Transcript V oc
Applications of Photovoltaic Technologies 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 Antireflection coating Fingers Emitter Antireflection texturing Base (grid pattern) Rear contact 2 I-V Tester 3 Illumination Sources Class Spectral Match Irradiance inhomogeneity Temporal Instability Long Term Short Term A 0.75 - 1.25% 2% 0.5% 2% B 0.6 - 1.4% 5% 2% 5% C 0.4-2.0% 10% 10% 10% Table: Solar simulator classification according to IEC 60904-9 Ed. 2.0. 4 No Resistive Losses Solar Cell model • The I-V relation is given as: I IL V I I L I0 q(V ) exp 1 nkT ID Io －dark saturation current , IL －light generated current. , n －ideality factor . 5 Solar Cell I-V Curve • A P-N junction in the dark consumes power, as it can be operated in 1st or 3rd quadrant I I (diffu.) V I0 • 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 • Current in the illuminated solar cell is negative, flows against the conventional direction of a forward diode 6 Solar Cell I-V Curve Solar cell parameters I Isc Im Pm • Voc - open circuit voltage, • Isc - short circuit current, • Pm - maximum power point Vm Voc V • Im, Vm – current and voltage at maximum power point Usual I-V plot of solar cell – Current is shown on positive y - • FF – fill factor axis • η – efficiency • Rs – series resistance • Rsh – shunt resistance 7 Short-Circuit Current, Isc I • 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). Pm X Im Vm Voc I I L I 0 (e qV / kT At V=0 I = -IL= Isc 1) •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. 8 Open Circuit Voltage: Voc I Isc Im Pm X Vm Voc I I L I 0 (e qV / kT At I=0 V = Voc • The open-circuit voltage, Voc, is the maximum voltage available from a solar cell, and this occurs at zero current. • The open-circuit voltage corresponds to the amount of forward bias on the solar cell junction due to illumination. 1) kT IL Voc ln( 1) q I0 9 Maximum power: Pm I Im Isc Pm X • Power out of a solar cell increases with voltage, reaches a maximum (Pm) and then decreases again. Vm Voc Pm = Im x Vm • Remember we get DC power from a solar cell 10 Fill Factor: FF I Ideal diode curve Isc Pm Im • 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 Vm Voc • Graphically, the FF is a measure of the "squareness" of the solar cell Max power from real cell Vm I m FF Max power from ideal cell Voc I sc 11 Efficiency: η I Im Isc Pm X • Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. Vm I m Max. Cell Power Incident light Intensity Pin Vm Voc Voc I sc FF Pin • The efficiency is the most commonly used parameter to compare the performance of one solar cell to another. • Efficiency of a cell also depends on the solar spectrum, intensity of sunlight and the temperature of the solar cell. 12 Four Point Probe Resistivity Measurements 13 Effect of Rs and FF Isc Characteristic resistance, Rch Medium Rs RCH Large Rs I Vmax I max Normalized series resistance, rs Voc V • Slope of the I-V curve near Voc gives indication about Rs Rs rs RCH • Effect of series resistance on the FF and maximum power Pm, s P0 (1 rs ) FFs FF0 (1 rs ) 14 Effect of Rsh on FF Normalized shunt resistance, rsh Isc Rsh rsh RCH Medium Rsh I • Effect of series resistance on the FF and maximum power Voc V • Slope of the I-V curve near Isc gives indication about Rsh Pm, sh 1 P0 (1 ) rsh 1 FFsh FF0 (1 ) rsh 15 ScienceTech 150W太陽光模擬器 • • • • 機型：ScienceTech 150W太陽光模擬器與IV 量測系統 Substrate：> 5.0 cm x 5.0 cm 可量測範圍：0.1V to 1.0V I-V曲線中之各項性能參數：開路電壓（Voc）、短路電 流（Isc）、最大輸出功率（Pmp）、並自動計算填充因子 (fill factor) 、太陽電池效率(efficiency) • 具有溫控功25℃ 16 ScienceTech 17 IV measurement 18 IV Analysis 19 全波段入射光子轉換效率光度計 (IPCE) 20 Quantum Efficiency 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. 21 Quantum Efficiency 22