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High Performance PbS Colloidal Quantum Dot Solar Cells by Employing Solution-Processed CdS Thin Films from a Single-Source Precursor as the Electron Transport Layer Long Hu, Robert J. Patterson, Yicong Hu, Weijian Chen, Zhilong Zhang, Lin Yuan, Zihan Chen, Gavin J. Conibeer, Gang Wang,* and Shujuan Huang* optimal ligands for CQD thin film surface passivation have evolved from organic thiols to halide anions, and currently lead iodide (PbI2) is considered to provide the best passivation.[9,10] The device architecture has evolved from Schottkytype devices to recent reports of p–i–n heterojunction structures[7,11–13] enabled by doping through tailoring the QD surface termination. Due to these attractive strategies, a certified power conversion efficiency (PCE) of 11.28% in PbS/ZnO CQDs solar cells has been reported.[14] Most highly efficient PbS CQD solar cells were prepared using a solution process such as spin-coating, dip-coating, or spraying,[9,15,16] which are low-temperature manufacturing processes that offer lowcost and compatibility with a variety of colloidal semiconductors. Wide bandgap n-type semiconductors such as ZnO,[3,6,17] TiO2,[16,18] and CdS[19–21] have been extensively employed in photovoltaic devices as they readily crystalize with good semiconducting properties from a wet-chemical fabrication process. CdS is a typical n-type semiconductor with higher photostability compared to ZnO and TiO2[22–24] and the CdS films have been prepared by various methods such as Chemical Bath Deposition (CBD),[25–28] sputtering,[29] and evaporation.[30] CdS films have been extensively studied in Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4, and CdTe solar cells as the electron transport layer. The most common approach for CdS film deposition is CBD. Although this method is low cost, it is inefficient because only a fraction of the CdS nanoparticles in the bath are deposited onto the substrates. In addition, the sputtering and evaporation processes are not the most efficient way to fabricate the CdS films due to the capital expense for these deposition processes relative to chemical synthesis. It is highly desirable to develop a method that is straightforward and has a high material conversion rate for the deposition of thin films for solar cell applications. Though the ZnO/PbS solar cells have achieved great success, some researcher have tried to fabricate CdS/PbS CQD heterojunction solar cells to take advantage of the outstanding electrical properties inherent in CdS films, and meaningful results have been obtained. Ellingson and co-workers prepared a CdS/PbS CQD solar cell by employing a CdS film prepared by RF-magnetron sputtering, and obtained a 3.3% PCE.[31] Bawendi’s group used a CdS film prepared by CBD to fabricate

CdS thin films are a promising electron transport layer in PbS colloidal quantum dot (CQD) photovoltaic devices. Some traditional deposition techniques, such as chemical bath deposition and RF (radio frequency) magnetron sputtering, have been employed to fabricate CdS films and CdS/ PbS CQD heterojunction photovoltaic devices. However, their power conversion efficiencies (PCEs) are moderate compared with ZnO/PbS and TiO2/ PbS heterojunction CQD solar cells. Here, efficiencies have been improved substantially by employing solution-processed CdS thin films from a singlesource precursor. The CdS film is deposited by a straightforward spin-coating and annealing process, which is a simple, low-cost, and high-material-usage fabrication process compared to chemical bath deposition and RF magnetron sputtering. The best CdS/PbS CQD heterojunction solar cell is fabricated using an optimized deposition and air-annealing process achieved over 8% PCE, demonstrating the great potential of CdS thin films fabricated by the single-source precursor for PbS CQDs solar cells.

1. Introduction Lead chalcogenide (such as PbS or PbSe) colloidal quantum dots (CQDs) have attracted much attention in optoelectronic[1,2] and photovoltaic devices,[3–8] because of the remarkable advantages solution-processing, tunable material bandgaps into the near-infrared, and multiple exciton generation have for advanced photovoltaic cells. In particular, PbS CQD solar cells have achieved a remarkable improvement in efficiency as well as ambient stability due to rapid progress in surface passivation and beneficial modifications to the device architecture. The L. Hu, R. J. Patterson, Y. Hu, W. Chen, Z. Zhang, L. Yuan, Z. Chen, Prof. G. J. Conibeer, Dr. S. Huang Australian Centre for Advanced Photovoltaics University of New South Wales 2032 Sydney, Australia E-mail: sj.huang@unsw.edu.au Prof. G. Wang State Key Laboratory of Rare Earth Resource Utilization ChangChun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022, P. R. China E-mail: wsu@ciac.ac.cn The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201703687.

