1 Introduction to Solar Cell Materials 22 February 2011; Ø186 P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
Photovoltaic cell: short history 2 1941 Russell Ohl (Bell Labs) discovered the silicon p-n junction and the effect of light on the junction 1954 Bell Labs researchers Pearson, Chapin, Fuller demonstrated the photovoltaic cell with 4.5% efficiency P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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Solar Cell Efficiency Limits 4 What limits the efficiency of a p-n solar cell? P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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Modern solar cell 7 P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
Materials for photovoltaic cells 8 Bulk semiconductors Silicon Single crystal Multi crystalline Gallium arsenide (GaAs) Other III-V semiconductors CdTe Thin Films semiconductors Amorphous silicon (a-si) Cadmium telluride (CdTe) Copper-Indium diselenide (CuInSe2, o CIS) Coper-Gallium-Indium diselenide (CIGS) Organic and hybrid materials - Small molecules - Polymers - Dye Sensitized Solal Cell P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
Efficiency 9 One of the most important parameters of the photovoltaic cell is the efficiency defined as: EFFICIENCY = = Max electrical power produced by the cell Total solar power impinging on the cell Example: 10 W/dm 2 = 10% 1 W 1dm = 20% 2 W 1dm It is important to increase as much as possbile the efficiency. P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
Figures of merit 10 Important features of the I-V curves The intersection of the curve with the y-axis (current) is referred to as the short circuit current I SC. I SC is the maximum current the solar cell can put out under a given illumination power without an external voltage source connected. The intersection with the x-axis (voltage) is called the open circuit voltage (V OC ). V OC is the maximum voltage a solar cell can put out. I MP and V MP are the current and voltage at the point of maximum power output of the solar cell. I MP and V MP can be determined by calculating the power output P of the solar cell (P=I*V) at each point between I SC and V OC and finding the maximum of P. Fill form factor FF P I V SC max OC I I MP SC V V MP OC The overall efficiency of a solar cell is larger for larger FF P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
Which are the factors influencing the cell efficiency? 11 EFFICIENCY MATERIALS Silicon GaAs CdTe Organic.. TECHNOLOGY Single junctions Multiple junctions Hybrid cell Up/Down conversion P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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High Efficiency Solar Cell: GaAs 16 High conversion efficiency: 25-27% at 1000X High throughput manufacturing process (MOCVD) Present technology for space application Small size (1mm 2 active area) allows for... reduced series resistance losses efficient heat extraction High cost affordable by means of high concentration level P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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Beyond the Shockley-Queisser limit 20 The maximum thermodynamic efficiency for the conversion of unconcentrated solar irradiance into electrical free energy in the radiative limit, assuming detailed balance, a single threshold absorber, and thermal equilibrium between electrons and phonons, was calculated by Shockley and Queisser in 1961 to be about 31%. W. Shockley and H. J. Queisser. J. Appl. Phys. 32 (1961) 510. What do we do to achieve efficiencies > 31 %? Concentration Multijunction Up/Down Conversion Nanotechnology P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
21 The thermalisation loss can be largely eliminated if the energy of the absorbed photon is just a little higher than the cell bandgap. Tandem or multijunction cells tackle this problem by stacking the cells with the highest bandgap uppermost to achieve the desired filtering effect. Increasing the number of cells in the stack improves the performance. The limiting conversion efficiency for direct sunlight amounts to 86.8% for an infinite stack of independently operated cells P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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29 CdTe/CdS Solar Cell 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 cm 2 ) P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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39 Intermediate band solar cells (Multiple photon harvesting) P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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Limiting Efficiency 42 P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I
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Future Paths 45 Cheaper & reasonable efficiency conventional cells Expensive & higher efficiency concentrator cells Cheaper & roll-to-roll processable flexible low efficiency cells New technologies Intermediate band solar cells Nanophase engineered solar cells Up/Down conversion & other technologies. P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials February 2011 Introduction to Solar Cell Materials-I