TWI546412B - Metal sulfide deposition using alkylated thioureas - Google Patents

Description

Metal sulfide deposition using alkylated thiourea

The invention described herein is an N-monoalkylated thiourea using, for example, N-methylthiourea, N-ethylthiourea, N-isopropylthiourea, or N-tert-butylthiourea, A chemical bath deposition method in which a metal sulfide film is grown or deposited on a substrate.

Cadmium sulfide (CdS), considered to be a p-type cadmium telluride (CdTe), p-type disilicide copper indium gallium nitride (C, In, Ga) due to its wide band gap (2.42 eV), photoconductivity, and high electron affinity An excellent heterojunction of Se ₂ or copper indium bisulfide CuInS ₂ (CIS). It is widely used as a window material for high-efficiency thin film solar cells based on CdTe or CIGS. CdS has been used in other applications including electronic and optoelectronic devices. Many techniques have been used to form CdS films, such as thermal evaporation, RF sputtering, physical vapor deposition, pulsed laser evaporation, molecular beam epitaxy, electrodeposition, spray pyrolysis, metal organic chemical vapor deposition, continuous Ion layer adsorption reaction, screen printing, compact space vapor transport, and chemical bath deposition (CBD). CBD has the advantage of being simple, low temperature, and inexpensive large area deposition technology. In fact, the CBD improves the performance of the CdS window used in the previously mentioned solar cells, compared to all other methods mentioned previously. It has been reported that when chemical bath deposition is used to grow CdS windows, the highest efficiency of CdTe and CIGS solar cells can be obtained.

The latest technology for the photovoltaic industry to grow CdS, ZnS and InS films to be incorporated into CdTe and chalcopyrite solar cells involves immersing the substrate in water, CdSO ₄ , NH ₄ at about 60-80 ° C. In solution of OH, and thiourea. This method is commonly referred to as CBD or chemical bath deposition. The method has a narrow processing range before the colloidal CdS begins to aggregate, ultimately resulting in particle formation and a low quality film having at least one or more of the following undesirable features, namely having a pothole in the film and having a desired band gap with the film. Undesirable, undesired, generally speaking, the band gap is too thin. At the same time, this process produces a large amount of CdS waste because the bath is typically used only for one deposition and must then be rinsed from the bath. There is currently no known method for eliminating particle formation under CBD conditions.

The CBD process CdS well studied and has been directed at generally about 60-80 ℃, the substrate is immersed containing NH ₄ OH, CdSO _4, and thiourea solution. Most of the research focused on the physical properties of the membrane during membrane growth, but there are some studies on the deposition mechanism. It has been shown that homogeneous CdS precipitation can cause the formation of discrete particles by consuming cadmium and sulfides to prevent film growth. Ortega-Borges and Lincot, J. Electrochem. Soc., Vol. 140, No. 12, 3464-3473, December 1993, propose a specific surface reaction mechanism based on the formation of intermediates on the surface of the substrate. Adsorption, as an initial step in the formation of CdS by decomposition of such intermediates. Membrane growth is thermally activated by activation energy of about 85 kJ/mol, which may correspond to chemical steps associated with thiourea decomposition. Later, Dona and Herrero, J. Electrochem. Soc., Vol. 144, No. 11, 4081-4091, November 1997, using experimental data, improved this surface reaction model at least within the concentration range employed. Others have attempted to optimize film deposition by modeling these processes and using various chelating agents, cadmium salts, sulfide sources, and surfactants to manipulate the existing balance. It has not been shown to date to optimize CdS deposition by exploiting differences between reaction mechanisms to form metastable Cd surface complexes at the expense of homogeneous CdS formation.

The invention provides a chemical bath deposition method for forming a metal sulfide film on a substrate, which reduces the formation of colloidal metal sulfide on the substrate relative to the metal sulfide film growth, thereby allowing a wider processing range and growth. Large-area, high-quality metal sulfide films, and generally reduce the production of metal sulfide waste. This method adjusts the film growth to the desired rate. The method of the present invention is particularly useful for improving the quality of CdS and other metal sulfide films used in photovoltaic and other devices incorporating metal sulfide films. The present invention is highly suitable for integration into common industrial processes involving solar cells of CdS, ZnS, or InS layers. According to the present invention, there is provided a method for producing a metal sulfide film on a substrate by chemical bath deposition by immersing the substrate in a temperature suitable for producing a metal sulfide film on the surface of the substrate. a water-soluble source of hydroxide ions, a water-soluble metal salt of a sulfide metal to be deposited on a substrate, and an aqueous chemical bath solution of N-monoalkylthiourea, and having been removed from the solution for a period of time A substrate of a metal sulfide film. The N-alkyl thiourea will be wherein the R is an alkyl thiourea.

