In this report, we explore the underlying mechanisms by which doped organic thin films as a top hole-selective layer (HSL) improve the performance and stability of colloidal quantum dot (CQD)-based solar cells. Molecular dynamics-based theoretical studies prove that the hydro/oxo-phobic properties of the HSL serve to efficiently passivate the CQD solid. Furthermore, the robust and outstanding electrical properties of the HSL, simultaneously ensure a high power conversion efficiency (PCE) and increase the stability performance of CQD-based solar cells. As a result, a best PCE of 11.7% in a lead sulfide (PbS)-based CQD solar cell is achieved and over 90% of the initial performance is retained after 1 year storage under ambient conditions

Baek, Se-Woong

Ha, Ye-Seol

Jeong, Sohee

Kim, Changjo

Kim, Hyungjun

Lee, Jung-Yong

Lee, Sang-Hoon

Shin, Hyeyoung

Song, Jung Hoon

Caltech Authors - Main

1Supporting Information 
A hydro/oxo-phobic top hole-selective layer for efficient and stable 
colloidal quantum dot solar cells 
Se-Woong Baek,a,† Sang-Hoon Lee,a,† Jung Hoon Song,b Changjo Kim,a Ye-Seol Ha,a Hyeyoung Shin,a,d Hyungjun 
Kim,a Sohee Jeong,b,c and Jung-Yong Leea,*
Optical simulations: The charge generation profile and device absorption can be calculated using a 
transfer matrix formalism (TMF) simulation.1 For optical simulations, the refractive indices of the 
organic materials and quantum dots were acquired by spectroscopic ellipsometry (Alpha-SE, J. A. 
Woollam Co.). The electric field at each position inside the device could be calculated using optical 
constants and thicknesses of all layers. The number of charge generation, or excited states, was 
directly related to the energy absorbed in the materials. The number of charge generation could be 
obtained by calculating the energy absorption at each position using the square of the electric field 
and the optical constants. Molecular dynamics (MD) simulations: MD simulations were performed 
using the LAMMPS software program.2 To achieve full equilibration, we performed MD simulations 
for 10.21 ns by following the procedure from the cohesive energy density (CED) method3, which 
combines temperature annealing, volume compression, and NPT ensemble simulations to correctly 
estimate the equilibrium density and structure of amorphous materials. In the simulation box, we 
included 27 MeO-TPD molecules, the interatomic potential was described by means of the 
DREIDING generic force field potential,4 and quantum mechanical partial charges were used to 
describe the electrostatics. For the description of O2 molecules, we used a rigid three-site oxygen 
model.5 For the calculation of diffusion constant, we further included 7 O2 molecules in the simulation 
cell and performed NVT ensemble simulation for 16 ns at 300 K to calculate the mean squared 
displacement (MSD). For the calculation of solubility constant (kH), we separately performed grand 
canonical ensemble Monte Carlo (GCMC) simulation using Cerius2 program6 and 107 GCMC steps 
Electronic Supplementary Material (ESI) for Energy & Environmental Science.
This journal is © The Royal Society of Chemistry 2018
2were used to obtain averaged values at 300 K. To obtain kH, the O2 adsorption isotherm calculated 
from the GCMC simulations was fitted using dual-mode theory7: C = kH p + C bp /(1+bp ), where C 
is the concentration of the gas, kH is the Henry’s constant, and p is the partial pressure. The last term 
represents a Langmuir-type isotherm with the saturation concentration of gas (C∞) and b is the ratio of 
the rates of adsorption/desorption of the gas molecules.
Fig. S1. Transport properties of water in MeO-TPD layer. The water solubility (kH) and 
diffusion coefficient (D) of MeO-TPD layer calculated from GCMC and MD simulations, 
respectively.
The H2O solubility (kH) of MeO-TPD was calculated as 6.66 cm3 (STP) cm-3 atm-1 similar to the kH 
value of polyvinylidiene chloride (PVDC), 6.5 – 10.2 cm3 (STP) cm-3 atm-1, from the previous 
theoretical study by Coxmuta et al.8 The diffusion coefficient (D) determined by MSD as a function of 
simulation time to be 5.67 × 10-7 cm2 s-1, yielding permeability P = kH × D = 3.78 × 10-6 cm3 (STP) 
cm-1 atm-1 s-1. This is comparable to the permeability of 0.5 mm thick polyacrilonitrile (PAN), 2.28 × 
10-6 cm3 (STP) cm-1 atm-1 s-1, water barrier property as well.
3Fig. S2. Hydrophobicity of a p-MeO-TPD layer. The contact angle analysis of a p-MeO-TPD 
