Fabrication Of Crystalline Silicon Solar Cell In .

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International Journal of Scientific & Engineering Research, Volume 7, Issue 5, May-2016ISSN 2229-5518581Fabrication of Crystalline Silicon Solar Cell inBangladesh: Limitations and RemediesGalib Hashmi, Md. Abdur Rafiq Akand, Mohammad Khairul Basher, Mahbubul Hoq and Md. Habibur RahmanAbstract-- For the first time in 2015, single crystalline solar cell was fabricated in Bangladesh and the fabricated mono‐crystallinesilicon solar cell efficiency is near 6.89%. This paper discuss about the reasons and limitations of achieving 6.89% efficient solar celland also remedies to achieve to high efficiency crystalline solar cell. Clean room class 100-1000 was not achieved during solar cellfabrication. Moreover Czochralski (CZ) silicon wafer, which contains higher impurity was used instead of Float Zone (FZ) siliconwafer. Inefficient technique like edge isolation barrier paste was used to isolate the edges of solar cell, instead of stacking the waferson top of each other and using CF 4 , O 2 and generating plasma with microwave frequency 2.45 GHz. While achieving good surfacepassivation requires thermal oxidation technique which requires temperature set at 900 C - 1200 C, could not be achieved due toRapid Thermal Annealing furnace whose maximum temperature is around 1000 C. Moreover shunt resistance and minority carrier2diffusion length (88 µm) was found low. High series resistance was found (6.197 Ω.cm ).Though line resistance of a tabbing wire istypically 20 times better than that of a screen printed busbar, no tabbing wire was soldered on top of each busbar. Remedies toimprove efficiency of mono crystalline solar cell is to use TiO 2 or ZnO layer as anti-reflecting coating, as no anti-reflecting was used2during solar cell fabrication process. Also for 12.5 X 12.5 cm silicon wafer, it is suggested to use 3 bus bars in front contactmetallization process because applying 3 busbar layout design improves current collection because of lower electric resistance. Asfor back contact metallization, not covering the back fully covered with Ferro FX53-038 aluminum paste, it is proposed to usediscontinuous busbars (2 strips with 3 gaps or 3 strips with 3 gaps) on the back side made with Ferro CN33-462 silver paste.Keywords: Crystalline silicon solar cell; Float Zone (FZ); Edge isolation; ARC layer; Busbars; Thermal Oxidation; RTA furnace;Shunt resistance; Series resistance—————————— ——————————1. INTRODUCTIONIJSERThe demand for energy is everlasting and it is increasingday by. Oil is not found everywhere [1], Coal is stillabundant and cheap but polluting [2], natural gas isscarce and nuclear energy production is costly and risky.Seeing that oil, coal, gas, nuclear power is not greenenergy, not abundant and will not last forever the onlyoption is to turn to renewable energy. By looking at thecurrent technical, environmental and economic aspectsolar cell is the best option for utilizing renewableenergy. But by looking the solar cell efficiency table(version 46) [3], it is seen that on an average solar cellefficiency is not more than 20%. So making the solar cellhighly efficient is a challenging task, and necessarybecause the sources of conventional energy is depletedday by day, resorting to alternative sources of energylike solar and making solar cell highly efficient hasbecome essential of the hour. So, for the first time in2015, single crystalline solar cell was fabricated inBangladesh and the fabricated mono‐crystalline siliconsolar cell efficiency is near 6.89%. [4, � Galib Hashmi1, Md. Habibur Rahman11 Department of Electrical and Electronic Engineering, University of Dhaka Md. Abdur Rafiq Akand 2, Mohammad Khairul Basher 2, Mahbubul Hoq22 Institute of Electronic, AERE, Bangladesh Atomic Energy Commission,Savar, Dhaka. Corresponding author: Galib Hashmi,Email: galib 90@yahoo.comDue to vast demand of solar panel and as solar cell, thussolar panel are not made locally in Bangladesh, the maingoal is