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

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 class. • 2.627 is the undergrad version of the class. Please ensure you’re signed up for the right version! 2 Buonassisi (MIT) 2011

2.626/2.627: Roadmap You Are Here 3 Buonassisi (MIT) 2011

2.626/2.627: Fundamentals Every photovoltaic device must obey:    Output Energy Conversion Efficiency  Input Energy For most solar cells, this breaks down into: Inputs Outputs Charge Light Charge Charge Charge Solar Spectrum Drift/Diff Absorption Excitation Separation Collection usion  total   absorption   excitation   drift/diffusion   separation   collection 4 Buonassisi (MIT) 2011 ฀

Liebig’s Law of the Minimum 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).  total   absorption   excitation   drift/diffusion   separation   collection 5 Buonassisi (MIT) 2011 ฀

Another system in which all parts must be optimized http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/ligabs.html http://en.wikipedia.org/wiki/Photosynthetic_efficiency Image by Bensaccount on Wikipedia. License: CC-BY-SA. This content is excluded from 6 our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. Buonassisi (MIT) 2011

Learning Objectives: Illuminated Solar Cell 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 ( V bi ) • Bias voltage ( V bias ) • Open-circuit voltage ( V oc ) • Short-circuit current ( J sc ) • Saturation (leakage) current ( J o ) • Maximum power point (MPP) • Fill factor (FF) 7 Buonassisi (MIT) 2011

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. 8 Buonassisi (MIT) 2011

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 ( I o ). 9 Buonassisi (MIT) 2011

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 ( V A ). • 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. 10 Buonassisi (MIT) 2011

2.626/2.627 Lecture 5 (9/22/2011) pn -junction, in the dark No Bias Forward Bias Reverse Bias + - - + Model Circuit - + - + - + + - P N P N P N - + - + + - - + - + - + E E E p-type n-type p-type n-type p-type n-type Band Diagram x x x e - diffusion: e - diffusion: e - diffusion: e - drift: e - drift: e - drift: I I I I-V X X Curve V V V X Buonassisi (MIT) 2011 11

2.626/2.627 Lecture 5 (9/22/2011) pn -junction, under illumination No Bias Forward Bias Reverse Bias + - - + Model Circuit - + - + - + + - P N P N P N - + - + + - - + - + - + E E E p-type n-type p-type n-type p-type n-type Band Diagram x x x e - diffusion: e - diffusion: e - diffusion: e - drift: e - drift: e - drift: ill. current: ill. current: ill. current: I I I I-V Curve V V V X X X 12 Buonassisi (MIT) 2011

Ideal Diode Equation Following the derivation in Green (Ch. 4, Eq. 4.43): I  I o e qV / kT  1   Dark Illuminated I  I o e qV / kT  1    I L Curves designed using ideal diode equation, with I o = 0.1 (a.u.), and I L = 0.6 (a.u.). 13 Buonassisi (MIT) 2011

Graphical Representation of Variables I I  I o e qV / kT  1   Dark I o I L Illuminated I  I o e qV / kT  1    I L I o V Curves designed using ideal diode equation, with I o = 0.1 (a.u.), and I L = 0.6 (a.u.). 14 Buonassisi (MIT) 2011

Graphical Representation of Bias Conditions Reverse Bias: Unbiased: Forward Bias: V applied < 0 V applied = 0 V applied > 0 I  I o e qV / kT  1   Dark Illuminated I  I o e qV / kT  1    I L V applied Curves designed using ideal diode equation, with I o = 0.1 (a.u.), and I L = 0.6 (a.u.). 15 Buonassisi (MIT) 2011

Readings are strongly encouraged • Green, Chapter 4 • http://www.pveducation.org/pvcdrom/, Chapters 3 & 4. 16 Buonassisi (MIT) 2011

Learning Objectives: Illuminated Solar Cell 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 ( V bi ) • Bias voltage ( V bias ) • Open-circuit voltage ( V oc ) • Short-circuit current ( J sc ) • Saturation (leakage) current ( J o ) • Maximum power point (MPP) • Fill factor (FF) 17 Buonassisi (MIT) 2011

Hands-On: Measure Solar Cell IV Curves 18 Buonassisi (MIT) 2011

Overall Circuit Layout IR OFF Yellow Light Switch Printed Solar Circuit Cell Board 19 Buonassisi (MIT) 2011

Printed Circuit Board Layout USB Microcontroller I/O DAC Op Amps Resistors 20 Buonassisi (MIT) 2011

PCB Section 1a: Sweep Voltage – Program 21 Buonassisi (MIT) 2011

PCB Section 1b: Sweep Voltage – Sweep 22 Buonassisi (MIT) 2011

PCB Section 1c: Sweep Voltage – Read 23 Buonassisi (MIT) 2011

PCB Section 2a: Measure Current – Change to Voltage 24 Buonassisi (MIT) 2011

PCB Section 2a: Measure Current – Rescale 0 - 5V - Summing Amplifier 25 Buonassisi (MIT) 2011

PCB Section 2a: Measure Current – Read Current 26 Buonassisi (MIT) 2011

Learning Objectives: Illuminated Solar Cell 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 ( V bi ) • Bias voltage ( V bias ) • Open-circuit voltage ( V oc ) • Short-circuit current ( J sc ) • Saturation (leakage) current ( J o ) • Maximum power point (MPP) • Fill factor (FF) 27 Buonassisi (MIT) 2011

How Solar Conversion Efficiency is Determined from an IV Curve 28 Buonassisi (MIT) 2011

Terminology • Often, PV researchers will report a “current density” (current per unit area, e.g., mA/cm 2 ) 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. 29 Buonassisi (MIT) 2011

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 ( J sc , the maximum current density of the device in short- circuit conditions), open-circuit voltage ( V oc , the maximum voltage produced by the device, when the two terminals are not connected), and fill factor (ratio of “maximum power” to the J sc * V oc product). 30 Buonassisi (MIT) 2011

Efficiency Calculations Illuminated JV Curve Current Density (J) J  J o e qV / kT  1    J L 31 Buonassisi (MIT) 2011

Efficiency Calculations V oc Illuminated JV Curve Current Density (J) J sc MPP J  J o e qV / kT  1    J L 32 Buonassisi (MIT) 2011

Efficiency Calculations Open-circuit voltage (maximum voltage, zero Short-circuit current, zero current (maximum power) V oc current, zero voltage, zero Illuminated JV Curve power) Maximum Power Point (maximum Current Density (J) power, i.e., current- voltage product) J sc MPP J  J o e qV / kT  1    J L 33 Buonassisi (MIT) 2011

Efficiency Calculations Industry Convention: Quadrant flipped! Illuminated JV Curve J sc Current Density Power Density (mW/cm 2 ) Current Density (mA/cm 2 ) MPP V oc 34 Buonassisi (MIT) 2011

Efficiency Calculations Illuminated JV Curve J sc Current Density Power Density (mW/cm 2 ) Current Density (mA/cm 2 ) MPP V oc Power In  V mp  J mp Efficiency    Power Out  35 Buonassisi (MIT) 2011 ฀