The experiment of Figure 1 illustrates the existence of a competitive process that produces optimum conditions, such as ammonia concentration. Ammonia, which is formed by complexes of Cd to prevent undesirable heterogeneous precipitation, by also pushing Cd (NH _{3) 4} ⁺² and _{Cd (OH) 2 (NH 3} ) ₂ balance towards Cd (NH ₃ ) ⁺² to slow the surface reaction. The equilibrium shown in Figure 1 is also carried out by using a multidentate ligand such as triethanolamine, ethylenediamine, ethylenediaminetetraacetic acid, nitrilotriacetic acid, cyano-complex, tartaric acid. The main reason for mixing these results with these chelating agents is that they are not only slowing down the formation rate of CdS particles by lowering the concentration of free Cd ⁺² , but also increasing the concentration of Cd(NH ₃ ) ₄ ⁺² and lowering Cd(OH). Concentration of ₂ (NH ₃ ) ₂ -thiourea complex and its rate of degradation to CdS on the surface.

In accordance with the present invention, it has been demonstrated that the difference in mechanism between the two sulfide delivery paths can be utilized to increase the heterogeneous process at the expense of the homogeneous process. The particle formation process, as shown in Figure 1, requires free S ^-2 from the catalyzed hydrolysis of a base of thiourea. Alternatively, the surface reaction of thiourea with Cd(OH) ₂ (NH ₃ ) ₂ is carried out by a mechanism similar to the nucleophilic substitution of an alkyl halide with a thiourea. As a nucleophile, sulfur forms an intermediate product of isothiouronium. The present invention demonstrates that the use of N-monoalkyl thiourea inhibits the rate of thiourea hydrolysis, but still allows reaction with cadmium hydroxide. It is believed that the steric hindrance of the alkylation of thiourea by the alkylation of adjacent nitrogen will slow the alkaline hydrolysis of thiourea, but still allow for the easy formation of the isothiouronium salt and its subsequent decomposition.

The invention is characterized in that the sulfide delivery system can also be used to grow ZnS and InS films other than CdS films. ZnS and InS films are generally grown in a similar manner to CdS films by utilizing an ammonium hydroxide buffer and a sulfide source CBD. Different from the growth of CdS films, there are much less studies on the growth of ZnS and InS films, but it is believed that both ZnS and InS have homogeneous and heterogeneous paths. The main difference is that for heterogeneous processes, clusters are used to form precipitates rather than molecules that precipitate on molecules. ZnS and InS are expected to be commercially available buffer layers due to the higher band gap and sales issues associated with the use of cadmium in solar cell manufacturing.

In the chemical bath deposition method of the present invention, any suitable N-monoalkylated thiourea of the following formula may be used.

Wherein R is an alkyl group. The R group in the N-alkylated thiourea generally comprises an alkyl group of 1 to 4 carbon atoms, preferably 2 to 3 carbon atoms and most preferably 3 carbon atoms. Examples of such a preferred N-monoalkylthiourea may include N-methylthiourea, N-ethylthiourea, N-isopropylthiourea, and N-t-butylthiourea. Mixtures of two or more N-monoalkylated thioureas may also be used if desired. In general, the amount of N-monoalkylthiourea in the aqueous chemical bath deposition solution is from about 0.01% to about 10% by weight based on the total weight of the bath.

The process of the invention may employ a water soluble source of any suitable hydroxide ion. Examples of such water-soluble sources of suitable hydroxide ions include, but are not limited to, ammonium hydroxide, alkyl ammonium hydroxide, and alkali metal and alkaline earth metal hydroxides, with ammonium hydroxide being preferred. In general, the water soluble source of hydroxide ions is present in the aqueous chemical bath solution in an amount from about 0.01% to about 10% by weight based on the total weight of the bath solution.

Any suitable water soluble metal salt can be employed in the process of the invention. Examples of such metal salts include, but are not limited to, cadmium salts, zinc salts, and indium salts such as sulfates, chlorides, bromides, iodides, nitrates, nitrites, acetates, phosphates, and perchlorines. The acid salt is preferably a sulfate of cadmium, zinc or indium. Generally, the amount of water soluble metal salt in the aqueous chemical bath solution will range from about 0.01% to about 1% by weight based on the total weight of the bath solution.

