Article pubs.acs.org/cm
A Novel and Versatile Strategy to Prepare Metal−Organic Molecular Precursor Solutions and Its Application in Cu(In,Ga)(S,Se)2 Solar Cells Gang Wang,† Shuyang Wang,‡ Yong Cui,† and Daocheng Pan†,* †
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China ‡ College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China S Supporting Information *
ABSTRACT: A novel and versatile metal−organic molecular precursorbased solution approach and the fabrication of high efficiency Cu(In,Ga)(S,Se)2 solar cells are presented. Many types of metal oxides, hydroxides, and acetylacetonates (acac), such as Cu2O, ZnO, SnO, Sb2O3, MnO, PbO, In(OH)3, Cd(OH)2, Ga(acac)3, and so forth, can be easily dissolved in butyldithiocarbamic acid, forming thermally degradable metal−organic molecular precursor solutions. By developing a simple and green ethanol solution-processed route and tuning the chemical composition of the Cu(In,Ga)(S,Se)2 thin film, as-fabricated solar cells exhibit an average power conversion efficiency up to 8.8%.
KEYWORDS: CIGS, solar cells, molecular precursor, solution process, chalcopyrite
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INTRODUCTION Copper indium gallium selenide (CIGSe) thin film solar cells are regarded as the next generation solar cells due to their high conversion efficiency and prominent stability.1 Recently, the CIGSe solar cell reached a record efficiency of 20.3%, which is close to the efficiency of a multicrystalline silicon solar cell (20.4%).2 However, these high efficiency CIGSe solar cells are usually fabricated by a vacuum-based coevaporation approach,1,3−6 and their extremely high manufacturing cost has been a major obstacle to CIGSe commercialization. Today, a low-cost solution-based deposition technique is playing an increasingly important role in the fabrication of high efficiency CIGSe thin film solar cells.7−18 Both nanoparticle-based inks7−12 and molecular precursor-based solutions13−18 are particularly attractive for large-scale manufacturing due to their compatibility with ultrahigh-throughput deposition techniques, such as printing and casting, which are wellestablished in industry. Recently, several groups have exerted great efforts to fabricate CIGSe solar cells on the basis of solution-processed inorganic nanocrystals.7−12 The highest power conversion efficiency (PCE) up to 12% was reported in the laboratory for CIGS nanoparticle-based solar cells by Agrawal’s group.12 However, the nanoparticle-based deposition approach is encountering some problems, such as solubility, long-term stability, yield, ligand exchange, removal of ligands, and mass production.7−12 Compared with nanoparticle inks, the molecular precursor-based solution route has several advantages.19 The chemical composition of the thin film can be more precisely controlled via a true solution processing © 2012 American Chemical Society
route. More importantly, the synthetic and purification processes are relatively simple compared to those of nanoparticles. Its low temperature processability also provides a high potential for cost-reduction and minimization of environmental impact.13−20 The most successful molecular precursor-based solution route in terms of solar cell efficiency is the hydrazine-based deposition approach reported by Mitzi and co-workers.13−16,21,22 The hydrazine-based solution has excellent dissolubility for metal sulfoselenide in the presence of excessive selenium, which avoids the incorporation of unexpected impurities (e.g., carbon, oxygen, and chlorine) and a hightemperature postselenization step using toxic gases (e.g., H2Se).13−16,20−22 Recently, Mitzi et al. reported that the highest conversion efficiency of 15.2% was achieved for a pure solution deposition approach.16 However, the hydrazine-based deposition approach suffers from a fatal disadvantage that hydrazine is a highly toxic and explosive solvent that requires extreme caution during handling and storage. Other molecularbased solution approaches, on the other hand, usually require the addition of the organic binders in order to obtain a crackfree thin film, which leads to carbon contamination in the final absorber layer and is a severe detriment the efficiency of the solar cells.23−25 Therefore, it is highly desirable to develop a simple, safe, green, low-cost, and robust molecular precursorReceived: August 25, 2012 Revised: September 23, 2012 Published: September 26, 2012 3993
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Supporting Information Figure S2). The light intensity was calibrated to 100 mW/cm2 using a Newport optical power meter (model 842PE) certified by Newport. The external quantum efficiency curve was measured using a Zolix SCS100 QE system equipped with a 150 W xenon light source and a lock-in amplifier.
based solution approach to fabricate high efficiency CIGSe solar cells.
