05_Illuminatedpnjunction

05_Illuminatedpnjunction - The Illuminated p-n Junction...

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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner The Illuminated p-n Junction
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner The Illuminated pn Junction Generation re-visited – Basic requirements – Optical Generation – Absorption Coefficient – Optical Generation Rate The Illuminated pn Junction – IV equation – Physical Meaning of I 0 – What is going on? Different Operating Conditions – Short circuit condition – Open circuit condition – Forward bias – Reverse bias
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Generation revisited Basic requirements: Need to move carriers from one band to the other, so require: – Energy increase greater than the band gap – Must be a carrier available for excitation – Must be a vacant state available for carrier to move to Energy can be provided by any means, however: – Thermal energy only gives nett increase with thermal gradients across device. – Optical absorption does not have such a restriction – will consider ONLY optical absorption Important to remember that each absorption process has its own inverse process (we may not be able to observe them easily but they are there) – In thermal equilibrium they balance exactly – Steady state they are both present though one will be much stronger than the other
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Optical Generation Absorption of photons where the energy of each photon is given by: Energy of photon is primary determinant of what happens when it hits the semiconductor – If E < E G then (ideally) no absorption – If E E G then absorption Energy above the band gap is lost as heat to the crystal lattice (phonons) Major fundamental losses for a solar cell excess holes
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Optical Generation Recall that the bands are actually more complex and vary with crystal momentum – Direct and indirect band gaps arise – Absorption for indirect band gap requires phonons, a three particle process meaning lower absorption
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Absorption Coefficient Absorption coefficient, α , is a measure of the probability that a photon is absorbed – Varies with wavelength – Material specific Absorption depends on likelihood of transition – lower around the band edge increasing further away Direct band gap materials generally have more rapid increase in the absorption – higher α Absorption depth 1/ α is often defined – intensity of light has dropped to 1/ e of initial
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Absorption Coefficient Absorption coefficient of direct materials has the form: Not so straight-forward for indirect band gap materials, like Si, also get absorption below E G
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  • Fall '08
  • Honsberg,C
  • Solar cell, solar electric systems, Electric Systems University, Solar Electric Systems University of Delaware

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05_Illuminatedpnjunction - The Illuminated p-n Junction...

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