The metal sulfide film can be deposited on any suitable substrate depending on the intended use of the substrate. For example, metal sulfide films can be deposited on substrates intended for use in electronics, microelectronics, optoelectronics, and optoelectronics, and are particularly contemplated for use in substrates in thin film solar cells. Examples of such substrates include, but are not limited to, glass, ruthenium, p-type cadmium telluride (CdTe), p-type copper indium gallium diselide (Cu(In,Ga)Se ₂ ), and copper indium disulfide (CuInS). ₂ ) Substrate.

The process of the invention is carried out at any suitable temperature for any suitable period of time. In general, the process will be carried out at a temperature below 100 ° C, usually at a temperature of from about 50 ° C to about 90 ° C, more preferably from about 60 ° C to about 80 ° C. Generally, the process will be carried out for a period of from about 0.5 minutes to about 20 minutes, preferably from about 0.5 minutes to about 10 minutes, and even more preferably from about 0.5 to about 2 minutes.

The invention is illustrated by the following examples, but is not limited to the following examples.

The film was deposited on a commercial glass slide (25 x 75 mm). The glass slides were cleaned with detergent shortly before use, degreased with ethanol in an ultrasonic bath, and etched in a 4% HF solution. The bath solution used in the chemical bath deposition CdS method consists of 36.6 ml DI H ₂ O, 6.5 ml NH ₄ OH (28-30%), 5 ml CdSO ₄ (0.015 M CdSO ₄ ), and 2.5 ml thiourea (1.5 M). )composition. N-isopropylthiourea was dissolved in methanol/water solution due to solubility problems. A plumbing vessel is used to hold the bath solution, which is continuously stirred by a magnetic stir bar during the deposition process. The combined water, CdSO ₄ solution and NH ₄ OH were combined and warmed to 80 °C. At this time, each thiourea was added and the reaction start time = 0 minutes was set. The process is allowed to proceed for 2-10 minutes. The reaction was monitored by a UV-Vis spectrophotometer and stopped when the attenuation at 550 nm reached 0.8 AU (absorption unit) in a 1 cm cell. The resulting film on the glass substrate was rinsed in DI water, dried in air for 8 hours, and weighed. The microparticles were collected in a 1.6 um glass fiber filter paper and air dried at 100 ° C for 8 hours. The quality is recorded in Table 1.

The mass ratio of the film/particulate increases monotonically with increasing spatial volume of the monoalkylated thiourea. It is expected that the mass ratio will be further increased by the use of more sterically hindered monoalkylated thioureas.

The film was deposited on a commercial glass slide (25 x 75 mm). The glass slides were washed with detergent prior to use, degreased with ethanol in an ultrasonic bath, and etched in a 4% HF solution. The bath used in the chemical bath deposition CdS method was designated as solutions 154-1, 154-2 and 154-4. These solutions are the following components.

Solution 154-1: 36.6 g water, 5 ml 0.015 M CdSO ₄ , 6.5 ml NH ₄ OH (30%) and 5 ml 0.75 M thiourea (comparative solution).

Solution 154-2: 36.6 g water, 5 ml 0.015 M CdSO ₄ , solution 1545-1: 36.6 g water, 5 ml 0.015 M CdSO ₄ , and 5 ml 0.75 M thiourea and 5 ml 0.75 M N-methyl thiourea .

Solution 154-4: 36.6 g water, 5 ml 0.015 M CdSO ₄ , solution 1545-1: 36.6 g water, 5 ml 0.015 M CdSO ₄ , and 5 ml 0.75 M thiourea and 5 ml 0.75 M N-ethyl thiourea .

A plumbing vessel is used to hold the bath solution, which is constantly stirred by a magnetic stir bar during deposition. The combined water, CdSO ₄ solution and NH ₄ OH were combined and warmed to 80 °C. At this time, each thiourea was added and the reaction start time = 0 minutes was set. When CdS is deposited as described in the steps of "In Situ thickness measurements of chemical bath-deposited CdS", JR Mann et al., Solar Energy Materials & Solar Cells, 94, 333-337, (2010), by observing bare molybdenum (Mo Color change to monitor CdS deposition. When the molybdenum surface turns green, the glass substrate is removed. The CdS deposition time is approximately 3 minutes. Comparison of the X-ray photoelectron spectroscopy (XPS) of the grown film, the data in Figure 3 (using N-methylthiourea-containing solution 154-2) and the data in Figure 2 (using the thiourea-containing solution 154-1) It was shown that N-methylthiourea was used to incorporate the same amount of carbon and less nitrogen and oxygen as the prior art thiourea. Thus, the presence of an alkyl group in the N-alkyl thiourea unexpectedly did not result in an increase in carbon in the film. The SEM photograph in Figure 4 shows the use of solutions 154-2 and 154-4 containing N-methylthiourea and N-ethylthiourea to produce a smoother CdS film surface with solution 154-1 containing thiourea. A smoother surface absorbs light better, increases current density, and makes the battery more efficient.

