Fischer, D.

Goetz, M.

Niquille, X.

Pernet, P.

Shah, A.

Infoscience - École polytechnique fédérale de Lausanne

FRONT CONTACT AND SERIES CONNECTION PROBLEMS OF a-Si:H SOLAR CELLS ON POLYMER FILMSUBSTRATESPascal Pernet, Michael Goetz, Xavier Niquille, Diego Fischer, Arvind ShahH. Althaus‡, A. Haller‡Institut de Microtechnique, Université de Neuchâtel, Breguet 2, CH-2000 NeuchâtelTelephone: xx41-32-718-33-52 Fax: xx41-32-718-32-01E-mail: pascal.pernet@imt.unine.ch‡ E. SCHWEIZER AG, Metallbau, CH-8908 HedingenABSTRACT: In this paper the authors present results of different investigations concerning the structuring and modulefabrication of a-Si:H solar cells on polymer film substrates. More precisely, this paper will discuss the evaluation of ITO andZnO as top contacts for n-i-p a-Si:H solar cell modules and the possibilities of implementing monolithic serial connections in aone step, post deposition process by using selective laser scribing.These features were successfully implemented in a small solar module (4 cells) on a polyimide film(PI/Al/ZnO/n-i-p/ZnO/met). The following performance values were achieved: Active area: 4*9.2 cm2, Voc = 3.42 V,FF = 62.3%, Isc = 12.34 mA/cm2.KEYWORDS: a-Si - 1: Laser scribing - 2: Polymer film - 31. INTRODUCTIONIn the past, in most cases glass or metal sheets wereused as substrate materials for amorphous silicon solarcells. Polymer films could in the future, however, turn outto be more suitable substrates, for the following reasons:At the present state of development it looks as if theefficiency of industrially produced amorphous siliconsolar modules will remain for the near and midterm futureclearly below 10%, in a range from 6-9%. The only wayto compete at this level with the dominating crystallinesilicon technology is by offering radically lower costs andunique application advantages.Regarding costs, only the use of polymer films allowsone to combine both the roll-to-roll continuousfabrication process, and an easy possibility of directmonolithic series connection, both factors being key costreduction elements. Neither glass nor metal substrates canoffer this combination. In addition, the polymer substratematerials themselves can potentially be very inexpensive.Regarding applications, polymer substrates, are light,allows for large area modules with electrical seriesconnection, flexible, and unbreakable, offer additionalunique advantages, in particularly in the domain ofbuilding integration, where they can be laminated withoutusing glass onto low-cost metal facades or roofs.In order to connect the cells together, it is necessary toseparate the cells on the same substrate, then to connectthe back contact of one cell to the top contact of itsneighbour. To achieve this, the laser scribing is the usualtechnique [3-9] and the most common way needs a 3 stepprocess of laser scribing: deposition and scribing,respectively of the back contact, of the n-i-p structure andof the top TCO. This process demands specialisedequipment for the alignment of the different steps of thescribing process and can be very problematic in the caseof the use of a plastic film as substrate.Another solution which allows one to reduce thenumber of laser scribing steps to a single step, only,consists of depositing a metallic layer on the other side ofthe substrate and then collecting the current through holesmade in the substrate [4].Here, we propose an alternative method of laser scribingwhich allows one, also, to reduce the structuring processof the device to a single step.2. EXPERIMENTALThe solar cells were deposited with the VHF-GDtechnique at a plasma excitation frequency of 70MHzwithin a single chamber deposition system [1]. The twocoplanar electrodes have a surface of 133 cm2, thesubstrate is fixed face down to the upper electrode. Theexcitation frequency is applied on the lower electrode.The effective temperature was kept between 170ºC and240ºC depending on the layer.The metallization of the back contacts (Al) wasdeposited by evaporation. ZnO (back and top contact) andITO (top contact) were deposited by Chemical VapourDeposition (CVD) and by sputtering, respectively.The structuring of the cells was performed with adoubled Nd-YAG pulsed laser (532nm), the beam wasdeviated with prisms onto a lens which was mounted on amicrometric screw. This allows one to control thefocusing (or defocusing) of the beam as it arrives on aXY-table onto which the samples are fixed. The power ofthe laser and the speed of the table are controlled by acomputer.Up to now, the flexible isolating paste was manuallydeposited with a syringe. The series connection wasperformed with Cr-Ag deposited by e-beam and by