New Concepts and Materials for Solar Energy Conversion Wladek - PowerPoint PPT Presentation

New Concepts and Materials for Solar Energy Conversion Wladek Walukiewicz Lawrence Berkeley National Laboratory, Berkeley CA Rose Street Labs Energy, Phoenix AZ In collaboration with EMAT-Solar group http://emat-solar.lbl.gov/ 1

Collaborators K. M. Yu, L. Reichertz, Z. Liliental-Weber, J. Ager, V. Kao, J. Denlinger, O. Dubon, E. E. Haller, N. Lopez, J. Wu LBNL and UC Berkeley R. Jones, K. Alberi, X. Li, M. Mayer, R. Broesler, N. Miller, D. Speaks, A. Levander Students, UC Berkeley W Schaff ( Cornell University ), P. Becla ( MIT ), C. Tu ( UCSD ), A. Ramdas ( Purdue University ), J. Geisz ( NREL ), M. Hoffbauer ( LANL ), S. Novikov and T. Foxon ( University of Nottingham ), J. Speck ( UCSB ), T. Tanaka ( Saga University ) 2

The Energy Challenge ● With a projected global population of 12 billion by 2050 coupled with moderate economic growth, the total global power consumption is estimated to be ~28 TW. Current global use is ~13 TW. ● To cap CO 2 at 550 ppm (twice the pre-industrial level), most of this additional energy needs to come from carbon- free sources. ● A comprehensive approach is required to address this difficult and complex issue facing humankind. 3

Solar Energy Potential • Theoretical: 1.2x10 5 TW solar energy potential (1.76 x10 5 TW striking Earth; 0.30 Global mean) •Energy in 1 hr of sunlight  14 TW for a year • Practical: ≈ 600 TW solar energy potential (50 TW - 1500 TW depending on land fraction etc.; WEA 2000) Onshore electricity generation potential of ≈ 60 TW (10% conversion efficiency): • Photosynthesis : 90 TW 4

Energy Production by Source Energy Production by Source 5

Energy Reserves and Resources 200000 150000 (Exa)J 100000 Unconv Conv 50000 0 Oil Oil Gas Gas Coal Coal Rsv=Reserves Rsv Res Rsv Res Rsv Res Res=Resources There is a growing consensus that continued use of carbon based fuels for energy production will irreversibly change planets climate 6

Energy dilemma Fossil fuels Abundant, inexpensive energy resource base Potentially destructive to environment and survival of humankind Renewable Energy Sources Safe and environmentally friendly Still relatively expensive, cumbersome technology Needs major scientific/technological/cost breakthroughs 7

Why should one work on Why should one work on renewable energy? renewable energy? 8

To Save Humankind To Save Humankind Global Warming and CO 2 Emission Over the 20th century, human population quadrupled and energy consumption increased sixteenfold. Near the end of the last century, a critical threshold was crossed, and warming from the fossil fuel greenhouse became a dominant factor in climate change. Hoffert, DOE workshop 9

To make money 10

To do exciting multidisciplinary To do exciting multidisciplinary science science Intersection of physics, chemistry Intersection of physics, chemistry and material science and material science 11

Solar Energy Utilization Fuel Light Electricity Fuels Electricity CO H O 2 2 2 e e Sugar sc M sc M H O 2 H O 2 O 2 Semiconductor/Liquid Photovoltaics Photosynthesis Junctions Adapted from Nathan S. Lewis, 1998 12

Fundamentals of Photovoltaics (single p/n junction) 1 1. Thermalization loss 2 illumination 2. Junction loss 3 4 3. Contact loss usable qV 4. Recombination loss 1 I • Dark and light I-V curves dark • V open-circuit light • I short-circuit V 0 • V open-circuit Maximum power P m  V m P m =I m • Fill factor (squareness)  I short-circuit FF=P m /(V open-circuit ) I short-circuit 13

How to improve the power conversion efficiency? multijunction multijunction multiband multiband The intermediate band serves as a “stepping stone” to transfer electrons from the valence to conduction band. Each of the cells efficiently converts photons from a narrow energy range. Photons from broad energy range are Band gaps are selected for optimum absorbed and participate in coverage of the solar energy spectrum. generation of current. Strict materials requirements Complex, expensive technology 14

Best Research- -Cell Efficiencies Cell Efficiencies Best Research Current record - Current record - 43.5% 43.5% 15

Three-Junction Solar Cells Structure of Triple-Junction (3J) Cell Front Contact AR Coating n + (In)GaAs • Efficiencies up to 41% n + AlInP [Si] n + InGaP [Si] InGaP p InGaP [Zn] Top Cell • Six different elements p AlInP [Zn] p ++ AlGaAs [C] Tunnel Junction n ++ InGaP [Si] n + AlInP [Si] • Three different dopants n + (In)GaAs [Si] InGaAs p (In)GaAs [Zn] Middle Cell p + InGaP [Zn] • Practically used: p ++ AlGaAs [C] Tunnel Junction n ++ InGaP [Si] 3-junction cells n+ (In)GaAs [Si] Buffer Layer n+ GaAs : 0.1µm • Ge n Research: p Ge Substrate Bottom Cell 4 to 5 junctions Back Contact Could this be simplified? Yamaguchi et. al., 2003 Space Power Workshop 16

