Improved stability of depletion heterojunction solar cells employing cation-exchange PbS quantum dots

Improved stability of depletion heterojunction solar cells employing cation-exchange PbS quantum dots

Solar Energy Materials & Solar Cells 164 (2017) 122–127 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homep...

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Solar Energy Materials & Solar Cells 164 (2017) 122–127

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Improved stability of depletion heterojunction solar cells employing cationexchange PbS quantum dots

MARK



Xudong Yaoa, Zihang Songa, Longfei Mia, Guopeng Lia, Xiaoyan Wanga, Xiaoming Wangb, , ⁎ Yang Jianga, a b

School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China School of Engineering Technology, Purdue University, West Lafayette, IN 47907, USA

A R T I C L E I N F O

A BS T RAC T

Keywords: PbS quantum dots Solar cells Cation exchange Air stability Power conversion efficiency

PbS colloidal quantum dots (QDs) have emerged as one of the most promising photovoltaic solar cells material. The synthesis of PbS QDs generally uses TMS2S as sulfur source. However, the volatile and environmentally unfriendly properties of TMS2S will limit its industrial applications. This paper presents a new method of synthesizing PbS QDs employing a CdS QDs/ODE solution as the sulfur precursor by exploiting a cation exchange reaction. The resulting QDs show a performance comparable with previous work for solar cells. Through optimizing the architecture of the photoactive layer, excellent stability in air is achieved. An efficiency of 7.89% is retained in air after 110 days.

1. Introduction PbS quantum dots (QDs) have attracted increasing attention for photovoltaic applications because of their quantum-size-effect-tuning matching their absorption with the sun's broad spectrum [1,2]. The power conversion efficiency has experienced a rapid increase to more than 10% over the past decade as a result of advances in QD surface passivation and improvements in device architecture [3]. For examples, Lan incorporated high amounts of iodide on CQDs to achieve improved passivation and deliver a certified efficiency of 10.6% [4]. Meanwhile, Liu presented a new solution-phase ligand-exchange method that enable closely packed CQDs films with flat energy landscapes, with a certified power conversion efficiency of 11.28% [5]. From early PbS QD Schottky junction solar cells to recent depleted heterojunction (DH) solar cells (such as a rectifying junction of TiO2, ZnO, CdS with PbS QDs film) [6–8], the photovoltaic performance has been significant improved [9]. While broad efforts have been dedicated to improve the device architectures [10,11], the surface chemistry is another major factor that affects the photovoltaic performance [12,13]. The well-known short-chain-organic ligand exchange process has been tuned to optimize inter-dots distance to enhance carrier transport and to passivate those high-density trap states attributed to a large amount of surface dangle bonds [14]. Recently, halide surface passivation treated PbS QDs have shown a great improvement in photovoltaic performance as a result of a lower density of trapped carriers than in their organic ⁎

ligands counterparts [15]. Furthermore, halide ion balances the excess charge caused by the nonstoichiometric ratio during the synthesis of the QDs [16]. In the synthesis of PbS QDs, an optimum size between 3 and 4 nm is desired for photovoltaic devices in general. And a lot of efforts have been made to the synthesis of PbS QDs [17,18]. In previous research, bis (trimethylsilyl) sulfide (TMS2S), a reactive sulfur source, was commonly injected into a Pb precursor solution to form PbS QDs [19]. The resulting QDs have good mono-dispersity and stability, and the size is suitable for solar cells. However, the TMS2S is volatile, toxic and expensive, which limit its industrial applications for solar cells. Sulfur powder is of particular interest in some chalcogenide (such as CdS, ZnS) QDs [20] due to its less toxicity, low price and good stability. However, it is not capable to achieve monodisperse PbS QDs with diameters smaller than 4 nm attributed to a relatively low nucleation threshold in Pb precursor system. Whereas, cation exchange techniques provide a powerful synthetic tool to overcome the limitations of traditional colloidal syntheses [21,22]. A typical example is to apply a two-step reaction process to covert CdS QDs/NRs to PbS QDs/NRs in a Cu2S structure [23]. Zhang et al. and Kim et al. also developed a direct cation exchange synthesis method to obtain PbSe QDs employing presynthesized CdSe and ZnSe QDs as raw materials. These PbS QDs offer a power conversion efficiency exceeding 6% with improved air stability [24,25]. In this paper, a direct pathway toward PbS QDs using CdS QDs as starting material via a cation exchange reaction is presented. In a

