1

Heterojunction PbS Nanocrystal Solar Cells

with Oxide Charge-Transport Layers

Byung-Ryool Hyun*, Joshua J. Choi+, Kyle L. Seyler*, Tobias Hanrath

+, and Frank W. Wise*

School of Applied and Engineering Physics*, School of Chemical and Biomolecular Engineering+,

Cornell Univeristy, Ithaca, NY 14853

Bh73@cornell.edu

PbS NC Synthesis and Purification : PbS NCs were synthesized based on a variation of a previous

literature report1. Briefly, 20 mL of 1-octadecene (ODE) was added to a three-neck flask, which was

vacuumed and purged with nitrogen gas 3 times to allow it to degas. The ODE was then heated to 110

°C, followed by two vacuum/purge cycles for 5 and 2 minutes to remove water and allow it to degas

further. Afterwards, the ODE was allowed to cool to room temperature in a nitrogen environment. 420

µL of bis(trimethylsilyl)sulfide (TMS) was added to the 20 mL of ODE and allowed to mix thoroughly.

In another three-neck flask, PbO, oleic acid, and ODE were mixed, degassed, and purged at 110 °C as

before. Once the solution turned clear, indicating the formation of lead oleate, the flask temperature was

lowered to a certain temperature. The mixture of TMS and ODE was then extracted and rapidly injected

into the lead oleate solution using a syringe. After injection, the temperature dropped and the solution

turned dark brown, indicating the formation of PbS NCs. The nanocrystal solution was left on the

heating mantle for 3 minutes and afterwards, was cooled rapidly to room temperature with an ice bath.

The nanocrystals were collected into centrifuge tubes, precipitated by adding twice the amount acetone

as NC solution, and centrifuged for 5 minutes under ambient conditions. The supernatant was discarded

and the NCs were redispersed in hexane. Acetone was once again added until the entire solution turned

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cloudy, and then the NCs were centrifuged, rinsed with acetone, and dried. The NCs were washed this

way one more time for a total of 3 precipitations, and finally, they were redispersed at 40 mg/mL in

chlorobenzene for solar cell fabrication and filtered through a 0.2 µm PVDF syringe filter to remove any

dust, particulates, and NC aggregates.

PbS NC-sensitized mesoporous SiO2 films: 4 g of colloidal SiO2 nanoparticles in water (Nalco 2327,

diameter = 20 nm, 40 wt. % aqueous dispersion) was diluted with 3.2 mL of water and then, 0.64 g of

polyethylene glycol (MW 20000) was added into the diluted dispersion of SiO2 nanoparticles. The

mixture was stirred for at least 2 hrs. Thin films of SiO2 nanoparticles were prepared on the glass

substrate by using a spin coating technique (1000 rpm, 30 sec). Films were subjected to 450 °C heat

treatment for 2 h. The temperature was then lowered to room temperature. Repeated steps of spin

coating and temperature treatment yielded the transparent mesoporous SiO2 film of ~10 µm-thicknesses.

At the last thermal treatment, films were lowered to 120 °C. The hot films were placed directly in a

0.20M 3-mercaptoacetic acid (MPA) (or 3-mercaptopropyl-trimethoxysilane, MPS) in toluene solution

for 12 hrs. Films were washed three times with toluene and placed directly in PbS NCs in toluene for 24

hrs to sensitize PbS NCs. The PbS NC-SiO2 films were washed three times with a toluene to remove the

unbound PbS NCs and then, were dipped in a 1M 2-mercaptoethanol (MEtOH) in toluene to remove the

oleic acid at the surface of PbS NCs. NiO nanoparticles solution was prepared by dispersing 1 g of NiO

nanopoweder (Inframat Advanced Materials, diameter = 20 nm) in 10 mL of water. After 0.5 hr

sonication, the NiO dispersion was left overnight to settle down the big aggregations. The light brown

supernatant was carefully pipetted. To fill NiO nanoparticles out inside the PbS NC-SiO2 mesoporous

film, a few 100 µL of NiO nanoparticles in water was dropped into the PbS NC-SiO film and water was

removed by an evaporation vacuum pump. It was repeated two or three time to fill the empty space

inside of mesoporous film. The NiO nanoparticle covered film was sealed by putting another cleaned

glass slide on top and sealing the sides with UV-curable epoxy. Encapsulated samples were stable

against oxidation for several months.