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EXPERIMENTAL SECTION
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Chemicals. Copper(I) oxides (Cu2O, 99.99%), indium hydroxide (In(OH)3, 99.99%), and gallium acetylacetonate (Ga(acac)3, 99.99%) were purchased from Aldrich. Zinc oxide (ZnO, 99.99%), tin(II) oxide (SnO, 99.9%), cadmium hydroxide (Cd(OH)2, AR), lead(II) oxide (PbO, 99.99%), antimony(III) oxide (Sb 2 O 3 , 99.99%), and manganese(II) oxide (MnO, 99.99%) were procured from Aladdin. Carbon disulfide (CS2, 99.9%), ethanol (CH3CH2OH, AR), 1butylamine (CH 3 (CH 2 ) 3 NH 2 , 99%), ammonium hydroxide (NH4OH, 25%), cadmium sulfate (CdSO4, 99%), and thiourea (NH2CSNH2, 99%) were obtained from Aladdin. All chemicals were used as received without any further purification. Preparation of Molecular Precursor Solution. First, 5.0 mL of chloroform, 1.8 mL of CS2 (∼30 mmol), and 3.0 mL of 1-butylamine (∼30 mmol) were mixed in a 25 mL conical flask under magnetic stirring at room temperature. Such solution was prepared in eight flasks. Afterward, Cu2O (0.2146 g, 1.5 mmol), In(OH)3 (0.4975 g, 3 mmol), ZnO (0.2442 g, 3 mmol), SnO (0.4041 g, 3 mmol), Cd(OH)2 (0.4393 g, 3 mmol), PbO (0.6696 g, 3 mmol), Sb2O3 (0.4373 g, 1.5 mmol), and MnO (0.2128 g, 3 mmol) were respectively loaded into each flask with heat treatment at 60 °C until all the solid dissolved. Preparation of CIGS Precursor Solution. First, 10.0 mL of ethanol, 3.6 mL of CS2 (∼60 mmol), and 6.0 mL of 1-butylamine (∼60 mmol) were mixed in a 50 mL conical flask under magnetic stirring at room temperature. Afterward, In(OH)3 (0.5804g, 3.5 mmol) and Ga(acac)3 (0.5505g, 1.5 mmol) were loaded into the flask with heat treatment at 60 °C for 30 min until all the solid dissolved. Then, Cu2O (0.3220 g, 2.25 mmol) was added to the above solution under stirring for 60 min, yielding a homogeneous light red solution. Finally, a sticky solution was obtained by removal of organic solvents under vacuum in a water bath at 80 °C and diluted to a total metal concentration of 0.4 M with ethanol. The solution was kept under continuously vigorous stirring for 8 h at room temperature, followed by centrifugation at 12 000 rpm for 5 min to remove a thimbleful of insoluble solid. These procedures were conducted in the air, and the mixed CIGS precursor solution was stored in a sealed glass vial and can be used in six months. Deposition of CIGSSe Absorber Layer and Fabrication of Solar Cell Device. CIGS precursor thin film was spun on a Mocoated coverslip (20 × 20 × 0.5 mm) at 2000 rpm for 30 s, followed by a sintering process on a 350 °C hot plate for 3 min to form CIGS nanocrystal thin film. The CIGSSe thin film with a thickness of ∼1.2 μm was obtained by seven spin-castings. The films were then annealed at 400 °C for 10 min prior to selenization. Subsequently, two films and 20 mg of selenium powder were sealed in a test tube (25 mm in diameter and ∼30 mL in volume) under vacuum, followed by a selenization process at 540 °C for 1 h in a furnace. The solar cell device was completed by chemical bath, depositing 60 nm of CdS thin film.26 The CdS buffer layer was grown in an aqueous solution containing 100 mL of deionized H2O, 12.5 mL of NH4OH, 50 mL of cadmium sulfate (0.006 M), and 50 mL of thiourea (3.0 M) in a 65 °C water bath for 15 min. This was followed by RF-sputtering 70 nm of intrinsic ZnO, DC sputtering 250 nm of indium tin oxide (ITO), and thermally evaporating 2 μm of Al grid electrode. The four devices with an active area of 0.368 cm2 were isolated by mechanical