Flexible and lightweight solar cells using exfoliated single crystal silicon foil basis and incorporating 2D materials

1. Flexible and lightweight solar cells using
exfoliated single crystal silicon foil basis and
incorporating 2D materials
Harry Chou1,3, Jae Hyun Ahn1,2,
Rajesh A Rao4, Leo Mathew4,
and Sanjay K Banerjee1,2,3
Thrust Area
Silicon Absorbers and Cells
Motivation
To create a lightweight and flexible solar cell with high efficiency and
low cost based on single crystal silicon utilizing emerging materials
1Microelectronics Research Center, The University of Texas at
Austin, Austin, Texas 78758, USA
2Department of Electrical and Computer Engineering, The
University of Texas at Austin, Austin, Texas 78758, USA
3Materials Science and Engineering Program, University of
Texas at Austin, Austin, Texas 78712, USA
4Applied Novel Devices Inc., Austin, Texas 78717, USA
The Basis of Our Device Past Work with All-IC Processes DHJ Cell
Experimenting with New Materials and Roll-to-Roll Compatible Processes
We demonstrated a dual heterojunction (DHJ) cell
fabricated by additional thin film and surface
processes to form junctions and contacts. The α-Si
dual heterojunction cell had a peak 19.4% efficiency
(14.9% PCE on Si foil).
Onyegam, E. U., Sarkar, D., Hilali, M. M., Saha, S.,
Mathew, L., Rao, R. A., … Banerjee, S. K. (2014).
Realization of dual-heterojunction solar cells on
ultra-thin ∼ 25 μ m, flexible silicon substrates.
Applied Physics Letters, 104(15), 0–4.
http://doi.org/10.1063/1.4871503
Hilali, M. M., Saha, S., Onyegam, E., Rao, R., Mathew, L., & Banerjee, S. K. (2014).
Light trapping in ultrathin 25 μm exfoliated Si solar cells. Applied Optics, 53(27),
6140–7. http://doi.org/10.1364/AO.53.006140
Process begins with a single
crystal Si wafer.
Seed layer of Ti is deposited on
one side of the wafer for
electroplating.
Thick film of Ni is plated onto the
thin Ti film. The structure is heated
and the coefficient of thermal
expansion (CTE) difference
between the Ni and the Si induces
stress.
Cracking initiates and spreads
laterally to relieve the stress and
the foil is exfoliated off of the
wafer surface. The parent wafer
can return to the beginning of
the process to be exfoliated
again. Multiple foils can be
made from a single wafer.
Onyegam, E. U. (2014). Remote
Plasma Chemical Vapor
Deposition for High Efficiency
Heterojunction Solar Cells on
Low Cost , Ultra-Thin ,
Semiconductor- on-Metal
Substrates. The University of
Texas at Austin.
To reduce the number of vacuum and high temperature process steps
that the Si foil experiences (thereby lowering cost) we experiment with
other materials, 2 dimensional materials. Graphene is a mechanically
flexible, optically transparent, and high mobility semimetal. We can
grow single layer graphene (SLG) or multi layer graphene (MLG) by
CVD. Others have demonstrated that graphene growth and transfer
are roll-to-roll compatible.
Li, X., Cai, W., Colombo, L., & Ruoff, R. S. (2009). Evolution of graphene
growth on Ni and Cu by carbon isotope labeling. Nano Letters, 9(12),
4268–72. http://doi.org/10.1021/nl902515k
Kobayashi, T., Bando, M., Kimura,
N., Shimizu, K., Kadono, K.,
Umezu, N., … Hobara, D. (2013).
Production of a 100-m-long high-
quality graphene transparent
conductive film by roll-to-roll
chemical vapor deposition and
transfer process. Applied Physics
Letters, 102(2), 1–5.
http://doi.org/10.1063/1.4776707
Work by Sony Advanced Materials Laboratories
Process cost analysis Research needed to transition
technology to industry
Summary of progress versus plans Metrics for the next year
On one path of study, we
incorporate multiple layers
of 2D materials. Here,
insulating h-BN.
On another path, we
deposit a thin film Al2O3,
with superior passivation
and dielectric properties.
Graphene acts as both the transparent conducting electrode in
the device as well as forming the junction. It is doped p-type
with AuCl3 coating as a charge transfer dopant.
Acknowledgement: This work is supported by the Department of Energy through the Bay Area Photovoltaic Consortium under Award Number DE-EE0004946
Directly forming a junction between graphene and Si gives high
recombination. By inserting an insulating electron blocking layer
between graphene and Si, recombination is suppressed. This film
can also serve to passivate the Si surface and improve VOC.
Doping the graphene layer increases the carrier concentration
and further improves device performance.
At left, the amount of doping (spin coating
AuCl3 in Nitromethane) correlates positively
with device performance. The dopant reduces
the resistance of the graphene layer, thereby
enhancing the device ability to separate charge.
To further boost the graphene current-carrying
ability we introduced multi-layer graphene
which further improved performance. We also
compare the oxide insulating layer GIS cell with
the h-BN insulating layer GIS cell.
• Fabricate a graphene and exfoliated single crystal
silicon foil single junction device with > 15% PCE
• Improve growth and transfer of still more 2D
materials for use as the I-layer in the GIS cell (or
GSS cell)
• 2D materials such as h-BN, MoS2, MoSe2, etc.
Considerable resources will be needed to improve
process integration (roll-to-roll, etc.) but these are
considered engineering development, rather than
research. Research is needed to find trap and
recombination sites at surfaces and interfaces, to
characterize dopant distribution and uniformity, and to
improve material quality.
The above milestones in blue have been reached. We
have DHJ cell with peak 19.4% PCE (14.9% PCE on Si foil)
and GIS cell with peak 7.8% PCE (4.8% on Si foil)