Cu2ZnSnSe4 nanocrystals capped with S2− by ligand exchange: utilizing energy level alignment for efficiently reducing carrier rec ombination
© Wang et al.; licensee Springer. 2014
Received: 13 March 2014
Accepted: 8 May 2014
Published: 24 May 2014
In this work, we employed a convenient one-step synthesis method for synthesizing Cu2ZnSnSe4 (CZTSe) nanocrystals (NCs) in an excess selenium environment. This excess selenium situation enhanced the reaction of metal acetylacetonates with selenium, resulting in the burst nucleation of NCs at relatively low temperatures. The phase morphology and surface and optoelectronic properties of NCs before and after ligand exchange were discussed in depth. It was found that pure tetragonal-phase structure CZTSe NCs with approximately 1.7-eV bandgap could be synthesized. The removal of large organic molecules on CZTSe NCs after ligand exchange by S2− decreased the resistivity. The bandgap of the films after ligand exchange by 550°C selenization was also decreased due to better crystallinity. For potential application in CZTSe solar cells, we constructed an energy level diagram to explain the mutual effect between the absorption layer and CdS layer. Using cyclic voltammetry (CV) measurement, we found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of CZTSe films shifted down after ligand exchange. After energy level alignment at the CdS/CZTSe interface, a type I band alignment structure was more conveniently formed after ligand exchange. This structure acted as the barrier against injection electrons from ZnO to the CZTSe layer, and recombination would subsequently be depressed.
KeywordsOne-step synthesis Ligand exchange CZTSe NCs Energy level alignment
Currently, nontoxic and earth-abundant I2-II-IV-VI4 quaternary compounds such as Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) have been considered as the most promising ‘next-generation’ photovoltaic materials to substitute for CIGSe absorber materials, due to their excellent properties such as high absorption coefficients (1 × 105 cm−1) [1–3], suitable absorption bandgap for the solar spectrum, high radiation stability, and considerable cell efficiency [4–6]. Various methods have been used for the preparation of CZTSe materials, including physical methods [7–10] and wet chemical routes [11–15]. Wet chemical routes are more prevalent due to their convenient operability, achievable by using traditional instruments, and low cost. CZTSe nanocrystals (NCs) are usually covered with long alkyl chain ligands to shield the surface of the NC, which can realize homogeneous nucleation and enable easy solution processibility for fabrication. However, these ligands also act as an insulating layer that impedes efficient charge transfer and the dissociation of photogenerated excitons and lead to poor conductivity, limiting NC application in electronic and photoelectronic devices [16–18]. Many results had been recently published regarding the development of new ligand strategies to minimize interparticle spacing. Zhang et al. reported that optical absorption of NCs could be effectively improved after ligand removal . Lauth et al. reported that 3 orders of magnitude conductivity increase of CIGS NC films could be achieved after ligand removal and conductivity enhancement depends on the NC size accentuating the role of trap states and internal grain boundaries in ligand-free NC solids for electrical transport . Carrete et al. and Stolle et al. performed ligand exchange on CZTSe nanoparticles, finding that crystallization of NCs and cell performances could be promoted [21, 22]. Their works focused on improving the optical and electrical properties of CZTSe films to increase the photocurrent of the device, but there is no detailed study clarifying the band alignment between the CdS layer and the absorption layer after ligand exchange.
Herein, we employed a convenient one-step method to synthesize CZTSe NCs. The key feature of this synthesis was to use excess Se relative to Cu, Zn, and Sn and conduct the reaction at a relatively low temperature. All-inorganic CZTSe NCs were obtained by ligand exchange strategy using a simple metal-free chalcogenide compound [(NH4)2S] as the inorganic ligand. We showed the energy level movement of CZTSe films before and after ligand exchange. Using cyclic voltammetry (CV) measurements, we found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of CZTSe films shifted down after ligand exchange. Utilizing energy level alignment at the CdS/CZTSe interface, we constructed an energy level diagram to explain the physical mechanism of reducing recombination in CZTSe solar cells. This provides a different approach to the design of the absorption layer, which is generally not afforded by previous reports applying interface passivation and the control of trap states, focuses on the problem of recombination, and holds for a more convenient way to optimize interface properties.
