April 14, 2005 EE 666 Advanced Semiconductor Devices Solar Cells --- frontiers in materials and devices Ning Su

April 14, 2005 EE 666 Advanced Semiconductor Devices Outline  Introduction  Market & technology comparison  Low cost solar cells  thin film solar cells (TFSC)  High efficiency solar cells  Advanced Si solar cells  Tandem cells  Thermophotovoltaic  other strategies  Conclusions

April 14, 2005 EE 666 Advanced Semiconductor Devices Introduction  Why PV ?  Average power incident upon continental United states is ~ 500 times of national energy consumption ( total, not just electricity)  Environmentally-friendly renewable energy source  Quiet  Reliable  Applications  Residential Cost-effective way to provide power to remote area  Space applications satellite, space stations

April 14, 2005 EE 666 Advanced Semiconductor Devices Photovoltaic Cells, Modules and Systems  Solar cell is the basic building blocks of solar PV  Cells are connected together in series and encapsulated into models  Modules can be used singly, or connected in parallel and series into an array with a larger current & voltage output  PV arrays integrated in systems with components for charge regulation and storage Cell module array system

April 14, 2005 EE 666 Advanced Semiconductor Devices Market for Solar PV  PV market grows at fast rate especially in recent years  Cumulatively, about 2GW of solar cells are being used in a variety of applications

April 14, 2005 EE 666 Advanced Semiconductor Devices Comparison of PV Technology  main technologies available: single & multi- cystalline Si, a-Si, CuInSe 2, CdTe….  Bulk cystalline Si remains dominant  Different technology comparison in efficiency & cost World PV module production in 2003

April 14, 2005 EE 666 Advanced Semiconductor Devices Low costHigh efficiency Thin filmOrganic SCtandem Terrestrial Space TPV Light weight Radiation resistance High efficiency Applications: Demands: Technology: Materials: Multicystalline SiIII-V a-Si ; CIS; CdTe Single crystalline Si Low Cost vs. High Efficiency SC

April 14, 2005 EE 666 Advanced Semiconductor Devices Thin Film Solar Cells “ thin film” refers more to solar cell technologies with mass-production possibilities Rather than the film thickness. requirement for suitable materials: low cost, high absorption, doping, transport, robust and stable leading materials for TFSC: CdTe, CuInSe 2, (CIS),a-SI… advantages: -- low material requirement -- variety of processing methods -- light weight modules disadvantages: -- low achieved efficiency

April 14, 2005 EE 666 Advanced Semiconductor Devices CIS & CdTe TFSC  CIS, direct band gap with Eg~ 1eV, α>10 5 cm -1 high cell efficiency (19.2 %), model efficiency (13.4%) comparatively long lifetime Current complicated and capital intensive fabrication  CdTe, direct band gap with Eg~ 1.45eV, α>10 5 cm ideal suited for PV applications  Record cell efficiency 16.5 % (NREL)  Numerous promising processing techniques

April 14, 2005 EE 666 Advanced Semiconductor Devices  Effect of bandgap on efficiency  GaAs, InP have E g close to the optimum, favored for high η cells  Si less favorable E g but cheap & abundant Solar Cell Efficiency  Ideal cell efficiency  Effect of spectrum on efficiency  improving η by concentrating light 100 suns or more illumination Parabolic reflector Fresnel lens

April 14, 2005 EE 666 Advanced Semiconductor Devices Minimize Losses in Real SC  Optical loss  Electrical loss  Concentration of light  Trapping of light: AR coatings Mirrors ( metallization rear surface or growing active layers on top of a Bragg stack) textured surface  Photon recycling reabsorption of photons emitted by radiative recombination inside the cell Rear metal reflector Double path length in metallized cell  Surface passivation  Resistive loss ……

April 14, 2005 EE 666 Advanced Semiconductor Devices Advanced Si Solar cells Martin A. Green etc.,” Very high efficiency silicon solar cells-science and technology,” IEEE Trans. Electron Devices,vol. ED-46,pp ,1999. PERL cell Burried contact sc  large improvement in the last 15 years 1) textured surface & AR coating 2) Improved surface passivation  PERL cell ( 24% in 1994 )  Buried contact cell commercialized by BP Solarex advantage: fine grid– reduced shading–J sc reduced contact recombination – V oc series resistance – concentrator sc Crystalline Si efficiency

April 14, 2005 EE 666 Advanced Semiconductor Devices Tandem Cells – beyond efficiency limit  Concept  Intrinsic efficiency limit using single semiconductor material is 31%  Stack different band gap junctions in series larger band gap topmost  efficiency of 86.8% calculated for an infinite stack of independently operated cells * * A. Marti, G. L. Araujo, Sol. Energy Mater. Sol. Cells 43 (1996) 203.

April 14, 2005 EE 666 Advanced Semiconductor Devices  Advantages : high efficiency  Practical approaches  Cover wider range of solar spectrum  reduce thermerlisation loss (absorbed photon with energy just little higher than E g )  individual cells grown separately and mechanically stacked  monolithically grown with a tunnel-junction interconnect Tandem Cells -- Practical approaches

April 14, 2005 EE 666 Advanced Semiconductor Devices GaInP/GaAs/Ge Dual- and triple-junction SC  Dual-junction (DJ) * N. H. Karam etc. Solar Energy Materials & Siolar cells 66 (2001) **N. H. Karam etc. Trans. Electron Dev. 46 (10) 1999 pp  GaInP/GaAs cells on Ge (average AM0 η 21.4 %) *  small-area lab cells large-scale manufacturing approach megawatt level **  Triple-junction (TJ)  efficiency of 27.0% under AM0 illumination at 28 0 C *

April 14, 2005 EE 666 Advanced Semiconductor Devices Multiple Junction Cells  Four-junction cells under development  addition of 1-eV GaInNAs subcells under GaAs to form 4 junctions  InGaN – potential material for MJ cells  Direct energy gap of InGaN cover most of the solar spectrum*  MJ solar cells based on this single ternary could be very efficient * LBNL/Conell work: J. Wu et al. APL 80, 3967 (2002).

April 14, 2005 EE 666 Advanced Semiconductor Devices Thermophotovoltaic (TPV)  TPV solar energy conversion Photovoltaic conversion with the addition of an intermediate thermal absorber/emitter is known as thermophotovoltaic (TPV) energy conversion. Solar radiation is used to heat absorber/emitter to temperature of K emitter radiates photons PV cell converts the energy of radiation into electrical power.  Advantage By matching the spectrum of the emitter to the PV cells, efficiency improved.

April 14, 2005 EE 666 Advanced Semiconductor Devices All TPV systems include: 1) heat source 2) radiator 3) PV converter 4) means of recovering unusable photons TPV Configuration  Components of a TPV system Selective emitter matched to PV cells

April 14, 2005 EE 666 Advanced Semiconductor Devices Other Strategies – for high efficiency  Intermediate band solar cells A.Luque and A. Marti,”Increasing the effiency of ideal solar cells by photon Induced transitions at intermediate levels”, Phys. Rev. Lett. 78, 5014 (1997) Low-dimentional strucutrues, QWs, QDs  Impact ionization solar cells P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995).  Hot carrier solar cells P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995). ……

April 14, 2005 EE 666 Advanced Semiconductor Devices Conclusions  Remarkable progress made in synthesis, processing and characterization leads to major improvement in PV efficiency and reduction in cost  Silicon continues to dominate the PV industry  Thin film and organic solar cells offer promising options for substantially reducing the cost, competitive for terrestrial applications  Very high efficiency achieved in multiple junction III-V semiconductors presently commercialized for space applications  New device concept for high efficiency facing challenges and prospects