Corresponding authors. E-mail addresses: [email protected] (X. Wang), [email protected] (Y. Jiang).

http://dx.doi.org/10.1016/j.solmat.2017.02.006 Received 8 November 2016; Received in revised form 17 January 2017; Accepted 6 February 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

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0.09 cm2.

comparatively green synthetic environment employing sulfur powders as anion source, CdS QDs of desired sizes are produced by injecting a sulfur/ODE solution into a cadmium oleate/ODE mixture. Then the pre-synthesized CdS QDs in ODE are injected into a PbCl2/OLA mixture to form PbS QDs. This method produces chloride-terminated (in situ) PbS QDs, preventing the QDs from oxidative attack and balancing excess charge from nonstoichiometric surface termination. These QDs are applied to an improved architecture in DH solar cells. PbS-TBAI exchanged QDs film serve as the main photoactive region whereas PbS-EDT exchanged one serve as the electron-blocking layer. This improvement results in a high power conversion efficiency (PCE) of 7.89% and a Jsc of 30.96 mA/cm2, the highest of PbS QDs heterojunction solar cells.

2.4. Device characterization The morphologies and crystal structures of the QDs were analyzed by using a JEM-2100 high-resolution transmission electron microscope. Optical absorption spectra were measured by using a Shimadzu UV–VIS–NIR 3600 spectrophotometer. X-Ray diffraction patterns were recorded with a Rigaku D/MaxrB diffraction using Cu Kα radiation with a wavelength of 0.1540 nm. The sectional view of the device was analyzed by a SU8020 field-emission scanning electron microscope. Photocurrent density-voltage (J-V) curves were conducted with a Keithley 2636 sourcemeter under N2 atmosphere using a xenon lamp solar simulator equipped with an AM 1.5 filter as the light source. IPCE was tested by using a Zolix spectrograph, the light source was also provided by a xenon lamp, and a standard silicon cell (OPRC185Si. QG-CAL S/N #1138) was used as a reference.

2. Experimental details 2.1. Synthesis of CdS QDs In a typical synthesis, CdO (0.28 g) and oleic acid (OA 1.5 ml) were loaded into a 50 ml three-neck round-bottom flask with 18 ml of octadecene (ODE). The mixture was heated to 300 °C under vigorous stirring and argon bubbling, yielding a yellowish brown homogeneous solution. Then 0.032 g sulfide powers in 10 ml ODE solution was injected into the flask. Before the injection of sulfur precursor, the heating mantle was removed and the solution was cooled down to room temperature naturally. After the synthesis, the CdS QDs were purified through a method that was reported in a previous work [26]. The isolation procedure was repeated by using hexane, ethanol, and acetone, CdS QDs were sedimented through centrifugation and were dried in an oven in following steps. Butylamine was added in the first extraction step to enhance the ability to remove the fatty acid and carboxylate salts into ethanol and chloroform was added in the precipitation step to retain the non-polar liquid [26].

3. Results and discussion The typical transmission electron microscopic (TEM) images of the as-synthesized CdS and PbS QDs are shown in Fig. 1a-b. It is clear from the images that the particles are monodisperse particles with similar shapes. The mean size of CdS QDs is ~2.7 nm, while the PbS QDs size is ~3.5 nm. The high-resolution TEM (HRTEM) image of a single CdS QD, as shown in Fig. 1c, demonstrates a single crystal with a high crystallinity. A single crystal PbS QD with an injection temperature of 150 °C is shown in the Fig. 1d. There is not an apparent structural defect in the QD and the continuous fringes are well-resolved. It is an indication that the elevated temperature provides sufficient thermal energy to remove any lattice stress or disorder that may be generated in the ion exchange reaction and CdS QDs are fully converted to PbS QDs in a directed cation exchange reaction. The cation-exchange process depends on the thermodynamic driving force and the activation barrier. The solvation of cations plays an important role in determining the thermodynamics of the reaction, and which could be controlled by varying the solvent environment. On the other hand, many factors (for instance, the anion sublattice structure, the iconicity of the cationanion interaction, and the structural difference between the reactant and product phases) would impact on the activation barrier for the diffusion and exchange of the cations. In our experiments, the cation exchange reaction was triggered by using PbCl2 in oleylamine. Based on above-mentioned facts, the solvent environment could provide enough thermodynamic driving force for the reaction, which would be caused by that the CdCl2 is easier to solvation in oleylamine than PbCl2. And being as a support, the similar results have been also reported in the reference [27]. Furthermore, the observed 0.336 nm and 0.296 nm fringe spacing in the HRTEM image correspond to the separation of the (111) and (200) lattice plane of zinc blende CdS QDs and rock salt PbS QDs. An absorption spectrum of toluene dispersions of CdS QDs is shown in Fig. 2a. The sharp first excitonic absorption peak is an evidence of a narrow size distribution of the QDs, in a good agreement with the TEM image. It has been reported that a uniform CdS QDs size is beneficial to the formation of reasonable monodisperse PbS QDs [27]. Typical absorption spectra of PbS QDs with various CdS/ODE injection temperatures are shown in the Fig. 2b. It can be found that the position of the first excitonic absorption peak is shifted from 850 nm to 1202 nm (red) with the increase of injection temperature from 80 °C to 160 °C, an evidence of that a higher injection temperature results in larger PbS QDs. It is attributed to an increased width of the reaction zone and the increase of the diffusion rate of the cation with temperature. A narrow size distribution is achieved in lower injection temperature and a slow red-shift is observed. With the increase of injection temperature, a large red-shift occurs accompanied by a broad size distribution, which means that the QDs grow rapidly at a high

