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Abstract
The effect of energy level alignment between the hole transport layer (HTL) and active layer in PbS quantum dot (QD) solar cells was investigated. Here, a great variation in device performance was observed when employing different hole transporting materials. Devices using HTLs that could not block electrons only show poor device behaviors, while those employing wide band-gap hole transporting materials with shallow lowest unoccupied molecular orbital (LUMO) energies to block electrons exhibit reduced dark currents as well as enhanced device efficiencies. A power conversion efficiency of 4.4% was obtained by utilizing Poly-TPD as the HTL due to the optimized energy level alignment. These improvements were realized by preventing current leakage and consequent counter diode formation. The efficiency can be further improved to 4.9% by inserting EDT-treated PbS QD film (PbS-EDT) hole transporting materials with higher hole mobility as well as suitable energy levels that can increase the collection efficiency.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, the power conversion efficiency (PCE) of the best performing silicon solar cells has reached 25%, which is close to the Shockley-Queisser limit [1]. The efficiencies for thin-film solar cells based on CdTe or Cu(In,Ga)Se2 (CIGS) are surpassing 20% and becoming the mainstream in current photovoltaic (PV) systems [1,2]. Solution-processed PV materials such as polymers and dyes have been developed as low-cost alternatives [3]. Recent studies on perovskite solar cells have shown certified efficiencies over 20% [4]. All devices mentioned above have shown great potential in utilization of solar energy; however, each of these cells leaves a large portion of the available solar spectrum untapped.
Colloidal quantum dots (CQDs) are solution-processed semiconductor materials of interest in low-cost photovoltaics. Tuning of the bandgap of PbS CQD films via the quantum size effect enables customization of solar cells’ absorption profile to match the sun’s broad visible- and infrared-containing spectrum [5]. Ever since the first solar cell employing CQD films for both absorption and charge transport was demonstrated in 2005, the device PCE increased rapidly to certified values over 10%, which is enabled by both improved materials and developments in device architectures [6–8]. Recent progress in CQD solar cell efficiency has been accompanied by improvements in device stability, which further increases their promise [9,10].
Charge collection is an important factor for increasing solar cell efficiency. Significant improvements in charge extraction have been achieved as the device architecture progressed from Schottky-junction to depleted heterojunction and bulk heterojunction devices [11,12]. In heterojunction solar cells, the introduction of interfacial layers help to suppress the contact resistance between active layer and charge-collecting electrodes, create ohmic contacts and efficiently transport charges. With suitable electron and hole blocking layers, both open-circuit voltage and photocurrent can be improved. So far, various electron-transporting/hole-blocking materials have been used at the CQD layer/cathode interface to improve device performance, such as cadmium sulfide (CdS), zinc oxide (ZnO) and titanium dioxide (TiO2) [13–15]. Besides, studies on modifying the n-type metal oxide electron transport layer (ETL) including doping and incorporation of metal ions, and related working mechanism on electron transfer and interfacial recombination have also been developed [16,17]. In contrast, investigation of the hole transport layer (HTL) is rarely seen. P-type semiconductor nickel oxide (NiO) and copper iodide (CuI) have been used as the hole-transport (electron-blocking) layer in CQD solar cells [18,19], while the n-type molybdenum oxide (MoO3) was proved to be an effective HTL for PbS devices even though the physical mechanism behind the cell efficiency enhancement is still in debate [20,21].
This paper highlights the importance of HTL selection in improving device efficiencies, which has received much less attention as compared to the electron accepting component in CQD solar cells. A series of organic and inorganic semiconducting hole-transporting materials was adopted in the device architecture. Through improving hole transport, reducing interfacial barriers, and minimizing shunt pathways, we demonstrate an overall improvement in PbS cell performance. The device open-circuit voltage (VOC) values were found to be dependent on highest occupied molecular orbital (HOMO) energies of the HTLs; by achieving the balance between driving force for hole-extraction and the photo-voltage, the CQD solar cell performance was obviously improved.
2. Experimental section
2.1. Synthesis of colloidal PbS QDs
The PbS QDs were synthesized by adapting previously reported methods [22]. Lead acetate (11.38 g) was dissolved in 21 ml of oleic acid and 300 ml of 1-octadecene at 100 °С. The solution was heated to 150 °С under the protection of nitrogen. Sulphur precursor (a mixture of 3.15 ml hexamethyldisilathiane and 150 ml 1-octadecene) was rapidly injected into the lead precursor solution. After synthesis, a mixture of methanol and butanol was added into the QDs solution, and then purified by centrifugation. The NCs were dispersed in hexane and stored in a glove-box.