DOI: 10.1002/adfm.201703687

Adv. Funct. Mater. 2017, 1703687

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a CdS/PbS CQD heterojunction solar cell and achieved a 3.5% PCE.[10] Jiang and co-workers fabricated CdS/PbS CQD bulk and planar heterojunction solar cells by using CBD for the CdS film and achieved 4.7%[32] and 5.2%[33] PCEs, respectively. This work demonstrated that CdS thin films can act as the electron transport layer in PbS CQD solar cells, but the device performance is mediocre and significant further progress is still possible. Herein, we demonstrate a new solution-processed method to fabricate the CdS thin films, and demonstrate a most significant improvement in CdS/PbS heterojunction solar cells. Uniform and compact CdS thin films were prepared from a simple single source precursor using a low-cost and materialsaving spin-coating process. By optimizing the CdS layer thickness and annealing conditions, we achieve a champion PCE of 8.3%, which is the most efficient device fabricated using a CdS electron transport layer to date. Furthermore, we have investigated the temperature-dependent behavior of PbS CQD solar cells from 110 to 350 K at 30 K intervals and observed that the device has the best performance at 260 K. We discovered that the CdS/PbS structure has better temperature-dependent stability compared to the ZnO/PbS structure probably due to cadmium sulfide’s improved electronic properties, such as high conductivity.

2. Results and Discussions The proposed chemical reaction for the preparation of the CdS precursor is shown in Formula 1–3. Carbon disulfide (CS2) was mixed with 1-butylamine in ethanol to form butyldithiocarbamic acid under vigorous stirring (Formula 1). Subsequently, the Cd(OH)2 powder was then added into this solution and stirred at 70 °C overnight (Formula 2). A uniform yellow solution containing the cadmium butyldithiocarbamic salt complex was then obtained, filtered, and diluted with ethanol for use as the single-source CdS precursor solution. A photograph of CdS precursor solution is shown in the inset of Figure 1a. The solution was stored in a refrigerator and was still found to be stable after 180 days.

SH

S

C

S

CH

S

H 2N

S

Formula 1

HN

C

SH

Cd(OH)2

HN

Cd

S

CH HN

Formula 2 2

S

Cd

S

CH S

HN

Annealing

CdS

Organics

Formula 3

2

Since the sol-gel deposition process used to fabricate the CdS thin films requires a low temperature thermal anneal, we applied thermogravimetric analysis (TGA) measurements to determine the temperature at which mass is lost and dissociation of the precursor produces CdS. As shown in Figure 1a, the TGA curve suggests that weight loss began at 180 °C and the total mass sharply decreased to 15% of the original value by 220 °C. This decomposition process is demonstrated in Formula 3, similar to the results from other groups.[34,35] Based on this result, we fabricated CdS thin films by spincoating the CdS precursor, followed by annealing at 220 °C for various times. A photograph of a variety of CdS films is shown in Figure S1 in the Supporting Information. A detailed description of the film fabrication can be found in the Experimental Section. After the CdS film preparation, we have carried out the transmittance spectrum, field emission scanning electron microscopy (FE-SEM), and atom force microscopy (AFM) measurements to examine the CdS film’s bandgap and surface morphology. The ultraviolet–visible (UV–vis) transmittance spectra of the CdS films on soda lime glass substrates are shown in Figure 1b. A larger blue-shift of the absorption edge and increased transmittance with longer annealing time were observed, indicating a gradual broadening of the optical band gap in the CdS film. By plotting (αhυ)2 versus (hυ), we extrapolated the linear fitting to the x-axis and obtained a band gap of 2.45 eV for 10 min, 2.49 eV for 20 min, 2.51 eV for 30 min, 2.55 eV for 45 min, and 2.64 eV for 60 min of annealing, respectively. The blue-shift probably can be attributed to oxidation of the surface of CdS to form CdO evidenced by an increase in the O 1s peak intensity[36,37] shown in the

Figure 1. a) TG curve of the CdS precursor; inset: digital photograph of the CdS precursor solution. b) Optical transmittance spectra of the CdS films annealed for various times in air.


