Se OR S BASED THIN FILM SOLAR CELL AND METHOD FOR FABRICATING THE SAME

The present disclosure relates to a Se or S based thin film solar cell and a method for fabricating the same, which may improve the structural and electrical characteristics of an upper transparent electrode layer by controlling a structure of a lower transparent electrode layer in a thin film solar cell having a Se or S based light absorption layer. In the Se or S based thin film solar cell having a light absorption layer and a front transparent electrode layer, the front transparent electrode layer comprises a lower transparent electrode layer and an upper transparent electrode layer, and the lower transparent electrode layer comprises an oxide-based thin film obtained by blending an impurity element into a mixed oxide in which Zn oxide and Mg oxide are mixed (also, referred to as an ‘impurity-doped Zn—Mg-based oxide thin film’).

[Detailed Description of Main Elements]1: substrate2: rear electrode3: light absorption layer4: buffer layer5′: amorphous lower transparent electrode layer6: upper transparent electrode layer

DETAILED DESCRIPTION

The present disclosure relates to a front transparent electrode layer of a so-called Se or S based thin film solar cell, which uses Se or S based material as a light absorption layer.

The front transparent electrode layer may be implemented as a double-layer structure composed of an upper transparent electrode layer and a lower transparent electrode layer, and the upper transparent electrode layer plays a role of collecting carriers generated by photoelectric transformation.

Generally, in a Se or S based thin film solar cell using a ZnO-based thin film doped with impurities as an upper transparent electrode, in order to improve carrier collecting efficiency of the upper transparent electrode layer, an electrical conductivity characteristic, namely a specific resistivity, should be excellent, and the specific resistivity has close relationship with the structural properties of the thin film. In other words, for ZnO-based thin films having the same free carrier concentration, if the crystallinity of the thin film improves, factors disturbing the movement of free carrier at grain boundaries and crystallographic defects decreases, which increases Hall mobility and thus improves the specific resistivity of the thin film. Therefore, in order to improve the carrier collecting efficiency of the upper transparent electrode layer, the crystallinity of the upper transparent electrode layer should be enhanced. However, since the upper transparent electrode layer is formed on the lower transparent electrode layer, the structural properties of the upper transparent electrode layer is affected by the structure of the lower transparent electrode layer.

In the present disclosure, a ZnO-based thin film doped with impurity elements is applied as the upper transparent electrode layer, and a Zn—Mg-based oxide thin film obtained by blending impurity elements to a mixed oxide in which Zn oxide and Mg oxide are mixed is applied as the lower transparent electrode layer for improving crystalline structure with (002) preferred orientation of the upper transparent electrode layer.

Looking into the overall configuration of the Se or S based thin film solar cell to which the upper transparent electrode layer and the lower transparent electrode layer5according to the present disclosure are applied (seeFIG. 9), a rear electrode2, a light absorption layer3, a buffer layer4, a lower transparent electrode layer5′, and an upper transparent electrode layer6are sequentially formed on a substrate1. It is worth mentioning that components other than the lower transparent electrode layer5′ and the upper transparent electrode layer6may be selectively modified if necessary. The rear electrode2is made of opaque metallic material such as molybdenum (Mo), the light absorption layer3is made of Se or S based material such as Cu(In1-x,Gax)(Se,S)2(CIGS) and Cu2ZnSn(Se,S)4(CZTS), and the buffer layer4may be made of material such as CdS and ZnS. In the Se or S based thin film solar cell as described above, the light absorption layer3and the buffer layer4make a p-n junction to induce photoelectric transformation, and carriers (electrons and holes) generated by the photoelectric transformation are respectively collected by a front transparent electrode layer and a rear electrode2to generate electricity.

In order to ensure high light transparency, suppress recombination of carriers and enhance carrier collecting efficiency, both the upper transparent electrode layer and the lower transparent electrode layer should have a photonic band-gap over a certain level. In addition, the upper transparent electrode layer should have low specific resistivity, and the lower transparent electrode layer should have relatively high specific resistivity. Moreover, in order to reduce an absorption loss, both the upper transparent electrode layer and the lower transparent electrode layer should have excellent light transparency.

In the present disclosure, a Zn—Mg-based oxide thin film doped with impurities is applied as the lower transparent electrode layer, and a ZnO-based crystalline thin film doped with impurity elements is applied as the upper transparent electrode layer. The Zn—Mg-based oxide thin film doped with impurities is used as the lower transparent electrode layer in order to ensure a good crystalline structure with (002) preferred orientation of the upper transparent electrode layer over a certain level when the upper transparent electrode layer is deposited. A ZnO thin film doped with impurities may be used as the upper transparent electrode layer in order to stably ensure a free charge concentration over 1020cm−3.

