Technological aspects of flexible CIGS solar cells and modules
Friedrich Kessler
a,*
,Dominik Rudmann
b
a
Zentrum fuer Sonnenenergie-und Wasserstoff-Forschung,ZSW,Industriestrasse 6,70565Stuttgart,Germany
b
Swiss Federal Institute of Technology (ETH)Zurich,Laboratory for Solid State Physics,Thin-Film Physics Group,Technopark,
8005Zurich,Switzerland
depositionReceived 17December 2003;received in revised form 14April 2004;accepted 26April 2004
Available online 25May 2004
Communicated by:Associate Editor T.M.Razykov
Abstract
This paper describes the technological status of and some challenges in the manufacturing of Cu(In,Ga)Se 2(CIGS)-based solar cells on flexible polymer and metal substrates.Substrate characteristics such as thermal expansion prop-erties and stability,surface roughness,or substrate composition,strongly influence growth and properties of the following layers.For example,adhesion failure,cracking,or contamination by diffusion of impurities from the sub-strate may occur with some substrates.Aspects of (external)sodium incorporation into CIGS are discussed for high and low CIGS deposition temperature.Low-temperature deposition processes are particularly important when polyimide substrates are used.The electrical insulation of metal foils by dielectric barriers (e.g.SiO x or Al 2O 3)allows the fab-rication of monolithically integrated modules.A soft and selective patterning technique based on laser scribing and mask-free photolithography is described.Working modules as large as 20cm ·30cm were achieved with these methods.
Ó2004Elsevier Ltd.All rights reserved.
Keywords:Flexible;CIGS;Cu(In,Ga)Se 2;Solar cells;Modules;Monolithic cell integration
1.Introduction
The use of flexible substrates offers new possibilities for the application of solar cells,for example for building integration by application on uneven surfaces such as tiles.In addition,flexible cells are very thin and lightweight,which makes them also more flexible in use than rigid cells.Also for space applications,flexible solar cells are very attractive,since simpler deployment mechanisms can be used,which saves weight and therefore reduces launch costs significantly.In addition,CIGS (also CdTe)solar cells show good stability under and recovery from electron and proton irradiation,
whereas,for example,high-efficiency Si or GaAs solar cells are more prone to radiation damage (Jasenek et al.,2001;La Roche et al.,2000).Another advantage of flexible solar cells,and maybe the most important one,is the potential to reduce production costs.Roll-to-roll deposition is considered more favourable from a pro-duction point of view than processing with rigid (glass)substrates.Furthermore,most of the energy and cost required to produce CIGS solar cells on glass is used for the substrate and cover glasses.A low-cost,glass-free,and thin substrate together wi
th a thin and flexible en-capsulant,would combine the advantages of flexible solar cells and cost-effective production.Finally,envi-ronmental benefits arise from the expected lower energy-pay-back time and reduced module weight if mounted on cars,caravans or RVs.Smaller batteries (e.g.for cars,consumer products)can be used and battery lifetime will be enhanced if recharged and supported by PV sources.
*
Corresponding author.Tel.:+49-711-7870-201;fax:+49-711-7870-230.
E-mail address:friedrich.kessler@zsw-bw.de (F.Kessler).
0038-092X/$-see front matter Ó2004Elsevier Ltd.All rights reserved.
doi:10.1016/j.solener.2004.04.010
Solar Energy 77(2004)
685–695
www.elsevier/locate/solener
2.Substrates
Today,the most commonly used substrate for the manufacturing of CIGS-based solar modules is soda-lime glass(SLG),which allowed champion efficiencies of up to19.2%(Ramanathan et al.,2003).This low-cost and well-known material is fabricated on a large scale and with reproducible quality mainly for the window industry.The demands on a proper CIGS substrate are manifold:
•Vacuum compatibility.The substrate should not de-gas during the various vacuum deposition steps,espe-cially during CIGS deposition,when the substrate must be heated.
•Thermal stability.For the growth of high-efficiency CIGS absorbers,the substrate temperature should reach500–600°C during at least a part of the depo-sition process.Substrate temperatures of less than about350°C usually lead to severely degraded ab-sorber quality and cell performance.Therefore,sub-strates should withstand temperatures exceeding 350°C.