2.2. Synthesis of ZnO nanocrystals
The synthesis of ZnO nanocrystals (NCs) was adapted from the literature [22]. Zinc acetate dihydrate (2.95 g) was dissolved in 125 ml methanol at 60 °С. Potassium hydroxide (1.48 g) was dissolved in 65 ml methanol. The potassium hydroxide solution was slowly added to the zinc acetate solution and kept stirring at 60 °С for 150 min. ZnO NCs were extracted by centrifugation and then washed twice using methanol followed by centrifugation. Finally, the solution was filtered with a 0.45 μm filter.
2.3. Device fabrication and testing
Patterned ITO glass was cleaned with detergent, isopropyl alcohol, and acetone successively. The MoOx film was obtained by spin-coating the MoO2(acac)2 isopropanol solution (5 mg/ml) at 4000 rpm, and then annealed in air at 150 °С for 10 min. The 40 nm poly-(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) film was deposited by spin-coating and annealed for 10 min at 150 °С in air. The poly (3-hexylthiophene) (P3HT) film was deposited by spin-coating and annealed for 10 min at 110 °С. The (poly (N,N’-bis(4-butylphenyl)-N,N’-bis (phenyl)benzidine) (poly-TPD) film was deposited by spin-coating and annealed for 10 min at 120 °С. Both P3HT and poly-TPD were dissolved in chloroform with a concentration of 1 mg/ml. PbS QD layers were fabricated by layer-by-layer spin-coating. For each layer, 50 mg/ml PbS solution was spin-coated onto the substrate at 2500 rpm, and then soaked in a tetrabutylammonium iodide (TBAI) solution (10 mg/ml in methanol) for 30 s, followed by three rinse-spin steps with methanol. All the HTL materials are crystallized after the heat treatment. Therefore the influence of TBAI can be ignored. In addition, The TBAI ligand treatment method has been proved to be an efficient method without affecting the hole transport layer. For 1,2-ethanedithiol (EDT) treated CQD films, a 0.02 vol% EDT acetonitrile solution was used. A 40 nm ZnO film were spin-coated onto the PbS-TBAI layer then. Finally, 100 nm Al (1 Å/s for the first 10 nmand 3 Å/s for the remaining 90 nm) was thermally evaporated at a pressure of 1 × 10−6 mbar through a shadow mask (active area 4 mm2).
2.4. Characterization
The phase structures of the PbS QDs and ZnO nanocrystals were characterized by X-ray diffractometer (XRD), using a monochromatized Cu target radiation resource. The transmission electron microscopies (TEM) were measured on a JEOLJEM-2100 microscope operated at 200 kV. UV-vis-NIR absorption spectra were measured on a Perkin Elmer model Lambda 750. Ultraviolet photoelectron spectroscopy (UPS) spectra are collected using a PREVAC system. Fourier transform infrared (FTIR) spectra were measured on a Bruker Tensor spectrometer. The current density-voltage characteristics of the CQD solar cells were measured using a Kheithey 2400 digital source meter under a simulated AM 1.5G solar irradiation at 100 mW/cm2 (Newport, Class AAA solar simulator, 94023A-U).External quantum efficiency (EQE) was measured using a Crowntech Q Test Station1000AD.
