Photoluminescence and Photocurrent in a Blue LED
Ben Stroup & Timothy Gfroerer, Davidson College, Davidson, NC
Yong Zhang, University of North Carolina at Charlotte, Charlotte, NC
Motivation: Solar Cells
Abstract
Model
Solar cells are semiconductor devices that convert light from the solar
spectrum into electricity. LED lights work in reverse; they convert electricity
into light. In this study, we use optical excitation to examine the
photoluminescence and photocurrent generated by blue LEDs across a
range of temperatures and laser powers. In images of the
photoluminescence, we observe contrast related to the localization of
charge carriers and the propagation of light through the device.
Photoluminescence spectra from different regions of the device
demonstrate that propagation depends on the wavelength of emitted light.
And our model of the underlying physics helps us identify the mechanisms
contributing to changes in photoluminescence and photocurrent with
changing temperature and optical power.
Contrast Imaging
The high energy portion of the solar spectrum can be converted into
electricity most efficiently with a semiconductor that has a bandgap
Eg in the blue region of the spectrum. Blue LEDs (light emitting
diodes) work in reverse: they convert electricity into blue light. We
study the behavior of blue LEDs across different temperatures and
optical powers to assess their promise as solar cell devices.
Rate Equations (in steady state):
Open Circuit
𝑑𝑛
𝐵 2
= 𝐺 − 𝑛 − 𝐴𝑛𝑟 𝑛 = 0
𝑑𝑡
𝑁𝑟
Short Circuit
𝑑𝑛
𝐵 2 𝐼𝑜𝑢𝑡
𝐴𝐼 𝑛 − 𝑁𝐼
=𝐺− 𝑛 −
1−
𝑑𝑡
𝑁𝑟
𝑞𝑉
𝑛
𝑛 = density of free carriers
𝐺 = carrier generation rate
𝐵 = radiative recombination
coefficient
𝐼𝑜𝑢𝑡 = measured current out
𝑞𝑉 = charge * volume
=0
𝐴𝑛𝑟 = non-radiative recombination
coefficient
𝐴𝐼 = current activated non-radiative
recombination coefficient = 0.2
𝑁𝑟 = recycling factor = 3
𝑁𝐼 = density threshold for 𝐴𝐼 = 2*1014
Medium Temperature, 155K
.009 W/cm2
7.87 W/cm2
4
Mechanisms
3
_ _
_
Conduction Band
_
2
_ _
100 μm
_ _
Energy
1
100 μm
High Power, Open Circuit
Low Power, Open Circuit
Defect
Level
Eg
2
.009 W/cm2
7.87 W/cm2
The decrease in PL efficiency of the low power, open circuit
configuration as temperature increases is due to an increase in nonradiative (heat) recombination. The decrease in efficiency of the high
power, closed circuit configuration as temperature increases is due to a
decrease in the rate of radiative (light) recombination.
4
Valence Band
1 Absorption
Current
3
2 Emission
+
+ +
3
4
Heat
1) A photon with energy greater than Eg is absorbed and an electron
is excited to the conduction band, leaving a hole in its place.
2) An electron recombines with a hole, producing a photon with
energy hn ≈ Eg (a process called photoluminescence or PL)
3) Electrons and holes can also drift along the built-in electric field
of the device producing current.
4) Electrons can become trapped in lower energy defects, producing
heat when they recombine with holes.
𝐴𝑛𝑟
+ + + + +
100 μm
100 μm
High Power, Short Circuit
Low Power, Short Circuit
The contrast, defined as the difference in intensity between bright and dark regions,
visibly changes with power and temperature. This allows us to analyze the diffusion
of charge carriers (which can depend on trapping), as well as the propagation, reabsorption, and re-emission of light throughout the device.
This Arrhenius Plot shows how the non-radiative recombination
coefficient, 𝐴𝑟 , changes with temperature. The slope of the fitted line
is the thermal activation energy, which is comparable to the energy of
a phonon (one unit of heat).
Spectrum Analysis
Experimental Setup
3
Conclusions
 Spatial inhomogeneity in the PL emission demonstrates the
existence of regions of higher and lower quality (i.e. defects) that
may be related to LED droop.
 Lower energy light recycling largely contributes to the dispersion of
light across the LED device, leading to more uniform emission.
 Two major factors involved in the temperature-dependent PL
efficiency and current generation are radiative (light) and nonradiative (heat) recombination.
 For blue solar cells at room temperature:
• The radiative loss decreases with increasing temperature and
should not inhibit current generation.
• A non-radiative loss mechanism (proportional to the current)
will limit the conversion efficiency above a threshold carrier
density.
Ammeter
4
1
Semiconductor
Low Pass Filter
Side View of LED Device
Camera Spectrometer
1
_
_
2
+
+
_
+
_
2
Slope: Ea ≈ 41 meV
+
2
Semiconductor Edge
Higher energy photons can be re-absorbed and re-emitted locally. Lower energy
photons are less likely to be reabsorbed and travel further, so we see more low
energy light from the edge of the device.
Acknowledgements
We would like to thank the Faculty Study and Research Committee for
supporting this research.