30 Years Trajectory of a Solar Photo Voltaic Research

8/6/2019 30 Years Trajectory of a Solar Photo Voltaic Research

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30YEARS TRAJECTORY OF A SOLAR PHOTOVOLTAIC RESEARCHYoshihiro Hamakawa

Deparrment of Photonics, Faculty o f Science and Engineering, Ritsumeikan University1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577 Japan

Te1:+81-77-561-2871 Fax:+81-77-561-2613e-mail:[email protected]

ABSTRACTA review is given on a research trajectory of the solar

photovoltaic PV science and engineering in recent 30 years.Firstly, an episode of starting the PV research related to theenvironmental issue is confided, then briefly overviewedwith a story how it started the Sunshine Project in 1973.Secondly, a motivation to start the amorphoussemiconductor research is explained to fabricate a syntheticsemiconductor which could be design the band gap energy.As an example of the material processing, fabrication of theternaly Si-As-Te amorphous semiconductor in the space bythe Space lab J and also amorphous Silicon alloys areintroduced together with their essential advantages of thematerial system in view of both physics and technology asnew type of physics semiconductors. Thirdly, as anapplication of the amorphous silicon material system, aseries of high efficiency R & D efforts are demonstratedsuch as inventions of the a-SiCia-Si heterojunction solarcell and of the stacked solar cells having the junctionstructures of a-Siiipoly-Si, a-Siiinc-Si etc. .. In the final partof the paper, some new strategies to develop photovoltaicindustry in Japan are introduced. The project milestone upto 2010 in the new Sunshine project and present status ofthe industrializations are summarized and discussed.1. MOTIVATION TO START PVRESEARCH

In November 1972, I had an oppomnity to stay at theUniversity of Arizona, Tucson Arizona for attending theI International conference on Modulation Spectroscopyfor a week[l]. 1 delivered there an invited review talk onAn Oscillatory Franz-Keldish Effect and ModulationSpectroscopy which I established measurement system asan useful new tool for the Characterization of the electronichand structure parameters of semiconductor which wasdone mostly at the University of Illinois from 1965 to 1967

The tale begins at a beautiful evening in the poolsidegarden party of Professor B. 0. Seraphins home. In themiddle of dinner party, I found a bright moving flightobject acrossing the twilight sky, and pointed out what isthat? Oh, that is an artificial satellite immediatelyanswered the person wh o sat with next seat of my chair. Hisname is Professor Aden. B. Meinel, the director of the

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Optical Science Center, University of Arizona, and he isfamous Astronomer as the discoverer of the Meinel nebula.Then, he explained many things about the newastronomical observatory of the Kit Peak, and mostimpressive one was an increasing of the sky hazy in recentseveral years by pollutant gas, even in Tucson Arizona. Wehad enjoyed so exciting discussions about environmentissue with the promotions of industrial development inrecent 10 years. Finally he suggests me that that pleasedevelop more efficient solar energy conversion technologyby semiconductor which should be suhstihlted to fossil fuel.Next morning, He kindly gave me, a kit of his publicationson quest of solar energy [3].

On the way back to Japan, I studied very hard toreview the materials in the flight seat, and published somereview paper entitled Solar Electric Power Generationand A New Role of Semiconductor Technology -Challenges to the New Energy - on the Solar PV, and I Ion the Solar Photo ~ Thermal conversion [4].

2. THE SUN - SHINE PROJECT ANDNEW SUNSHINE PROJECT

In relation to the State of the Union Message byPresident Nixon in January and 1973, Japanese AIST, MITl(Agency of Industrial Science and technology, Ministry ofInternational Trade and Industry), had proposed to organizea new initiative namely Sunshine Project (SS) in May

Fig.1 3E-Trilemma, the most important task assigned to21 centurys civilization. Only way to solve it is developclean technoloav

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..............................New hwhw RojatBy1560

