Photovoltaics is considered as one of the most promising new energy technology, because its energy source is omnipresent, pollution-free and inexhaustive. However, photovoltaic technology has not yet reached the level where it can economically compete with conventional power generation system, except for special application sectors such as power source of remote areas or islands. Therefore, the development of low cost and high efficiency solar cells is prerequisite for the wide application of photovoltaic technology. It is agreed that these solar cells must be thin film type because thin film process is cost-efficive in the fact that it uses much less raw materials and can be continuous.
$CuInSe_2$ (CIS) and related compounds such as $(Cu(In_xGa_{1-x})Se_2$(CIGS) have been studied by their potential for use in photovoltaic devices. CIS thin film materials which have high absorption coefficient and wide bandgap, have attracted much attention as an alternative to crystalline and amorphous silicon solar cells currently in use. The intensive studies on CIS solar cells have achieved efficiency record of about 17% for the cell size of 0.4 ㎠ and about 10% for modules size of 3,892 ㎠, in 1995. These are the highest efficiency ever achieved for thin film solar cells. This fact shows the potential of CIS solar cells to achieve the goals of solar cells with high efficiency, low-cost, and high reliability. However, there still exist problems to resolve the materials, fabrication technology, and control of film properties.
In this study, CIGS and the other films have been deposited and characterized to find optimum deposition conditions and ZnO/CdS/CIGS/Mo thin film solar cells were fabricated using these films.
CIGS thin films have been prepared by three-stage sequential coevaporation of In-Ga-Se, Cu-Se, and In-Ga-Se elements by co-evaporation. First, an In(Ga)-selenide was deposited on glass or Mo-coated glass substrates at 350℃ by the co-evapration from elemental sources. Second, the CIGS film was formed at 550℃ by the reaction between the pre-deposited In(Ga)-selenide layer and an incoming Cu and Se flux from Cu and Se elemental sources. Finally, In-Ga-Se were added on the top of the CIGS films at the same temperature to yield an In(Ga)-rich CIGS surface layer or $Cu(In_xGa_{1-x})_3Se_5$ layer.
In the first part of this studey, $CuInSe_2$ thin films with a thin layer of the CuIn3Se5 phase were prepared and characterized for photovoltaic applications. The $CuIn_3Se_5$ pahse is also known as ordered vacacncy compound(OVC). By the formation of the $CuIn_3Se_5$ phase on the CIS surface, the absorption edge was shifted from 1200 to 1000 nm wavelength and the XRD peaks were shifted to higher 2θ values. The $In_2Se_3$/CIS solar cells fabricated with a thick OVC layer on the CIS film displayed a kink effect, while the cells with a thin OVC layer exhibited a normal diode curve. The solar cells with a thin OVC layer yielded efficiency of 8.46% with an active area of 0.2 ㎠.
In the second part of this study, $Cu(In_xGa_{1-x})Se_2$ thin films were prepared and characterized with various Ga contents. Also, sodalime glass and Corning glass were used as substrates to investigate the effect of Na diffusion into CIGS film. The resistivity of CIGS films was not changed in the Cu-poor region by diffusion of sodium from sodalime glass. Also, secondary phases formed in CIGS films deposited on the sodalime glass substrate. These secondary phases were unidentified, but they could be in the form of $Cu_uNa_v(InGa)_wSe_z$ compoud. As the Ga content increased, the grain size of CIGS film became smaller. The 2θ values in XRD patterns were shifted to larger values and the overlapped peaks were splitted, indicating that the c/a ratio decreased. The energy bandgap increased from 1.04 to 1.67 eV and the resistivity decreased. The solar cell fabricated with ZnO/CdS/$Cu(In_0.7Ga_0.3)Se_2$/Mo structure yielded an efficeincy of 14.48% with an acitive area of 0.18 ㎠. The efficiency decreased with further increase of Ga content.
In the third part of this study, $Cu(In_xGa_{1-x})_3Se_5$ thin films have been grown on sodalime glass substrates by coevaporation of elemental sources. Micrographs of the films showed a substantial change in morphology as the Ga content increased. The high Ga-conent materials showed stronger 220 peak intensity than the 112 peak. The optical bandgap increased from 1.2 to 1.9eV as the Ga content increased. The carrier density of the film with the Ga content of 50at% was of the order $10^15 Ωㆍcm$. The solar cell with $Cu(In_0.5Ga_0.5)_3Se_5$ film as an absorber layer showed the high value of $V_oc$ and low value of $J_sc$. The results indicated that it is necessary tto increase the carrier concentration to improve the $J_sc$ value.
In the last part of this study, the other films which are required for CIS and CIGS solar cells were prepared and characterized at various deposition conditions. The resistivity of Mo film deposited by bi-layer films was $5 × 10^{-5} Ωcm$. $In_2Se_3$ buffer layers were deposited on the CIS films in the same co-evaporator without breaking the vacuum. The $In_2Se_3$ buffer layer had its lowest resistivity of $3 × 10^4 Ωcm$ at 350℃, which the $γ-In_2Se_3$ phase was obtained. The CdS films were prepared by process using cadmium salts and thiouria solution. At the condition of pH=12 and solution temperature of 70℃, the CdS films showed above 90% transmittance and $5 × 10^4 Ωcm$ resistivity. The resistivity and transmittance of the i-ZnO film deposited by RF magnetron sputtering were $~ 10^6$ and above 90%, respectively, which those of n-ZnO film were $4 × 10^{-4} Ωcm$ and above 85%, respectively.
