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Improving Performance in Colloidal Quantum Dot Solar Cells by Tuning Band Alignment through Surface Dipole Moments Pralay Kanti Santra, Axel F Palmstrom, Jukka T. Tanskanen, Nuoya Yang, and Stacey F. Bent J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00341 • Publication Date (Web): 15 Jan 2015 Downloaded from http://pubs.acs.org on January 20, 2015

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The Journal of Physical Chemistry

Improving Performance in Colloidal Quantum Dot Solar Cells by Tuning Band Alignment through Surface Dipole Moments

Pralay K. Santra,1 Axel F. Palmstrom,1 Jukka T. Tanskanen,1,§ Nuoya Yang2 and Stacey F. Bent.1* 1

Department of Chemical Engineering, Stanford University, Stanford, California, 94305, USA

2

Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States

§

Current affiliation: The Academy of Finland, Helsinki, 00531, Finland

*Address correspondence to this author [email protected]

ACS Paragon Plus Environment

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ABSTRACT Colloidal quantum dots (CQD) 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 (4nitrothiophenol, 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 improve the photovoltaic performances. TABLE OF CONTENT (TOC) GRAPHIC

KEYWORDS Band alignment; colloidal quantum dot solar cell; thiophenols; lead sulfide; dipole moment;


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INTRODUCTION Solid-state colloidal quantum dot (CQD) solar cells have gained much attention recently as they show promise towards next generation photovoltaic devices.1-15 These solar cells are solution processable7,

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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 studies1-12, 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 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 photoelectric current. This transport process is enhanced by the electric field in the depletion region.


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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 grey) 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 towards their respective electrodes.

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 condition, 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 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 cm2V-1s-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


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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 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 carriers. Such band alignment will also create an effective 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


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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, lead precursor was prepared by degassing and dissolving 2 mmol of PbO in 10 ml ODE and 1 .6 ml 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 glove box with 1 mmol of TMS in 2 ml ODE. The sulfur precursor was swiftly injected to the reaction mixture. The reaction was continued for 30 s and was stopped by removing the heating mantle. After cooling down 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 < 1.0) of sample in hexane was used to carry out UV-visible absorption using a Varian Cary 6000i spectrometer. A thin film was prepared by drop casting the QDs on glass slide for x-ray diffraction measurement using PANanalytical X’Pert PRO instrument. TEM images were collected using FEI Tecnai G2 F20 X-TWIN Transmission Electron Microscope operating at 200 kV. The TEM samples were prepared by drop casting a dilute QD solution on carbon coated Cu-grid. For FTIR and XPS


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measurements, QD films were prepared by spin coating on Si wafer. FTIR measurements were carried out in a Bruker Vertex 70. The X-ray photoemission experiments were performed using a SSI S-Probe XPS Spectrometer. Photoelectron Spectroscopy in Air (PESA) was carried out using photoelectron spectrometer from RKI Instruments (Model AC-2). In this technique, the electrons are emitted from the material surface by the photoelectric effect. For semiconducting materials, a linear relationship exists between the cube root of photoelectric quantum yield and the incident photon energy. The ionization energy is determined as the incident photon energy at the inflection point in a plot of the cube root of the photoelectric yield versus incident energy.

Ligand Exchange. A solution of 0.05 M thiophenols in methanol has been used to replace the native long chain oleic acid ligand molecules. Fresh thiophenols were always prepared in methanol just before use. The ligand exchange was performed during spin coating.