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Table 1. The physical parameters of CdS films annealed for various time. Time [min]

Band gap [eV]

Fermi level [eV]

VBM [eV]

10

2.45

4.26

6.56

CBM [eV] 4.11

20

2.49

4.30

6.55

4.06

30

2.51

4.35

6.55

4.04

45

2.55

4.40

6.50

3.95

60

2.64

4.45

6.45

3.81

X-ray photoelectron spectra (XPS) in Figure S2 (Supporting Information). The enhanced transmission with longer annealing time could be attributed to the improved material quality upon annealing which reduces structural defects and sub-band absorption, hence increasing transmission. Another possible reason is the suppressed free carrier absorption in longer wavelength region. The mechanism for this is that the Fermi level shifts away from the conduction band minimum (CBM) after the sample has been annealed for longer times, as demonstrated by ultraviolet photoemission spectroscopy (UPS) results in Figure S3 (Supporting Information). The distance between the Fermi level and valence band maximum (VBM) can be obtained by fitting the onset region of the UPS spectra. Combining the UPS results with the estimated band gaps from the transmittance measurement, the VBM and CBM of the CdS films of different annealing time were obtained, as shown in Table 1. The Fermi level of CdS film annealed for a longer time is further away from the CBM, which indicates that carrier concentration of this film has been reduced. The transmittance and band gap of the CdS annealed for longer time increased. The solar cells benefit from the enlarged band gap and higher transmittance, as these increase the incident light into the PbS CQDs absorber layer. The representative top-view SEM and AFM images are shown in Figures S4 and S5 (Supporting Information), respectively. The SEM image shows that the CdS film is uniform and crack free. The AFM result shows the CdS film was smooth, crack and pin-hole free with a 2.5 nm average surface roughness. Such a smooth surface is beneficial in reducing leakage current at the n–i material interface because of the reduced junction area.[6]

To test the performance of the CdS films in photovoltaic devices, we fabricated the PbS CQDs solar cells with the CdS film as the electron transport layer, PbI2 treated PbS CQDs as the intrinsic absorber layer, and a thin layer of PbS CQDs treated with 3-mercaptopropionic acid (MPA) as the hole collection layer. Fabrication details are shown in the Experimental Section. The PbS CQDs have an excitonic peak at 890 nm (1.4 eV) as shown in Figure S6 in the Supporting Information. All processes including CQDs purification, film spin-coating, and device characterization were performed in ambient conditions. Figure 2a shows the cross-sectional SEM image of a complete solar cell with a device architecture consisting of ITO/ CdS/PbS-PbI2/PbS-MPA/Au layers. From this image, the thickness of the CdS film is about 65 nm, the PbS-PbI2/PbS-MPA layer is about 210 nm, and the Au electrode is about 70 nm. Five batches of devices were prepared using CdS films with various annealing times, with 12 devices in each batch. Figure 2b shows the J–V curves of the champion devices, under the AM 1.5 illumination. A substantial Voc enhancement was observed with increasing annealing time for all samples due to the reduced number of defects in the CdS film and hence reducing nonradiative recombination. This is in agreement with the carrier lifetime results as shown in Table 3. The devices using CdS films with 30 min annealing time have the best performance. The champion device has a PCE of 8.3%, with a Voc of 0.62 V, a Jsc of 21.5 mA cm−2 and a fill factor (FF) of 0.62. The device using a CdS film annealed for 10 min has the lowest performance (PCE of 6.5% with a corresponding Voc of 0.58 V, a Jsc of 19.8 mA cm−2, and an FF of 0.57), which is likely due to the poor quality of the CdS layer. After annealing for 60 min, the device performance decreased to 6.9%, probably due to excessive oxidation of CdS which results in an unfavorable band alignment as shown in Figure 3d. The device parameters are given in Table 2. To verify the Jsc, an external quantum efficiency (EQE) measurement was carried out on the devices, as shown in Figure 2c. The integrated photocurrent from the EQE results was 18.9, 19.5, 20.2, 19.1, and 18.6 mA cm−2 based on devices fabricated with CdS films annealed for 10, 20, 30, 45, and 60 min, respectively. This is in good agreement with the J–V measurement. We carried out steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements on

Figure 2. a) Cross-sectional SEM image of a complete solar cell. b) J–V characteristics and c) EQE spectra of devices that employed CdS films annealed for various times.


