Presented at the 3rd World Conference on Photovoltaic Energy Conversion; Osaka, Japan; May 11-18, 2003.Light induced degradation (LID) in boron doped Czochralski (Cz) silicon with high oxygen content is known to degrade solar cell efficiency. Multicrystalline Si
crystals also have oxygen and use B doping, but LID effects are largely unknown. In this paper, ribbon, Cz, and cast multi-crystalline Si crystals with a resistivity of 1-3 Ωcm were investigated for LID. 15-16% efficient EFG, String Ribbon, and cast mc-Si solar cells, fabricated by manufacturable screen printed technology, show small but
measurable LID (0.2% absolute efficiency loss). In less than 15% efficient devices, LID was not detectable in ribbon Si crystals. However, >16% efficient photolithography ribbon Si degraded >0.5% absolute. Analysis of the bulk lifetime using photoluminescence mapping, after cell processing, supports the presence of LID in the good regions of the ribbon materials while the defective regions remained essentially unaffected

Damiani, Benjamin Mark

Kim, Dong Seop

Nakayashiki, Kenta

Ostapenko, Sergei

Rohatgi, Ajeet

Tarasov, Igor

Yelundur, Vijay

Scholarly Materials And Research @ Georgia Tech

LIGHT INDUCED DEGRADATION IN PROMISING MULTI-CRYSTALLINE SILICON MATERIALS FOR SOLAR CELL FABRICATION  B. Damiani1, K. Nakayashiki1, D. S. Kim1, V. Yelundur1, S. Ostapenko2, I. Tarasov2 and A. Rohatgi1 1. Georgia Institute of Technology, Atlanta, GA 30332-0250 University Center of Excellence for Photovoltaics (UCEP) 2. University of South Florida, Tampa, FL 33620 Nanomaterials and Nanomanufacturing Research Center (NNRC)    ABSTRACT  Light induced degradation (LID) in boron doped Czochralski (Cz) silicon with high oxygen content is known to degrade solar cell efficiency.  Multicrystalline Si crystals also have oxygen and use B doping, but LID effects are largely unknown.   In this paper, ribbon, Cz, and cast multi-crystalline Si crystals with a resistivity of 1-3 Ωcm were investigated for LID.  15-16% efficient EFG, String Ribbon, and cast mc-Si solar cells, fabricated by manufacturable screen printed technology, show small but measurable LID (0.2% absolute efficiency loss).  In less than 15% efficient devices, LID was not detectable in ribbon Si crystals.  However, >16% efficient photo-lithography ribbon Si degraded >0.5% absolute.  Analysis of the bulk lifetime using photoluminescence mapping, after cell processing, supports the presence of LID in the good regions of the ribbon materials while the defective regions remained essentially unaffected.   INTRODUCTION  Boron doped Czochralski (Cz) silicon with high oxygen content is known suffer LID, decreasing the stabilized solar cell efficiency [1].  This results from a decrease in the minority carrier lifetime after exposure to carrier injection.  The LID-induced loss in efficiency for screen-printed Cz Si cells can lead to a 10% drop in relative efficiency or >1.5% absolute.  In order to decrease the cost of solar cells for manufacturing, many different silicon crystal growth techniques are being explored.  All the materials currently use boron as the bulk dopant and incorporate various levels of oxygen.  A linear relationship between the substitutional Boron (Bs) concentration and LID trap formation has been reported by various researchers [2].  The relationship to interstitial oxygen (Oi) concentration has not been so straightforward.  As high as a fifth order relationship and as low as a first order relationship has been proposed [3].  The most recent proposal is a quadratic relationship with the fast diffusing oxygen dimmer (O2i) [3].  The extent or existence of LID on lower cost mc-Si is largely unknown.  In these materials, the as grown lifetimes are in the range of 1-10 µs and are dominated by other impurities and defects.  LID has been observed in Cz and some cast mc-Si with greater than 5-10 µs minority carrier lifetime [4].  For example, in order to observe LID in cast mc-Si a phosphorus gettering step is employed to increase the bulk lifetime so LID data is not obscured.  De Wolf et al. demonstrated LID in very low resistivity (<0.2 Ωcm) cast mc-Si as a function of wafer position in an ingot [4].  Oxygen content was shown to increase from undetectable (25oC) for the top of the ingot to 4x1017 cm-3 near the bottom of the ingot.  Solar cells fabricated from the top of the ingot showed little efficiency degradation while cells from the bottom showed increased LID despite the decreasing B doping concentration with ingot depth that would act to decrease LID.  