Tandem Colloidal Quantum Dot Solar Cells Employing a ... of Electrical and Computer Engineering, University of Toronto, 10 King’s College Rd., ... SI1 Tandem CQD solar cell cross-sectional

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Supplementary information

Tandem Colloidal Quantum Dot Solar Cells Employing a Graded Recombination Layer

Xihua Wang1,†, Ghada I. Koleilat1,†, Jiang Tang1, Huan Liu1, 2, Illan J. Kramer1, Ratan Debnath1,

Lukasz Brzozowski1, D. Aaron R. Barkhouse1, Larissa Levina1, Sjoerd Hoogland1, and Edward

H. Sargent1,*

†. These authors contributed equally to this work

1. Department of Electrical and Computer Engineering, University of Toronto, 10 King’s

College Rd., Toronto, Ontario M5S 3G4, Canada

2. Department of Electronic Science & Technology, Huazhong University of Science &

Technology, Wuhan 430074 P. R. China

*e-mail: [email protected]

Table of ContentsSI1 Tandem CQD solar cell cross-sectional SEM image ............................................................... 2SI2 Materials................................................................................................................................... 2

Chemicals.................................................................................................................................... 2Colloidal Quantum Dot Synthesis and Purification.................................................................... 2

SI3 Device modeling....................................................................................................................... 3Spatial band diagrams of constituent devices and tandem device .............................................. 3

In the case of the bottom single-junction solar cell with PbS CQD (1.6 eV) film ................. 4 In the case of the top single-junction solar cell with PbS CQD (1 eV bandgap) film............ 4

Estimation of the resistance of a suitably-designed GRL, and of a non-optimal RL ................. 5SI4 Current matching...................................................................................................................... 6SI5 I-V curves of tandem CQD solar cells ..................................................................................... 6SI6 Characterization of GRL materials........................................................................................... 7

UPS results.................................................................................................................................. 7XPS results.................................................................................................................................. 9Optical absorption results ......................................................................................................... 11Cyclic voltammetry results ....................................................................................................... 11FET results ................................................................................................................................ 12

SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2011.123

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SI1 Tandem CQD solar cell cross-sectional SEM image

SI2 Materials

Chemicals Lead oxide (PbO) (99.9%), oleic acid (90%), bis(trimethylsilyl)sulphide (TMS, synthesis grade), 1-octadecene (90%), 3-mercaptopropionic acid (99%), terpineol, Triton-X and all solvents (anhydrous grade) were obtained from Sigma-Aldrich. Titanium dioxide (TiO2) sputtering target, aluminium-doped zinc oxide (AZO) and indium tin oxide (ITO) sputtering target were obtained from Kurt J. Lesker, Inc. ITO-coated glass substrates were obtained from Delta Technology. MoO3 was obtained from Alfa Aesar.

Colloidal Quantum Dot Synthesis and Purification TMS (0.18 g, 1 mmol) was added to 1-octadecene (10 mL), which had been dried and degassed by heating to 80°C under vacuum for 24 hours. A mixture of oleic acid (1.34 g, 4.8 mmol), PbO

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SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2011.123

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(0.45 g, 2.0 mmol), and 1-octadecene (14.2 g, 56.2 mmol) was heated to 95°C under vacuum for 16 hours then placed under Ar. The flask temperature was increased to 120°C and the TMS/octadecene mixture was injected. After injection, the temperature dropped to ~95°C, and the flask was allowed to cool to 36°C. The nanocrystals were precipitated by adding 50 mL of distilled acetone and were centrifuged under ambient conditions. After discarding the supernatant, the precipitate was redispersed in toluene. The nanocrystals were precipitated again using 20 mL of acetone, centrifuged for 5 min, dried, and finally dispersed in toluene (~200 mg mL-1). The PbS nanocrystals were then brought into a N2-filled glove box. They were precipitated twice using methanol, and then finally redispersed at 50 mg mL-1 in octane.

SI3 Device modeling

Spatial band diagrams of constituent devices and tandem device To generate the spatial band diagrams of Figure 1c-e, we carried out simulation using the software PC1D [University of New South Wales]. We used electronic materials parameters (see below) obtained from direct measurement of our materials or, when values are well-accepted, from published references. In the table, values measured by us are shown in black; values measured by others are shown in blue. All values measured by others are accompanied by references to the literature. Band parameters in the table below are subject to a 0.1-0.2 eV uncertainty in light of the measurement methods used. The band diagrams are subject to the same uncertainties as a result. We simulated the band diagram of individual single-junction semiconductor devices. We then drew the band diagram of tandem CQD solar cell under the premise that an effective GRL achieves alignment of the Fermi levels in the ITO with that in the TiO2.

