TWM471360U - A precursor structure of cigs thin film solar cells - Google Patents

Abstract

We report a precursor structure of CIGS to resolve the problem of overly higher roughness of Cu(In,Ga)Se2 (CIGS) forming in the CIGS absorber layer caused by sputtering and the inhomogeneous distribution of elemental Ga during selenization in the same time period. Results demonstrate that the proposed precursor structure of CIGS is superior to conventional sputtering structure for its ability to reduce surface roughness in the CIGS absorber layer following a two-stage selenized annealing process and resolves inhomogeneous distribution elemental Ga in CIGS cells. Using experimental analysis, we explored the mechanism underlying the increase in grain size in the CIGS absorber layer.

Description

Precursor layer structure of CIGS thin film solar cell

The present invention relates to the fabrication of a solar cell component, and more particularly to a pre-layered structure of a CIGS thin film solar cell.

A thin film solar cell is a material having a chalcopyrite phase as its absorber layer, such as copper indium gallium selenide (CuIn _x Ga _1-x Se ₂ ). Therefore, a thin film solar cell using a material of such a chalcopyrite crystal phase as a light absorbing layer is also referred to as an I-III-VI compound semiconductor. Copper indium gallium diselenide (Cu(In,Ga)Se ₂ ,CIGS) is a material property of a direct energy gap of a chalcopyrite phase in a thin film solar cell. It has stability, high absorption coefficient, high efficiency, low cost and flexibility. The advantages of the song, etc., were identified as the most potential thin film solar cells.

In the near future, high-efficiency CIGS solar cells can be obtained by three-stage co-evaporation method. However, in order to improve the application surface and expand mass production, large-area and low-cost are inevitable, so the development of the pre-layer-selenization process has been carried out. Most of the products are made of metal and alloy layers, such as pure elements and alloys such as copper, indium, gallium and selenium, and finally crystallized in the environment of H ₂ Se and H ₂ S, but the CIGS is currently completed by this process. The film, its crystal structure still has the following problems: (1) the surface roughness of the precursor layer film is too high, resulting in the post-selenization film crystal growth can not obtain a good flat surface and growth direction control, (2) to the subsequent buffer layer CdS In contrast to the coating of the light window layer, etc., it must be covered with a considerable thickness, otherwise the open circuit voltage (Voc) and the fill factor (FF) of the device will be lowered, and (3) the selenization of the front layer will be carried out. The Ga atoms are prone to diffusion unevenness and cause a large amount of accumulation at the back electrode, so that the surface energy gap of the CIGS film is too low. The relevant source numbers correspond to the following:

[related source]

(1) Tung-Po Hsieh, Chia-Chih Chuang, Chung-Shin Wu, Jen-Chuan Chang, Jhe-Wei Guo, Wei-Chien Chen, Effects of residual copper selenide on CuInGaSe ₂ solar cells, Solid-State Electronics, 56 (2011), 175-178

(2) G.M. Hanket, W.N. Shafarman, B.E. McCandless, R.W. Birkmire, J. Appl. Phys. 102 (2007), 074922.

(3) Lundberg, M. Edoff and L. Stolt, The effect of Ga-grading in CIGS thin film solar cells. Thin Solid Films, 2005, 480-481: 520-525.

This paper proposes a pre-layered structure of CIGS thin film solar cells. The main purpose is to obtain a flattened CIGS thin film solar cell absorber film and improve the deposition of Cu(In,Ga)Se ₂ ,CIGS precursor layer by sputtering. After selenization, the surface roughness of the previous drive layer after selenization is too high and the Ga atoms are unevenly diffused.

The creation structure mainly stacks the CIGS thin film solar cell precursor layer through two layers of different process In layers and one CuGa layer; two layers of different process In layers, one of which is represented by In _(E) after vapor deposition, and the other is splashed. After plating In, it is represented by In _(S) , and its precursor layer CIG film stack structure is In _(E) /In _(S) /CuGa, and the surface of the absorption film of CIGS thin film solar cell is observed through two-stage selenization annealing. Morphology and the influence of Ga concentration distribution, it is found that the In _(E) /In _(S) /CuGa structure can effectively reduce the surface roughness of the CIGS thin film solar cell absorber film after selenization, mainly by evaporation ( In _(E) ) flat surface improves the original Sp-In island structure and greatly reduces the surface roughness of CIGS. In addition, the flatter In surface reduces the diffusion of In onto the surface of the absorption layer of the CIGS thin film solar cell, thereby reducing the generation of a large amount of In _x Se _y compound during the selenization process, which not only causes the liquefied Cu _2-x Se phase to help the CIGS crystal grow. It can also help the Ga atoms to uniformly diffuse and promote the growth to a larger grain size and flattened chalcopyrite structure.