..................................F i g 2Project Orgnnizations

and objectives of the Ne w Sunshine

1973, and formally started the project with budget in April1974. While the first oil shock was in the Middle Eastbegan October 1973. The main purpose o f the proposal is tobring back clean air and water to our land by new kinds oftechnological development which is presently said - asolution of 3E-Trile'nma as shown in Fig.1. That is, theregular way along capitalism, for the activation ofeconomical development (E : Economy), we do need anincrease of the energy expense ( E Energy). However, itinduces environmental issue ( E Environment) by moreemissions of pollutant gases. On the contrary, if th epolitical option choses a suppression of pollutant gasemission, it inactivates the economical development. Thisis 3E-Trilenma. The solar photovoltaic (PV) technologywas involves in the SS project, and R & D'of the low costsolar cell materials with high efficiency solar cells weresettled as the first 10years subproject.Considering the two sided nature of the energystrategy, that is, even, in &e period of 1980, continuousgrowth o f mass consumption of the limited fossil ,fuels oneside, and becoming severe the global environmental issueon the other side, AIST in MITl in Japan has decided toestablish "New Sunshine Program" for the development ofclean energy technology and environmental technology.Figure 2 illustrates a comprehensive structure of the newprogram an d its relation to 1he;Sunshine Project which w asformulated in May 1973, prior to the first energy crisis, andstarted in 1974, the Moonlight Project which was initiatedin 1978 for the energy saving technological development,and also environmental technology project progressed from1989[5]. The past injected budgets are also inserted in thefigure. While the new program consists of three parts; a)Renewable Energy Development Technology, b) HighEfficiency Utilization of Fossil Fuels a nd Energy Storage,and c) International Energy Cooperation, so-called WENET(World Energy Network) utilization hydrogen fuelproduction technology by PV and distributions with a widearea energy utilization network system namely as"Eco-energy City".

3. AMORPHOUS SEMICOND UCTOR RESEARCHFOR LOW COST SOLAR CELL PRODUCTION

In the several years from 1965, the band structureparameters of more than ten kinds of new semiconductorswere characterized by the modulation spectroscopy, andcomputerized measurement system on the energy spectra ofoptical absorption and reflectance were well established inthe year period of I970lh. Then next s tep of my researchdream was focused on an attempt to realize a syntheticsemiconductor, which has a controllability of bandparameters, for example, energy gap Eg. The experimentalattempt had been realized by a ternary Si-As-Te amorphoussemiconductor in the micro gravity environment by the TT50 0 A Rocket [6] and FMPT (First Material ProcessingText) in the Space lab J [7]. This Si-As-Te amorphoussemic ondu ctor system has various interesting p oints inviews of both basic physics and technological applications.Since the Si-As-Te system consists of mixed combinationso f IV-Ill-I1 hedral bonding, it has a large structuralflexibility to construct a high possibility o f random network,which yields a very large glass formation region with awide physical constant controllability a s shown in Fig. 3.For exa mple, the energy gap can be controlled continuouslyin the range from 0.6eV to 2.5eV, which covers the energygap s of conventional crystalline semiconductors; 'G e(0.66eV), Si (I.OeV), &A s (1.43eV) and GaP(2.25eV).

Figure 4 show s an effect of Ni doping on the electricalconductivity of Si9As,4Te21. dramatic enhancement of theelectrical conductivity, more than 7 orders of magnitude, isseen for the FMPT material, while an increase of only 3orders on terrestrial material. Also found is almost zeroactivation energy for the doped FMPT material, whichcontrasts highly with a slight decrease (25%) in theactivation energy upon doping in the terrestrial material. Inboth cases, a change in the conduction type from p-type(hole in undoped material) to n-type (electron in Ni dopedmaterial) is confirmed by means of thermoelectric power

Si

Fig.3amorphous semiconductor in the Gibb s friungle.Eqqui-energv gap confour lines of the Si-As-Te

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0.772svd3 3.1 3.2 3. 3 3.A

ReciprecalTemwrJIura l r r ( r lo 3 M )