Jouleeffect, respectively, through a mask.The cells were characterised by measuring illuminatedj-V curves; thereby, a two-source solar simulator wasused. Voc and FF were determined from the j-V curves.The short circuit current (Isc) was determined byintegration of the spectral response curve of a selectedcell.3. TOP CONTACT OF n-i-p a-Si:H DEVICES WITHLARGE AREASA proposed structure for a n-i-p device has beenpreviously given by us [2]. Because the cell is directlyexposed to light, the generated current must be collectedthrough the top TCO (Transparent Conductive Oxide)(Fig. 1). It is well known that for a cell of a few squarecentimetres or more, the top TCO will behave as a serialresistance and may substantially decrease the FF (fillfactor) of the cell.If an ITO (Indium Tin Oxide) top contact is used, theoptimal thickness, from the optical point of view, is lowerthan 100 nm due to the fact that the ITO is usually flatand the optimisation of the thickness depends oninterferometric effects.Metallic gridTCOn-i-pBack contactPlastic filmFIGURE  1: Structure of a n-i-p device. Thephotogenerated current is collected through the topcontact.In this case, for distances more than about 1 or 2 mm,the ITO layer is not thick enough to carry the current of asingle cell without significant losses due to the serialresistance of this material. In other words, this means thatit becomes very rapidly necessary to use a metallic gridwith fingers to collect this current at the surface; however,the shadow effect of such a metallization will reduce theeffective surface (i.e. the active area).Another solution consists of using ZnO (Zinc Oxide)which has a higher conductivity than ITO, the thicknessof such a TCO can be increased (1 to 2 µm) due to its highoptical transmission coefficient. In this case, the losses inthe top TCO are much lower than in the case of ITO. Thedistances between the metallic fingers can be increased, orthe latter even be removed.From this point of view, the ZnO top contacts clearlyappear to be a more advantageous solution.4. SELECTIVE STRUCTURING BY LASERSCRIBINGIn our case, we tried to reduce to only one step of laserscribing the structuring process of the device. Thefunction required is to separate the unit cells electrically,not to make series-connection. Figure 2 resumes thedifferent kinds of laser scribing processes needed toperform this.One should distinguish between:· 1 Total structuring: all the layers are removed as far asthe substrate. This structuring allows one to separatethe cells deposited on the same substrate.· 2 Deep structuring: the top TCO and the n-i-p structureare removed but not the back contact. The sweepingof a small surface with such a deep structuringmethod allows one to "open a window" as far as theback contact.· 3 Top structuring: only the top TCO is removed. Thiskind of structuring can be used to define the area ofeach cell.1 2 3Plastic filmAl / ZnOnipZnO or ITOFIGURE  2: Selective laser scribing processesimplemented with a 532nm laser (Nd-YAG frequencydoubled). 1: Total structuring, all the layers are removed.2: Deep structuring, the top contact and the n-i-p deviceare removed. 3: Top structuring: only the top TCO isremoved.5. RESULTS ATTAINED AND DISCUSSION ABOUTLASER SCRIBINGTotal structuring is possible with both ITO or ZnO astop contact (see Fig. 4). We note that, for an appropriatepower, all the layers are removed without damage of theplastic film. Nevertheless, the n-i-p structure is stronglydeteriorated in the borders of the path of the scribe. Thisdeterioration is due to the spallation and crystallisation ofthe material [5,7].This structuring is appropriate for a separation of theback contacts of the cells but needs an other structuringprocess to define the area of each cell.On Fig.4, the borders of the path (in black) show that thestructure of the cell is deteriorated. The white region is theback contact (Al / ZnO) and, in the middle of the path, theplastic film.Deep structuring shows the same effect (a deteriorationof the n-i-p structure).In Figure 5, the borders of the path (in black) show thatthe structure of the cell is damaged. The white region isthe back contact of Al-ZnO.No damage of the n-i-p structure were observed afterthe top structuring of an ITO top contact. This can beexplained by the thin thickness (• 70nm) of this layer andthe low energy needed to remove it.FIGURE 4: Photography (optical microscope) of a totalscribe of the device PI / Al-ZnO / n-i-p / ZnO. The lengthof a side is 1 mm.FIGURE 5: Photography of a deep scribe of thestructure PI / Al-ZnO / n-i-p / ITO.FIGURE 6: Photography of a top scribe of the structurePI / Al-ZnO / n-i-p / ZnOTop structuring of ZnO