DFT Calculations. The dipole moments of the ligand molecules were determined from density functional theory (DFT) calculations carried out using PBE042 functional and standard def-SVP43 basis set, as implemented in the utilized software Gaussian09.44 The dipole moments were determined by fully optimizing the ligands and by performing population analyses on the optimized structures with the Natural Bond Orbital (NBO) method.45 The pristine and functionalized quantum dots (QDs) were simulated computationally by density functional theory (DFT) calculations utilizing BP8646-47 with the resolution of identity approximation48-50 (RI-BP86) and PBE51 functionals. Optimizations without symmetry constraints of the QDs and the ligand molecules (NTP, FTP and NTP) were performed using the RI-BP86 method and def2-SVP49,

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basis set with the TURBOMOLE54-55 software. The


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electronic structures of the QDs were analyzed by both RI-BP86/def2-SVP calculations and PBE calculations utilizing the projector-augmented-wave (PAW) method.56-57 The latter calculations were performed by the Vienna Ab initio Simulation Program (VASP).58-59 The DFT calculations were performed at 0 K. However, a standard computational technique called Fermi smearing has been used in our calculations, which excites electrons into the virtual space, thus improving SCF convergence and shifting the Fermi level above the highest occupied band at 0 K. The finite temperature has been determined by the width of the smearing, which was set to 0.2 eV.

Solar Cell Fabrication and Characterization. A thin layer (~ 15 nm) of anatase TiO2 was deposited on cleaned SnO2:F (FTO; Pilkington TEC 15, Hartford Glass) glasses by atomic layer deposition using a custom-made hot wall ALD reactor controlled by LabVIEW software.60 Titanium tetrachloride and water was used as precursor. Quantum dot films were deposited on TiO2 substrate by spin coating a 40 mg/ml of QD in hexane under ambient condition. Each QD layer was deposited at 2500 rpm for a period of 25 s followed by a treatment with thiophenol ligand solutions of 0.05 M concentration and spin coated from another 25 s at 2500 rpm. Finally the films were washed twice with methanol and hexane to get rid of any extra thiophenol and unexchanged PbS QDs. This processes was repeated for a total of 8 cycles. A 60 nm thick gold contacts were prepared on top of the QD layer by thermal evaporation with a rate of 0.2-2 Å/s. The active areas of the solar cell were considered to be the overlapped region of the gold and QD/TiO2 layer, which was typically around 0.1 cm2. Photovoltaic characteristics were measured under ambient condition using a Keithly 2420 source-meter. The solar spectrum of AM 1.5 G was simulated using an Oriel Sol 3A Class-AAA


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solar simulator from Newport. The intensity of the light was set to 1 sun (100 mW/cm2) using a calibrated Si-photodiode. Solar Cell Simulation. 1D solar simulations were performed using Solar Cell Capacitance Simulator (SCAPS 3.0.01) software. Details of the simulation parameter are provided in the supporting information.


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RESULTS AND DISCUSSION The synthesis of PbS QDs was carried out following the method in an earlier report.29 The absorption spectrum of the PbS QDs employed in this study is plotted in the main panel of Figure 2. The QDs show an excitonic peak at ~894 nm. The sharp excitonic feature suggests a narrow size distribution of the PbS QDs. Transmission electron microscopic (TEM) analysis was carried out in order to gain a better understanding of the size distribution of the PbS QDs. A representative TEM image is shown in the inset of Figure 2. The average particle size was calculated to be 3.2±0.5 nm, consistent with the position of the excitonic peak near 900 nm.61 The size distribution histogram is shown in Figure S1a in the Supporting Information.

Figure 2. UV-Vis absorption spectrum of PbS QDs dispersed in hexane. Inset shows a TEM image of the PbS QDs.

As-synthesized, PbS QDs are passivated with long chain ligand molecules of oleic acid which are insulating; films of QDs passivated with such ligand molecules are known to be highly resistive.62 It is necessary to replace the long chain molecules with smaller ligand molecules to improve the conductivity of the films, and earlier reports have suggested that the use of various ligands can improve the conductivity of different lead chalcogenide films.10, 63-67