layer and a MoO3 layer using DI water as test solution. 
Fig. S2 shows a hydrophobic property of a p-MeO-TPD layer. To confirm the hydrophobicity of a 
p-MeO-TPD layer, the contact angle analysis was performed using DI water as test solution on glass 
substrates. The p-MeO-TPD layer (79.88) showed more hydrophobicity compared to the MoO3 layer 
(13.35). Those results support our MD simulation as described in Fig. 1.
4Fig. S3. The time of flight secondary ion mass spectroscopy (TOF-SIMs) of CQDs-based solar 
cells with and without p-MeO-TPD. a, The TOF-SIMs depth profile results of CQDs-based solar 
cells without HSL before (0 day) (left) and after (60 days) (right) degradation. b, The TOF-SIMs 
depth profile results of CQDs-based solar cell with a p-MeO-TPD layer before (0 day) (left) and after 
(60 days) (right) degradation: (purple) O-, (pink) S-, (blue) F-, (green) ZnO-, (gray) Ag-, (red) PbS-, 
respectively. 
 Fig. S3 depicts the TOF-SIMs depth profile results of CQDs-based solar cells with and without a 
p-MeO-TPD layer. The TOF-SIMs results were obtained from full device configurations 
(ITO/ZnO/PbS/with or without p-MeO-TPD/Ag). The anion elements depending on the sputter time 
clearly confirmed the overall device structure. We detected PbS- and S- to finding the PbS region (red 
area) and F- to locate the p-MeO-TPD layer (blue area). The green and orange areas in Fig. S3 denote 
5the Ag and ZnO layers, respectively. The element level of the device without HSL clearly showed that 
the O- content at the PbS layer (red area, Fig. S3a) significantly increased after 60 days. On the other 
hand, the O- content of the device with p-MeO-TPD at the PbS layer (red area, Fig. S3b) did not 
notably change over time. 
Fig. S4. The doping mechanism and properties of p-MeO-TPD layer. a, Schematics of the p-
type doping mechanism and the molecular structure of MeO-TPD (host material) and F6-TCNNQ (p-
type dopant). b, The XPS result of a p-MeO-TPD layer: O1s (red) and F1s (blue) peaks, respectively. 
The energy level of the lowest unoccupied molecular orbital (LUMO) in the F6-TCNNQ dopant 
allows the transfer of electrons from the highest occupied molecular orbital (HOMO) energy level of 
the host material (MeO-TPD), generating an excess of free holes in the bulk region, resulting in a 
highly conductive p-doped HSL. Fig. S4 depicts the XPS results of p-MeO-TPD layer to confirm the 
doping concentration. The F1s could be detected only in F6-TCNNQ dopant and O1s could be detected 
6only from MeO-TPD host; therefore, the doping concentration could be determined by O/F atomic 
ratio. We calculated the p-doping concentration of p-MeO-TPD layer as approximately 8.4 wt%.
Fig. S5. The ultraviolet photoelectron spectroscopy (UPS) and band energy levels of PbS 
layers. a, The binding energy spectra: ZnO layer (black), ZnO/PbS-PbI layer (red), ZnO/PbS-
PbI/PbS-MPA(1 LBL) layer (blue), and ZnO/PbS-PbI/PbS-MPA(2 LBL) layer (green), respectively, 
b, The energy level diagram estimated from the results of Fig. S5a. 
Fig. S54 shows UPS results and the energy level diagram for PbS QDs layers and two hole 
selective layers (HSLs) (i.e. MeO-TPD and MoO3). The samples were prepared with the same 
fabrication conditions. A PbS-PbI layer showed large Ev-Ef (0.93 eV). As the number of PbS-MPA 
layer increased, the Ev-Ef was gradually reduced, turning to the p-type. The final valence band of PbS 
layer was 5.26 eV, which is well aligned with a p-MeO-TPD layer (5.1 eV).
7900 1000 1100 1200 1300
 
 
In
te
ns
ity
 [a
.u
.]
Wavelength [nm]
 PbS only
 MoOx
 p-MeO-TPD
Fig. S6. The PL intensity of CQDs layers without (black) and with a MoOx (red) and a p-MeO-TPD 
(blue) layer. 
Fig. S6 depicts the PL measurement results of CQDs with HSL layers. For the PL measurement, 
both p-MeO-TPD and MoOx layers were deposited on the CQDs (300 nm) / glass substrates. PL 
spectrums were acquired using LabRAM HR Evolution Visible-NIR model from HORIBA. A 633 nm 
laser was used as an excitation source. The p-MeO-TPD film decreased the PL intensity further, 
implying more efficient charge extraction of p-MeO-TPD compared to MoOx. 
8Fig. S7. The statistical histogram of devices performance without and with HSL (MoO3, p-
MeO-TPD). The statistical histogram for the device performance based on the 25 devices (black) 
without HSL, (red) with MoO3 layer, and (blue) with p-MeO-TPD layer, respectively.
 
