Interest in photovolatics has grown rapidly during the last of the current centry. The importance of alternative energy sources has increased in significant both for energy supply and ecological conservation reasons. Photovoltaics(PV) is considered as one of the most promising new energy technology, because its energy source is inexhaustible and its applicability is universal. However, since the PV technology has not yet reached the level where it can economically compete with conventional power generation system, today the exploitation of PV is limited and PV`s contribution to the world`s energy demand is negligible. Therefore, 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-effective in the fact that it uses much less raw materials and can be feasible for deposition on large area substrates of module size.
Cu(In,Ga)Se$_2$(CIGS) thin films with direct band gap and high absorption coefficient are one of the promising absorbing materials for heterojunction solar cells. Technical advances in polycrystalline thin film technology have demonstrated photovoltaic devices with measured efficiencies of 18.8% for the cell size of 0.45 cm$^{2}$, employing the coevaporation of Cu, In, Ga, and Se elements through a three-stage process. These are the highest efficiency ever achieved for thin film solar cells. This fact shows the potential of CIGS 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. CIGS thin films can be prepared by several methods such as co-evaporation, selenization, molecular beam deposition and so forth. For high efficiency CIGS solar cell, the CIGS film usually fabricated by co-evaporating elemental materials through three-stage process. But the elemental co-evaporation method requires delicate control of the deposition rate of each element source by using knudsen cells and high power for Cu, Ga and In evaporation. Instead of elemental sources, selenide compounds such as $Cu_2Se$, $Ga_2Se_3$ and $In_2Se_3$ can be used as evaporation source. The selenide compounds can be sublimated at relatively low temperature and the sublimation rate is relatively high and easily controlled, compared to the evaporation temperature and rate of Cu, Ga, and In.
In this study, CIS and CIGS films have been prepared in a "3-stage process" using binary selenide compounds, $In_2Se_3$, $Ga_2Se_3$ and $Cu_2Se$. Using those films, a metal/ZnO/CdS/CIGS/Mo/Glass thin film solar cells were fabricated and characterized.
$CuInSe_2$ thin films with a thin layer of Cu-poor surface region including the $CuIn_3Se_5$ phase were prepared and characterized for photovoltaic applications. In the first stage, $In_2Se_3$ and Se elements were evaporated on the substrates at 150℃ to form a $In_2Se_3$ layer with a thickness of 1 ㎛. In the second stage, Cu$_2Se$ was evaporated and reacted directly with the $In_2Se_3$ layer at 440 ℃, so that a Cu-rich CIS film was formed. The deposited CIS films were annealed at 440℃ for 10 min in a Se atmosphere in the same evaporation chamber. In the third stage, $In_2Se_3$ and Se elements were evaporated on the CIS film with various deposition times in order to convert the surface region into an In-rich CIS composition or to form a thin $CuIn_3Se_5$ layer. The Cu-poor surface region including a $CuIn_3Se_5$ phase layer was controlled by co-evaporating $In_2Se_3$ and Se in the third stage. The thickness of the $CuIn_3Se_5$ layer was precisely analysed using AES, microEDS, and RBS. The $CuInSe_2$ solar cell with a 100 nm-thick $CuIn_3Se_5$ layer showed the best active area efficiency of 9.59% with open circuit voltage $(V_oc)=417mV$, short-circuit current density $(J_sc)=37.9mA/㎠$ and FF=0.606 in the area of 0.16 ㎠.
CIGS films were prepared by adding Ga element to CIS, using evaporation of $Ga_2Se_3$. In the first and third stage, $Ga_2Se_3$. was evaporated with $In_2Se_3$ and Se elements in the same time. In the first stage The substrate temperature was at 150℃. In the second stage, $Cu_2Se$ was evaporated and reacted directly with the $(In,Ga)_2Se_3$ layer at 500℃, so that the Cu-rich CIGS film was formed. The in-situ formed CIGS films were annealed at 500℃ for 10 min in a Se atmosphere in the same evaporation chamber. In the third stage, a small amount of $In_2Se_3$, $Ga_2Se_3$. and Se elements were evaporated on the CIGS layer. In the first and third stage, the evaporation rates of I$In_2Se_3$ and $Ga_2Se_3$ were varied, followed in variation of composition(Ga/(In+Ga) ratio) of CIGS film. The solar cell fabricated with CIGS film(Ga/(In+Ga)=0.33) showed the best efficiency of 9.36%. As the Ga content increased, CIGS exhibited the higher bandgap and smaller grains. All CIGS films showed a bi-layer morphology, regardless of the Ga/(In+Ga) ratio. To eliminate the demarcation in film bulk and increase the grain size, the substrate temperature was either at 150℃ or at 325℃. The increase of first-stage temperature to 325℃ made the $Cu(In_{0.67}, Ga_{0.33}Se_2$ have large grains without bi-layer morphology. The change of microstructure improved the CdS/CIGS solar efficiency from 9.3% ($V_oc=529 mV$, $J_sc=28.15mA/㎠$, and FF=0.625) to 11.7%($V_oc=550mV$, $J_sc=31.25 mA/㎠$, and FF=0.682). As the first-stage temperature increased, the hole density of CIGS, reverse saturation current, and spectral response were increased, reduced, and higher. These results were due to the increased grain size with the increase in the first-stage temperature and they were responsible for the improvement in device performance.
