Thin Film Solar Cells from Earth Abundant Materials Thin Film Solar Cells from Earth Abundant Materials Growth and Characterization of Cu ZnSn(SSe) Thin Films 2 4 and Their Solar Cells Dr. Subba Ramaiah Kodigala Department of Physics & Astronomy, California State University, Northridge, CA, USA AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON(cid:129)NEWYORK(cid:129)OXFORD PARIS(cid:129)SANDIEGO(cid:129)SANFRANCISCO(cid:129)SINGAPORE(cid:129)SYDNEY(cid:129)TOKYO Elsevier 32JamestownRoad,LondonNW17BY,UK 225WymanStreet,Waltham,MA02451,USA Copyright©2014ElsevierInc.Allrightsreserved Nopartofthispublicationmaybereproducedortransmittedinanyformorbyanymeans, electronicormechanical,includingphotocopying,recording,oranyinformationstorage andretrievalsystem,withoutpermissioninwritingfromthepublisher.Detailsonhowto seekpermission,furtherinformationaboutthePublisher’spermissionspoliciesandour arrangementwithorganizationssuchastheCopyrightClearanceCenterandtheCopyright LicensingAgency,canbefoundatourwebsite:www.elsevier.com/permissions Thisbookandtheindividualcontributionscontainedinitareprotectedundercopyright bythePublisher(otherthanasmaybenotedherein). 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BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress ISBN:978-0-12-394429-0 ForinformationonallElsevierpublications visitourwebsiteatstore.elsevier.com ThisbookhasbeenmanufacturedusingPrintOnDemandtechnology.Eachcopyis producedtoorderandislimitedtoblackink.Theonlineversionofthisbookwillshow colorfigureswhereappropriate. DedicatedtoMyBelovedParents MrsSampoornammaKodigala MrSankaraiahKodigala Preface Thisbookdescribescurrentstatusofphotovoltaictechnologyintheprocessofearth abundant solar cells and deals several things about fabrication and characterization of solar energy materials, including thin film solar cells. The Cu2ZnSn(S1-xSex)4 based solar cells are technologically premature, compared with CuIn1-xGaxSe2 (CIGS)basedthinfilmsolarcells.Therefore,thisbookillustratesbasicpropertiesof materialsandthinfilmsolarcellsandhasfivechaptersmainlydealingCu2ZnSn(S1-x Sex)4systemforcellapplications. First chapter deals utilization of renewable energy in many ways for the man- kind. To produce electricity by solar energy, the achievements of different compa- nies working in the thin film solar cell industry are emphasized to understand overall situation of the market. The usage of flexible thin film solar cells in the remote areas is explained. The possible earth abundant solar energy materials in the earth crust for low-cost thin film solar cells are mentioned in this chapter. The role of contemporary solar cells, such as Si, CIGS, CdTe, along with Cu ZnSnS 2 4 (CZTS) solar cells are nicely described by providing high priority for the thin film solar cells. The basic working principles of different types of solar cells such as conventionalthinfilmsolarcells,quantumdotsolarcells,andplasmonicsolarcells are quietly illustrated with schematic diagrams. The physics behind the conven- tionalsolarcellswithbilayerstructuresisdescribedwelltounderstand.Acompari- sonbetweenCIGSandCZTSthinfilmsolarcellsisalsoexploited. There are possibilities of generation of secondary phases in the Cu2ZnSn(S1-x Sex)4 system while growing it. Therefore, the properties of all secondary phases, which would probablyarrive from this compound, such as Cu S, SnS, SnS , Sn S , 2 2 2 3 ZnS, ZnSe, Cu Se, are distinguished in the second chapter. The growth of these 2 phasesbyseveraltechniquesiscited.Inaddition,thestructural,optical,andelectri- cal properties of the materials are elaborated in this chapter. Some of the binary compoundsareusefulasabsorbersandothersaswindowsforsolarcellapplications. Forexample,theCu SandSnSareusedasabsorberswhilstZnSandZnSeareused 2 as window layers in the solar cells that they depend on the basis of their band gaps or other properties. The fabrication processes of Cu S and SnS thin film solar cells 2 by different techniques are emphasized. The I(cid:1)V measurements, stability, efficien- cies of the solar cells are described. So far, the SnS-based cells exhibit low effi- ciency for which the reasons are elaborated. The Cu S-based cells show reasonable 2 solar energy conversion efficiency of around 10% but the efficiency decreases with ageowingtoinstability. x Preface Third chapter mainly describes how to grow Cu ZnSnS (CZTS) thin films by 2 4 vacuum and nonvacuum processes because the optimum growth process is very important to have good quality thin film to achieve high efficiency thin film solar cells. The CZTS thin films are grown by different techniques such as chemical solution process, ink jet printing, screen printing, spin coating, spray, thermal vacuum evaporation, reactive sputtering, sputtering, pulsed