Arizona Institute for Renewable Energy & the Solar Power Laboratories

Arizona Institute for Renewable Energy & the Solar Power Laboratories International Photovoltaic Reliability Workshop July 29-31, Tempe AZ Christiana Honsberg, Stephen Goodnick, Stuart Bowden Arizona State University IPRW 2009 C. Honsberg 1

Needs of energy systems Why change energy system? Environmental issues New energy sources which reduce pollution and CO2 generation Increase in per capita usage Technical characteristic limit applications. IPRW 2009 C. Honsberg 2

2. Enable New Applications IPRW 2009 C. Honsberg 3

2. Enable New Applications What are characteristics needed? SMART grid applications Self-contained, autonomous power Use arbitrary inputs and outputs IPRW 2009 C. Honsberg 4

3. Increase per capita usage Many other problems are related to energy, eg water Magnitude of the solar resource allows it to uniquely meet world energy needs in 2050 IPRW 2009 C. Honsberg 5

Needs on Energy System First integrate with existing energy system, then develop newer approaches IPRW 2009 C. Honsberg 6

Path for transforming energy system First fit into existing technologies, then develop new applications. Electricity generation - reasonably direct fit from PV or concentrating solar thermal generation. Thermal applications; Geothermal, solar thermal Transport & storage; direct replacements are difficult; examine a range of new technologies Fuel replacements; ethanol, new approaches compatible with existing infrastructure New fuels New fuels and conversion approaches; fuel cells IPRW 2009 C. Honsberg 7

Path for transforming energy system IPRW 2009 C. Honsberg 8

Sustainable Energy at ASU IPRW 2009 C. Honsberg 9

Sustainable Energy at ASU IPRW 2009 C. Honsberg 10

Sustainable Energy at ASU IPRW 2009 C. Honsberg 11

Sustainable Energy at ASU IPRW 2009 C. Honsberg 12

Sustainable Energy at ASU IPRW 2009 C. Honsberg 13

Sustainable Energy at ASU IPRW 2009 C. Honsberg 14

Bio Fuels Production IPRW 2009 C. Honsberg 15

Electrochemistry and Fuels IPRW 2009 C. Honsberg 16

Sustainability at ASU IPRW 2009 C. Honsberg 17

Policy and Economics IPRW 2009 C. Honsberg 18

ASU Campus Solarization IPRW 2009 C. Honsberg 19

Arizona State University Center of US Solar Radiation ASU IPRW 2009 C. Honsberg 20

Solar Cell Efficiency IPRW 2009 C. Honsberg 21

Crystalline Silicon at ASU Current and emerging PV Compatibility with current industrial processes (6 inch ) MacroTechnology Works (7700 S River Pkwy) Dedicated solely to solar: 4000 ft 2 of clean room space MBE systems for nanostructures Actively seeking collaborations IPRW 2009 C. Honsberg 22

PV Industry Breakdown Recent resurgence of high efficiency crystalline PV 3000 2500 2000 1500 1000 500 0 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 rest TF 6 7 1 4 3 5 7 19 37 74 ribbon Si 0 0 6 15 23 26 7 41 53 55 a-si 15 19 25 27 36 36 26 55 71 119 mono c-si 62 60 85 102 139 201 244 455 608 1100 multi c-si 43 67 85 140 202 289 460 687 991 1179 Total 126 155 201 288 401 560 744 1257 1759 2535 IPRW 2009 C. Honsberg 23

Challenges in Photovoltaics Challenges Need higher efficiency, but in all materials higher efficiency gives high cost. Transistors increase performance and decrease cost why not photovoltaics? Moore s Law of Photovoltaics How do we increase the efficiency to thermodynamic limits in a material efficient approach. New Materials New Micro-System Approaches New Physical Approaches IPRW 2009 C. Honsberg 24

Moore s Law of Photovoltaics Moore s Law is an observation which accounts for the rapid development of IC s, enabled by gate width simultaneously increasing performance and reducing Si area. Equivalent Moore s Law is in W/cm 3 Reducing thickness allows simultaneously more Watts per gram of Si, increased performance and reduced cost. IPRW 2009 C. Honsberg 25

