Diode Under Illumination ( a.k.a . IV Curve Lecture) Lecture 6 - - PowerPoint PPT Presentation

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Diode Under Illumination ( a.k.a . IV Curve Lecture) Lecture 6 - - PowerPoint PPT Presentation

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Charge Separation Part 2: Diode Under Illumination ( a.k.a . IV Curve Lecture) Lecture 6 9/27/2011 MIT Fundamentals of Photovoltaics 2.626/2.627 Fall 2011 Prof. Tonio Buonassisi 1 Kind Reminder 2.626 is the graduate version of the

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Charge Separation Part 2: Diode Under Illumination (a.k.a. IV Curve Lecture)

Lecture 6 – 9/27/2011 MIT Fundamentals of Photovoltaics 2.626/2.627 – Fall 2011

  • Prof. Tonio Buonassisi

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Buonassisi (MIT) 2011

Kind Reminder

  • 2.626 is the graduate version of the class.
  • 2.627 is the undergrad version of the class.

Please ensure you’re signed up for the right version!

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2.626/2.627: Roadmap

You Are Here

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2.626/2.627: Fundamentals

Charge Excitation Charge Drift/Diff usion Charge Separation Light Absorption Charge Collection

Outputs

Solar Spectrum

Inputs

Conversion Efficiency 

   Output Energy

Input Energy

Every photovoltaic device must obey: For most solar cells, this breaks down into:

฀ total absorptionexcitation drift/diffusion separation collection

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Liebig’s Law of the Minimum

฀ total absorptionexcitation drift/diffusion separation collection

  • S. Glunz, Advances in

Optoelectronics 97370 (2007)

Image by S. W. Glunz. License: CC-BY. Source: "High-Efficiency Crystalline Silicon Solar Cells." Advances in OptoElectronics (2007).

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Another system in which all parts must be optimized

http://en.wikipedia.org/wiki/Photosynthetic_efficiency http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/ligabs.html

Image by Bensaccount on Wikipedia. License: CC-BY-SA. This content is excluded from

  • ur Creative Commons license. For more information, see http://ocw.mit.edu/fairuse.

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1. Diode in the Dark: Construct energy band diagram of pn- junction. 2. Diode under illumination: Construct energy band

  • diagram. Denote drift, diffusion, and illumination

currents. 3. In class exercise: Measure illuminated IV curves. 4. Define parameters that determine solar cell efficiency:

  • Built-in voltage (Vbi)
  • Bias voltage (Vbias)
  • Open-circuit voltage (Voc)
  • Short-circuit current (Jsc)
  • Saturation (leakage) current (Jo)
  • Maximum power point (MPP)
  • Fill factor (FF)

Learning Objectives: Illuminated Solar Cell

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Key Concept: The current-voltage response of an ideal pn-junction can be described by the “Ideal diode equation”. We plot the ideal diode equation for dark and illuminated cases. Forward, reverse, and zero bias conditions are represented on the same curves.

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Exercise: pn-junctions under bias

  • For a pn-junction under different bias conditions,
  • Draw I-V curves for the solar cell.
  • With a dot, denote the “operating point” for each bias

condition.

  • With arrows, denote the magnitude of the the saturation

current (Io).

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Exercise: pn-junctions under bias

  • For a pn-junction under different bias conditions,
  • Draw equivalent circuit diagrams for each bias condition.
  • Draw the external bias (VA).
  • Draw the relative width of the space-charge region.
  • Draw an arrow for the electric field (x). Relative

magnitudes of the arrows correspond to relative magnitudes of the electric fields.

  • Draw the direction of current flow (I).
  • Draw the direction of electron flow.