This paper reports on the degree of solar cell efficiency degradation resulting from LID in some of the promising materials for industrial manufacturing of solar cells.  String Ribbon (SR) from Evergreen, Edge Defined Film Feed Growth (EFG) ribbon, cast mc-Si grown by the Heat Exchanger Method (HEM), B doped Cz, Ga doped Cz, and B doped web Si from Ebara were analyzed.  In selected cases cells with photolithography contacts were also made.     EXPERIMENTAL  In order to check for the characteristic degradation / recovery cycle due to LID effect in the promising silicon materials, first phosphorus gettering with in-situ oxidation for passivation was performed on Si wafers.  Subsequently SiNx was deposited on both sides of the wafer for enhanced passivation. Lifetime measurements were made using the quasi-steady state photoconductance (QSSPCD) lifetime measurement technique.  Additional samples were prepared for lifetime measurement by etching the solar cell down to bare Si and coating it with a SiNx layer for surface passivation on both sides.  Solar cell efficiencies were measured under AM 1.5 global conditions after various exposure to illumination and annealing to check for degradation and annealing.  Oxygen data was obtained by Fourier Transform Infra-Red (FTIR) spectroscopy, using a Digilab FTS40-pro. Scanning room-temperature photoluminescence was applied for lifetime mapping of wafers with and without LID, using a novel photoluminescence scanning setup (SPEX-500M) [5].   RESULTS     Initial measurements were made on phosphorus gettered mc-Si samples.  Figure 1 shows the area averaged lifetime data obtained via QSSPCD for the ribbon and cast mc-Si materials at 5x1014 cm-3 injection level.  After phosphorus gettering step, different materials had different lifetimes ranging from 3-30 µs.  Cast mc-Si, EFG, and web showed detectable degradation / recovery.  Only the SR sample did not show LID, probably because the very low lifetime, due to the absence of defect hydrogenation, acted to obscure the LID.   Figure 2 shows the efficiency dependence on LID for manufacturable screen-printed (SP) solar cells on these materials.  Initial efficiencies for all materials (except web, where the ~100 µm thickness and somewhat lower efficiency obscures degradation) was over 15%.   Along with the B doped Cz (0.8 Ωcm) solar cell, the HEM (1.5 Ωcm) , EFG (~3 Ωcm), and SR (~3 Ωcm) solar cells also degrade but not as much (see Fig. 2), partly due to the lower base resistivity of the substrates.  The Cz sample showed a ~1% drop in absolute efficiency compared with only 0.2% absolute for the other materials.  The ribbon material, after the phosphorus gettering, did not show significant degradation (see Fig. 1).  The bulk lifetime in the finished devices is much higher than the as-grown or even phosphorus gettered samples due to hydrogenation [6].  Table I shows the most recent data available to our knowledge on interstitial oxygen (Oi) content in these promising crystalline Si materials.  Table I.  Current Oi concentration in promising crystalline Si materials [9-11].  Interstitial Oxygen Concentration (Oi) cm-3  FZ <1015  Web 1018 SR 4-8x1016  (B) Cz ~1018 EFG <5x1016  (B) MCz 1016-5x1017 HEM <4x10-17  (Ga) Cz ~1018  It is important to note that there is small variation in oxygen content of the  ribbon materials.  The oxygen concentration has varied over the years as the ribbon growth techniques evolved.  It is possible that oxygen concentration now maybe different than reported in this table.  In the case of  cast mc-Si ingot, the Oi concentration is dependent on position within the ingot.  Therefore, in this study, thick mc-Si wafers were obtained to determine the oxygen concentration by FTIR as a function of ingot position.  Figure 3 shows the room temperature FTIR spectroscopy data for the oxygen interstitial peak in a directionally solidified HEM ingot as a function of position.  The oxygen concentration increased from top to bottom with a quenching of oxygen at the very bottom.  Despite the varying Oi content and base doping level the ribbon and cast mc-Si screen printed (SP) samples showed similar degradation (0.2%) for ~15% SP cells.  In order to understand the reason for this, we investigated the post processing lifetime in the ribbon materials and performed spatially resolved photoluminescence measurements.  Figure 4 shows the characteristic degradation curve of the minority carrier lifetime in a EFG sample after solar cell fabrication, including hydrogenation.  