06_Solar_Cells - Solar Cells ELEG620: Solar Electric...

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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Solar Cells
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Solar Cells We want to maximize the power generated by an illuminated pn junction solar cell, this is done in three major ways: 1. Generate a large short circuit current 2. Generate a large open circuit voltage 3. Maximize the Fill Factor, this means minimizing parasitics as much as possible A lot of the time these requirements overlap Some times they fight against each other Must compromise
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Short circuit current J SC depends on two things : 1. Generation of carriers by light absorption – Absorb light in semiconductor to generate carriers – Minimize reflection – These two depend on the incident light, optical properties of the solar cell, the band gap and thickness of the solar cell 2. Collection of light generated carriers – Depends on material and device parameters – Depends on the surfaces
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Light Absorption 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 Light Absorption Band gap is the primary determinant of the amount of photons available for generating carriers – In general we have one band gap – Lower band gap means more photons available but…. .
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Reflections Two causes of reflection on top surface – Reflection from contact (we’ll come back to this) – ‘Natural’ reflection from exposed semiconductor
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Anti-reflection Use coatings that will ensure there is destructive interference between the incoming and outgoing light waves - need thickness that is ¼ of the target wavelength - refractive index must be geometric mean of air and semiconductor
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Anti-reflection • Can go to more anti-reflection coating • Works to reduce reflection significantly – has a target wavelength, we have a spectrum – need material with optimum refractive index that also can be incorporated with other processing • Can go to more than one anti-reflection coating • Still are targeting a wavelength - can we reduce reflections for all incident wavelengths?
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Texturing • What about texturing of surfaces (front and back)
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This note was uploaded on 12/14/2011 for the course ELEG 620 taught by Professor Honsberg,c during the Fall '08 term at University of Delaware.

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06_Solar_Cells - Solar Cells ELEG620: Solar Electric...

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