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In this study, we considered three different para-functionalized thiophenol molecules to passivate the surface of PbS QDs as shown in Figure 3a. Although they are not typical ligands for enhanced conductivity in CQD films, these linear molecules were chosen because they allow for systematic variation of the surface dipole moment on the QDs. First, they have only one active functional group, -SH, which can bind to the QD surface, fixing the directionality of the Figure 3. Thiophenols used to passivated the PbS QD surface: NTP; 4-nitrothiophenol, FTP; 4fluorothiophenol and MTP; 4-methylthiophenol. The dipole moments of the free ligands are mentioned under the chemical structure within the parentheses.

ligand. Second, by changing the functional group (nitro, fluoro and methyl) at the para-position of the thiophenol, it is possible to tune the dipole

moment of the free ligand molecule. We have calculated the dipole moments of the free ligand molecules using DFT calculations. Nitrothiophenol (NTP) has a dipole moment of +4.4 D due to the strong electron withdrawing nature of the nitro group. 4-Methylthiophenol (MTP) shows a dipole moment of -1.82 D, which is opposite in direction and of a different magnitude compared to NTP, due to the electron donating nature of the methyl group. Fluorothiophenol (FTP) is intermediate to the other two ligands, with a dipole moment of nearly zero (+0.04).

A ligand exchange procedure was carried out during the spin coating of the QDs onto the substrate, as discussed in the experimental section, to attach the various thiophenol ligands to the QDs. To verify the exchange of the native oleic acid ligand with the thiophenol molecules at the surface of PbS QDs, we performed X-ray photoelectron spectroscopy (XPS) of films of the PbS QDs before and after ligand exchange. Survey scans of PbS QDs passivated with different ligand


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molecules are shown in Figure 4a. The XPS spectra are normalized with respect to the Pb 4f7/2 feature at 138 eV. The survey scans show distinct peaks corresponding to different core levels of Pb at 644 eV (Pb 4p3/2), 435 eV (Pb 4d3/2), 412 eV (Pb 4d5/2) and 19 eV (Pb 5d). A weak signal from the S 2p core level is observed in each spectrum at 162 eV. The ratio of Pb to S peak intensities remains the same within error for the PbS QDs passivated with different ligands. In addition to Pb and S peaks, a signal corresponding to C 1s is observed at 285 eV in each spectrum. However, the relative intensity ratio of C 1s with respect to Pb 4f is higher for oleic acid-passivated PbS QDs than for each of the thiophenol-passivated PbS QDs. Since oleic acid has 18 carbon atoms, whereas the thiophenols used in this study have only 6 or 7 carbon atoms, the decrease in C 1s signal intensity suggests replacement of oleic acid with thiophenol molecules. There can be potential contamination of samples with carbonaceous materials. However, the intensity of C 1s signal intensity of PbS QDs passivated with either NTP or FTP is almost an order of magnitude lower than that of oleic acid-passivated PbS QDs. The significant change in intensity of C 1s is only a qualitative measure of replacement of the long chain native ligand. Additional evidence comes from XPS peaks originating from heteroatoms on the thiophenol ligands. We observe a distinct signal from F 1s at 687 eV for the FTP-passivated PbS QDs as shown in the high resolution spectrum of F 1s (Figure S3a in supporting information). The survey scan on NTP-passivated PbS QDs reveals a weak signal of N 1s at 405 eV. The presence of N was further verified from high-resolution spectra collected in that region as shown in Figure S3b in the Supporting Information. The binding energy of N 1s matches with the previously reported68 binding energy of N 1s from nitro compounds. Hence, the XPS studies qualitatively support successful replacement of the native oleic acid ligands with the corresponding thiophenol molecules.


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Figure 4. (a) Survey scans of X-ray photoelectron spectra and (b) Fourier transform infrared spectra of PbS QDs passivated with oleic acid (black), MTP (red), FTP (green) and NTP (blue). The prominent peaks are labeled. In (a), the O 1s core level peak at 532 eV for MTP and FTP originates from contamination from ex-situ analysis. In (b), the characteristic stretching frequencies of different functional groups are highlighted in the FTIR spectra.