3. Results and discussion
Figure 1(a) shows the TEM and its high resolution TEM (HR-TEM) images of the obtained PbS QDs. It can be seen that PbS QDs are homogenous and have good mono dispersities. The lattice fringes observed in HR-TEM images confirms the QDs are well crystallized. Figure 1(b) shows the XRD patterns of the PbS QDs and ZnO nanocrystals. It can be seen that the XRD patterns of both samples are in exact agreement with the corresponding standard cards. No impurity peaks appear implying that both samples are in pure phase. The diffraction peaks of PbS QDs are in exact agreement with the standard cards (JCPDS No.05-0592), implying that the PbS QDs crystallized in a single cubic structure. The peaks of ZnO nanocrystals were assigned to the diffractions of hexagonal structure (JCPDS No.36-1451) as shown in the Fig. 1(b). In addition, the average grain size of PbS QDs was calculated to be about 4nm by Scherrer Formula, this is consistent with the TEM images as shown in Fig. 1(a). Figure 1(c) shows the absorption spectrum of the as-synthesized PbS QDs in octane and tetrabutylammonium iodide (TBAI) treated PbS films. Oleic-acid-capped PbS QDs with the first exciton absorption peak of 902 nm are used to fabricate the devices. The TBAI treatment causes the first exciton absorption peak to red-shift by 33 nm, corresponding to an optical bandgap of 1.33 eV. The red shift in optical transitions of the PbS film originates from delocalization of electron and hole wave functions due to electronic coupling arising from the reduced inter particle distance, combined with the effect of the increased dielectric constant [23,24]. At the same time, absorbance is enhanced across the spectrum, and the color of the CQD film becomes darker. Absorption spectra of ZnO nanocrystals, P3HT, and poly-TPD are presented in Fig. 1(d).
Fig. 1 (a) TEM image of PbS QDs, the inset shows the HR-TEM image of PbS QD. (b) XRD patterns of PbS QDs and ZnO nanocrystals. (c) Absorption spectra of PbS QDs in octane solution and a PbS QD film treated using TBAI. (d) Absorption spectra of ZnO NCs, poly-TPD, and P3HT in chloroform solution.
The solid-state treatment using TBAI salt dissolved in methanol to treat the oleic-acid capped PbS QD film was firstly employed in CQD solar cell fabrications by Sargent and associates, which is called the atomic-ligand passivation, and produces halide-passivated PbS QD films [25]. The PbS QD films treated TBAI (PbS-TBAI) were found to exhibit superior air stability (nearly unchanged stored in air for 20 days) and high charge mobility (due to the low average trap energy) [26,27]. Fourier transform infrared spectroscopy (FTIR) spectra (Fig. 2(a)) show the complete removal of the oleate ligands (C-H vibrations at 2922 cm−1 and 2852 cm−1, and COO- vibrations at 1545 cm−1 and 1403 cm−1, are all eliminated) after the TBAI treatment.
Fig. 2 (a) FTIR spectra of oleic acid capped PbS film and PbS-TBAI film. (b) UPS spectrum of the PbS-TBAI film.
The band energies of CQDs can be tuned by ligand exchange, as different ligands would cause changing in the energy level positions of the same QD material, which is due to the contributions from both the QD ligand interface dipole and the intrinsic dipole moment of the ligand molecule itself [28]. Here, we determined the band edge energies with respect to vacuum in PbS-TBAI films using ultraviolet photoelectron spectroscopy (UPS). The PbS-TBAI exhibits a Fermi level energy (EF) of 4.77 eV, and HOMO energy of 5.59 eV. As the optical bandgap of PbS-TBAI is 1.33 eV, we are able to determine the lowest unoccupied molecular orbital (LUMO) energy of 4.26 eV. The difference between EF and HOMO energy (0.82 eV) in the film is greater than that between EF and LUMO energy (0.51 eV), indicating the PbS-TBAI showing n-type behavior.
The CQD solar cells were fabricated with a device structure of ITO/hole transport layer (HTL)/PbS-TBAI/ZnO/Al, and the device architecture is schematically shown in Fig. 3(a). For CQD solar cells using MoOx or PEDOT:PSS as hole transporting materials, the HTLs are directly deposited on patterned ITO glass; for devices using P3HT or poly-TPD as hole transporting materials, the polymer layers are spin-coated onto PEDOT:PSS covered ITO substrates. After that, devices were fabricated by sequentially depositing PbS-TBAI layers (~100 nm) and ZnO NC films (~40 nm). Finally, 100 nm Al were thermally evaporated through a shadow mask as the device cathode. There is a large energy offset between the cathode work function and the HOMO energy of the ZnO electron transporting material to block hole injection from the cathode under reverse bias. Figure 3(b) shows the energy level alignment of the PbS-TBAI together with the HOMO and LUMO levels of used HTLs. It can be observed that these hole transporting materials have energy levels vary in a large range, which is sufficient to investigate the effect of energy level alignment on device performance in CQD solar cells [29–31]. Among these hole transporting materials, MoOx is the only n-type semiconductor; since its LUMO level is much deeper than that of PbS-TBAI, MoOx layer can never help in blocking electrons [22]. This will lead to charge recombination at the anode/CQD interface, which is proved by the current density-voltage (J-V) characteristics from the corresponded CQD cells as discussed later.