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No Bias Forward Bias Reverse Bias Band Diagram I-V Curve Model Circuit

pn-junction, in the dark

E

e- diffusion: e- drift:

x

p-type n-type

E

e- diffusion: e- drift:

x

p-type n-type

E

e- diffusion: e- drift:

x

p-type n-type

N P

+ + +

  • N

P N P

2.626/2.627 Lecture 5 (9/22/2011) +

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I V I V I V X X X

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No Bias Forward Bias Reverse Bias Band Diagram I-V Curve Model Circuit

pn-junction, under illumination

E

e- diffusion: e- drift:

  • ill. current:

x

p-type n-type

E

e- diffusion: e- drift:

  • ill. current:

x

p-type n-type

E

e- diffusion: e- drift:

  • ill. current:

x

p-type n-type

I V I V I V N P

+ + +

  • N

P N P

2.626/2.627 Lecture 5 (9/22/2011) +

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+

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+ +

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X X X

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Ideal Diode Equation

I  Io eqV /kT 1

 

I  Io eqV /kT 1

  IL

Curves designed using ideal diode equation, with Io = 0.1 (a.u.), and IL = 0.6 (a.u.).

Dark Illuminated

Following the derivation in Green (Ch. 4, Eq. 4.43):

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Graphical Representation of Variables

Curves designed using ideal diode equation, with Io = 0.1 (a.u.), and IL = 0.6 (a.u.).

IL Io

I  Io eqV /kT 1

  IL

I  Io eqV /kT 1

 

Dark Illuminated

V I Io

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Graphical Representation of Bias Conditions

I  Io eqV /kT 1

 

I  Io eqV /kT 1

  IL

Curves designed using ideal diode equation, with Io = 0.1 (a.u.), and IL = 0.6 (a.u.).

Dark Illuminated

Forward Bias: Vapplied > 0

Reverse Bias: Vapplied < 0

Unbiased: Vapplied = 0

V

applied

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Readings are strongly encouraged

  • Green, Chapter 4
  • http://www.pveducation.org/pvcdrom/,

Chapters 3 & 4.

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1. Diode in the Dark: Construct energy band diagram of pn- junction. 2. Diode under illumination: Construct energy band diagram. Denote drift, diffusion, and illumination currents. 3. In class exercise: Measure illuminated IV curves. 4. Define parameters that determine solar cell efficiency:

  • Built-in voltage (Vbi)
  • Bias voltage (Vbias)
  • Open-circuit voltage (Voc)
  • Short-circuit current (Jsc)
  • Saturation (leakage) current (Jo)
  • Maximum power point (MPP)
  • Fill factor (FF)

Learning Objectives: Illuminated Solar Cell

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Hands-On: Measure Solar Cell IV Curves

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Overall Circuit Layout

Solar Cell Light Switch

OFF Yellow IR

Printed Circuit Board

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Printed Circuit Board Layout

Microcontroller

USB I/O

DAC Op Amps Resistors

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PCB Section 1a: Sweep Voltage – Program

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PCB Section 1b: Sweep Voltage – Sweep

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PCB Section 1c: Sweep Voltage – Read

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PCB Section 2a: Measure Current – Change to Voltage

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PCB Section 2a: Measure Current – Rescale 0 - 5V

  • Summing Amplifier

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PCB Section 2a: Measure Current – Read Current

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1. Diode in the Dark: Construct energy band diagram of pn- junction. 2. Diode under illumination: Construct energy band diagram. Denote drift, diffusion, and illumination currents. 3. In class exercise: Measure illuminated IV curves. 4. Define parameters that determine solar cell efficiency:

  • Built-in voltage (Vbi)
  • Bias voltage (Vbias)
  • Open-circuit voltage (Voc)
  • Short-circuit current (Jsc)
  • Saturation (leakage) current (Jo)
  • Maximum power point (MPP)
  • Fill factor (FF)

Learning Objectives: Illuminated Solar Cell

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How Solar Conversion Efficiency is Determined from an IV Curve

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Terminology

  • Often, PV researchers will report a “current density”

(current per unit area, e.g., mA/cm2) in lieu of “total current”. Normalizing for geometry makes it easier to compare the performance of two or more devices of similar semiconductor materials but different sizes.