To further characterize the surface of PbS QDs, FTIR spectroscopy was carried out on films of PbS QDs passivated with oleic acid and the thiophenol ligands as shown in Figure 4b. Distinct aliphatic ν(C-H) stretching frequencies between 2800 – 3000 cm-1 were observed for oleic acidpassivated PbS QDs. A more detailed analysis of the aliphatic ν(C-H) for PbS QDs passivated with oleic acid and aromatic ν(C-H) stretching of thiophenol ligands is shown in Figure S4 in supporting information. The large reduction in intensity of these aliphatic stretching frequencies for the thiophenol-passivated PbS QDs suggest that substantial replacement of the native oleic acid ligands has been achieved during the ligand replacement process. In addition, new modes associated with the para-substituted thiophenols provide additional evidence for ligand exchange. For all of the thiophenol-passivated PbS QDs, aromatic C-C stretching modes were detected in the range of 1475 – 1485 cm-1. Moreover, the C-F stretching frequency at 1227 cm-1 was observed from FTP ligands, and both symmetric and asymmetric stretching frequencies of the NO bond were observed at 1342 cm-1 and 1512 cm-1 from the NTP ligands.


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Figure 5. (a) Variation of negative ionization energy of PbS QDs passivated with different thiophenol ligands with their dipole moment. (b) Positions of the valence band maximum (VBM) and conduction band minimum (CBM) of PbS QDs as determined from PESA and UV-visible absorption spectroscopy. Blue and green lines indicate the position of VBM and CBM, respectively.

To determine the effect of the ligand dipole moment on the electronic structure of the PbS QDs, we have experimentally measured the band positions of PbS QDs using photoelectron spectroscopy in air (PESA).69 PESA determines the ionization energy of a material.

The

ionization energy can be extracted from plots of the cube root of photoelectric quantum yield versus the photon energies, as shown in Figure S5 in Supporting Information. The resulting ionization energies for PbS QDs passivated with different thiophenol molecules versus the dipole moment of molecule are plotted in Figure 5a. NTP-passivated PbS QDs show the highest ionization energy of 5.48 ± 0.02 eV, followed by FTP and MTP with values of 5.22 ± 0.05 eV and 4.81 ± 0.04 eV, respectively. The ionization energy of PbS QDs varies nearly linearly with the dipole moment of the ligand molecule. This suggests that the dipole moment of the passivating ligand molecule directly affects the electronic structure of the QD, consistent with earlier reports from other QD systems.38-39


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In the case of a semiconductor, the ionization energy measures the position of the valence band maximum (VBM).70 We have plotted in Figure 5b the VBM of PbS QDs for different thiophenol molecules as shown by the blue lines. To determine the conduction band minima (CBM), shown by green lines, we have added the band gap, measured from the point of inflection in absorption spectra (Figure S6 in supporting information), to the VBM energy. It is evident from the experimental data in Figure 5 that by changing the passivating ligand, both the VBM and CBM change in such a way that they form type-II band alignment with each other. We also noticed that there is a change in band gap of the PbS QDs with different passivating ligands, which is due to the dipole moment of the passivating ligands. The electric field of electron withdrawing ligands, NTP, helps in the extent of delocalization of the electron wave function of the QD thus decreasing the band gap (1.25 eV), while the electron donating ligands, MTP, confine the electron wave function, thus increasing the band gap (1.37 eV).

To better understand the experimental results in Figure 5, DFT calculations were performed. Figure 6a shows the computational results for the VBM, CBM and Fermi energy (EF) of PbS QDs passivated with different thiophenols. Based on X-ray diffraction (Figure S2), TEM (Figure 1) and the nominal chemical composition of the PbS QD, we simulated the QDs as spherical, rock salt crystalline particles with excess surface Pb, and the resulting pristine QDs had a chemical composition of Pb80S68. Functionalized QDs were simulated by binding up to 16 ligands on the particles and fully optimizing the functionalized QD (as shown schematically in Figure S7 in supporting information). The attachment of the ligand molecules takes place via formation of a chemical bond between Pb in the QD and S in the ligand. The hydrogen of the


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thiol group in the ligand structure is bound to a neighboring surface S on the QD in the simulations for charge neutrality.