Fig.4 A comparison of the doping eflciency by Nidoping on both terrestrial and micro gravity (FMPT)made Si-As-Te7].measurement. Such large existence of the effect of dopingon electrical conductivity has never been observed formelt-quenched chalcogenide glass system. This high dopingeffiency in the FMPT sample implies either a lower nativedefect density or a greater dopin g efficiency of Ni additives.More detailed analyses and theoretical consideration on thisitem are discussed in OU previous paper [7]. All theexperimental results suggest the realization ofhomogeneous amorphous network structure with aminimized disorder by the material processing in a micragravity environment. The material processing in spacewould contribute largely to the physics of disorderedmaterials as well as to the development of new materials forsemiconductor electronics.

As has been mentioned above, the valency electroncontrol to the Si-As-Te is q uite difficult in principle d ue tothe shuctural flexibility in chalcogenide. In 1975, abeautiful success of valency electron control to amorphous

silicon (a-Si) has been reported by Spear and Lu Comber[SI.AAer ow own preliminally investigations on a-Si 191.we have started a series of basic research such a s depositioncond ition, valenc y electron control in a-Si, a-SiC and theirmicrocrystallines ( c) .

As has been reported elsewhere, a-Si alloys can hedeposited onto any inexpensive substrates with lowtemperamre less than 300C. Moreover, it is possible toform very wide area thin films due to a vapor phase growthonto non-crystalline substrates. Another noticeable propertyof these a-Si alloys is existence of valency controllahilityby doping of substitutional impurity atoms with mixing ofimpurity gasses such as B2H6 and PH I, etc. Therefore, inview of production sequence, this material system has anexcellent massproduceability with large scale merit.Significances of a-Si alloys as the solar cell materials aresummarized in Table 1.

As the preparation technology for a-Sic alloy, theplasma CV D is now widely utilize every where. While ECRC W and Ion-heam CVD (1s-CVD) are intensivelyinvestigated in a recent few years. The un ique advantage ofthe ECR CVD is that the growing surface receives almostno bombardment damages by electrons an do r other heavyspecies soft landing having an energy of several tens of eV[ IO ] . This effe ct might result not only in prevention of we akbonds from heiug introduced into the network but alsosuppression of the diffusion of long lifetime radical spe ciesdue to the raised surface temperature. It is expected thatfilms with dense network and low defect density areformed.

For the deposition of a-SiC and U c-Sic, hydrogen isused as an ECR plasma excitation gas, with a mixture ofS i b , CHI and B2H6 or PH, are usually employed as areaction gas for the growth of p- and n-type SiC:H. Detailsof the preparation cond itions are reported in reference [ l l ] .Since the operation pressure is in the range of IO and IO4Torr, the lifetime o f chemically active hydrogen radicals arequite long, so that a large amount of hydrogen radicals willreach the growing surface and play an important role in

Toble1. Significances of a-Si:H alloy as a so/ar cell materialA Physical Properties B Fabrication Technologya-1) Excellen t photocond uctivities with b-1) Wide area thin film

high absorption c oefficient far sun light (Plasma CVD. ECR, CVD. etc)a-2) Low dark conductivitya-3) High stru ctura l sensitivitya-4)

(Valency electron controllability)Wide range of energy gap controllabilityI.OeV < EO < M e Va-SiGe - a-Si - a-Sic(Relatively low interface states)(because of amorphous network)

a-5) Easy to make heterojunc tiona-6) Mechanically strong

b-2) Low temperature depositionb-3)b-4)

100C < Ts < 380Cp-n control can be accomplishedonly by mixture gas regulationsCan be deposited on any inexpensivesubstrates

b-5)b-6)

Easy to apply integration technology(HJ. super lattice, tandem cells)Lo w cost and good massproduceability

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Fig.5 Dependence o optical energv gaps and darkconductivitier o p and n- type pc-SiC:H on hydmgendilution radio in reaction gosser in ECR CVD [I987 by D.Knrngom et 01.. r e f l o ]

determining the properties of growing films. Therefore, thedependence of the material properties on the hydrogendilution ratio in the reaction gas has been investigated.Figure 5 shows the dependence of the optical energy gapand dark conductivity of the samples on the Hz dilutionratio. As the ratio increases, the optical gap (Eo) and alsothe dark conductivity ( a of both p- and n-type filmsincrease.