to fabricate high efficiency solar cell in Bangladesh.Because the efficiency of the fabricated solar cell for the firsttime in Bangladesh, is too low for commercialization, thispaper discuss about what are the reasons for achieving suchlow efficiency and also the possible remedies to improve theefficiency of solar cell in Bangladesh.Limitations like ISO-8 class clean room, lack of FZ wafer,use of edge isolation paste, high series resistance low shuntresistance and rapid thermal annealing equipment (maxtemperature around 1000 C) etc. which are the causes for ofachieving 6.89% efficient solar cell are discussed in thispaper. In the old approach (The solar cell which wasfabricated in Bangladesh) did not use any kind of antireflecting coating or discontinues busbars, whereas in thenew approach it is proposed to use of ZnO (Zinc Oxide) orTiO 2 (Titanium Oxide) anti-reflecting coating, discontinuesbusbars (2 strips with 3 gaps or 3 strips with 3 gaps), use oftabbing wire, clean room, FZ wafer, Microwave plasmasystem for edge isolation, thermal oxidation at 1200 C withO 2 gas and different texturing methods to achieve to highefficiency crystalline solar cell.2.REASONS FOR ACHIEVING 6.89%EFFICIENT SOLAR CELL2.1 Clean RoomIJSER 2016http://www.ijser.org

International Journal of Scientific & Engineering Research, Volume 7, Issue 5, May-2016ISSN 2229-5518Modern solar cell manufacturing is performed in asophisticate facility known as a clean room. A cleanroom is a facility that is isolated from outsideenvironment and free from pollutants such as particles,metals, organic molecules, electrostatic discharge (ESD)and chemical vapors [6]. A typical room in urbanenvironment contains 35,200,000 particles per cubicmeter (Size range of 0.5 μm and larger in diameter) andknown as ISO 9 cleanroom, where as an ISO 1 cleanroomhas no particles in that size range and only 2 particlesper cubic meter of 0.2 μm and smaller [7]. U.S. GeneralService Administration’s standards (known as FS209E)class 100 to 1000 equivalent to International StandardsOrganization (ISO) ISO 5 – ISO 6 class requirement wasnot meet during the solar cell fabrication in Bangladesh.By using met one a2400 laser particle counter it is seenthat ISO class is in between ISO 7 and ISO 8.2.2. Wafer SelectionThe first step of making silicon solar cell is choosing the ptype silicon wafer. For single crystalline silicon solar cellthere are two ways that p type silicon wafer can bemanufactured. They are of Czochralski (CZ) method andFloat Zone (FZ) method and the wafers are called CZsilicon wafers and FZ silicon wafer. It is seen that in FZsilicon wafer, oxygen is less than 1016 atom/cm3 where asin Grade A type CZ wafer it is 9X 1017 atom/cm3. Alsocarbon concentration is less than below 1016 atom/cm3 inFZ where as in CZ, it is 1X 1017 atom/cm3 [8]. These oxygenand carbon are the main impurities of solar cellprocessing and the precipitation of oxygen and carbonoccurs during crystal growth [9]. Moreover oxygendefects reduces the lifetime of the bulk. Since the meltdoes not come into contact with a quartz crucible, and nohot graphite container is used during FZ crystal growth,FZ wafer has lower oxygen and carbon concentrationcompared to CZ silicon [10]. In fact FZ wafer is superiorover CZ wafer is based on low impurity, minority carrierbulk lifetime and low resistivity variation. Still fabricatingsolar cell in Bangladesh, CZ wafer was used instead of FZwafer.582thus reduces the efficiency of solar cell. Here inBangladesh for the edge isolation process, edge isolationpaste was used before phosphorous diffusion. Little isknown about the edge isolation paste which was boughtfrom the market and it is assumed as one kind of fluxrosin. Using edge isolation paste is one of the main reasonfor achieving low efficiency solar cell in Bangladesh. Butfor better performance in the edge isolation process it isproposed to use Microwave Plasma System [27]. Whereedge isolation is done by stacking the wafers on top ofeach other and using, and generating plasma withmicrowave