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Ternary Composites with PbS Quantum Dots For Hybrid Photovoltaics Aleksandr P. Litvin, Ivan D. Skurlov, Iurii G. Korzhenevskii, Aliaksei Dubavik, Sergei A Cherevkov, Anastasiia V. Sokolova, Peter S. Parfenov, Dmitry A. Onishchuk, Victor V. Zakharov, Elena V. Ushakova, Xiaoyu Zhang, Anatoly V. Fedorov, and Alexander V. Baranov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11685 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019

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Ternary Composites with PbS Quantum Dots for Hybrid Photovoltaics Aleksandr P. Litvin,a,∗ Ivan D. Skurlov,a Iurii G. Korzhenevskii,a Aliaksei Dubavik,a Sergei A. Cherevkov,a Anastasiia V. Sokolova,a Peter S. Parfenov,a Dmitry A. Onishchuk,a Victor V. Zakharov,a Elena V. Ushakova,a Xiaoyu Zhang,b Anatoly V. Fedorov,a Alexander V. Baranova

aDepartment of Optical Physics and Modern Natural Science, ITMO University, 49 Kronverksky Pr., St. Petersburg, 197101, Russia bCollege of Materials Science, Jilin University, Changchun 130012, China


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ABSTRACT: Ternary hybrid photovoltaics based on polymers, fullerenes, and colloidal semiconductor quantum dots (QDs) bring together high efficiency and technological flexibility. In order to increase the performance of such hybrid solar cells, thorough control over both film morphology and charge transport is required. In the present work, the morphology and optical properties of films of binary and ternary blends of PbS QDs with poly(3-hexylthiophene-2,5diyl) and [6,6]-phenyl C71 butyric acid methyl ester have been investigated by steady-state and time-resolved photoluminescence spectroscopy, atomic force microscopy, and confocal laser scanning microscopy. The influence of postdeposition and liquid-phase ligand exchange procedures on film properties have been considered. It is shown that liquid-phase iodide-passivation improves both film quality and charge transfer efficiency. The mixture of three components facilitates charge transfer in the hybrid material, but a thorough control over QD size and ligand type is required.


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INTRODUCTION

Hybrid photovoltaics based on conjugated polymers and fullerene derivatives combine technological flexibility and high performance. However, these materials usually suffer from low absorption efficiency. Colloidal semiconductor quantum dots (QDs) may be used instead of fullerene derivatives, acting both as light absorbers and electron acceptors.1,2 In this case, light absorption can be tuned over a wide spectral range and cover the nearinfrared (NIR) region. Semiconductor nanocrystals of different chemical composition and shape have been considered as a substitute for fullerenes.3–6 Nonetheless, devices based on QDs still suffer from low efficiency, which is mostly caused by inefficient charge transfer. To overcome this limitation, the concept of ternary composites7,8 has been further developed. Thus, CdSe QDs have been added to a polymer/fullerene blend9 to enhance light absorption and charge separation, and this has been followed by a growing interest in ternary systems with QDs.10–16 In order to capture the NIR part of solar spectrum, PbS QDs have been considered as an additional light-harvester.17,18


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In organic and hybrid solar cells, the morphology of thin layers and the quality of their interfaces must be controlled,6,19–22 including solvent control23,24 and post-annealing procedure.25 In order to create efficient hybrid solar cells based on semiconductor nanostructures, additional control should be achieved over the ligands used for nanocrystal passivation because both the morphology of thin films and their optical properties are dependent on ligand type and nanocrystal surface coverage.26,27 Assynthesized QDs are typically coated with long stabilizing organic ligands which ensure their solubility in organic solvents and prevent aggregation. However, the ligands also act as an insulating layer. PbS QDs are usually coated with oleic acid (OA) ligands. The length of the OA molecule (about 1.8 nm), which determines the minimal distance between close-packed QDs, makes the thickness of the ligand insulating layer excessive for efficient transfer of charge carriers between the QDs and other components in the active layer of a solar cell.28 In order to improve charge transfer in polymer/QD blends, a number of procedures for removing/replacing ligands from the surface of the QDs have been developed. Ligand replacement can be carried out in a liquid phase (solution-phase ligand exchange) using,


