Optoelectronic
Devices
Haris Mehmood
Optoelectronic devices
• Optoelectronic devices convert one energy form to another. These devices
produce light by expending electrical energy. They can also detect light
and transform light signals to electrical signals.
• Examples: Photodiode, solar cell, laser diode, Light-emitting diode (LED)
• Optoelectronic devices consist of a p-type and n-type regions, just like a
regular p-n diode.
• Key differences:
• Additional interaction between the semiconductor and light.
• Combination of various semiconductor layers on the substrate to optimize light
absorption and emission, resulting in a high conversion efficiency.
Optoelectronic devices
In optoelectronic devices, electric fields in the form of applied voltages or
band-bending due to doping are usually utilized to separate the charge
carriers.
Solar Cells
Also known as Photovoltaic (PV) Cell
Video on Global Warming
Energy concerns
Future biggest challenges:
• Energy security
• depleting resources
• climate change
Renewables emerging!
PV growth: 40 GW in 2010 to
>500 GW in 2019 1,2
Approximately 5% of world
population going PV-powered
by 2020 3
Fig.1.1. Oil consumption worldwide [1]
1. BP, “BP Statistical Review of World Energy 2018,” London UK, 2018.
2. REN21, “Renewabale Energy Policy Network for the 21st Century: Renewables 2018 Global Status Report,” Paris, France, 2018.
3. “International Energy Agency (IEA) 2014 Technology Roadmap: Solar Photovoltaic Energy Edition,” 2014, Online: http://www.iea.org/, Accessed on: 21 August, 2018.
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Fig. 1.2. Solar PV Global capacity as recorded in terms of Country/region from 2007-2017
Solar Cell Operation
Sunlight
Cathode
Solar cell converts sunlight into electricity by n-type
Photovoltaic effect:
• Absorption of sunlight Electric field
• Generation of Electron-hole pairs (EHPs)
• Separation of carriers
• Collection at electrode
p-type
Anode
Hole Electron 7
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Maximum theoretical efficiency for Si PV
Si as an absorber has a band gap
of 1.12 eV hc
E ph
Maximum η of 33% according to
revised Quessier Shockly Limit 1,2
• Of the solar light, which does reach a solar
cell, only photons with energy larger than
the energy band gap of the semiconductor
generate electron-hole pairs
• Much portion of the sunlight is wasted as
heat in solar cell (non-radiative)
Sun spectrum and absorption limitation by
Silicon
1. William Shockley and Hans J. Queisser, Journal of Applied Physics, 32 (3), pp. 510–519, 1961
2. S. Rühle, Solar Energy, 130, pp. 139–147, 2016.
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Best efficiency
Max conversion efficiency (theoretical vs achieved)
Crystalline Silicon solar cell
Highest efficient Si solar cell has the conversion
efficiency (η) of 26.6% (IBC-SHJ configuration) 1
Another structure: SHJ HIT proffer η of 24.7% 2
Employ amorphous silicon (a-Si:H) layers
• Improvement in the passivation
• Enhanced voltage (Voc) of the device
• High η potential
Device configuration of record 26.6%
efficient Interdigitated Back-contact solar
cell 1
1. K. Yoshikawa, H. Kawasaki, W. Yoshida, et al., Nat. Energy, 2, p. 17032, 2017
2. M. Taguchi et al., IEEE J. Photovoltaics, 4, 1, pp. 96-99, 2014.
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State-of-the-art configuration of 10
HIT device with 24.7% efficiency
Band diagram of simple solar cell
Band bending physics with MoOx
Band diagram of complete solar cell
Used in research
papers related to
solar cells
IV characteristics of solar cell
(Vm, Pm)
Light Emitting Diode (LED)
• Light emitting diodes are p-n diodes in which
the recombination of electrons and holes
yields a photon.
• This radiative recombination process occurs
primarily in direct band gap semiconductors
where the lowest conduction band minimum
and the highest valence band maximum
occur at k = 0, where k is the wavenumber.
• Examples of direct bandgap semiconductors
are GaAs, InP, and GaN
Composition of LEDs
Band diagram of blue LED
Band diagram of LED
Characteristics of LEDs
• Indium gallium nitride (InGaN): blue, green
and ultraviolet high-brightness LEDs
• Aluminum gallium indium phosphide
(AlGaInP): yellow, orange and red high-
brightness LEDs
• Aluminum gallium arsenide (AlGaAs): red
and infrared LEDs
• Gallium phosphide (GaP): yellow and green
LEDs