<Example>

1 is a schematic cross-sectional view showing a front layer structure of a CIGS thin film solar cell. The precursor layer seeks to reduce surface roughness by means of vapor deposition of indium. The preferred structure comprises the following structure: (a) a substrate, The substrate 101 of a bi-layer Mo back electrode layer 102 is sputtered in a vacuum environment, wherein the substrate may be glass, stainless steel or other hard materials, or may be a flexible substrate such as a plastic substrate or a titanium substrate. a metal substrate; (b) a deposited layer of indium (in _(E)), can be deposited through the Mo back electrode layer 102 a in a vacuum environment, a in _(E) layer 103a is completed, the creation of which evaporation The metal layer is not limited to the indium layer, and may be a copper zinc tin sulfide layer, or may be a binary powder composed of copper indium gallium selenide or copper zinc tin sulfide (eg, copper selenide, indium selenide, gallium selenide, sulfurized). Zinc, copper sulfide, tin sulfide, etc.), ternary powder (such as copper indium gallium, copper indium selenide, copper zinc tin, zinc tin sulfur, etc.); (c) a CuGa layer, which can be transmitted through a vacuum environment In _(S) In a layer 103b or sputtering a layer 104 is completed on CuGa _(E) layer 103a, which may be a sputtering layer composed of a sulfur copper zinc tin bis Targets (such as copper selenide, indium selenide, zinc sulfide, copper sulfide, tin sulfide, etc.), ternary targets such as copper indium gallium, copper indium selenide, copper zinc tin, zinc tin sulfur, etc., the number of stacked layers The layer may be a single layer or more; and (d) the precursor layer may be annealed in a vacuum environment to form a CIGS thin film solar cell absorber layer 105. The annealing atmosphere may be selenium vapor, sulfur vapor, hydrogen selenide, sulfurization. Hydrogen or a combination of the above.

After the preparation of the CIGS thin film solar cell absorption layer 105 film of the present embodiment, the buffer layer cadmium sulfide (CdS) can be stacked by chemical bath deposition (CBD), and then the window layer is intrinsic zinc oxide (intrinsic-ZnO). ), the aluminum oxide zinc oxide (AZO) is sputtered on the window layer with a low-resistance transparent conductive layer, and finally the nickel-aluminum wire is sputtered as an upper electrode to complete the components of the CIGS thin film solar cell.

<Comparative example>

2 is a schematic view showing a comparative example of a front layer structure of a CIGS thin film solar cell. The precursor layer is a comparative example of sputtering through indium. The comparative example precursor layer comprises the following steps: (a) a vacuum environment Sputtering a bi-layer Mo back electrode layer 202 on the substrate 201; (b) sputtering an In _(S) layer 203 on the Mo back electrode layer 202 in a vacuum environment; (c) in a vacuum environment The In _(S) layer 203 is sputtered with a CuGa layer 204; and (d) annealed in the same atmosphere as in the embodiment to form a CIGS layer 205.

<Analysis data>

From the comparative example of the SEM Fig. 3(a) example (b), it can be seen from the cross-sectional morphology that there are two distinct types of microstructures before the selenization, and In uses the evaporation technique and from Figure 3 ( a) The example shows that In has no island growth, so it has a smoother surface, and the comparative example of Fig. 3(b) is easy to produce this island structure because of the low melting point temperature and high surface tension property during deposition (Island). Structure). Figure 4 (a) Example (b) Comparative Example is the surface morphology and cross-sectional surface topography of the film after heat treatment The SEM image shows that after two-stage selenization annealing heat treatment, the island growth is still quite obvious. The surface roughness was observed by the Atomic Force Microscope (AFM) from the example of Fig. 5(a) to be RMS: 61.17 nm; however, the roughness of the Sp-plated In structure was as shown in the comparative example of Fig. 5(b). After the selenization of CIGS, the roughness RMS is as high as 117.86 nm. Therefore, the island structure caused by sputtering of indium is likely to form an uneven and rough surface morphology to reduce the film quality, which is disadvantageous for the fabrication of the rear component stack. The effects of the innovations of the examples and comparative examples are confirmed, and the reason for the formation is further explained below by the schematic diagram of the process of selenization annealing.

Figure 6 (a) Example SLG / Mo / In _(E) / CuGa and (b) Comparative Example SLG / Mo / In _(S) / CuGa is a schematic diagram of the process after two-stage post-selenization annealing, because Figure 6 (b) In the comparative example, sputtering has a rough surface, so In is easily diffused to form an InSe phase, and a process of sputtering In forms a large number of In _x Se _y phases (InSe ₂ and In ₂ Se ₃ ), and Cu _{2- x} Se is the liquefied phase and the In ₂ Se ₃ solid phase will exist simultaneously in the CIGS crystal growth process, since In _x Se _y will hinder CIGS nucleation; Figure 6 (a) embodiment is SLG/Mo/In _(E) / After the crystallization process of CuGa after two-stage selenization annealing, since the surface formed by vapor deposition of In is relatively flat, In is less likely to diffuse to form the In _x Se _y phase, so in the case where there is no In _x Se _y phase, steaming is used. Plated In can achieve better quality for the surface of CIGS.