•Suitable thermal expansion.The coefficient of thermal expansion(CTE)of the substrate must lie in the range of the CTE of CIGS(see Table1),otherwise CIGS adhesion problems may be encountered.Addi-tionally,cracking of the Mo back contact can occur due to its comparably low CTE(see Table1).•Chemical inertness.The substrate should not corrode, neither during processing nor during use.In particu-lar,it should not react(strongly)with Se during the CIGS deposition process or decompose during aque-ous solution deposition of buffer layers(CdS).Also,a
good substrate should not release any impurities that can diffuse into the absorber,except when this is explicitly desired(such as in the case of Na diffusing from SLG).
•Sufficient humidity barrier.The substrate should pro-tect the active solar cell layers during long-term oper-ation against environmental attack from the back,
vapour.
•Surface smoothness.A smooth substrate surface is re-quired for two reasons.First,abrupt changes in the surface topography such as spikes or cavities may lead to shunts between the front and the ba
ck con-tact.Second,the deposition of impurity diffusion barriers or insulation layers may be easier and more successful on a smooth substrate.
•Cost,energy consumption,availability,weight.Obvi-ously,the ideal substrate is cheap,requires little en-ergy for its manufacture,consists of available and abundant materials,and is lightweight.These points are of course correlated with the thickness of the sub-strate.
Evidently,SLG is the most interesting and thus preferred substrate material for an industrial manufac-turing of rigid CIGS-based modules since it fulfils most of these requirements excellently.Additionally,the re-lease of sodium has a beneficial effect on the CIGS quality,superseding the need for any external Na.Since glass substrates are also available at rather low thick-nesses(down to30l m),even the fabrication of light-weight modules on glass is possible.The main,but grave,disadvantages of glass substrates are their high brittleness and their non-flexibility which restrict the
Table1
Properties offlexible substrate materials,active solar cell,and barrier layers
Material CTE(10À6KÀ1)T s;max(°C)Notes
SLG9(20–300°C)$600Standard glass substrate,contains Na,K,etc. Corning7059 4.6>600Alkali-free glass
Cr steel10–11>600Diffusion of Fe,Ni,Cr,etc.;low-cost substrate
Ti8.6>600Low Ti diffusion,restricted Ti purity
Ni/Kovarâ5–11>600CTE can be well-matched
Al23–24600Low cost,low weight,very high CTE
KaptonâE17(20–200°C)<500a Polyimide
UpilexâS12–24(20–400°C)<500a Polyimide
ETH-PI3<500b Polyimide b
Mo 4.8–5.9(20–600°C)>600Back contact
CuInSe211.2–11.4/7.9–8.6(20°C)^c-axis/k c-axis
ZnO 4.75/2.9^c-axis/k c-axis
SiO x1–9>600Insulation layer and/or diffusion barrier
Al2O36–8>600Insulation layer and/or diffusion barrier
CTE is the coefficient of thermal expansion,and T s;max denotes the approximate maximum substrate temperature that can be used during CIGS deposition.
a The applicable maximum substrate temperature depends also on the exposure time.
b Removable polyimide layer spun on a glass carrier.
686 F.Kessler,D.Rudmann/Solar Energy77(2004)685–695
application areas considerably.The numerous benefits arising from the availability of thin,lightweight,flexible, and efficient modules warrant the efforts for evaluation of novel(non-glass)substrates suitable for roll-to-roll production.
The influence of the selected substrate material,its surface morphology,and pre-cleaning on the properties of the adjacent active layers andfinally on the solar cell performance are tremendous and––even for glass––only partially understood.
2.1.Flexible substrates
Prime candidates are metals or polymers.The main criteria for the selection of a certainflexible substrate are well-matched 5–12·10À6KÀ1),sufficient thermal(T P400°C)and chemical resistivity(especially against Se),suitability for roll-to-roll deposition,and costs.There is a variety of suitable metals,especially if only physical and chemical requirements are taken into account.Among the tested metals are austhenitic and Cr–steels,titanium,molybdenum,aluminium,and some alloys.Especially Cr–steel(low cost)and Ti(low weight) seem to be appropriate candidates,whereas aluminium foils and austenitic steels failed during the CIGS co-evaporation process due to their too high CTEs and subsequent CIGS adhesion problems.The advantage of metal foils is the high mechanical stability and thermal stability exceeding600°C.On the other hand,they need a diffusion barrier to suppress impurity diff Fe)into the absorber.For the case of monolithically integrated modules,an electrically insulating coating is also required,but this may be the same layer as the diffusion barrier.The demands on the diffusion barriers and dielectric insulation layers increase for higher sub-strate temperatures due to higher diffusion rates and increased thermal stress,therefore low-temperature CIGS growth processes can be interesting also for metallic substrates(see Section4).