frequency 2.45 GHz and having an etchingenvironment consists of combination of fluorinated gas(CF 4 ) and oxidant gas (O 2 ).2.4. Thermal OxidationOne of the reason that the efficiency in silicon solar cellscontinues to be limited is, by the recombination ofphoto-excited electron-hole pairs that takes places atsurfaces and interfaces. The process for producingsemiconductor surface with very low recombination rateis called surface passivation [12].To achieve high surfacepassivation, thermal oxidation technique is used and thetemperature most appropriate for thermal oxidation is900 C - 1200 C[13]. Rapid thermal annealing (RTA)furnace was used for thermal oxidation technique butthe required temperature 900 C - 1200 C could not beachieve because the RTA furnace maximumtemperature is around 1000 C. Moreover no oxygen(O 2 ) and nitrogen (N 2 ) gas was used as the furnaceprovide no option to utilize these gases.IJSER2.5 Other Factorsi) Shunt ResistanceSignificant power loss can be seen in solar cell due tolow shunt resistance, R SH. Shown in Figure 1 . For anideal cell, R SH should be infinite and should notprovide an alternate path for current to low, and if R SHis low, the value of V oc will also be low and in ourfabrication process shunt resistance was found low.2.3. Edge IsolationEdge isolation is a process that removes the phosphorousdiffused area around the p-type wafer, so that the frontemitter (N-type layer) is isolated from the rear (P-typelayer) [11]. The removal diffused layer around the edge ofthe solar cell is absolutely necessary because the diffusedlayer acts as short for p-n junction of the solar cell andIJSER 2016http://www.ijser.org

International Journal of Scientific & Engineering Research, Volume 7, Issue 5, May-2016ISSN 2229-55185833(a), and by using two-curve method [15] and thefollowing series resistance equation the seriesresistance of the fabricated solar cell wasdetermined.Rs 𝚫𝐕𝚫𝐈 𝐕𝟏 𝐕𝟐𝐈𝟐 𝐈𝟏I-V Curve of Solar CellFigure 1. Shunt resistanceUnder low light conditions the shunt resistance wasfound 234 ohm for a 12.5X12.5 cm2 wafer, whereasgood solar cell has more than 1000 ohm of shuntresistance [14]. It is seen that at 850 C to 900 C thesolar cell broke in the RTA furnace, if not shuntresistance was lower. A dryer oven (whose maxtemperature is 250 C) was used (shown in Figure 2),which improved the shunt resistance but still it wasmuch lower than commercial solar cell’s shuntresistance.Current (mA)605040302010000.10.20.30.40.5IJSERVoltage (V)Figure 3. (a) I-V curve of fabricated solar cell.Figure 3. (b) I-V curve of ideal solar cell showingof increased series resistance. [16]the effectFigure 2. Dryer Oven (Photo courtesy: Institute of Electronic,AERE, Bangladesh Atomic Energy Commission, Savar.)iii) Series ResistanceThe light (L) intensity current (I)-voltage (V) (LIV)measurement instrument was used to measure thecurrent and voltage of the fabricated solar cell. Afterobtaining the current voltage data the I-Vcharacteristics curve was drawn, shown in the figureThe series resistance, Rs of the fabricated solar cellwas found 6.197 Ω.cm2. Which is much higher ashigh efficient solar cell series resistance value variesfrom 0.3 Ω.cm2 to 1 Ω.cm2 [17]. Meanwhile the highseries resistance of the fabricated solar cell can alsobe seen from the I-V curve, as series resistanceincreases the I-V curve of the solar cell deviates fromIJSER 2016http://www.ijser.org0.6

International Journal of Scientific & Engineering Research, Volume 7, Issue 5, May-2016ISSN 2229-5518the ideal I-V curve of the solar cell shown in figure3(b).iv) Minority carrier diffusion length & Minoritycarrier Life TimeLight-induced surface photo voltage (SPV) measurementsystem was used to measure the minority carrierdiffusion length. It was seen that minority carrierdiffusion length is 88 µm which lacks the requireddiffusion length because typical minority carrierdiffusion length is 100-300 µm [18].Measurement of minority carrier life time was also doneusing the following equation.L 𝐷𝜏Where L is the diffusion