TUTORIAL COURSE International Conference on Solar Cells January

TUTORIAL COURSE International Conference on Solar Cells January 21 -23, 2008 Centro de Investigación en Energía, UNAM PRINCIPLES OF SOLAR CELLS Cochin University of Science and Technology 20 th January, 2008 1 Aarón Sánchez Juárez Depto. Materiales Solares asj@cie. unam. mx www. cie. unam. mx

Centro de Investigación en Energía, UNAM GOALS To give some fundamentals phenomena about the photovoltaic effect and an outlook of solar cells devices. 2 www. cie. unam. mx

SOME IMPORTANT DEFINITIONS Centro de Investigación en Energía, UNAM ¿What is the Photovoltaic effect? IT IS THE POWER ELECTRICITY GENERATION IN AN OPTOELECTRONIC DEVICE DUE TO THE LIGHT ABSORPTION. DEVICES THAT ARE ABLE TO TRANSFORM LIGHT IN ELECTRICITY ARE COMMON CALLED PHOTOVOLTAIC GENERATORS THE MINIMUM UNIT IN WHICH PV EFFECT IS PRODUCED IS CALLED SOLAR CELL EFFICIENCY CONCEPT POWER IN 3 SOLAR CELL POWER OUT = Po / Pi www. cie. unam. mx

Physical evidence of PV effect Centro de Investigación en Energía, UNAM SOLAR LIGHT: PHOTONS SOLAR CELL Photon energy Ep = h E = 1. 240/ : wavelenght in µm STANDARD TEST CONDITIONS Light intensity: 1. 0 k. W/m 2 Cell temperature: 25ºC Air Mass: 1. 5 Photo-generate Voltage: photovoltage 4 Photo-generate current: photocurrent www. cie. unam. mx

Centro de Investigación en Energía, UNAM How solar cell works? NO LIGHT SOLAR LIGHT ~~ 0. 60 0. 00 (-) Negative charge accumulation SOLAR CELL Positive charge accumulation 5 Voltmeter (+) N Ei field zone Electron and Hole P www. cie. unam. mx

Centro de Investigación en Energía, UNAM How a solar cell works? Borrowed from: 6 www. cie. unam. mx

Electric parameters in a solar cell device Centro de Investigación en Energía, UNAM The open circuit voltage Voc: maximum voltage photo-generated The short circuit current Isc: maximum current photo-generate VOLTMETER (High resistance) STC 0. 7 Voc = 0. 7 Volt AMPERMETER (Zero resistance) (-) I = 0 amp 2 Area 100 cm Isc = 4. 0 Amper V= 0 volt 2 Area 100 cm (+) UNSW PERL: Best m-Si solar cell; Green et al. Prog. Photovolt: Res. Appl. 2007; 15, 35 -40 7 4. 2 STC (+) www. cie. unam. mx

IMPORTANT FACTS Centro de Investigación en Energía, UNAM Ø VOC only depends on the solar cell material. Ø ICC varies proportionally with intensity of light. Ø ICC varies proportionally with the solar cell area. Ø Temperature affects the electrical properties of the solar cell: reduce VOC, power and efficiency 8 www. cie. unam. mx

Centro de Investigación en Energía, UNAM Electric parameters for a coupling solar cell-load Iop and Vop are the current consumed and voltage operation for the electric load coupled directly to a solar cell SOLAR RADIATION POWER ( Pi ) VOLTMETER Vop = 0. 54 volts 0. 54 (-) AMPERMETER (Impedancia=0) 2. 5 Electric load Area 100 cm 2 (+) I I 1 Iop = 2. 5 A Isc Im Maximum power point PM = IM VM Maximum area rectangle I 2 Voc 9 v 1 Vm v 2 V www. cie. unam. mx

Ohm Law Voltmeter Centro de Investigación en Energía, UNAM Less value for R R I Bigger value for R - + V V I Ampmeter Linear behavior indicates: Where V= Voltage (Volt ) I V I= m. V m= 1/R I = Current ( Amper) R = Resistence (ohm) 10 V =RI www. cie. unam. mx

I-V CHARACTERISTIC FOR A SOLAR CELL UNDER DARK CONDITIONS Equivalent circuit Centro de Investigación en Energía, UNAM Typical I vs V curve I ID=I 0 ( P e q. V/Ak. T – 1) Ri N + V I I V Vc Typical p-n junction Diodo Electrical characteristic 11 www. cie. unam. mx

I-V characteristic for a solar cell under illumination Typical I vs V curve Equivalent circuit Centro de Investigación en Energía, UNAM I IS VL V Ri + V IL IL ID IS = ID-IL 12 www. cie. unam. mx

