H. Wang1, J. X. Wang1, A. B. Prakoso1,2, and Rusli 1,2

1School of Electrical and Electronic Engineering,

Nanyang Technological University, Singapore 2CINTRA UMI CNRS/NTU/THALES 3288, Research Techno

Plaza, NTU, Singapore

email: erusli@ntu.edu.sg

Thin Film Silicon Nanowire/PEDOT:PSS

Hybrid Solar Cells with Surface Treatment

1

Nanyang Technological University

Singapore 2

Outline

Introduction

SiNWs/PEDOT:PSS hybrid cell

Experiments, results and discussions

Fabrication of thin film SiNWs/PEDOT:PSS cells

Two-step surface treatment

Characterization of hybrid cells

Conclusions

3

Inorganic solar cells High carriers mobility

High PCE & stability

High material cost

Complex fabrication

Organic solar cells Low material cost

Simple solution process

Low carriers mobility

Low PCE & poor lifetime

Hybrid solar cells Combined advantages

Introduction

4

Si/PEDOT:PSS hybrid cell

Si is abundant, non-toxic

Eg = 1.1 eV, large absorption spectrum

PEDOT:PSS is transparent and conductive

Si/PEDOT:PSS solar cells belong to a type of selective carriers contact (SCC) solar cell. PEDOT:PSS serves as hole SCC and plays passivation role.

Large area, low temperature and solution based low cost

fabrication process. Greatly simplified compared to conventional

Si solar cells.

Introduction

5

Si nanostructures (e.g. nanowires, nanocones and nanoholes) are

commonly introduced for light trapping and anti-reflection.

Excellent light trapping allows use of thinner Si.

Provide large heterojunction area.

SiNWs/organic hybrid structure with

shorter diffusion distance

SiNWs and SiNHs are typically fabricated

using large area, solution based, low cost

metal-catalyzed electroless etching (MCEE) technique.

Efficiency has improved significantly in recent years from 5% to

13.0% for front-junction cell, reached 17.4% for back-junction cell.

Introduction

Si

PEDOT:PSS

6

Si

structure Organic material Group/year PCE (%)

Jsc

(mA/cm2)

SiNW Poly(3-octylthiophene)

(P3OT) G. Kalita et al. /2010 [1] 0.61 7.85

SiNW PEDOT: PSS W. Lu et al./2011 [7] 6.35 21.2

SiNW PEDOT: PSS S.-J. Syu et al./2011 [8] 8.4 24.2

SiNW (P3HT) F. Zhang et al./2012 [2] 9.2 37.6

SiNW Spiro-OMeTAD/PEDOT:

PSS L. He et al./2011 [3] 10.3 30.9

SiNW Spiro-OMeTAD/PEDOT:

PSS X. Shen et al./2011 [4] 9.7 31.3

SiNW PEDOT: PSS P.C. Yu et al./2013 [5] 13.0 34.3

SiNW PEDOT: PSS R. Y. Liu et al./2014 [6] 12.0 32.3

SiNW PEDOT: PSS J. X. Wang et al./2015 [9] 12.4 30.8

SiNW PEDOT:PSS (back

junction) D. Zielke et al./2014 [10] 17.4 39.7

7

References 1 G. Kalita, S. Adhikari, H. R. Aryal, R. Afre, T. Soga, M. Sharon, W. Koichi, and M.

Umeno, Journal of Physics D: Applied Physics 42, 115104 (5 pp.) (2009).

2 F. Zhang, X. Han, S.-T. Lee, and B. Sun, Journal of Materials Chemistry 22, 5362-5368

(2012).

3 L. He, C. Jiang, Rusli, D. Lai, and H. Wang, Applied Physics Letters 99, 021104-6

(2011).

4 X. Shen, B. Sun, D. Liu, and S.-T. Lee, Journal of the American Chemical Society 133,

19408-19415 (2011).

5 P. C. Yu, C. Y. Tsai, J. K. Chang, C. C. Lai, P. H. Chen, Y. C. Lai, P. T. Tsai, M. C. Li, H.

T. Pan, Y. Y. Huang, C. I. Wu, Y. L. Chueh, S. W. Chen, C. H. Du, S. F. Horng, H. F.

Meng, Acs Nano 7, 10780-10787 (2013).

6 R. Y. Liu, S. T. Lee, and B. Q. Sun, Advanced materials 26, 6007-6012 (2014).

7 W. Lu, C. Wang, W. Yue, and L. Chen, Nanoscale 3, 3631-3634 (2011).

8 H.-J. Syu, S.-C. Shiu, and C.-F. Lin, Solar Energy Materials and Solar Cells 98, 267-272

(2012).