Commercially available metal sheets or foils nor-mally show a distinct surface texture(roughness)arising from the rolling procedure during the foil production process.In order to get rid of these cavities and spikes, the foils often have to be either polished or coated by a levelling layer.Normally,however,shunts are rather due to abrupt changes(steps or cavities)in the surface than by the average roughness.
Certain polyimides(PI)are the only commercially available polymers that withstand temperatures of400°C or more.Polyimidefilms are lightweight,insulating, and have smoother surfaces than metal foils,but suffer from reduced thermal stability and comparatively high CTE.Only few of them exhibit CTEs below30·10À6 KÀ1and are compatible with vacuum processing at the same time.Typically,the CTE of PIs increases at tem-peratures above their glass transition temperature,which usually lies well below their decomposition tem-perature.Therefore,for some PIs the CIGS deposition temperature may be limited by thermal expansion mis-match rather than by thermal stability.Commercially available PIfilms suitable for CIGS deposition UpilexâS and KaptonâE,both with high decomposi-tion temperatures exceeding500°C and relatively low CTEs compared to other PIs(see Table1).
Both metal foils and PIs are generally Na-free,which calls for external Na addition.This necessity ma
y be a disadvantage in terms of process simplicity,but can be an advantage in terms of homogeneity and therefore reproducibility.Furthermore,Na-free substrates may be desirable anyway for low-temperature CIGS processes, where Na addition after CIGS growth may be beneficial (see Section5).
3.Barrier layers on metal substrates
Metallic substrates such as stainless steel(Cr–Steel, SS)or Ti offer the possibility to deposit the CIGS ab-sorber at temperatures similar to or even higher than those used for high-quality CIGS on SLG at T s;max P550°C.A high deposition temperature has a beneficial effect on absorber growth,but,on the other hand,the rates of undesired substrate elements diffusing into the growing CIGS layer increase.The exact influ-ence of a certain element or a mixture of them on cell performance is not very clear to date.Furthermore,the impact may depend on deposition parameters such as the CIGS coevaporation 3-stage or2-stage),the temperature profile and the duration of the high-temperature step.Solar cells fabricated on very pure foils of titanium or chromium work even without any barrier layers since Ti and Cr seem to have no detrimental effects on the conversion efficiencies,at least if the diffusion rates into the absorber are comparably low(Hartmann et al.,2000).The costs of commercially available metal foils are,however,lower for substrates with a lo
wer purity level.Even grade1titanium has a purity of only99.5%Ti,whereas stainless steel is much less defined.The composition of Cr–steel,which is one of the preferred candidates at ZSW,is81wt.%Fe, 17wt.%Cr and traces of Mn,Si,C,P,S.Thus81%of the Cr–steel substrate consists of an element which is known to have a very detrimental effect on absorber quality.In order to become independent of the chemical composition of the substrate material,diffusion barriers SiO x,Al2O3or Cr can be applied(Herz et al., 2003)which effectively reduce contamination by Fe and other elements.