scribing with a tungsten needle. Characterization. The XRD patterns were recorded using a Bruker D8 X-ray diffractometer. The scanning electron microscope (SEM) images were taken on a Hitachi S-4800 equipped with an energy dispersive X-ray (EDX) analyzer (Bruker AXS XFlash detector 4010). The sample for EDX measurement was prepared on a glass slide. Thermogravimetric (TG) analysis was performed on a TGA/ DSC 1 STARe of Mettler-Toledo. The thickness of the thin film was measured by a step profiler (AMBIOS, XP-100). I−V curves were measured with a Keithley 2400 source meter and a solar simulator (Abet Sun 2000; AM 1.5) by a homemade probe station (see
RESULTS AND DISCUSSION Here, we present a novel and versatile solution route to prepare butyldithiocarbamic acid-based metal−organic molecular precursor solution. Butyldithiocarbamic acid (BDCA) is relatively nontoxic, inexpensive, and thermally degradable and can be easily synthesized by the reaction of 1-butylamine with carbon disulfide in situ.27−29 Figure 1, top, shows the reaction
Figure 1. (top) Reaction mechanism of metal oxide with butyldithocarbamic acid. (bottom) Digital photograph of Cu2O, ZnO, SnO, In(OH)3, Sb2O3, Cd(OH)2, MnO, and PbO dissolved in chloroform with the aid of butylamine and carbon disulfide (∼0.35 M metal concentration).
mechanism of metal oxides with butyldithiocarbamic acid. The chloroform solution of BDCA can be directly used to dissolve many types of metal oxides and hydroxides, such as Cu2O, ZnO, SnO, Sb2O3, MnO, PbO, In(OH)3, Cd(OH)2, and so forth, forming a variety of metal−organic precursor solutions (see Figure 1). The multiple component semiconductor absorber layer can be fabricated by mixing different metal precursor solutions. Hereby, we develop a simple, green, versatile, and low-cost butyldithiocarbamic acid-based ethanol solution route to deposit high quality CIGSSe absorbers, and as-fabricated solar cells exhibit an average power conversion efficiency up to 8.8% which is the highest value for hydrazinefree molecular precursor solution-processed CIGSSe solar cells. In this paper, Cu2O, In(OH)3, and Ga(acac)3 (acac = acetylacetonate) were used as the starting materials, and the mixed Cu(In,Ga)S2 (CIGS) precursor solution can be directly used to deposit CIGSSe thin film without special purification. Note that CIGS precursor solution is stable for at least six months in the air and highly soluble in common organic solvents; thus, nontoxic ethanol was used as the solvent. The butyldithiocarbamic acid-based technique involved three simple steps performed to produce high quality CIGSSe thin film: (1) dissolved In(OH)3, Ga(acac)3, and Cu2O in a ethanol solution of butyldithiocarbamic acid at 60 °C and then adjusted total metal concentration to 0.4 M using ethanol; (2) deposited the CIGS thin film on a Mo-coated sodalime glass substrate at 2000 rpm for 30 s followed by annealing on a 350 °C hot plate for 3 min under an inert atmosphere; and (3) formed CIGSSe absorber layer by a selenization process at 540 °C for 60 min in a sealed test tube. Figure 2 presents thermogravimetric analysis (TGA) data for the mixed CIGS precursor (Cu/(Ga + In) = 0.9, Ga/(Ga + In) = 0.3). The TGA sample was prepared from the CIGS precursor solution by removal of all the small organic molecules 3994
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Figure 2. TG curve of the CIGS precursor (Ga/(Ga + In) = 0.3, Cu/ (Ga + In) = 0.9). Inset: digital photograph of the CIGS precursor dissolved in ethanol (a total metal concentration of 0.4 M).