Materials thickness dielectric constant

electronaffinity

Ionization energy

doping concentration Fermi-level

Bottom TiO2 500 nm 301 4.1 eV2 NA 1 x 1016 cm-3 2 4.3 eV2

PbS CQD (1.6 eV) 200 nm 432 3.6 eV 5.2 eV 2 x 1016 cm-3 2 5.0 eV MoO3 10 nm 163 5.4 eV 8.5 eV ~1019 cm-3 5.4 eV ITO 50 nm 44 NA NA > 1020 cm-3 4.8 eV5

AZO 50 nm 86 4.1 eV 7.4 eV ~5 x 1019 cm-3 4.1 eV TiO2 40 nm 301 4.0 eV 7.7 eV ~5 x 1016 cm-3 4.1 eV PbS CQD(1 eV) 300 nm 432 4.0 eV 5 eV 2 x 1016 cm-3 2 4.8 eV



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In the case of the bottom single-junction solar cell with PbS CQD (1.6 eV) film The device modeled using PC1D consisted of 500 nm TiO2, 200 nm PbS CQD (1.6 eV bandgap), 10 nm MoO3, and 20 nm ITO. The ITO-TiO2 contact was treated as ohmic. We found that, for these materials parameters, the entire device was fully depleted, including both TiO2 and PbS film. A further simulation of expanded PbS CQD (1.6 eV) film to 500 nm thick shows that more than 300 nm of PbS is depleted. The electric field is plotted below for the simulated 500 nm thick cell.

In the case of the top single-junction solar cell with PbS CQD (1 eV bandgap) film The device modeled consisted of 30 nm ITO, 50 nm AZO, 40 nm TiO2 and 300 nm PbS CQD (1 eV bandgap). The ohmic contact condition was applied to the PbS CQD film. We found that the entire device was fully depleted, including both TiO2 and PbS film. A further simulation of expanded PbS CQD (1 eV) film to 500 nm showed that more than 350 nm of PbS is depleted. The electric field is plotted below for the simulated 500 nm thick cell.

The above analysis of depletion width agrees with our previous report (results obtained from capacitance-voltage measurement) for a similar type of PbS CQD (1.3 eV) film2.



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Estimation of the resistance of a suitably-designed GRL, and of a non-optimal RLWe sought to assess whether the energetic barriers to electron flow in the GRL device could reasonably be expected to be compatible with the flow of solar current densities without imposing excessive resistive loss. To begin we obtained spatial band diagrams using the same methods as described above. The graded recombination layer (left figure) consisted of 10 nm MoO3, 50 nm ITO, 50 nm AZO, and 40 nm TiO2. Spikes appeared in the conduction band at the MoO3/ITO and ITO/AZO interfaces (Figure below, left panel). The larger barrier along the path for electrons to flow from right to the left was a 0.5 eV triangular barrier of width ~ 5 nm. An abrupt, or nongraded, recombination layer, shown on the right below, consists of 10 nm MoO3, 50 nm ITO, and 40 nm TiO2. Omitting the shallow work function heavily-doped AZO caused a wide depletion region in TiO2. The barrier height here of close to 1 eV was expected to impede significantly the flow of appreciable electrical current via thermionic emission. The calculations below show that, for the abrupt recombination layer, a parasitic voltage drop of multiple tenths of an eV would be required to support solar current densities. This would lead to a significantly lowered operating voltage Vm at the maximum power point. The calculations below show that, for the GRL, solar current densities can be supported at the expense of a minimal (less than kT) voltage drop.

We now provide a more detailed quantitative estimation of the current that can flow in the presence of a ~ 0.5 eV triangular barrier. The tunnel current is usually described with Fowler-Nordheim type equation7. J= a b

-1F2exp(- b b3/2/F)

Here a ~ 1.5x10-6 A eV V-2 and b ~ 6.8 eV-3/2 V nm-1, =1, =(me*/me)3/2, and eF= b/xd. We use the reference value8 me*= 0.2me for AZO. For the case of b=0.5 eV high and xd=5 nm as obtained from our spatial band diagram calculations above, the barrier can readily support a