The sample showed a drop in lifetime down to 22 µs at 5x1014 cm-3 injection level.  The recovered lifetime at 5x1014 cm-3 was ~30 µs.  Ribbon materials are known to benefit greatly from processing with hydrogenation, which may increase the 01020304050FGA 10 min. LID FGA 48 hours LID FGAEffective Lifetime (us)EFGHEMWebSR5x1014 cm-3Fig. 1   QSSPCD effective lifetime data for ribbon and cast mc-Si  Fig. 2    Average efficiency change after 4 degradation / recovery cycles for mc-Si solar cells 16.515.315.115.615.716.314.915.216.515.414.816.314151617FZ Cz (B) EFG SR HEM Cz (Ga)Efficiency (%)RecoveredDegraded1E-051E-041E+13 1E+14 1E+15 1E+16Injection level (cm-3)Effective Lifetime (us)EFG RecoveredEFG DegradedFig. 4   Effective lifetime verses injection level for processed hydrogenated solar cell  Fig. 3  Room-temperature  FTIR spectra show Oi concentration in HEM mc-Si as a function of wafer position from the top of the ingot (0%) to the bottom of the ingot (100%).   Undetected - 4x1017  overall lifetime from ~1-5µs to over 30µs [6]. Additional samples were examined to understand and quantify the LID effect in very good and average cells.  Table II shows the change in important solar cell parameters for these materials.  Preliminary investigations of LID in a very high efficiency SR (>17%) and EFG (~16%) samples with photolithography front contacts (initial Voc= 619 and 612 mV respectively) showed a higher (0.5% and 0.9%) efficiency degradation compared to the >15% SP cells (see Fig. 2).  Further investigation is required to determine if this was the result of higher than normal oxygen in these ribbon samples or higher starting efficiency and other factors.  Additionally, screen printed SR and EFG cells with efficiencies in the range of ~13.5-15% showed negligible degradation because the lower lifetime obscures the LID.    Table II.  Efficiency degradation after 3 days illumination of ~0.5 suns for various efficiency mc-Si wafers. mc-Si type ∆Voc (mV) ∆Jsc (mA/cm2) ∆Eff. (%) SR 17.1% 9.1 0.44 0.50 SR 13.3% 0.0 0.0 0.00 EFG 16.0% 19.0 0.93 0.90 EFG 14-15% 0.0 0.01 0.03 HEM 15.6% 4.0 0.47 0.28 HEM 15.3% 2.0 0.13 0.04     Interestingly, the HEM samples with comparable efficiencies show varying efficiency degradation (0.3% and 0.04%).  This is possibly due to difference in Oi content of these wafers, as shown in Fig. 3 and [4].  Thus more accurate knowledge of Oi is needed to properly explain the variation in the LID behavior observed in these materials.   In order to improve the understanding of the effect of LID in the ribbon materials, a spatially resolved PL mapping technique was applied.  This technique gives a map of the band to band PL maximum intensity at the energy of 1.09eV and independently the intensity map of a “defect” PL band with maximum at ~0.8eV.  Using Ibb values it is possible to quantify the minority carrier lifetime through the following relationship [5] :   ( )111~ −−−+ nrradradbbI τττ        (1)  where Ibb is the intensity of the band to band recombination, and τrad and τnr are the radiative band-to-band lifetime and non-radiative lifetime respectively.  The non-radiative lifetime is dominant in silicon at room temperature and is proportional to the effective lifetime (τeff).  The τrad is a material constant and much larger than the τnr therefore:  effradeffradnrbbI τττττ ~~~            (2)  which enables the same analysis of the LID trap (NLID*) used in photoconductance lifetime measurements [3]:  11* )((deg) −− −= recIIN bbbbLID          (3)  where NLID* is the effective normalized trap concentration determined from the change in band to band PL intensity, Ibb(deg) is band to band intensity of a Si wafer in a degraded state and Ibb(rec) is the intensity in the recovered state.  Figure 5 shows two PL maps of a B doped Cz-Si wafer degraded and recovered.  The PL map shows a uniform distribution of the intensity (~NLID*) across the entire wafer in the degraded state as well as the recovered state.  PL maps were also obtained for ribbon Si wafers passivated with SiNx on top and bottom surfaces.  Analogous to the band to band recombination in Si, we observe a defect band associated with areas of reduced lifetime, which is typical for ribbon materials [5].  Radiative recombination in defective materials give rise to a ~0.8 eV spectra.  Ribbon materials often have this quality and therefore can provide additional information.  In Figure 6, a line scan of lifetime area map of a EFG wafer before and after degradation of the Ibb as well as the defect band Idb is shown.  