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for example, lead iodide (PbI2)29 or methylammonium iodide30 for iodide passivation, or after the deposition of a thin hybrid film (post-deposition ligand exchange), using acetic acid (AA), 1,4‐benzenedithiol (BDT), 1,2‐ethanedithiol (EDT), 1,3‐mercaptopropionic acid (MPA), methylammonium (MA) or tetrabutylammonium iodide (TBAI).31–34 An alternative approach is the direct synthesis of QDs in the medium of a conjugated polymer.35,36 The most widely used procedure is the replacement of the ligands after the creation of an active layer, but a serious disadvantage of this post-deposition ligand exchange is the possible alteration of the morphology of the active layer. To overcome this problem, several liquid-phase ligand exchange procedures have been developed in recent years.37 For further development of hybrid photovoltaics, thorough control over the morphology of the active layer and its optical properties after the ligand exchange procedures must be performed. In the present work, we have studied active layers for hybrid solar cells based on PbS QDs. These QDs have a broad absorption band from the UV to the NIR spectral regions and have become an indispensable light absorber in hybrid and heterojunction solar cells.38–40 We have studied the influence of post-deposition and liquid-phase ligand


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exchange on the morphology and optical properties of hybrid polymer/PbS QD thin films. We have introduced a fullerene derivative into the poly(3-hexylthiophene-2,5-diyl) (P3HT) polymer/PbS QD blend as the third component to improve charge separation and transfer. We show that the design of ternary blends for hybrid photovoltaics should take ligand exchange procedures into consideration. We demonstrate that a proper ligand exchange approach allows to improve the quality of the thin films, the compatibility of their components, and charge transfer efficiency.

MATERIALS AND EXPERIMENTS

P3HT (poly(3-hexylthiophene), Mw 54,000-75,000, 99,995%), PCBM ([6,6]-Phenyl C71 butyric acid methyl ester, 99%), PMMA (poly(methyl methacrylate), Mw 120,000), PEDOT:PSS

(Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate),

3.0-4.0%

aqueous solution), ZnO (Zinc oxide ink), MPA (3-mercaptopropionic acid, ≥ 99.0%), AA (acetic acid, ≥ 99.5%), PbI2 (99.999%), PbO (>99.9%), oleic acid (90%), 1-octadecene (90%), and (TMS)2S (hexamethyldisilthiane) were purchased from Sigma-Aldrich and


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used as received. OA-capped PbS QDs with mean diameters of 3.9±0.3 nm and 2.8±0.3 nm were synthesized from the PbO and (TMS)2S precursors by the hot-injection method as described elsewhere.41,42 P3HT and PMMA solutions (12 mg/ml) were prepared by stirring the polymers for 15 hours at 70 °C in 1,2-dichlorobenzene under argon atmosphere. Solutions of PbS QDs and PCBM (12 mg/ml) were prepared in 1,2-dichlorobenzene. To obtain iodide-capped (I-capped) PbS QDs, solution-phase ligand exchange was performed with PbI2 in a methanol/ dimethylformamide (DMF) mixture as reported by Lu et al.29 After the treatment, the QDs can be dispersed in non-polar solvents with the addition of butylamine. Before blending with the polymers, the QD and PCBM solutions were ultrasonicated for 30 minutes. The blends were prepared by stirring the component solutions for 3 hours at 1500 rpm. All the blends were prepared with a 1/1 or 1/1/1 weight ratio. Glass substrates were cleaned with acetone and isopropanol under ultrasonication, and then treated by plasma cleaning. Pure and mixed solutions were spin-coated at 1000 rpm. For post-deposition ligand exchange of OA by MPA ligands in the OA-capped PbS QDs, the film was immersed into a 0.1 M solution of MPA in acetonitrile for 30 seconds, after


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which the film was rinsed with pure acetonitrile while rotating at 2000 rpm. This cycle was repeated three times. For the ligand exchange of OA by AA ligands, the film was immersed into a 0.1 M solution of AA in acetonitrile for 10 minutes under ultrasonication. After that, the film was rinsed with pure acetonitrile several times. Steady-state and time-resolved photoluminescence (PL) measurements were performed using purpose-built setups.43,44 The NIR PL spectra were obtained by excitation with a LED at 530 nm and power 5 mW. The collected radiation was passed through an Acton-2500 monochromator and focused on an InGaAs/InP avalanche photodiode (Micro Photon Devices) connected to a photon counting board. The VIS PL spectra were taken under 532 nm excitation using an Andor iDus 401A CCD camera. NIR PL decay curves were obtained using excitation by laser radiation with a wavelength of 532 nm, average power of 1 mW, and a pulse repetition frequency of 5 kHz. The excitation radiation was blocked by a monochromator and two bandpass NIR filters. Atomic-force microscopy (AFM) measurements were performed using a Solver Pro-M (NT-MDT) microscope in semi-contact mode. Confocal laser scanning microscopy