As described in the “Description of the Related Art” section and the “Summary” section above, since the intrinsic ZnO (i-ZnO) used as the lower transparent electrode layer has a relatively high specific resistance and a low free charge concentration, the photonic band-gap has a value near 3.3 eV. Therefore, if the material of the buffer layer is changed, it is difficult to suitably cope with an absorption loss and a band structure. In the present disclosure, since the lower transparent electrode layer includes Zn oxide and Mg oxide and the composition of Mg is controlled, the photonic band-gap of the lower transparent electrode layer may be selectively adjusted. Further, since an impurity element is blended into a mixed oxide of Zn oxide and Mg oxide, when the upper transparent electrode layer is formed, the structural properties of the upper transparent electrode layer may be improved. The impurity element blended into the mixed oxide of Zn oxide and Mg oxide plays a role of mineralizer or surfactant to help crystal growth of the thin film.

The lower transparent electrode layer uses an oxide-based thin film obtained by blending impurity elements to a mixed oxide of Zn oxide and Mg oxide, namely ‘a impurity-doped Zn—Mg-based oxide thin film’, and satisfies a condition of an oxide semiconductor in which the photonic band-gap is 3.2 to 4.5 eV.

The impurity elements doped in the impurity-doped Zn—Mg-based oxide thin film may be at least one of group-III elements, group-IV elements, transition metals, and their mixtures. The group-Ill elements include B, Al, Ga and In, the group-IV elements include Si, Ge and Sn, and the transition metals include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ag and Cd. In addition to the elements above, a halogen element, F, and a glass metal, Sb, may be doped to the impurity-doped Zn—Mg-based oxide thin film.

Specifically, the impurity-doped Zn—Mg-based oxide thin film may have an atom % of Mg/(Zn+Mg) of 45% or below with respect to Zn and Mg which are metal elements other than oxygen and impurity elements. If the atomic ratio (atom %) of Mg/(Zn+Mg) exceeds 45%, the crystal structure of the lower transparent electrode layer starts deviating from a ZnO crystal structure of hexagonal system, and cubic MgO crystals start appearing, which does not help the improvement of (002) peak intensity of the ZnO-based upper transparent electrode layer which grows thereon.

Meanwhile, in the impurity-doped Zn—Mg-based oxide thin film, a composition ratio of both Zn and Mg, among elements except for oxygen, namely an atom % of (Zn+Mg)/(Zn+Mg+impurity elements), may be 90% or above and 99% or below. If the concentration of impurities is small, it is not easy to improve crystallinity of the lower transparent electrode layer. If the concentration is too high, compounds of the impurity elements appears, which disturbs crystallinity of the lower transparent electrode layer.

Even though the lower transparent electrode layer contains a small amount of impurity elements in addition to Zn oxide and Mg oxide, a photonic band-gap may be selectively controlled by adjusting Mg content, similar to the mixed oxide thin film of Zn oxide and Mg oxide. Referring to examples of the present disclosure below, it may be found that the photonic band-gap of the lower transparent electrode layer may be controlled in various ways by adjusting the relative composition of Zn and Mg. If the Mg content increases, the photonic band-gap increases and the light transparency in the short-wavelength region is improved. Both the upper transparent electrode layer and the lower transparent electrode layer may be formed by means of sputtering and vapor deposition.

Hereinafter, the characteristics of the lower transparent electrode layer applied to the Se or S based thin film solar cell according to the present disclosure will be described by means of examples.

A pure ZnO target and a MgO target have been co-sputtered to prepare a thin film made of a mixed oxide of ZnO and MgO, and a Ga-doped ZnO target (GZO) and a pure MgO target have been co-sputtered to prepare a thin film made of a mixed oxide of ZnO—MgO blended with Ga. After that, structural characteristics of the thin films have been observed. Table 1 shows a Mg atomic ratio (Mg/(Zn+Mg+Ga, atom %) of the prepared thin films. S1 series are samples free from MgO, S2 series have Mg composition ratios of about 10%, S3 series have Mg composition ratios of about 22%, and S4 series have Mg composition ratios of about 33%. In this way, ZnO—MgO mixture thin films have been prepared to be compared with the Ga-blended ZnO—MgO mixed oxide thin films at similar Mg composition ratios. Table 1 shows Ga composition ratios of the Ga-blended ZnO—MgO mixed oxide thin films.

FIG. 10ashows the X-ray diffraction spectra of the (002) peaks obtained from ZnO—MgO mixed oxide thin films, andFIG. 10bshows the X-ray diffraction spectra of the (002) peaks obtained from Ga-blended ZnO—MgO mixed oxide thin films. InFIG. 10,10-1and10-5represent S1-series samples,10-2and10-6represent S2-series samples,10-3and10-7represent S3-series samples, and10-4and10-8represent S4-series samples. As shown inFIG. 10a, it may be found that if the Mg composition ratio increases, (002) peak intensity of the ZnO—MgO-based mixed oxide thin film becomes stronger, and if the Mg composition ratio increases further, (002) peak intensity decreases again. For the Ga-blended ZnO—MgO mixed oxide thin films, the tendency with respect to the Mg composition ratio is somewhat different from the ZnO—MgO mixed oxide thin films. In other words, it may be found that (002) peak intensity is strongest when Mg is absent, and if the Mg composition ratio increases, (002) peak intensity gradually decreases. However, ifFIGS. 10aand10bare compared, it may be found that the Ga-blended ZnO—MgO mixed oxide thin films have stronger (002) peak intensities than the ZnO—MgO mixed oxide thin films in all composition ranges, and therefore the crystallinity is more excellent. For reference, the y axes ofFIGS. 10aand10bhave the same scale.

