Charge Carrier Conduction Mechanism in PbS Quantum Dot Solar

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Charge Carrier Conduction Mechanism in PbS Quantum Dot Solar Cells: Electrochemical Impedance Spectroscopy Study Haowei Wang, Yishan Wang, Bo He, Weile Li, Muhammad Sulaman, Junfeng Xu, Shengyi Yang, Yi Tang, and Bingsuo Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03198 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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ACS Applied Materials & Interfaces

Charge Carrier Conduction Mechanism in PbS Quantum Dot Solar Cells: Electrochemical Impedance Spectroscopy Study Haowei Wang1, Yishan Wang2, Bo He2, Weile Li2, Muhammad Sulaman2, Junfeng Xu3, Shengyi Yang2,4*, Yi Tang3 and Bingsuo Zou2. (1 Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China) (2 Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China) (3 Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, P. R. China) (4 State Key Lab of Transducer Technology, Chinese Academy of Sciences, P. R. China)

Abstract: Being the properties of bandgap tunability, low cost and substrate compatibility, colloidal quantum dots (CQDs) are becoming promising materials for optoelectronic applications. On the other hand, solution-processed organic, inorganic and hybrid ligand-exchange technologies have been widely used in PbS CQDs solar cells, and currently the maximum certified power conversion efficiency (PCE) of 9.9 % has been reported by passivation treatment of molecular iodine. Till now, however, there are still some challenges and the basic physical mechanism of charge carriers in CQDs-based solar cells is not clear. Electrochemical impedance spectroscopy (EIS) is a monitoring technology for current by changing the frequency of applied alternating current (AC) voltage, and it provides an insight to its electrical properties that cannot measured by direct current (DC) testing facilities. Therefore, in this work we used EIS to analyze the recombination resistance, carrier lifetime, capacitance and conductivity of two typical PbS CQD solar cells Au/PbS-TBAl/ZnO/ITO and Au/PbS-EDT/PbS-TBAl/ZnO/ITO, in this way, to better understand the charge carriers conduction mechanism behind in PbS CQD solar cells, and it provides a guide to design high-performance quantum-dots solar cells.

1 Environment ACS Paragon Plus


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Keywords: Lead sulfide (PbS), colloidal quantum dots (CQDs), solar cell, electrochemical impedance spectroscopy (EIS), recombination resistance.

1. Introduction Colloidal quantum dots (CQDs) have obtained quite a number of attention during the past few years for its efficient, low-cost and facile implementation of optoelectronic devices, such as light-emitting diodes1-4, photovoltaics5-8, photodectors6,9-11 and field-effect transistors12-14. Being its tunable energy bandgap which can cover the infrared region in the sun light which is barely absorbed by organics, narrow band-gap lead chalcogenide such as lead sulfide (PbS) and lead selenide (PbSe) CQDs have been considered as attractive materials for photovoltaic devices. Recently, interests in PbS CQDs are particularly intense for the properties of PbS CQDs, such as its small bulk bandgap (0.41 eV), excellent size monodispersity and high dielectric constant with a broad spectral response from the ultraviolet (UV) to the near infrared (NIR) region. Moreover, recent studies have validated that PbS CQDs showed a strong multiple exciton generation (MEG) effect and possessed a significantly larger bulk exciton Bohr radius (18 nm for PbS), which makes PbS CQDs with a larger bandgap tunability and higher carrier mobility. Currently, PbS CQDs-based solar cells top the photovoltaic efficiency in CQDs-based solar cells, benefiting greatly from the progress of improving junction architecture, light absorption and charge collection efficiency. However, the surfaces of as-made PbS CQDs are usually covered with long-chain ligands which were used to synthesize and stabilize the CQDs. Therefore, the device performance of CQDs-based optoelectronic devices depends on the electronic coupling among neighboring nanocrystals in the CQD film. To reach this goal, the long native ligands are usually exchanged with short and more strongly bonded ligands, making the CQDs into more electronic 2 Environment ACS Paragon Plus