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Absorption and Emission Spectrum Measurements: Absorption spectra were measured on a

Shimadzu UV-3101PC spectrophotometer at room temperature. Emission spectra were recorded at room

temperature with an infrared fluorometer equipped with a 200-mm focal length monochromator, a single

mode fiber coupled laser source (S1FC635PM, 635 nm, Thorlabs, Inc) as the excitation source, and an

Si and InGaAs photodiode (New Focus Femtowatt model 2151 and 2153).

Fluorescence lifetime measurements: The samples were excited at a repetition rate of 1 kHz by

femtosecond pulses of a Ti:sapphire laser system with an optical parametric amplifier (OPA) (at 680

nm) or regenerative amplifier (at 800 nm). The sample was exposed to intensity levels below 10

mW/cm2 so that the excitation level was always well below one electron-hole pair per dot. Fluorescence

below 1000 nm was monitored with a Si avalanche photodiode (APD) single photon counting module

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(PerkinElmer, SPCM-AQRH-44-FC). The time–resolved fluorescence measurements were performed in

the time-correlated single photon counting (TCSPC) mode with a TCSPC board (PicoQuant, TimeHarp

200) and a digital delay generator (Stanford Research Systems, DG645) under right-angle sample

geometry. The fluorescence decay curves were analyzed by means of iterative re-convolution using the

FluoFit. Fluorescence lifetime measurements between 1000 and 1600 nm were performed with an

InGaAs photomultiplier tube module (H10330-75, Hamamatsu, Inc.). The output was fed into a digital

oscilloscope (Tektronix TDS 520A) and averaged, which provides adequate temporal resolution of

decay times in the microsecond range.

Figure S2. Absorption and emission spectra of PbS NCs with diameters of 4.0, 4.9 and 6.0 nm in TCE

Figure S3. Typical transient fluorescence traces of PbS NCs in TCE and SiO2-PbS NC-NiO composites.

The time constants of decay for PbS NCs with D = 4.0, 4.9, and 6.0 nm in TCE were 2.2, 1.6, and 1.4

5

µs, respectively. The 1/ decay times for SiO2-MPA-PbS NC composites were 0.082 (green line), 0.42

(red line), and 0.16 (blue line) µs, respectively.

Solution processed solar cells:

a) NiO layer : We followed the procedure of ref [2]. Briefly, the nickel acetate tetrahydrate (Aldrich)

was dissolved in methanol to a concentration of 0.4 M. An equimolar quantity of diethanolamine

(Aldrich) was added under stirring. ITO substrate was cleaned by successive ultrasonic treatment in

detergent, purified water, acetone and propanol before drying under nitrogen. Thin films were

prepared by spin-coating the sol–gel precursor onto ITO substrate under ambient conditions at a spin

speed of 3000 rpm for 30 s. Samples were then placed immediately into a tube furnace and annealed

under air at 425 °C for 10 min, forming a dense, polycrystalline material. Film thicknesses were

measured by a stylus profiler to be 30–40 nm thick.

b) ZnO layers : ZnO nanoparticles were synthesised in ambient air. 1.76 g of zinc acetate dihydrate

was dissolved in 150 mL of ethanol with vigorous stirring and heated to 60 oC for 1 hour. Parafilm

was used to block solvent evaporation during the heating and the entire reaction time. In a separate

container, 6.4 mL of a tetramethyl ammonium hydroxide (TMAH) solution (28% in MeOH) was

added to 50 mL of ethanol. Over 10 minutes, the TMAH solution was slowly added to the zinc

acetate solution at regular intervals. During this time the temperature of the zinc acetate solution was

maintained at 60 oC with continuous stirring. Subsequently, the solution was heated at 60 oC for 30

minutes after which the heater and stirrer were removed. After cooling to room temperature, the

solution was kept refrigerated at ~ 5 oC. This mother-liquor solution was stable for over ~5 months.

ZnO nanoparticles were collected from the refrigerated solution immediately before device

fabrication. To prepare devices, 5 mL of the solution was typically mixed with 20 mL of hexane to

precipitate the ZnO particles. After centrifugation, the supernatant was removed and the white

precipitate was dissolved in isopropyl alcohol.

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c) EDT-treated PbS NC Layer Deposition: PbS NCs at 40 mg/mL in chlorobenzene were deposited by

spin-coating from a Laurell WS-400A-6NPP-LITE. All spin steps were carried out at 1000 rpm for

30 seconds. First, 250 µL of NC solution was dispensed onto the substrate using an automatic

pipette and then the substrate was spun. Second, 1 mL of 0.1 M ethanedithiol (EDT) in acetonitrile

was swiftly dispensed on the substrate, left for 30 seconds, and then spun. The short EDT ligands

replace the longer oleic acid ligands, improving conductivity through improved inter-NC coupling.