is more problematic. On onehand, the wavelength of 532nm is inappropriate becausethe ZnO is highly transparent at this wavelength; thereforethe ablation of this layer is performed by a local increaseof the pressure in the a-Si:H layer. On the other hand, thethickness of the ZnO top contact is 2 to 3 time higher thanthe thickness of the cell, so that the power needed for its"top structuring" must be increased with respect to thepower needed for ITO "top structuring". Such a powercan lead to the deterioration of the a-Si:H structure (Fig.6). We observed that the laser scribing of ZnO cangenerate certain "micro-shunts" all along the scribingpath; however, the micro-shunts can be removed with asoft chemical etching step [6].6. RESULTS OF SELECTIVELY LASER-SCRIBEDSOLAR CELLS ON PLASTIC FILM SUBSTRATESFigure 7 shows the design of the serial connection of thecells deposited on a plastic film and structured with thetechnique of selective scribing. Note that the thicknessesof the different layers are deliberately increased in thefigure and the width of the scribes are relativelydecreased.Plastic filmIsolating pasteMetallizationDead areaActive areaFIGURE 7: Monolithic serial connection of a n-i-p solarcell with the selective laser scribing process.Figure 8 shows the process flow of the cells depositedon a plastic film. Note that all the depositions of the layersare performed before the laser scribing step. A substantialeconomic advantage of this method is that no specialalignment is required for the different steps.Substr. cleaningDep. metallization+ dep. ZnO back contactDep. a-Si:H nip single cellDep. TCO top contact (ZnO or ITO)Laser scribingChemical etchingIsolationMetallizationSteps of processZnO ITOFIGURE 8: Process flow for a n-i-p solar cell depositedon a plastic film with the selective laser scribing process.Note the "one-step process" used here for laser scribing.Figure 9 shows a prototype fabricated by our laboratory:the module consists of 4 single n-i-p cells according to theprocess flow previously described (see Fig.8). The area ofeach cell is about 11.5 cm2 with an active area of 9.2cm2per cell. The active area can be increased with theautomatization of the deposition of the isolating paste(manually performed up to now) and a reduction of thegaps between the different scribes (see Fig. 7). Thecharacteristics of this module are: Voc = 3.42 V,FF = 62.3%, Isc = 12.34 mA/cm2.FIGURE 9: Cells deposited on plastic film. Thestructure is PI/Al-ZnOCVD/n-i-p/ZnOCVD.The structuring is performed in a single pass, using theselective laser structuring process. The dimension of eachcell are 7cm * 1.5cm.7. CONCLUSIONSWe propose a new "one-step" laser scribing process fora-Si:H solar cell deposited on polymer films. First, wedeposit all the different layers (back contact-n-i-p solarcell-top contact) then we selectively scribe the differentlayers with a frequency doubled Nd-YAG (532nm) laser.The interest of this method resides in the fact that there isno special alignment needed anymore between thedifferent laser scribe steps, as was the case for the"conventional" process flow chart. We were able toimplement a small module made up of 4 single cells of9.2cm2 each; the power is approximately 250mW underAM1.5.8. ACKNOWLEDGMENTSFinancial support by the Project and Study Fund of theSwiss Electric Power Utilities (PSEL), under grantNo. 88, is gratefully acknowledged.REFERENCES[1] H.Curtins et al., Electronics Lett. 23, 228 (1987)[2] P.Pernet, R.Felder, M.Goetz, H.Keppner, D.FischerA.Shah, 14th EPVSEC, Barcelona, Spain (1997),p2339[3] Y.Ichikawa, T.Ihara, S.Saito, H. Ota, S.Fujikakeand H.Sakai, Proc. 11th E.C.PVSEC, Montreux,Switzerland, 1992, p203[4] Y.Ichikawa, K.Tabuchi, S.Kato, A.Takano,M.Tanda, S.Saito, H.Sato, S.Fujikake, T.Yoshidaand H.Sakai, IEEE 1st WCPEC, Hawaii, 1994,p441.[5] S.R.Praschek, W.Riedl, H.Hoermann andH.G.Goslowsky, Proc 22nd IEEE PVSC, LasVegas, Nevada (1991), p1285.[6] R. van den Berg et al., Solar Energy Materials andSolar Cells 31, 253 (1993).[7] J.Wallinga, O.L.Muskens, R.E.I.Schropp and W.F.van der Weg, 14th EPVSEC, Barcelona, Spain(1997), p533[8] S.Avagliano, M.L.Addonizio, G.Conte, C.Privato,12th EPVSEC, Amsterdam, The Netherlands,1994. p1218.[9] S.Avagliano, M.L.Addonizio, N.Angelucci,N.Bianco, E.Terzini, 13th EPVSEC, Nice, France,1995. p179.

Front Contact and Series Connection Problems of a-Si:H Solar Cells on Polymer Film Substrates

http://infoscience.epfl.ch/record/133810/files/paper_284.pdf

Front Contact and Series Connection Problems of a-Si:H Solar Cells on Polymer Film Substrates

Abstract

Similar works

Full text

Available Versions

Infoscience - École polytechnique fédérale de Lausanne