eamta keynote 2014

Quantum Dot Solar Cells:

A Simulation Approach

Ing. Ariel Cedola

GEMyDE, Departamento de Electrotecnia, Facultad de Ingeniería, Universidad Nacional de La Plata

Calle 48 y 116, 1er Piso, La Plata, 1900, Buenos Aires, Argentina

&

Dipartimento di Elettronica e Telecomunicazioni, Politecnico di Torino

Corso Duca degli Abruzzi 24, 10129, Torino, Italia

2014

Ing. Ariel Cedola – UNLP & POLITO

Quantum Dot Solar Cells (QDSC) have gained

attention during the last years as one of the most

feasible semiconductor structures for the

implementation of the intermediate band solar cell

(IBSC) concept.

IBSCs are p-i-n structures with a narrow band of

energy levels within the bandgap of the intrinsic

region.

According to theoretical predictions, based on

idealized considerations, IBSCs maximum

efficiencies could reach values >60%, due to:

• Absorption of low energy photons (processes 1

and 2 in the figure), generation and escape of

extra carriers.

• Increment of cell short-circuit current (Jsc) with

no degradation of open-circuit voltage (Voc).

Intermediate band (IB)

Barrier

Luque A. et. al, Phys. Rev. Lett.,

Vol. 78, N. 26, p. 5014 (1997)


• Semiconductor nanostructures with quantum mechanical properties (e.g. InAs)

¿What are QDs?

• Size: Base 10-60 nm; Height 4-10 nm

• x, y, z confinement (0D DOS)

• Discrete energy levels


• Bandgap and absorption spectrum depend on materials, sizes and shapes

• Non uniformity: absorption spectrum broadening

4 nm

9 nm

8 nm

InAs/GaAs InAs/GaAs GaN QDs


QDs fabrication

• Stranski-Krastanow method

• QDs layer stacking ND = 1010 – 1011 cm-2


EQE

Quantum efficiency of solar cells with embedded QDs layers

Without QDs

QDs (x, ND)

QDs (x, ND)

QDs (x, ND)


InAs/GaAs Quantum Dot Solar Cells (QDSCs)

InAs QDs

layers

p GaAs n GaAs

i GaAs

Energ

ía [

eV

]

0

-1.5

1.5

x

Energy bands diagram



InAs QDs

layers


< 900 nm

Efot > EG GaAs

Photogeneration

Recombination



> 900 nm

Efot < EG GaAs

Photogeneration

Recombination

Escape

Capture

JQD (EscWLB – CapBWL)



Jolley 2012 Prog. Photovolt.

Guimard 2010 APL Bailey 2011 APL

Yang 2013 SEM&SC


InAs/GaAs QDSCs: Experimental IV curves


QDSCs modeling: State-of-the-art approaches

Our work: Drift-Diffusion + QDs carrier dynamics modeling

2

2 i i i i i id a WL WL ES ES GS GS

i

V qp n N N p n p n p n

x

1

i iWL B B WLnB B nESC nCAP

i

JnR G R R

t q x

1

i ip WL B B WL

B B pESC pCAP

i

JpR G R R

t q x

n n n

V nJ q n qD

x x

p p p

V pJ q p qD

x x

• Poisson equation

• Continuity equations for holes and electrons

Drift-diffusion transport model

i = QDs layer


i i i i i i i

i i

WL B WL WL B WL ES ES WL

nCAP nESC nCAP nESC WL WL

nR R R R R G

t

i i i i i i i i i

i i

ES WL ES ES WL ES GS GS ES

nCAP nESC nCAP nESC ES ES

nR R R R R G

t

i i i i i

i i

GS ES GS GS ES

nCAP nESC GS GS

nR R R G

t

( )

( )

( )( )1n p ESC

n p ESC

n pn pR

DOS

( )

( )

( ) ( )1n p CAP

n p CAP

n p n pR

DOS

=WL, ES, GS; =B, WL, ES

• Rate equations for electrons at each energy level at each QD layer

• Escape and capture rates for electrons (holes)

fe(h)i

fe(h)i


QDSCs modeling

i = QDs layer

• Recombination rates

ddxffffxGi

WLiWLiWLiWLii

x

heWLGAMheWLWL

0

5.1 ',,exp,,,

ddxffffxGi

ESiESiESiESii

x

heESGAMheESES

0

5.1 ',,exp,,,

ddxffffxGi

GSiGSiGSiGSii

x

heGSGAMheGSGS

0

5.1 ',,exp,,,

• Photogeneration rates at each QD energy level

0.2 0.4 0.6 0.8 1 1.2 1.40

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength [nm]

AM1.

5G S

pect

ral I

rradi

ance

[kW

/m2/

um]

GaAs InAs

IEEE ARGENCON 2014


QDSCs modeling

Results: comparison with experimental measurements

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-0.016

-0.014

-0.012

-0.01

-0.008

-0.006

-0.004

-0.002

0

Tensión [V]

Densid

ad d

e c

orr

iente

[A

/cm

2]

Celda de GaAs Ref. [5]

QDSC Ref. [5]

Simulación de la celda de GaAs

Simulación de la QDSC

[5] K. Sablon et al, Strong enhancement of solar cell efficiency due to quantum dots with built in charge,

NanoLetters, vol. 11, pp. 2311-2317 (2011).