I …Less… I Iqual to… I IL N I 13 CC IM P V I Corriente (A) Centro de Investigación en Energía, UNAM Graphic interpretation Vc V Voltaje (V) M CA www. cie. unam. mx V

Centro de Investigación en Energía, UNAM Electric characteristics IS = IL - ID Voltage source I A RS IL V Power sources 14 RS ID Current source (a) IS (b) H RL (c) Resistances: shunt and series www. cie. unam. mx

Centro de Investigación en Energía, UNAM ¿What kind of materials and Which devices are suitable to build solar cells? Junctions between two or more solid, liquid and gases materials are able to present the photovoltaic effect. THICK SILICON SEMICONDUCTORS SINGLE AND POLICRYSTAL AMORPHOUS Ga. As THIN Cd. Te SINGLE AND POLICRYSTAL Cu. In. Se 2 Best efficiencies are obtained using solid state semiconductors like silicon (Si), Gallium arsenide, Cadmium telluride, Cooper indium selenide. 15 www. cie. unam. mx

Traditional Solar Cells Structures N N Centro de Investigación en Energía, UNAM P P N P I P Metal-Semiconductor SCHOTTKY BARRIER HOMOJUNCTION HETEROJUNCTION S-I-S TYPICAL SOLAR CELL P-N HOMOJUNCTION Collection grid ARC N Layer N P Layer P Rear metallic contact 16 Collector-Emiter Window layer Absorber-Generator Absorber layer www. cie. unam. mx

Centro de Investigación en Energía, UNAM Physical process LIGHT ABSORPTION: Photons that reach the cell with energy grater than bangap are absorbed generating electron-hole pairs, those are the photocarries which are able to produce the photocurrent. PHOTOCARRIERS SEPARATION: It is necessary to build an electric field inside the solid for separing the photocarries (electron-hole) before they recombine. The junction of two material with different type electrical conductivity can build an internal electric field: Example the P-N junction CHARGE COLLECTION: Before photocarriers have a chance to recombine and lost its energy obtained by the photon absorption, it is necessary to collect them. Thus, life time and diffusion length for the photocarries must be bigger to collect in electric metallic contacts before they recombine. 17 www. cie. unam. mx

Centro de Investigación en Energía, UNAM Some physics semiconductors properties Nucleus Orbitals Electron Energy Band Formation E 1 E 2 Atom E 2 Energy band E 1 Energy band 1 atom 18 Bring 2 atoms Bring many together atoms together www. cie. unam. mx

Semiconductor Energy Bands Band Gap Concept Centro de Investigación en Energía, UNAM E Empty Bands Conduction Band Valence Band gap Filled Bands Semiconductor: Full valence band, Empty conduction band Electrical conductivity is thermal activate. At ºK, semiconductors are like insolators. At room temperature, electrons at the top of the balance band acquire the enough energy to jump the energy band gap 19 www. cie. unam. mx

Absorption process Photon energy Ep = h Centro de Investigación en Energía, UNAM h < Eg h = Eg h > Eg No absorption electro-hole photogeneration Band Gap Eg 20 www. cie. unam. mx

How thick to absorb the light? Centro de Investigación en Energía, UNAM Io 21 Direct transition (h ) = C(h -Eg)1/2 > 104 cm-1 I I = Io exp (- x) Indirect transition (h ) = C(h -(Eg-Eph))2 ~ 104 cm-1 www. cie. unam. mx

How thick to absorb the light? Centro de Investigación en Energía, UNAM Ga. As Eg=1. 42 Direct a> 104 cm-1 90% absorption for a thick of 2. 3µm Si Eg=1. 12 Indirect a~102 cm-1 90% absorption for a thick of 230. 0 µm 22 www. cie. unam. mx

Electrical conductivity modification Doping: Silicon case Centro de Investigación en Energía, UNAM Pure Silicon Doped Silicon N-type No mobile carriers Hole (missing electron) extra electron Si Si Si Si P Si Si B Si Si Si Group IV 4 valence electrons 23 P-type Introduce Phosphorus Group V 5 valence electrons Introduce Boron Group III 3 valence electrons www. cie. unam. mx

How to build an Internal electric Field? Centro de Investigación en Energía, UNAM P-N Junction N P Electron diffusion N Hole diffusion + + + - P Internal Electric Field 24 www. cie. unam. mx

Materials combination to form junctions Centro de Investigación en Energía, UNAM Homojunction with indirect band gap material Heterojunction with indirect band gap material 25 Borrowed from: A. L. Fahrenbruch and r. H. Bube; Fundamentals of Homojunction with direct band gap material Heterojunction with direct band gap material Solar Cells; PV Solar Energy Conversion www. cie. unam. mx