Cupric(II) acetylacetonate [Cu(acac)2], zinc(II) acetylacetonate [Zn(acac)2], tin(IV) chloride tetrahydrate (SnCl4 · 4H2O), 2,4-pentanedione, triethylamine, perchlorethylene 1-dodecanethiol (DDT), and oleylamine (OLA) were purchased from Alfa Aesar (Ward Hill, MA, USA). Tetrabutylammonium hexafluorophosphate (TBAPF6) and sodium hydroxide (NaOH) were purchased from Aldrich (St. Louis, MO, USA). Toluene, N,N-dimethylformamide (DMF), and ethanol are of analytical grade. All water used was obtained from a Millipore Milli-Q purification system (Darmstadt, Germany). The chemicals were used in an as-received condition without further purification.
For the synthesis of Sn(acac)4, 20 mmol SnCl4 · 4H2O was dissolved in 15 ml deionized water. Under magnetic stirring, 90 mmol of 2,4-pentanedione (9 ml) was added and kept stirred for another 15 min. Then Sn(acac)4 was precipitated by the addition of triethylamine (6 ml). The resulting Sn(acac)4 was washed for several times by ethanol and water, then dried in the vacuum.
A typical synthetic procedure of CTZSe NCs is briefly described as follows: 1 ml OLA, 1 ml DT, and 2 mmol Se powder were placed in a three-neck flask and stirred to dissolve the Se powder. Once the Se powder was completely dissolved, 0.5 mmol Cu(acac)2, 0.25 mmol Zn(acac)2, 0.25 mmol Sn(acac)4, 1 ml DT, and 10 ml OLA were added under vigorous stirring. Then the mixture was placed in an oil bath at 240°C and maintained for 0.5 h. After that, the flask was rapidly cooled to room temperature, and the as-synthesized NCs were separated by precipitation with ethanol and collected by centrifugation at 9,500 rpm for 4 min. The supernatant was decanted. The precipitates were dispersed in hexane and further purified by ethanol for several times. The precipitates were dried under vacuum at room temperature.
The ligand exchange process was carried out according to the literature with some modification . Colloidal dispersion of CZTSe NCs with organic ligand was prepared in toluene, while the solution of CZTSe NCs with inorganic ligand was prepared in polar formamide (FA) immiscible with toluene. For a typical ligand exchange, 20 mg CZTSe NCs was dispersed into 3 ml toluene and 0.1 ml (NH4)2S was dissolved into 3 ml FA. Then the (NH4)2S solution in FA was mixed with the CZTSe NC dispersion in toluene. The mixture was stirred for about 10 min leading to a complete phase transfer of CZTSe NCs from toluene to the FA phase. The phase transfer can be easily monitored by the color change of toluene (black to colorless) and FA (yellow to black) phases. The FA phase was separated out followed by triple washing with toluene to remove any remaining nonpolar organic species.
The morphology of CZTSe NCs was characterized by transmission electron microscopy (TEM; JSM-2010, JEOL Ltd., Akishima-shi, Japan). The phase and crystallographic structure of the products were identified by X-ray diffraction (XRD; X'Pert Pro, Philips, Amsterdam, The Netherlands). The UV-visible (UV-vis) absorption spectra were obtained by using a UV-vis spectrometer (Lambda 35, PerkinElmer, Waltham, MA, USA). Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 360 FTIR spectrometer (Thermo Fisher Scientific, Inc., Waltham, MA, USA) using KBr pellets in the range of 4,000 of 400 cm−1. The Raman spectrum was recorded using a LABRAM-1B confocal laser micro-Raman spectrometer (HORIBA, Kyoto, Japan) with the wavelength of 632.8 nm. The resistivity was tested by the four-probe method on a digital source meter (Keithley 2400, Keithley Instruments, Inc., Cleveland, OH, USA). Mott-Schottky plots and CV curves were measured by using an electrochemical test station (Novocontrol Technologies, Montabaur, Germany).
Results and discussion
The successful synthesis of high-quality monodisperse quantum dots (QDs) must start with a swift and short nucleation from supersaturated reactants, followed by growth without further nucleation [24, 25]. In this study, this excess selenium situation significantly enhanced the reaction of the metal acetylacetonates [Cu(acac)2, Zn(acac)2, and Sn(acac)4] with selenium, resulting in a short nucleation stage. This synthetic tactic is advantageous over the typical hot-injection synthesis , which requires a relatively high injection temperature (usually above 250°C) to generate burst nucleation.
Figure 1c shows a high-resolution transmission electron micrograph (HRTEM) of CZTSe NCs. The average size of CZTSe NCs is about 3 nm. CZTSe NCs have better dispersibility. Figure 1d shows the UV-vis absorption spectrum of CZTSe NCs and the corresponding bandgap of CZTSe NCs. The bandgap of CZTSe NCs was estimated to be 1.76 eV by extrapolating the linear region of a plot of the squared absorbance versus the photon energy. This is mainly attributed to the small size and quantum confinement effect of CTZSe NCs .