laser deposition, and electrodeposition techniques. More or less similar techniques are employed to grow Cu ZnSnSe (CZTSe) or CZTSSe thin films. All the grown metal stacks or 2 4 semiconductor layers such as CZT, CZTS, or CZTSe are sulfurized or selenized under S or Se atmosphere to obtain device quality thin films for solar cell appli- cations. This postannealing process is well illustrated by pointing optimized conditions. Fourth chapter contributes a lot of things of characterization techniques, which are highly helpful to assess the quality of the device layers such as absorbers, win- dows, or other layers. The techniques, which are employed for characterization of the samples, are EDS, XRF, SIMS, ICP, XPS, XRD, Raman, etc. In particular, the EDS, SIMS, and ICP techniques provide how to characterize the samples in detail. The SEM and AFM scan the surface of the samples to determine roughness, grain sizes, etc. that are detailed. The XRD analysis characterizes different kind of sam- ples such as CZTS, CZTSe, and CZTSSe but fails to determine structure of whether kesterite or stannite of them but Raman spectra distinguishes the struc- tures. The defect levels determined by photoluminescence analysis are explicitly illustrated. Based onthesecharacterization results, the CZTS, CZTSe, and CZTSSe samples are analyzed, which determine the quality of the samples that fit well to the solar cells. The characterization results provide compositional, chemical states of compounds, structural, optical properties of the samples. The electrical proper- tiesofCZTS,CZTSe,CZTSSecompoundsarealsogiveninthischapter. The fabrication of thin film solar cells using CZTS, CZTSe, and CZTSSe absor- bers is explained in the last chapter. The band structures of heterojunctions such as CZTS/CdS and CZTS/ZnS with different orientations of buffer layers are dis- cussed. The cells made with different kinds of techniques are explained with sev- eral examples. The I(cid:1)V characterization of the samples provides efficiency, series resistance, shunt resistance, open circuit voltage, short circuit current, fill factors, etc. The reasons for occurring low and high efficiencies in the thin film solar cells are explained. How the composition of the absorber affects the efficiency of the cellsiswelldescribed.Theannealingrecipeontheabsorberisoneoftheimportant factors to decide photovoltaic parameters. The carrier concentration and resistances of the samples are also play a big role in the thin film solar cells to achieve high efficiency. The cells made with other environment-friendly window layers are explained.Thebandgapsofabsorber,buffer,andwindowlayerscanbedetermined fromquantumefficiencymeasurements. Acknowledgements I would like to thank Professor Henk W. Postma, California State University, Northridge, CA, for his kind and spontaneous help and fruitful discussionsto shape all the materials as a book. The incredible help by his laboratory team members is unforgettablewhilepreparingthemanuscript. I am highly grateful to Professor Jerry Stinner, Dean, College of Science and Mathematics whose encouragement is unimaginable to complete the book writing onimportanttopicwithinastipulatedtime. I am thankful to Professor Lim Say-Peng, Chair, Department of Physics and AstronomyforhishelpandoccasionaldiscussionsonvariousPhysicstopics.Ithank Professors Cristina Cadavid, Debi Choudhary, Damian Christian, Miroslov Peric, Radha Ranganathan, Yohannes Shiferaw, Duane Doty, Donna Sheng, Nicholas Kioussis, Igor Beloborodov, Bhat, Morkoc, Johnstone, Rajendra Prasad, Prabhakar Rao, Narayana Rao, Bhuddudu, Shalini Menezes, Pilkington, Hill, Tomlinson, Sudharsan,Y.K.Su,S.J.ChangandF.S.Juangfortheirincrediblehelp. I am highly indebt to Professor Somanath Chattopadhyay, Department of Electrical Engineering, California State University, Northridge, CA, for his help andunpredictablelengthydiscussionsondevicesandsimulations. I thank mybeloved teacher Professor Sundara Raja and his team for fruitful dis- cussionsonthesubjectmatters.IamhighlyindebttomyrighthonorablefriendDr. MesfinTayeforhisadvicesduringwritingthisbookandmoralsupport. Nonetheless, I thank my wife Mrs Mitra Vinda from my bottom of heart for her help and cooperation during writing this book and caring me at all the time. I would like to thank my lovely children Mr Ashok and Mr Sri Hari for their help in assisting me in computer graphics even though they are out of home for their Universityeducation. I am highly grateful to my loving mother Mrs Sampoornamma, brother Mr Chandraiah,mother-in-lawMrsPakkiramma,brother-in-lawsMrPrasad,Rajagopal, Venkatesh, Rangaiah. I thank all my relatives and friends Mrs and Mr Ramaiah, Jagnatham, Kodandaramaiah, Balasubramanyam, Subramanyam, Dhanalakshmi, Revathi, Ramatulasi, Lakshmidevi, Menaka, Ramana Reddy, Madhu Reddy, Ajaya Babu, Rajendra Naidu, Nagaiah, Sivaiah, Venkataramana Raju, Appala Raju, Venkata Raju, Sanjeevi, Narasimhalu, Kailasam, Jayarama Setty, Manohar Naidu, Dwarakanath, Nagaraja Naidu, Krishnamachari, Kistaiah, Sarojamma, Kistamma, Ramesh, Venkatasubbaiah, Ramamurthy, Babu, Radhakrishna, Mohan, Vani, Vidhyasagar, Vasantha, Aruna, Prabhakar Rao, Nagaraj, Munirathanam, Sarswat, Parag,Avag,SubbarajuandSubbammafortheirsupport. 