  • The variable “I” is typically used to represent “current”,

while the variable “J” represents “current density”. Thus, you may well see “JV curves” reported in the literature.

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Key Concept: “Conversion efficiency” of a solar cell device can be determined by measuring the IV curve. Just three IV-curve parameters are needed to calculate conversion efficiency: Short-circuit current density (Jsc, the maximum current density of the device in short- circuit conditions), open-circuit voltage (Voc, the maximum voltage produced by the device, when the two terminals are not connected), and fill factor (ratio of “maximum power” to the Jsc*Voc product).

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J  Jo eqV /kT 1

  JL

Efficiency Calculations

Current Density (J) Illuminated JV Curve

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Efficiency Calculations Voc Jsc MPP

Illuminated JV Curve Current Density (J)

J  Jo eqV /kT 1

  JL

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Efficiency Calculations Voc Jsc MPP

Open-circuit voltage (maximum voltage, zero current, zero power) Maximum Power Point (maximum power, i.e., current- voltage product)

J  Jo eqV /kT 1

  JL

Illuminated JV Curve Current Density (J)

Short-circuit current (maximum current, zero voltage, zero power)

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Voc Jsc MPP

Current Density

Industry Convention: Quadrant flipped!

Efficiency Calculations

Illuminated JV Curve Current Density (mA/cm2) Power Density (mW/cm2)

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Efficiency Calculations Voc Jsc MPP

Current Density

฀ Efficiency   Power Out Power In  Vmp Jmp 

Current Density (mA/cm2) Power Density (mW/cm2) Illuminated JV Curve

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Efficiency Calculations Voc Jsc MPP

Current Density

฀ Efficiency   Power Out Power In  Vmp Jmp A 

Sunlight (Input) Solar Cell Output Power at MPP

Current Density (mA/cm2) Power Density (mW/cm2) Illuminated JV Curve

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Efficiency Calculations MPP

฀ Fill Factor  FF  Vmp Jmp Voc Jsc

Jmp*Vmp Jsc*Voc

Illuminated JV Curve Current Density (mA/cm2) Power Density (mW/cm2)

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Efficiency Calculations MPP

฀ Fill Factor  FF  Vmp Jmp Voc Jsc

Jmp*Vmp Jsc*Voc

Ratio of areas

  • f two boxes,

defined by JscxVoc, and JmpxVmp.

Illuminated JV Curve Current Density (mA/cm2) Power Density (mW/cm2)

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Efficiency   Power Out Power In  Vmp Imp  Fill Factor  FF  Vmp Imp Voc Isc  Vmp Jmp Voc Jsc

Efficiency Calculations

By combining equations 1 and 2… We obtain:

฀ Efficiency   Power Out Power In  Vmp Imp   FF Voc Isc 

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Why does Efficiency matter?

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Key Concept: “Conversion efficiency” effectively determines the area of solar collectors needed to produce a given amount of power. Since many costs scale with area (e.g., glass, encapsulants, labor, mounting, framing… pretty much everything but the inverter), increasing conversion efficiency is a highly leveraged way to reduce the cost of solar.

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The Area Needed to Produce A Certain Amount of Power Scales with Efficiency

100% efficiency (impossible to achieve) 33% efficiency (space-grade solar cells) 20% efficiency (monocrystalline silicon solar cells) 10% efficiency (thin film material)

Very Expensive material Expensive material Relatively Inexpensive material

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The Cost of Materials (Glass, Encapsulants…) Scales with the Area, i.e., Inversely with Efficiency

See: T. Surek, Proc. 3rd World Conference on Photovoltaic Energy Conversion (WCPEC), Osaka, Japan (2003)

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MIT OpenCourseWare http://ocw.mit.edu

2.626 / 2.627 Fundamentals of Photovoltaics

Fall 2013 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.