The fabrication of monolithically integrated modules on a conductive substrate requires the electrical insula-tion by a thin andflexible dielectric layer.These require-ments are much higher than for a diffusion
F.Kessler,D.Rudmann/Solar Energy77(2004)685–695687
insulation layers are normally good diffusion barriers but not vice versa.They should survive high tempera-tures in Se atmosphere without crack formation and withstand all patterning steps without losing the insu-lating properties,all on a large area.Sufficient insulating qualities were achieved at ZSW by ca.3l m thick SiO x and Al2O3layers which were deposited by sputtering (SiO x,Al2O3),by pl
asma-enhanced chemical vapour deposition PE-CVD(SiO x),or by sol–gel deposition (SiO x).Pinholes often result from the formation and precipitation of particles during the barrier growth. Particles should be prevented in any case by an accurate pre-cleaning procedure for the substrate and/or the sputtering or CVD reactor.The quality of the dielectric was tested on commercially available unpolished Cr–steel,Ti,and Kovarâfoils by measuring the resistivity before and after the CIGS process and by the applica-tion of a galvanic method which is able to detect pin-
holes quantitatively via the generation of bubbles. Details are described elsewhere(Kessler et al.,2001, Herrmann et al.,2003).The PE-CVD-deposited SiO x layer was grown by microwave decomposed hexame-thyldisiloxane diluted in a high amount of oxygen.The dip-coated SiO x sol–gel layer offers the possibility of an intrinsic Na doping of the absorber by adding Na to the sol–gel liquid.The minimum insulator thickness is determined by the desired breakdown voltage and the surface roughness(homogeneity,scratches,defects).A total thickness of about3l m was a good compromise as thinner layers showed a higher probability for pin-hole formation whereas thicker barriers have the tendency to form cracks.Best small-area cells(0.5cm2)with13% efficiency were fabricated in the in-line CIGS reactor at ZSW under standard conditions on metal foils insulated by a double layer of SiO x:Na/SiO x deposited by sol–gel and PE-CVD.
The scanning electron microscopy(SEM)micro-graph of Fig.1shows the cross-section of a solar cell grown on a Cr steel substrate which was covered by a1-l m-thick SiO x layer.Obviously,there is a highfilm adhesion between SiO x/Mo and CIGS/ZnO,but a lower adhesion between Cr–steel/SiO x and Mo/CIGS.
4.CIGS co-evaporation at low temperatures
Polymeric substrates restrict CIGS growth tempera-tures to about450°C due to their limited thermal sta-bility.However,low-temperature CIGS deposition processes can also be interesting for metallic substrates, since impurity diffusion from the substrate during growth may be easier to suppress and since the demand on layers with differing thermal expansion coefficient (insulation layers or diffusion barriers,back contact) decreases.Furthermore,lower substrate temperatures reduce energy consumption during processing and hence module cost.The lowest temperatures used for the synthesis of CIGS absorbers using physical vapour deposition(PVD)are usually above300°C.Laboratory cell efficiencies exceeding9%were achieved on SLG substrates using substrate temperatures as low as310°C (Bodeg ard et al.,2000).Several groups have
reported Fig.1.SEM cross-section view of a solar cell on a Cr–steel substrate.
Table2
Selected AM1.5efficiencies(g)of small-area CIGS solar cells prepared at low substrate temperatures at different laboratories Laboratory T sub(°C)g(%)Na incorporation method AR a Reference
Uppsala Univ.3109.1Diffusion from SLG?Bodeg ard et al.(2000) Matsushita/Ryukoku Univ.35012.4Diffusion from SLG No Nishiwaki et al.(2001) ZSW40011.7Na coevaporation towards end Lammer et al.(2001) IEC,Univ.Delaware40012.8Diffusion from SLG Yes Shafarman et al.(1997) ETH40013.8Post-deposition in-diffusion No Rudmann et al.(2004a) Uppsala Univ.42514.3NaF precu
rsor?Bodeg ard et al.(2000) IEC,Univ.Delaware45013.5Diffusion from SLG Yes Shafarman et al.(1997) ZSW45014.1NaF precursor/SLG No Kessler et al.(2001) ETH45014.4Post-deposition in-diffusion No Rudmann et al.(2004b) Ryukoku Univ./Matsushita45014.8Diffusion from SLG No Wada et al.(2000)
a With anti-reflection coating.
688 F.Kessler,D.Rudmann/Solar Energy77(2004)685–695
promising efficiencies between12and15%for substrate temperatures ranging from350to450°C,achieved with rigid glass substrates(see Table2).
The application of such growth processes on samples with polyimide substrates generally resulted in lower efficiencies so far.There are several possible reasons for that.On the one hand,the handling offlexible samples in the laboratory is often awkward and may pose problems,for example,due to curling.On the other hand,there may be negative influences on absorber properties due to stress(also affecting adhesion of CIGS to Mo),for example originating from the relatively high thermal expansion coefficients of most polyimides,and due to impurities diffusing from the substrate.Further-more,polyimides usually do not contain sodium,and Na-free absorbers or external Na incorporation i
n non-optimum doses may additionally be responsible for lower efficiencies compared with cells on SLG(see also Section5).
