Volume 19 / Issue 4 December, 2009 Thin Film Photovoltaic Solar Cells Solar Energy and Cell Figure 1 Response The generation of solar electric power con- siders the available solar energy spectrum and the efficiency of converting solar pho- tons to electric current. Efficient conversion requires matching solar cell sensitivity re- sponse to the solar spectrum reaching the Earth’s surface, i.e., the ground-level solar irradiance as it is transmitted by the atmo- sphere. The optical path at noon at the equa- tor is defined as 1 atmosphere (AM1). An average path length over typically collected sun exposure angles is closer to 1.5 – 2 at- mospheres. Figure 1 shows the spectral ir- radiances for top of the atmosphere (AM0) and through 1.5 atmospheres at Earth’s sur- face [1]. The AM1.5 model has been adapted by the solar industry for evaluating solar cell modules, and is used by the National Re- Figure 1. ASTM solar spectral irradiance at top of the atmosphere and at the earth’s newable Energy Lab (NREL) as a test bed. surface through Air Mass 1.5 [ Ref 1]. Atmospheric absorption (ozone, water va- por, etc) and scattering by aerosols elimi- nate solar terrestrial energy at wavelengths tion bands in the irradiance spectrum. Glass Silicon-Based Solar shorter than ~350 nm, and impose absorp- covers also absorb below 320 nm. Most of Cells the solar irradiance energy distribution is at wavelengths short of ~1000 nm, where Sili- The photovoltaic (PV) solar cell is undergo- con-based and CdTe materials absorb highly ing rapid development and commercialization Contents and possess high conversion efficiency. Spe- as a source of renewable energy. The Solar cial materials and layer combinations have America Initiative (SAI) program under the Solar Energy and Cell been developed to extend the spectral re- direction of the Department of Energy has as Response sponse to collect more energy and thereby one of its goals the increase in solar power increase efficiencies. Figure 2 (see page 2) generation for commercial and residential en- Silicon-Based Solar Cells shows the spectral responses for different ergy needs at a greatly reduced $/KWhr cost. solar cell materials [1]. Achieving this goal requires greater conver- Thin-Film Solar Cells: sion efficiencies continuing research and de- Advances Beyond Silicon Spectral bandwidths for materials are: amor- velopment of materials and processes. The phous silicon α-Si:H, 400-650 nm; μ-Si:H first high efficiency PV cells were made from Thin Film Development: 650 – 850 nm, so layered the response cov- single crystal silicon (“first generation” tech- “Second Generation” PV Cell ers 400 to 850 nm. CdTe cells respond over nology). Ribbon silicon generated by poly- Technology 400 – 850 nm with higher quantum efficiency crystalline or amorphous growth technology than Silicon compositions. CIGS (CuInGaSe/ has lower production cost at the sacrifice of Development Continues S ) cover 400-1100 nm. These are discussed lower light-to-electric current conversion ef- further in the following sections. Continued on page 2 3 Silicon-Based Solar Cells from page 1 low absorption of visible and near-IR energy, Thin-Film Solar Cells: ficiency. Silicon-based technology has a fun- therefore thicknesses of at least 150 μm are Advances Beyond damental conversion limitation due to its rela- required to obtain efficient PV output. Han- tively narrow spectral absorption width, dling and processing of thinner silicon is also Silicon ~400 to ~850 nm even with the compound inhibited by fragility issues of large area wa- In an effort to circumvent the materials re- structure mentioned above. This range cap- fers. Cell size (area) is restricted by practi- source limitations and production costs as- tures only ~50% of the total available solar cal and economical limitations associated with sociated with the first generation silicon terrestrial irradiance for wavelengths <1500 the production of large diameter wafers from technology, effort is being applied toward nm. The classical solar cell based on crystal- crystal ingots. Solar concentrator optics are the development and production of PV cells line silicon, achieves conversion efficiencies being employed to increase the efficiency per based on advanced technologies. Multi- near 20% and this technology accounts for unit area, thereby permitting smaller areas of junction cells built of thin layers of III-V perhaps 95% of current module installations. silicon to be used. Such optics are also used composition and with their wider spectral However, physical limitations drive produc- with the other thin-film materials discussed response have achieved twice the efficiency tion costs. Crystalline silicon has a relatively below. of silicon cells under 10X solar concentra- tion [2]. Structures having 3 and 4 junc- tions provide spectral response from UV Figure 2 wavelengths approaching 1500 nm. Multi- junction cells are fabricated in low volume under high production cost, and find appli- cation in orbiting