Foundations of photovoltaics: II. the photovoltaic effect, materials for applications

Transcription

1 Foundations of photovoltaics: II. the photovoltaic effect, materials for applications Maria L. Calvo Department of Optics Faculty of Physical Sciences Complutense University of Madrid, Spain

2 Outline of the topic Luminous sources: radiation properties The Sun Fundamentals of radiometry Important key parameters Photovoltaic effect Materials for photovoltaic cells

3 The photogalvanic effect A. Edmond Becquerel discovered that when exposing certain materials to sunlight he could generate a weak electrical current. He named this phenomenon the "photovoltaic effect". Alexander-Edmond Becquerel, discovered the first evidence of light energy-electricity conversion effect in 1839 (at the age of nineteen). The photovoltaic effect is the basic process in which a solar cell converts sunlight into electricity. Diagram of apparatus described by Becquerel (1839). Platinum plates immersed in nitric acid. Comments: He said that: << electrical effects arisen with an origin other than a heater one>>.

4 Basic structure: p-n junction p-n junctions are formed by joining n-type and p-type semiconductor materials diffusion diffusion p-type region as a high hole concentration n-type region has a high electron concentration

5 Key input: to know energy levels in a semiconductor

6 The photovoltaic Effect Under open circuit conditions, the light-generated carriers forward bias the junction, thus increasing the diffusion current.

7 Basic equations for photovoltaic Carrier concentration in thermal equilibrium Electron density: Hole density: Effective density of states in the conduction band Effective density of states in the valence band If we take the product: n i : intrinsic carrier concentration (low mass action). T

8 Intrinsic carrier concentration n i strongly dependends on the absolute temperature T. Consequences: solar cells have to operate at a given constant T. Taken from in each type of semiconductor.

9 Solar Cell structure Basic steps in the operation of a solar cell: Generation of light-generated carriers. Collection of the light-generated carries to generate a current. Generation of a large voltage across the solar cell. Dissipation of power in the load and in parasitic resistances. Short circuit current

10 Types of Photovoltaic Cells First Generation PV Cell: Single crystalline silicon Multi-junction cell (different band-gap materials) Second Generation PV Cell: Thin film silicon (amorphous silicon) CdTe (Cadmium Telluride) CuInSe 2 (Copper Indium Diselenide) Third Generation PV Cell Ultra-High Efficiency concepts (>80%) Ultra-low Cost Polymer cells with quantum dots or nanostructures CdTe rods in polymer Carbon nanotube on Si for more efficient solar power

11 A standard Silicon Solar Cell This device structure is used by most manufacturers today. The front contact is usually formed by POCl 3 diffusion The rear contact is formed by firing screen-printed Al to form a back-surface field The cell efficiencies for screen-printed multicrystalline silicon cells are typically in the range of 14 17%.

12 Research Needs for Photovoltaics Lower Costs Efficiency improvements will help to lower costs of PV electricity Low-cost storage required for significant penetration of the grid (> 10%) In the case of silicon solar cells, there is a need for a high-quality, thin silicon wafer or sheet (10 50 m) that can be produced at low cost Automated high throughput process with intelligent process control Low-cost (i.e. abundant) materials must be incorporated into reliable, high performance PV modules and systems

13 Delft Univ. of Technology Thin Film Silicon Solar Cells Companies such as Sharp and Mitsubishi are developing variants of the micromorph solar cell. Applied Materials and Oerlikon have each sold several manufacturing lines that can produce single-junction amorphous silicon and micromorph solar cells.

14 Cadmium Telluride Solar Cells CdTe : Bandgap 1.5 ev; Absorption coefficient 10 times that of Si CdS : Bandgap 2.5 ev; Acts as window layer Limitation : Poor contact quality with p-cdte (~ 0.1 Wcm 2 ) The CdS/CdTe heterojunction solar cell is typically formed by using a chemical bath technique to deposit the CdS and close space vacuum sublimation to deposit the CdTe. Toxicity of Cd is perceived by some to be an issue. Best lab efficiency = 16.5%.

15 Copper-Indium-Gallium-Diselenide Cell NREL has demonstrated an efficiency of 19.9% for the CIGS solar cell. Typically requires relatively high temperature processing (> 500 C).

16 Triple-Junction Solar Cell Spectrolab has reported a conversion efficiency of 40.7% with this solar cell structure operating at ~ 250 suns. (1 sun: irradiance of one solar constant). More recently Fraunhofer ISE has obtained an efficiency of 41.1% with a triple-junction cell operating at ~ 454 suns.

17 Toward the next generation of solar cells 3D solar configuration. Generating up tp 20 times more energy than conventional flat cells. Using standard Monte Carlo simulations and genetic algorithms, a series of triangular structures was arranged to optimize energy density by minimizing cell shading and maximizing the re-absorption of light reflected by surrounding cells (MIT).

18 Research-Cell Efficiencies

19 References References in Internet

20 Largest photovoltaic park in the world: Olmedilla Solar Park, Spain Thank you for your attention Muchas gracias por su atención 60 MWp photovoltaic park installed by Nobesol with modules from Silikin