Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar

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Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar Cell Performance Mohamed Abdellah, Rebecca Marschan, Karel Zidek, Maria E Messing, Abdallah Abdelwahab, Pavel Chabera, Kaibo Zheng, and Tonu Pullerits J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5086284 • Publication Date (Web): 13 Oct 2014 Downloaded from http://pubs.acs.org on October 19, 2014

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The Journal of Physical Chemistry

Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar Cell Performance Mohamed Abdellah1,2, Rebecca Marschan3, Karel Žídek1, Maria E. Messing4 Abdallah Abdelwahab1, Pavel Chábera1, Kaibo Zheng1*,and Tõnu Pullerits1* 1

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Department of Chemical Physics, Lund University, Lund, Sweden

Department of Chemistry, Qena Faculty of Science, South Valley University, Qena 83523, Egypt 3

Department of Physical Chemistry, Christian Albrechts University Kiel, Kiel, Germany 4

Department of Solid State Physics, Lund University, Lund, Sweden

ACS Paragon Plus Environment


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Abstract

The performance of the current quantum dot (QD) solar cells is limited by several deficiencies. One of them is the existence of surface traps, especially hole traps, which are blocking the hole injection into electrolyte. The trapping can be efficiently suppressed by growing a shell of wider band-gap material around the core dot. Optimum parameters of such shell layer for photovoltaic applications are, however, not established. We study effects of the shell formation on the ultrafast carrier dynamics and the performance of QD-sensitized solar cells. We can disentangle electron and hole dynamics and demonstrate that the QD shell diminishes surface hole trapping. By combining the knowledge about the hole trapping and electron injection into metal oxide we can clearly correlate the electron and hole dynamics with the solar cell efficiency as a function of the shell thickness. We conclude that the optimal shell thickness is 1.3 nm for this system.

Keywords: Core–shell CdSe–ZnS quantum dots, hole traps, PL time-resolved spectroscopy, electron collection efficiency, quantum yield, and transient absorption.


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Introduction Obtaining an optimum between solar cell efficiency and cost has been the main motivation for the research and development of third generation solar cells.1 Among them, quantum dot (QD)based solar cells have been one of the most promising candidates actively studied over the last decade.2,3 QDs have simple synthesis procedure, high light harvesting efficiency, easy tunability of absorption range, and prospects for breaking the Shockley-Queisser thermodynamic limit by using multiple exciton generation.3-8 Still, achieving the efficiencies of other similar novel photovoltaic devices, like dye sensitized solar cells,9,10 has been difficult. As a result of numerous studies, we understand now many important limiting factors in QD solar cells like hole transfer2 and surface trapping11 Surface defects lead to carrier trapping and can contribute to photodegradation.12 Progress in atomic-ligand passivation of QD surfaces has given promising results13 and the recent band alignment engineering by different ligand treatments has achieved a certified efficiency of 8.55%.8 Surface passivation can be also accomplished by growing shell layers of wider band gap semiconductors around the core, i.e by using core–shell QDs (CSQDs).14-16 Two strategies exist for growing a shell: step-like CSQDs and gradient CSQDs.17,18 Gradient CSQDs with smooth composition change from the core to the shell material, in general show superior quality: they contain a minimum number of interfacial defects, feature enhanced fluorescence quantum yield and, at the same time, the Auger recombination rates in such QDs are reduced.19,20 Previous studies have shown that the performance of QD-based solar cells does improve by using shellpassivated QDs as building blocks.21,22 However, the detailed knowledge about the influence of the shell on the charge carrier dynamics (electrons and holes) and how the dynamics affects the solar cell performance are yet to be revealed.2


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In our previous work,14 we have reported on an optimum shell thickness in the CdxSeyZn1-xS1-y gradient core–shell QDs, where both high degree of QD passivation and efficient electron injection from QDs to metal oxide acceptor can be achieved. In this article we study the effect of the shell thickness on the hole trapping dynamics in the system. By using time-resolved photoluminescence (PL) measurements we observe clear correlation between shell thickness and the hole trapping process. We also demonstrate that the dynamics is one of the factors, which determines the real solar cell performance. Combining the knowledge about electron injection14 and hole trapping, we can explain the dependence of the solar cell performance on QD shell thickness. This provides an opportunity to optimize the core–shell QD materials for photovoltaics prior to making real devices.