 Fig. S7 depicts the statistical histogram for the power conversion efficiency (PCE) based on the 25 
devices. The p-MeO-TPD-deposited device clearly showed improved performance compared to the 
MoO3-deposited devices. All devices were fabricated using the same PbS CQDs batch and size. 
9-1.0 -0.5 0.0 0.5 1.0
10-5
10-3
10-1
101
103
 
 
Cu
rr
re
nt
 d
en
si
ty
 [m
A/
cm
2 ]
Voltage [V]
 MoO3
 p-MeO-TPD
Fig. S8. J-V characteristics of solar cells with deposited MoO3 (black squares) and p-MeO-
TPD (red circles) layers under dark conditions.
We used a double diode model to obtain the series resistances from the J-V curves under a dark 
condition as shown in Fig. S8. The J-V curves are fitted according to the equations, [I = 
(Rp/(Rs+Rp))(Is(exp(q(V-IRs)/nkBT)-1)+V/Rp)-Iph] from previous studies.9 The best fit gave a series 
resistance of 1.97 Ω cm2 for a p-MeO-TPD device and 365 Ω cm2 for a MoO3 device. Detail fitting 
parameters were Js = 1.62×10-15 mA/cm2, n = 1.64, Rs= 1.97 Ω∙cm2, Rp = 3749 kΩ∙cm2 for a p-MeO-
TPD device, and Js = 6.57×10-6 mA/cm2, n = 1.26, Rs= 365 Ω cm2, Rp = 1995 kΩ∙cm2 for a MoO3 
device.
10
Fig. S9. The normalized EQE of a p-MeO-TPD-deposited device at various forward bias 
conditions. The normalized EQE results measured under a dark condition a, and under a bias light 
condition b.
The wavelength dependent EQE reduction of p-MeO-TPD-deposited device under a forward bias 
was observed in such conditions as high forward bias voltage and without bias light illumination 
during EQE measurements. Current values from the EQE light source are similar to dark current 
values of the devices. Therefore, wavelength dependent EQE reduction was clearly observed under a 
forward bias condition. As explained in the manuscript, the wavelength dependent EQE reduction 
occurred due to highly injected holes from the p-MeO-TPD layer. These high injection of holes under 
a forward bias condition creates a charge accumulation region near the HSL and generated charges 
near the p-MeO-TPD layer in the longer wavelength (700 ~ 1000 nm) easily recombine with those 
injected charges, resulting in the wavelength dependent EQE reduction. On the other hand, both under 
a low forward bias voltage and bias light conditions, the injected charge accumulation disappeared 
due to the low carrier injection and the bias light currents, respectively. As a result, the wavelength 
dependent EQE reduction was not clearly observed under the low forward bias voltage and the light 
bias condition as shown in Fig. S9. 
11
Fig. S10. The XPS result of a MoO3 layer for stability assessment. The XPS results of MoO3 
layer corresponding to Mo3d (black) and O1s (red) peaks before (dark) and after (bright) degradation, 
respectively.
Fig. S10 depicts the XPS results of a MoO3 layer to observe the stoichiometry change of O/Mo 
ratio. Air exposure of the MoO3 layer leads to change the binding energy of their components. The 
binding energy spectra of O1s and Mo3d were shifted to the higher binding energy. These data clearly 
showed the change of O/Mo ratio increased from 2.7 to 3.2, implying the increased oxygen contents 
after air exposure. It can be implied the reduction of conductivity originated from the variance of 
oxidation state in the MoO3 layer. 
Fig. S11. The XPS results of p-MeO-TPD layer for stability assessment. The XPS result of p-
MeO-TPD layer each corresponding to a, O1s, b, F1s and c, C1s peaks before (0 day) (black) and after 
(20 days) (red) degradation, respectively.
 
