Article pubs.acs.org/JPCC
Improving Performance in Colloidal Quantum Dot Solar Cells by Tuning Band Alignment through Surface Dipole Moments Pralay K. Santra,† Axel F. Palmstrom,† Jukka T. Tanskanen,†,§ Nuoya Yang,‡ and Stacey F. Bent*,† †
Department of Chemical Engineering and ‡Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: Colloidal quantum dots (CQDs) have received recent attention for low cost, solution processable, high efficiency solid-state photovoltaic devices due to the possibility of tailoring their optoelectronic properties by tuning size, composition, and surface chemistry. However, the device performance is limited by the diffusion length of charge carriers due to recombination. In this work, we show that band engineering of PbS QDs is achievable by changing the dipole moment of the passivating ligand molecules surrounding the QD. The valence band maximum and conduction band minimum of PbS QDs passivated with three different thiophenol ligands (4-nitrothiophenol, 4-fluorothiophenol, and 4-methylthiophenol) are determined by UV−visible absorption spectroscopy and photoelectron spectroscopy in air (PESA), and the experimental results are compared with DFT calculations. These band-engineered QDs have been used to fabricate heterojunction solar cells in both unidirectional and bidirectional configurations. The results show that proper band alignment can improve the directionality of charge carrier collection to benefit the photovoltaic performance.
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INTRODUCTION Solid-state colloidal quantum dot (CQD) solar cells have gained much attention recently as they show promise toward next generation photovoltaic devices.1−15 These CQDs are solution processable7,16 which makes them ideal candidates for large area, low-cost, high efficiency photovoltaic devices. Variation of the size17 or composition18,19 of quantum dots (QDs) allows for control over the band gap, thus the absorption range of the material. QDs based on lead chalcogenides have been used extensively in previous studies 1−12,20−28 because they possess high absorption coefficients and can be easily tuned to absorb in the infrared region of the solar spectrum.29 A power conversion efficiency greater than 8.5% has been achieved from PbS CQD solar cells.7 The basic structure of a CQD solar cell is similar to that of other established thin-film photovoltaic devices, e.g., CdTe,30 CIGS,31 and CZTS32 solar cells. In brief, CQD solar cells consist of a p−n junction in which a high band gap, transparent metal oxide such as TiO2 or ZnO is generally used as the n-type material and the QDs serve as the p-type material. Unlike in QD sensitized solar cells, QDs are deposited as a multilayer film (approximately 200−300 nm thick) on top of the metal oxide in CQD solar cells. Due to the nature of the p−n junction, the QD layer near the interface is depleted.6 Upon illumination with photons of sufficient energy, electron−hole pairs (excitons) form within the QD layer. These excitons must be split and the electrons and holes transported to the metal oxide layer and counter electrode, respectively, in order to achieve a photocurrent. © 2015 American Chemical Society
Transport and hence harvesting of charge carriers in CQD solar cells depends on the electric field of the depletion region. Earlier reports6,33 have shown that the solid CQD film remains completely depleted under short-circuit conditions, as shown schematically in Figure 1a. The operating point of interest of a solar cell is near the maximum power point condition (MPP), which can be achieved at forward bias, i.e., with a certain load applied in the circuit. The load decreases the width of the depletion region (WD) as shown in Figure 1b. With a decrease in the depletion width, charge carrier collection outside the depletion region will rely only on diffusion. Due to low mobilities (10−3−10−2 cm2 V−1 s−1) of the charge carriers,26,34 they are not efficiently collected outside the depletion region before recombining. One strategy to improve photovoltaic performance of CQD solar cells is, therefore, to extend the depletion region within the CQD solid film operating at the MPP. The schematic energy diagram of the QDs in a typical CQD film is shown in Figure 1c. All the QDs will possess the same energy levels if they have the same size and composition. A more advantageous scheme is one in which the band positions of the QDs within the film are altered to form a type-II band alignment with each other (Figure 1d). This type-II alignment will improve the charge separation by creating a favorable energy cascade between the electrodes for both the electron and hole, thus improving the directionality of the charge transport. Such band alignment will also create an effective Received: January 12, 2015 Published: January 15, 2015 2996
DOI: 10.1021/acs.jpcc.5b00341 J. Phys. Chem. C 2015, 119, 2996−3005
Article
The Journal of Physical Chemistry C
Figure 1. Band diagram of a CQD solar cell at (a) short-circuit condition and (b) maximum power point (MPP). The left side (shaded with blue) shows the n-type material and the right side (shaded with gray) shows the p-type material. The region under the red lines indicates the depleted region of the CQD solar cell. WD represents the depletion width. Schematic energy diagram of (c) QD film without band engineering and (d) QD film with band engineering having a type-II alignment. The band-engineered energy levels create an effective electric field within the QD layer, which can enable both electron and hole transport toward their respective electrodes.