Since the CIS film has a small forbidden band width and cannot absorb high-energy photons, Ga is added to the film to increase the energy gap of the film. However, in the process of CIGS solar cells, Ga is diffused toward the back electrode. It is easy to cause the content of Ga on the surface of the film to be insufficient, and it is difficult to increase the energy gap of the surface of the film. Figure 7(a) and (b) Comparative Example SIMS depth profile, Figure 7(a) In the embodiment, it can be seen that there is a fixed content of Ga on the surface of the film, and a stable concentration distribution is maintained from the surface of the film to the back electrode, and the content of Ga is increased at a depth of 1 μm. This gradually increasing band gap forms an electric field. Pushing electrons into the space charge region, thereby reducing the recombination of electrons in the back electrode, the minority carriers that generate current increase, and the current increases; as shown in Figure 7(b), the surface Ga content of the film is obviously insufficient, because both Diffusion in the direction of the back electrode, this composition gradient forms a reverse electric field which is not conducive to the collection of carriers, so in this case, the short-circuit current (Jsc) and the fill factor (FF) cannot be improved; The structure can avoid the increase of Ga to the back electrode during the selenization process, and can maintain the content of Ga on the surface of the film, which can improve the absorption efficiency of photons.

In summary, through the creation of the CIGS thin film solar cell precursor layer structure, the grain growth of the absorption layer can be improved, thereby improving the problem that the crystal of the CIGS absorption layer is too small after selenization of the precursor layer.

101‧‧‧Stained glass substrate (Soda-lime glass, SLG)

102‧‧‧bi-layer structure back electrode (molybdenum electrode) film layer

103a‧‧‧Indium (In _(E) ) coating

103b‧‧‧spray indium (In _(S) ) film

104‧‧‧copper gallium alloy (70-30at%) film layer

105‧‧‧CIGS thin film solar cell absorption layer

201‧‧‧Stained glass substrate (Soda-lime glass, SLG)

202‧‧‧bi-layer structure back electrode (molybdenum electrode) film layer

203‧‧‧Indium (In _(S) ) coating

204‧‧‧copper gallium alloy (70-30at%) film layer

205‧‧‧ CIGS thin film solar cell absorption layer

1 is a schematic cross-sectional view showing a structure of a precursor layer of a CIGS thin film solar cell according to a preferred embodiment of the present invention.

2 is a schematic view showing a comparative example of a structure of a precursor layer of a CIGS thin film solar cell according to a comparative example of the present invention.

Figure 3 SEM image of the structure before selenization in different In processes

(a) SLG/Mo/In _(E) /CuGa (b)SLG/Mo/In _(S) /CuGa

Figure 4 SEM image of the structure after selenization in different In processes

(a) SLG/Mo/In _(E) /CuGa (b)SLG/Mo/In _(S) /CuGa

Figure 5 AFM diagram of different In process structures after selenization

(a) SLG/Mo/In _(E) /CuGa (b)SLG/Mo/In _(S) /CuGa

Figure 6 (a) SLG / Mo / In _(E) / CuGa (b) SLG / Mo / In _(S) / CuGa

Comparison of the growth changes of selenium CIGS in different In process structures

Figure 7 (a) SLG / Mo / In _(E) / CuGa (b) SLG / Mo / In _(S) / CuGa

SIMS depth analysis after annealing in different In processes

101‧‧‧Stained glass substrate (Soda-lime glass, SLG)

102‧‧‧bi-layer structure back electrode (molybdenum electrode) film layer

103a‧‧‧Indium (In _(E) ) coating

103b‧‧‧spray indium (In _(S) ) film

104‧‧‧copper gallium alloy (70-30at%) film layer

105‧‧‧CIGS thin film solar cell absorption layer

Claims (8)

A pre-layered structure of a CIGS thin film solar cell comprises: a flat surfaced indium oxide layer (In _(E) ) and a CuGa layer; the vapor deposited indium layer can be deposited on a substrate coated with a Mo film, and a CuGa layer is deposited On the indium-plated layer (In _(S) ), the precursor layer structure In _(E) /CuGa is formed. The layer structure is as described in claim 1, wherein the material of the precursor layer is copper, indium, gallium, zinc or tin applied to the material of the thin film solar cell. The layered structure is as described in claim 1, wherein the indium-deposited layer may also be a binary plating layer of copper selenide, indium selenide, gallium selenide, zinc sulfide, copper sulfide, tin sulfide, or the like. Or ternary coatings such as copper indium gallium, copper indium selenide, copper zinc tin, zinc tin sulfur. The layered structure may be as described in claim 1, wherein the stacking may be indium/spray indium/copper gallium, indium/copper gallium, copper gallium/evaporation, indium/copper gallium/evaporation Different precursor layer stacking combinations such as indium/copper gallium, copper gallium/evaporation indium/copper gallium/evaporation indium. The layered structure is as described in claim 1, wherein the indium plating process can be prepared by sputtering, evaporation or non-vacuum coating. The thickness of the indium-deposited layer may be 80-500 nm as in the layered structure described in claim 1. The thickness of the surface of the vapor-deposited layer is 20-90 nm as in the layered structure described in claim 1. As described in claim 1, the selenization annealing method required for forming the CIGS structure may be a furnace mode annealing, a rapid heating furnace annealing, or a H ₂ Se annealing.