under vacuum at 80 °C. We found that these CIGS precursors start to decompose at 150 °C, and their weight loss occurs relatively fast between 170 and 260 °C. Additionally, the TGA profile shows that the CIGS precursors are completely decomposed into CIGS at 400 °C. The thermal decomposition mechanism of the metal butyldithiocarbamates has been extensively investigated in the literature.30,31 As-fabricated CIGS thin film is selenized to form CIGSSe absorber layer under selenium vapor at 540 °C for 60 min. The crystal structures of as-prepared CIGS thin film and selenized CIGSSe thin film were examined by X-ray diffraction (XRD). As-fabricated CIGS thin film exhibits very broad diffraction peaks, as shown in Figure 3, indicating that the
Figure 4. (a) SEM top view image of CIGSSe absorber layer after selenization at 540 °C for 60 min. (b) SEM cross-sectional image of CIGSSe absorber layer after selenization.
extremely dense and compact, which is desirable for high efficiency devices. Note that the pinhole and crack have not been observed even under low resolution SEM observation (see Supporting Information Figure S1). For the fabrication of butyldithiocarbamic acid-based CIGSSe solar cells, metal stoichiometry was selected to yield a Cu/(Ga + In) ratio of 0.9 and a Ga/(Ga + In) ratio of 0.3, similar to those of previously reported record devices.1,3−6,16 Typically, the absorber layer of Cu-poor and In(Ga)-rich is desirable in a CIGS device in order to eliminate the unwanted secondary phase CuxSe and improve the separation of the photogenerated charge carriers.7−18 The chemical composition of the as-fabricated CIGS thin film is Cu0.88In0.68Ga0.32S1.80, determined by averaging 10 random spots using energy dispersive X-ray spectroscopy (EDS) (Figure 5), and the selenized CIGSSe film has a Cu/(In + Ga) ratio of 0.95 and a Ga/(In + Ga) ratio of 0.3, which are close to those starting precursor ratios and consistent with those previously reported high efficiency CIGSSe devices.1,3−6,13−16 The ratio of S/(S + Se) is approximately 0.45, which is significantly higher than those of CIGSSe thin films in the literature,13−16 indicating that a larger band gap is obtained. The CIGSSe solar cell device was fabricated according to the conventional configuration of glass/Mo/CIGSSe/CdS/i-ZnO/ ITO/Al. The current density−voltage (J−V) characteristics for a typical CIGSSe solar cell measured in the dark and under AM 1.5 illumination are shown in Figure 6a. The device performance parameters are reported based on the active area
Figure 3. X-ray diffraction patterns for as-prepared CIGS thin film (red line) and CIGSSe absorber layer after selenization under Se vapor (black line).
CIGS thin film is comprised of CIGS nanocrystals. The crystal structure of CIGS nanocrystal thin film is undistinguishable by XRD pattern owing to their weak and broad diffraction peaks. After selenization, the diffraction peaks systematically shift to the lower angle due to the expansion in the unit cell volume with the replacement of S by Se (Figure 3a). Note that CIGSSe thin film possesses a chalcopyrite structure. Also, the X-ray diffraction peaks evidently sharpen, indicating that large densely packed grains have been obtained after annealing under Se vapor (Figure 3, black line), which is highly desirable for high efficiency devices. In addition, numerous minor peaks such as (101), (211), and (105) become observable after selenization. The replacement of S by Se can shrink the band gap of CIGSSe, which causes an increase in the short-circuit current.7−18 Figure 4 displays top-view (a) and cross-sectional (b) fieldemission scanning electron microscope (FE-SEM) images for the selenized CIGSSe thin film. The surface morphology of the selenized CIGSSe thin film reveals that the film surface is 3995
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al. has an absorber thickness > 1.9 μm.16 The calculated Jsc from EQE spectrum is 22.12 mA/cm2, which is slightly higher than the measured Jsc under AM 1.5 illumination. The band gap of CIGSSe absorber estimated from the EQE curve is around 1.36 eV, which is significantly larger than those reported for high efficiency CIGSSe solar cells (∼1.15 eV).1,3−6,13−16 The increase in band gap energy is due to the replacement of Se by S compared to poor-sulfur CIGSSe solar cells.13−16 It is expected that a significant improvement in device performance could be achieved by increasing the thickness and decreasing the band gap of the absorber layer,16 and further work in these topics is in progress in our lab. In addition, a graded band gap absorber can also effectively improve the PCE of the CIGSSe solar cells.1,3−6,13−16
Figure 5. EDS spectra and chemical compositions of as-prepared Cu(In,Ga)S2 thin film and Cu(In,Ga)(S,Se)2 thin film after selenization.