1 Introduction 1.1 Current Trends in Utilization of Solar Energy The solar, wind, and thermal energies are normally come under the umbrella of the renewable energy, which is one of the alternatives to the conventional energy. The hydro, thermal by coal, and nuclear can be treated as conventional energy sources. The nuclear disasters of Chernobyland Fukushima are cautioningworld about dan- gers of nuclear plants and consequences tomankind. The nuclear energy shares 7% in world energy and 15% in production of electricity. In order to govern the safety of nuclear power plants, the international energy agency revamps the safety regula- tions and guidelines. France, Japan, EU, and the United States depend on nuclear power plants for electricity in their energy resources of 75, 30, 28, and 19%, respectively [1,2]. So far, China, Russia, Korea, and Latin America have 28, 11, 5, and 8 nuclear power plants, respectively. A lot of countries promised that they gradually abandon the nuclear power plants in order to reduce risk factor. Capacity of total energy of the world is 4742GW in which share of the solar energy is 37GW nothing but 0.78% in 2010. In 2009, the new installation of solar energy is 7.1GW that is doubled in 2010 as 17.5GW. The solar energy produced by differ- ent countries like Germany, Italy, Czech Republic, Japan, and the United States is 7.5, 3.8, 1.2, 0.8, and 0.8GW, respectively. In the existing global renewable energy, the production of hydroelectricity is 0.5TW, tides and ocean currents of 2TW, geothermal of 12TW, wind power of 2(cid:1)4TW, and solar energy of 120,000TW.Ofallthese,thecontributionofsolarenergyisthehighest[3]. The top 10 companies such as Q-cells, Sharp, Suntech, Keyocera, First Solar, Motech, Solar World, Jasolar, Yingli, and Sanyo produce solar energy of 9, 8, 8, 5, 5, 4, 4, 3, 3,and 4%, respectively.The remaining 47%is covered bythe restof the world. In fact, the conventional electricity costs around $0.39/kWh or less. In recent years, a lot of efforts have been initiated to develop low-cost thin-film solar cells, which are alternative to high-cost silicon (Si) solar cells. The reduction of cost is easier innon-Si thin-filmsolar cells than inSisolarcells. We can obviously play as much as alternations in thin solar cells to improve performance of them whereas Si solar cells do not give much room to tailor the parameters to enhance the efficiency. The main drawback with the Si solar cells is that it is an indirect band gap semiconductor and needs a thick layer around 180(cid:1)300µm to absorb photons [4]. The band gap of 1.1eV for Si does not absorb more than 50% of the visible spectrum, i.e., blue and green regions. These factors undermine to reduce the cost of Si solar cells. The low-cost and high-quality chalcogenide-based ThinFilmSolarCellsFromEarthAbundantMaterials.DOI:http://dx.doi.org/10.1016/B978-0-12-394429-0.00001-9 ©2014ElsevierInc.Allrightsreserved. 2 ThinFilmSolarCellsFromEarthAbundantMaterials thin-film solar cells have to be developed, which will potentially reduce manufacturingcost ofsolarenergy from $3(cid:1)5/W to $0.60/W. Recently, FirstSolar Company proclaimed that the current cost of electricity by its CdTe solar panel is $0.70(cid:1)0.72/Wandaimstodevelopsolarcellsatthecostof$0.6(cid:1)0.5/W[5]. The search for suitable band gap materials for the applications of solar cells is essential. Therefore, scientists have initiated to fabricate novel and new absorbers by identifying the earth’s abundant solar energy materials to reduce the cost of thin-film solar cells. Recently, Cu(In1(cid:1)yGay)(S1(cid:1)xSex)2 (CIGSS) based thin-film solar cells are technologically developed in which the Zn/Sn replaces In/Ga that reducescostofthesolar panelspartially.Thereplacementchangesthesystemfrom Cu(In1(cid:1)yGay)(S1(cid:1)xSex)2 to Cu2(ZnSn)(S1(cid:1)xSex)4. In every year, the cost of In or Ga doubles its original value owing to high demand in the market. In the earth crust, the existences of Cu, Zn, Sn, S, and Se are 50, 75, 2.2, 260, and 0.05ppm, respectively whereas availability of In is 0.049ppm (Figure 1.1) [6,7]. It is learned that 30 tons of In is necessary to produce 1 GW power [8,9,10]. The indium tin oxide (ITO) is one of the main players in the realm of optoelectronic screen