In order to improve the cell efficiency, an efficient CIGS absorber film should be prepared, that was possible in consideration of controlling film bulk properties. In the former section, we had known that the substrate temperature had a great influence on the microstructure of CIGS which were closely related with electrical properties. The Cu content in CIGS, exerted an effect on the film bulk properties. Therefore, the 1st and 2nd substrate temperature were increased from 325 ℃ and 500℃ to 350℃ and 525℃, respectively. The Cu/(In+Ga) ratio in CIGS film bulk was varied from 0.76 to 1.03. When the Cu/(In+Ga) ratio was below 0.83, the efficiency decreased slightly, due to decrease of $V_oc$. The decrease of $V_oc$. was mainly due to the rise of interface and junction recombination current, and decrease of effective carrier concentration. In a dark log J-V measurement, CIGS solar cell with Cu/(In+Ga) ratio of 0.76 showed the high recombination current density at near zero field and 0.3∼0.4 V. Thus, the decrease of efficiency was ascribed as the increase of recombination current density in the junction and interface. When the Cu/(In+Ga) ratio was above 0.92, the efficiency decreased rapidly due to decrease of $V_oc$. and $J_sc$. In this case, the $Cu_{2-x}Se$ phase was observed on the CIGS surface. The $Cu_{2-x}Se$ was known as semi-metallic phase and had an adverse effect on CIGS solar cell performance. Hence, the highly efficient CIGS solar cells could be prepared in the range of Cu/(In+Ga) ratio=0.83∼0.88.
The concept of a shallow buried junction between a Cu-poor n-type surface phase and the stoichiometric p-type bulk material can provide a better understanding of polycrystalline thin film $Cu(In,Ga)Se_2$ device operation. The homojunction in the CIGS films can avoid the recombination at the CdS/$Cu(In,Ga)Se_2$ interface, resulting in the higher-efficiency CIGS cells. The compositional variation in the surface region of CIGS film has an important effect on the device performance.
In this part, the effect of compositional variations near the surface region on CIGS solar cells was focused. Firstly, the composition of the CIGS surface was adjusted by the control of the evaporation rate of $In_2Se_3$, $Ga_2Se_3$, and Se at the third stage. As the third-stage evaporation time increased, the junction interface moved from the CdS/CIGS interface to bulk CIGS by the formation of n-type CIGS on the p-type CIGS film by EBIC analysis. The depletion of Ga atoms was observed below the CdS/CIGS interface. The formation of homojunction below the CdS/CIGS interface improved the quantum collection efficiency both in red light and longer wavelength region and reduced the recombination. $J_sc$, $V_oc$, and FF are all improved by the formation of shallow n-type junction on p-type CIGS film. When the location of minimum Ga content was too deep, the series resistance became higher and recombination current density at the interface and junction increased, resulted in the decrease of $V_oc$ and FF.
Secondly, the Ga/(In+Ga) ratio on CIGS surface was varied from 0.11 to 0.68. As the Ga/(In+Ga) ratio increased, $J_sc$ decreased and $V_oc$ increased. $J_sc$ in CIGS solar cell is related with minimum Ga content, because the CIGS with lower Ga content had a smaller bandgap. High value of $V_oc$ is correlated with a low dark saturation current density $J_sc$. The CIGS solar cells with a very low Ga content at the surface had a lower $V_oc$, which may be limited by interface recombination rather than by recombination via recombination centers in the space charge region of the absorber. Though CIGS solar cells with high Ga content should be expected to have a higher value of $V_oc$, $V_oc$ decreased slightly. This was attributed to the increase of interface recombination current. FF decreased due to higher series resistance with increasing the Ga content at the CIGS surface. The efficient solar cell was fabricated in the range of Ga/(In+Ga) ratio of 0.25∼0.34.
Last of all, the effect of CdS buffer layer on CIGS solar cells was characterized. The thickness of CdS layer had an influenced on the uniformity of solar efficiency. In our experiment, we could achieve more uniform efficiency in cell with 60 nm-thick CdS than 30 nm-thick CdS layer and had known that the proper thickness of CdS layer was required. The CIGS cells with CdS layer prepared in condition of Cd-rich solution showed the higher efficiency and lower space charge recombination current density. This indicated that type-inversion of CIGS surface could occur by the CIGS surface doping by Cd, resulted in forming the p-n junction better.
The best efficiency of 13.4% with $V_oc=574 mV$, $J_sc=33.1mA/㎠$, and FF=0.705 was achieved in the active area of 0.21 ㎠ using binary selenides as evaporation sources.