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(CLSM) was used to obtain PL and reflection images using 405 nm laser radiation in back-scattering mode with a Carl Zeiss LSM 710 confocal laser scanning microscope. Ultraviolet photoelectron spectroscopy (UPS) spectra were collected using a PREVAC system. For the UPS measurements, a ~20 nm thick QD film was spin-coated onto a 10×10 mm2 ITO-covered glass substrate. The UPS spectra allowed the determination of the valence band maximum of PbS QDs. To determine the conduction band minimum, the QDs bandgap from absorption measurement and the Coulomb stabilization energy were taken into account.45 For solar cells fabrication, the pre-patterned ITO substrates were ultrasonically cleaned with different solvents and then treated with plasma. ZnO layer with a thickness of 30±5 nm was deposited by spin-coating, then annealed at 120 °C and treated by UV light. An active layer of the reference sample composed of 2.8 nm PbS QDs and P3HT at 9/1 weight ratio. For the ternary sample, 4 wt.% of PCBM was added into the QD/polymer blend. After stirring, the blends were deposited by spin-coating at 1000 rpm for 60 s. 18±3 nm thick PEDOT:PSS layer was then spin-coated, followed by 100 nm thick layer of Au anode deposited by magnetron sputtering through a shadow mask. The devices


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composed of 6 pixels of 3.0 mm2. The fabricated devices were transferred to Ar atmosphere and annealed at 120 °C for 20 minutes. J-V measurements were carried out using Solar Cell I-V Test System provided by Ossila. The 100 W halogen lamp was used to obtain I-V curves under light illumination. To obtain a reasonable match with a solar simulator, a set of optical glass filters was used to correct the lamp spectrum. The output focused beam was adjusted to provide irradiation of 100 mW/cm2.

RESULTS AND DISCUSSION

Binary Blends.

Post-Deposition Ligand Exchange. Since charge transfer from the QDs to P3HT and exciton trapping after ligand exchange modify the optical properties of the films, we prepared initially films of PbS QDs blended with an insulating polymer (PMMA) to investigate the effect of the ligand exchange procedure on the properties of the films without the interference of those processes. The OA native ligands from PbS QDs of average diameter 3.9 nm were exchanged by MPA and AA after deposition of the


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PMMA/OA-capped PbS thin films. The morphology of a typical sample of PMMA/OAcapped PbS film (PMMA/PbS-OA) is shown in Figure 1a. The AFM image shows a smooth surface and uniform relief. After the treatment with AA the film (PMMA/PbS-AA) exhibits an island-like structure (Figure 1b), while after the MPA treatment the film (PMMA/PbS-MPA) consists of small aggregates in addition to larger islands (Figure 1c).

Figure 1. AFM images of a PMMA/PbS-OA film (a) and the films after post-deposition ligand exchange with AA (b) and with MPA (c).

Figure 2 illustrates how the treatments affect the optical properties of the QDs in the films. In can be seen that, after the ligand exchange, the PL intensity decreases and the average PL lifetime increases. An increase in the decay time can be the result of damage


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of the QD surface due to the treatment, which causes more efficient recombination involving carrier traps, which is characterized by longer decay times.46

Figure 2. PL decay of a PMMA/PbS-OA film (black line) and PMMA/PbS-AA and PMMA/PbS-MPA films after post-deposition ligand exchange with AA (blue line) and with MPA (red line).

Liquid-Phase Ligand Exchange. To perform liquid-phase ligand exchange, the assynthesized OA-capped PbS QDs were treated with PbI2 in a methanol/DMF mixture.29 After the treatment, the absorption and PL spectra show a synchronic blue shift, indicating a change of the local environment around the QDs, as shown in Figure 3.


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The OA-capped and I-capped PbS QDs were blended with P3HT. The morphology of the hybrid films was investigated by CLSM and AFM. Typical images of the surface of the P3HT, P3HT/PbS-OA, and P3HT/PbS-I films are shown in Figure 4.

Figure 3. Absorption (solid lines) and PL (dotted lines) spectra of solutions of OA-capped PbS QDs (red) and I-capped PbS QDs.