2.2. Synthesis of PbS QDs via cation exchange PbCl2 (0.773 g) and oleylamine (9.2 ml) were mixed to a three-neck round-bottom flask and degassed under vacuum at 80 °C for 30 min. The flask was then heated to 140 °C by bubbling with argon and was maintained at this temperature for another 30 min. Then 200 mg CdS QDs in 6.67 ml ODE solution were injected swiftly into the reaction flask at a temperature range from 80 °C to 160 °C. The solution changed to a brownish color immediately as the evidence of the formation of PbS QDs. After the injection, the heating mantle was removed and the reaction was stopped naturally. At 75 °C, 9.3 ml toluene and 7.4 ml OA were added to purify QDs solution to replace the weakly bound oleylamine (OLA) at 40 °C followed by vigorous stirring for 20 min. Then the PbS QDs were purified several times using toluene and acetone, finally dispersed in toluene for device fabrication. 2.3. Device fabrication Commercial conducting FTO/glass substrates were cleaned ultrasonically with emulsifier, acetone, ethanol, and deionized water sequentially. After drying in nitrogen, a TiO2 compact layer was deposited on the well-cleaned FTO substrate. 0.2 M titanium diisopropoxide bis (acetylacetonate) ethanol solution was spin-coated on the clean substrate followed by sintering at 500 °C for 30 min, followed by immersion in a 40 mM TiCl4 aqueous solution for 30 min at 70 °C and drying at 500 °C for another 30 min 5–6 layers of PbS-TBAI exchanged QDs films were spin-coated onto the TiO2 layer. Onto the multiple TBAI layers, two layers of PbS-EDT exchanged QDs films were deposited. Finally, Au electrode was deposited by thermal evaporation at a rate of 0.02–0.03 nm/s under a pressure of 1–5×10−5 Torr to a thickness of 150 nm. An active size was defined by the mask as 123

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Fig. 1. Typical TEM images of the as-synthesized CdS QDs (a) and PbS QDs (b) with the injection temperature of 150 °C. HRTEM images of a single particle of CdS (c) and PbS (d).

Fig. 2. Absorption spectrum of CdS QDs (a) and PbS QDs at different injection temperatures in the solution phase (b). The short wavelength absorption was shown in the inset (b). The standard XRD patterns of resulting CdS QDs (c) and PbS QDs (d).

