Residual and Bending Stress Measurements by X-Ray Diffraction and Synchrotron Diffraction Analysis in Silicon Solar Cells

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1 Residual and Bending Stress Measurements by X-Ray Diffractin and Synchrtrn Diffractin Analysis in Silicn Slar Cells V.A. Ppvich\ N. van der Pers\ M. Janssen!, 1.1. Bennett 2, K.M.B. Jansen 3, J. Wright 4, I.M. Richardsn! l. Delft University f Technlgy, Department f Materials Science & Engineering, Delft, The Netherlands, Phne: +31 () , v.ppvich@tudelft.nl 2. Energy Research Centre f the Netherlands, PV Mdule Technlgy, Petten, The Netherlands 3. Delft University f Technlgy, Department f Design Engineering, Delft, The Netherlands 4. ESRF, 6 Rue Jules Hrwitz, BP 22,3843 Grenble, France Abstract - The presence f residual stresses in multicrystalline silicn slar cells has becme a prblem f grwing imprtance, especially in view f silicn wafer thickness reductin. Withut increasing the wafer strength, this leads t a high fracture rate during subsequent handling and prcessing steps. The mst critical prcessing step during the manufacture f screen-printed slar cells is the firing f metallic cntacts. In this wrk we evaluate the develpment f mechanical stresses in metallic cntacts (AI, Ag and AI/Ag bus bars) with respect t different prcessing steps. Fr this purpse we cmbine X-ray diffractin (XRD) stress measurements, Synchrtrn measurements, cell bwing measurements with a laser scanning device and in-situ bending tests. Synchrtrn diffractin analysis shwed that there is a stress gradient in bth Ag and AI layers. It was fund that the AI back cntact layer represents a very prusllse micrstructure, which des nt affect the mechanical stability f the slar cell. It was als fund that the thickness and cmpsitin f the eutectic layer are the mst imprtant factrs influencing the bwing f a cmplete slar cell. Furthermre, residual stresses and stresses develping during cell bending in Ag, All Ag bus bars are measured and discussed in detail in this wrk. Index Terms - residual stress, multicrystalline silicn slar cells, metallic cntacts. I. INTRODUCTION Nwadays slar cells and slar panels represent a cmplex intercnnected system with different interfaces in a multilayer/multi-stacked package. Residual stresses are frmed within the cell due t mismatch f thermal expansin cefficients and different mechanical behavir f the materials used in the metallic cntacts and sldered intercnnectins. Residual stresses have a large effect n the mechanical behaviur f the materials, such as fracture, wear and frictin. Cracking f slar cells has becme ne f the majr surces f slar mdule failure. Therefre, it is nt nly imprtant t investigate the electrical prperties f silicn slar cells, but als the stress state develpment during the manufacturing f slar cells. In rder t take int accunt the effect f the residual stress during the design and prcessing f slar cell, the actual level and sign f stress in the material has t be determined. This research gives a deeper understanding n metallic cntact build up and prvides cncrete infrmatin regarding the stress distributin in slar cells. The results can be used t enhance prductin yields, imprve cell reliability and establish mechanical criteria that lead t a reductin in cell csts. In this wrk several aspects related t slar cell prcessing cnditins and metallizatin are described in relatin t residual and bending stresses. II. EXPERIMENTAL CONDITIONS Residual stress measurements were perfrmed n rectangular (1x3 mm 2 ) neighbring single crystalline (Czchralski silicn - CZ) silicn slar cell specimens which were laser cut frm cmplete slar cells. Stresses in metallic layers f CZ-silicn slar cells were measured using cnventinal XRD with Bruker D8 diffractmeter, where the sin 2 \jf stress evaluatin methd was used t deduce the in-plane stress frm the slpe f d (spacing f diffracting planes planes) vs. sin 2 \jf [1]. XRD was chsen amng ther nn-destructive investigatin techniques, because it is the