dis- plays where In is the prime component to make its oxide layer. On the other hand, the usage of Ga in the light emitting devices is high. Therefore, the optoelectronic industry has high impact for demand of In and Ga. In this context, the search for alternative solar energy materials has to be done in order to reduce the cost. The main objective of the solar industry is to make the laboratory sodalime glass (SLG)/Mo/Cu (ZnSn)S /CdS/ZnO/ZnO:Al thin-film solar cell with the efficiency 2 4 of .15% and size of less than 1cm2 at initial stage that will lead to prototype thin-film solar cell module indicating that the laboratory technology will be trans- lated into industrial scale. The advantage of chosen chalcogenide-based thin-film solar cells is quite profitable to the mankind because it relays on low-cost and abundant Cu (ZnSn)S absorber. The size of prototype module can then be 2 4 increased to meter by meter size as an industrial thin-film solar cell panel. The low-cost thin-film solar cell panels with solar to electrical conversion efficiency of 80 Figure1.1 Estimatedcontentof Zn Earth crust content Cu,Zn,Sn,In,andGaintheearth 70 m) crust. p 60 p nt ( 50 Cu e nt o 40 c st u 30 cr arth 20 Ga E 10 Sn In 0 25 30 35 40 45 50 Atomic number Introduction 3 B13% or more can adequately be commercialized in the market. The research and development (R & D) supports to grow various stack layers as a sandwich as well as monolithic integration of cells for modules, which is a main constituent to the industry to address the technical problems during the fabrication of thin-film solar cellseitherinthelaboratoryorintheindustry. The motto of companies is to develop low-cost solar cells, which potentially mitigate over cost of electricity generated by present Si or CuIn1(cid:1)xGaxSe2 (CIGS) based solar cells. For example, the current cost of electricity .$1/W by thin-film solar cells is higher than B$0.37/kW of conventional electricity. A lot of compa- nies target to reduce solar power cost from present cost of $1 to 0.60 by 2014. Today, the laboratory CIGS thin-film solar cells lead to the highest efficiencies of 20.3% with an active area of 0.5cm2 made by Center for Solar-Energy and Hydrogen Research (ZSW) Company and 18.7% on glass and flexible substrates, respectively, as close to that of Si indicating that understanding of full depth of each layer in the sandwich of thin-film solar cells to some extent has been done [11]. The First Solar company took a decade to develop high-efficiency panel that the giant CdTe thin-film solar cells and their solar panels show efficiencies of 17.3 and 14.4%, respectively [5]. The Germany-based Avancis Company develops monolithically integrated CIGS-based thin-film solar cell panel with size of 30330cm2, which delivers efficiency of 12% and power of 30W. A number of panels connected in series produce power of 20MW in Torgau, Germany. The active area cell presumably produces efficiency of 15.5%. The scientists at Empa, the Swiss Federal Laboratories for Materials Science and Technology tout record efficiency of 18.7% on flexible substrates for CIGS by surpassing their own effi- ciency of 17.6%. Honda Soltec developed 13% efficiency CIGS thin-film solar cell panels. Several companies such as First Solar, Nanosolar, Globalsolar, Muosolar, Solopower, and Solexant have been immensely involving to develop and produce CIGS-based thin-film solar cell and mini-modules to target production of several gigawatt per year range around the world. The Ascent Solar Inc. Company devel- opsCIGSmonolithicallyinterconnectedthin-filmsolarcellsonflexibleplasticsub- strates with module aperture efficiency of 11.9% and module efficiency of 10.5% while Solopower Company made CIGS thin-film solarcell panel on the metal flex- ible substrates, which exhibits aperture efficiency of 11%. However, the In and Ga metals used for CIGS cells by these companies are expensive in the international metal markets in London. A brilliant new approach uses Zn and Sn or Ge in the place of In and Ga to mitigate cost of the materials. The energy generated by solar panels is obviously pollution-free whereas the electricity generated by coal thermal power plant or nuclear reactor produces pollution of carbon particles, such as CO , 2 as green house effect gases or radiation hazard. Recently, we have learned many lessons from the Fukushima nuclear reactor disorder due to Tsunami in Japan. On the other hand, solar energy creates more jobs and steady economic growth that is why the government and private sectors immensely involve to developing renew- ableenergyatlowercost. The quaternary p-Cu ZnSnS (CZTS) absorber is an excellent semiconductor 2 4 and a serious candidate for thin-film solar cells owing to suitable band gap of