length in meters, D is thediffusivity in m²/s and τ is the lifetime in seconds.Diffusivity is considered as 27 cm2 /s and from theequation it was found that the minority carrier life timeis 2.8681 µsec, which is much below 10 µsec.3. REMEDIES584narrow lines of metallization perpendicular to thebusbars are called fingers shown in figure -4 (a)."Busbars" are connected directly to the externalinterconnection, while "fingers" collect current anddeliver to the busbars [20]. The solar which wasfabricated in Bangladesh has two busbars, but as 12.5X12.5 cm2 solar cell is quite big [21] it is assumed thatusing of 3 busbars instead of 2 busbars in front contactmetallization process will increase the efficiency of solarcell.Applying 3 busbar layout design will improve currentcollection because of lower electric resistance. Also forfront contact metallization process no tabbing wire wasused and it is suggested to use tabbing wire, which willbe soldered on top of each busbar. These tabbing wiresserve mainly two functions. The first is to provideelectrical interconnection to other cells in a module. Thesecond function is to provide extra conductivity for thebusbar. The line resistance of a tab is typically 20 timesbetter than that of a screen printed busbar [22].IJSERApart from having clean room class 100 -1000, using FZwafer, thermal oxidation at 900 C - 1200 C andmicrowave plasma system for edge isolation there areseveral other options also to improve the efficiency ofsingle crystalline solar cell in Bangladesh. They aredescribed below.3.1 Anti-reflecting coatingAntireflection coating (ARC) is a layer on the surface ofsolar cell (On n-type layer) to reduce the reflection ofsunlight. But no anti-reflection coating was used infabricating solar cell in Bangladesh. Spin Coater ACE-200can be used to deposit TiO 2 (Titanium Oxide) layer onemitter as anti-reflection coating. ZnO (Zinc oxide) layercan also be used as ARC as the transmittance of ZnOlayer is more than 85% [19] at wavelength range of600nm–900nm. Though silicon nitride (SiNx) layer is themost widely used materials as ARC, cannot be appliedon emitter of solar cell in Bangladesh because ofinsufficient equipment.Figure 4. (a) Busbars and Fingers.3.2 Front and Back Contact MetallizationMetallic top contacts are essential for collecting thecurrent generated by a solar cell. The metallic topcontacts which is of wide strips and vertical inarrangement is called busbars shown in Figure -4(a). TheIJSER 2016http://www.ijser.orgFigure. 4(b) Discontinuous busbars on the back side solarcell.

International Journal of Scientific & Engineering Research, Volume 7, Issue 5, May-2016ISSN 2229-5518For back contact metallization, the fabricated solar cellwas fully covered with Ferro FX53-038 aluminum paste.Remedy includes not covering the back fully with FerroFX53-038 aluminum paste but to have discontinuousbusbar on the back side. Discontinuities should be 2strips with 3 gaps on each busbar, or 3 strips with 3 gapson each busbar on the back side. Shown in figure 4(b).On all discontinues busbars Ferro CN33-462 silver pasteshould be used instead of Ferro FX53-038 aluminumpaste. The reason for using silver paste instead ofaluminum paste in the back contact is that, silver haslower resistivity 1.59x10-8 Ω-m and higher conductivity6.29x107 S/m whereas aluminum has higher resistivity2.65x10-8 Ω-m and lower conductivity 3.77x107 S/m at200 C. [23]. Also same kind of tabbing wire can be usedfor front and back contact metallization. Suggested frontcontact metallization for 3 busbars specification is,Fingers Spacing: 0.1831 cm, Fingers Width: 0.0182 cm,Bus Bar Width: 0.1419 cm. and Bus Bar Gap: 3.125 cm.For back contact metallization with discontinuous 3busbars, and 3 gaps in each busbar the specification is,Bus Bar Width: 0.1419 cm, Bus Bar Gap: 3.125 cm,Vertical Gap Distance: 2.1224 cm.the efficiency of solar cell can be improved by usingthese resources. Remedies like discontinuous busbarswith silver paste on the back contact metallizationprocess and using of tabbing