半导体物理学半导体 (33).pdf

Solar Cell（太阳能电池）pn junction solar cell01Heterojunction solar cell02Photovoltaic property03Solar cell material and structure04Solar cell applicationsMini-modules for personal electronicsLarge modules for station,airplane and satellite10200 cm2200800 cm2800 cm2Sub-modules for road or traffic lampspn junction solar cellEven zero bias,an electric field exists in the space charge region.Incident photon illumination can create electron-hole pairs in the space charge region that will be swept out producing the photocurrent IL in the reverse-biased direction.n The photocurrent IL produces a voltage drop across the resistive load which forward biases the pn junction.The forward-bias voltage produces a forward-bias current IF.n The photocurrent is always in the reverse-biased direction and the net solar cell current is also always in the reverse-biased direction.Net current=IL-IF Heterojunction solar cellpN heterojunction energy-band diagram in thermal equilibrium.n Photons with energy less than EgN will pass through the wide-bandgap material,which acts as an optical window,and photons with energies greater than Egp will be absorbed in the narrow bandgap material.n If EgN is large enough,then the high-energy photons will be absorbed in the space charge region of the narrow-bandgap material.It should have better characteristics than a homojunction cell,especially at the shorter wavelengths.Heterojunction solar celln The wide bandgap material acts as an optical window for photon energies hEg1.n Photons with energies Eg2hEg1 will create excess carriers in the window type material.Photovoltaic parametersn Short-circuit condition:R=0,V=0,IF=0,ISC=ILn Open-circuit condition:R=,I=0.The photocurrent is just balanced by the forward-biased junction current.The power delivered to the load Photovoltaic parametersThe maximum power to the loadDefine fill factor(FF)Define power conversion efficiency(PCE)Solar spectral and simulatorAir mass zero represents the solar spectrum outside the earths atmosphere.Air mass one is the solar spectrum at the earths surface at noon.3A standard:spectral match,irradiance uniformity,temporal stabilityAM 1.5Efficiency limitationShockley-Queisser limit(S-Q l i m i t),w h e r e t h e o n l y recombination path was assumed to be radiative recombination and photon energy above band g a p w a s a s s u m e d t o b e converted into electron-hole pair with quantum efficiency of 100%.The maximum PCE(S-Q limit)for a solar cell operated under AM 1.5G illumination as a function of the band gap.Record PCEs of different materialsThe highest PCE of single junction solar cell was reported to be 28.8%from GaAs with Eg=1.42 eV,where Jsc=29.68 mA/cm2,Voc=1.122 V and FF=86.5%.The reported PCE of 28.8%is 87.5%of S-Q limit.n With increasing Eg,there are decreased JSC,but increased VOC and FF.n The larger the bandgap,the narrower light absorption,resulting in a decreased photocurrent.n The larger the bandgap,the larger quasi Femi-level splitting,contributing to a increased VOC.Photovoltaic parameters of record efficiency solar cells with respect to S-Q limit(black line).Band gap-dependent efficiencyMonocrystalline Silicon,Amorphous SiliconSolar cell materialIII-V group Semiconductors:GaAs,InP,AlGaAs,InGaP,InGaAsCopper-Indium Gallium Diselenide(CIGS)CdTeFirst-generationSecond-generationThird-generationDye-Sensitized Perovskite:ABX3OrganicSolar cell efficiency recordNREL accessed at March,2020Planar structure,Mesoscopic structure,N-i-p formal structure,P-i-n inverted structure Solar cell structure