Thin Film Photovoltaics By Justin Hibbard Thin Film Photovoltaics • What is a thin film photovoltaic? • Thin film voltaics are materials that have a light absorbing thickness that is less than 10 μm. • The thin film material can converting sunlight to electricity. • Thin films are predicted to be as efficient as monocrystalline Si wafer cells with more R & D. • Need more R & D to evolve from the laboratory to the marketplace. • Two competing materials are (CIGS):Cu(In,Ga)Se2 and CdTe. Thin Film Photovoltaics Thin film photovoltaic devices take advantage of absorption of ir-vis-ir light by semiconductors and converts the light to power. What is a semiconductor? Semiconductors are materials that require a “turn on” energy to conduct energy ≈ current. Semiconductors share the electrons respectively with the individual atomic nuclei and require a minimum voltage to make the electrons mobile. Whereas metals are materials that are referred to as conductors and have “free” electrons that respond to a small voltage. Semiconductors are materials that prefer a Lewis/closed shell structure. This rule may be deviated with the addition of element(s) that differ by group number ±1,0. Which may give the material an extra or a missing (hole) valence electron. This atomic substitution allows the material scientist to “tune” the material by manipulation of the conduction properties of the material. Semiconductors are preferable because of the ability to tune the eV with the right elemental mixture and the small amount of material needed. Thin Film Photovoltaics Thin films technologies have a common device/module structure: substrate, base electrode, absorber, junction layer, top electrode, patterning steps for monolithic integration and encapsulation but in a reverse order. Thin film photovoltaics are cheap if the elements are not rare. They are becoming cheaper to produce and will become cheaper with time be R & D. Monocrystalline Si is expensive to grow. Amorphous and polycrystalline silicon are not very efficient. Thin film CIGS cell and modules are 19.5% and 13% efficient and CdTe cells and modules are 16.5% and 10.2% efficient. CdTe and CIGS PV modules have the potential to reach cost effective PV-generated electricity. They have transitioned from the laboratory to the market place. Pilot production/first-time manufacturing (US) ~ 25 MW. CdTe technology ramping to 75 MW. Enjoying a flux of venture capital funding.Transitioning from the lab to manufacturing has been much more difficult than anticipated. CIGS and CdTe Device Structure CdTe Glass CTO/ZTO,SnO2 ZnO, ITO 0.2-0.5 µm 2500 Å CdS CdS 700 Å 600-2000 Å CIGS 1-2.5 µm CdTe 2-8 µm Mo 0.5-1 µm Glass, C-Paste Metal Foil, with Cu, Plastics or Metals CIGS CTO/ZTO = Cd2SnO4/ZnSnOx CIGS Deposition System Thin Film Photovoltaics Thin films challenges: 1)Science and Engineering support. 2)Long Term Stability. 3)In-Situ Process Diagnostics and control. 4)Thinner CIGS and CdTe Abosrbers. 5)Need for High-Throughput , Low-Cost Processes. 6)Inproved Voc in CdTe devices. Thin Film CIGS Solar Cells Efficiency Area Area VOC JSC FF Efficiency (cm2) (cm2) (V) (mA/cm2) (%) (%) Comments CIGSe/CdS/Cell CIGSe 0.410 0.697 35.1 79.52 19.5 NREL, 3-stage process CIGSe/ZnS (O,OH) CIGSe 0.402 0.67 35.1 78.78 18.5 NREL, Nakada et al Cu(In,Ga)S2/CdS CIGS 0.409 0.83 20.9 69.13 12.0 Dhere, FSEC Cu(In,Al)Se2/CdS CIAS – 0.621 36.0 75.50 16.9 IEC, Eg = 1.15eV CTO/ZTO/CdS/CdTe CdTe 1.03 0.845 25.9 75.51 16.5 NREL, CSS CdTe – 0.840 24.4 65.00 13.3 SnO2/Ga2O3/CdS/CdTe IEC, VTD CdTe 0.16 0.814 23.56 73.25 14.0 ZnO/CdS/CdTe/Metal U. of Toledo, sputtered Thin Film Photovoltaics Justin Hibbard thanks the nrel.gov webpage for pictures and information.
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