PVIC research into the CdTe absorber layer in photovoltaic devices.

1. Development of CdCl2 Activation to Minimize Zn Loss from Sputtered Cd1-xZnxTe Thin Films for Use in
Tandem Solar Cells
Fadhil K. Alfadhili, Geethika K. Liyanage, Adam B. Phillips, and Michael J. Heben
Increasing the band gap of cadmium telluride (CdTe) from 1.48 eV
to > 2 eV can be achieved by alloying CdTe with ZnTe. Like CdTe, the
alloyed films are expected to allow for low cost production,
suggesting that Cd1-xZnxTe could be an ideal top cell for mass
produced tandem devices. However, the CdCl2 activation of the
alloyed films results in a significant loss of Zn, thereby reducing the
bandgap. In this study, we demonstrate a novel CdCl2 activation
method that does not result in significant Zn loss. By performing the
activation step in a closed, inert environment we are able to avoid
oxidation of the Zn in the Cd1-xZnxTe film; furthermore, by
including sacrificial Zn in the container, an overpressure of ZnCl2
forms limiting the amount of ZnCl2 formed in the film. Both x-ray
diffraction, optical measurements, and Auger spectroscopy show
that the CdCl2 treatment with no flowing gas minimizes the loss of
Zn from the CZT alloy.
Click here for the paper.
MRS Advances 3, 2018 , pp. 3129-3134. DOI: 10.1557/adv.2018.521
PL of CdCl2 of CdZnTe

2. Impact of Infrared Optical Properties on Crystalline Si and Thin Film CdTe Solar Cells
Indra Subedi, Timothy J Silverman, Michael G. Deceglie, and Nikolas J. Podraza
Photons with energies below the photovoltaic (PV) absorber band
gap do not generate current and adversely impact performance
when absorbed in other solar cell components to produce heat.
Here we incorporate infrared (IR) optical response in simulations
for understanding thermal losses. Spectroscopic ellipsometry is
used to measure Al-Si interface optical properties in Si PV.
Reflectance of an Si module has been analyzed to account for
encapsulant and Al-Si interface contributions. IR extended
quantum efficiency simulations calculate efficiency gains / losses
arising from variations in current generated and total reflectance
as functions of transparent front contact material in thin film
CdTe PV.
Click here for the paper.
44th IEEE Photovoltaic Specialists Conference (PVSC), Washington DC, 2017.

3. Glancing Angle Deposited CdTe: Optical Properties and Structure
Dipendra Adhikari, Prakash Koirala, Maxwell M. Junda, Robert W. Collins, and Nikolas J. Podraza
7th World Conference on Photovoltaic Energy Conversion, Waikoloa, HI. 2018.
Optical and microstructural properties of as-deposited CdTe films deposited on
soda lime glass by magnetron sputtering at various source flux angles have been
investigated using GIXRD, SEM, unpolarized transmittance / reflectance, and
spectroscopic ellipsometry. Influence of deposition angle on resultant crystalline
grain size and orientation are tracked for these films. All CdTe films studied are
found to have cubic crystal structure and (111) preferential grain orientation.
Films deposited at 0° and 45° are almost entirely (111) oriented, whereas films
deposited at higher angles exhibit a wider variety of competing grain
orientations, suggesting that deposition angle can be used as an effective
parameter towards controlling grain orientation. With increasing numbers of
grain orientations, grain size is found to decrease. Ex-situ spectroscopic
ellipsometry is used to obtain the structural and optical properties. Stress
induced in the film is calculated based on shifts of critical point energies.
Click here for the paper.

4. Low Temperature Photoluminescence Spectroscopy of Defect and Interband Transitions
in CdSexTe1-x Thin Films
Niraj Shrestha, Corey R. Grice, Ebin Bastola, Geethika K. Liyanage, Adam B. Phillips, Michael J. Heben, Yanfa Yan, and Randy J. Ellingson
We present the defect analysis by photoluminescence (PL)
spectroscopy of CdSexTe1-x thin films, grown with varying Se content
by a co-sputtered deposition method. We observe a
peak at 1.203 eV in the CdSexTe1-x film for x = 0.21, which shifts
towards higher energies with increase in laser power. This peak was
assigned to a donor-to-acceptor (DAP)
transition, with a measured j-shift of ~4.7 meV/decade.
Temperature dependent PL intensity measurements confirm that
the observed DAP peak involves a shallow defect state
of binding energy ~34.7 meV. In contrast, a free-to-bound (FB) peak
at 1.294 eV involving a shallow defect of binding energy ~18.3 meV
was observed in the CdSexTe1-x film for x = 0.14.
Additionally, we observe band edge emission at 1.452 eV and 1.448
eV in CdSexTe1-x films for x = 0.14 and x = 0.21 respectively. Our
analysis shows that the Se concentration not only changes the band
gap energy of the resulting CdSexTe1-x alloy thin film, but also
modifies the nature of the dominant observed defect emission.
Click here for the paper.
MRS Advances 2018,1-7. doi: 10.1557/adv.2018.516
PL of CdSeTe