From the above result, it may be understood that in case of the Ga-blended ZnO—MgO mixed oxide thin films, Ga plays a role of promoting crystallization of the mixed oxide thin film.

Optical characteristics of the ZnO—MgO mixed oxide thin films and the Ga-blended ZnO—MgO mixed oxide thin films, prepared in Example 1, have been analyzed.FIG. 11shows light transmittance spectrums, obtained from the S1-series mixed oxide thin films11-1,11-3and the S3-series mixed oxide thin films11-2,11-4in Table 1. It may be found that both the Ga-blended ZnO—MgO mixed oxide thin films11-2,11-4and the ZnO—MgO-based mixed oxide thin films11-1,11-3have excellent light transparency. It can be seen that the films with similar Mg content exhibit similar fundamental absorption edges which are located at ultraviolet region where the light transparency rapidly decreases. In addition, it may be found that the absorption edges of the S3-series thin films are substantially shifted toward a short wavelength when compared with those of the S1-series thin films.FIG. 12shows the change of photonic band-gaps, obtained from thin films made of the ZnO—MgO mixed oxide and thin films made of the Ga-blended ZnO—MgO mixed oxide, shown in Table 1, as a function of the change of a Mg composition ratio. In both the ZnO—MgO mixed oxide thin films12-1and the Ga-blended ZnO—MgO mixed oxide thin films12-2, the photonic band-gap increases with increasing Mg composition ratio. Therefore, it may be understood that the photonic band-gap of the mixed oxide thin films doped with impurity elements may be easily controlled by simply adjusting Mg content, as in the case of the ZnO—MgO-based mixed oxide thin films.

Electric characteristics of GZO samples obtained by using the thin films made of a ZnO—MgO mixed oxide and the thin films made of a Ga-blended ZnO—MgO mixed oxide as the lower transparent electrode layer have been compared with those of GZO thin films deposited on glass substrate under the same condition.

InFIG. 13, the changes in the ratios of electric conductivity13-2(σon buffer/σon glass) and Hal mobility13-4(μon buffer/μon glass) of GZO thin films, for which Ga-blended ZnO—MgO mixed oxide thin films are used as the lower transparent electrode layer, are compared with those of GZO thin films, for which ZnO—MgO mixed oxide thin films are used as the lower transparent electrode layer, as a function of the change in the amount of Mg (Mg/(Zn+Mg+Ga), atom %) among metal components in the lower transparent electrode layer. InFIG. 13,13-1and13-3represent the ratios of electric conductivity and Hall mobility in case of using the ZnO—MgO-based mixed oxide thin films as the lower transparent electrode layer, which are the same graphs as7-1and7-2shown inFIG. 7, and they are depicted again inFIG. 13for comparison. FromFIG. 13, it may be understood that the GZO thin films grown on Ga-blended ZnO—MgO mixed oxide thin films as the lower transparent electrode layer exhibit excellent electric conductivity and Hall mobility when compared with the GZO thin films grown on ZnO—MgO-based mixed oxide thin films as the lower transparent electrode layer, in all Mg composition ratios. The GZO thin films formed on ZnO—MgO-based mixed oxide thin films as the lower transparent electrode layer give much inferior electric characteristic in comparison to the GZO thin films deposited on the glass substrate when the Mg composition ratio is low. However, the GZO thin films formed on Ga-blended ZnO—MgO mixed oxide thin films as the lower transparent electrode layer give excellent electric conductivity in comparison to the GZO thin films deposited on the glass substrate. In particular, it may be found that in all Mg composition ratios tested, the GZO thin films deposited on Ga-blended ZnO—MgO mixed oxide thin films as the lower transparent electrode layer show excellent Hall mobility in comparison to the GZO thin films formed on ZnO—MgO-based mixed oxide thin films as the lower transparent electrode layer, which may be more clearly understood from the X-ray diffraction characteristic depicted inFIG. 14.

InFIG. 14, the X-ray diffraction spectra of the (002) peaks obtained from the GZO thin films (14-2,14-4) deposited on the lower transparent electrode layers made of Ga-blended ZnO—MgO mixed oxide thin films are compared with those (14-1,14-3) deposited on the bare glass substrate. The Mg composition ratios shown inFIGS. 14aand14bare S2 and S3 series, which is comparable to the X-ray diffraction results of the GZO thin films (S2 and S3 series) using ZnO—MgO-based mixed oxide thin films as the lower transparent electrode layer as shown inFIGS. 8band8c. As shown inFIG. 14, (002) peak intensities (intensity) of the GZO thin films deposited on Ga-blended ZnO—MgO mixed oxide thin films are greater than those of the GZO thin films grown on the glass substrate. This is clearly in contrast with the result shown inFIGS. 8band8c, in which (002) peak intensities of the GZO thin films grown on ZnO—MgO-based mixed oxide thin films are much weaker than those of the GZO thin films grown on the glass substrate.