mst accurate and best develped methd, and it can be applied t a wide variety f sample gemetries. Synchrtrn XRD measurements were carried ut n the slar cell samples at beamline ill 11 f the Eurpean Synchrtrn Radiatin Facility (ESRF) in rder t measure residual and applied stresses in Si bulk substrate and metallic cntacts with a much higher penetratin depth cmpare t that f labratry based XRD. A mnchrmatic X-ray beam with an energy f 44.4 key (wavelength A), and a beam size f SOx 1 /-lm was used t illuminate the samples. T investigate the effect f the maximum firing temperature f the AI back cntact, three neighbring wafers were prcessed with identical cnditins, but with different peak temperatures, i.e. 75, 85, and 95 C. 442

2 In rder t examine the influence f the aluminum layer thickness n the residual stresses f the cells, tw different cells with 2 and 4 11m AI layers were investigated (a cmmercially available AI paste was used). Measurements f the amunt f bwing that results frm metallizatin were made by an ptical methd ver the full length f the slar cell (156 mm), using a Quick Visin Mituty system. In rder t measure residual stresses in the eutectic layer, the aluminum prus layer was partially remved with Ar ins by using a Gatan precisin in plishing system, nrmally used fr transmissin electrn micrscpy. Creep tests were perfrmed at a cntrlled cnstant temperature and frce using a DMA Q8 setup with 3-pint bending clamp in rder t cnfirm the rigin f the stress drp. Scanning electrn micrscpy (SEM) was used t analyze the surface and crss-sectin mrphlgy f slar cells. XRD was als used t examine the phase and element cmpsitin. Specifically fr this wrk, an in-situ bending device was built t fit inside the X-ray gnimeter (Figure 1). A parallel beam gemetry was used, thus allwing t exclude the influence f the sample shape. Different in-situ bending XRD stress experiments were perfrmed using cnventinal and synchrtrn radiatin n specimens f lo x 3 mm 2 cut ut f cmplete cells. Fig. I. In-situ 4-pint bending device fr XRD: Bending device inside the diffractmeter. III. RESULTS AND DISCUSSION A. Stress measurements via cnventinal X-ray Diffractin. Residual stresses in aluminum rear side and silver frnt side cntacts Frm ur previus investigatins in was fund that the prus aluminum layer has a cmplex cmpsite-like micrstructure, cnsisting f three main cmpnents: 1) spherical (3-5 I-1m) hypereutectic AI-Si particles, surrunded by a thin aluminium xide layer (15-2 nm); 2) a bismuthsilicate glass matrix (3.3%) 3) pres (14%) (Figure 2) [2]. It is knwn that when fired, the aluminum layer creates large slar cell bwing [3]. Hwever, it is nt entirely clear what effect the prus aluminum part has n the strength f slar cells. X-ray stress measurements were cnducted n tw slar cell samples with different thicknesses f the aluminum back cntact. Results shwed that residual stresses in the prus part f the AI back cntact layer are very lw, i.e. in the range f 1 MPa (Figure 3, Table I). It was als fund that a 2 11m thick AI layer shws higher stresses than a 4 11m layer (the X-ray infrmatin depth in AI is 2 11m). This result culd indicate that AI prus part f the rear side cntact is very lse and the majr part f the slar cell bwing is generated by the AI-Si eutectic reactin layer. It shuld be pinted ut that residual stresses were fund t be equal in lngitudinal and transverse directins, fr bth AI and Ag layers. 111'11 c -... c -II Fig. 2. Mdel f the rear face f a silicn slar cell with crrespnding micrstructural features [2]. In rder t cnfirm this hypthesis a part f the aluminum layer was gradually remved, resulting in a crss sectin, as shwn in Figure 4. The stresses in the remved layer, and thus in the eutectic layer, were fund t be -3 MPa. Hwever, this value is nt representative fr the entire eutectic layer, because the scan partially cvered the edges f the prus part f the AI layer. Further investigatins, including mre precise and gentle layer remval methds, are required t btain an actual stress value fr the AI-Si eutectic layer c ".9576 ō N « t:..4 sin2psi AI layer - 4 micrns AI layer - 2 micrns.6.8 Fig. 3. Effect f AI layer thickness n residual stresses (X-ray penetratin depth m). 443