The computational results show that there is a significant shift in the energy positions with different ligand molecules, and the trend is consistent with our experimental results. The calculated band gaps of different PbS QDs are smaller compared to the experimentally observed band gap which is a well known drawback of DFT as shown earlier in literature.71 The dipole moment of the ligand molecule creates an electric field around the QD, which alters its band positions. The magnitude and direction of the dipole moment on the QD is different, as shown schematically in Figure 6c, for NTP and MTP ligands; the resulting electric field, in turn, is mainly responsible for changing the band positions of the QDs. Earlier theoretical work38 on CdSe QDs also showed systematic shifts in VBM and CBM positions due to the dipole moment at the surface of the QD, which was associated with the ligand molecules. The total dipole moment is a combination of the dipole moment of the bare ligand molecule and the induced dipole moment at the QD-ligand interface.


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In addition to the VBM and CBM, it is important to examine the position of the Fermi energy. We have calculated the EF of the PbS QDs passivated with different thiophenol molecules as indicated in Figure 6a. The Fermi energy of the QDs also shifts with the dipole moment of the ligand molecule. The position of the EF has been found to be located closer to the VBM for PbS QDs passivated with three thiophenols, indicating the QDs to be p-type. To find out if the

Figure 6. (a) VBM, CBM and EF positions of PbS QDs passivated with different thiophenol ligand molecules as calculated on Pb80S68 cluster passivated with 16 ligands. (b) Variation in VBM positions with respect to EF of the QDs passivated with different thiophenols shows the change in doping density with the dipole moment of the ligand. (c) Schematic diagram showing the dipole moment of the ligand on PbS QDs passivated with MTP and NTP.

passivating ligands offer any change in doping density of the QDs, we have calculated the difference between the EF and VBM. In Figure 6b, we plot the position of the VBM with respect to the EF of the system. It is evident that there is a small shift in the relative positions as a


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function of ligand. The decrease in the difference between the EF and VBM from NTP to MTP indicates that the QDs are becoming more p-type. To explore the effect of band alignment of the QDs on photovoltaic performance, we have used PbS QDs passivated with two different ligands, NTP and FTP, to fabricate CQD solar cells in three different configurations. A total of eight cycles of QDs were deposited in each case to maintain the same thickness of the absorber layer. Each configuration of the solar cells is shown in the inset of Figure 7a. In the first configuration, we have made a control solar cell containing PbS QDs passivated with only NTP. As all the QDs are treated with the same ligand, it is expected to have the same energy levels for individual QDs as shown in Figure 7b. In the second configuration, we have made solar cells consisting of the first four cycles with NTP-passivated PbS QDs (corresponding to ~350 nm in thickness), followed by another four cycles with FTPpassivated PbS QDs. The schematic band positions of such a configuration are shown in Figure 7c. The NTP and FTP passivation is expected to change the band positions of the QDs in a typeII fashion with respect to each other. We have denoted this as a unidirectional configuration. The third configuration of the solar cell was constructed with FTP-passivated PbS QDs followed by NTP-passivated PbS QDs. The schematic band energy position for this configuration is shown in Figure 6d. The band positions of the QDs are aligned in such a way in this configuration that some of the electrons and holes will encounter a potential barrier to reach their respective electrodes. This is the opposite configuration of the previous one and we call it a bidirectional configuration.


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Figure 7. (a) J-V characteristics of solar cells having three different configurations (i) control, (ii) unidirectional, and (iii) bidirectional. The configurations are shown in the inset. Schematic energy diagram of (b) control, (c) unidirectional, and (d) bidirectional configurations.