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current density J of order 106 A/cm2 at a cost of less than kT/q of applied bias (corresponding to well under kT/q parasitic voltage drop). Negligible tunneling current density is supported in case of the abrupt (nongraded) device in view of its extremely wide depletion in the TiO2. We now examine the themionic emission current density:

J=A**T2exp(-e b/kT)(exp(V/nkT)-1) A** is the Richardson constant, b is the barrier height, n is the ideal factor of the diode. Using A**=120 Acm-2K-2 (an optimistic value intended to give an upper bound on supported current density), b = 0.9 eV for the ITO/TiO2 junction, we find only 10-7 mA/cm2 of current density are supported for kT/q applied bias. We conclude that, from these calculations, the GRL concept is necessary to achieve flow of solar current densities at minimal cost to operating voltage.

SI4 Current matching In order to determine the proper film thickness to achieve current matching in our structure, we considered the following in our evaluation:

We began by fixing the thickness of the front cell at 200 nm. We had found this thickness to provide the best single-junction visible-cell performance experimentally. The average internal quantum efficiency was up to ~70% in the visible region. We determined the total expected current density available from the single-junction visible-cell (with transparent top contact) to be ~9.2 mA/cm2. From the absorption coefficient of the large bandgap CQDs (figure 2a), we determined the remaining AM1.5 flux illuminating the back cell by employing the single-junction visible-cell (with transparent top contact) as the visible light filter. We found the average internal quantum efficiency of ~35% for the 1.0 eV cell under the remaining AM1.5 flux. Thus we estimated the expected current density for a double-pass for various thicknesses of the small-bandgap CQD film and plotted the calculated values in figure 2b. Current matching occurs for 250-300 nm thick films

SI5 I-V curves of tandem CQD solar cells Our tandem devices as well as single-junction solar cells need 5-10 minutes of light soaking to achieve their maximum efficiency. Similar phenomena were observed by Grätzel et al. in dye-sensitized solar cells9. We provide in the figure below the forward and reverse scan of a typical device. The curves overlap closely, evidencing minimal hysteresis.



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Below we plot the I-V characteristics of a typical device on a log-linear scale (left panel) and show the full I-V scan beyond open-circuit voltage (right panel).

SI6 Characterization of GRL materials

UPS results Ultraviolet photoelectron spectroscopy (UPS) allows determination of the absolute value of work function (Fermi level, Ef) and ionization potential (equivalent to valence band edge, Ev) of semiconductor materials. UPS was carried out using He I (21.22 eV) photon lines from a discharge lamp. The GRL materials are deposited on a commercial ITO substrate (from Delta Technology). The thickness of MoO3, AZO and TiO2 were all 50 nm in order to eliminate background signal from the ITO substrate. The full UPS spectra for MoO3, AZO and TiO2 are shown in Figure 4a of the main text. The following plots show the regions of interest. The Ef is extracted by subtracting the cut-off value



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of the curve from the kinetic energy of He I (21.22 eV) photon. The Ev is extracted from the cut-off value of the curve, and it represents the energy below Fermi level of the material.

-16.2 -16.0 -15.8 -15.6 -15.4

MoO3

Inte

nstiy

(arb

. u.)

energy w.r.t. Fermi level (eV)-6 -5 -4 -3 -2 -1 0

Inte

nstiy

(arb

. u.)

energy w.r.t. Fermi level (eV)

MoO3

The red lines in the above two plots show the cut-off position, and UPS analysis gives values of 15.84 eV and 3.13 eV. We conclude that MoO3 has Ef of ~5.4 eV and Ev of ~8.5 eV.

-18.0 -17.5 -17.0 -16.5 -16.0

Inte

nsity

(arb

. u.)


AZO

-6 -5 -4 -3 -2 -1 0

AZO

Inte

nsity

(arb

. u.)

energy w.r.t. Fermi level (eV) The red lines in the above two plots show the cut-off position, and UPS analysis gives the value of 17.16 eV and 3.34 eV. We conclude that AZO has Ef ~4.1 eV and Ev ~7.4 eV.

-18.0 -17.5 -17.0 -16.5 -16.0

Inte

nsity

(arb

. u.)


TiO2

-6 -5 -4 -3 -2 -1 0

TiO2

Inte

nsity

(arb

. u.)




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The red lines in the above two plots show the cut-off position, and UPS analysis gives the value of 17.10 eV and 3.55 eV. We conclude that TiO2 has Ef ~4.1 eV and Ev of ~7.7 eV.

