1. Growth & Characterization of GaSb
Denise Montesdeoca
Cardenes
Growth & Characterization of GaSb
Quantum Dots for Intermediate Band Solar Cell
Supervisors: Anthony Krier, Peter Carrington
Mid Term Meeting
London 7th-8th Dec 2016
Spain, Canary Islands

2. Madrid 2009-2013: Physics degree (UAM).
Project: “P-doping with Zn on GaAs by MBE” (Basilio J. Garcia).
Spain, Canary Islands
P-doping in Zn was required for building mismatched alloys with Intermediate band behaviour.

3. Madrid 2013-2014: Physics Master Degree
(UAM+ICMM): Advanced Materials.
Project: “Optical properties of
Photonic glasses” ( Alvaro Blanco).
: Employer in the Photonic Crystal group (ICMM).
Spain, Canary Islands

4. Growth & Characterization of GaSb Quantum Dots
Workpackage 3: Materials for energy
Growth & Characterization of GaSb Quantum Dots
for Intermediate Band Solar Cell
MBE growth of quantum structures for increasing efficiency.
Characterization of electrical and spectral properties.
Study of Intermediate band behaviour.
Top Cap: 100nm
Spacer Layer
GaSb
QD GaSb
Spacer Layer
GaSb
QD GaSb
Buffer: 100nm
GaAs(n) Substrate

5. Understanding efficiency limit in Single JunctionCB
1. VOC = EG
2. ISC = Isc(EG)
Single Junction Ideal SC limited by:
Transmission losses
Thermalization losses EG
Theory: ηSJ < 31%
Real: ηSJ = 25%
Si technology VB

6. Alternative to Single Junction: Multijunction Solar Cell
Theory: ηMJ < 50%
Real: ηMJ = 30%
Tandem/stacked cells have produced record efficiencies by splitting the solar spectrum.
Expensive and limited to concentrator systems.

7. Looking for high efficiency: Intermediate Band
Two operating principles of IBSC:
Voc Conservation:
3 Fermi levels( EF,C EF,V EF,IB)
Partial filling of IB
2) Two-Photon Subbandgap Photo-Current (TPPC):
- No overlapping of absorption coefficient
- Partially filled IB
Looking for high efficiency: Intermediate Band CB
1. VOC = EG
2. ISC = Isc(EG) + Isc(EH) + Isc(EH+EL)
Intermediate Band Gap Ideal SC:
More photons absorbed.
Less photons thermalized.
Theory: ηSJ< ηIB=47%
IB needs to be partially filled EH EG IB EL VB
The Solar Energy Institute in Madrid
Luque and Marti, Phys. Rev. Lett. 78 (1997) 5014

8. MBE growth of GaSb QR/QD stacks for SC

9. Characterization of GaSb/GaAs QD SCWL
VOC drops
EQE(%) QD
GaAs
Ref x5
x10
Wavelength (nm)
VOC drops when GaSb QD are incorporated.
Sub-bandgap photocurrent is detected: WL & QD.

10. Intermediate Band behaviour on GaSb/GaAs SC
16K EG EH
Source
1+2
Source 1
With a single Monochromatic light the photoresponse is limited until 0.8eV (1550nm).
Source 1

11. Intermediate Band behaviour on GaSb/GaAs SC
16K EG EL
+ EH
Source
1+2
Source 1
With a secondary source the IB is photofilled enhancing the EQE beyond 0.6eV (2070nm).
Source 1 +2

12. Future work
Ideal intermediate band requires interaction between QD layers. As closer the stacks are larger is the coupling and miniband can be formed.
Redshift of PL emission
By decreasing the Cap the energy of the GL drops. Similar behaviour is expected when QD states couple.

13. Summary Work in progress Solar Cells grown containing GaSb QR by MBE.
GaSb QR identified by TEM, XRD and PL revealing WL.
IB behaviour observed by illuminating with a secondary source.
IV curves confirm that Voc is not conserved by adding QR in the SC.
Work in progress
PL study of Miniband formation by decreasing QD Cap.
Lifetime and photoreflectance study at Tyndall(Secondment 2017).
Modelling IBSC with Tyndall (Secondment 2017).
TEM characterization of the Stack of QDs (UCA, Cadiz).
Study the effect of the incorporation of nitrides in the QD.

14. Skills Acquired GROWTH MODELLING PROCESSING XRD PL (4K) IV EQE
2 PHOTON
SET UP
PROCESSING

15. Outputs Talks at PROMIS Networks Posters Outreach
“GaSb/GaAs Quantum Dot Solar Cell” at Cadiz PROMIS Workshop, Cadiz (Spain), 4th-6th May 2016.
“Two photon photocurrent in GaSb/GaAs Intermediate Band Solar Cell” at Montpellier PROMIS Workshop, Montpellier (France), 30th Aug-3rd Sept 2016.
Posters
“GaSb/GaAs Quantum Dot Solar Cell” at Material Science Institute, Lancaster University (Lancaster), 5th-6th April 2016.
“Two photon photocurrent in GaSb/GaAs Intermediate Band Solar Cell” at MBE16, Montpellier, 1st-5th Aug 2016;.
“GaSb/GaAs Intermediate Band Solar Cell” at Photovoltaic Workshop, Imperial College (London), 15th-16th Sept 2016.
Outreach
Webcast on Photonics: “Solar cells”.
Participation on “Day of Photonics”.

16. Future aspirations Publish high impact papers.
Present talks at international conferences:
PVTC2017: The Photovoltaic Technical Conference, April (Marseille).
MBE2018: International conference in Molecular Beam Epitaxy Sept, Sept (Shanghai).
Finding a postdoc/ fellowship application/ researcher in company.
Obtain a good job in academic/industry.