wire are proposed toimprove the efficiency of solar cell. Last but not theleast, as no anti-reflecting coating was used forfabricating solar cell in Bangladesh, TiO 2 (TitaniumOxide), ZnO (Zinc oxide) or silicon nitride (SiNx) layershould be used as anti-reflection coating to improvethe efficiency of solar cell. Despite the lack of somefabrication equipment and characterization equipmentfor the first time in Bangladesh solar cell wasfabricated. Though the efficiency is still 6.89% it isbelieved that the efficiency will be improved afterobserving the limitations and taking the proposedsuggestions discussed in this paper.5. ACKNOWLEDGEMENTThe authors thanks Bangladesh Atomic EnergyCommission, for granting access to the only solar cellfabrication laboratory at Savar, Bangladesh and to doresearch in the solar cell fabrication laboratory, andalso providing logistics support. The authors alsothanks Department of Electrical and ElectronicEngineering, University of Dhaka for taking suchinitiative to do this kind of research.IJSER3.3 Saw damage removal and TexturingToday’s high efficiency PERL (Passivated Emitter,Rear Locally-diffused) solar cell uses inverted pyramidstructure which could not be achieved due to lack oflithography equipment. Wet chemicals were used forsaw damage removal and texturing process. The recipeused for saw damage removal was NaOH solution andDI water and for texturing, KOH-IPA solution. Piranhaor RCA solution should also be used for saw damageremoval process. Various timing, concentration ordifferent solution should also be tried to achieve bettertexturing which will increase the efficiency of solarcell.4. CONCLUSIONIn this paper various reasons for achieving solar cellefficiency 6.89% is discussed. Using of edge isolationbarrier paste is one of the main reason for achievinglow efficiency. Moreover, Rapid thermal annealing(RTA) furnace with maximum temperature around1000 C alongside not using oxygen (O 2 ) and nitrogen(N 2 ) gas are the reasons for not having proper thermaloxidation and having high series resistance and lowshunt resistance. Lagging of FZ wafer, ISO 5- ISO 6class clean room and Microwave Plasma System foredge isolation, reduced the efficiency of solar cell and5856. REFERENCES[1] John Lorié, “Atradius Oil Market Outlook”, AtradiusGroup, 2013[2] Coal polluting, The Colombus Dispatch, [Online]Available: s-stillabundant-and-cheap.html[3] Martin A. Green, Keith Emery, Yoshihiro Hishikawa,Wilhelm Warta and Ewan D. Dunlop, “Solar cell efficiencytables (version 46)”, Progress in Photovoltaics: Research andApplications, vol. 23, pp. 805–812, 2015[4] M. K. Basher and K. M. Shorowordi, “Fabrication ofMonocrystalline Silicon Solar Cell using PhosphorousDiffusion Technique”, International Journal of Scientific andResearch Publications, Vol. 5, Issue 3,pp. 1-7, 2015[5] Md. Abdur Rafiq Akand, Mohammad Khairul Basher, Md.Asrafusjaman, Nusrat Chowdhuyi, Atia Abedin, andMahbubul Hoq, “Study and Fabrication of Crystalline SiliconSolar Cell in Bangladesh; Using Thermal DiffusionTechnique”, International Journal of Innovation and ScientificResearch, Vol. 18 No. 2, pp. 417‐426, 2015[6] Michael Quirk and Julian Serda (2001), “SemiconductorManufacturing Technology”, (Prentice Hall).[7] Terra Universal.com, ISO Cleanroom Standards, [Online]Available:IJSER 2016http://www.ijser.org

International Journal of Scientific & Engineering Research, Volume 7, Issue 5, May-2016586ISSN iso-classification[26] M.R.I. Ramadan, “Effect of minority carrier lifetime incleanroom-standards.phpsolar cells”, Solar & Wind Technology, Elsevier,Vol. 6, Issue 5,[8] M.-A. Trauwaert, J. Vanhellemont, H.E.

Abstract--For the first time in 2015, single crystalline solar cell was fabricated in Bangladesh and the ‐crystalline fabricated mono silicon solar cell efficiency is near 6.89%. This paper discuss about the reasons and limitations of achieving 6.89% efficien t solar cell and also remedies to achieve to high efficiency crystalline solar cell.

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