Method for manufacturing light absorption layer

Abstract

Provided is a method of fabricating a CIGS absorption layer which, may have improved material utilization and productivity and have excellent thin film uniformity even in a large area by depositing and heat treating a precursor having a multilayer structure by a sputtering method using a compound, target of In.sub.xGa.sub.ySez(IGS) and Cu.sub.xSe.sub.y (CS).

Claims

1. A method of fabricating a solar cell, comprising: forming a lower electrode layer on a substrate; forming a first precursor layer on the lower electrode layer by performing a first sputtering using a target consisting of a compound of IIIb group elements and Se; forming a second precursor layer on the first precursor layer by performing a second sputtering using a target consisting of a compound of Ib group elements and Se; forming a third precursor layer on the second precursor layer by performing a third sputtering using the target consisting of a compound of IIIb group elements and Se; forming the light absorption layer by forming the third precursor layer and then performing a Se atmosphere heat treatment process; forming a buffer layer on the light absorption layer; and forming an upper electrode layer on the buffer layer, wherein the first precursor layer is formed at temperature T.sub.1, the second precursor layer is formed at temperature T.sub.2, the third precursor layer is formed at temperature T.sub.3 which is higher than T.sub.2 and 500 C. or less, the T.sub.1 is higher than T.sub.2 and 500 C. or less, and the T.sub.2 is 20 C. or more and lower than 250 C.

2. The method of claim 1, further comprising: after the performing of the Se atmosphere heat treatment process, performing heat treatment using H.sub.2S.

3. The method of claim 1, wherein the first precursor layer has a Ga/(Ga+In) composition ratio of 0.2 to 0.6.

4. The method of claim 1, wherein the first precursor layer is a single layer or a plurality of layers of two layers or more and has a reduced gallium (Ga) content toward a thickness direction of the second precursor layer.

5. The method of claim 1, wherein the third precursor layer has a Ga/(Ga+In) composition ratio of 0.2 to 0.6.

6. The method of claim 1, wherein the third precursor layer is a single layer or a plurality of layers of two layers or more and has an increased gallium (Ga) content toward a thickness direction of the buffer layer.

7. The method of claim 1, wherein a ratio of the gallium (Ga) content of the first precursor layer to the gallium (Ga) content of the third precursor layer is 1:1 to 3:1.

8. The method of claim 1, wherein a ratio of a thickness of the first precursor layer to a thickness of the third precursor layer is 1:1 to 5:1.

Description

DESCRIPTION OF DRAWINGS

(1) FIGS. 1 and 2 are cross-sectional views illustrating a solar cell according to a first exemplary embodiment of the present invention.

(2) FIGS. 3 to 5 are diagrams illustrating a deposition temperature profile of each precursor layer according to an exemplary embodiment of the present invention.

(3) FIG. 6 is a cross-sectional view of a solar cell according to an exemplary embodiment of the present invention.

(4) FIG. 7 is a cross-sectional view of a solar cell according to a comparative example.

(5) FIG. 8 is a diagram illustrating a process of fabricating a solar ceil according to an exemplary embodiment of the present invention.

BEST MODE

(6) Hereinafter, a method of fabricating a solar cell according to an exemplary embodiment of the present invention will be described in detail. The drawings to be provided below are provided by way of example so that the idea of the present invention can be sufficiently transferred to those skilled in the art to which the present invention pertains. Technical terms and scientific terms used in the present specification have the general meaning understood by those skilled in the art to which the present invention pertains unless otherwise defined, and a description for the known function and configuration obscuring the present invention will be omitted in the following description and the accompanying drawings.

(7) According to an exemplary embodiment of the present invention, in a solar cell including a substrate, a lower electrode layer, a light absorption layer, a buffer layer, and an upper electrode layer, a first precursor layer is deposited on a lower electrode layer by performing sputtering using a target consisting of a compound of IIIb group elements and Se, a second electrode layer is deposited on the first precursor layer by performing the sputtering using a target consisting of a compound of Ib group elements and Se, and a third precursor layer is deposited on the second precursor layer by performing the sputtering using a target consisting of the IIIb element and the Se compound to form a reserved light absorption layer configured of the first precursor layer, the second precursor layer, and the third precursor layer and then a Se atmosphere heat treatment process is performed thereon to form the light absorption layer.

(8) According to the exemplary embodiment of the present invention, the IIIb group element is at least one element selected from aluminum (al), gallium (Ga), and indium (In) and the Ib group element is at least one element selected from copper (Cu) and silver (Ag), in which selenide of these metal elements is used as a sputtering target.