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coupling via reducing the inter-dot spacing. In this way, carrier mobility in CQDs film mainly depends on the inter-coupling of QDs, while the recombination process and lifetime of charge carriers depend strongly on the ligand materials and the process of ligands exchange. Therefore, to maximize the transport properties of carriers, recombination resistance and carriers lifetime of CQDs-based solar cells, are essential to achieve high device performance. Great progress in the surface passivation of CQDs, such as by short-chain ligands15, by organic ligands16-18 and/or by inorganic ligands19,20, particularly by halide ions21,22, have led to rapid improvements in PbS CQDs solar cells and the maximum certified power conversion efficiency (PCE) has reached 9.9 %23. However, there are still some challenges remained, and the charge carriers conduction mechanism in the CQDs solar cells are still unclear. Much more attention should be paid to the fundamental understanding of the PbS CQDs-based solar cell since this is the basis on how to enhance the performance of CQDs-based solar cells. Usually, current density-voltage (J-V) characteristics are used to analyze the cell performance and the open-circuit voltage (VOC), short-circuit current (JSC) and fill factor (FF) of the solar cells. However, the full physical mechanism beneath the active layer and solar cells cannot be derived only from its J-V characteristics. As we know, electrochemical impedance spectroscopy (EIS) is applied for current change monitoring as a response of alternating frequency of alternating current (AC) voltage, which permits an investigation of electrical characteristics other than those obtained by direct current (DC) testing equipments. In order to achieve the better performance of devices, EIS measurement has been applied for the study of carriers’ relaxation and transport properties for many types of optoelectronics devices, such as dye-sensitized solar cells24,25, perovskite solar cells26 and organic light-emitting diodes27,28. Some recent papers have reported the electrical



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property of organic solar cells by EIS method29-31, attempting to explain the mechanism of recombination resistance and lifetime of carriers in the active layer of organic device. Nevertheless, few published results werededicated to revealing the properties of CQDs-based solar cells and more attention is devoted to improve the device performance other than understanding the physical mechanism of CQDs-based solar cells.Therefore, in this work, we explored the carriers conduction mechanism of the PbS CQDs-based solar cells by the EIS to better understand the carriers accumulation and transport mechanism underneath. Our experimental data showed the convincible information for the PbS CQDs-based solar cells on how to optimize the device configuration and the device performance. Moreover, in this way, we can better understand the conduction mechanism in PbS CQDs-based solar cell and then know how to enhance the performance of CQDs-based solar cells.

2. Experiments 2.1 Materials. Lead oxide yellow (PbO, 99.9%), Oleic acid (OA, 90%), 1-octadecene (ODE, 90%), bis(trimethylsilyl) sulfide ((TMS)2S, synthesis grade), 1,2-ethanedithiol (EDT>98%) and tetrabutylammonium iodide (TBAI, >98%) were purchased from Sigma-Aldrich. Hexane (99.9%), acetone (99.98%), methanol (99.99%) and toluene (98%) were purchased from the Beijing Chemical Works. All these chemicals were used as received. 2.2 Synthesis of PbS CQDs.


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PbS CQDs were synthesized following the procedure developed by Hines and Scholes32 and performed under inert conditions with standard Schlenk-line techniques. Details of the synthesis are described in the Supporting Information. 2.3 Synthesis of ZnO Film. ZnO films were synthesized following the method reported by White et al33. 2.4 Fabrication of Solar Cells. PbS were deposited using a layer-by-layer spin-coating process with a solid-state ligand exchange process, and all these processes were carried out in an ambient atmosphere. Substrates were prepared by pre-patterning ITO on glass, and cleaned by ultrasonic in de-ionized water, acetone and isopropanol sequentially, then a layer of ZnO film was prepared by spin-coating at 3000 rpm and then annealed at 200 °C for at least 30 min before its use. For PbS layer, PbS CQDs solution (50 mg/mL in octane) was spin-coated onto the substrate with a rate of 2500 rpm. The PbS QD film on the ITO substrate, was prepared by soaking TBAI/Methanol solution (10mg/ml) for 30s, and then by spin-washing 3 times with pure methanol. The same process was applied for organic ligand-exchange with EDT solution (0.05 vol% in acetonitrile). All the device fabrication processes were performed under air condition at room temperature. In our experiments, all the samples were deposited for 10 times, consisting of multiple layers of CQDs, and it displays a thickness of about 210 ± 20 nm. The PbS CQDs layer were stored in air for ten hours and then transferred to a nitrogen-filled glove box for Au anode evaporation to complete the device, and the Au anodes were thermally evaporated in a vacuum of 4 × 10−4 Pa at an evaporating rate of 0.1~0.3 nm/s.