3 T ABLE I RESIDUAL STRESSES IN AL LAYER. AI layer Stress, Stress thickness, (!-1m) (MPa) errr (MPa) Specimens with different firing temperatures f the AI back cntact, and as a result with different thicknesses f the eutectic layer, shwed that higher firing temperatures lead t larger amunts f cell bwing (Figure 5). Firing AI cntact at 75 C allws nly small amunt f eutectic 'islands' t be frmed. Firing at 85 C and 95 C gives a much mre unifrm and thick eutectic layer entirely cvering the silicn wafer. Thus the bw increase culd nly be explained by an increased thickness f the eutectic layer, because the ttal AI layer thickness was the same fr all samples. The XRD patterns f AI layers fired at different temperatures shwed that there is an increased amunt f Si in the AI layer with increasing firing temperature, indicating a higher diffusin f Si int the liquid AI particles. a) AI prus layer remved using a Precisin In _ '? Plishing System (Ar+, 5 kev) b) Thus, it can be cncluded, that bth thickness and cmpsitin f the eutectic layer can be cnsidered as imprtant parameters cntrlling mechanical stability f silicn slar cells. XRD stress measurements were als perfrmed n silver frnt cntact and Ag/AI bus bars, Figure 6. The stress in the Ag/AI bus bars was fund t be lwer (41 MPa), cmpared t the stress in the Ag frnt side cntact (7 MPa). This culd be related t the different cmpsitin f the silver busbars, which als cntain aluminum. 75 C Firing temperature (C) Amunt f bwing (mm) Fig. 5. a) Effect f maximum firing temperature n micrstructure f AI back cntact layers, shwing different thicknesses f the eutectic layer b) Resulting amunt f bwing > " '" Silver frnt Cntact Fig. 4. Remval f AI prus layer: a) Draft crss sectin f the layer; b) Resulting "hle" in the prus part f the AI layer. TABLE II EFFECT OF FllUNG TEMPERATURE (EUTECTIC LAYER THICKNESSES) ON RESIDUAL STRESSES (X-RA Y PENETRATION DEPTH - 4 M) Firing Stress Stress errr Temperature (C) (MPa) (MPa) Furthermre, XRD stress measurements in the AI layer shwed that there is nly a minr stress increase with increasing firing temperature (Table II). This increase culd be a result f a higher fractin f Si phase inside the AI particles, leading t a higher degree f aluminum defrmatin. Despite the lw value f stresses, there is a clear increase f the amunt f bwing, which culd result frm the eutectic layer itself, rather than frm the prus part f the aluminum layer !::. Ag/AI bus bar a sin'psi Fig. 6. Residual stresses in silver frnt cntact and silver bus bar (Xray penetratin depth m). B. Stress measurements in cmbinatin with bending tests In-situ lading f the Ag layer using the bending device installed inside the diffractmeter was perfrmed in rder t measure bending stresses in Ag layer. Lading f the Ag layer shwed an increase in bending stresses (Figure 7 a), indicating that it is pssible t measure bending stresses by X-ray diffractin using an in-situ bending clamp. The stress in the Ag layer was fund t be 86 MPa after lading t 3 N. Furthermre, it was fund that hlding the sample at a lad f 3 N during 8 h resulted in a 1 MPa stress drp, while n significant change in deflectin culd be bserved (Fig. 7 b). It shuld be nted that in these stress measurements the X-ray penetratin depth in the Ag is nly 2 /-lm. 444

