Electronic Supplementary Information

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.
This journal is © The Royal Society of Chemistry 2014
Electronic Supplementary Information
Inverted Schottky PbS colloidal quantum dot solar cells with
enhanced carrier extraction and air-stability
Xuan-Dung Mai,ab Hey Jin An,a Jung Hoon Song,ac Jihoon Janga, Sungwoo Kima, and Sohee
Jeong*ad
Nanomechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon
305-343, Republic of Korea. Fax: +82 42-868-7884 ; Tel: +82 42-868-7939 ; E-mail:
[email protected].
a
b
Department of Chemistry, Hanoi Pedagogical University No2, Vinh Phuc, Vietnam.
c
Graduate School of Nanoscience and Technology, KAIST, Daejeon 306-701, Republic of Korea.
Department of Nanomechatronics, Korea University of Science and Technology, Daejeon 305-350,
Republic of Korea.
d
a)
b)
6
100
3
Nor
Inv
2
1
PNJ
VOC (V)
60
300 nm-PbS thick
40
20
0.6
0
0.5
400
Nor
Inv
PNJ
0.4
0.3
JSC (mA/cm2)
Normal Schottky
Inverted Schottky
80
4
EQE (%)
PCE (%)
5
Nor
PNJ
20
15
500
600
700
800
900 1000 1100
Excitation wavelength (nm)
Inv
10
5
FF (%)
70
60
Nor
Inv
PNJ
50
40
0
100
200
300
400
Thickness (nm)
Figure S.1 SCAP simulation results for normal Schottky, (abbreviated as Nor), inverted Schottky
(abbreviated as Inv), and p-n junction (abbreviated as PNJ), solar cells with varying the thickness of
PbS QD film. a) from top to bottom: AM1.5 power conversion efficiency (PCE), open circuit voltage
(VOC), short circuit current density (JSC), and fill factor (FF). b) calculated external quantum efficiency
(EQE) for normal and inverted Schottky devices with a 300nm-thick PbS CQD layer. The PbS CQDs
have the first excitonic peak at 970 nm.
The SCAP calculations were carried out as reported for CQD solar cells.1,2 We used doping
density of PbS QD film NA = 5 x 1016 cm-3, conduction band (EC) and valence band (EV)
energy to be -3.8 eV and -5.1 eV (with respect to the vacuum level), respectively.
Workfunction (wf) was assumed to be -3.9 eV for LiF/Al and -4.8 eV for ITO in the normal
Schottky cell and -3.9 eV for low-wf TCO and -4.8 eV for high-wf metal in the inverted
Schottky cell. To calculate the performance of p-n junction solar cell, TiO2 is used as n-type
semiconductor whose doping density (NC), EC, and EV were assumed to be 1019 cm-3, 4.1 eV
(with respect to the vacuum level), and 7.3 eV (with respect to the vacuum level),
respevtively.2 The PCE of inverted Schottky cell is comparable to PNJ cell.
a)
b)
5
9
AM1.5
4
3
2
1
0
1.5
PbS QD (Eg=1.35 eV)
Generation rate (s-1.nm-3)
Abs coefficiency (x103/cm)
Photon Flux (x1018/s.m2)
400 500 600 700 800 900 1000 1100
Thickness of PbS QD layer
100nm
150nm
200nm
250nm
300nm
8
7
6
5
4
3
2
0
50
100
150
200
250
300
Distance from TCO (nm)
1.0
0.5
0.0
Transmittance (%)
80
70
60
FTO
PEI/FTO
50
40
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Figure S.2 (a) Photon flux (top), absorption coefficiency (middle) and transmittance of FTO and PEImodified FTO, noted as low-work-function FTO in the main text, (bottom) data used to calculate the
carrier generation rate. (b) The total carrier generation rate within PbS QD layer of different thickness
(shown as legend) obtained by integrating equation (1) shown in the main text over 400-1100 nm
range. It is apparent from (b) that the carrier generation rate at the front of CQD layer, near TCO, is
always higher than elsewhere.
b)
a)
35
500
PbS Thickness (nm)
PEI Thickness (nm)
30
25
20
15
10
5
0
400
300
200
100
0
0.0
0.2
0.4
0.6
0.8
PEI Concentration (wt%)
1.0
2
4
6
8
10
12
14
16
18
20
Coating Cycles
Figure S.3 a) Concentration-dependence of the thickness of the PEI layer. The coating conditions
were: room temperature; coating speed at 5000 rpm with 5 second acceleration time; the films were
annealed at 100oC in N2-filled glove box for 10 minutes. The thickness was measured by mean of
ellipsometry spectroscopy (SE). b) The dependence of PbS QD layer thickness on the number of
coating cycle. The concentration of oleic acid capped PbS CQDs in octane is 30 mg/ml. 1,2ethanedithiol 1 volume percent in acetonitrile was used to perform solid-state ligand exchange with
the oleic acid. The coating was carried out while the substrate pinned at a speed of 2500 rpm.
PCE (%)
2.0
1.5
1.0
0.5
0.0
40
FF (%)
30
20
JSC (mA/cm2)
10
9
6
3
VOC(V)
0
0.6
0.4
0.2
0.0
0
5
10
15
20
25
30
35
Thickness of PEI layer (nm)
Figure S.4 Effects of PEI thickness on the performance of inverted Schottky solar cell. Cells have a
common structure: Low-wf FTO/100 nm-thick PbS QD (Eg = 1.49 eV)/10 nm MoOx/80nm Au-Ag.