(9) According to the exemplary embodiment of the present invention, the light absorption layer is configured of an IGS layer, a CS layer, and an IGS layer, in which each layer is formed by a sputtering method using the selenide of metal and thus previously forms a stable phase in a depositing process. In the case of performing the sputtering using a metal target according to the related art, two targets of a CuGa mixture and In due to a low melting point (about 30 C.) of Ga are used. In this case, a volume of a sequentially deposited CuGaIn is expanded by selenization heat treatment and Se having a large atom volume is diffused up to a lower portion of thin film and thus a heat treatment process time may be increased. However, according to the exemplar embodiment of the present invention, since metal selenide is sputtered and a composition of the deposited thin film is deposited to be approximately the same as that of the target, heat treatment for re-crystallization may be easy.

(10) Further, according to the exemplary embodiment of the present invention, as compared with co-evaporation, a thin film having very excellent surface roughness may be obtained, due to an ion bombarding effect by plasma.

(11) According to the exemplary embodiment of the present invention, the process of forming the first precursor layer, the second precursor layer, and the third precursor layer may be performed within a temperature range between 20 C. and 500 C.

(12) According to the exemplary embodiment of the present invention, the deposition processes of the first precursor layer, the second precursor layer, and the third precursor may be performed within the same temperature range or different temperature ranges.

(13) According to an aspect, when the precursor deposition is performed within the same temperature range, the deposition process may be performed within a substrate temperature ranging from 150 C. to 450 C. According to another aspect, after the first precursor, when the second precursor layer is deposited at lower temperature, a natural cooling method may be used by radiation under vacuum atmosphere to lower temperature. Further, after the second precursor layer, when the third precursor layer is deposited at higher temperature, a rapid heat treatment method may be used or an isothermal oven may be used. In this case, a beating rate may be controlled to a range between 1 C./S and 10 C./s.

(14) According to the exemplary embodiment of the present invention, in a process of forming a light absorption layer, a chalcogenization heat treatment process is performed to perform selenization or sulfurization. In this case, the chalcogenization heat treatment is performed at 400 C. to 600 C. for 5 to 60 minutes under the atmosphere of any one or more selected from selenium (Se) and sulfur to perform crystallization.

(15) In the process of forming the light absorption layer, after the chalcogenization heat treatment process is performed, the heat treatment using hydrogen sulfide (H.sub.2S) may be additionally performed to control a band gap. In the case of performing the H.sub.2S processing, Se is substituted into S on a CIGS surface. In this case, the band gap of the CIGS is increased, in particular, a valence band offset, not a conduction band, is increased and thus the band gap is increased in a state in which a band-alignment with a buffer layer is maintained, thereby increasing an open circuit voltage Voc.

(16) After the heat treatment process, each precursor layer is changed to a CIGS single layer structure in a three layer structure of IGS-CS-IGS and has a form of a final absorption layer.

(17) According to the exemplary embodiment of the present invention, the first precursor layer may have a Ga/(Ga+In) composition ratio of 0.2 to 0.6. When the composition ratio is less than 0.2, the open circuit voltage is reduced and when the composition ratio exceeds 0.6, a short circuit current is reduced, such that solar cell efficiency may be reduced.

(18) Further, the first precursor layer may be configured of a single layer or a plural layer of at least two layers and when a gallium (Ga) content is reduced in a thickness direction from the substrate toward a buffer layer, a charge may be easily transferred.

(19) According to the exemplary embodiment of the present invention, the third precursor layer may have the Ga/(Ga+In) composition ratio of 0.2 to 0.6 to prevent conversion efficiency from reducing. When the composition ratio is less than 0.2, the open circuit voltage is reduced and when the composition ratio exceeds 0.6, a short circuit current is reduced, such that solar cell efficiency may be reduced.

(20) Further, the third precursor layer may be configured of a single layer or a plural layer of at least two layers and the gallium (Ga) content may be increased in the thickness direction from the substrate toward the buffer layer. When the charge is transferred at the time of being bonded with the buffer layer, a harrier is formed and thus recombination probability of electron-hole is reduced in a defect present in a bonded boundary, such that the drop of the open circuit voltage may be prevented, thereby obtain higher efficiency.

(21) According to the exemplary embodiment of the present invention, the ratio of the gallium (Ga) content of the first precursor layer to the gallium (Ga) content of the third precursor layer may be 1:1 to 3:1, more preferably, 1:1 to 2:1 to uniformly maintain the entire Ga concentration.