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Figure 3. a) PL peaks from band-to-band emission at ≈500 nm and defect level emission at ≈680 nm in CdS films annealed for various times. b) PL spectra of PbS CQD films treated with PbI2 on CdS films annealed for various times. c) TRPL probed at 512 ± 25 nm of the CdS films showing that 60 min of annealing results in the longest lifetime, and d) schematic diagram of band energy alignment and electron injection between PbS CQDs and CdS films annealed for 10, 30, and 60 min. The dashed line indicates unfavorable electron injection due to slightly higher conduction edge for films annealed for 60 min.

CdS films with various annealing times, as shown in Figure 3. The Figure 3a shows that the intensity of the peak in the PL from the CdS film was located at about 500 nm and increased slightly with increasing annealing times. The PL peak located at 680 nm is related to the sulfur vacancies in the CdS film, which decreased with longer annealing time suggesting a decrease in the defect density.[38–40] Figure 3b shows that the PL intensity of PbS CQDs treated with PbI2 deposited on top of the CdS films Table 2. Device parameters of the solar cells with CdS films annealed for various times. Time [min]

Voc [V]

Jsc [mA cm−2]

FF [%]

Best PCE [%]

Average PCE [%]

10

0.58

19.8

57

6.5

6.2 ± 0.3

20

0.60

20.6

59

7.3

7.1 ± 0.2

30

0.62

21.5

62

8.3

8.1 ± 0.2

45

0.63

20.3

60

7.7

7.4 ± 0.3

60

0.64

19.5

58

7.2

6.9 ± 0.3


was higher when CdS films were annealed for longer times. Charge separation and extraction were less efficient with these CdS films, which is otherwise similar to that of lead trihalide perovskites in terms of transport to electron or hole collection layers.[41–44] The TRPL measurements were performed (probed at 512 ± 25 nm) on CdS films with various annealing times to investigate charge carrier lifetimes, as shown in Figure 3c. The TRPL curves compare well with biexponential decay curves obtained from fitting to the data using Equation (1): IPL (t ) = A1exp (−t /τ 1 ) + A2exp (−t /τ 2 ) (1) where IPL(t) is the time dependent PL intensity, τ1 and τ2 are the short and long PL lifetime, respectively, and A1 and A2 are constants corresponding to the amplitudes of the PL components. We define the effective lifetime of the charge carries as [τ] = (A1τ1 + A2τ2)/(A1 + A2), and use [τ] as a signature of merit for comparing the CdS films with different annealing times.[45] The fitting results are shown in Table 3. The effective

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Table 3. The carrier lifetime of CdS films annealed for various times. A1

τ1 [ns]

A2

τ2 [ns]

τ [ns]

10

1.35

0.09

0.27

1.12

0.26

20

0.83

0.43

0.19

2.10

0.74

30

0.67

0.66

0.32

2.95

1.40

45

0.68

0.73

0.33

3.18

1.53

60

0.53

0.74

0.40

3.50

1.92

Time [min]

carrier lifetimes in the CdS films are inversely proportional to the defect density of the material including both surface and bulk. The effective carrier lifetime of the CdS film annealed for 60 min is the longest at 1.92 ns, suggesting lowest nonradiative recombination, hence lowest defect density and higher Voc. The carrier lifetime of the CdS film annealed for 10 min is the shortest probably suggesting it having the highest defect density, hence lower Voc. These TRPL results are consistent to the increase of Voc when annealed for a longer time as shown in Table 2. To help explain the variation in device performance, a schematic diagram showing the electron transport between PbS CQDs and CdS films annealed for 10, 30, and 60 min, is given in Figure 3d. Luther’s group has calculated the Fermi level, CBM and VBM of PbS CQDs treated with PbI2 from a UPS measurement.[9] We used their method since our PbS CQDs have the same ligand and a similar band gap. Generally, the device performance depends on the balance between carrier lifetime and effective electron injection rate from the PbS CQDs to the CdS films. For the CdS annealed for 10 and 30 min, the electron can readily be injected into the CdS film due to additional field provided by the conduction band offset. For the CdS annealed for 60 min, the electron injection becomes relatively difficult due to the CBM of the CdS film being slightly higher than the CBM of the PbS CQDs. The devices fabricated from CdS films annealed for 10 min showed the poorest performance due to serious nonradiative recombination caused by the highest defect density as suggested by the shortest carrier lifetime, though with favorable injection. The devices fabricated from CdS films annealed for 60 min performed moderately due to unfavorable injection at the interface between the PbS CQDs and the CdS films, though with the longest carrier lifetime and lowest nonradiative recombination. Finally, the devices fabricated from CdS films annealed for 30 min show the best performance due to favorable injection and relatively long carrier lifetime. We further investigated the relationship between the thickness of the CdS layer and the device performance. The performance of the devices employing one, two, three, and four layers of spin-coated CdS is summarized in Table 4. These CdS layers were annealed at 220 °C for 30 min in air. The devices employing one layer of spin-coated CdS film showed poor performance, which may result from the existence of pinholes in such a thin film. This can cause shunting between the PbS CQD layer and the indium tin oxide (ITO). As the number of the CdS layers increases from two to four, there is no change in the Voc of the devices, probably due to the fixed energy level of the CdS layer. While the device with two layers


