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Supplementary Information Solution processed inorganic bulk nano-heterojunctions and their application to solar cells Arup K. Rath, Maria Bernechea, Luis Martinez, F. Pelayo Garcia de Arquer, Johann Osmond, Gerasimos Konstantatos * * [email protected] ICFO - Institut de Ciències Fotòniques, Mediterranean Technology Park Castelldefels, Barcelona 08860 Spain SUPPLEMENTARY INFORMATION DOI: 10.1038/NPHOTON.2012.139 NATURE PHOTONICS | www.nature.com/naturephotonics 1 © 2012 Macmillan Publishers Limited. All rights reserved.

Transcript of Supplementary Information - Nature Research › original › nature-assets › nphoton › journal...

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Supplementary Information

Solution processed inorganic bulk nano-heterojunctions and their application to

solar cells

Arup K. Rath, Maria Bernechea, Luis Martinez, F. Pelayo Garcia de Arquer, Johann

Osmond, Gerasimos Konstantatos*

*[email protected]

ICFO - Institut de Ciències Fotòniques, Mediterranean Technology Park

Castelldefels, Barcelona 08860 Spain

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NPHOTON.2012.139

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S1. Two-dimensional EDX mapping of BNH device and TEM characterization

For EDX analyses a lamella extracted from a blend photovoltaic device of approximately

60 nm width was prepared by focused-ion beam milling (FIB).

Figure S1.1 a) SEM image of the lamella prepared by FIB; b) bidimensional analysis showing the distribution of silver (purple), bismuth (green), lead (red) and indium (light blue); c) One dimensional analysis showing the distribution of Ag (purple), sulfur (cyan), bismuth (green) and lead (red) along the device.

a) b)

c)

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Figure S1.1 summarizes the distribution of the different elements present in the sample. Taking together all the data it is clearly observed how there is a preponderance of lead near the ITO layer, then in the blend layer both elements (bismuth and lead) are detected and only bismuth is detected next to the silver layer. These three layers (only bismuth sulfide, BNH, and only lead sulfide) can also be easily detected in the TEM micrographs (see Figure S1.2), thanks to the different packing of the nanocrystals.

Figure S1.2. TEM micrograph of a lamella extracted from a blend photovoltaic device clearly showing the only bismuth sulfide layer (top), the BNH layer (middle) and only lead sulfide layer (bottom).

In Figure S1.2 the packing of the PbS nanocrystals forming a compact layer is clearly seen along with the four LBL layers of PbS. The Bi2S3 layer shows a slightly more disordered structure with some voids, probably due to the elongated shape of the nanocrystals, which precludes a homogeneous packing, while in the BNH layer those voids seem to be occupied by the PbS nanocrystals which conformally coat the Bi2S3 NC network. To gain more insight into the nanostructure of the nanocrystals in the BNH layer we prepared devices with different ratios of PbS:Bi2S3 (10:1, 1:2 and 1:4), we mechanically scratched them, sonicated the material in MeOH and placed the material on TEM grids. In Figure S1.3 we present the TEM micrographs.

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Figure S1.3. TEM micrographs of the three devices with three different PbS:Bi2S3 ratios on the BNH layer (a) 10:1, (b) 1:2 and (c) 1:4.

In the PbS rich sample (Figure S1.3a) we can see the close packing of the small lead sulfide nanocrystals with sporadic inclusions of bismuth sulfide particles. On the other hand, the Bi2S3 rich sample (Figure S1.3c) exhibits groups of bismuth sulfide

a

c

b

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nanocrystals where some PbS particles are present. Finally, the optimal ratio found for our devices (1:2, Figure S1.3b) shows the PbS QDs to fill the voids between the Bi2S3nanoparticles and the formation of bicontinuous channels that would allow for a carrier percolation path in the two media.

The lamella was prepared by focused-ion beam milling (FIB) at the CRNE (UPC) laboratories. EDX mapping as well as TEM imaging in Fig S1.3 was performed on a JEOL JEM 2100 at 200 kV in the Scientific and Technological Centers of the University of Barcelona (CCiT-UB). The maps were acquired with an INCAx-sight of Oxford Instruments, placed at 25 degrees. The high-resolution cross sectional TEM image, shown in Fig. S1.2, was acquired using a Titan STEM at The Institute of Nanoscience of Aragon in University of Zaragoza.

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S2. Absorbance spectra of Bi2S3 NCs and PbS QDs and absorption of devices

Fig. S2 (a) Absorbance in solution of PbS QDs and Bi2S3 NCs. (b) Total absorption of the best performing bilayer and BNH devices used for the determination of IQE in the manuscript.