Although the present invention has been described with reference to the specific embodiments thereof, it is understood that changes, modifications and variations may be made without departing from the spirit and scope of the invention. Accordingly, it is intended to embrace all such changes, modifications and changes in the

Figure 1 depicts the competitive process involved in depositing a metal sulfide film on a substrate;

Figure 2 is a spectrum of X-ray photoelectron spectroscopy (XPS) of a film grown using thiourea as a sulfur source;

Figure 3 is a spectrum of X-ray photoelectron spectroscopy (XPS) of a film grown using N-methylthiourea as a sulfur source;

Figure 4 is an SEM image of a CdS film on glass deposited using thiourea, deposited with N-methylthiourea, and deposited with N-ethylthiourea.

(no component symbol description)

Claims (20)

A method for producing a metal sulfide film on a substrate by chemical bath deposition, the method comprising immersing the substrate in an aqueous chemical bath solution at a temperature suitable for producing a metal sulfide film on the surface of the substrate The aqueous chemical bath solution contains a water-soluble source of hydroxide ions, a water-soluble metal salt of a sulfide metal to be deposited on a substrate, and an N-monoalkylthiourea of the formula wherein R is an alkyl group. , And removing the substrate having a metal sulfide film thereon from the solution, wherein the amount of N-monoalkylthiourea in the aqueous chemical bath solution is from about 0.01% to about 10% by weight based on the total weight of the aqueous chemical bath solution weight%. The method of claim 1, wherein the R group of the N-monoalkylated thiourea is an alkyl group having 1 to 4 carbon atoms. The method of claim 1, wherein the R group of the N-monoalkylated thiourea is an alkyl group having 2 to 3 carbon atoms. The method of claim 1, wherein the N-monoalkylated thiourea is N-isopropylthiourea. The method of claim 1, wherein the metal sulfide film produced on the substrate is selected from the group consisting of CdS, ZnS, and InS films. The method of claim 1, wherein the metal sulfide film produced on the substrate is a CdS film. The method of claim 1, wherein the water-soluble metal salt is metal sulfuric acid salt. The method of claim 1, wherein the water-soluble source of the hydroxide ion is ammonium hydroxide. The method of claim 1, wherein the water-soluble metal salt is a metal sulfate and the water-soluble source of the hydroxide ion is ammonium hydroxide. The method of claim 9, wherein the metal salt is cadmium sulfate. The method of claim 2, wherein the water-soluble metal salt is a metal sulfate and the water-soluble source of the hydroxide ion is ammonium hydroxide. The method of claim 3, wherein the water-soluble metal salt is a metal sulfate and the water-soluble source of the hydroxide ion is ammonium hydroxide. The method of claim 4, wherein the water-soluble metal salt is a metal sulfate and the water-soluble source of the hydroxide ion is ammonium hydroxide. The method of claim 5, wherein the water-soluble metal salt is a metal sulfate and the water-soluble source of the hydroxide ion is ammonium hydroxide. The method of claim 6, wherein the water-soluble metal salt is a metal sulfate and the water-soluble source of the hydroxide ion is ammonium hydroxide. The method of claim 1, wherein the substrate is a substrate for a photovoltaic cell substrate. The method of claim 16, wherein the substrate is a substrate for a solar cell. The method of claim 17, wherein the substrate is a substrate selected from the group consisting of p-type cadmium telluride, p-type copper indium gallium diselide, and copper indium copper sulfide substrate. The method of claim 9, wherein the substrate is a substrate selected from the group consisting of p-type cadmium telluride, p-type copper indium gallium diselide, and copper indium disulfide substrate for solar cells. The method of claim 1, wherein the method is carried out at a temperature of from about 50 ° C to about 90 ° C for a period of from about 0.5 to about 20 minutes.