Transcript No Slide Title

A Recombination Model for GaAs Solar Cells
Keyuan Zhou and Tim Gfroerer, Davidson College
Yong Zhang, UNC Charlotte
Abstract
Solar cells convert sunlight into electricity. But defects in solar cells are one of the major factors inhibiting conversion
efficiency. Defects allow for the recombination of charge carriers, so they fail to contribute to the electrical output.
Measurements and preliminary analysis by Ashley Finger (’14) show illumination and temperature-dependent trends in
GaAs that help clarify the role of defects. My research aims to develop a new way to analyze the data. In particular, I seek
to improve the model describing the recombination process under the influence of defects in the semiconductor material.
Previous Experiments
What is a GaAs Solar Cell?
Interface (macroscopic defect) region
GaInP
Upper Confinement Layer
Bulk defect region
GaAs
Active Layer
GaInP
Fig 5a: GaAs sample diagram
Lower Confinement Layer
• Ashley Finger (’14) and Dr. Gfroerer made
Fig 5b: Experimental setup diagram from thesis file of Ashley Finger (’14)
prior measurements on a GaAs sample
Fig 1: Picture of a solar cell and diagram of its working mechanism
• A device that converts light energy directly into electricity
• By shining a laser on the sample, electron-hole pairs are generated, and a
camera and oscilloscope are used to study the recombination process
• GaAs is a crystalline compound of elements gallium and arsenic
Our Study and Preliminary Model
• High efficiency but high cost!
What are Defects?
• Our focus is the recombination process of the electrons
and holes around a macroscopic defect.
MODEL
• Previous work included measurements of response under
different illumination at different temperatures
ranging from 77K-295K
• We have constructed a physical model and we compare
theoretical curves with the experimental results
Fig 2a: Illustration of macroscopic defects
• Occur during the manufacturing process
1.0
• Microscopic or bulk defects are misplaced
or alternative atoms in the crystal
0.8
• Macroscopic defects are extended mismatch
features in the crystalline structure
Fig 2a: Illustration of microscopic defects
Carrier Generation and Recombination
• Electrons absorb the energy from a
photon and jump to a higher energy
level
Energy
Conduction Band
Photon in
Photon out
Valence Band
Electrons
Emission Efficiency
• Statistically, defects are unavoidable
0.6
Temperature
77K
131K
185K
239K
295K
0.4
0.2
• The vacancies left behind will have
an effective positive charge – these
vacancies are called holes
15
10
16
17
10
10
-3
Steady-State Carrier Density (cm )
Holes
Fig 6a: Radiative efficiency analysis
Fig 3a: Illustration of Carrier generation and recombination
• This absorption process is called
carrier generation
50
• When the electrons fall down to the lower energy level, a photon may be re-emitted
• Carriers can diffuse before
recombining, and the distance
traveled is called the diffusion
length
Fig 3b: Illustration of diffusion and laser excitation
Why do defects matter?
• Energy from the incident light is dissipated as heat
• Reduces electrical output
• Heat generation can also damage the cell
40
Effective Diffusion Length (m)
• The electron fills the hole (a process called recombination) and the time between
generation and recombination is called the lifetime
Temperature
77K
131K
185K
239K
295K
Fig 6b: Transient analysis of carrier lifetime
• Diffusion lengths are calculated by
the equation
where
• μ is the mobility of electron with
literature value of 8500 cm2V-1s-1
30
20
• k is the Boltzmann constant and q is
the electron charge
10
Note: Initial densities for theoretical
calculations over-estimate experimental
steady-state conditions
0
1014
1015
1016
1017
1018
Carrier Density (cm-3)
Fig 6c: Diffusion length analysis, open symbols are theoretical results
Working Model and Future Work
Late this summer, we discovered a new
method that uses the differential equation
model to fit the data directly without any
assumptions/simplifications
Fig 7: Comparison of the transient data fits with the preliminary model
(red and green) and working model (dark green) at 239K
Fig 4a: Thermal picture of a solar panel by Dr. Gfroerer
Fig 4b: Diagram of defect-related loss