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Applications of Photovoltaic Technologies

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2 Solar Cell-structure A solar cell is a P-N junction device Light shining on the solar cell produces both a current and a voltage to generate electric power. Busbar Fingers Emitter Base Rear contact Antireflection coating Antireflection texturing (grid pattern)

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I-V Tester 3

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Illumination Sources ClassSpectral Match Irradiance inhomogeneity Temporal Instability Long TermShort Term A0.75 - 1.25%2%0.5%2% B0.6 - 1.4%5%2%5% C0.4-2.0%10% 4 Table: Solar simulator classification according to IEC 60904-9 Ed. 2.0.

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5 No Resistive Losses ILIL I IDID V Solar Cell model The I-V relation is given as: I o dark saturation current, I L light generated current., n ideality factor.

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6 Solar Cell I-V Curve V I I (diffu.) I0I0 Effect of solar radiation on the I-V curve Under illumination solar cell can be operated in the fourth quadrant corresponding to delivering power to the external circuit A P-N junction in the dark consumes power, as it can be operated in 1 st or 3 rd quadrant Current in the illuminated solar cell is negative, flows against the conventional direction of a forward diode

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7 Solar Cell I-V Curve I sc I VmVm ImIm V oc PmPm V Usual I-V plot of solar cell Current is shown on positive y - axis Solar cell parameters V oc - open circuit voltage, I sc - short circuit current, P m - maximum power point I m, V m current and voltage at maximum power point FF fill factor efficiency R s series resistance R sh shunt resistance

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8 Short-Circuit Current, I sc The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). The short-circuit current is due to the generation and collection of light-generated carriers. The short-circuit current is the largest current which may be drawn from the solar cell. At V=0 I = -I L = I sc I VmVm ImIm V oc PmPm X

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9 Open Circuit Voltage: V oc The open-circuit voltage, V oc, is the maximum voltage available from a solar cell, and this occurs at zero current. I sc I VmVm ImIm PmPm X V oc The open-circuit voltage corresponds to the amount of forward bias on the solar cell junction due to illumination. At I=0 V = V oc

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10 Maximum power: P m I sc I VmVm ImIm PmPm X V oc Power Power out of a solar cell increases with voltage, reaches a maximum (P m ) and then decreases again. P m = I m x V m Remember we get DC power from a solar cell

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11 Fill Factor: FF I sc I VmVm ImIm V oc Ideal diode curve PmPm The FF is defined as the ratio of the maximum power from the actual solar cell to the maximum power from a ideal solar cell Graphically, the FF is a measure of the "squareness" of the solar cell

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12 Efficiency: Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. The efficiency is the most commonly used parameter to compare the performance of one solar cell to another. I sc I VmVm ImIm PmPm X V oc Power Efficiency of a cell also depends on the solar spectrum, intensity of sunlight and the temperature of the solar cell.

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Four Point Probe Resistivity Measurements 13

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14 Effect of R s and FF I sc V oc V I Medium Rs Large Rs Characteristic resistance, R ch Normalized series resistance, r s Effect of series resistance on the FF and maximum power Slope of the I-V curve near V oc gives indication about R s

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15 Effect of R sh on FF I sc V oc V I Medium R sh Slope of the I-V curve near I sc gives indication about R sh Normalized shunt resistance, r sh Effect of series resistance on the FF and maximum power

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ScienceTech 150W ScienceTech 150W IV Substrate > 5.0 cm x 5.0 cm 0.1V to 1.0V I-V V oc I sc P mp (fill factor) (efficiency) 25 16

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ScienceTech 17

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IV measurement 18

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IV Analysis 19

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(IPCE) 20

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Quantum Efficiency 21 Quantum efficiency (Q.E.) is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell. Internal quantum efficiency (IQE) refers to the efficiency with which photons that are not reflected or transmitted out of the cell can generate collectable carriers. External quantum efficiency (EQE) of a silicon solar cell includes the effect of optical losses such as transmission and reflection.

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Quantum Efficiency 22