Third, the substrate was rinsed with pure acetonitrile, spun, rinsed with pure chlorobenzene, and

spun to remove residual ligand molecules and NCs. This three-step spin process of deposition,

ligand exchange, and rinsing was one cycle. Three cycles were performed to produce ~ 100 nm thick

NC films.

d) Fabrication solar cells: The samples were loaded into a metal evaporator and the evaporation

chamber was pumped down to ultrahigh vacuum (~10-6 Torr). Au, Ag or Al was thermally

evaporated at a rate of 0.5 Å/second for the first 100 Å and at 2~3 Å/second for the rest of

deposition. Typically total of 400 ~ 600 Å of metal electrode was deposited.

e) Device characterization: Device testing was performed with a source measurement unit (Keithley

236). Samples were irradiated with 100mW/cm2 AM1.5 illumination from a Solar Light 16S-002

solar simulator. Light output power was calibrated with a Newport 818P-010-12 thermopile high

power detector. Small spectral mismatch was not taken into account in these measurements.

ITO Substrate Cleaning: Pre-patterned ITO coated glass substrates (1" × 1", Kintec) were cleaned by

scrubbing for over 30 seconds with a swab soaked a solution of 1% Alconox in distilled H2O. The

following steps were then taken, with the substrate kept wet at all times. Rinse with distilled H2O,

sonicate in 1% Alconox solution for 10 min, rinse with distilled H2O, rinse with acetone, sonicate in

acetone for 10 min, rinse with acetone, rinse with isopropanol (IPA), sonicate in IPA for 10 min, and

rinse with IPA. Finally, the IPA-wetted substrates were dried with a blast of nitrogen gas.

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Air annealing and photodoping

Room-temperature sputtered ZnO and NiO layers: ZnO was deposited at a rate of 1.5 Å/s by

reactively sputtering at a base pressure of 10-6 Torr with a 2″ diameter Zn target in a 4:1 Ar:O2 pressure

ratio at 5 mTorr and 65W of DC power. NiO was deposited at a rate of 1.7 Å/s by reactively sputtering

at a base pressure of 10-6 Torr with a 2″ diameter Ni target in a 99:1 Ar:O2 pressure ratio at 10 mTorr

and 57W of DC power. Substrates sat on a rotating stage with a mask over the edges to keep the ITO

bare for good electrical contact.

X-ray diffraction (XRD):

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Transmittance spectrum:

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AFM image of DC-sputtered NiO film: RMS = 3 nm

AFM image of DC-sputtered ZnO film: RMS = 3 nm

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Material Characterization: Surface morphology measurements were obtained by using a Veeco

Dimension 3100 AFM in tapping mode. Transmittance and absorbance spectra were obtained using a

Shimadzu UV-Vis-NIR Spectrometer. Although film thicknesses were not measured for each device,

care was taken to calibrate tooling factors. Film thicknesses were measured using a Tencor Alpha Step

500 profiler.

Performance data for PbS NCs-based solar cells:

Series resistance and shunt resistance were taken as the inverse slope at 0.9-1V and as the inverse slope

of dark JV curve near 0V before it breaks down, respectively. The values in the table were obtained

from averaging values from several devices and at least 2 measurements have been performed for each

device.

1) Sol-gel processed devices

Diameter

[nm]

VOC

[V]

JSC

[mA/cm2]

FF Efficiency

[%]

Series resistance

[Ω⋅cm2]

Shunt resistance

[Ω⋅cm2]

2.75

0.65

± 0.01

3.82

± 0.15

0.38

± 0.004

0.94

±0.03

16.9

± 0.6

6925000

± 14035000

3

0.58

± 0.01

8.15

± 1.42

0.44

± 0.01

2.08

±0.36

8.5

± 0.1

3688000

± 1053000

3.5

0.49

± 0.01

9.53

± 0.33

0.47

± 0.02

2.21

±0.16

9.80

± 0.02

150000

± 8000

4.4

0.36

± 0.01

7.67

± 0.50

0.43

± 0.03

1.19

±0.12

10.2

± 0.01

57000

± 3000

5.7

0.32

± 0.01

5.38

± 0.28

0.43

± 0.023

0.73

±0.07

12.95

± 0.03

6400

± 100

6.4

0.19

± 0.01

2.60

± 0.26

0.32

± 0.01

0.16

±0.03

13.17

± 0.05

400

± 100

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2) The sputtered devices

The thickness of ZnO = 10 nm

NiO

thickness

[nm]