The J-V characteristics for the CQD solar cells with the three different configurations are shown in Figure 6a, and the photovoltaic parameters are summarized in Table 1. The control solar cell with PbS QDs passivated with NTP delivers a short circuit current (JSC) of 0.041 mA/cm2 with an open circuit voltage (VOC) and fill factor (FF) of 0.59 V and 27.4 respectively. The total power conversion efficiency is 0.0067 %. We note that the low efficiency reflects the use of these particular ligands with a single functional group capable of attaching to the QD surface which, although selected for their controllable dipole moment, makes the lead sulfide quantum dot multilayer films highly resistive73 and these are not optimized for overall performance. The unidirectional configuration has JSC of 0.048 mA/cm2 and FF of 30.5. Both the JSC and FF are higher than that for the control solar cell, although the VOC for the unidirectional


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Table 1. Photovoltaic parameters of solar cells having different configurations Relative Configuration

JSC

VOC

(mA/cm-2)

(V)

Solar Cells

Efficiency FF

Enhancement (%) (%)

0.041

0.594

27.43

0.0067

±

±

±

±

0.005

0.005

0.25

0.0003

0.048

0.562

30.51

0.0082

22.78

±

±

±

±

±

0.001

0.004

0.14

0.0003

5.81

0.037

0.538

31.55

0.0062

-6.41

±

±

±

±

±

0.002

0.005

0.05

0.0003

5.76

NTP only§ −

(Control)

NTP/FTP

§

(Unidirectional)

FTP/NTP

§

(Bidirectional) §

The error bars were obtained by averaging over 5, 4 and 2 solar cells for control, unidirectional and bidirectional configuration, respectively.

configuration is smaller by ~ 40 mV compared to the control configuration. We note that the change in VOC between these two configurations, which have a heterojunction device architecture, is small compared to that expected for a Schottky device architecture, in which the change in VOC would be more prominent as shown earlier.72 The change in VOC in the heterojunction device architecture is expected to be significantly less compared to the changes in band positions relative to the vacuum energy level, as measured experimentally (Figure 5b) by PESA because a change in VOC results from Fermi energy level shifts. When these semiconductor materials are brought in contact, the Fermi energy levels equilibrate. If there is no shift in the


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Fermi energy level relative to the valence and conduction bands of each semiconductor, a band offset observed by PESA will only result in a shift of the vacuum level when the semiconductors are brought into contact. However, if there is a change in the Fermi level of the semiconductor caused by doping or surface dipole effects, there will be an offset in the conduction and valence bands when the semiconductors are brought together in a device. This offset due to a shift in the Fermi energy level can cause larger splitting of the quasi Fermi energy levels under solar illumination, leading to a larger device VOC. As shown in Figure 6b, the shift in Fermi level is predicted to be on the order of hundredths of an eV, which is similar to the experimentally observed VOC changes in our devices.

Nevertheless, the overall power conversion efficiency of the unidirectional configuration is 0.0082 % which is almost 23 % higher relative to the control solar cell. The improvement in both the short circuit current and fill factor indicate that the band alignment indeed improves the charge collection for the unidirectional configuration. In contrast, the solar cell with bidirectional configuration exhibits the lowest JSC among all three configurations. The decrease in JSC provides support that the opposite band alignment in the bidirectional configuration creates a barrier for both electrons and holes. The total power conversion efficiency of this bidirectional configuration is 0.0062 %, which is 6 % relatively poorer compared to the control solar cell.


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Figure 8. (a) Schematic energy level diagrams of three different materials used in simulating the photovoltaic performances. The bandgap, CBM (EC), VBM (EV) and Fermi energy (EF) are indicated in the same figure. (b) Simulated J-V spectra for three different configurations of solar cells as shown schematically in the inset.