12
 Fig. S11 depicts the XPS results of a p-MeO-TPD layer to carefully study the stability 
performance. Compared to the MoO3 layer, O1s peak was not notably changed after 20 days. In 
addition, doping concentration also showed marginal change. These results suggest that the p-MeO-
TPD layer is relatively stable compared to the MoO3 layer. 
0 20 40 160 180
0.0
0.2
0.4
0.6
0.8
1.0
 
 
No
rm
al
ize
d 
ef
fic
ie
nc
y
Time [day]
 MoO3
 p-MeO-TPD (20nm)
 p-MeO-TPD (50nm)
Fig. S12. The stability performance of HSL-deposited CQDs device. The normalized efficiency 
of MoO3 (black) and p-MeO-TPD layer depending on the thickness; 20 nm (red) and 50 nm (blue)
Fig. S12 depicts the stability performance of CQDs devices depending on the p-MeO-TPD layer 
thickness. The p-MeO-TPD layer devices always showed higher stability performance than the MoO3 
device. In addition, even higher stability was achieved with a thicker p-MeO-TPD, suggesting that the 
blocking effect of the p-MeO-TPD layer boosted the CQDs device stability. 
13
Fig. S13. The J-V characteristics of EMII-based CQDs device using various sizes of PbS QDs. 
The J-V curves of p-MeO-TPD deposited CQDs devices at various first excitonic peaks of PbS QDs: 
(black) 818 nm, (bright yellow) 853 nm and (dark yellow) 878 nm, respectively. 
Fig. S13 shows the J-V characteristics of EMII-based device with a p-MeO-TPD layer using 
various sizes of PbS QDs. We tested three PbS QDs batches such that first excitonic peaks are 818 nm, 
853 nm, and 878 nm. The detailed photovoltaic characteristics are described in Table S1. As shown in 
Fig. S13 and Table S1, The Voc was reduced and Jsc was improved as the size of PbS QDs became 
larger. The highest PCE of 10.7 % was achieved with 878 nm. 
14
9.0 9.5 10.0 10.5 11.0 11.5 12.0
0
2
4
6
8
10
12
 
 
Co
un
ts
PCE [%]
p-MeO-TPD
 w/o V-CPT
 w V-CPT
Fig. S14. The statistical histogram of device performance. The histogram of p-Meo-TPD-
deposited devices without (black) and with (red) V-CPT light trapping schemes. We characterized 25 
devices at each condition.
Fig. S14 depicts statistical PCE histograms of 25 devices. A 818 nm PbS CQDs was used for p-
MeO-TPD-deposited devices and those devices performance were characterized with and without V-
CPT. The results clearly showed that the V-CPT light trapping schemes improved the PCE of p-MeO-
TPD-deposited devices significantly. 
15
Table S1. Photovoltaic characteristics of EMII-based device depositing p-MeO-TPD with 
various sizes of PbS QDs.
1st excitonic peak Voc [V] Jsc [mA/cm2] FF [%] η [%]
818 nm 0.675 22.02 68.31 10.15
853 nm 0.647 23.25 69.72 10.49
878 nm 0.611 25.03 69.84 10.68
References
1. L. A. Pettersson, L. S. Roman and O. Inganäs, J Appl Phys, 1999, 86, 487-496.
2. M. Belmares, M. Blanco, W. A. Goddard, 3rd, R. B. Ross, G. Caldwell, S. H. Chou, J. Pham, P. M. 
Olofson and C. Thomas, J Comput Chem, 2004, 25, 1814-1826.
3. S. Plimpton, J Comput Phys, 1995, 117, 1-19.
4. S. L. Mayo, B. D. Olafson and W. A. Goddard, J Phys Chem-Us, 1990, 94, 8897-8909.
5. L. Zhang and J. I. Siepmann, Theor Chem Acc, 2006, 115, 391-397.
6. Cerius2 v 4.10, Accelrys, San Diego, CA, (2005).
7. V. T. Stannett, W. J. Koros, D. R. Paul, H. K. Lonsdale and R. W. Baker, in Chemistry, Springer Berlin 
Heidelberg, Berlin, Heidelberg, 1979, DOI: 10.1007/3-540-09442-3_5, pp. 69-121.
8. I. Cozmuta, M. Blanco and W. A. Goddard, 3rd, J Phys Chem B, 2007, 111, 3151-3166.
9. B. P. Rand, D. P. Burk and S. R. Forrest, Physical Review B, 2007, 75, 115327.


A hydro/oxo-phobic top hole-selective layer for efficient and stable colloidal quantum dot solar cells

https://authors.library.caltech.edu/86652/2/c7ee03184j1_si.pdf

A hydro/oxo-phobic top hole-selective layer for efficient and stable colloidal quantum dot solar cells

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main