Third generation solar cells - How to use all the pretty colours?

Transcription

1 Third generation solar cells - How to use all the pretty colours? Erik Stensrud Marstein Department for Solar Energy

2 Overview The trouble with conventional solar cells Third generation solar cell concepts Third generation solar cell research at IFE

3 Sunlight Sunlight consists of a large number of photons distributed across a large wavelength and energy range. The photon energy depends on the photon wavelength in the following manner: E phot = hc/λ The variation in photon energy makes efficient utilization of all sunlight in one solar cell difficult. Irradiance Wavelength [um]

4 The trouble with conventional solar cells Conventional solar cells are made from one material exhibiting only electronic band gap. Band gap energy E g This makes it possible for such a solar cell to only utilize one photon energy optimally. E g

5 The trouble with conventional solar cells A photon with sufficient energy can excite an electron across the band gap creating a mobile electron and a mobile hole. If the photogenerated electron and hole is collected at the external solar cell terminals, it will contribute to the current from the solar cell. The voltage related to this process is determined by the energy difference at which the electron and hole can be extracted at the respective external terminals. E phot - + ΔE = qv E g

6 The trouble with conventional solar cells Only photons with sufficient energy can excite e - across the band gap E g. Insufficiently energetic photons with E phot < E g will not contribute to the photocurrent generation (1). Photons with E phot > E g will initially generate energetic excited charge carriers (2). Any energy in excess of E g will be wasted heating up the solar cell through thermalization (3). E phot E g

7 Thermalization Thermalization within solar cell materials normally occurs on an extremely rapid time scale Thermalization in silicon: 1: Initial concentration of charge carriers 2: Initial exitation of charge carriers after exposure to laser pulse 3-4:Carrier-carrier collisions lead to an energetic redistribution around carrier temperature 5-6:Carrier-phonon interactions cool down carriers 7-8:Recombination across band gap M.A. Green, UNSW

8 Additional fundamental loss mechanisms 1. Insufficient photon energies E < E g 2. Thermalization of energetic carriers If E > E g, E E g 3. Energy loss across p-n junction 4. Energy loss at contacts 5. Recombination M.A. Green, UNSW

9 Consequences Large current densities can be obtained by selecting a material with a low band gap energy However, due to thermalization, the band gap puts an efficient upper limit to the extractable voltage. If we should be even so lucky! Si bandgap: 1.12 ev Good open circuit voltage: 650 mv

10 The Shockley-Queisser limit The Shockley-Queisser limit is a measure of the upper obtainable efficiency of a perfect solar cell based on only one solar cell material with only one electronic band gap. Perfect solar cell: All photons incident on cell captured Complete absorption of all photons with E > E g Complete thermalization occurs Lossless transport and collection of charge carriers Ideal materials: Only Auger or radiative recombination The efficiency limit of a perfect, conventional Si solar cell is 29%

11 Third generation photovoltaics Useful definition: Solar cell concepts that allow for a more efficient utilization of the sunlight than solar cells based on only one electronic band gap. Main approaches: Modification of the photonic energy distribution prior to absorption in a solar cell Utilization of materials or cell structures incorporating several band gaps Reducing losses due to thermalization

12 Modifying the photon energy distribution Photon energy down-conversion: one energetic photon creates two or more less energetic ones. Nanocrystals/quantum dots Photon energy up-conversion: two or more low energetic photons create one sufficiently energetic one. Nanocrystals/quantum dots Phosphors Thermophotovoltaics: The sunlight heats a thermally radiating object, which again powers a suitable solar cell. Down conversion Up conversion Marstein IFE/UiO

13 Photon energy conversion Photon energy down-conversion in front of the solar cell Anti-reflection coating Transparent conductive layer Plastic material Cover glass Separate layer (plan B) Photon energy up-conversion on the rear of the solar cell Surface passivated dielectric Rear local contact openings Separate layer (plan B) Down conversion Up conversion

14 Thermophotovoltaics Principle of operation: 1. Sunlight heats a layer above cell. 2. This layer radiates at a temperature more suited to the (LOW) band gap of the solar cell beyond. Features: Fewer high energy photons are generated. Low energy photons are reflected from the solar cell and recycled.

15 Multiple band gap solar cells Multiple band gap solar cells can be realized in a number of ways Physical splitting of the solar spectrum onto different solar cells Renaissance: DARPA (a) Tandem/multi-junction solar cells Current world record holder! >40% efficient! Two or more terminals (b) Intermediate band gap solar cells In Norway: NTNU

16 Multi-junction solar cells Considerations Matching of current throughput is essential, since all cells in the tandem stack are series connected Very vulnerable to variations in spectral density, which will distort the current matching Less of a problem in space Often difficult to grow the desired materials on top of each others due to mismatched lattice constants. Many realizations: III-V semiconductor technology, a-si:h technology, possible dye sensitized solar cells Emcore

17 Countering thermalization Collect energetic ( hot ) charge carriers before significant thermalization occurs Hot carrier solar cells Use materials wherein hot carriers collide and subsequently ionize atoms, increasing the number of charge carriers Impact ionization solar cells

18 Theoretical efficiency limits Theoretical upper efficiencies for third generation cell concepts have been calculated Cell concept Efficiency (under direct/global irradiation) Possible realization (selected examples) Landsberg 93.3%/73.3% Multicolour 86.8%/68.2% Black-body 85.4%/53.6% Hot carrier, infinite cell tandem, photon energy conversion, thermo-pv 3-level 63.8%/49.3% 3 cell tandem 2-level 55.7%/42.9% 2 cell tandem Homojunction 40.8%/31.0% Homojunction Real cell records ~40%/32.0% 3 cell tandem M.A. Green: Third generation photovoltaics, Springer 2003

19 3 rd generation solar cell research at IFE Main idea Develop and test silicon-based 3 rd generation solar cells Use techniques with a potential for mass-production of large area structures Every single percent is valuable Topics Novel multi-junction materials Nanocrystals Photon energy conversion materials Novel processes for thin and highly efficient silicon-based solar cells Foss (IFE/UiO)

20 3 rd generation solar cell research at IFE Laboratory infrastructure New solar cell laboratory building under construction A complete R&D process line for silicon wafer-based solar cells Recent additions Large-arera sputtering New PECVD chamber Variable angle spectral ellipsometer

21 Conclusion There is much to gain from substantial increases in solar cell efficiency Solar cell efficiencies well in excess of the Shockley-Queisser limit are obtainable, at least in theory The only functioning 3 rd generation solar cells today are multijunction solar cells IFE has started several research projects within this very interesting and fun field

22 Black body radiation (5762 K) VISIBLE

23 Silicon-based solar cells USEFUL PHOTONS WASTED PHOTONS

24 One-step up-conversion USEFUL PHOTONS UPCONVERSION POTENTIAL

25 Down-conversion USEFUL PHOTONS WASTED PHOTONS