4 ;;c- 8 Cl.. e- 7 If) If) 6 U i. &....., I Frce (N). 'I. 3 Stress relaxatin after hlding laded sample fr -8 hurs at 3 N.8 E.s.6 '"B 4 Q) Frce (N) Fig. 7 a) Bending stresses in Ag frnt cntact. Creep tests were perfrmed at a cntrlled cnstant temperature in rder t cnfirm the rigin f the stress drp. A bending sample with an Ag layer n the tensile side was laded t 1.8 N at 5 C and kept in the DMA chamber fr 8 hurs, while displacement changes were measured with an accuracy 1 nm (figure 8). It was fund that there is nly a minr displacement change f 5 /lm after 8 h, which can nt explain the 1 MPa stress drp bserved during the XRD measurements (Figure 8). It is cncluded that stesses d nt relax in the bulk f the Ag layer during the 8 h hlding, but that sme relaxatin takes place in the near-surface part (upper few /lm) f the Ag layer. This culd be due t plasticity/creep and/r cracking f the Ag layer. An in-situ bending test n the diffractmeter with the aluminum layer laded in tensin did nt result in a significant stress increase in the AI prus layer. This result indicates that the prus part f the AI back cntact is t lse t shw any Fig. 7 b) Deflectin f the sample with Ag frnt cntact as a functin f lading and hlding time fs hurs. buildup f residual r bending stresses and cnsequently will nt give any cntributin t the fracture strength f slar cells. The btained data are cnsistent with results frm previusly perfrmed bending tests [4]. The btained XRD stress data will be used in a multi scale mdel and based n the results it shuld be pssible t find a relatin between metallizatin prcessing cnditins, paste micrstructures and mechanical stresses. Hwever, it shuld be pinted that due t a lw penetratin depth f abut 1 /lm in AI and nly 1.5 /lm in Ag, this technique can determine nly near-surface residual stresses. Furthermre, it was fund that using labratry XRD it is impssible t measure stresses in each significant layer, especially in the eutectic layer, which frm previus investigatins [4] was cnsidered as the mst imprtant parameter cntrlling mechanical stability f silicn slar cells. High energies (3-8 key) and small beam sizes (5x5 /lm 2 ) are required in rder t achieve specific bjective. Therefre, the use f synchrtrn X-ray diffractin measurements was slicited. 51. Silver layer n CZ-Si wafer () :::J (J) (J) I Temperature, C Displacement (micrns) Frce, (N) Time, min Fig. S. DMA creep bending test n silver layer at cnstant temperature with Sh hlding time 445

5 C. Residual stress measured by synchrtrn diffractin analysis A series f synchrtrn X-ray diffractin residual stress measurements were perfrmed at beam line ID 11 f the Eurpean facility fr synchrtrn radiatin (ESRF) in Grenble, France. The specimens were irradiated by a mnchrmatic beam f 44.4 ke V phtns (crrespnding t a wavelength f A), and a beam size f SOx 1 f.lm 2 psitined in the centre f the sample incident perpendicular t the sample thickness (the beam travels in the thickness directin) and a transmissin gemetry was used. A reference pwder sample (W) was affixed t the specimen t mnitr any drift in the diffractin peaks as a result f changes in the beam energy prfile and t detect sample psitin relative t detectr. W pwder was chsen because its diffractin rings d nt verlap with thse f AI, Si r Ag. The diffractin peaks were btained by integrating diffractin ring and fitted using a Gaussian prfile. The internal strain fr each reflectin was then calculated using: d -d E = ---,- I _ I d where d is the "unstressed" lattice parameter. Estimatins fr the stress-free lattice parameters fr AI and Ag layer were btained by remving pwder frm the metalized Si wafer. Residual stresses were calculated using: (1) It was fund that the measured prfile f the as-fired Ag layer exhibits a significant peak shifting (Figure 9 a). It shuld be nted that the cmpsitin f the as-fired Ag layer and remved/stress free reference Ag pwder is identical, thus the peak shifting can nly result frm a residual stress. As can be seen frm figure 9, all fur Ag peaks shift twards lwer 28 angles, crrespnding t a larger d spacing. This indicates a tensile stress, which is expected after the firing prcess in view f a higher thermal expansin cefficient f silvercmpared t silicn. Furthermre, there is a peak splitting f Ag (Ill), indicating strain variatins alng the thickness f the Ag layer. The results shw that there are tw different stresses in the Ag layer (see fig. 8b): peak 1, crrespnding t a tensile stress similar t that btained via cnventinal XRD; peak 