Group III-Nitrides before 2002 17

Fundamental Bandgap of Wurtzite InN 10  MBE-grown high- InN(250nm)/GaN(buffer)/sapphire 2 /Vs, n=5.5x10 18 cm -3 quality InN  =615cm 8  Photoluminescence (295K) absorption (10 4 cm -1 ) All characteristic PL or PR signal band gap 6 features lie near absorption (295K) 0.7 eV 4  No energy gap is observed around Photo-modulated 2 eV 2 Reflectance (77K) 0 0.5 1 1.5 2 2.5 E (eV) 18

In 1-x Ga x N Alloys 3.5 1 our data In 1-x Ga x N 3.0 Shan 0.8 295K Pereira bowing b=1.43eV 2.5  2 (10 10 cm -2 ) bowing b=2.63eV 0.6 E g (eV) 2.0 0.4 0 1.5 17% In 1-x Ga x N 31% 0.2 43% 1.0 50% 0 0.50 0.5 1 1.5 2 2.5 3 0 0.2 0.4 0.6 0.8 1 x E (eV) • Small bowing parameter in In 1- x Ga x N: b = 1.43 eV • The bandgap of this ternary system ranges from the infrared to the ultraviolet region! 19

Full solar spectrum nitrides The direct energy gap of In 1-x Ga x N covers most of the solar spectrum  20

What is unusual about InN?  InN has electron affinity of 5.8 eV, larger than any average energy of native defects other semiconductor  Extreme propensity for 0.9 eV native n-type conduction InN electron affinity = 5.8 eV and surface electron accumulation for InN and In-rich In x Ga 1-x N 21

Integration of InGaN with Si 22

Band diagram of In .46 Ga .54 N/Si In .46 Ga .54 N Si 2.00 1.50 p-type Na=1e18 1.00 F (eV) p-type 0.50 Energy, relative to E Na=1e17 0.00 n-type n-type -0.50 Nd=1e17 Nd=5e19 -1.00 -1.50 Ec (eV) Ev (eV) -2.00 EF -2.50 0 1000 2000 3000 4000 5000 6000 7000 8000 Depth from Surface (Angstroms) 23

Si Two-junction hybrid solar cell N Ga .54 In .46 24 E

Si  Two-junction hybrid solar cell h N Ga .54 In .46  h 25 E

e- h+ Si Two-junction solar cell N Ga .54 e- h+ In .46 26 E

e- Si h+ Two-junction hybrid solar cell N Ga .54 e- h+ In .46 27 E

Two-junction hybrid solar cell In .46 Ga .54 N Si e- e- e- e- e- e- E h+ h+ h+ h+ h+ h+ 28

e- e- e- Si Two-junction hybrid solar cell N Ga .54 In .46 h+ h+ h+ 29 E

Two-junction hybrid solar cell In .46 Ga .54 N Si e- e- e- e- e- V oc,Si E V oc,InGaN V oc,cell = h+ h+ h+ h+ h+ V oc,InGaN + V oc,InGaN 30

InGaN/Si tandem InGaN-Si tandem p-InGaN n-InGaN No barrier p-Si for e-h recombination n-Si - - Optimum top cell bandgap for a dual junction tandem solar cell with a Si bottom cell: 1.7~1.8 eV Adding InGaN top cell boosts a 20% - Thermodynamic efficiency limits efficient Si cell into more than 30% (1x sun AM1.5G) Si single junction: efficient tandem cell ! 29%, with additional top cell: 42.5% No tunnel junction needed 31

InGaN/Si MJ efficiency estimates Calculated 300 K AM1.5 direct efficiency of a 2J InGaN/Si tandem solar cell. Assumed InGaN parameters  e = 300 cm 2 V -1 s -1  h = 50 cm 2 V -1 s -1 m e = 0.07m 0 m h = 0.7m 0 The surface recombination velocities assumed to be zero. The maximum efficiency is 35% using InGaN with a bandgap of 1.7 eV (In 0.5 Ga 0.5 N). 32

GaN-Si tandem cell GaN/Si hybrid tandem GaN GaN Not current matching ! Illumination: 1x AM1.5G plus 325 nm HeCd laser Top cell greatly restricts the current Voc= 2.5 V, Jsc= 7.5 mA/cm2 , fill factor = 61% Eg = 3.4 eV → max. Jsc= 0.6 mA/cm 2 Demonstration of GaN-Si tandem (1 sun, 100% QE) (developed with funding by RSLE) W. Walukiewicz, K.M. Yu, J. Wu, U.S. Patent No. 7,217,882, “Broad spectrum solar cell,” (issued May 15, 2007). W. Walukiewicz, J.W. Ager, K.M. Yu, Patent Application No. PCT/US2008/004572,“Low-resistance Tunnel Junctions for High Efficiency Tandem Solar Cells.” W. Walukiewicz, J.W. Ager III, K.M. Yu, Patent Application No. PCT/US2008/067398, “Single p-n Junction Tandem Photovoltaic Device.” 33

External Quantum Efficiency Clear evidence for tandem PV action 34

Intermediate band solar cells Intermediate band solar cells Multi-junction solar cell The intermediate band serves as a Each cell converts photons from a “stepping stone” to transfer electrons narrow energy range. from the valence to conduction band. Band gaps are selected for optimum coverage of the solar spectrum Photons from broad energy range Strict materials requirements are absorbed and participate in Complex, expensive technology generation of current. 35