As can be seen from Fig.5, there are two main factorswhich determine the optical energy gap; one is thecomposition ratio of S i:C H corresponding to the source gasratio CH JS ih , and the other is the grade of Hz dilutionwhich might be related to the details of the networkstructure. The film properties are strongly dependent notonly on the substrate temperature and microwave power but

also on the ratio of hydrogen to reaction gasesH2/(Ch+Si&) during deposition. Although the opticalenergy gap increases with the flow rate of CHa, the effect isnot as remarkable as the dependence of hydrogen dilution.Hydrogen dilution has the effect of reducing the hydrogencontent in the film, and also of enhancing the degree ofmicrocrystallinity.

The formation of Si and Sic micro crystallites areconfirmed by Raman spectra as mentioned in the originalwork [ IO]. The Raman spectrum of the films prepared atmicrowave powers higher than 250 Watts exhibits distinctstructures at around 520 and 74 0 cm?, w hich correspond toTO phonon'modes crystalline Si and S i c clusters.

4. R & D E F F O R T S T O P R O DU C E HIGHEFFICIENCY WITH LOW COST SOLAR

C E L L STh e first application to solar cell with a-Si:H has been

achieved by RCA group in 1977 with a structure ofSchottky barrier type [12]. Since we have concentrated ourattention on the valency electron control in a-Si:H, the p-i-njunction type solar cells are fabricated in 1978 [13]. Figure6 shows a picture of laboratory made solar cells in 1978including and integrated type high voltage cell whichdirectly applied to Solar Calculator by Sanyo Electric Co.in 1980. With an invention of a-Sic by the author's groupin 1980, an 8% efficiency barrier, which was proposed bythe photovoltaic system as the feasibility line for the powerapplication, had been broken through as 8.25% obtained in1981 and become 9.39% in 1983 as shown in Fig.6 [14].With a-Si:H solar cell, 13.2% efficiency was reported byMitsui Chemical in 1993.

In 1979 another new concept to imp rove a-Si:H solarcell efficiency has been proposed by the author's group[ IS] , that is, a multi-band gap stacked solar cell. A series of

Fig.6 Laboratory-made early stage a-Si:H solar cells [I978 by H. Okamoto, I:Nitta, and I:H a m a h w a , rex151

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:r, :*. ,.., ... _. .

, . . ...> -v Iv :

Fig. 7 Phoiovo l to ieper/*~~ances.unction stmcfureondj:Vchoraeterisrics o/a-SiC/cl-Si heierojuncrion Solorcells (U) and0).ornporing with tho1 o/ordinoq 0-Si homojunction solar. .ell (c). , , .

. . I . .systematic investigations wa s initiated on. the optimumdesign theory with s elections of material combinations [16].It has been suggested from the result that the structure ofa-Si:W/poly-Si is the .bes t comb ination., For , the , two,-terminal a-Si;H solar cell stacked with poly-Si [ Il l, .aconversion efficiency ,of 15.04% with Voc=1.478V,lsc=16.17mAicm2 and FF=63% has,'beeh obtained underA M I illumination: Figure 8, sh ow s the structure and anexample of output characteristics of a proto-type 4-terminalstacked cell having 21% efficiency [17]. Along with thisconcept, wide 'varieties of experimental trials are inprogress on the multi-band gap stacked solar cells. For'example, on large area (1 200 cm*) a-Si:W a-Si:H attackedsolar cells, the efficiency of 10% is reported by FujiElectric [18]. Recently, the band profiling design study bycomputer simulation has also been made as an optimumdesign of the am bipolar carrier transport in' i-layer of themulti band'gap junction.'As an 'experimental trial o f, heband profiling sh ldy a-SiC:H/ a-Si:W a-SiGe:H triple band

. .

