.: Analysis of internal quantumefficiency and a new graphical evaluation scheme.Solid-State Electronics, Volume 38, Number 5(1995) pp. 1009-1015[18] R. Brendel et al.: Effective diffusion lengths forminority carriers in solar cells as determined frominternal quantum efficiency analysis. Journal ofApplied Physics, Volume 85, Number 7 (1999)pp. 3634-3637[19] A. Wolf et al.: Investigation of the doubleexponential in the current-voltage characteristicsof silicon solar cells. IEEE Transactions onElectron Devices, Volume 24, Number 4 (1977),pp. 419-4282042。