XPS results X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique to ascertain the elemental composition and chemical state of thin films.

The above XPS survey of MoO3 shows the existence of Mo and O. Further quantitative analysis gives the atomic concentration of Mo/O=28%/72%.

0100200300400500600700800900100011000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 105 q1014059.spe

Binding Energy (eV)

c/s

-O K

LL

-O2s

-O1s

-C1s

-Mo4

p

-Mo3

s

-Mo3

p3 -M

o3p1

-Mo3

d5 -M

o3d3

-Mo3

d



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The above XPS survey of AZO shows the existence of Zn, O, and Al. Further quantitative analysis gives the atomic concentration of Zn/Al=98%/2%.

The above XPS survey of AZO shows the existence of Ti and O. Further quantitative analysis gives the atomic concentration of Ti/O=31%/69%.

0100200300400500600700800900100011000

2

4

6

8

10

12

14

16

18x 104 q1014062.spe

Binding Energy (eV)

c/s

-C1s

-O K

LL

-O2s

-O1s

-TiL

MM

1

-Ti L

MM

-Ti3

s -Ti3

p

-Ti2

s

-Ti2

p3 -T

i2p1

-Ti2

p

0100200300400500600700800900100011000

1

2

3

4

5

6

7

8

9x 105 q1014053.spe

Binding Energy (eV)

c/s

-Al2

s

-Al2

p

-C1s

-O K

LL

-O2s

-O1s

-Zn

LMM

3 -Zn

LMM

2

-Zn

LMM

1 -Zn

LMM

-Zn3

s

-Zn3

p

-Zn3

d

-Zn2

p3 -Z

n2p1



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Optical absorption results Optical absorption measurements were carried out using a Varian Cary 500 UV-Vis-IR Scan spectrophotometer. Each material was deposited onto a transparent glass substrate. The same thickness of 50 nm was used for each of the following three samples; MoO3, AZO and TiO2. The results were shown in the graphs below. The spectra show that each material is highly transparent. We determined the optical bandgap of these materials to be 3.1 eV, 3.3 eV, and 3.4 eV for MoO3, AZO and TiO2, respectively. We observed a 0.3 eV discrepancy in the bandgap of TiO2 obtained using UPS and cyclic voltammetry.

Cyclic voltammetry results Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement. It has been applied to obtain the LUMO and HOMO levels of organic materials and quantum dots10, as well as the electron affinity of semiconductors11. Here we use it to measure the electron affinity (equivalent to conduction band edge Ec) of AZO and TiO2. The results are shown in the following figures.



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-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-40

-30

-20

-10

0

10

20

30

40C

urre

nt (

A)

E vs. Ag/AgNO3 (V)

TiO2

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-40

-30

-20

-10

0

10

20

30

40AZO

Cur

rent

(A

)

E vs. Ag/AgNO3 (V) We use the Ag/AgNO3 (0.01M acetonitrile) reference electrode in the measurement, which has the absolute value of -4.7 eV. The position of the reduction peak reflects the Ec of the materials. For AZO and TiO2, we obtained electron affinities of 4.1 eV and 4.0 eV respectively. The left graph in below shows the same sample went through multiple scans in CV measurement. In order to test the TiO2 surface response to chemical treatment, we broke a sample into two pieces and applied chemical (MPA/methanol) treatment only on one of them. In the right graph in below, we indicated the two pieces from the same sample as ‘before chemical treatment’ and ‘after chemical treatment’. Since the sample was contaminated after CV measurement, there is a limit to test exactly the same sample before and after chemical treatment. The graph shows the electron affinity of the surface of the TiO2 is not significantly affected by the MPA/methanol treatments which we use to fabricate our devices.

FET results We built field effect transistor (FET) test structures with the goal of estimating the order of magnitude of the free carrier densities within each layer of the GRL.



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We use the similar method as in the reference12 to fabricate FET devices. In the following figures, our FET devices show good modulation along with applied gate bias.