5. Development of CdCl2 Activation to Minimize Zn Loss from Sputtered Cd1-xZnxTe Thin Films for Use in
Tandem Solar Cells
Fadhil K. Alfadhili, Geethika K. Liyanage, Adam B. Phillips, and Michael J. Heben
MRS Advances, 1-6 (2018); doi: 10.1557/adv.2018.521
We have developed a CdCl2 activation method that does not result
in significant Zn loss for Cd1-xZnxTe thin films. By performing the
activation step in a closed, inert environment we are able to avoid
oxidation of the Zn in the Cd1-xZnxTe film; furthermore, by including
sacrificial Zn in the container, an overpressure of ZnCl2 forms
limiting the amount of ZnCl2 formed in the film. The results suggest
that CdCl2 activation of Cd1-xZnxTe films in a static, inert atmosphere
is well behaved and can be used for a controlled device processing.
Click here for the paper.

6. Current Enhancement of CdTe-Based Solar Cells
We report on the realization of CdTe solar cell photocurrent enhancement using an
n-type CdSe heterojunction partner sputtered on commercial SnO2/SnO2:F coated
soda-lime glass substrates. With high-temperature close-space sublimation CdTe
deposition followed by CdCl2 activation, this thin-film stack allows for substantial
interdiffusion at the CdSe/CdTe interface facilitating a CdSexTe1-x alloy formation.
The bowing effect causes a reduced optical bandgap of the alloyed absorber layer
and, therefore, leads to current enhancement in the long-wavelength region and a
decrease in open-circuit voltage (VOC). To overcome the VOC loss and maintain a
high short-circuit current (JSC), the CdTe cell configuration has been modified using
combined CdS:O/CdSe window layers. The new device structure has demonstrated
enhanced collection from both short-and long-wavelength regions as well as a VOC
improvement. With an optimized synthesis process, a small-area cell using
CdS:O/CdSe window layer showed an efficiency of 15.2% with a VOC of 831 mV, a
JSC of 26.3 mA/cm2, and a fill factor of 69.5%, measured under an AM1.5
illumination without antireflection coating. The results provide new directions for
further improvement of CdTe-based solar cells.
Click here for the paper.
Naba R. Paudel, Jonathan D. Poplawsky, Karren L. Moore, Yanfa Yan
IEEE Journal of Photovoltaics, 5, 1492-1496 (2015); DOI: 10.1109/JPHOTOV.2015.2458040

7. High temperature CSS processed CdTe solar cells on commercial SnO2:F/SnO2 coated soda-lime glass
substrates
We report on the realization of CdS/CdTe solar cells grown on commercial SnO2:F/SnO2-
coated soda-lime glass substrates with conversion efficiencies >16 % by careful
optimization of fabrication processes, which consist of sequential deposition of
magnetron sputtered CdS window layer and high temperature close-space sublimation of
CdTe absorber, CdCl2 activation, and thermal evaporation of elemental Cu/Au bilayer
back contacts. The optimization of CdS and CdTe depositions include the incorporation of
small fraction of oxygen during the growth. In our fabrication, the optimal amount of O in
the deposition ambient is 5 % for CdS deposition and 0.5 % for CdTe depositions. The
incorporation of small amounts of O is found to enhance the short circuit current (JSC)
and open circuit voltage (VOC) of CdTe solar cells. We further identified that the flow rate
of the ambient gas during the CdS sputtering can also affect the performance of the CdTe
solar cells. The optimal flow rate is 40 sccm for our fabrication process. With careful
optimization of our synthesis steps, the best small-area cell has shown an efficiency of
16.4 % with a VOC of 858 mV, a JSC of 26.5 mA/cm2, and a fill-factor of 72.0 % measured
under AM1.5G illumination. This cell was coated with a 100 nm thick magnesium fluoride
anti-reflection coating layer on the glass side.
Click here for the paper.
Naba R. Paudel, Corey R. Grice, Chuanxiao Xiao, Yanfa Yan
Journal of Materials Science: Materials in Electronics, 26, 4708-4715 (2015);DOI:10.1007/s10854-015-2923-5

8. Photoluminescence Spectroscopy of Cadmium Telluride Deep Defects
Deep defect states of Cadmium Telluride deposited via close space
sublimation and magnetron sputtering are evaluated via steady
state and time resolved photoluminescence. Intensity dependent
photoluminescence measurements for as-grown and cadmium
chloride treated samples reveal the recombination mechanism
associated with each transition. The as-grown sputtered film
photoluminescence is weak with broad features while the close
space sublimation film photoluminescence is comparatively bright
and dominated by a deep donor acceptor pair recombination.
Unlike excitonic or free-to-bound transitions, donor acceptor pair
recombination exhibits a distance dependence that determines the
distribution of transition energies and recombination rates. We
measure the PL lifetime with respect to energy as a direct
observation of the increasing donor acceptor pair recombination
rate with decreasing donor-acceptor separation.
Click here for the paper.
Paul J. Roland, Naba R. Paudel, Chuanxiao Xiao, Yanfa Yan, Randy J. Ellingson
2014 IEEE 40th Photovoltaic Specialist Conference, Denver, CO, 2014.
PL of CdTe Deep Defects