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However, in the current structure, the photogenerated electrons may sweep into the metal electrode instead of the TiO2 layer, reducing the carrier collection efficiency. Therefore, two different ligand-exchanged PbS QDs were applied as the photoactive layer. As depicted in the energy band diagram, PbS-TBAI exhibits a deeper work function than PbS- EDT because the band energies of QDs can be modified by ligand exchange as discussed elsewhere [26]. But the difference between the Fermi level and the conduction band in PbS-EDT is larger than that in PbS-TBAI. As a result, the formation of Fermi level balance between two different QDs at the PbS-TBAI/PbS-EDT interface will lead to a downward band-bending in EDT region while an upward bandbending in TBAI region. The band alignment demonstrates the role of PbS-TBAI layer as being the main absorption layer [31]. Under illumination, the high conduction band in the EDT layer provides energy barrier to prevent the photogenerated carriers (electrons) flow into the EDT layer, whereas the valence band bending in TBAI layer promotes photogenerated holes injection into the EDT layer. As a result, an increased photocurrent and enhanced device performance are obtained in this structure. On the basis of this structure, the PbS QDs obtained in cation exchange with the first excitonic absorption peak at 1040 nm were used to fabricate the heterojunction solar cells. The performances of PbSTBAI/PbS-EDT structure with different layer combination were presented in the Fig. 5. It can be seen that the Voc of the device in the 7layer TBAI is significantly lower and the Jsc in the 7-layer EDT layer is lowest, resulting in lower power conversion efficiency. In the combination layer of TBAI and EDT, the performance is increased by this tandem structure, the TBAI layer is considered to be the main absorber layer and the Jsc tends to be larger with the increase of the number of TBAI layers, when the TBAI layer reaches to 5, the optimal performance is achieved. TBAI-device is not efficient at the break-in vacuum condition, but the power conversion efficiency increase with time and become stabilized. This is an interesting phenomenon and the origin of this initial increase in performance as a result of short air exposures is still under investigation. For the PbS-TBAI/ PbS-EDT device, 5 layers of PbS-TBAI films were spin-coated onto the TiO2 layer followed by 2 layers of PbS-EDT films. Through optimization, a high power conversion efficiency of 7.56% was obtained in this improved architecture, the Jsc and Voc are measured to be 30 mA/cm2 and 0.49 V respectively with a FF of 52.8%, as shown in the Fig. 6. Using the cation-exchanged QDs, a very high Jsc approaching 30 mA/cm2 was reported by us, which is explained by the following reasons. First, the band offset that is caused by the insertion of PbS-EDT layer not only prevents electron flow from PbS-TBAI to the anode but also facilitates hole extraction to the anode as being discussed in the energy level diagram. Secondly, at the PbS-TBAI/ PbS-EDT interface, a small depletion region is formed owing to the band bending and provides a longer depletion region in the light

temperature. This phenomenon is similar to the reactions in a PbCl2/ TMS2S system [16]. The inset shows a PbS absorption spectrum with the injection temperature of 80 °C in the short wavelength region. It is obvious that no CdS characteristic peak (as shown in Fig. 2a) appears in this region, an evidence that CdS QDs converted to PbS QDs completely even at a relatively low temperature. Meanwhile, a high temperature is essential for a complete exchange reaction in a CdSe/ PbSe system. The phenomenon is consistent with the reports of faster Cd inter-diffusion in CdS than in CdSe [28]. In the cation exchange process, a reaction zone width will exist in which the Pb cation is the entering species and the Cd cation is the exiting species. The reaction zone width is larger for the CdS/PbS system than for the CdSe/PbSe system due to the faster Cd inter-diffusion, so a higher temperature is essential for increasing the reaction zone width to achieve a complete exchange reaction in CdSe/PbSe system. The corresponding XRD patterns (Fig. 2c–d) confirm the complete transformation from CdS (zinc blende) to PbS (rock salt) as no CdS residual peaks can be found in the PbS XRD spectrum. All of the diffraction peaks are consistent with CdS (JCPDS 75–0581) and PbS (JCPDS 65–0241). In the experiment, the different sizes of CdS QDs were achieved by varying the concentration of oleic acid in ODE as shown in Fig. 3. For 360 nm, 398 nm and 413 nm CdS QDs, the position of the first excitonic absorption peak of PbS QDs is shifted from 870 nm to 1247 nm at the same injection temperature of 150 ℃, indicating that size-tunable PbS QDs by varying the size of the starting CdS QDs was achieved. The structure of device is presented in Fig. 4a with illumination from the TiO2 side. The high density photocarriers are mainly produced in the combined diffusion length and depletion width region in the interfacial area between photoactive layer and electron acceptation layer. As shown in the cross-sectional FESEM image in Fig. 4b, a 40 nm-thick TiO2 layer is deposited onto the FTO, followed by the deposition of a PbS photoactive layer. Finally, the devices are completed by the deposition of an Au electrode in thickness of ~150 nm. The energy levels of the constituent materials are taken from the literature, as shown in the Fig. 4c [29,30]. In a classic heterojunction structure, electrons transfer from TiO2 to PbS QDs while holes transfer in an opposite direction due to the diffusion of the majority carriers caused by electron concentration gradient, inducing a built-in electric field with the direction from TiO2 to PbS QDs. The built-in electric field leads to minority carrier drift in a direction opposite to majority carrier diffusion direction, eventually, a balance between the drift current and the diffusion current is reached, and a space charge region is formed at the TiO2/PbS interface. Under illumination, the balance is broken by photogenerated carriers in the combined diffusion length and space charge region. Under the built-in electric field effect, the photogenerated electrons sweep into the TiO2 layer, while the holes move in an opposite direction, producing a photocurrent from TiO2 to PbS.