Icm' Hybrid cellAMl.5! ,2YegKaneka Dual-lightsource simulatorJsc: 14.4mN.cm2Vac: 1.41VF.F.': 0.728eff.: 14.7% . .

. .

1 : . : . I . : . : . . I I :. I i0 0.5 1.0 1.5

: Voltage(V). , . .

Fig.9 , V-I .characteristic. of the a-Si/ 0 -Si two iterminal stacked solar cell rep~orredby Kcneka gmup ,,.. . . .[20l , , , , . . .gaptandem solar cell has been made by Sharp group, andan initial conversion efficiency of 12.4% has been achieved

Another important idea to realize the'next generationsolar cell is continuous ail plasma CV D production system,noto nlyth e a-Si:H top cell but also JL c-Si the bottom cell..A tremendous R & D effort has been inprogress. Figure 9shows an example ,o f recent top data repor ted by kmeka,(Kaneka'hchi Chemical Industry) group 14.7% efficiencyfor R.& D . lcm2 cell area [20], and also 11.6% efficiencyfor'large area mass production module [20]. Canon has alsoreported l4:49 % for R & D small cell and 1 3.37% for massproduction module w ith a-SGp c- S i/ i c-Si three tandems&ic tuk cell [ZI]. Kaneka has ,recently started the, mpss:production of , this structure solar cell module with aproduction s cale of ZOMWiyear in 2003.

~ 1 9 1 . . .

, . .

.. . .. . . . . . ..5. NEW STRATEGY AND KEY ISSUES FOR PV .. , . . INDUSTRIALIZATIONIn spite of various advantages in photovoltaic.power

generation as . , entioned above, a big barrier impeding the

Fir.8 Cell shuchlre (a )and V-Icharacter is t ics(6)of a-Si:H//poly-Si 4 terminol stacked solar cell by Osaka Universi?,[U]

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expansion of large-s cale powe r source application was thehigh price of solar cell module, which was more than$30/W p (peak watts) in 1974 . There fore, the cost reductionof the solar cell is of prime importance. To achieve thisobjective, tremendous R&D efforts have been made in awide variety of technical fields, from solar cell material,device smcture, and mass production processes tophotovoltaic systems over the past 20 years. As a resultof recent. fifteen y ears R& D efforts, about one order ofmagnitude price decrease has been achieved, and nowcome down less than $ 4 N p in a firm hid for the large scalepurchase.

In December 1994, a new initiative of Japanesedomestic renewable energy strategy, so called.Fundamental Pr inciples to Promote New EnergyDevelopments an d U tilization has been identified by theCabinet Meeting. Related action planning by law such astax reduction, government subsidy etc. are approved by theCongress on April IO , 1997. The strategies are applied tono t only whole ministries and governme nt offices but alsolocal g ov em e nt authori t ies and pr ivate enterpr ises.

With this government new policy, development andpromotion of PV technologies have been complied as themost promised project. An integrated installation volumeof 4OOMWp PV modules by FY2000 and 4.82CWp byFY2010 for Japanese dom estic use are sched uled as a milestone in the program. A special regulation of tax reductionfor investment to the renewable energy plant, a governm entfinancial support of 1/2 subsidy on the PV system forpublic facilities so called PV Field Test Experiments, 113subsidy for the private solar houses as the PV HousePlan ning o f the field testing etc., are in progress. Num berof government subsidy accepted PV house increases