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3. Results and Discussion Followed by the procedure developed by Hines and Scholes32, we obtained well-defined size of PbS CQDs with high-quality particles by the hot injection method after optimizing the synthesis conditions. Fig. 1(a) shows the transmission electron microscopy (TEM) and its high resolution TEM (HR-TEM) image of PbS CQDs. From here, one can see that the average particle size is 3～4 nm. Fig. 1(b) shows the absorption spectrum of PbS in n-hexane and its photoluminescence under excitation wavelength of 325 nm. Fig. 1(c) shows the X-ray diffraction patterns of PbS CQDs, the broadened peaks imply that the size of PbS nanocrystals are really in nanometers, indicating the high quality of the PbS CQDs. To better understand the conduction mechanism of PbS CQDs-based solar cells, we fabricated two typical kinds of PbS CQDs-based solar cells, i.e. Au/PbS-TBAl/ZnO/ITO (device A) and Au/PbS-EDT/PbS-TBAl/ZnO/ITO (device B), and their schematic diagrams are shown in Fig. 2, as well as the energy band alignment of device B and the cross-sectional SEM image of device A. For all related experiments in this work, we chose Tetrabutylammonium iodide (TBAI) and 1,2-ethanedithiol (EDT) as the inorganic and organic ligands for ligand exchange process of the PbS CQDs, respectively.The J–V characteristics of photovoltaic devices with PbS-EDT layer (i.e. device B) and without PbS-EDT layer (i.e. device A) are shown in Fig. 3(a). From here, one can see that there is an obvious improvement in current density for device B as compared with that of device A. The PCE of device A and device B reached 5.23 % and 7.85 %, respectively, implying a 50% enhancement. Also, there is a 20 % improvement for JSC after insetting a PbSEDT layer for device B as compared with that of device A. Therefore, device B reveals a higher external quantum efficiency (EQE), as shown in Fig. 3(b), showing better photo-carrier transportation and carriers collection efficiency as compared with device A. Fig. 2(c) and Fig.


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2(d) indicate the band alignment of the PbS CQDs based solar cell, the role of PbS-EDT layer for blocking electrons is clearly demonstrated. This electron-blocking effect results in a remarkable improvement of photocurrent collection efficiency. It is probably that the photo-generated electrons transport from PbS-TBAI layer to anode in device A, which is the opposite direction of the desired photocurrent; In Fig.2(d), the electron-blocking principle is shown after introducing PbS-EDT layer in device B. All the device performance parameters for those two typical kinds of PbS CQDs solar cells are summarized in Table 1. Table 1. Performance parameters for two typical kinds of PbS CQDs solar cells (device A and device B) under AM 1.5 illumination (100 mW/cm2)*.

Devices

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

Device A

0.588±0.022

16.9±1.6

50.4±1.8

5.12±0.2

Device B

0.606±0.08

21.3±2.0

58.7±1.6

7.71±0.3

*

These experimental results are averaged with standard deviation for 9 samples on 3 different substrates.

By comparing the device performance of devices A and B, it is natural for one to ask why the device performance is enhanced magnificantly after inserting a PbS-EDT layer between the Au/PbS-TBAl interface? In order to explore the physical mechanism beneath, here, the EIS was used to study the charge carriers accumulation and transport in these two kinds of PbS CQDsbased solar cells. The impedance spectra of these two kinds of devices under different measurement conditions are shown in Fig. 4. Here, Z denotes the complex impedance of the device, Z' is the real part and Z'' is the imaginary part, and it can be described by Eq. (1). Z (ω ) = Z ' + iZ ''

(1)



