Charge Extraction Lecture 9 – 10/06/2011 MIT Fundamentals of Photovoltaics 2.626/2.627 – Fall 2011 Prof. Tonio Buonassisi Buonassisi (MIT) 2011
2.626/2.627 Roadmap You Are Here 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 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 Buonassisi (MIT) 2011
Learning Objectives: Charge Extraction 1. Describe the purpose of contacts, and their most common types. 2. Describe the impact of good and poor contacts on IV characteristics. 3. Sketch the IV characteristics of Schottky and Ohmic contacts. 4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction. 5. Sketch common band alignments (Types 1, 2, 3 junctions). 6. Sketch common solar cell device architectures. Buonassisi (MIT) 2011
Contacts • …extract carriers from device. • …prevent back -diffusion of carriers into device. • …are studied extensively in the semiconductor industry (several good review papers) for “common” semiconductors. • …are semiconductor -specific: While fundamentals generally apply universally, the devil is in the details, and each material system requires individual optimization. • … are influenced heavily by surface states (i.e., repeatable surface preparation is a must !) Buonassisi (MIT) 2011
Materials Commonly Used for Contacts • Metals – Optically opaque. – Electrically conductive. • Transparent Conducting Oxides (TCOs) – Optically transparent. – Electrically conductive. Buonassisi (MIT) 2011
Properties of TCOs Conductivity ( ) Transparency Quartz Glass Si Ge ITO Ag 1 Visible Transparency Semi Insulator Metal conductor 6 -18 -14 -10 -6 -2 2 log (S/cm) 0 0 1 2 3 = n e Energy of light (eV) Transmittance: > 80% (Films) n - carrier conc. (cm -3 ) Range: 400 ~ 700 nm - mobility (cm 2 /Vs) Band gap > 3.1eV e - charge per carrier Buonassisi (MIT) 2011
How TCOs Work CB E 1 = Large E E 3 = very small E F E 2 = Large VB x Buonassisi (MIT) 2011
Learning Objectives: Charge Extraction 1. Describe the purpose of contacts, and their most common types. 2. Describe the impact of good and poor contacts on IV characteristics. 3. Sketch the IV characteristics of Schottky and Ohmic contacts. 4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction. 5. Sketch common band alignments (Types 1, 2, 3 junctions). 6. Sketch common solar cell device architectures. Buonassisi (MIT) 2011
Equivalent Circuit: Simple Case Lin Scale I-V Curve Current Density (mA/cm2) 1.E+00 8.E-01 J 0 6.E-01 V ja V 4.E-01 2.E-01 0.E+00 0 0.2 0.4 0.6 0.8 Voltage (V) Log Scale I-V Curve J J 0 exp qV Current Density (mA/cm2) 1 J L 1.E+00 kT 1.E-02 1.E-04 1.E-06 1.E-08 1.E-10 0 0.2 0.4 0.6 0.8 Voltage (V) Buonassisi (MIT) 2011
Equivalent Circuit: Simple Case I-V Curve Current Density (mA/cm2) 5.E-02 R s 4.E-02 J 0 3.E-02 V ja V 2.E-02 1.E-02 0.E+00 0 0.2 0.4 0.6 0.8 Voltage (V) J J 0 exp q V JR s I-V Curve Current Density (mA/cm2) 1 J L 1.E+00 kT 1.E-02 1.E-04 1.E-06 1.E-08 1.E-10 0 0.2 0.4 0.6 0.8 Voltage (V) Buonassisi (MIT) 2011
Equivalent Circuit: Simple Case I-V Curve Current Density (mA/cm2) 5.E-02 R s 4.E-02 J 0 3.E-02 R sh V ja V 2.E-02 1.E-02 0.E+00 0 0.5 1 Voltage (V) I-V Curve J J 0 exp q V JR s V JR s Current Density (mA/cm2) 1 J L 1.E+00 kT R sh 1.E-02 1.E-04 1.E-06 1.E-08 1.E-10 0 0.5 1 Voltage (V) Buonassisi (MIT) 2011
Equivalent Circuit: Simple Case R s J 0 R sh V ja V Courtesy of PVCDROM. Used with permission. Firing contacts? Three possibilities: J J 0 exp q V JR s V JR s 1. Contact just right: low R s , large R sh . 1 J L 2. “Underfired” contact: Poor contact with kT R sh Si, large R s . 3. “Overfired” contact: Metal drives too deep into Si, low R sh . Buonassisi (MIT) 2011