2, crrespnding t a much higher tensile stress value. The relatively brad width f the tw peaks suggests that there is a rather smth strain change ver the entire Ag layer thickness. It can be suggested, that the first lwer stress cmes frm the uter surface f the Ag layer, whereas the secnd stress represents the rest f the thickness f the Ag layer. Figure 1 represents the synchrtrn diffractin spectrum f the as-fired AI layer, shwing peak splitting fr AI (111) and AI (2). The splitting f the peaks and its sharpness suggests a relatively abrupt strain change ver the thickness f the AI layer. The lwer stressed peak, giving a stress value f 3 MPa, is similar t that btained by cnventinal XRD with a lw penetratin depth (table 1). This peak results frm the prus part f the AI layer Silver (111) 29, dspacing, A Stress, MPa Ii) C 1 ::J a).2-8 'iii b) Ag reference pwder fired Ag - peak fired Ag - peak c 2 c 6 -- Silver Layer fired n Silicn -- Reference Silver layer (remved frm silicn) Fig. 9 a) Diffractin spectrum f the reference Ag layer and as-fired silver layer, shwing peak splitting fr Ag (III) b) Crrespnding d spacing and stresses fr 2 peaks f Ag (31 I). 446

6 AI (2) lng. 29, d spacing, A Stress, MPa AI reference pwder b) fired AI - peak fired AI - peak AI (111) AI (2) a) 4 AI (111) 35 AI (2) 3 AI (22) Ii) 25 :::>.. 2 'iii 2!- 15 Si (22) 2 1 Si (311) 29 AI (311) 5 AI4Si AI4Si Si Fig. 1 a) Diffractin spectrum f the as-fired Al layer, shwing peak splitting fr Al (III) and Al (2) b) Crrespnding d spacing and stresses f Al (2). The secnd much higher stressed peak ( = 185 MPa) riginates frm the eutectic layer underneath the prus AI layer. This result further supprt the cnclusin that the eutectic layer can be cnsidered as the mst imprtant factr cntrlling mechanical stability f silicn slar cells. VI. CONCLUSION The residual stresses and stresses resulting frm bending in multicrystalline silicn cells were investigated using cnventinal and synchrtrn X-ray diffractin measurements, bwing measurements and bending tests. The study shwed that: - It is necessary t cmbine cnventinal XRD, synchrtrn diffractin and bw measurements in rder t btain a cmplete picture f the stress state in silicn slar cells. - The thickness f the eutectic layer as well as the cmpsitin f the aluminum rear-side cntact layer can be cnsidered as imprtant parameters cntrlling mechanical stability f silicn slar cells. - There is a strng crrelatin between maximum firing temperature, amunt f bwing and residual stress level f slar cells, i.e. the higher the firing temperature the higher the residual stresses and the amunt f bwing. - It is pssible t measure bending stresses by X-ray diffractin using an in-situ bending clamp specially designed fr thin slar cell specimens. - It was fund that hlding samples at a lad f 3 N results in stress relaxatin in the uter surface layer due t plasticity/creep r cracking f the Ag layer. - Synchrtrn diffractin analysis shwed that there are large stress gradients in bth the Ag and AI layers. - The X-ray diffractin technique, in cmbinatin with bw measurements and bending tests, prved t be a pwerful nn-destructive qualitative and quantitative experimental technique that prvides infrmatin abut fracture behavir and stress states f silicn slar cells. REFERENCES [I] Withers PJ, Bhadeshia HKDH, Overview: Residual Stress, Part I-Measurement Techniques, Materials Science and Technlgy 17, pp. 355, 2. [2] V.A. Ppvich, M. Janssen, LM. Richardsn, T. van Arnstel and U. Bennett, Micrstructure and mechanical prperties f aluminium back cntact layers, Slar Energy Materials and Slar Cells, Vlume 95, Issue I, pp , January 211. [3] M. Hilali, Understanding and develpment f manufacturable screen printed cntacts n high sheet resistance emitters fr lwcst silicn slar cells, 25. [4] V.A. Ppvich, A. Yunus, M. Janssen, LM. Richardsn, U. Bennett, Effect f silicn slar cell prcessing parameters and crystallinity n mechanical strength, Slar Energy Materials and Slar Cells, Vlume 95, Issue 1, pp. 97-1,