Fig. 3 (a) Device architecture. (b) Energy level diagram. HUMO and LUMO levels of the various components are taken from refs [22,26,29–31].
When two different semiconductors are brought together, a rectifying junction would form between them due to the differences in carrier concentration. Figure 4(a) shows the dark J-V characteristics for devices using different HTLs. The key difference among these hole transporting materials is that the film of MoOx couldn’t prevent electrons within the PbS-TBAI from flowing to the anode, while films of the others could act as efficient electron blockers. A weak rectifying behavior is observed for the MoOx based device, while strong rectifying behaviors are observed for other devices. It is notable that in the MoOx based device the dark current has a strong bias dependence, while the dark current in other devices is fairly independent of the bias voltage. The difference becomes more pronounced at higher biases, indicating that the use of electron blocking layer can significantly reduce the dark current under reverse bias.
Fig. 4 (a) J–V curves obtained under dark conditions for MoOx, PEDOT:PSS, P3HT, and poly-TPD based devices; (b) and (c) are representative J–V characteristics and power density versus applied bias measurements for the same devices, respectively.
The representative J−V characteristics of PbS CQD solar cells using different HTLs are plotted in Fig. 4(b), and their photovoltaic parameters are summarized in Table 1. The CQD cell using MoOx as hole transporting material shows a best PCE of 1.0%, which is the lowest value among all devices in our study. Such a poor device performance is caused by the back-transportation of the photo-generated electrons, and the charge recombination at the PbS/anode interface. The highest open-circuit voltage (VOC) values for devices using PEDOT:PSS, P3HT, and poly-TPD as HTLs are 0.45, 0.51, and 0.54 V, respectively. Since these three wide band-gap hole transporting materials all have shallow LUMO energies to block the electron transport at the QD/HTL interfaces, the VOC values of their corresponded devices are all significant improved than that of the MoOx based device. We find that although the unwanted back-transfer of photo-generated electrons has been prevented in devices using these p-type polymers as HTLs, the VOC values of PEDOT:PSS, P3HT, and poly-TPD based devices are still not the same. This is originated from the difference in HOMO energies of these polymers. Analogy from organic PV device theory, the VOC can be limited by the relative energy difference between the HOMO/LUMO of the HTL/ETL [32,33]. In other words, to maximize the VOC while still maintaining favorable charge extraction, the HOMO/LUMO levels of the chosen HTL/ETL must approach but not exceed those of the CQD layers, respectively. As expected, greater VOC values were observed while the HOMO energy of the hole transporting materials increases, and the highest VOC of 0.54 V was achieved from the poly-TPD cell.
Table 1. Summary of photovoltaic performances of PbS CQD solar cells using different HTLs. The cell parameters are the champion cell performances and the standard deviations (marked in italic) are based on 12 solar cells.
We have tried to explain the experiments by PL and TRPL, nevertheless the fluorescence is too weak to detect due to TBAI treatment. Efficient exciton separation and charge extraction are also key factors to ensure the performance of a photovoltaic device. In our devices using polymer as HTLs, the effective blocking layers could increase the exciton separation efficiency within the QD region. Among all the three polymers, poly-TPD owned the lowest HOMO energy of 5.2 eV, which is 0.39 eV higher than that of PbS-TBAI. Thus, the HTL/QD HOMO energy differences in all the three polymer HTL based cells are positive and sufficiently large to allow efficient hole extraction; and this has been proved by the short-circuit current density (JSC) values of these devices. The highest JSC of PEDOT:PSS, P3HT, and poly-TPD based devices are 16.06, 16.75, and 16.34 mA/cm2, respectively. All these values are significantly improved than those of MoOx based cells, indicating that the recombination at QD/anode interfaces have been suppressed, and there does have a sufficient driving force for efficient hole extraction. There is not much difference between the JSC values of PEDOT:PSS, P3HT, and poly-TPD based cells. In fact, the absorption in the visible region of these HTL materials can be ignored excepted for the P3TH because it is a well-known photovoltaic material, the device employing P3HT as HTL has only shown a slight increase in the JSC. This phenomenon is reasonable since theP3HT films employed here is very thin (~10 nm, the concentration of the solution is 1 mg/mL). The highest fill factor (FF) of 55% was obtained from the PEDOT:PSS based device. Figure 4(c) represents the power density versus applied bias characteristics for all devices, with the maximum power point power conversion efficiency of 4.4% achieved for the poly-TPD based cells.