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The implicit parameter in Fig. 4 is the frequency and it increases in the direction from right to left34. In the low frequency region, the intersection point of EIS curve with X-axis is the recombination resistances35, which indicates the difficulty of exciton recombination. The larger recombination resistance suggests more difficult for exciton recombination36. We derived the recombination resistances vs. applied voltage from Fig.4 and it was shown in Fig. 5. Also, the basic equivalent circuit of the PbS CQDs-based solar cells is shown in the inset of Fig. 5(a). Excepting for a small series resistance RS (in tens Ohms), the recombination resistance (Rrec) and the capacitance (C) are necessary elements for PbS CQDs-based solar cells, in this sense, the recombination resistance should be as large as possible to allow accumulated carriers in the capacitor element to flow through the external circuit when it returned to the equilibrium state. Further, we estimate the capacitance of CQDs-based solar cells by using EIS. The capacitance is another important parameter to study the conduction mechanism of PbS CQDsbased solar cell. The capacitance can be defined as following, C (ω ) =

1 iω Z (ω )

(2)

Capacitance is important for PbS CQDs-based solar cells since it gives a description of the basic mechanism whereby photo-generated carriers store energy and produce a voltage and current in the external circuit. The capacitance for the two PbS solar cells at different frequency in dark and under illumination are shown in Fig.6. From Fig. 5 and Fig. 6, the recombination resistance and capacitance of device B are ~1.85×107 Ω and 3.33×10-8 F at the frequency of 50 Hz at 0 V, the recombination resistance and capacitance of device A are ~8.13×106 Ω and 1.67×10-8 F, respectively. From here, one can see that the recombination resistance and capacitance are much more enhanced by insetting a PbSEDT layer. When the circuit is open, the capacitance of the PbS CQDs-based solar cells is fully


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charged and there is no external current. The larger capacitance means the larger ability of storing charge carriers even for the same thicknesses of the PbS CQDs film in device A and device B. When the circuit is connected, the carriers are forced by the built-in electric field and there is a current through the external circuit. For an ideal solar cell, Rrec tends to infinity. All the stored charge carriers in the capacitance can be fully used by the external current when it is connected to the external circuit. In practice, however, it is impossible for Rrec to be an infinite value. The larger the recombination resistance, the more carriers stored in the capacitance to be used by the external circuit. Since the recombination resistance and capacitance are the key elements determining how the photogenerated carriers recombine and store, they are the parameters that will limit the maximum performance attainable for the solar cells. From here, one can understand that the recombination resistance exhibits an exponential decay with increasing the applied voltages because the current density increases with applied voltages, and it results in a high possibility for carriers recombination37. The recombination resistance of device B is much higher than that of the device A no matter in dark or under illumination and whatever voltages applied, indicating it gets much more difficult for charge carriers to recombine after insetting the PbS-EDT layer. From Fig. 5, also one can see that the recombination resistances in dark are three-orders of magnitude higher than that under illumination. This is mainly because the quantity of the photogenerated carriers gets much more under illumination and the probability of carriers recombination is increased33. The capacitance for the two PbS solar cells at different frequency are derived at 0 V. From Fig. 6 (a) and Fig. 6 (b), one can see that the capacitance of device B is much higher than that of device A no matter in dark or under illumination. Under illumination, more photogenerated charge carriers will be generated and collected at the force of the built-in



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electric field, which is the reason why the capacitance under illumination is three-orders of magnitude higher than that in dark. The inset in Fig. 6 (b) shows the capacitance of the two typical PbS CQDs based solar cells at various applied voltage in dark. As we know, the capacitance of the device C = ε S / 4π kd , where C is the capacitance of the device, ε is the dielectric constant, S is the area of the solar cell, k is a constant and d is the depletion width. Therefore, from here, one can see that the capacitance is inversely proportional to the depletion width of the device and it depends greatly on the applied voltages. The capacitance increases with increasing the applied voltages and the value of ε , S and k stay invariant, so the depletion width decreases as increasing the applied voltages. Also, one can see that the capacitance of device B is higher than that of device A no matter any voltages applied. Further, the carrier lifetime and the conductivity of CQDs-based solar cells were estimated by using EIS, which are also found to be crucial parameters to describe the dynamic mechanism of PbS CQDs-based solar cells. As an elementary relaxation process, in general, the relaxation process of carriers can be described as following38,

I (t) = Ae

−t τ1

(3)

where A is a constant and τ 1 is defined as the lifetime of the carriers. t is the variation of time. The relaxation process described by Eq. (3) has a characteristic frequency39, and it can be described by the following equation,

ω1 =

1 τ1

(4)

where the frequency ω1 can be derived from Fig. S1 directly, and it is the value of corresponding frequency when the imaginary part Z'' reaches its maximum. As shown in Fig. 7, the carrier lifetime of device B is much longer than that of the device A no matter in dark or under


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illumination, which means the charge carriers in device B can be transported much longer and more efficiently, in this way, facilitating the electrodes to collect the carriers. The conductivity ( σ ) can be defined as following,

σ (ω ) =

L AZ (ω )

(5)

L is the thickness of the PbS CQDs film, A is the effective area of PbS CQDs-based solar cells.