P3HT forms a characteristic domain structure.47 The addition of OA-capped PbS QDs leads to the formation of aggregates that can be clearly seen in the AFM image. The formation of a large number of aggregates induces segregation of polymer and QD phase,


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which worsens the efficiency of a solar cell.48 In contrast, the addition of I-capped PbS QDs seems to produce a smooth surface. The AFM image shows that P3HT/PbS-I films form large domains with a much smaller number of aggregates. Such a balance between aggregation and phase-separated polymer and QDs domains is expected to be beneficial for a solar cell.49


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Figure 4. Confocal reflection (left) and luminescence (center) images and AFM (right) images of the films of P3HT, P3HT/PbS-OA, and P3HT/PbS-I.


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Figure 5 shows PL decays of PMMA/PbS-OA, P3HT/PbS-OA, PMMA/PbS-I, and P3HT/PbS-I blends and the corresponding average PL lifetimes calculated from fitting the decays to two exponentials. Average PL lifetimes for OA-capped PbS QDs are quite close for the cases of PMMA and P3HT, indicating inefficient charge transfer between species, and the calculated hole transfer rate (Table 1) is in agreement with literature data.17 On the contrary, a strong decrease of the PL lifetime is observed for the I-capped QDs. The decrease is caused by effective hole transfer from the QDs to the P3HT polymer, and the rate, ktransfer, and efficiency, Etransfer, of this process can be estimated by

―1 ―1 𝑘𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 = 𝜏𝑏𝑙𝑒𝑛𝑑 ― 𝜏𝑟𝑒𝑓

𝐸𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 = 1 ―

𝜏𝑏𝑙𝑒𝑛𝑑 𝜏𝑟𝑒𝑓

,

(1)

(2)

where 𝜏𝑟𝑒𝑓 and 𝜏𝑏𝑙𝑒𝑛𝑑 are the average PL lifetime for the QDs blended with PMMA and P3HT, respectively. Hole transfer rates and efficiencies for these samples are shown in


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Table 1 together with the values obtained for an additional sample prepared by postdeposition ligand exchange of P3HT/PbS-OA with AA. We therefore conclude that solution-phase ligand exchange using PbI2 yields a comparable hole transfer efficiency, but with the advantage of providing a more desirable surface morphology for the active layer of a hybrid solar cell, as mentioned previously.

Figure 5. PL decay of I-capped (a) and OA-capped (b) PbS QDs blended with PMMA and P3HT. The corresponding intensity-weighted average PL lifetimes calculated from the fit to two exponentials are indicated.

Table 1. Calculated Average PL Lifetimes and Rates and Efficiencies of Hole Transfer for the Films Investigated


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Sample

PMMA/PbS-I

P3HT/PbS-

PMMA/PbS-OA

P3HT/PbS-OA

P3HT/PbS-AA

108 ± 1

86 ± 1

2.5 ± 0.5

1.7×108

2.4×106

3.9×108

94 %

20%

98%

I Average

PL

90 ± 1

5.5 ± 0.3

lifetime (ns) Hole transfer rate (s-1) Hole transfer efficiency

Ternary Blends. The addition of carbon allotropes into the active layer of a hybrid solar cell facilitates electron extraction and transfer. C60 and C70 fullerenes, with their high electron affinity, are very promising candidates as a third component for hybrid solar cells with QDs.50–53

In order to investigate the efficiency of such ternary blends for

photovoltaics, we have performed steady-state and transient PL measurements of binary and ternary blends of P3HT, I-capped PbS QDs, and PCBM. Figure 6 shows the PL spectra obtained for I-capped PbS QDs and their blends with P3HT and PCBM. Effective quenching is observed for the P3HT/PbS-I blend. However, an increase of the NIR PL is observed for 3.9 nm PbS QDs mixed with PCBM. The PL enhancement is accompanied by a slight increase of PL lifetime shown in the inset of


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Figure 6. In the ternary blend P3HT/PCBM/PbS-I the quenching of the PbS QDs PL is also not as efficient as in the binary blend P3HT/PbS-I. The increase of PL lifetime and intensity in the PCBM/PbS-I blend can be induced by non-radiative energy transfer,54 since the PL band of PCBM lies in the absorption range of PbS QDs. Non-radiative energy transfer becomes more efficient than the desired electron transfer from QDs to PCBM.

Figure 6. PL spectra of 3.9 nm I-capped PbS QDs (black circles) and blends P3HT/PbSI (red circles) and PCBM/PbS-I (blue circles). The inset shows the PL decay for I-capped PbS QDs (black circles) and their blend with PCBM (blue circles).