Moore s Law of Photovoltaics Thinner solar cells Thickness increases efficiency and uses less silicon, so decreases cost; efficiency production per installed fab line This is the holy grail in PV. What has changed? Surface passivation Processing challenges; need to shift paradigm from a solar cell being a simplified integrated circuit; no diffusion; no AL, possibly no nitrides Growth technologies Potential of new designs; nanotechnology Defect Engineering IPRW ERC Draft 2009 C. C. Honsberg 26

IC vs Solar Cell Processing Process Past IC Heritage Present Solar cells Future Solar Cells Feed stock Siemens process Siemens process Fluidized bed reactor Contacts Photolithography/ lift-off Silver Screen printing Many Options (Copper Plating) Wafers 600 um, blade sawn <200um wire sawn 50-100 um Passivation Thermally grown silicon dioxide Silicon nitride Wide band gap Doping High temperature diffusions High temperature diffusions Low temperature depositions of a-si IPRW 2009 C. Honsberg 27

Why Thin? Silicon solar cell peak efficiency is 10-100 um Only if there is sufficient surface passivation Potential to achieve 25% Technologies trend to higher performance and lower cost (cf CMOS gate dimensions) Sunpower achieves 22% on 147um wafers future present IPRW 2009 C. Honsberg 28

Silicon Heterojunction Solar Cells IPRW 2009 C. Honsberg 29

Silicon Heterojunction (SHJ) Diffusions replaced with low temperature depositions Wide bandgap semiconductor forms a heterojunction with the Si substrate Surface passivation (intrinsic buffer layer) Industrial Production (HIT) intrinsic a-si p-type a-si n-type c-si intrinsic a-si n-type a-si TCO Al IPRW 2009 C. Honsberg 30

SHJ Band Diagram Surface Inversion Depletion Region E V Depletion region located in c-si Inversion layer introduced Electrical junction separated from material junction Large valence band offset governs device operation IPRW 2009 C. Honsberg 31

New solar cell structures High Jsc and FF of IBC High Voc of SHJ New solar cell structure implements an interdigitated back contact solar cell using silicon heterojunctions (IBC-SHJ) IPRW 2009 C. Honsberg 32

Materials other than asi Silicon heterojunctions for c-si solar are a new area and there are many other possibilities: amorphous SiC:H crystalline SiC (band gap 2.2 ev) ZnS has a close lattice match to Si and band gap > 3.5eV Polymers IPRW 2009 C. Honsberg 33

Efficiency Limits of Solar Energy Conversion Existing solar cells have achieved < ½ of their thermodynamic efficiency Single junctions relatively well optimized, and reach 70-80% of theoretical efficiency. IPRW 2009 C. Honsberg 34

Materials for MJ Solar Cells 5 or 6 junction solar cell introduces significant material constraints, particularly for monolithic configuration. Requires good lattice matching and current matching. Requires 2 Egs in between GaAs and Ge. Requires a high band gap solar cell > 2.4 ev. Requires highly efficient tunneling connections IPRW 2009 C. Honsberg 35

InGaN Solar Cell Results Optimization of material, thickness, polarization allowed high open circuit solar cells with band gap of 2.5 ev V OC = 2.1V Thin test structures: 100nm Strong photovoltaic effect. Band gap: 2.5eV V OC 2.1V; corresponds to E G. IPRW 2009 C. Honsberg 36

New Materials IPRW 2009 C. Honsberg 37

New Materials and Approaches IPRW 2009 C. Honsberg 38

New Materials and Approaches IPRW 2009 C. Honsberg 39

Spectral Splitting Approaches Spectral splitting and low-concentration static concentrators allow >50% efficiency using existing semiconductor materials. Eliminates the need for all current matched solar cells Substantially increases the material choices for each of the solar cells Low concentration reduces heating, defect dependence Developed in DARPA VHESC program IPRW 2009 C. Honsberg 40

Transformative Photovoltaics What has changed? Static efficiencies in conventional technologies New theory/thermodynamics for solar cells New materials Nanotechnology Potential for ideal solar cell high theoretical efficiency, practical device achieves large fraction of theoretical limit, device is implemented in low cost, scalable, material efficient manner. Enable new physical mechanisms Circumvent existing cost and performance trade-offs Closer match between thermodynamics and physical device structures IPRW 2009 C. Honsberg 41