9 Jianxiong Wang et. al, Nanoscale, Vol 7, 4559-65 (2015)

10 D. Zielke, A. Pazidis, F.Werner and J. Schmidt, Solar Energy Materials &Solar Cell 131,

110-116 (2014).

8

Hybrid Si/PEDOT:PSS Solar Cells Year Strucutre PCE (%)

2011 SiNW/SPIRO hybrid cell1 10.3

2011 SiNW/PEDOT hybrid cell2 9.0

2012 Pymarids/SiNWs/PEDOT hybrid solar cells3 9.9

2012 Planar Si/PEDOT hybrid cell4 10.6

2012 SiNWs/ PEDOT hybrid cell with 2.2um epitaxial Si5 5.6

2014 SiNH/PEDOT hybrid cell6 8.3

2015 SiNW/PEDOT hybrid cell with surface treatment7 12.4

2016 Planar Si/PEDOT hybrid cell with 15.5um epitaxial Si8 8.7

2016 SiNH/PEDOT hybrid cell9 12.9 1 L. He et. al, Applied Physics Letters 99, 021104-6 (2011). 2 L. He et. al, IEEE Electron Device Letters, 32, pp. 1406-8 (2011). 3 L. He et. al, Small, vol 8, pp. 1664-1668, (2012). 4 L. He et. al, Applied Physics Letters, vol. 100, pp. 073503-5 (2012). 5 L. He et. al, Applied Physics Letters, vol 100, pp. 103104-7 (2012) 6 L. Hong et. al, Applied Physics Letters, vol 104, 053104 (2014) 7 J. X. Wang et. al, Nanoscale, Vol 7, 4559-65 (2015) 8 H. Wang et al, IEEE Journal of Photovoltaics 6, 217 (2016). 9 Z. Y. Li et. al, EU PVSEC (2016)

9

Hybrid Si/PEDOT:PSS Solar Cells

SiNWs/PEDOT:PSS

Solar Cells

Pyramid/SiNWs/PEDOT:PSS

Solar Cells

SiNHs/PEDOT:PSS

Solar Cells

10

The efficiencies of hybrid solar cell based on Si

nanostructures are not much better than planar cell,

despite their excellent light trapping properties.

Jsc is increased but Voc is substantially lowered.

Attributed to the severe carriers recombination

associated with the defective surface.

Introduction

11

Defective SiNWs surface

Dry etching ion bombard damage of the surface

Metal catalyze electroless etching (MCEE)

Large surface area of the nanostructures

PEDOT:PSS layer is not fully penetrated into the gaps

of SiNWs

Long molecular chain of PEDOT:PSS and its fast

drying process

Agglomeration of the long nanowires

Introduction

12

Various approaches can reduce the recombination of

nanostructures

Surface passivation

Attaching some chemical molecules to the Si surface.

E.g. CH3-temination

Using native oxides as passivation layer. SiOx have

been commonly used for passivation.

Optimized nanostructures

Shrinking surface area but not sacrificing their light

absorption. (e.g. inverted nanopyramid, nanoholes)

Introduction

13

Nanowires/PEDOT:PSS

hybrid cells on thin film Si with

surface treatment

14

We applied a two-step surface treatment to hybrid

SiNWs/PEDOT:PSS solar cells.

Treated with O2 plasma surface treatment to grow oxide,

which embeds the Ag nanoparticles. Oxide is then

partially removed and leave behind a thin layer for

passivation.

Si/PEDOT:PSS hybrid cells are demonstrated on thin

film Si to lower the material cost.

SiNWs with different lengths are investigated.

Introduction

15

Fabrication Process

16

Fabrication Details 10.6 µm epitaxial single crystal (100) Si film grown by HTCVD on

n++ Si (100) substrates using dichlorosilane precursor and

phosphine (PH3) dopant gas at 1000 oC.

MCEE: etching solution of 4.6 M HF and 0.02 M AgNO3 at room

temperature to fabricate the SiNWs

SiNWs

Length: 0.4 to 2.7 µm

Diameter: 20 to 100 nm

Gap between SiNWs: < 50 nm

O2 plasma treatment 480 sec : 30 sccm, RF power of 30 watts and

pressure of 200 mTorr.

SiNWs were etched in a 5% HF solution for 85 s to reduce the

thickness of the surface oxide and obtain a cleaner surface

PEDOT:PSS thickness: 80 mn

Back contact Ti/Pd/Ag: 50nm : 50nm : 1000 nm

Front contact Ag: 1000 nm

Active area of cell: 0.95 cm2. 17

Epitaxial Si films

DSIMS profile XRD spectrum

Doping concentration of Si film: of ~ 1.5×1016 cm-3.

Thickness (T) of Si film is 10.6 µm

Doping concentration of substrate: of ~ 1×1020 cm-3.