space platforms where weight/kW is a significant cost and pay- load capacity factor. Deposition processes use expensive equipment such as MOCVD (Metal-Organic Chemical Vapor Deposi- tion) or MBE (Molecular Beam Epitaxy). CERAC Coating Materials News is a quarterly publication of CERAC, inc. Figure 2. Spectral responses for various solar cell materials [1]. A subsidiary of Williams Advanced Materials Inc. P.O.Box 1178 Thin Film Development: “Second Generation” Milwaukee, WI 53201-1178 Phone: 414-289-9800 PV Cell Technology FAX: 414-289-9805 web: www.cerac.com Thin-film PV (TFPV) cells are under devel- multi-junction α-Si /α-Ge. In the CdTe TFPV e-mail: [email protected] opment by several companies in Europe and cell, the light absorbing layer is CdTe 10 μm the US. New compositions based on thin- thick and the CdS heterojunction 0.1 μm Editor: film structures have overcome the limitations David Sanchez thick (Figure 3). Both layers can be depos- Sr. Materials & Applications Scientist of crystalline silicon specifically by produc- ited by evaporation or CVD. CERAC, inc. ing high absorption and efficient conversion over a larger portion of the solar spectrum in For compound materials, Copper Indium Principal Contributor: thicknesses of a few micrometers. Large area Sulfur/Selenide (CIS) and CuInGaSe/S Samuel Pellicori modules can be economically produced us- (CIGS), the absorber thickness need only be Pellicori Optical Consulting ing roll-to-roll vacuum web coating process- ~2 μm, leading to lower materials cost and P.O. Box 60723 ing on non-rigid substrates. This “second introducing a variety of fabrication processes. Santa Barbara, CA 93160 generation” technology is capable of produc- Phone/FAX: 805-682-1922 CIGS cells with efficiency ~20% have been e-mail: [email protected] ing cell efficiencies greater than crystalline fabricated. TFPV cells are constructed of silicon because of their extended spectral re- multiple layers as modules, then divided into For a free subscription to CMN, please sponse, and they are more economical to pro- cells as shown in Figure 3. Between the trans- E-mail your name and address to duce in high volume. Production-line mod- parent conductor (TCO) and the back metal [email protected] or send ules currently achieve typical efficiencies contact are buffer and absorber layers. The us a fax at 414-289-9805. Guest articles ~14% [3]. The lower efficiency compared fabrication process, and therefore the illumi- or topic suggestions are welcome. with crystalline silicon is traded against the nation direction, determines the order in larger area in light of the lower production An electronic version of this publication which these layers are deposited, as illus- can be accessed from the Technical cost of TFPV cell. trated. The absorber / conversion layer can Publications page of the CERAC web consist of a stacked series of multi-layers of site at www.cerac.com. From there, link Compositions of the solar photon absorbers controlled composition to expand the spec- to the CMN Archives to view back is- in TFPV modules include: amorphous sili- tral absorption range and thereby increase sues. con (α-Si:H), micromorphous silicon (μ-Si) continued on page 3 and vertically stacked layers of both, and ©Copyright 2009-2010, CERAC, inc. 2 Figure 3 Figure 3. Alternate configurations for TFPV solar cells. With the construction on the right, the substrate can be a flexible metal such as Kapton, polyimide film or stainless steel foil, in place of glass. PV Cell Technology from page 2 the cell conversion efficiency. Process temperatures for the CVD of reducing recombination sites through the growth of a microstructure CIGS and CIS absorbers range from 400 to 600°, necessitating the with longer range order. The focus directed by, for example projects presence of barrier layers to prevent diffusion of metal impurities sponsored by SAI, will decrease our dependence on non-renewable such as Fe that will reduce conversion efficiency. For example, if the energy resources. cell is grown on a stainless steel foil substrate, a diffusion barrier is needed between the steel and the Mo conductor. That high-dielectric barrier often is a several-μm thick layer of an oxide such as SiOx. Greater stability to years (actually decades) of exposure to the envi- ronment in terrestrial installations is a concern that is receiving atten- tion. Improved barrier or protective coatings and greater tolerance to temperature and humidity cycles are needed. The second-surface construction (left) in Figure 3 provides better immunity to environ- mental effects. The TCO used with TFPV cells is Al:ZnO (AZO), a more economi- cal alternative to ITO with higher transmission. We have discussed AZO coating material previously [4]. TFPV cells and modules are built using high temperature PECVD, or by plasma-enhanced evapo- ration, sputter deposition, and other techniques. DC and pulsed-DC magnetron sputtering is commonly used to deposit all of the layers from TCO to Mo shown in Figure 3.