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Supplementary Information for “Depleted-Heterojunction
Colloidal Quantum Dot Photovoltaics Employing Low-Cost
Electrical Contacts’’
Ratan Debnath,1 Mark T. Greiner,2 Illan Kramer,1 Armin Fischer,1 Jiang Tang,2 Aaron
Barkhouse,1 Xihua Wang,1 Larissa Levina,1 Zheng-Hong Lu,2,3 and Edward H. Sargent1,*
1
Department of Electrical and Computer Engineering, University of Toronto, 10 King’s
College Road, Toronto, Ontario M5S 3G4, Canada
2
Department of Materials Science and Engineering, 184 College Street, Toronto, Ontario
M5S 3E4, Canada
3
Department of Physics, Yunnan University, 2 Cuihu Beilu, Yunnan, Kunming 650091,
People’s Republic of China
*
e-mail: ted.sargent@utoronto.ca
Lead oxide (PbO) (99.9%), bis(trimethylsilyl)sulphide (TMS, synthesis grade), oleic acid
(90%), 1-octadecene (ODE, 90%), 3-mercaptopropionic acid (MPA, 99%), Ethanol, Triton
X-100 and all solvents (anhydrous grade) were purchased from Sigma-Aldrich.
Fluorine-doped tin oxide (FTO) coated glass substrates (2.5×2.5 cm2, Pilkington TEC 15)
were obtained from Hartford Glass. TiO2 nanoparticles (DS-90) were obtained from Dyesol,
Inc. Au (99.99%) and Ni (99.99%) sputtering targets were obtained from Kurt J. Lesker, Inc.
1.
PbS Nanocrystal Synthesis
210 ul of TMS was added to 10 ml of ODE and the mixture had been dried and degassed by
heating to 80°C under vacuum for 24 hrs. A mixture of 0.45 g PbO, 1.5 mL oleic acid and
16.5 mL ODE were mixed and heated to 95°C under vacuum for 16 hours then placed under
Argon. The flask temperature was increased to 120°C and the TMS/ODEe mixture was
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swiftly injected. After injection, the temperature dropped to ~ 95°C, and the solution was
slowly cooled down to 36 °C.
The nanocrystals were precipitated with 50 mL of distilled acetone by centrifugation under
ambient conditions. The supernatant was discarded and the precipitated nanocrystals were
redispersed in toluene. The nanocrystals were then precipitated with 20 mL of acetone,
centrifuged for 5 min, dried, and finally dispersed in toluene (~180-200 mg/ mL). The PbS
nanocrystals were then stored into a N2-filled glove box, washed twice with methanol and
then finally redispersed in octane at a concentration of 50 mg/ mL.
2.
Porous TiO2 Electrode Preparation
The FTO glass substrates were first rinsed with toluene and then sonicated for 20 minutes in
a mixture of Triton in de-ionized water (1 to 3% by volume) followed by sequential
sonication in isopropanol and de-ionized water.
TiO2 nanoparticle paste (Dysol) was mixed with Ethanol at a 1:3 ratio(by mass) and vortexed
thoroughly. The diluted paste was spin cast at 1000 RPM for 45s on the FTO substrates. Two
edges of the substrate were then wiped free of the titania nanoparticle paste with a swab
soaked in Ethanol to expose a region of clean FTO for metal contacts. The substrates were
sintered for 1 hour on a hotplate at 400 ºC to get rid of all the organics and the allowed to
cool down at room temperature. In order to improve the bridging between TiO2 nanoparticles,
FTO substrates were further soaked in 120 mM TiCl4 in de-ionized water at 70 ºC for 30
minutes, followed by rinsing with de-ionized water and drying with nitrogen. Finally, all the
substrates were heated to 520ºC in a tube furnace for 1 hour.
3.
Device Fabrication and Characterization
PbS CQD films were spin-coated under ambient conditions using a layer-by-layer deposition
technique with the following steps: (a) 2 drops of PbS CQDs at 50 mg/ml and then spin, (b)
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10 drops of 10% 3-MPA in Methanol while spinning and (c) finally rinse with 10 drops of
Methanol while spinning. These steps were repeated 8 times at 2500 rpm for 10 s to achieve
the desired film thickness. The devices were then stored a nitrogen glove box and left
overnight.
For electrode deposition, the devices were then transferred to an Edwards 306 system. 1.2 nm
LiF was thermally evaporated at a rate of 0.04-0.08 nm/s with a base pressure of 10-5 Torr
whereas both Au and Ni were DC sputtered with 5 mTorr Argon at a rate of 0.1 nm/s. A
shadow mask was used to define a 4×4 array of 2.78 mm diameter circular contacts. In order
to eliminate any variation due to device fabrication and substrate preparation, various metal
depositions took place on similar substrates which were then tested side by side.
Current-voltage data were measured using a Keithley 2400 source-meter under ambient
conditions. The solar AM1.5G was simulated to within class A specifications (less than 25%
spectral mismatch) with a Xe lamp and filters (Solar Light, Inc.). The source intensity (94
mW/cm2) was measured with a Melles-Griot broadband power meter (responsive from 300
nm to 2000 nm), through a circular 2.5 mm aperture at the position of the sample and
confirmed with a calibrated solar cell (Newport, Inc.). The accuracy of the power
measurements was estimated to be ± 7%. The EQE spectrum was measured with a Keithley
2400 source meter and the illumination was achieved by a white light source which was
dispersed by a Jobin-Yvon Triax 320 monochromator.
XPS was carried out on a PHI 5500 system using a monochromated Al Kα source with a
photon energy of 1486.7 eV. The binding energy scale was calibrated by setting the Au 4f7/2
peak to 83.98 eV and the Cu 2p3/2 peak of sputter-cleaned Cu to 932.67 eV. 2 nm of Ni and
Au were deposited on PbS and PbS-LiF films for XPS in order to investigate the interface
reaction between PbS and metal contacts.