(a)

(b)

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S3. Statistical data on performance of BNH and bilayer devices

(a)

(b)

(c) Fig S3 Data for Jsc (a), FF (b) and PCE (c) from 6 devices from each class of device: bilayer, BNH 1:2 and BNH 1:3. The graphs show the higher performance obtained from the BNH structures compared to the bilayer structures. Voc is not plotted since both BNH and bilayer structures yielded statistically similar Voc values.

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S4. Carrier lifetime and Jsc intensity dependence at laser excitation of 980 nm

Fig. S4 (a) Carrier lifetime for a bilayer, BNH and control device upon illumination of 980 nm laser light at various intensities. Control device consist of 5:1 PbS:Bi2S3 BNH layer of 240 nm and Bi2S3 layer of 180 nm. PbS:Bi2S3 ratio of 5:1 was the maximum ratio value that preserved the formation of a junction – as evidenced by rectification – and allowed the measurement of transient Voc. (b) Short-circuit current density of a bilayer, BNH and control device as a function of intensity at 980 nm wavelength illumination. The power law dependence of this curves fits to a factor α of ~0.9. Schematic representations of the device structures (c) bilayer, (d) BNH, (e) control device.

We monitored the carrier lifetime and photocurrent power dependence of the devices at excitation wavelength of 980 nm where only the PbS phase absorbs light. In this case, the Bi2S3 layer acts merely as an electron transport layer (electrode), since there is not minority carrier photogeneration in Bi2S3. In this condition, both bilayer and BNH devices showed similar lifetimes equal to previously reported lifetimes measured in PbS

a b

c d e

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Schottky devices2. To further elucidate the effect of a BNH in carrier lifetime prolongation in the PbS quantum dots we fabricated devices that consist of a BNH layer atop ITO followed by an overlayer of Bi2S3. This device is deprived of a pure PbS layer and therefore would allow us to estimate the carrier lifetime of photogenerated carriers in the PbS phase within the BNH layer. We then monitored the carrier lifetime at excitation wavelength of 980 nm. We found that carrier lifetime in this structure is by 60% longer when compared to the bilayer device demonstrating thus the benefit of the BNH in the prolongation of the photogenerated carriers’ lifetime in PbS.

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S5. PL lifetime measurements

The fluorescence dynamics of PbS, Bi2S3 and various blends were measured as follows: 1 LBL of the solutions were spin casted onto 200 µm thick glass cover slips and EDT ligand exchanged following the device fabrication procedure. Samples are illuminated with 50 ps pulsed laser light at 532 nm, 50 MHz repetition rate, and 6 mW of average power. The pump laser is focused at the sample back face through a 100x microscope oil immersion objective with 1.46 numerical aperture. The light emitted from the samples is collected via the same microscope objective through dichroic and highpass filters which ensure a detection window of 550–1100 nm and a better than 1012 suppression of the pump laser. A confocal technique is used to direct the emitted light from spots of 1.5µm diameter to the avalanche photodiode. The fluorescence decay profile is reconstructed by time-correlated single-photon counting with an ~100 ps response time. The Figure shows the resultant PL decay dynamics. An ultrafast decay appears with response on the order of 300-400 ps attributable to Auger recombination and limited by the response of the setup.

Fig S5 Photoluminescence dynamics of PbS, Bi2S3 and 10:1 5:1 1:2 normalized to their maximum values. The inset table shows the extracted values of the lifetimes from the curves. BNH films possess longer lifetimes than individual PbS and Bi2S3 NC films. The data are averaged over 5 different measurements per sample.

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S6. I-V characteristics at light and dark conditions for different blend ratios in ITO/PbS/BNH/Au and ITO/BNH/PbS/Ag configurations.

Fig. S6 Schematic representation of control devices (a)ITO/ PbS (180 nm)/BNH (240 nm)/Au (b) ITO/BNH(240 nm)/Bi2S3 (180 nm)/Ag. I-V characteristics of the devices having different blend ratios at dark (c) and (d) and under light condition are shown in (e) and (f). Different PbS/Bi2S3 ratios are indicated by different colors shown in the graphs.

a b

c d

e f

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As evidenced by the graphs strong rectification at the best performing ratio of PbS:Bi2S3for the BNH is observed in contact with the PbS layer, whereas there is no junction formation of this BNH with the Bi2S3 NC layer. (e) Shows a progressive decrease of Vocwith increasing concentration of Bi2S3 in a PbS-dominated BNH followed by a decrease in the turn-on voltage under dark (shown in c).

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S7. Capacitance at zero bias for different PbS:Bi2S3 ratio in BNH and bilayer device configurations.

