6.973 Semiconductor OptoelectronicsLecture 8: Photodetectors & PhotovoltaicsRajeev J. RamElectrical EngineeringMassachusetts Institute of TechnologyOutline:•Review of Last Time• Open-circuit Voltage & Short-circuit Current• Maximum Power & Efficiency• Some Practical IssuesIntroduction to Introduction to pp--ii--nnDiodesDiodes• primary transport by diffusion• slow carrier movement by diffusion• small depletion with - high capacitance• can’t change bandgap to reduceparasitic absorption• primary transport by drift• low capacitance• can engineer bandgap so absorption is only in depletion regionnn--typetypeintrinsicintrinsicppAR AR coatingcoatingnn--typetypepp--typetypeAR AR coatingcoatingDesign for high quantum efficiency:• Long intrinsic absorbing region• Low reflectivity surface• Small doped p-regionnn--typetypeintrinsicintrinsicppAR AR coatingcoatingPhotodetectorPhotodetectorQuantum Quantum EfficieincyEfficieincyBandwidth of Bandwidth of PhotodetectorPhotodetectorSo, for a transit time limited device, the bandwidth is…Including the RC limit due to the depletion capacitance…Jang, J.-H. et al.Device Research Conference, 200000.20.40.60.811.210 100 10003 dB Bandwidth (GHz)Equivalent Device Slope Efficiency at λ = 1300 nm (A/W)UCSB / Colorado State, 1993PIN – 1300 nmOrtel, 1994UCSB / Colorado State, 1995GainGain--Bandwidth Product of Bandwidth Product of PhotodetectorPhotodetectorIncident Solar RadiationIncident Solar RadiationIncident intensity (AM1.5):844 W/m2= 5.2 x 1017eV/cm2/s(AM1.5 is through atmosphere at 37o)- The average power incident upon the surface of the Earth from sunlight is ~10,000 timesthe average global power consumption. -The average power incident upon the continental United States is ~500 times our national consumption (total energy consumption, not just electricity). 10% efficient PV systems over 2% of the US.Henry, JAP (1980)-1-0.500.511.52-0.0100.010.020.030.040.050 0.05 0.1 0.15 0.2 0.25 0.3J (A/cm2)VoltagePhotovoltaicsPhotovoltaics• Photocurrent opposes bias currents• Since IV <0 in this case, then the diode is delivering power to the circuit !Incident Solar RadiationIncident Solar RadiationGaAsFor Eg= 1.35 eV:Area Under Curve (1):844 W/m2= 5.2 x 1017eV/cm2/s• Lowering the bandgap increases JLHenry, JAP (1980)PhotovoltaicsPhotovoltaicsOpen-circuitShort-circuit-1-0.500.511.52-0.0100.010.020.030.040.050 0.05 0.1 0.15 0.2 0.25 0.3J (A/cm2)Generated Power (I V )VMaximum PowerMaximum PowerJ (A/cm2)Generated Power (I V )VEabsorptionthermalizationuseful current at Vincomingspectrum• Emis energy per electron delivered to the load at max powerMaximum PowerMaximum Power• Increasing bandgap, increases maximum voltageMaximum Power and EfficiencyMaximum Power and EfficiencyFor Eg= 1.35 eV:= 31%Henry, JAP (1980)Maximum Power and EfficiencyMaximum Power and Efficiency•Increasing Egimproves voltage for a given total input power•Reducing Egimproves current since more light is collectedAPS (1979)Practical Issues: Silicon Practical Issues: Silicon nn--ppPhotovoltaicPhotovoltaicpp--typetypenn--typetypenn--typetypepp--typetypelong λshort λHovel, IEDM (1979)Practical Issues: Silicon Practical Issues: Silicon nn--ppPhotovoltaicPhotovoltaicBoundary Condition at Ideal Ohmic Contact: Boundary Condition at Semiconductor Surface: • Spis the surface recombination rate or velocitynn--typetypenn--typetypePractical Issues: Silicon Practical Issues: Silicon nn--ppPhotovoltaicPhotovoltaicBoundary Condition at Semiconductor Surface: • Recombination at surface reduces the contribution of the high energy photoncsHovel, IEDM (1979)HeterojunctionHeterojunctionPhotovoltaicPhotovoltaicThermophotovoltaicThermophotovoltaice-reflectorh+reflector• Long narrow bandgap absorber• Heterobarriers to guide transport• Wide bandgap increases VocThomas Surek, NRELPhotovoltaic Materials and DevicesPhotovoltaic Materials and DevicesE1st absorptionthermalizationthermalization2nd absorptionuseful current at Vlowuseful current at VhighMultijunctionMultijunctionCellsCellsMaximum Power and EfficiencyMaximum Power and EfficiencyHenry, JAP