REFERENCES[111 international Cant on Modulation Speetrorcopy, University ofArizona, Tucson,Arizona (Nov.1970)[Ilsee far example, Y. Hamakawa and T.Nishino: Reccnt advancer inmodulation spec~ oscop y hspt.6 of Optical Pmpenics of Solids -New Developments, Ed. by B. 0. Seraphin, N o h Holland Pub. Co.(1976) pp.255-353P I A . B.Meincl: Physics Today (1972) p.44 and many othm.14lY. Hamakawa: A New Role o f Semiconductor Technology ~Challenges IO Efficient Solar Energy Utilizations 1 EIeermnicMoreriols (1974) pp.524-529. I1 bld (1974) pp.530-534.[SIK. Miyarawa, K. Kato, K. Kawamura: Repan of Solar EnergyDivision Meting, Technology & Industrial Council, MlTI March(1997)pp. 1-8.[6]Y. Hamakawua: Ceramics, 22 (1987) p. 277. [in apanese][7] Y. Hamakawa. W Shams-Kolahi, K. Hanori, C. Sada and H.Okamoto: Journal ofApp lied Microgravity vol. I2 (1995) pp. 27-37.181W. E. pear and P G LeComber, Philos. Mag. 33 (1976) 935.191 H. Okamoto, Y. ina, T. Adachi and Y. Hamakawa, Surf Sei. 86(1979) 486.[ I O ] Y. Hamakawa,Y Malsumoto, G Hiram and H. Okamoto, MUSSymposium 164(199 1)pp.823-829.

300 2 0IProgress of PV Module Productton tn Japan 14 1200 Utility Power Use

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3rd World Conference on PhorovolroicEnergy Conversion May 11-18.2003 Osokn.Japan

0

95 96 97 98 99 00 01 02Calendar Year

Fig. I O Annual Pmducrions ofsolar Cell Module.doub ling year by year. On the other hand, in the PV FieldTest Experiment totals of 259 sites with 6.84 MW has beeninstalled during 7 years. As the result of acceleratedpromotion strategy, in fact, sales price for the 3kW solarphotovoltaic system for private house decreases verysharply about one third for example, from 2M TkW in 94down t i 0.72 M W W in 02.

Figure 10 shows transitions of the solar cell moduleannual production in Japan since 1995 as surveyed by PaulM ay cw k [221. As can be seen from the figure, aremarkable increase of the annual production has been seensince starting of the New S unshine Project in 1993.

Let us embark on a new energy revolution withclean-energy photovoltaics. Might it be possible toaccomplish the clean-energy revolution within the next 25years? It would be a worthy challenge, no longer simply afanciful dream.

[ I l l W. Ma , T. Horiuchi, M. Yorhimi, K. Hanari, H. Okamoto. Y.Hamakawa, h o c . PVSEC-6 (New Delhi, 1992) p. 463.[12]D. E. Carlson and C. R. Wronrki: Appl. Phys. Len.28.671 (1977)(131H. Okamoto, Y. Nina. T. Adachi and Y. Hamakawa, SurfaceScience86(1979)pp. 486-491.[I41 Y. Hamakawa. K. Fujimoto,S. Nonomura and H. Okamoto: Newtypes of high efficiency solar cell based on a-SiAppl. Phys. Len. 43(1983) pp.644-646.[ISIY. Hamakawa. H. Okamoto and Y. Nina: Appl. Phyr. Len.35, IS(1979) and Japanese Patent Sho-54-32993 and U.S. Patent 4,271.328.March 20, 1979.[16]H. Takakura, Jpn. J.App l. Phys.31 (1992) 2394.[17]W. Ma , T. Honuchi, C.C. Lim, M. Yoshimi. H . Okamota and Y.Hamakawa. Roe. 23d IEEE PVSC, 1993, p.338.[18]V.IchikawaandH.Sakai,Pmc.WCPEC,Hawaii(1994)p.441[19]Y. Nakata, S. Monguchi, Y. noue, K. Nomoto, A. Yokota, M. ltohan d T.Truji: Optoslectronics- Devices andTechnol. (1990)-5,2 09[20]K. Yam amoto and Y. Tawada. Tech. Dig. WCPEC-111to be reponed(2003) S20-B9-03 and 5PL-DI-03.[21] K. Saito and K. Ogawa: to be reponed in WCPEC-Ill (2003)50-D14-04.[22] Paul Maycock PV NEWS vol. 22, No.5 May (2003) pp. 1-3.

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30 Years Trajectory of a Solar Photo Voltaic Research