Table 4. Device parameters of solar cells that employ various layers of CdS. Layers

Voc [V]

Jsc [mA cm−2]

FF [%]

PCE [%]

Rs [Ω]

Rsh [Ω]

1

0.54

15.3

46

3.8

45

612

2

0.62

21.5

62

8.3

48

3952

3

0.62

19.1

58

6.8

52

3982

4

0.62

17.5

56

6.1

57

3966

of CdS (≈65 nm) showed the best performance as also shown in Figure 2, Jsc and FF start to decrease gradually with more layers. This is because a thicker CdS film can result in higher series resistance and lower transmittance. The device series resistance increased with increase of CdS film thickness as showed in Table 4. The optical transmittance decreased with increase of CdS film thickness as shown in Figure S7 (Supporting Information). In addition, we explored the temperature dependent behavior of the PbS CQD solar cells. Temperature dependent J–V characteristics were measured on the champion solar cell in this study, from the range of 110 to 350 K with 30 K intervals, as shown in Figure 4a and b. The overall effect on the device efficiency originates from the changes in Voc, Jsc, and FF. The Voc gradually decreased with increasing temperature from 110 K (Voc = 0.72 V) to 260 K (Voc = 0.62 V), then sharply decreased to Voc = 0.47 V at 350 K. Both Jsc and FF also show the similar trend, increasing from 110 to 260 K, and then decreasing from 260 to 350 K. The overall device performs the best at 260 K, which is similar to Bawendi[13] and Loi’s results[46] of ZnO/PbS CQDs solar cells. However, our CdS/PbS CQDs device performance decreases less with temperature changes compared to ZnO/PbS CQDs solar cells probably due to the better electronic properties of CdS, such as higher conductivity (as shown in Figure S8, Supporting Information) and shallower defects that remain thermally activated as dopants across a broader temperature range. The device performance of CdS/PbS CQDs solar cells at 110 K has a 25% decrease compared with a 38% decrease for ZnO/PbS CQDs,[13] which suggests CdS has a broader optimal temperature range in applications. We use Equation (2) to express the relationship between the device parameters: Voc = nkT/q × ln( J sc/J 0) (2) where J0 is the reverse saturation current density, q is the elementary charge, T is temperature, k is the Boltzmann constant, n is the ideality factor, and Jsc is photogenerated current density. Both of Jsc and J0 decrease with decrease of temperature because there is less thermal excitation of free carriers from shallow traps. However, the J0 decreases much more than Jsc,[46] thus increasing the net value of Voc. Given these mechanisms, the optimal point for device performance was found to be 260 K. The FF depends on series resistance and leakage current at the p–n junction, and increases from 110 to 260 K due to decreased series resistance, then decreases due to increased leakage current.

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Figure 4. a) Temperature-dependent light J–V curves showing a maximum Voc at the minimum temperatures measured and an optimal Jsc, FF, and PCE at 260 K. b) Temperature dependence of the characteristic device parameters (PCE, FF, Jsc, and Voc).

3. Conclusion In summary, we have demonstrated efficient PbS CQD heterojunction solar cells employing a CdS electron transport layer deposited from a single-source precursor. By optimizing the CdS film quality with various annealing times, a CdS/PbS CQD planar heterojunction solar cell with 8.3% PCE was achieved. Exploring the temperature dependent behavior of the solar cells, the best device performance is demonstrated at 260 K from the variable temperature J–V test, and CdS demonstrates better electronic property applied in solar cells compared with ZnO at low temperature. The single-source CdS precursor solution can be potentially used in other thin film solar cells such as Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4, and CdTe solar cells, which could significantly reduce fabrication-cost and improve material conversion rate.