electric field within the CQD solid film, which may, in turn, increase the depletion region and enhance carrier collection, thereby reducing recombination losses. Earlier, it was shown that the energy levels of quantum dots depend strongly on the passivating ligand molecule and that the energy levels can shift depending on the dipole moment of the passivating ligand.35−39 Zaban and co-workers have demonstrated in quantum dot sensitized solar cells a systematic shift of the CdS QD energy levels with respect to TiO2 using the molecular dipole of passivating ligand molecules.40,41 Very recently, Bawendi et al. have demonstrated high performance CQD solar cells through band engineering of QDs.7 The band offset between different QD layers effectively blocks electron flow to the anode while facilitating hole extraction. In this work, we aim to alter the energy positions of the PbS QDs by using dipolar ligands and to apply them to CQD solar cells in different configurations in order to explore the effect of band alignment on photovoltaic performance. The energy levels of the PbS QDs are controllably tuned by changing the dipole moment of the passivating ligands. Photoelectron spectroscopy in air (PESA) together with UV−visible absorption spectroscopy is employed to measure the band positions of the QDs. We perform density functional theoretical (DFT) calculations to understand the role of the ligands in tuning the band positions as well as the Fermi energy of the PbS QDs. Finally, we use these band engineered PbS QDs to make colloidal quantum dot solar cells in different configurations and provide a proof of concept that band alignment using dipolar ligands can control the photovoltaic performance of colloidal quantum dot solar cells depending on the direction of the band grading. The results are further verified with 1D solar cell simulations using Solar Cell Capacitance Simulator (SCAPS) software performed on thin-film solar cells having similar configurations. This systematic variation of ligands allows us to controllably change the band positions of the QDs,
which is further supported by our theoretical calculations. The photovoltaic performance of devices in “unidirectional” and “bidirectional” configurations shows direct evidence of the usefulness of proper band alignment, which can ultimately be applied to make high efficiency devices.
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EXPERIMENTAL SECTION Materials. Lead oxide (PbO), oleic acid (OA), bis(trimethylsilyl)sulfide (TMS), 1-octadecene (ODE), 4-methylthiophenol (MTP), 4-fluorothiophenol (FTP), and 4-nitrothiophenol (NTP) were purchased from Sigma-Aldrich. Methanol, hexane, and ethanol were purchased from Fisher Scientific. All chemicals were used as received. Synthesis. PbS QDs were synthesized following an earlier report by Hines et al.29 In brief, the lead precursor was prepared by degassing and dissolving 2 mmol of PbO in 10 mL of ODE and 1 0.6 mL of OA at 100 °C under vacuum. After the reaction mixture became clear, the temperature was raised to 110 °C under nitrogen. The sulfur precursor was prepared inside a N2-filled glovebox with 1 mmol of TMS in 2 mL of ODE. The sulfur precursor was swiftly injected into the reaction mixture. The reaction was continued for 30 s and was stopped by removing the heating mantle. After cooling to room temperature, ethanol was added to the reaction mixture to precipitate the PbS QDs. The QDs were purified three times by precipitation and dissolving in methanol and hexane, respectively. Finally, the QDs were stored in hexane under dark for further use. In this reaction, excess Pb precursor has been used intentionally to yield QDs with a Pb-rich surface.18 Characterization of Quantum Dots. A dilute concentration (absorbance
