EELE408 Photovoltaics Lecture 19: Characterization

Transcription

1 EELE408 Photovoltaic Lecture 9: Characterization Dr. Todd. Kaier Department of Electrical and Computer Engineering Montana State Univerity - Bozeman Electrical Characterization 3 baic technique Illuminated IV: the cell i illuminated at one un and baic cell parameter are meaured Dark IV: cell i in the dark and the cell IV are traced c Voc: c and Voc are meaured at different illumination level Meaurement of Solar Cell Efficiency Baic Structure for IV Teting Air Ma.5 for terretrial cell and AM0 for pace cell Intenity of 00mW/cm = kw/m (one un) Cell temperature of 5⁰C ( not 300K) 4 point probe to remove effect of contact reitance 3 4 Illumination Source Halogen Lamp with Dichroic Reflector The intenity a well a the pectrum mut be matched to un light Ideal artificial ource feature Spatial nonuniformity of le than % Variation in total irradiance with time le than % Filtered for a given reference mimatch of % Give an accuracy of % Cot: <$00,

2 Spectrum from typical olar imulator ource Probing 7 8 Error Automated Electronic Difficult to exactly match olar pectrum Deviation in AM.5 caue error in c One-un illumination i quite intene and heat the cell Poor temperature control introduce error in V oc Contact error in probing Probing error primarily caue Fill Factor error but can alo caue c and V oc error Two weep 0-70% of Voc then 70%-00% of Voc in maller increment 9 0 IV Dark (linear) IV Dark (emilog) Semilog IV plot reveal more information about the diode. Different region of the IV curve are dominated by different lo mechanim

3 Limitation of Dark IV Meaurement Current Comparion (Light) Aume the light IV curve i jut the dark IV curve hifted by the photogenerated current Not alway true, if ha high erie reitance Or if carrier lifetime i a function of voltage in ome multicrytal material Meaure the diode with current in the oppoite direction Under illumination the current croing the junction i nearly uniform and then travel along the emitter to the contact 3 4 Current Comparion (Dark) Model for Effect of Serie Reitance on Part of the Solar Cell In the dark mot of the current croe the junction directly under the contact and doe not travel along the emitter It will have a lower erie reitance than the lighted cae 5 6 Double Diode Model Single diode model aume a ingle value for the ideality factor In reality the ideality factor i a function of the voltage acro the device High voltage Recombination dominated by urface and bulk recombination n Low voltage Recombination in the junction dominate n 7 Double Diode Equation Light R qv R q V kt L 0 exp 0 exp Dark kt R qv R q V kt 0 exp 0 exp kt Small fluctuation in the light intenity overwhelm the effect of the econd diode. More common to ue the double diode equation in the dark V R R hunt hunt V R R 8 3

4 Ideality Factor Meaurement Semilog plot of Dark Current-Voltage High Injection region qv I I0exp nkt qv I I 0 exp nkt q ln I ln I0 nkt V Plot the natural log of the current v the voltage give a lope proportional to q/nkt and an intercept at the ln(i 0 ) Ln(I) R hunt (n= ): e qv/kt R erie (n = ): e qv/kt Bia Voltage (V) 9 0 Problem Meauring the Ideality Factor At low voltage the hunt reitance dominate the device performance and caue a large peak (can not be compenated for) At high voltage the erie reitance dominate caue a large peak in the ideality factor Ideality factor come from a difference of ignal and i prone to noie Temperature change during meaurement introduce error Carrier Lifetime Carrier lifetime in the bulk i compoed of variou recombination mechanim Band to Band or Radiation Recombination Auger Recombination Shockley-Read-Hall (SRH) Recombination via trap bulk rad A SRH For indirect bandgap material uch a ilicon the rad i very large and uually neglected Surface Lifetime Effective Lifetime Include the urface recombination and urface lifetime Function of Surface Recombination Velocity (S), the minority carrier Diffuivity (D), and width of the wafer (W) eff bulk urface W W S D W D : IdenticalSurface S S S : Perfectly Paivated S S : High Recombination LargeS S S 0 Combination of bulk lifetime and urface lifetime If correctly paivated urface then eff = bulk If high bulk lifetime then effective i dominated by the urface: eff = urface 3 4 4

5 Lifetime Meaurement Pule a light ource on the urface and meaure the minority carrier denity give an exponential dependence. ne -t/ t eff ln n 5 5