Experimental CdxSeyZn1-xS1-y gradient CSQDs were prepared by following a method described by Bae et. al.18 and reproduced in our recent work.6,14 Briefly, we use a single-step hot injection method. Both Cd2+ and Zn2+ oleate in 1-octadecene solution were heated to 325 °C. Then Se2- and S2- in 3 ml of trioctylphosphine (TOP) solution were swiftly injected into the cation precursor solution. To obtain gradient CSQDs with different shell thicknesses the growth process was terminated by cooling the solution down using ice bath after 5 sec, 1, 3, 5, and 10 min. For sensitization, the surface capping agent of purified QDs was exchanged from oleic acid (OA) to a bifunctional linker molecule, mercaptopropionic acid (MPA) and re-dissolved in ethanol which is used as a solution to sensitize the photoanode.14,23. To fabricate the photoanode for solar cells, TiO2 paste was coated on the surface of fluorine-doped SnO2 (FTO) by doctor-blading technique. After annealing at 450 °C for one hour, TiO2 is suitable for QD loading. The TiO2 – FTO film was


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then sensitized with QDs. The sensitization time was varied for different QDs to keep the same optical density. Finally, freshly prepared photoanode was scratched to area ~ 0.25 cm2. In order to fabricate the solar cell, 1 M Na2S/1M S solution used as an electrolyte and FTO glass coated with 200 Å Platinum was used as a cathode.24 Size of the QDs was analysed by the highresolution transmission electron microscopy (HR-TEM) images obtained by JEOL3000F microscope equipped with an Oxford SDD X-ray analyzer. The excitation for the time-resolved PL setup is drawn from a Titanium:Sapphire passively mode-locked femtosecond laser (Spectra-Physics, Tsunami), emitting at 820 nm with the 80 MHz repetition rate and 150 fs pulse length. The laser pulses were frequency-doubled to 410 nm by a harmonic generator (Photop technologies, Tripler TP-2000B). The excitation photon flux was 1.1×1012 photons/cm2/pulse. Time-resolved PL spectra were detected by a streak camera (C6860, Hamamatsu) coupled to a Chromex spectrograph, triggered by Ti:Sapphire laser. A long-pass wavelength filter from 490 nm was used in front of the spectrograph to cut off the scattering from the excitation pulses. The current – voltage (I-V) characteristics curves for the solar cell was measured using Keithley 2400 source meter under a simulated AM1.0G solar irradiation with a light intensity of 100 mW/cm2. Transient absorption (TA) spectra were recorded using a pump-probe setup described in our previous study.25 Briefly, laser pulses (800 nm, 80 fs pulse length, 1 kHz repetition rate) were generated by a regenerative amplifier (Spitfire XP) seeded by a femtosecond oscillator (Tsunami, both Spectra Physics). Excitation pulses at the wavelength of 450 nm were acquired using an optical parametric amplifier (Topas C, Light Conversion). The used excitation photon flux was 2×1014 photons/cm2/pulse. The probe pulses (a broad supercontinuum spectrum) were generated from the 800-nm pulses in a sapphire plate and split by a beam splitter into a probe pulse and a


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reference pulse. The probe pulse and the reference pulse were dispersed in a spectrograph and detected by a diode array (Pascher Instruments). Thin film samples were measured in a nitrogen atmosphere to avoid possible oxidation of QDs.12

Results and discussion In our previous work,14 we studied the effect of the shell thickness on the electron injection efficiency from the CSQDs to the metal oxide (MO), namely ZnO. Here we investigate the effect of hole dynamics on parameters of the real solar cells for the same CSQDs attached to TiO2. Unlike conventional step-like core–shell structure, the one-pot hot injection method in our work would produce alloy QDs with combination of all Cd, Zn, Se, and S elements. However, due to different reactivity of the precursors, composition gradient would form in the radial direction during the growth of QDs, making a Cd-rich core and Zn-rich shell structure (see EDX analysis in SI). In addition, the gradual growth of the material in radial direction is epitaxial where no obvious distortion or crash of the lattice can be found in HR TEM images. (See supporting information). We call the QDs extracted immediately after injection as ‘core’ and we use them as a reference. The core is compared to five different shell thicknesses ~ “0”, 0.6 1.3, 1.6, and 1.9 nm obtained by growth time of 5 sec, 1 min, 3 min, 5 min, and 10 min (see supporting information for TEM images and the shell thickness determination). For the sample of 5 sec growth time the shell is negligible and we cannot provide an exact value for the thickness. Therefore we call it “0” nm shell as it has intermediate properties between the pure core and the sample with 0.6 nm shell. First the steady-state spectroscopy was used to characterize the pure core and the gradient CSQDs. Figure 1A shows the absorption and the emission spectra of purified oleic acid capped


