Diana Shvydka, Ph.D - The University of Toledo

induced separation of electrons and holes and their non-radiative recombination and can be
used as a very sensitive indicator of device degradation.
Low-light mesoscopic properties of thin-film solar cells
Developed database approach to statistical analysis of nominally identical solar cells.
Discovered a crossover light intensity, below which the relative fluctuations of the PV
parameters diverge. Identified a mesoscopic nature of this effect (correlation length vs. cell
diameter). This introduced a simple uniformity diagnostic technique.
Device degradation and accelerated life testing
For the first time observed that in a polycrystalline CdTe/CdS solar cell, illuminated with a
laser beam of constant power, junction PL gradually decreased over time. Two
independent mechanisms were short-time PL decrease due to local heating and more
gradual change related to the material degradation (PL fatigue). This was attributed to
defect creation by the light-generated electrons and holes. Simple analytical model that fits
the data also provides basis for accelerated life testing using PL fatigue.
Non-uniformity diagnostics
Identified nano-scale nonuniformity as a major problem reducing device efficiency and
stability. Numerically simulated system of random diodes describing effects of
nonuniformity and successfully applied it to the thermography data, in collaboration with
the Max-Plank Institute (Germany). Introduced several other mapping techniques such as
PL, open-circuit voltage, and STM current, in particular, with an industry partner, First
Solar, LLC (co-PI in the grant “Non-uniformity losses”). Within the National CdTe team,
organized collaboration in the framework of the “Micro-nonuniformity” sub-team.
Computer modeling
For the first time applied computer modeling to simulate integral parameters of composite
systems of many nano-particles possessing random diode characteristics. Commercial
software PSpice was used to investigate effects of nonuniformities on cells and large-area
integrated photovoltaic modules. This resulted in several important observations, such as
disorder topology having almost no effect on the macroscopic characteristics, scaling
dependencies, and formation of closed current loops.
INDUSTRIAL RESEARCH
At First Solar, LLC, designed and implemented a testing procedure for CdTe
photovoltaic modules where optical absorption measurements were used to characterize
CdTe/CdS alloy composition vs. probe position (composition mapping). Lead X-ray
diffraction (XRD) measurement project and developed associated software aimed at
studying microcrystalline structure of CdTe films. Developed the photoluminescence (PL)
mapping technique to test the device uniformity. Developed bias-dependent PL techniques
as a tool for characterizing the device stability at the earliest stage of degradation.
Established the use of frequency-dependent capacitance techniques to determine the
density of defect states in the device.
SECOND ORDER PHASE TRANSITIONS
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