4. Experimental Section CdS Precursor and Film Preparation: 2 mL 1-butylamine was added to 5 mL ethanol in a 20 mL vial, then 1.2 mL CS2 was added into the vial drop by drop while stirring at the room temperature to form the reaction solution. 0.256 g Cd(OH)2 was then added into the reaction solution followed by overnight stirring at 70 °C. A clear yellow precursor solution formed after filteration and being diluted with 5 mL ethanol. Patterned ITO glass was cleaned with detergent, DI water, isopropanol, and acetone under ultrasonication. The CdS precursor solution was spincoated on the cleaned ITO glasses at 3000 rpm for 30 s and annealed on a preheated hotplate at 220 °C to form one layer. Device Fabrication: PbS CQDs were synthesized according to the published recipe.[4] 0.9 g PbO, 2.9 g oleic acid (OA), and 20 mL octadecene (ODE) (All materials were perchased from Sigma) were degassed in 100 mL three-neck flask at 85 °C under stirring for 6 h. 280 µL hexamethyldisilathiane (TMS) in 10 mL degassed ODE was fast injected into flask and cooled to room temperature slowly. The PbS CQDs were purified with precipitation. 30 mg mL−1 PbS CQDs in hexane were spin-coated on CdS films at 3000 rpm for 20 s, treated with 10 mmol L−1 PbI2 dimethylformamide solution for 45 s by dip-coating, rinsed with acetonitrile by immersing for 5 s, and dried with N2 gas. This process was repeated five times to achieve a 150 nm thickness of PbS-PbI2 CQDs with one spin-coating yielding a layer of 30 nm in thickness. For the


PbS-MPA layers, 10% volume MPA methanol solution was applied to PbS-PbI2 CQDs layer, then followed by an acetonitrile rinse. Two layers of PbS treated with MPA were spin-coated for our devices. All fabrication processes were carried out in air. The PbS CQDs films were stored in air for 1 d before 70 nm gold electrodes were thermally evaporated on them. The contacts defined the active area of our devices as 0.067 cm2. Characterization: TGA was performed on a Setaram Setsys Evolution Thermal Analyzer. A mass of ≈30 mg of the CdS precursor was heated from ambient to 600 °C in an air atmosphere at a heating rate of 5 °C min−1. The time-resolved PL measurements were performed on a Micro Time 200 (Picoquant) confocal microscope using the time-correlated single photon counting (TCSPC) technique with a 470 nm excitation and detection through a 512 ± 25 nm band pass filter. The laser light was focused through a water immersion objective (NA 1.2). The steady state PL measurement was undertaken on a homemade system using a 405 nm laser as excitation source. The UPS measurement was carried out in a Kratos AXIS Ultra-DLD ultrahigh vacuum photoemission spectroscopy system with an Al Kα radiation source. Tapping mode AFM was performed using a Veeco multimode V instrument. SEM images were obtained using an FEI Nova Nano SEM 450. The optical transmittance of CdS films and optical absorbance of PbS CQDs were recorded by UV–vis–near IR spectrophotometer (PerkinElmer Instruments, Lambda 950 using an integrating sphere). X-ray photoelectron spectroscopy (XPS) measurement was carried out with a Genesis system (EDAX Inc.). The current density–voltage characteristics of devices were measured using Keithley 2400 (I–V) digital source meter under a simulated AM 1.5G solar irradiation at 100 mW cm−2 (Newport, AAA solar simulator, 94023A-U). Temperature dependent J–V tests were carried out in a custom-made stage with a liquid nitrogen cryostat.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the Australian Government through the Australian Research Council (ARC) and the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information, or advice expressed herein is not accepted by the Australian Government. The

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authors would also like to thank Dr. Bin Gong of UNSW for the XPS measurements and the Electron Microscopy Unit of UNSW for the microscopy imaging support.

Conflict of Interest The authors declare no conflict of interest.

Keywords CdS/PbS heterojunction, PbS colloidal quantum dots, photovoltaics, single-source precursors Received: July 5, 2017 Published online:

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