Electrical Characterization of Solar Cells,Dave Rose Keithley Instruments, Inc.,Agenda,Solar Cell Efficiency I-V measurements of efficiency Understanding the sources of efficiency loss Measuring key parameters of a cell Solar cell material research Advanced material techniques using I-V and C-V measurements Summary and Conclusions,2,Key Technical Challenge: Increase Efficiency,3,I-V Load Sweep of a solar cell,Solar Cell,4,I-V Load Sweep of a solar cell,5,I-V Measurements for solar cell efficiency,Fill Factor:,% efficiency vs. an ideal cell,6,I-V Measurements for solar cell efficiency,7,I-V Measurements for solar cell efficiency,Cell Efficiency ():,Power output as a ratio of power input to the cell. (PIN is the power of the light intensity on the cell),8,I-V Measurements for solar cell efficiency,Cell Efficiency ():,Power output as a ratio of power input to the cell. (PIN is the energy of the light on the cell),Solar Cell,Light Source,One SMU can be used to set and monitor power into a light source.,One SMU conducts the I-V curve on the cell.,Use a light source with a known output power and spectrum.,9,I-V Measurements for solar cell efficiency,10,Photons,Ideal Electrical Model of a Solar Cell,Isrc,Load on cell,Solar cell,Rload,11,Photons,Ideal Electrical Model of a Solar Cell,Isrc,Load on cell,Solar cell,Rload,12,Photons,Real World Electrical Model of a Solar Cell,Isrc,Rsh,Load on cell,Solar cell,Rs (cell + interconnect),Rload,+ V -,13,Photons,Real World Electrical Model of a Solar Cell,Isrc,Rsh,Load on cell,Solar cell,Rload,+ V -,Rs,14,Photons,Real World Electrical Model of a Solar Cell,Isrc,Rsh,Load on cell,Solar cell,Rs,Rload,+ V -,As RS increases; cell efficiency decreases,15,Photons,Real World Electrical Model of a Solar Cell,Isrc,Rsh,Load on cell,Solar cell,Rs,Rload,+ V -,As RSh decreases; cell efficiency decreases,16,Photons,Real World Electrical Model of a Solar Cell,Isrc,Rsh,Load on cell,Solar cell,Rs,Rload,+ V -,As T increases; cell efficiency decreases,17,Measuring Series Resistance (Rs) Low Rs low parasitic loss better efficiency,Rs can be roughly approximated from the slope of this portion of the curve, but there are errors!,18,Measuring Series Resistance (Rs) Low Rs low parasitic loss better efficiency,Parameter to be measured: Rs,Rs Measurement with Slope Method (from a series of forward IV sweeps),This differential method makes the measurement independent of n, I0, and RSh which are more difficult to accurately measure.,19,Measuring Series Resistance (Rs) Low Rs low parasitic loss better efficiency,20,Measuring Shunt Resistance (Rsh) high Rsh low internal loss (not consuming its own current) better efficiency,Rsh can be estimated from slope of a reverse I-V sweep,21,Measuring Characteristic Resistance: matching cells with each other and their loads for maximum output efficiency,Characteristic Resistance (Resistance at Pmax tells you how to connect (set R for load) to avoid external losses,22,Defect Density Measures Trapped Charges using Drive Level Capacitance Profiling (DLCP) technique,NDL = defect density (cm-3) C1, C0 = coefficients of quadratic fit of C-V data q = electron charge (1.60 x 10-19 C) = permittivity (F/cm) A = area of solar cell (cm2) e= charge density (C/cm3) p = hole density (cm-3) xe = distance from interface where EF-Ev=Ee,Measurement system simplifies complex equation to a single value,23,Example: Doping Density Measurement with 4200 CVU Doping Density measures density of P-type and N-Type dopants in the substrate,Plot of: CV sweep (red) 1/C2 (blue) - calculated doping density is derived From 1/C2,Measurement system simplifies complex equation to a single value,24,Solar Cell Pulsed I-V Measurements (Low Voltage),Lower-powered pulse measurements can be used to estimate carrier lifetime. A voltage pulse is applied to the cell. The resulting current and voltage is measured. The carrier lifetime is estimated from the x-intercept of the decay curveSolar cells are highly capacitive. Make sure the pulse width is long enough otherwise the readings will be unsettled.,25,Summary of Electrical Characterization for Solar Cells & Key Parameters,26,Summary of Electrical Characterization for Solar Cells & Key Parameters,27,Summary of Electrical Characterization for Solar Cells & Key Parameters,28,Conclusions (1),Precision I-V measurements of cell parameters help to inform and improve upon development processes Accurate understanding of sources of error can lead to more efficient cell production Cell tests can be done quickly and easily with a single instrument solution,29,Conclusions (2),Doping density, defect density, and resistivity can inform mobility and other fundamental material properties Difficult I-V/C-V measurements of properties of the semiconductor material and doping technologies can be made simple with the proper measurement system,