Voc=50 mV

Jsc=600 mA/cm2


300 400 500 600 700 800 900 1000 1100 120010

0

101

102

103

104

105

Longitud de onda [nm]

Respuesta

espectr

al [a

.u.]

Celda de GaAs Ref. [5]

QDSC Ref. [5]

Simulación de la celda de GaAs

Simulación de la QDSC

[5] K. Sablon et al, Strong enhancement of solar cell efficiency due to quantum dots with built in charge,

NanoLetters, vol. 11, pp. 2311-2317 (2011).


Results: comparison with experimental measurements (cont.)

Results: Dependence of I-V curves with number of QD layers and density

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-0.025

-0.02

-0.015

-0.01

-0.005

0

Tensión [V]

Densid

ad d

e c

orr

iente

[A

/cm

-2]

ND = 1.2e10 cm-2

ND = 4e10 cm-2

ND = 1e11 cm-2

20x

100x


20 30 40 50 60 70 80 90 10014.6

14.8

15

15.2

15.4

15.6

15.8

16

16.2

Número de capas de QDs

Eficie

ncia

[%

]

ND = 1.2 e10 cm-2

ND = 4e10 cm-2

ND = 1e11 cm-2

Eficiencia de la celda

de GaAs sin QDs

Results: Dependence of the efficiency with the number of QD layers and density


Results: Doping effects

80 nm (NA = 1018 cm-3)

70 nm (NA = 3x1017 cm-3)

600 nm (intr.)

300 nm (ND = 1018 cm-3)

10x

EB-WL

EWL-ES

EES-GS

EB-WL

EWL-ES

EES-GS

1420 meV

Parámetros QDSC-A QDSC-B

Capa p+ GaAs (1018 cm-3) [nm] 80 80

Capa p- GaAs (3x1017 cm-3) [nm] 70 70

Capa i GaAs [nm] 600 600

Capa n+ GaAs (1018 cm-3) [nm] 300 300

ND = Densidad sup. de QDs [cm-2] 6x1010 6x1010

Número de capas de QDs 10 10

Rango de captura de los QDs [nm] 5 5

n: EB-WL, EWL-ES, EES-GS [meV] 140, 62, 70 220, 40, 30

p: EB-WL, EWL-ES, EES-GS [meV] 28, 16, 16 140, 15, 15

n-capWL, n-capES, n-capGS [ps] 0.3, 1, 1 0.3, 1, 1

p-capWL, p-capES, p-capGS [ps] 0.1, 0.1, 0.1 0.1, 0.1, 0.1

rWL, rES, rGS [ns] 1, 1, 1 1, 1, 1


Results: Doping effects (cont.)



Results: Non-linear (additive) behavior & QD dynamics

Full solar spectrum illumination > 900 nm

K. Sablon et al, NanoLetters, vol. 11, pp. 2311-2317 (2011)

0 0.2 0.4 0.6 0.8-30

-25

-20

-15

-10

-5

0

X: 0.001

Y: -8.607

Voltage (V)

Curr

ent

density (

mA c

m-2

)

X: 0

Y: -20.74

X: 0

Y: -29.34

h/e follow hole (faster) dynamics

-> linear (additive) behavior

h/e follow electron (slower) dynamics

-> NON linear behavior


Results: Non-linear (additive) behavior & QD dynamics (cont.)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-40

-30

-20

-10

0

10

20

X: 0

Y: -7.09

Voltage (V)

Curr

ent

density (

mA c

m-2

)

X: 0

Y: -20.74

X: 0

Y: -25.3



010E+12

1,000E+12

010E+16

1,000E+16

010E+20

1,000E+20

010E+24

1,000E+24

010E+28

1,000E+28

0 5 10 15 20

Rat

e [c

m-3

s-1]

# QD layer

GS

tasa_cap_n_GS

tasa_esc_n_GS

tasa_cap_p_GS

tasa_esc_p_GS

g_sol_qd_gs

tasa_rec_GS

abs(net_cap_n_GS)

abs(net_cap_p_GS)

0 5 10 15 20

GS




100E-12

1,000E-12

001E-08

010E-08

100E-08

1,000E-08

001E-04

010E-04

100E-04

1,000E-04

001E+00

0 5 10 15 20

Ocup. factors

0 5 10 15 20

Ocup. factors

GS ES WL GS ES WL





-200E+20

-150E+20

-100E+20

-050E+20

0,000E+00

050E+20

100E+20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

NET ESCAPE

GS->ES ES->WL WL->Barrier

-060E+20

-040E+20

-020E+20

0,000E+00

020E+20

040E+20

060E+20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

NET ESCAPE

Conclusions

• A device-level model including QD intersubband carrier dynamics and transport has been developed

for simulation of QDSCs.

• Preliminary results agree very well with experimental data.

• Effects of doping and non-additive behavior of the QD photocurrent have been investigated.

• QD Photocurrent can be increased with optimal n-uniform doping, althoug the Voc degradation is still

a factor to investigate.

• Non-linearities can be associated to the de-synchronization of QD dynamics


Thankyou for your attention.

[email protected]


eamta keynote 2014

Documents

q p n n n p n p n p

qds qds x

wl wl es es gs gs

jn r g r r t q x

nd qds x

n structures

qds layer ing

ariel cedola gemyde