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QDs (OA-QDs). The mean diameter of pure core of 3.0 nm was determined by using transmission electron microscope (TEM) images (see supporting information). The measured diameter is in good agreement with the QD size calculated from the position of the lowest absorption band.26 The lowest band of the CSQDs is shifted towards red compared to the pure core QDs due to leakage of the electron and hole wave functions into the shell layers.8 The corresponding emission spectra consist of a single relatively narrow emission band with FWHM from 25 nm to 50 nm on the low energy side of the absorption. The size distribution of all CSQDs was less than 15 % as evaluated by TEM images (see the supporting information). In order to attach the QDs to the MO acceptor, OA capping agent is replaced by MPA. MPA linker has high chemical stability and its short chain length enables high electron transfer rate form QDs to MOs.27 In our earlier work14 we used ZnO NPs. TiO2 shows higher chemical stability than ZnO against high pH solution (resulting from linker exchange process)28 therefore in the current work TiO2 NPs are used as an acceptor. The absorption features of the QDs are unchanged by the attachment to TiO2 (Figure 1B). Only a blue shift of about ~ 7 nm appears due to the different permittivity of surrounding environment and because of QD-TiO2 coupling.29


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Figure1 A) absorption and PL of OA-core and OA-CSQDs with different shell thicknesses; B) absorption of neat TiO2 NPs, neat CdSe QDs and CdSe QDs attached to TiO2; C) trapped hole % (blue line), and PL QY before and after linker exchange as a function of shell thickness; D) the energy alignment of the MO/QD/electrolyte system.30 The linker exchange is necessary for QD-TiO2 attachment. However, it is known that the exchange introduces numerous surface defects acting as trapping centers.31 In the pure core QD system we found a significant drop in the PL quantum yield (PL QY) from 14 to 0.5 %. The trapping induced by linker exchange completely changes the recombination pathways. In contrast, for QDs protected by a shell layer of 1.3 nm, the PL QY drops from 50 % to 35 % after the linker exchange. (See Figure 1C). Clearly, the linker-induced trapping can be efficiently passivated by the shell. In particular, we have observed that the ~ 1.3 nm shell thickness leads to the highest PL QY both before and after the linker exchange process. The error bars in Figure 1C


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represent the spread of data over three different batches of samples showing good reproducibility of the results. The solar cell efficiency depends both on behaviour of photoexcited electrons and holes (see Figure 1D).2 We have shown in our previous work14 that the injection of electrons is very efficient in CSQDs up to the shell thickness of 1.3 nm. Here we focus on the competition between hole scavenging by electrolyte and the hole trapping on the QD surface, which is expected to highly affect the resulting solar cell performance. In order to investigate the hole trapping, we need to combine information from TA and timeresolve PL spectroscopy. The TA technique in visible spectral range is inconclusive about electrons and holes simultaneously since the dominating bleach signal is due to the electron population.32,33 On the other hand, PL intensity depends on population of both electrons and holes. Complementarity of the time-resolved PL and visible TA provides us the full picture for electron and hole dynamics in our system. Figure 2A demonstrates sensitivity of both TA and PL on the charge carriers. TA decay remains slow, regardless, whether the QDs are capped with OA or MPA. The PL of those samples has a totally different kinetics with significantly faster decay. For the details of multiexponential fitting see supporting information. Combination of TA and PL enables us to distinguish electron trapping from hole trapping. The former is negligible in both OA- and MPAcapped QDs, whereas the latter is highly pronounced for the MPA-capped QDs.34,35