In order to help explain the photovoltaic performances of the solar cells, we have performed 1D solar cell calculations73-75 of three configurations similar to those in our experiments. In the simulations, we have considered a p-n junction with 1.5 μm and 0.1 μm thick p-type and n-type materials, respectively. The n-type material (N) has a band gap of 3.2 eV, the same as TiO2, and the EF lies 0.08 eV below the CBM. To simulate the control configuration, we have considered only one p-type material (P1) with a bandgap of 1.3 eV, the same as PbS QD, and an EF that lies 0.14 eV above the VBM. The unidirectional configuration has been simulated with two different p-type materials, P1 followed by P2. P2 has the same band gap of 1.3 eV; however, the doping density is chosen to be higher compared to P1 so that the EF lies 0.12 eV above the VBM. The difference in the EF between P1 and P2 are chosen to match the difference in calculated EF as shown in Figure 7b after proper scaling. The schematic energy diagrams of these materials are shown in Figure 8a. Other characteristics of these materials are tabulated in Table S1 in Supporting Information. We have also simulated the bidirectional configuration by considering


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the P2 layer followed by P1. The schematic diagrams for the configurations are shown in the inset of Figure 8b. The simulated J-V curves of these three configurations are shown in Figure 8b, and the photovoltaic performances are summarized in Table S2 in Supporting Information. Notably, the results from our simulations have a very similar trend to that of our experimental observations. The unidirectional configuration shows both a higher JSC and fill factor compared to the control configuration. The bidirectional configuration has the worst performance among these three configurations, again consistent with experiment.

In a recent work,7 Bawendi et al. have used tetrabutylammonium iodide (TBAI) and 1,2ethanedithiol (EDT) as passivating ligands to alter the band energy positions of the PbS QD. EDT can shift up the energy levels of the QD compared to TBAI. By replacing the two topmost PbS-TBAI layers with PbS-EDT, a cascade band energy alignment, similar to our unidirectional configuration, was achieved which gave rise to a total power conversion efficiency of 8.5 %. Taken together, these results affirm the role of band alignment in CQD solar cell performance. The present work shows through a systematic study that the dipole moment of the passivating ligand can precisely tune the band alignment, which is critical for light energy harvesting applications.

CONCLUSIONS In summary, we have studied the effect of the dipole moment of the passivating ligands at PbS QDs on their band positions. The band position of the QD is linearly dependent on the dipole moment of the free ligands. By changing the dipole moment of the passivating ligand from 4.4D to -1.82 D, the band position could be shifted up by 0.7 eV. Similar trends were observed from


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DFT calculations on PbS clusters having different passivating ligands, further confirming the effect of the ligand dipole moment. The calculations also show a finite change in Fermi energy position of the QD due to the dipole moment of the passivating ligand. The use of these band engineered PbS QD allow us to make photovoltaic devices in both unidirectional and bidirectional configuration. In case of unidirectional configuration, the cascaded type-II band alignment improves the photovoltaic performances by increasing both the short circuit current and fill factor, whereas the bidirectional configuration yields lower short circuit current due to potential barrier formation for both electron and hole at the interface between the two different PbS QD layers. The results suggest that by altering the dipole moment using different passivating ligands or a mixture of different ligands around the QD, precise band engineering of the QD can be achieved, which further plays an important role in their photovoltaic performance.

ACKNOWLEDGMENT The authors would like to thank Adam Hultqvist for helpful discussions. This work was supported as part of the Center for Nanostructuring for Efficient Energy Conversion (CNEEC), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0001060.

SUPPORTING INFORMATION Size distribution histogram, HRTEM, XRD of PbS QDs, high resolution x-ray photoelectron spectra of Pb 4d and N 1s core levels, expanded FTIR around the C-H stretching, PESA data, absorption spectra of PbS QD films, space fill models of different PbS QDs, reproducibility of J-


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V characteristics, parameters and results for solar simulation. This material is available free of charge via the Internet http://pubs.acs.org.


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