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 Vg = 0 V Vg = 0.5 V Vg = 1.0 V Vg = 1.2 V Vg = 1.5 V Vg = 2.0 V Vg = 2.5 V Vg = 3.0 VI d (

A)

Vd (V)

MoO3

0.0 0.2 0.4 0.6 0.8 1.00

2000

4000

6000

8000

10000

Vg = 0 V Vg = 0.5 V Vg = 1.0 V Vg = 1.2 V Vg = 1.5 V Vg = 2.0 V Vg = 2.5 V Vg = 3.0 VI d (

A)

Vd (V)

AZO

0.0 0.2 0.4 0.6 0.8 1.00.00.51.01.52.02.53.03.54.04.5

I d (A

)

Vd (V)

Vg = 0 V Vg = 0.5 V Vg = 1.0 V Vg = 1.2 V Vg = 1.5 V Vg = 2.0 V Vg = 2.5 V Vg = 3.0 V

TiO2

We calculated mobility from the measured transconductance

LVWC

VI

g di

constVg

dm

d

by taking of slope of the Id vs. Vg curve at linear region. Here, the channel length L =2.5 m, the channel width W = 2 mm, the capacitance per unit area of the insulating layer is Ci = 100 F/cm2. We saw appreciable hysteresis in FET measurements that render our mobility measurements – and therefore our extracted free carrier densities – accurate to within somewhat better than one order of magnitude. We comment below to the effect that this is sufficient for the purposes of this work (eg providing reasonable accuracy in spatial band diagrams, and in supporting the assertion that MoO3 and AZO are both essentially degenerately-doped). From the measured 0.2 S/cm conductivity of AZO and the extracted mobility, we estimated AZO to have a free carrier density in the mid-1019 cm-3 range. From the measured 6 x 10-5 S/cm conductivity obtained for MoO3 and the extracted mobility, we estimated MoO3 to have free carrier density in the low 1019 cm-3 range.



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From the measured 1.4 x 10-7 S/cm obtained for TiO2 and the estimated mobility, we estimated a doping in the mid-1016 cm-3 range. In semiconductor physics, Ef=Ec+kTln(Nd/Nc), where Nd is close to the doping density n0, Nc is the density of states at conduction band and has the typical value of ~1019 cm-3 for wide-band gap metal-oxides. Considering the doping of our TiO2 in the mid-1016 cm-3 range, the Fermi-level is ~0.15 eV below the conduction band edge. 1 Matsunami, H. & Fuyuki, T. Electonic properties of the interface between Si and TiO2 deposited at very low temperatures. Jpn. J. Appl. Phys. 25, 1288-1291 (1986). 2 Pattantyus-Abraham, A. G. et al. Depleted-heterojunction colloidal quantum dot solar cells. ACS Nano 4, 3374-3380 (2010). 3 Simmons, J. G. & Nadkarni, G. S. Electrical properties of evaporated molybdenum oxide films. J. Appl. Phys. 41, 545-550 (1970). 4 Kulkarni, A. K. & Knickerbocker, S. A. Estimation and verification of the optical properties of indium tin oxide based on the energy band diagram. J. Vac. Sci. technol. A 14, 757-761 (1996). 5 Kim, J. Y. et al. Efficient tandem polymer solar cells fabricated by all-solution processing. Science 317, 222-225 (2007). 6 Takahashi, Y., Kanamori, M., Kondoh, A., Minoura, H. & Ohya, Y. Photoconductivity of ultrathin zinc oxide films. Jpn. J. Appl. Phys. 33, 6611-6615 (1994). 7 Forbes, R. G. Physics of generalized Fowler-Nordheim-type equation. J. Vac. Sci. Technol. B. 26, 788-793 (2008). 8 Jayaweera, P. V. V., Perera, A. G. U. & Tennakone, K. Why Gratzel’s cell works so well. Inorganica Chimica Acta 361, 707-771 (2008). 9 Hagfeldt, A., Bj rkstén, U. & Grätzel, M. Photocapacitance of nanocrystalline oxide semiconductor films: band-edge movement in mesoporous TiO2 electrodes during UV illumination. J. Phys. Chem. 100, 8045-8048 (1996). 10 Hyun, B.-R. et al. Electron injection from colloidal PbS quantum dots into titanium dioxide nanoparticles. ACS Nano 2, 2206-2212 (2008). 11 Kim, J. Y. et al. New architecture for high-efficiency polymer photovoltaic cells using solution-based titanium oxide as an optical spacer. Adv. Mater. 18, 572-576 (2010). 12 Kang, M. S., Sahu, A., Norris, D. J. & Frisbie, C. D. Size-dependent electrical transport in CdSe nanocrystal thin films. Nano Lett. 10, 3727-3732 (2010).



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Tandem Colloidal Quantum Dot Solar Cells Employing a ... of Electrical and Computer Engineering, University of Toronto, 10 King’s College Rd., ... SI1 Tandem CQD solar cell cross-sectional