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Figure 2 A)TA kinetics at the band-edge absorption and PL decay of MPA-CdSe and OA-CdSe QD films with multi-exponential fitting (for the TA kinetics on longer time scale see supporting information); B) PL decay and multi-exponential fitting of MPA-CSQDs with different shell thickness compared to MPA-CdSe QDs up to 1.5 ns; and C) PL decay of the early time for the pure core and different CSQDs. As the TA kinetics is not sensitive to the hole trapping, we will mainly discuss in the following sections the time-resolved PL data for our system, i.e. hole dynamics. Figure 2B represents the PL kinetics of MPA-capped QDs with different shell thicknesses in solution (before attachment to metal oxide). Multi-exponential function was used to fit the PL decay kinetics for the pure core and the CSQDs. Table 1 summarizes the fitted kinetic parameters. For the QDs with thin shell thickness ( 80%) cannot be collected due to the hole trapping. In these conditions, the trapped holes are not able to be injected to the electrolyte probably due to insufficient driving force.35 Passivation of the hole traps using core–shell structures provides an efficient way to enhance the hole collection as shown in Figure 3C. However, shells can also serve as energy barrier preventing the electron/hole transfer from the core. Fortunately, CSQDs with shell thickness up to 1.3 nm can still reach electron injection efficiency of 80%. This provides us a possibility to optimize the core–shell structures by simultaneous efficient surface passivation and charge transfer for higher performance device. The results are summarized in the Scheme A. In our case the optimum shell


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thickness of 1.3 nm with the best solar cell efficiency can be understood as a compromise providing efficient electron injection (~80%), while the hole trapping is practically eliminated.

Scheme A. Electron and hole pathways in case of different shell thicknesses. The vertical axis represents the relative efficiency of the cells. At 1.3 nm shell thickness the solar cell efficiency is the highest as a result of the surface passivation.

Conclusions We have studied effect of the shell thickness on performance of the core shell QD sensitized solar cells. From the study of hole dynamics in the system we were able to demonstrate, that formation of a shell layer can inhibit hole trapping process. In combination with knowledge about electron dynamics in the system presented in our previous work, we can clearly correlate the performance of the core–shell QD-based solar cells with the electron and hole dynamics in the system. Finding an optimum shell thickness (1.3 nm in our case) increases efficiency of the solar cells by 2.5-times compared to the core-only case. It shows that a careful control of the QD surface passivation can significantly enhance the efficiencies of QD-based solar cells. Our work


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correlates the carrier dynamics and solar cell efficiency enabling to understand the atomistic origin of the optimal core–shell QD properties for efficient photovoltaics.

Corresponding Author *E-mail: [email protected]. Phone: +46 46 222 81 31. Fax: +46 46 222 41 19. *E-mail: [email protected]. Phone: +46 46 222 81 31. Fax: +46 46 222 41 19.

ACKNOWLEDGMENT We gratefully acknowledge financial support of the Knut and Alice Wallenberg Foundation, the Swedish Energy Agency, the Swedish Research Council, and Erasmus Mundus program. Collaboration within nmC@LU is acknowledged.

Supporting Information Details of QDs structure characterization, shell thickness estimation, repetition rate effect, harvested energy, electron collection efficiency calculations, and Solar cells’ efficiency and its error estimation, and Multiexpontial fitting of TA and TRPL kinetics of Pristine CdSe QDs with different capping agent. This material is available free of charge via the Internet at http://pubs.acs.org.


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35. Zheng, K.; Žídek, K.; Abdellah, M.; Zhang, W.; Chábera, P.; Lenngren, N.; Yartsev, A.; Pullerits, T. Ultrafast Charge Transfer from CdSe Quantum Dots to p-Type NiO: Hole Injection vs Hole Trapping. J. Phys. Chem. C 2014, 118, 18462–18471. 36. Saba, M.; Aresti, M.; Quochi, F.; Marceddu, M.; Loi, M. A.; Huang, J.; Talapin, D. V.; Mura, A.; Bongiovanni, G. Light-Induced Charged and Trap States in Colloidal Nanocrystals Detected by Variable Pulse Rate Photoluminescence Spectroscopy. ACS Nano 2012, 7, 229– 238. 37. Abdellah, M.; Karki, K. J.; Lenngren, N. Zheng, K.; Pascher, T.; Yartsev, A.; Pullerits, T. Ultra Long-Lived Radiative Trap States in CdSe Quantum Dots. J. Phys. Chem. C 2014, 118, 21682–21686.


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Table of Contents


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140x129mm (300 x 300 DPI)


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103x87mm (300 x 300 DPI)



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

117x76mm (300 x 300 DPI)


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328x165mm (96 x 96 DPI)



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

328x165mm (96 x 96 DPI)


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Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar

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