For CIGS growth at lower substrate temperatures less thermal energy is available,which reduces the mobilities of the constituent atoms.Therefore,a worse crystal quality is obtained even if all other growth parameters are kept identical.Grain sizes are smaller (Shafarman and Zhu,2000),which is expected to reduce the diffusion length of photogenerated carriers––pri-marily due to recombination at the numerous grain boundaries.The3-stage process(Gabor et al.,1994) currently leads to best cell efficiencies at high substrate temperatures.But the advantages of that process may not be valid at lower substrate temperatures since the film must undergo phase transformations during the second stage,which requires efficient interdiffusion of the constituent atoms.
5.Sodium incorporation
The benefits of sodium‘‘contamination’’in CIGS solar cells were realised in1993(Hedstr€o m et al.,1993), but are still not understood in detail today.Solar cell performance is often found to be enhanced by30–50% as a consequence of Na incorporation in amounts of typically about0.1at.%.The enhancement is mainly due to improved open circuit voltage andfill factor,induced
by electronic and often also structural changes in the absorber(Granath et al.,2000;Contreras et al.,1997; Ruckh et al.,1994).For example,higher net carrier concentrations and improved in-plane conductivity were observed by several research groups(Holz et al.,1994; Granata and Sites,1998;Kimura et al.,1999;Contreras et al.,1997;Lammer et al.,2001;Ruckh et al.,1994). Influences of Na on absorber growth are quite common, but the results are controversial:Enhanced grain growth due to Na has been observed in some laboratories,while in others no influence or even hindered grain growth was found(Bodeg ard et al.,1994;Contreras et al.,2000; Granath et al.,2000;Contreras et al.,1997;Nakada et al.,1997;Rudmann et al.,2003a).The influence of Na on grain growth was furthermore observed to depend on the CIGS preparation recipe(Rudmann et al.,2003b).
CIGS texture improves due to the presence of Na in some cases(Bodeg ard et al.,1994;Contreras et al.,2000; Lammer et al.,2001),whereas in others no consistent change was observed(Nakada et al.,1997;Granata and Sites,1998)or the effect depended on the Na incorpo-ration method(Rudmann et al.,2003a).Similar effects of Na were obtained when CIGS was grown at low and high substrate temperatures(Bodeg ard et al.,2000).
Historically,Na was incorporated into CIGS during growth by diffusion from SLG substrates through t
he Mo back contact.Although this method is still widely used today and led to record cell efficiencies,it is obvi-ously not applicable whenflexible Na-free substrates are employed.Alternatives are deposition of Na or of a Na compound prior to or during back contact deposition (such as SiO x:Na dielectric diffusion barriers deposited on Ti foils;Herrmann et al.,2003),deposition of a Na-containing precursor layer onto the back contact prior to CIGS growth,coevaporation of a Na compound during CIGS deposition,and Na in-diffusion into as-grown absorbers.In particular,precursors have been successfully applied on stainless steel substrates,leading to cell efficiencies exceeding16%(Tuttle et al.,2000; Hashimoto et al.,2003).NaF,Na2S or Na2Se are used often as precursor materials,and coevaporated materials include metallic sodium as well.Incorporation of sulfur into CIGSfilms is sometimes used to optimise absorber properties(widening of the band gap near the absorber front).Therefore,by using Na2S,the advantageous ef-fects of both Na and S could be combined.On the other hand,NaF reacts less quickly with(moist)air than Na2S,Na2Se,or Na,and may therefore be easier to handle.No incorporation of F into CIGSfilms could be observed with several Na incorporation methods(Bod-eg ard et al.,2000;Rudmann et al.,2001).All of these Na incorporation methods require a controlled dosage of Na,since too much sodium causes absorber properties to degrade and CIGS adhesion problems can occur (precursors!),while with too low doses the benefits of Na cannot be exploited fully.
Sodium could play a different role during CIGS growth at low and at high substrate temperatures.For example,CIS phase formation in the range of350–400°C was found to be delayed by the presence of Na(Wolf et al.,1998).Thus,a hindering influence of Na during growth of CIGSfilms could become detrimental at low temperatures,where the mobilities of the constituent atoms during growth are already limited due to the low thermal energy available.Therefore,CIGSfilms grown at low temperatures without Na may have better crystal quality than correspondingfilms grown in the presence
F.Kessler,D.Rudmann/Solar Energy77(2004)685–695689
版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。
发表评论