Tel Aviv University | Faculty of Engineering

ISES-2015

1

Contacts Configuration for Novel Photo-Thermionic Solar Cells

Rama Sandovsky

Abraham Kribus School of Mechanical Engineering

Tel Aviv University

Support: Israel Ministry of Energy and Water

Gideon Segev

Yossi Rosenwaks School of Electrical Engineering

Tel Aviv University

Tel Aviv University | Faculty of Engineering

ISES-2015

2

The Race to Solar Electricity

PETE

CSP

PV

Tel Aviv University | Faculty of Engineering

ISES-2015

3

Thermionic Conversion

Load

Cat

ho

de

An

od

e e¯

e¯

Energy

Conduction Band

Valence Band

Fermi

Vacuum

Work Function

Waste heat

Electricity

Heat Input

Emission current density

Tel Aviv University | Faculty of Engineering

ISES-2015

4

Photon-Enhanced Thermionic Emission (PETE)

Cathode Anode

-

Schwede et al, Nature materials 2010

Semiconductor cathode

Tel Aviv University | Faculty of Engineering

ISES-2015

5

Contacts Geometry Front grid (as in PV cells) Shading Recombination

hv hv

Contact

Fraunhofer ISE 2-D model - COMSOL

Potential Electrons

concentration Holes concentration

Generation Recombination

Electrons current density

Holes Current density

Tel Aviv University | Faculty of Engineering

ISES-2015

6

Solution in COMSOL-Multiphysics

Electric Potential [V]

Inputs Cathode and anode parameters (materials, sizes, doping concentrations,..) Operation condition (cathode temperature, sun concentration,..) Contact size (fraction) Voltage

Outputs Current density Potential map Charge carrier concentration Recombination concentration

0

20

40

60

80

0 0.2 0.4 0.6

Cu

rren

t[A

/cm

²]

Voltage[V]

Tc=850 K

Tc=750 K

Tc=550 K

Tel Aviv University | Faculty of Engineering

ISES-2015

7

0

50

100

150

550 650 750 850 950

10% contact Sh

ort

-Cir

cuit

Cu

rren

t [A

/cm

²]

Cathode Temperature [K]

The Temperature Effect

PETE Thermionic

Electrons flow lines:

Electrons recombination At contact

Electrons injection (negative recombination)

Tel Aviv University | Faculty of Engineering

ISES-2015

8

Contact Size Effect: Moderate Temperature

Contact area 0.1% 1.0% 10.0% 100.0%

Cu

rren

t [

No

rmal

ized

]

Small contact = Barrier

Tel Aviv University | Faculty of Engineering

ISES-2015

9

Cu

rren

t [

No

rmal

ized

]

Contact area

0

2

4

6

8

10

12

0.1% 1.0% 10.0% 100.0%

Contact Size Effect: High Temperature

Normalized current > 1 Current > Photogeneration

Tel Aviv University | Faculty of Engineering

ISES-2015

10

0

10

20

30

40

50

500 600 700 800 900 1000

Effi

cien

cy[%

]

Temperature [K]

Efficiency

PETE Thermionic

Smaller contact is better Larger contact is better

Impact of contact size is significant

Tel Aviv University | Faculty of Engineering

ISES-2015

11

Conclusions

Contact size is important

PETE regime: Smaller contact

Thermionic regime: Larger contact

Further work

Validate experimentally

Investigate for other cathode materials