Download ANALYSIS OF AlUMINUM NITIRDE (AlN) AND GRADED

School of Electrical, Computer and Energy Engineering
PhD Final Oral Defense
NOVEL MATERIALS, GRID DESIGN RULE, AND CHARACTERIZATION
METHODS FOR MULTI-JUNCTION SOLAR CELLS
by
Jingjing Li
May 18, 2012
2:00 pm
GWC 487
Committee:
Dr. Yong-Hang Zhang (chair)
Dr. Meng Tao
Dr. Dieter Schroder
Dr. Dragica Vasileska
Abstract
This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells
from material development to device design and characterization.
Firstly, among the various methods to improve the energy conversion efficiency
of MJ solar cells using, a novel approach proposed recently is to use II-VI
(MgZnCd)(SeTe) and III-V (AlGaIn)(AsSb) semiconductors lattice-matched on GaSb or
InAs substrates for current-matched subcells with minimal defect densities. CdSe/CdTe
superlattices are proposed as a potential candidate for a subcell in the MJ solar cell
designs using this material system, and therefore the material properties of the
superlattices are studied. The high structural qualities of the superlattices are obtained
from high resolution X-ray diffraction measurements and cross-sectional transmission
electron microscopy images. The effective bandgap energies of the superlattices obtained
from the photoluminescence (PL) measurements vary with the layer thicknesses, and are
smaller than the bandgap energies of either the constituent material. Furthermore, The PL
peak position measured at the steady state exhibits a blue shift that increases with the
excess carrier concentration. These results confirm a strong type-II band edge alignment
between CdSe and CdTe. The valence band offset between unstrained CdSe and CdTe is
determined as 0.63 eV±0.06 eV by fitting the measured PL peak positions using the
Kronig-Penney model. The blue shift in PL peak position is found to be primarily caused
by the band bending effect based on self-consistent solutions of the Schrödinger and
Poisson equations.
Secondly, the design of the contact grid layout is studied to maximize the power
output and energy conversion efficiency for concentrator solar cells. Because the
conventional minimum power loss method used for the contact design is not accurate in
determining the series resistance loss, a method of using a distributed series resistance
model to maximize the power output is proposed for the contact design. It is found that
the junction recombination loss in addition to the series resistance loss and shadowing
loss can significantly affect the contact layout. The optimal finger spacing and maximum
efficiency calculated by the two methods are close, and the differences are dependent on
the series resistance and saturation currents of solar cells.
Lastly, the accurate measurements of external quantum efficiency (EQE) are
important for the design and development of MJ solar cells. However, the electrical and
optical couplings between the subcells have caused EQE measurement artifacts. In order
to interpret the measurement artifacts, DC and small signal models are built for the bias
condition and the scan of chopped monochromatic light in the EQE measurements.
Characterization methods are developed for the device parameters used in the models.
The EQE measurement artifacts are found to be caused by the shunt and luminescence
coupling effects, and can be minimized using proper voltage and light biases.
Novel measurement methods using a pulse voltage bias or a pulse light bias are invented
to eliminate the EQE measurement artifacts. These measurement methods are
nondestructive and easy to implement. The pulse voltage bias or pulse light bias is
superimposed on the conventional DC voltage and light biases, in order to control the
operating points of the subcells and counterbalance the effects of shunt and luminescence
coupling. The methods are demonstrated for the first time to effectively eliminate the
measurement artifacts.