Fig. 3. Absorption spectra of CdS QDs in different size (a) and corresponding exchanged PbS QDs (b).

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Fig. 4. (a) Schematic of the devcie. (b) Cross-sectional FESEM image of the heterojunction solar cells. (c) Energy band diagram along with photocarrier transport mechanism in this structure. (d) The photograph of a typical device.

Fig. 6. J-V curve of the highest efficiency FTO/TiO2/PbS-TBAI/PbS-EDT/Au device, the performance after 110 days and the highest efficiency FTO/CdS/PbS QDs/MoO3/Ag device.

Fig. 5. J-V curve of different combination layers in FTO/TiO2/PbS-TBAI/PbS-EDT/Au devices.

absorption layer. Hence, it is possible to increase the thickness of photoactive layer without a compromise between light absorption and carrier extraction. The third, compared to the previously reported high performance PbS solar cells, larger size QDs with narrower bandgap are used. The Jsc is higher while the Voc is lower due to the increase of light absorption and lower gap between the Fermi level positions for larger QDs. To prove the good electrical properties of the cation-exchanged QDs in solar cells, devices were also assembled using another architecture, where CdS is employed as the electron acceptor materials, and a FTO/ CdS/PbS QDs /MoO3/Ag structure was formed. To eliminate the role of iodine in the solid-state ligand exchange, we chose the FTO/CdS/PbS QDs /MoO3/Ag structure in which Mercaptopropionic acid (MPA) was performed for stacking the QDs. Two different sizes of QDs, with the

first excitonic absorption peak at 1000 nm and 1100 nm respectively, were employed in the photoactive layer to form a graded, tandem and multi-junction solar cells. From the J-V curve in Fig. 6, the Jsc, Voc, FF and PCE are determined as 25.53 mA/cm2, 0.47 V, 49.2% and 5.78%, respectively. In this work, the cation-exchanged QDs devices typically show a 10% improvements in PCE compared to TMS2S synthetic QDs devices (with the highest efficiency of 5.22%) in our previous work [32]. The IPCE spectra in Fig. 7 confirms that the increased absorption in the range from ultraviolet to near infrared caused by the increased thickness of the photoactive layer is successfully translated into a collected charge advantage, contributing to a higher current. The IPCE increases rapidly and reaches 90% at ~450 nm while drops slowly at longer wavelength, suggesting a low recombination loss in the bulk of the film before reaching the depletion region. The devices are more 126

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Fig. 7. IPCE spectra for the same device.

efficient at a wavelength ranging from 400 nm to 1100 nm and the estimated band gaps of PbS QDs are ~1.13 eV. By using the in situ halide-passivated PbS QDs and the improved architecture, the devices exhibit excellent air-storage stability for over 110 days without encapsulation. The tested devices were stored in air in the dark, and the highest performance cell was tested after 110 days. The excellent stability in our device depends on three main aspects. First, the twofold advances passivation exists in QDs surface, including the in situ chloride passivation in QD synthesis and the iodine passivation in solid-state ligand exchange. Secondly, the new depletion heterojunction structure of the PbS-TBAI/PbS-EDT architecture is employed to optimize electron unidirectional transferring. Thirdly, the PbS-TBAI absorber layer is insensitive to oxygen and moisture during storage compared to the previous MoO3 interfacial layer, whose work function may decrease under oxygen and moisture condition. As shown in Fig. 6, the Jsc, Voc, and FF are presented as 30.96 mA/ cm2, 0.49 V, and 51.9% respectively, and the PCE has improved to 7.89% incredibly. To the best knowledge of the authors, a Jsc of 30.96 mA/cm2 is a particularly high current in PbS QDs heterojunction solar cells. 4. Conclusion PbS QDs have been successfully synthesized by exploiting a direct cation exchange reaction. The resulting QDs show comparable electrical properties with the TMS2S synthetic QDs for solar cells. Using the cation exchanged QDs, DH solar cells by a FTO/TiO2/PbS-TBAI/PbSEDT/Au architecture were fabricated successfully. The solar cells exhibit an increased performance and excellent air-storage stability. And a high efficiency of 7.89% was achieved after air-storage for over 110 days. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. U1632151 and 61076040), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 2012011111006), and the Open

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