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CONCLUSIONS In summary, a novel and versatile solution approach has been demonstrated for fabricating metal−organic molecular precursor-based solution and high efficiency Cu(In,Ga)(S,Se)2 solar cells. Low-cost metal oxides and hydroxides, carbon disulfide, and butylamine were used as the starting materials, and nontoxic ethanol was chosen as the solvent. The asfabricated photovoltaic devices exhibit an average efficiency of 8.8%, which is the highest reported efficiency for hydrazine-free molecular precursor solution-processed CIGSSe solar cells. The chemical composition of the final CIGSSe absorber layer can be precisely controlled by changing the starting metal precursor ratio. In addition, a variety of metal cations, such as Li+, Na+, K+, Sb3+, Fe3+, Mn2+, Ni2+, Zn2+, Pb2+, Co2+ and so forth, can also be incorporated in this system in terms of precursor solution formation, allowing their effects on the CIGSSe device performance to be systematically investigated. Furthermore, this versatile solution approach could be extended to produce organic−inorganic hybrid solar cells, quantum dot-sensitized solar cells, and earth abundant element thin film solar cells Cu2(MII)(MIV)(S,Se)4 (MII = Zn, Cd, Fe, Co, Ni, Mn; MIV = Sn). Overall, we have already opened a broad avenue to fabricate many types of metal chalcogenide thin films via a simple and green solution-processing route. These air stable precursor solutions can be widely used for the fabrication of optoelectronic and electronic devices, including solar cells, light-emitting diode, transparent conductive sulfides, thin-film transistors, and nonlinear optical thin films.
Figure 6. (a) J−V curvers of one of the CIGSSe (Ga/(In + Ga) = 0.3) solar cells measured in the dark and under AM 1.5 illumination. (b) External quantum efficiency (EQE) spectrum of the corresponding CIGSSe (Ga/(In + Ga) = 0.3) solar cell; inset: the band gap of the absorber was calculated to be ∼1.36 eV by extrapolation.
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of 0.368 cm2, excluding the area of the Al grid electrode (∼10% of total device area). The as-fabricated device shows a PCE of 8.75% (Voc = 0.609 V, Jsc = 20.86 mA/cm2, FF = 68.9%), with a best value of 8.92%. The mean PCE is 8.8% by averaging the 10 best cells. The overall PCE is limited owing to a thin absorber layer, resulting in a substantial decrease in current density. The shunt conductane (G) and series resistance (Rs) obtained from a plot of dJ/dV against V and dV/dJ against (J + Jsh)−1 are 0.25 mS/cm2 and 2.8 Ω/cm2, respectively.32 Figure 6b shows the external quantum efficiency (EQE) spectrum of the corresponding CIGSSe solar cell device. The quantum efficiency in the visible range is promising (∼80%); however, it drops off significantly in the near-IR range. The loss of quantum efficiency in the longer wavelength is probably due to a nonoptimal thickness of the CIGSSe absorber layer in the current devices. Typical absorption coefficients reported for CIGSSe thin films are in the range of 104−105 cm−1, which require a thicker absorber to effectively absorb the solar light and reduce recombination at the back contact. The hydrazinebased high efficiency CIGSSe solar cell reported by Todorov et
ASSOCIATED CONTENT
* Supporting Information S
SEM image of selenized CIGSSe thin film and a simple and useful homemade probe station. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Corresponding Author
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21071142; 51172229; 51202241) and the Fund for Creative Research Groups (Grant No. 20921002). 3996
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