1. GRAPHENE IN SOLAR CELLS
Dr. Julio Cárabe
División de Energías Renovables
CIEMAT
Madrid – España
Workshop: Graphene Technology in Particle Physics
CIEMAT - Madrid
November 13th, 2017

2. GRAPEHENE IN SOLAR CELLS
INTRODUCTION
POSSIBLE ROLES OF GRAPHENE IN A SOLAR CELL
GRAPHENE-BASED SCHOTTKY DEVICES
GRAPHENE FOR TRANSPARENT ELECTRODES
CONCLUSIONS

3. Graphene has unique properties:
Flexibility
Conductivity
Transparency
Mechanical resistance
No energy gap

4. Solar cells have different elements:
Absorber
Emitter/s
Electrodes (transparent or not)
Anti-reflecting coating

5. Graphene for absorbers: zero energy gap

6. Graphene for emitters: Schottky-junction devices
Source: Dell’Olio, Palmitessa & Ciminelli, Electronics 2016, 5, 73; doi: /electronics

7. Source: Ahn, Chou & Banerjee, J. Appl. Phys. 121, 163105 (2017)

8. Source: Miao et al. Nano Lett. 2012, 12, 2745−2750

9. Graphene for electrodes (transparent electrodes).

10. Particular case: silicon-heterojunction cells (CIEMAT).

11. TCO sheet resistance R□
Particular case: silicon-heterojunction cells (CIEMAT).
TCO sheet resistance R□
Series Resistance Rs
Fill factor FF
Efficiency η

12. Particular case: silicon-heterojunction cells (CIEMAT).
TCO in silicon-heterojunction cells:
Carrier conduction
Optical interface (reflectance).
Thickness: nm
Avg. transmittance: >80%
Sheet resistance: 120 Ω/□

13. Particular case: silicon-heterojunction cells (CIEMAT).
Graphene for silicon-heterojunction-cell electrodes:
Carrier conduction
Optical interface (reflectance).
Thickness: 0.34 nm/monoatomic layer
Avg. transmittance: > 97% (1 layer), 95% (2 layers), 93% (3 layers)
Sheet resistance: 400 Ω/□ (monolayer)

14. Particular case: silicon-heterojunction cells (CIEMAT).
Graphene may replace or be combined with TCOs
Reduce sheet resistance
Reduce average reflectance
Contribute to better light trapping

15. BASIC RESULTS OBTAINED SO FAR AT CIEMAT ON THE APPLICATION OF GRAPHENE TO SILICON-HETEROJUNCTION SOLAR-CELL TRANSPARENT ELECTRODES

16. Quarz
Graphene
TCO (AZO)
TCO sputtered onto graphene

17. Rbilayer = 50-60 Ω/□ (expected)
Corning glass
TCO (AZO)
Graphene
Graphene transferred onto TCO
R□TCO = 120 Ω/□
R□Graph= 110 Ω/□
Rbilayer = Ω/□ (expected)
POSSIBLE REASON: AZO contains Al
(All contacts Al/graphene have shown negative interactions)

18. Reflectance. Effect of graphene.
AZO/Si
Graphene (2l)/AZO/Si

19. 1st test on solar cells. Effect of graphene.
AZO/Si
Graphene (2l)/AZO/Si

20. CONCLUSIONS
Graphene has unique properties which may be used to improve solar cells and photon detectors.
Its use as an absorber is not contemplated owing to its no-gap condition.
Applying it as an emitter opens the door to promising Schottky-junction cells
Graphene can contribute to improve solar-cell electrodes and particularly silicon-heterojunction-cell transparent electrodes.
Graphene/AZO tests were not satisfactory. Graphene/ITO is going to be tested subsequently.

21. We would like to ackowledge the support received from the Spanish Research Agency (AEI) through project GRAFAGEN (ENE C3-3-R).
Julio Cárabe