Conductivity is also an important parameter for PbS CQDs-based solar cells since it is an effective parameter to evaluate the ability of carriers transport. The conductivities of these two typical PbS-based solar cells at various frequencies are shown in Fig. 8. From Fig. 7 and Fig. 8, one can see that both the carriers lifetime and the conductivity of the device B are larger than those of device A, implying the inserted PbS-EDT layer in device B contributes to store electric power and it is more convenient for carriers to transport, as a consequence, the device performance is enhanced.

4. Conclusion In summary, the underneath physical mechanism to enhance the power conversion efficiency of PbS CQDs-based solar cells (Au/PbS-TBAl/ZnO/ITO and Au/PbS-EDT/PbS-TBAl/ZnO/ITO) was investigated by using the EIS. The recombination resistance, carrier lifetime, capacitance and conductivity were enhanced greatly after inserting the PbS-EDT layer as compared with the device without PbS-EDT layer. The increment of the recombination resistance, carrier lifetime, capacitance and conductivity can facilitate the extraction of photogenerated charge carriers, transportation and collection of carriers by electrodes, thus to improve the device performance.



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Our experimental data can contribute to boosting our understanding of the physical mechanism beneath for CQDs-based solar cells and paving a way to fabricate higher PCE solar cells.

Acknowledgements This project was partially funded by the project of State Key Laboratory of Transducer Technology (SKT1404), the project of the Key Laboratory of Photoelectronic Imaging Technology and System (2015OEIOF02), Beijing Institute of Technology, Ministry of Education of China, and Key Project of Chinese National Programs for Fundamental Research and Development (2013CB329202).

AUTHOR INFORMATION *Corresponding author E-mail: [email protected], Tel: 86-10-68918188

Present Address Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, School of Physics Beijing Institute of Technology No. 5, Zhongguancun South Street, Haidian District, Beijing, China


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions: Figure 1. (a) TEM image of PbS nanocrystals, and the inset shows its high resolution TEM image, (b) absorption and photoluminescence spectra of PbS CQDs in n-hexane solution, and (c) Normalized X-ray diffraction patterns of PbS nanocrstals.

Figure 2. Schematic diagram of the PbS CQDs-based solar cells Au/PbS-TBAl/ZnO/ITO (device A) (a) and Au/PbS-EDT/PbS-TBAl/ZnO/ITO (device B) (b), the energy level diagram of Au/PbS-EDT/PbS-TBAl/ZnO/ITO (c) and the schematic illustration of energy band alignment at the interfaces of ZnO/PbS-TBAI/PbS-EDT (d) and the cross-sectional SEM image of device A (e) and device B (f). The filled and open circles represent the electrons and holes, respectively, and the arrows represent charge carriers transition in the solar cell.

Figure 3. (a) J-V curves for the two typical PbS CQDs-based solar cells in dark and under AM 1.5 illumination and (b) EQE curves for devices A and B.

Figure 4. Impedance spectra for the two typical PbS CQDs-based solar cells measured at different conditions, (a) device B and (b) device A at different applied voltage in dark, (c) device B and (d) device A at different applied voltages under illumination.

Figure 5. The recombination resistances for the two PbS solar cells at various biases in dark (a) and under illumination (b), also the basic equivalent circuit of the PbS CQDs-based solar cells is shown in the inset of Fig.5 (a), and Rs is the series resistance.



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Figure 6. The capacitance for the two typical PbS CQDs-based solar cells at different frequency in dark (a) and under illumination (b), and the inset in Fig. 6(b) shows the capacitance for the two typical PbS CQDs-based solar cells at various biases in dark.