VOC

[V]

JSC

[mA/cm2]

FF Efficiency

[%]

Series resistance

[Ω⋅cm2]

Shunt resistance

[Ω⋅cm2]

0

0.63

± 0.01

2.1

± 0.2

0.28

± 0.01

0.36

± 0.03

33.46

± 3.03

113334

± 1520

5

0.61

± 0.01

10.7

± 0.1

0.43

± 0.01

2.83

± 0.02

8.51

± 0.01

870

± 4

10

0.625

± 0.01

13.9

± 1.3

0.29

± 0.01

2.53

± 0.27

4.83

± 0.30

1514

± 15

20

0.63

± 0.01

6.7

± 0.7

0.27

± 0.01

1.13

± 0.06

4.31

± 0.07

15761

± 268

30

0.62

± 0.02

5.1

± 0.1

0.26

± 0.01

0.82

± 0.07

8.60

± 0.10

595

± 1

40

0.58

± 0.01

1.7

± 0.2

0.30

± 0.01

0.31

± 0.03

7.90

± 0.10

870

± 7

The thickness of NiO = 10 nm

ZnO

thickness

[nm]

VOC

[V]

JSC

[mA/cm2]

FF Efficiency

[%]

Series resistance

[Ω/cm2]

Shunt resistance

[Ω/cm2]

5

0.35

± 0.04

3.9

± 1.1

0.28

± 0.01

0.25

± 0.04

4.83

± 0.04

469

± 5

10

0.63

± 0.01

13.9

± 1.3

0.29

± 0.01

2.53

± 0.27

4.83

± 0.30

1514

± 15

20 0.65 11.2 0.33 2.42 4.59 16641

12

± 0.01 ± 0.4 ± 0.01 ± 0.10 ± 0.10 ± 89

30

0.66

± 0.01

11.1

± 0.3

0.33

± 0.02

2.50

± 0.20

4.46

± 0.10

97359

± 4729

40

0.67

± 0.01

11.6

± 0.7

0.33

± 0.02

2.53

± 0.33

4.51

± 0.10

64334

± 1479

Optical model calculation : Scattering matrix formalism is based on (1) linearity of equations for

electric field propagation and (2) continuity of tangential component of the electric field at every

interface. A generalized 2 by 2 matrix is formulated by accounting for phase accumulation during

propagation and phase change upon reflection at various interfaces. Assuming that the structure to be

modeled is multi-layered thin films with perfectly flat interfaces and the plane wave is incident in the

direction perpendicular to the interfaces, reflection and transmission that occur at each interface is

described by an interface matrix:

1

1 1

where tjk and rjk are the Fresnel complex transmission and reflection coefficients at the interface jk. In

addition to the changes at the interfaces, the propagating wave accumulates phase while traveling

through each layer. This is described by the phase matrix:

00

where is the refractive index dependent wavevector and dj is the thickness of the layer j. Therefore,

is the amount of phase change due to the propagation through layer j. The scattering matrix, S, now

can be obtained by multiplying all the matrices required to account for every interface and layer in the

structure to be modeled. Using the scattering matrix, transmission and reflection coefficients and optical

field throughout the entire structure can be calculated. Detailed derivations and equations can be found

in Ref. [3]. For the device structure model, we used the stack of Glass (1.1 mm) / ITO (80 nm) / ZnO

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(variable thickness) / PbS_EDT (100 nm) / NiO (variable thickness) / Au (60 nm). The experimentally

measured complex refractive indices for each film were used as input for the calculation.

1. Hines, M. A.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater.

2003, 15, 1844-1849. 2. Mashford, B. S.; Nguyen, T. L.; Wilson, G. J.; Mulvaney, P. All-Inorganic Quantum-Dot Light-Emitting Devices Formed via Low-Cost, Wet-Chemical Processing. J. Mater. Chem. 2010, 20, 167-172. 3. Pettersson, L. A. A.; Roman, L. S.; Inganas, O. Modeling Photocurrent Action Spectra of Photovoltaic Devices Based on Organic Thin Films. J. Appl. Phys. 1999, 86, 487-496.