High Efficiency Photovoltaic Approaches Assumption in Shockley-Q ueisser Input is solar spectrum One photon = one electron-hole pair One quasi-fermi level separation Constant temperature = cell temperature = carrier temperature Steady state ( equilibrium) Approach which circumvents assumption Multiple spectrum solar cells: transform the input spectrum to one with same energy but narrower wavelength range Multiple absorption path solar cells: any absorption path in which one photon oneelectron hole pair Multiple energy level solar cells: Existence of multiple meta-stable light-generated carrier populations within a single device Multiple temperature solar cells. Any device in which energy is extracted from a difference in carrier or lattice temperatures AC solar cells: Rectification of electromagnetic wave. Examples Up/ down conversion Thermophotonics Impact ionization Two-photon absorption Intermediate band Quantum well solar cells Hot carrier solar cells Rectenna solar cells IPRW 2009 C. Honsberg 42

1. Multiple Spectrum Solar Cells Multiple spectrum devices: take the input solar spectrum, and change it to a new spectrum with the same power density Does not need to be incorporated into solar cell can use existing solar cells, and add additional optical coatings Approaches for multiple spectrum solar cells: up/down conversion, thermophonics IPRW 2009 C. Honsberg 43

2. Multiple absorption path solar cells Change absorption mechanisms to one photon one electron-hole pair Mechanisms include: Two-photon absorption Impact ionization/auger generation: generate n electrons for an incident photon energy of n Eg. Q uantum dot showing carrier multiplication Q uantum efficiency > 100% due to carrier multiplication R.J. Ellingson, et al Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots Nano Letters, 5, 5 p. 865-871 (2005) IPRW 2009 C. Honsberg 44

3. Multiple Energy Level Solar Cells Introduce more than a single quasi-fermi level separation by introducing additional energy levels or bands, such that extracted energy of photon energy of band gap Energy levels can be spatially localized (energy levels) or interacting to form mini-bands. IPRW 2009 C. Honsberg 45

4. Multiple Temperature Solar Cells Multiple temperature solar cells extract energy from temperature differentials in solar cell. Possible that temperature differentials are lattice temperature differential, but requires materials with substantially different thermal conductivity than electrical conductivity. Easier to maintain a temperature differential in carrier temperature by introducing variations in the band structure IPRW 2009 C. Honsberg 46

Hybrid Thermal/Quantum Devices Use gradient in carrier temperature, not lattice temperature, coupled with quantum converter Efficiency limit is same, but very different performance with concentration, number of unique conversion processes, etc. IPRW 2009 C. Honsberg 47

Thermal Power Sources Multiple energy level approaches allow for higher efficiencies (factor of 2 or more) and high power densities Allows use of higher band gaps with low temperatures, making devices practical 0.7 Barrier Material Material Bandgap (Eb) 1.1 1 0.9 0.8 Efficiency 0.45+ 0.7 0.4 to 0.45 0.35 to 0.4 0.3 0.3 to 0.35 0.6 0.25 to 0.3 0.2 0.2 to 0.25 0.5 0.15 to 0.2 0.1 to 0.15 0.1 0.4 0.05 to 0.1 0 to 0.05 0 0.3 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Well Material Material Bandgap (Ew) Efficiency 0.6 0.5 0.4 Efficiency 1273 K TPV Efficiency 1273 K Efficiency 1073 K 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Band Gap (ev) IPRW 2009 C. Honsberg 48

Multiple Quasi-Fermi Level Materials Requirements: 1. Three (or more) carrier populations in steady state that are not in thermal equilibrium with each other 2. Absorption/emission rates which of similar magnitude for all processes Multiple quasi-fermi levels enable unique material properties IPRW 2009 C. Honsberg 49

Opportunities for Solar Energy Conversion Fundamental science new physical processes, new materials & fundamental science which transcends a single discipline Photovoltaics, or more broadly quantum-based converters, are the newest of the energies and there remains substantial physics, chemistry and optics to be discovered. New engineering and systems opportunities by integrating different areas and expertise can achieve a system which is more than the sum of the parts New educational models energy is a multi-disciplinary field (including economics and policy). Revitalize engineering with important problem based in latest science and requiring strong engineering capabilities. IPRW 2009 C. Honsberg 50