P-N Junction under Illumination Centro de Investigación en Energía, UNAM LIGHT Charge Concentration Electric Field Energy Band Diagram Optically Generated Current!! 26 www. cie. unam. mx

Efficiency Centro de Investigación en Energía, UNAM IO IR Ø Photons Can be: Ø Reflected off surface Ø Transmitted through material Ø Absorbed by material: Useful light! IT In order to increase the efficiency, o Light must be absorbed at (or near) junction to be useful o Photocarriers must be collected at external metallic contact 27 www. cie. unam. mx

Efficiency Centro de Investigación en Energía, UNAM Max Theoretical Efficiency ~28% The current best (in Lab) Si = 24. 7% Ga. As = 26% 28 www. cie. unam. mx

Centro de Investigación en Energía, UNAM Consideraciones Tecnologias para Fabricación de Celdas Solares Existen varios materiales con los que se fabrican las Celdas Solares. Entre ellos, el que destaca es el SILICIO. El Campo Eléctrico Interno, responsable de la separación de los portadores fotogenerados, es el componente más importante de la celda solar. Este se puede lograr mediante diferentes uniones entre materiales. Destacan: HOMOUNIONES: La más popular Silicio tipo-n con silicio tipo-p HETEROUNIONES: Histórica Cd. S tipo-n / Cux. S tipo-p BARRERA SCHOTTKY: Unión rectificadora metal/semiconductor UNION M/I/S: Unión rectificadora metal/aislante/semiconductor. UNION S/I/S: Unión rectificadora semicond/aislante/semicond. Celda típica: SILICIO AMORFO 29 www. cie. unam. mx

Centro de Investigación en Energía, UNAM Technologic criteria to chose materials Absorber Window 30 ØBand gap around 1. 4 e. V (max abs of solar radiation and min ID). ØDirect Optical absorption ØLong minority-carrier diffusion lengths ØType P Semiconductor ØConductivity ~10 -3 (. cm)-1. ØEg as large as possible (Rs low) ØThickness like a thin film to reduce Rs. ØType N Semiconductor. ØElectrical conductivity bigger that the absorber. www. cie. unam. mx

Centro de Investigación en Energía, UNAM Technologic criteria to chose materials Both materials Similar value for the electron affinity. Minimum lattice mismatch. Suitable deposition methods for thin film formation and control should be available. Low electrical resistance contacts to both N- and P- type materials. Materials should be enough to allow large area cell production, no toxic, and cost competitive. Solar cell must have an operating lifetime sufficient to pay back economic and energy cost required to produce it. 31 www. cie. unam. mx

Centro de Investigación en Energía, UNAM Celda Solar de Silicio Espectro de la dencidad de energía Exceso de energía Generada 32 Brecha de banda prohibida de Si Energía efectiva Eg = 1. 1 (e. V) Perdida debido a la transmisión Longitud de onda www. cie. unam. mx

Centro de Investigación en Energía, UNAM Celda Solar de Ga. As Exceso de energía Generada Espectro de la dencidad de energía Brecha de banda prohibida de Ga. As Eg = 1. 43 (e. V) Brecha de banda prohibida de Ge Eg = 0. 65 (e. V) Longitud de onda 33 www. cie. unam. mx

Diagrama de Bandas para unión Multiple Ec Centro de Investigación en Energía, UNAM n+ - n -p - p+ Homouniones Ef Ev p n n+ p+ Semiconductor Grado Banda Heterounión 1 34 2 3 Semiconductores 4 www. cie. unam. mx

Celda Solar Tandem Centro de Investigación en Energía, UNAM E g 3< E g 2 < E g 1 35 www. cie. unam. mx

Efficiency vs Band Gap Centro de Investigación en Energía, UNAM Some new materials and new concepts 36 www. cie. unam. mx

Centro de Investigación en Energía, UNAM Factor de espectro para heterouniones a Base de Cd. S 37 www. cie. unam. mx

Centro de Investigación en Energía, UNAM CONCLUSIONS Ø Some concepts concern to the Properties of PV effect have been presented Ø The sequence of phenomena in the PV effect involves the following process: o Power radiation received from the sun, o Absorption process of light, o Generation and Transport of charge carriers in the semiconductor, o Separation of charge carriers by the junction, and o Collection of these carriers at the contacts to the device Ø PV devices represent a clean energy generator with minimal ambient impacts Ø For terrestrial application, the cost of PV devices must competitive with actually energy technology: large production, large area, thin films cells or concentrating 38 systems using m-Si, could be the solution www. cie. unam. mx

Centro de Investigación en Energía, UNAM Gracias 39 sgestec@cie. unam. mx www. cie. unam. mx