Learning Objectives: Charge Extraction 1. Describe the purpose of contacts, and their most common types. 2. Describe the impact of good and poor contacts on IV characteristics. 3. Sketch the IV characteristics of Schottky and Ohmic contacts. 4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction. 5. Sketch common band alignments (Type 1, 2, 3, and 4 junctions). 6. Sketch common solar cell device architectures. Buonassisi (MIT) 2011
Classes of Contacts Ohmic and Schottky Contacts • Ohmic: – Linear I-V curve. + – Typically used when Current (a.u.) charge separation is not a goal for metallization. 0 Schottky Ohmic • Schottky: - – Exponential I-V curve. – Used when charge - 0 + separation is desired. Voltage (a.u.) Buonassisi (MIT) 2011
Learning Objectives: Charge Extraction 1. Describe the purpose of contacts, and their most common types. 2. Describe the impact of good and poor contacts on IV characteristics. 3. Sketch the IV characteristics of Schottky and Ohmic contacts. 4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction. 5. Sketch common band alignments (Types 1, 2, 3 junctions). 6. Sketch common solar cell device architectures. Buonassisi (MIT) 2011
Step #1: Schottky Theory (the ideal case) Buonassisi (MIT) 2011
Contacts: Schottky Model E Vacuum q c q f M E C E F E V Semiconductor Metal x Buonassisi (MIT) 2011
Contacts: Schottky Model E Vacuum q f M q c E C E F E V Metal Semiconductor x Buonassisi (MIT) 2011
Contacts: Schottky Model • For Ohmic contact: f m > f s • Barrier Height: f b = f m - c • Contact Potential: V bi = f m - f s • Space-charge region width: 2 s W V o qN D Courtesy of Tesfaye Ayalew. Used with permission. http://www.iue.tuwien.ac.at/phd/ayalew/node56.html Buonassisi (MIT) 2011
Classes of Contacts • Ohmic: – Electron barrier Ohmic and Schottky Contacts height ≤ 0 (for n -type) – Linear I-V curve. + – Typically used when charge separation is Current (a.u.) not a goal for 0 metallization. Schottky Ohmic • Schottky: - – Electron barrier height > 0 (for p-type) - 0 + – Exponential I-V curve. – Used when charge Voltage (a.u.) separation is desired. Buonassisi (MIT) 2011
Evaluating Metals for Contacts - Schottky Model Courtesy of Tesfaye Ayalew. Used with permission. http://www.iue.tuwien.ac.at/phd/ayalew/node56.html Buonassisi (MIT) 2011
Reality: Deviations from Schottky theory • Substantial deviations from Schottky theory are possible, due to interface effects including: – Orientation-dependent surface states. – Elemental nature of surface termination in binary compounds ( e.g. , A or B element?). – Interface dipoles. – and more … Courtesy of Tesfaye Ayalew. Used with permission. http://www.iue.tuwien.ac.at/phd/ayalew/node56.html Buonassisi (MIT) 2011
Role of Surface States For related visuals, please see the lecture 9 video or the reference below. D.K. Schroder, IEEE Trans. Electron Dev. 31 , 637 (1984) Buonassisi (MIT) 2011
Contacts: Schottky Model • For Ohmic contact: f m > f s • Barrier Height: f b = f m - c • Contact Potential: V bi = f m - f s • Space-charge region width: 2 s W V o qN D Courtesy of Tesfaye Ayalew. Used with permission. http://www.iue.tuwien.ac.at/phd/ayalew/node56.html Buonassisi (MIT) 2011
Thermionic Emission & Field Emission Effects For related visuals, please see the lecture 9 video or the reference below. D.K. Schroder, IEEE Trans. Electron Dev. 31 , 637 (1984) Buonassisi (MIT) 2011
Evaluating Metals for Contacts - Practical • Sources: – Reference books – Review articles – Scientific articles – Trusted websites https://web.archive.org/web/20130818214213/ http://www.siliconfareast.com/ohmic_table.htm • NB: – Surface states matter!! Be sure you have repeatable surface preparation. Buonassisi (MIT) 2011
Recommend
More recommend