Fig. S7 Capacitance at zero bias (short circuit) Csc of BNH devices having different PbS/Bi2S3 ratios and the red point indicates capacitance at zero bias of bilayer device for comparison (which corresponds to infinite concentration of PbS:Bi2S3). Short-circuit capacitance scales inversely with the depletion width of the semiconductor, therefore, decrease in Csc correlates with decrease in doping density and indicates less-doped semiconductor layers. BNH device consists of 90 nm PbS/ 270 nm BNH/ 40 nm Bi2S3and bilayer device consist of 360 nm PbS/ 40 nm Bi2S3 layer. Total thickness of 400 nm as well as device area of 4 mm2, were kept constant for both structures. The capacitance decreases with increasing ratio of PbS QD to Bi2S3 NC up to 20:1 suggesting the formation of a less doped BNH layer.

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S8. EQE spectra for different thickness of BNH layer and performance dependence on the thickness of the PbS electron blocking layer.

Fig. S8.1 EQE spectra for BNH devices with blend thickness of 150 nm and 240 nm. The EQE increases in the long wavelengths with increasing thickness of the BNH as a result of more efficient absorption in the PbS QDs within the BNH layer. The onset of EQE for the devices is found at a wavelength of ~1.5 µm pointing to an effective bandgap of PbS QDs of 0.8 eV, significantly reduced from the initial bandgap of 1.3 eV (exciton peak at ~950 nm) as a result of efficient electronic coupling of the close-packed QD film.

Fig S8.2 Jsc and FF dependence of the BNH device on the thickness of the PbS electron blocking layer. With increasing thickness of the PbS layer we observe a reduction in Jsc as a result of photons absorbed in the PbS front layer which converts the photogenerated carriers less efficiently than the BNH layer. There is also an optimum thickness layer of PbS with respect to FF, therefore we have chosen a thickness of ~100 nm for the optimization of our best performing BNH devices.

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S9. Dependence on device area and electrode geometry

We have fabricated devices of different areas and geometry employing the cross electrode architecture forming a 4 mm2 area as well as devices with isolated electrodes having circular contacts with areas of 3.1 mm2, 7 mm2 and 12.5 mm2. We employed a BNH device having a 1:1 PbS:Bi2S3 ratio.

Area (mm2) Voc (V) Jsc (mA/cm2) FF η% 4 0.42 14.10 0.44 2.60

3.14 0.42 14.42 0.51 3.08 7.06 0.42 14.16 0.46 2.74

12.56 0.4 14.80 0.40 2.37

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S10. Determination of carrier lifetime from transient Voc

We used transient Voc measurements for the determination of the carrier lifetime in our devices as described elsewhere2. In brief: Transient open-circuit voltage measurements at visible and near infrared wavelengths were performed via amplitude modulation of two different lasers. First, a Newport LQA635-08C 635 nm laser was controlled using an Agilent 33220A waveform generator, to provide for pulsed monochromatic red light. Then, an Optoenergy 975033230M fiber coupled 980 nm singlemode laser module controlled by a Thorlabs ITC4000 was used to provide for near-infrared laser light. In both cases, the time profile was then recorded in a Tektronix TDS2024B digital oscilloscope. The very initial slopes of the decays were fit with first order polynomial curves1. Finally the carrier lifetimes (τ) were calculated according to the expression:

where k is the Boltzmann constant, q is the electron charge and T is the room temperature. Figure S10.1 shows the fittings of the transient Voc in the linear regime for a bilayer and a BNH device.

a b

Fig S10.1 Transient Voc measurements at 635 nm for a bilayer (a) and a BNH (b) device. The arrows indicate the decrease in light intensity.

To further support the results we also performed Transient Voc measurement based on small signal (ΔV~30mV) transient photovoltage measurements2. These were performed using a Thorlabs HRP050 continuous wavelength 633nm laser source and a Newport LQA635-08C 635 nm laser modulated with an Agilent 33220A waveform generator coupled together with a 50:50 beam splitter. The time profile was then recorded in a Tektronix TDS2024B digital oscilloscope. The transient profile was corrected by the offset, normalized and then, fit to a single exponential which decay rate is related to the carrier lifetime according to the expression2:

.

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Fig. S10.2 Small signal and large signal transient Voc decays at a given optical intensity of 635 nm laser for a bilayer device. The two approaches yield similar values of carrier lifetime. We observed a good agreement between the two techniques. As shown in Fig S10.2 the extracted carrier lifetime using the two approaches indeed yield carrier life time within a range of 10%.

References:

1 Johnston, K. W. et al. Efficient Schottky-quantum-dot photovoltaics: The roles of depletion, drift, and diffusion. Appl. Phys. Lett. 92, 122111, (2008).

2 Zhao, N. et al. Colloidal PbS Quantum Dot Solar Cells with High Fill Factor. Acs Nano 4, 3743-3752, (2010).

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