ABSTRACT
Beforegoingintothevarietyofmaterials,thefundamentalaspectsofPVare described. First, the sunlight spectra are shown in Figure 3.1. Due to the atmospheric absorption(mainlybyozonemoleculesintheUVregionandwaterandcarbondioxidemoleculesintheinfraredregion)andscattering,thesespectrahavesomewhat complicatedfeaturesontheearthground.AMisabbreviationofairmassandthe numbermeansthethicknessoftheatmosphere(unitycorrespondstoverticalthickness), while the letter “D” or “G” means that each spectrum is directly from the sun orfromtheentireskyincludingbluelightscatteredbytheatmosphere.InFigure3.1, photonnumber-basedspectrum(onthebasisoftheSibandgap)isalsopresented sinceaPVcellcurrentisproportionaltothetotalnumberofabsorbedphotons, nottothetotalabsorbedenergy.InFigure3.2,photoabsorptioncharacteristicsof various semiconductor materials are summarized. Recently reported corrections2 for CuInSe2 (CIS) and CdTe pro›les are compiled in Figure 3.2 along with the previous onesthatareshowninbrokencurves.Inaddition,acommonlyuseddyemolecule calledN719isalsoplottedinthis›gurebysimplyassumingthemolecularvolume
3.1 Introduction .................................................................................................... 57 3.2Crystal Silicon Solar Cells ..............................................................................60 3.3 Thin-Film Silicon Solar Cells ......................................................................... 67 3.4CIGS Solar Cells ............................................................................................. 71 3.5 CdTe Solar Cells ............................................................................................. 74 3.6 III-V Compound Semiconductor Tandem Solar Cells ................................... 75 3.7Emerging Materials for Solar Cells ................................................................ 79
3.7.1 Metal Silicides .................................................................................... 79 3.7.2Perovskites .......................................................................................... 79
3.8Inorganic Materials Employed for Dye-Sensitized and Organic Solar Cells .....80 References ................................................................................................................ 81
to be 1 nm3. In Figure 3.3, the maximum current corresponding to the bandgap cutoff is plotted. This is obtained simply by integrating the number of higher-energy photons and is basically the same as the one reported by Henry,3 except for two corrections of total intensity from 844 to 1000 W/m2 and the revision of standard solar spectra.4 This ›gure is useful for a brief estimation of current collecting ef›ciency for a semiconductor of a given bandgap. When it comes to the voltage, the situation is rather complicated. Generally speaking, half (or 2/3 at best) of the bandgap is usually obtained for open-circuit voltage. Refer to the Shockley-Queisser limit for estimating the upper limit of energy conversion for a single-junction solar cell.5 Two major origins to limit the energy conversion ef›ciency are shown in Figure 3.4. One is
longer-wavelength light that cannot be absorbed, and the other is shorter-wavelength light that has excess energy above the bandgap that cannot be utilized for conversion although absorbed. To partly improve this situation, a multi-bandgap PV cell, commonly called as tandem cell, is employed. The basic concept of a tandem cell is schematically shown in Figure 3.5. It should be noted that a semiconductor will not absorb the light whose energy is below the bandgap. Practical implementation of this tandem concept into PV cells is discussed in Section 3.6.