The optimal thickness of PEI is about 3 nm, which is obtained by using a 0.2 wt% solution of PEI in
methoxymethanol, see Fig. S3(a).
Photoluminescence (a.u)
Absorbance (a.u)
600
700
800
900
1000
1100
1200
Wavelength (nm)
Figure S.5 UV-vis absorption (solid spectra) and photoluminescent (PL) (dotted spectra) of PbS
CQDs dissolved in tetrachloroethylene (TCE) of different sizes in tetrachloroethylene. The
absorbance was adjusted to clarify the first excitonic peak. The PL spectra were normalized. The
energy bandgap (Eg) of CQDs were calculated from the PL peak.
C1s
Counts (a.u)
Counts (a.u)
Counts (a.u)
PEI+EDT
PEI
N1s
Counts (a.u)
289
288
287
286
285
284
283
404
168
166
164
162
160
158
402
400
398
396
Binding Energy (eV)
d)
b)
PEI+EDT
Counts (a.u)
PEI+EDT
Sn3d
O1s
PEI
Counts (a.u)
PEI
Counts (a.u)
170
Binding Energy (eV)
Binding Energy (eV)
500
PEI+EDT
average
S2p
172
PEI
Counts (a.u)
290
Counts (a.u)
e)
c)
a)
PEI+EDT
495
490
Binding Energy (eV)
485
534
532
530
528
Binding Energy (eV)
Figure S.6 XPS spectra of PEI or PEI treated with 1,2-ethanedithiol (EDT) thin layer on FTO
substrate showing a) C1s, b) Sn3d, c) N1s, d) O1s, and e) S2p region, respectively. Most of N atoms
in the PEI are in the neutral state whose binding energy is of 399 eV. The fact that the XPS spectrum
of PEI is largely unchanged (from panel a to d) and that the S2p peak is lacking (panel e) indicates
that the PEI layer maintains and does not react with PEI during the QD coating step.
In order to test whether the PEI layer is physicochemically affected by the QD coating step,
X-ray photoelectron spectroscopy (XPS) analyses were conducted on two sample sets: PEI on
FTO (PEI/FTO) and PEI+EDT/FTO that were obtained by treating the pre-formed PEI/FTO
5 cycles of with EDT solution (1 vol% in acetonitrile, 0.05 ml) - rinsing with acetonitrile
(0.08 ml). EDT and acetonitrile were used as cross-link ligand and solvent in the QD coating
process. XPS results indicate that the PEI layer is not chemically affected by EDT and that it
maintains on FTO after the QD coating.
3
3
PCE (%)
PCE (%)
b)
a)
4
2
1
1
50
FF(%)
40
30
40
20
30
15
13
JSC (mA/cm2)
FF (%)
50
JSC (mA/cm2)
2
12
9
6
12
11
10
9
0.50
VOC(V)
VOC (V)
0.6
0.5
0.45
0.40
0.4
50
100 150
200
250 300
350
Thickness of PbS QD layer (nm)
100
150
200
250
300
350
Thickness of PbS QD layer (nm)
Figure S.7 Effects of PbS QD layer thickness on the performance of inverted (a) and normal (b)
Schottky solar cell. The inverted cells have a common structure: Low-wf FTO/PbS CQD/10 nm
MoOx/80 nm Au-Ag. The bandgap of PbS CQDs is 1.49 eV. The normal Schottky cells have a
common structure ITO/PbS QD/1 nm LiF/100 nm Al. The thickness of the PbS QD layer was varied
by increasing the coating cycle as shown in Fig. S3(b).
b)
a)
700
Current Density (mA/cm2)
Open-circuit voltage (mV)
2
Inverted
600
500
Normal
400
300
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Energy gap of PbS CQDs (eV)
0
-2
-4
VOC= 0.75 V
JSC= 7.9 mA/cm2
FF= 33.1 %
PCE=1.96 %
Light
Dark
-6
-8
-10
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Figure S.8 a) Bandgap-dependence of VOC of the inverted Schottky (red points) and normal (blue
points) solar cells. The structure of the inverted cells is: Low-wf FTO/200±20 nm PbS QDs/10 nm
MoOx/80 nm Au-Ag. The structure of the normal cells is: ITO/100±10nm PbS QDs/1 nm LiF/100 nm
Al. Each data point was the average of six devices on a single substrate. b) J-V characteristics of the
device yielding the highest VOC (0.75 V) under dark (dashed curve) and simulated AM1.5 illumination
(solid curve) conditions. Insert in (b) is the cell performance under AM1.5.
100
150
450
0
50
100
150
450
400
200
600
400
200
FF (%)
PCE (%)
RS (cm2) RSh (cm2)
50
600
JSC(mA/cm2)
VOC (V)
0
0
4
3
2
1
0
60
40
20
0
15
10
5
0.6
0.5
0.4
Air-exposing time (hour)
Figure S.9 The variations in shunt-resistance (RSh), series-resistance (RS), Am1.5 power conversion
efficiency (PCE), fill-factor (FF), short-circuit current density (JSC), and open-circuit voltage (VOC) of
the champion inverted (black-square) and normal (gray-triangle) Schottky PbS CQD SCs after
exposing to air with different period of times.
ESI. References
1. M. Burgelman, P. Nollet and S. Degrave, Thin Solid Films, 2000, 361, 527.
2. D. Zhitomirsky, I. K. Kramer, A. J. Labelle, A. Fischer, R. Debnath, J. Pan, O. M. Bakr,
and E. H. Sargent, Nano. Lett., 2012, 12, 1007.

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