(22) According to the exemplary embodiment of the present invention, a ratio of the thickness of the first precursor layer to the thickness of the third precursor layer may be controlled within a range of 1:1 to 5:1, When the thickness of the first precursor layer is larger, a surface charge depletion layer is very deeply formed in the CIGS and as the efficiency is reduced due to the reduction in charge density, at least first precursor layer may be formed to be the same or thicker.

(23) FIG. 1 illustrates a cross-sectional view of a solar cell in which an IGS layer, a CS layer, and an IGS layer are each stacked on a lower electrode by performing the sputtering predetermined metal selenide as a sputtering target and FIG. 2 illustrates that the IGS layer is formed in a multi layer of at least two layers or more, in which a thin film deposited by using the sputtering target at the time of depositing a multi component-based thin film may precisely control the composition ratio and photoelectric efficiency may be maximized by controlling the gallium content.

(24) FIGS. 3 to 5 illustrate a temperature profile at the time of deposition using the sputtering method, FIG. 3 illustrates that all the layers are subjected to a process at one temperature condition without changing a temperature condition, FIG. 4 illustrates that the first precursor layer (first IGS layer) is formed at T.sub.1 (temperature at the time of depositing the first precursor) and then the second precursor layer (second CS layer) is formed at temperature T.sub.2 (temperature at the time of depositing the second precursor) lower than T.sub.1 and the third precursor layer (third IGS layer) is formed at temperature T.sub.3 (temperature at the time of depositing the third precursor) higher than T.sub.2, and FIG. 5 illustrates that the first IGS layer is formed at T.sub.1 and the second CS layer and the third IGS layer are formed at temperature T.sub.2 lower than T.sub.1.

(25) In FIG. 4 or 5, when temperature is reduced to form the second precursor layer, the reaction with the first precursor layer may be reduced at the time of deposition. It is known that a minimum temperature at which the CIGS phase is formed by the reaction of the IGS and the CS is about 250 C. and when the temperature is reduced than the temperature of 250 C. at the time of forming the second precursor layer, the reaction of the first precursor layer and the second precursor layer is minimized and then the re-crystallization may be easy by the high temperature heat treatment.

(26) FIG. 6 is a cross-sectional view of a solar cell according to an exemplary embodiment of the present invention, in which the IGS layerCS layerIGS layer are formed depending on the temperature profile and then the heat treatment is performed at 550 C. and the crystallization is performed, such that it may be confirmed that a void is partially present or a film which is small in size and uniform is formed. The first precursor layer is deposited at 350 C., the second precursor layer is deposited at 150 C., and the third precursor layer is deposited at 350 C.

(27) On the other hand, FIG. 7 is a cross-sectional view of a solar cell by an existing metal sputtering method, in which it may be confirmed that the inside of the CIGS formed by performing the sputtering according to the existing method using the CuGa and In target and then by the selenization and sulfurization processing at high temperature is provided with a plurality of large voids.

(28) FIG. 8 schematically illustrates an apparatus of fabricating a CIGS absorption layer using a metal selenide compound target according to an exemplary embodiment of the present invention. For example, the apparatus of fabricating a CIGS absorption layer may be configured to include a loading chamber into which a substrate is loaded and is subjected to a process under vacuum, a pre-heat chamber for increasing temperature up to a desired temperature prior to deposition, a deposition (DEP) 1 chamber for IGS deposition, a buffer chamber for a change in temperature or a standby state prior to transferring DEP2, a DEP2 chamber for CS deposition, a buffer chamber for a change in temperature or a standby state prior to transferring a DEP3 chamber, a DEP3 chamber for depositing the IGS layer again, a cooling chamber for reducing temperature to a normal, temperature, and an un-load chamber for faking out the completely deposited substrate at a normal pressure. Each deposition chamber (DEP1, DEP2, and DEP3 chambers) may be provided with the plurality of targets to control the composition and the deposition rate and the buffer chamber is not applied when the change in process temperature is not present and the DEP1 to DEP3 chambers may be immediately continuously configured.

(29) Hereinabove, although the present invention has been described by specific matters, exemplary embodiments, and drawings, they have been provided only for assisting in the entire understanding of the present invention. Therefore, the present invention is not limited to the exemplary embodiments. Various modifications and changes may be made by those skilled in the art to which the present invention pertains from this description.

(30) Therefore, the spirit of the present invention should not be limited to the above-described exemplary embodiments, and the following claims as well as ail modified equally or equivalently to the claims are intended to fall within the scope and spirit of the invention.