Herein, Raman spectroscopy was further employed for phase analysis, as shown in Figure 3b. Because (NH4)2S is used during ligand exchange, the CZTSe nanocrystal thin film shows one weak peak of Cu2ZnSnS4 at around 333 cm−1 after ligand exchange. There are no characteristic peaks of other impurities detected. CZTSe thin films prepared by selenization shows three peaks of CZTSe with Raman shift at 172, 192, and 232 cm−1, in agreement with previous reports . These results further confirmed that the ligand exchange did not change the structure of CZTSe NCs. There are no observable secondary phases such as Cu2Se, SnSe, and Cu2SnSe3. The intensity of the Raman peaks increased largely after annealing due to the recrystallization of CZTSe NCs.
Energy level and resistivity of CZTSe NC thin films before and after ligand exchange by 550°C selenization
Before exchange (550°C)
After exchange (550°C)
where ϵ is the relative permittivity (dielectric constant) of the CZTSe films, ϵ0 is the vacuum permittivity, e is the elementary charge of an electron, ND is the donor density in CZTSe films, Efb is the flat-band potential, k is the Boltzmann constant, and T is the temperature; the carrier concentration is inversely proportional to the slope of 1/C−2 vs. E. It can be seen that the slope of CZTSe films after ligand exchange is smaller than that before ligand exchange, indicating that the carrier concentration increases after ligand exchange and the conductivity of CZTSe NC thin films would be improved.
Figure 5b shows the individual energy level of ZnO, CdS, and the absorption layer used for CZTSe solar cells. The HOMO-LUMO levels of the absorption layer by selenization before and after ligand exchange listed in Table 1 are determined from the onset oxidation and reduction potentials according to Equations 2 and 3. It can be seen that the HOMO and LUMO energy levels of the CZTSe layer shift downwards after ligand exchange. If CZTSe solar cells are structured, CZTSe, CdS, and ZnO are in close contact with each other to form a heterojunction. The carrier will transfer between these semiconductors until the three kinds of materials form the unified Fermi level and the heterojunction is in thermal equilibrium state. After ligand exchange, the conduction band of the CdS layer is above that of the CZTSe layer, which is in accordance with the real condition of the CZTSe solar cell. A type I band alignment is more conveniently formed at the CdS/CZTSe interface. This structure acts as the barrier against injection electrons from ZnO to the CZTSe layer, and recombination between majority carriers is not formed . Meanwhile, this structure acts as the barrier against photogenerated electrons in CZTSe, too. Photogenerated electrons cannot cross over the barrier if the height of this barrier at the CdS/CZTSe interface becomes over 0.4 eV. The height should be modestly controlled to keep Jsc constant . However, before ligand exchange, the conduction band of the CdS layer is below that of the CZTSe layer and a type II band alignment is formed at the CdS/CZTSe interface. This structure will cause recombination between majority carriers at the interface, and the entire recombination increases with increasing absolute value of conduction band difference between CdS and CZTSe layer . As a result, the open circuit voltage of the CZTSe solar cell will become higher after ligand exchange due to the type I band alignment structure and the depression of recombination.
In conclusion, we synthesized pure tetragonal-phase structure CZTSe NCs with the size of about 3 nm by a facile one-step synthesis. For potential application in CZTSe solar cells, the physical mechanism of utilizing energy level alignment for reducing recombination was discussed in depth after ligand exchange. It was found that the removal of large organic molecules on CZTSe NCs after ligand exchange by S2− decreased the resistivity. The bandgap of the films after ligand exchange by 550°C selenization was also decreased due to better crystallinity. HOMO and LUMO energy levels of CZTSe films both shifted down after ligand exchange, and a type I band alignment structure was more conveniently formed at the CdS/absorption layer interface in CZTSe solar cells. This structure acts as the barrier against injection electrons from ZnO to the CZTSe layer, and recombination will subsequently be depressed. Overall, the cell efficiencies relatively depend on the energy level alignment and ligand exchange will make great contribution in this aspect.
This project is supported by the National Natural Science Foundation of China (21203053, 21271064, and 61306016), the Joint Talent Cultivation Funds of NSFC-HN (U1204214), the New Century Excellent Talents in University (NCET-08-0659), the Program for Changjiang Scholars and Innovative Research Team in University (PCS IRT1126), the Natural Science Foundation of Shandong Province (ZR2011BQ011), and the Scientific Research Foundation of Henan University (SBGJ090510 and B2010079).
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