Page - 65 - in Photovoltaic Materials and Electronic Devices

unoptimized n-layer/ZnO/Ag back reflector (BR) coated PEN in n-i-p configuration PVdevices [15]. Overall theseresultsandanalysisproceduresdevelopedhereare applicabletomoredirectlyrelatingpropertiesof layers inthedeviceconfiguration,as obtainedbynon-destructivemeasurements,withvariations indeviceperformance. Thesetypesofmeasurementshavebeendemonstratedfora-Si:Hsolarcellsdeposited onplanarsubstratesasdescribedhereandalsoonthose incorporatingmacroscopic roughnessor texturing[16–18]. Botha-Si:Handnc-Si:Hcomponentmaterialsareusedinstate-of-the-artSi:H based PV devices. nc-Si:H, either as individual layers or in PV junctions, has significantenhancementinnearIRabsorptionofthesolarspectrumandhighstability underprolongedillumination incontrast to itsamorphouscounterpart [19,20]. The quantitative analysis, characterization, and control of the relative nanocrystalline and amorphous volume fractions within mixed-phase films is also a major challenge in Si:H manufacturing. Most often the nanocrystalline fraction is estimated from x-ray diffraction or Raman spectroscopy, which can yield values ranging an order of magnitude[6,21,22]. Althoughthesemeasurementsarevaluable, limitationsexist. Typically ex situ x-ray diffraction measurements average information over the full depthofa thinfilmsample,and exsituRamanspectroscopyaverages information overafinitepenetrationdepthintothesamplethatisdependentuponthewavelength oftheprobinglaser, itspower,andtheabsorptioncoefficientof thematerial. Profiling these materials non-invasively is an even greater challenge due to probe penetration depth limitationsandlikelynon-uniformcrystallite fractionwithdepth intofilms. Si:Hfilmsmaybe inhomogeneouswith thicknessascrystallitesnucleate fromand coexist with the amorphous phase. Deconvolving gradients in crystallinity from exsitux-raydiffractionandRamanspectroscopymeasurementsrequiresmultiple samples, while in situ RTSE measurements applied during film deposition have been usedtoquantifystructuralgradients incrystallinitywithinasinglefilm. A wealth of information can be extracted from these types of RTSE measurements applied at a single spot on a sample surface, but additional property variations related to sample non-uniformity have also been obtained by ex situ mapping spectroscopic ellipsometry (SE) [23–26]. In mapping SE, the sample and multichannel ellipsometer, similar to the instrument used in RTSE studies, are mechanically translated with respect to the each other in one or more dimensions toobtainellipsometric spectraasa functionofspatialposition. In thecaseofSi:H, simplifiedstructuralmodelsbasedonresults fromRTSEmeasurementsareapplied toprobesubtlevariations inmaterialopto-electronic responsesuchas thebandgap ofa-Si:H,filmthickness, surfaceroughness thickness,andnanocrystallite fraction in mixed phase materials. These types of measurements have been applied to Si:H [23,24], cadmium telluride [26], and copper indium gallium diselenide [25] PV devices ranging from tens of square centimeters on the laboratory scale to full 65