Photocurrrent generation takes place only in the 10.6 µm Si

absorber layer 18

SEM image of the hybrid cells

top view

cross-section cross-section

Longer SiNWs suffer from agglomeration at the top resulting in the

formation of large bundles of SiNWs, ascribed to the Van Der Walls

and attractive capillary forces 19

SEM image of the hybrid cells

top view

Treated SiNWs reveal a smoother surface coverage of

PEDOT:PSS layer 20

untreated treated untreated treated

before treatment

(hydrophobic)

Contact angle measurement of SiNW

substrate

Smoother surface of treated cell is attributed to O2 plasma

treatment that resulted in a more hydrophilic surface.

This facilitates better coverage of PEDOT:PSS on the surface of the

SiNWs and better penetration into the gaps between the SiNWs

after HF etching

(hydrophilic)

after treatment

(hydrophilic)

21

Reflectance drops with increasing L.

The EQE peak increases with L up to L = 0.95 µm, with a maximum

value of 69.7% at λ = 625 nm. Attributed to the enhanced light

trapping and charge separation capability of the SiNWs.

As L further increases to 1.5 and 2.7 µm, EQE drops due to the

higher surface recombination with the SiNWs bundles.

External Quantum Efficiency

400 600 800 10000

20

40

60

80

EQ

E (

%)

Wavelength (nm)

0.4 µm

0.7 µm

0.95 µm

1.5 µm

2.7 µm

(a)

400 600 800 10000

10

20

30

40

50

Refl

ecta

nce (

%)

Wavelength (nm)

0.4 µm

0.7 µm

0.95 µm

1.5 µm

2.7 µm

(b)

22

J-V Characteristics of Hybrid Cells

0.0 0.2 0.4 0.6

-20

-15

-10

-5

0Voc (V) Jsc (mA/cm2) FF(%) PCE(%)

Untreated 0.496 20.6 68.6 7.02

Treated 0.572 20.6 65.7 7.74

Cu

rre

nt

De

ns

ity

(m

A/c

m2)

Voltage (V)

0.4µm Untreated

0.4µm Treated

(a)

0.0 0.2 0.4

-20

-15

-10

-5

0Voc (V) Jsc (mA/cm2) FF(%) PCE(%)

Untreated 0.416 20.3 51.7 4.36

Treated 0.542 21.0 63.7 7.24

(b)

Cu

rren

t D

en

sit

y (

mA

/cm

2)

Voltage (V)

0.95µm Untreated

0.95µm Treated

Voc of treated cell is recovered to 0.572 V and 0.542 V, much higher

than the untreated cell.

Jsc and FF also improved for longer wires.

23

0.5 1.0 1.5 2.0 2.5 3.00.2

0.4

0.6

(b)

Vo

c (

V)

SiNWs length (µm)

Untreated

Treated

0.5 1.0 1.5 2.0 2.5 3.00

2

4

6

8

10

(a)

PC

E (

%)

SiNWs length (µm)

Untreated

Treated

0.5 1.0 1.5 2.0 2.5 3.030

40

50

60

70

80

(d)

FF

(%

)

SiNWs length (µm)

Untreated

Treated

0.5 1.0 1.5 2.0 2.5 3.010

15

20

25

(c)

Jsc (

mA

/cm

2)

SiNWs length (µm)

Untreated

Treated

Max average

PCE: 7.27% at

0.7 µm.

PCE less

sensitive to

SiNW length

Improvement

in Voc for all L

Improvement in

Jsc and FF for

longer L due to

the large surface

area. Smoother

surface coverage

Performance of hybrid SiNWs Cells

25

TEM of SiNWs

EELS results

TEM and EELS results indicate silver particles attached on SiNWs after the

MCEE process. These are difficult to remove using nitric acid even for 4 hrs.

After O2 plasma treatment, SiOx will form and act as a sacrificial

layer to remove the silver particles and defects and achieve

cleaner SiNWs

Before treatment After treatment

26

TEM results

HRTEM characteristic of SiNWs

Before O2 plasma

treatment After O2 plasma

treatment After HF etching

As prepared SiNWs have H+ terminate surface with very thin SiOx

layer

O2 plasma treatment increases the thickness of oxide layer to 5 nm

After HF etching, a residual 1 - 2 nm thin SiOx layer is seen on the

SiNW surface for passivation.

27

Demonstrated a two-step surface treatment process to remove

residual impurities and defects near the SiNWs surface, and

effectively passivates the SiNWs to reduce the recombination

loss.

Treated SiNWs hybrid solar cell reveals a high PCE of 7.83%,

and the highest ever reported Voc of 0.572 V, for 0.7 µm SiNWs

based on a 10.6 µm thin Si film.

The proposed simple approach of surface treatment is

promising in boosting the efficiency of SiNW/organic hybrid cell

and will potentially lead to their practical use.

Conclusions

28

29