Introduction to Solar Cell Materials-I 23 July 2012 P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Photovoltaic cell: short history 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, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Photovoltaic cell: working principle Continuous Current P-type silicon N-type silicon Conventional photovoltaic cells are based p-n junction between semiconductors. P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Solar Cell Efficiency Limits What limits the efficiency of a p-n solar cell? P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Modern solar cell P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Materials for photovoltaic cells 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, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Solar Spectrum Spectral power density [(W/m 2 )/nm] Wavelength [nm] P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Efficiency One of the most important parameters of the photovoltaic cell is the efficiency defined as: EFFICIENCY = η = Max electrical power produced by the cell Total solar power impinging on the cell Example: 10 W/dm 2 η = 10% 1 W 1dm η = 20% 2 W 1dm It is important to increase as much as possbile the efficiency. P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Figures of merit 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 = max I SCVOC I I MP SC V V MP OC The overall efficiency of a solar cell is larger for larger FF P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Figures of merit PHOTORESPONSIVITY The photoresponsivity is defined as the photocurrent extracted from the solar cell divided by the incident power of the light at a certain wavelength. EXTERNAL QUANTUM EFFICIENCY The external quantum efficiency is defined as the number of charges N e extracted at the electrodes divided by the number of photons N ph of a certain wavelength incident on the solar cell POWER CONVERSION EFFICIENCY The power conversion efficiency is defined as the ratio of the electric power output of the cell at the maximum power point to the incident optical power. P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Which are the factors influencing the cell efficiency? EFFICIENCY MATERIALS Silicon GaAs CdTe Organic.. TECHNOLOGY Single junctions Multiple junctions Hybrid cell Up/Down conversion P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Solar Energy Map P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Solar Cell Spectral Response P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
High Efficiency Solar Cell: GaAs 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, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Max and module level efficiencies P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Beyond the Shockley-Queisser limit 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 Nanotecnology P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
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, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Multijunctions Eg=1.9eV Eg=1.42eV Cell 1 Eg1 Eg=0.7eV Cella 2 Eg2<Eg1 Cella 3 Eg3<Eg2 P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I
Introduction to Solar Cell Materials Continue next lecture P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I