Note that the LID degradation / recovery cycle is limited to the band to band spectra and occurs at different magnitudes according to crystal location.  LID was observed away from low intensity (low lifetime) defect regions.  Strong LID was observed in the high PL intensity regions.  The sharp drop in the spectral intensity corresponds to defect locations and causes the LID data to be obscured (or at least more noisy).  The Idb line scan shows the excellent spatial resolution in PL mapping.  The mean values of the NLID* across the whole Fig. 5  PL map of a quarter B doped Cz  Si wafer before and after LID.  UCz-Si and EFG wafers were 0.019 and 0.0061, respectively.  No additional peaks were observed in the full PL spectrum for the degraded samples.  Therefore the LID trap is not a radiative trap with > 0.8 eV energy, further supporting a near midgap trap location.  This also helps to explain the lack of efficiency degradation observed in lower efficiency (or low lifetime) ribbon solar cells that are dominated by defective regions. In an effort to gain a better understanding of the trap behavior using the PL mapping technique, a partial shading experiment was used to block light from being incident on a portion of a B doped Cz wafer.  A 3 cm thick aluminum block was used to shade a low intensity light for 60 minutes.  Figure 7 shows the resultant PL map.  The area exposed to the low intensity room light showed degradation, while the covered portion of the wafer maintained a much higher lifetime.  This was simultaneously confirmed using a surface photo-voltage diffusion length measurement.  The area of interest is the boundary between the shaded and exposed region.  By spatially confining the injected carrier concentration careful analysis of the transition region between the degraded section and  the shaded section should result in trap diffusivity information.   CONCLUSIONS  Boron doped Cz Si (~300 µm) showed significant LID, resulting in 1-1.5% degradation in absolute efficiency.  Ribbon Si materials are found to exhibit LID.  The LID effect was smaller in SP mc-Si materials due to slightly higher resistivities for ribbon crystals, lower oxygen concentrations, and higher densities of defective regions.  Cast mc-Si is shown to degrade 0.2-0.3% in some cases but not in the others (0.04%), depending on the position of the wafer in the ingot.  This is partly attributed to the variation in oxygen content in the ingot.  The top of the ingot has undetectable Oi but the bottom section can contain 4x1017 cm-3.  Photoluminescence maps were taken to provide a spatially resolved picture of the behavior of the trap responsible for LID.  Strong LID was observed away from defect regions in EFG Si.   REFERENCES  [1] J. Schmidt, A.G. Aberle, and R. Hezel, “Investigation of Carrier Lifetime Instabilities in Cz-Grown Silicon”, Proceedings of the 26th IEEE PVSC, p.13 (1997) [2] S. Glunz, S. Rein, J. Lee, and W. Warta, “Minority Carrier Lifetime Degradation in Boron-Doped Czochralski Silicon”, J. Appl. Phys., 90, 2397 (2001) [3] J. Schmidt, K. Bothe, and R. Hezel , “Formation and Annihilation of the Metastable Defect in Boron Doped Czochralski Si”, Proceedings of the 29th IEEE PVSC., p.432, (2002) [4] S. De Wolf, P. Choulat, et al., “Light-Induced Degradation of Very Low Resistivity Multi-Crystalline Silicon Solar Cells, Proceedings of the 28th IEEE PVSC, p.53 (2000) [5] S. Ostapenko, I. Tarasov, J. P. Kalejs, et.al “Defect Monitoring Using Scanning PL Spectroscopy in mc-Si Wafers”, Semicond. Sci. Technol. 15, 840 (2000) [6] J. Jeong, Y. Cho, A. Rohatgi, et al.,“Rapid Thermal Processing for PECVD SiN-Induced Hydrogenation of High-Efficiency EFG Silicon Solar Cells”, Proceedings of the 29th IEEE PVSC, 250 (2002) [7] Private discussion with Evergreen [8] J. Kalejs, “Silicon Ribbons and Foils-State of the Art” , Solar Energy Mat. & Solar Cells, 72, p.139 (2002) 10 20 30 40 50 60 70050100150200250Horizontal line scans taken at 40 mmArbitrary Units10 20 30 40 50 60 700246810Arbitrary UnitsLength (mm)Fig. 6  Line scans of EFG (after solar cell processing) before and after LID of the Ibb and Idb Fig. 7    PL map of a half wafer of B doped Cz Si subjected to shading by a aluminum block on the bottom ~42-43 mm. 

Light Induced Degradation in Promising Multi-Crystalline Silicon Materials for Solar Cell Fabrication

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Light Induced Degradation in Promising Multi-Crystalline Silicon Materials for Solar Cell Fabrication

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