In order to understand the mechanisms of interaction between PbS and PCBM, the HOMO and LUMO levels must be considered. UPS and absorption measurements allow


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to determine precisely the energy position of HOMO and LUMO in PbS QDs.45 The energy position of HOMO and LUMO levels for P3HT, PCBM and I-capped PbS QDs of two different sizes are shown in Figure 7a. A very small difference between the LUMO in 3.9 nm PbS QDs and PCBM may prevent effective charge separation and transfer at their interface, that leads to ineffective electron transfer and effective non-radiative energy transfer. Both size and ligand type affect the HOMO and LUMO energetic position in QDs. It can be seen that the use of smaller dots is preferable for a mixture of PCBM, P3HT and I-capped PbS QDs. To check this hypothesis, binary and ternary blends with 2.8 nm QDs were prepared. For all the blends we observed quenching of PL intensity as compared to the samples of the unblended components (I-capped PbS QDs, P3HT, and PCBM).


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Figure 7. Energy position of the HOMO and LUMO levels for P3HT, PCBM and I-capped PbS QDs of two different sizes (a) and NIR PL decays of 2.8 nm I-capped PbS QDs and their blends PCBM/PbS-I and P3HT/PCBM/PbS-I blends (b).

PL decays of 2.8 nm PbS-I QDs and their blends PCBM/PbS-I and P3HT/PCBM/PbS-I are shown in Figure 7b. The decrease of PL lifetime indicates an efficient charge transfer from the QDs. This confirms that the QDs size and ligand type must be accurately tuned to obtain a highly efficient hybrid material. A blend of 2.8 nm QDs with P3HT and PCBM also induce quenching of the VIS PL from both P3HT and PCBM, as shown in Figure 8. The highest quenching of both NIR and VIS PL is achieved in the ternary blend.


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Figure 8. VIS PL spectra of P3HT (a) and PCBM (b), their binary blends with 2.8 nm Icapped PbS QDs and the ternary blend P3HT/PCBM/PbS-I.

To prove the impact of a third component on photovoltaic devices performance, we fabricated ITO/ZnO/Active/PEDOT:PSS/Au solar cells. For the reference sample, the active layer composed of a binary 2.8 nm PbS QDs / P3HT blend with a 9/1 weight ratio. The J-V curve obtained for the sample under light illumination is shown in Figure 9 by black line. After addition of 4 wt.% of PCBM, the devices demonstrated an increase of both Voc and Jsc (Figure 9, red line). The enhanced performance of the active layer is attributed to the improved charge separation and transfer, that is in line with the spectroscopic data. Optimization of device fabrication, together with optical and morphological data, will bring an impact on the development of the ternary hybrid photovoltaic device.


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Figure 9. J-V curves for the ITO/ZnO/Active/PEDOT:PSS/Au solar cells with P3HT-PbS (black line) and P3HT-PbS-PCBM (red line) active layers. The inset demonstrates the architecture of the devices.

CONCLUSIONS

In summary, we have investigated the morphology and optical properties of films of binary and ternary blends of PbS QDs with P3HT and PCBM. We show that liquid-phase iodide passivation of the QD surface leads to a charge-transfer efficiency comparable to that obtained for a post-deposition ligand-exchanged sample. However, the film quality is


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drastically enhanced by liquid-phase ligand exchange. The addition of fullerene derivatives promotes more effective charge separation and transfer in polymer/QD hybrids, but a thorough control over QD size and ligand type is required. We show that for the larger I-capped QDs, non-radiative energy transfer from PCBM to the QDs will prevail over electron transfer and this would worsen the efficiency of devices. Reducing the QD size allows to tune the HOMO and LUMO energy positions in order to achieve optimal charge separation and transfer, which is of great importance to the development of hybrid optoelectronic devices operating in the NIR spectral region. We demonstrate that addition of the third component to an active layer leads to the enhanced performance of the hybrid solar cells.

AUTHOR INFORMATION

Corresponding Author


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* A.P.L. [email protected], [email protected], ITMO University, 49 Kronverksky Pr., St. Petersburg, 197101, Russia

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

ACKNOWLEDGMENT

The authors thank the Ministry of Education and Science of the Russian Federation (Goszadanie no. 16.8981.2017/8.9) for financial support. A.P.L. thanks the Ministry of Education of the Russian Federation for financial support (Scholarship of the President of the Russian Federation for young scientists and graduate students, SP-70.2018.1).


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