In photovoltaic devices, the mobilities for charge transporting layers also play important roles. We adopted ZnO NC films as ETLs in our devices. The electron mobility of ZnO films was reported to be 2 × 10−3 cm2/V s [34], which is about 1 order of magnitude higher than hole mobility of the organic HTLs we used here (3.30 × 10−4 cm2/V s for P3HT, and 1 × 10−4 cm2/V s for poly-TPD) [31,35]. Thus, we may further improve the device efficiency by using hole-transporting materials with higher hole mobilities. After treated by 1,2-ethanedithiol (EDT) in air, PbS QD films show p-type behavior, with a hole mobility of 1.7 × 10−3 cm2/V s [36], which is similar to electron mobility of ZnO NC films. For PbS QDs used here (first exciton absorption peak at 902 nm), the EDT treated PbS QD film (PbS-EDT) was demonstrated to have a HOMO level of 4.91 eV and a LUMO level of 3.58 eV [22], which is ideal for electron blocking and hole extraction as a HTL in our device. A layer of PbS-EDT with a thickness around 20 nm was inserted between PEDOT:PSS and PbS-TBAI films as the HTL.
Figure 5(a) shows the J-V characteristics of PbS-EDT and pure PEDOT:PSS based CQD cells. The device with PbS-EDT shows a PCE of 4.7%, which is higher than the PEDOT:PSS cells. Significant improvements in JSC and VOC have been achieved by introducing the PbS-EDT layers, resulting in a ~18% improvement in PCE. EQE spectra for devices with and without PbS-EDT are shown in Fig. 5(b). The JSC value calculated by integrating the EQE spectrum for the PbS-EDT device is 17.61 mA/cm2, which shows good agreement with the measured JSC of 17.68 mA/cm2. In fact, energy band diagram is also an important factor that influences the charge transport. The EDT treated PbS QD film (PbS-EDT) show p-type behavior with a HOMO level of 4.91 eV and a LUMO level of 3.58 eV as shown above,when it contact with PbS-TABI, a potential barrier formed due to the shallow LUMO energies in PbS-EDT, which will block the electron transport at the QD/HTL interfaces. The bending of the HOMO of PbS-TBAI can promote the injection of holes into PbS-EDT layer. In one word, this energy band alignment increases the collection efficiency of photo-excited carriers and therefore the efficiencies can be improved by inserting a PbS-EDT layer. The main issue of the low PCEs is the low EQE values in the range of wavelength larger than 600 nm, this may arise from the serious recombination in the PbS active layers and HTL materials, the quality of these materials should be further optimized. The PCE histogram distribution diagram of CQD solar cells using PbS-EDT as HTLs is given in Fig. 6. As shown, the best-performing device shows a PCE of 4.9%, and an average efficiency of 4.64% has been achieved.
Fig. 5 (a) The J-V characteristics of PbS-EDT and pure PEDOT:PSS based CQD cells. (b) EQE spectra for devices with and without PbS-EDT.
4. Conclusions
In summary, we showed the importance of HTL selection in improving PbS CQD solar cell efficiencies. A series of hole-transporting materials were investigated to study the influence of electron blocking, energy level alignment, and hole mobilities on the device performance. By utilizing poly-TPD HTL with optimized energy level alignments a promising PCE of 4.4% were obtained. Furthermore, the collection efficiency of photo-excited carriers and therefore the efficiencies can be improved by inserting a PbS-EDT layer with higher hole mobility. Our optimized device shows a PCE of 4.9%, corresponds to a 23% improvement over the device employing commonly used PEDOT:PSS HTL. Our study on HTL selection could be helpful for understanding the charge extraction process and designing efficient photovoltaic device structures.
Funding
National Natural Science Foundation of China (11504188, 61703216); Natural Science Foundation of Henan Province (U1504626); Program for Science & Technology Innovation Talents in Universities of Henan Province (19HASTIT019); Key Scientific and Technological Project of Henan Province (182102210469); Special Funded Projects of Nanyang Normal University (2018ZX002).
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