Figure 7. The carriers lifetime for the two typical PbS CQDs-based solar cells at various biases in dark (a) and under illumination (b).

Figure 8. The conductivities for the two typical PbS CQDs-based solar cells at various frequencies in dark and under illumination at 600 mV.


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Figure 1

(b)

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 600

700

800

900

Wavelength (nm)

(c)

20

30

1.2

40

50

60

2 theta(degree)

Fig. 1


70

80

1000

1100

Photoluminescence (a.u.)

Absorption (a.u.)

1.2

Normalized intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2

Fig. 2


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Page 23 of 29

Figure 3

2 J (mA/cm )

10

device B under illumination device B in dark device A under illumination device A in dark

0

-10

(a)

-20 0.0

0.2

0.4

0.6

Voltage (V)

75 device B device A

50 EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25

(b) 0 200

400

600

800

1000

Wavelength (nm)


1200


Fig. 3

Figure 4

1.0x10

7

8.0x10

6

6.0x10

6

4.0x10

6

2.0x10

6

-ImZ (Ω)

3x10

6

2x10

6

1x10

6

4.0x10

6

8.0x10

6

0mV 100mV 200mV 300mV 400mV 500mV 600mV 700mV

0

in dark

0.0 0.0

in dark

(b) 0mV 100mV 200mV 300mV 400mV 500mV 600mV 700mV

1.2x10

7

0.0

3.0x10

1200

(c)

6

6.0x10

6

ReZ (Ω)

ReZ (Ω)

500


under illumination

1000 800

(d)


under illumination

400

-ImZ (Ω)

-ImZ (Ω)

(a)

-ImZ (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600 400

300 200 100

200 0

0 0.0

8.0x10

2

1.6x10

3

2.4x10

3

0.0

3.0x10

2

ReZ (Ω)

6.0x10

2

ReZ (Ω)

Fig. 4


9.0x10

2

1.2x10

3

Page 25 of 29

Recombination resistance (Ω)

Figure 5

2.0x10

7

1.5x10

7

1.0x10

7

5.0x10

6

(a)

in dark RS

R rec

device B device A

C

0.0 0

100 200 300 400 500 600 700

Voltage (mV)

Recombination resistance (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

under illumination

2x10

3

1x10

3

device B device A

0 0

100 200 300 400 500 600 700

Voltage (mV)

Fig. 5



Capacitance (F)

Figure 6

5.0x10

-8

4.5x10

-8

4.0x10

-8

3.5x10

-8

3.0x10

-8

2.5x10

-8

2.0x10

-8

1.5x10

-8

(a)

10

0

-4

device B device A

in dark

1

10

(b)

1.0x10

2

3

10 10 10 Frequency (Hz)

4

5

10

-5

-8

Device B Device A

8.0x10

10

6

device B device A

under illumination

8.0x10

In dark

-8

7.0x10

-8

-5

6.0x10

6.0x10

C (F)

Capacitance (F)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-5

4.0x10

-8

5.0x10

-8

4.0x10

-8

3.0x10

-8

2.0x10

-5

2.0x10

-8

1.0x10

0

100

200

300

400

500

Voltage (mV)

0.0 10

0

10

1

10

2

10

3

10

4

10

5

Frequency (Hz)


10

6

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Fig. 6

Figure 7

(a)

-2

Lifetime (s)

9.0x10

in dark

device B device A

-2

6.0x10

-2

3.0x10

0.0 0

100 200 300 400 500 600 700 Voltage (mV)

1.6x10

-5

1.2x10

-5

8.0x10

-6

4.0x10

-6

(b) under illumination

Lifetime (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


device B device A

0.0 0

100 200 300 400 500 600 700 Voltage (mV)

Fig. 7



Figure 8

Conductivity (Ω-1cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.4x10

-3

1.2x10

-3

1.0x10

-3

8.0x10

-4

6.0x10

-4

4.0x10

-4

2.0x10

-4

device B in dark device B under illumination device A in dark device A under illumination

0.0 10

0

10

1

2

3

10 10 10 Frequency (Hz)

4

10

5

Fig. 8


10

6

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Table of Contents Graphic


Charge Carrier Conduction Mechanism in PbS Quantum Dot Solar

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