Copper Indium Gallium DiSelenide absorber layers are fabricated using a two stage manufacturing friendly process. The first step involves the sequential deposition of Copper and Gallium and co-deposition of indium and selenium at 275oC. This is followed by the second stage where the substrate is annealed in the presence of Selenium and a thin layer of copper is deposited to neutralize the excess Indium and Gallium on the surface to form the CIGS absorber layer. The top copper thickness as well as the time of deposition was varied to study the effect of Copper on the performance of the cells. Another recipe was developed for the precursor formation, where Gallium was co-evaporated with Indium and Selenium. A large bandgap shift was seen with this recipe and the open circuit voltage was increased. The performance of CIGS/CdS/ZnO solar cells thus fabricated was characterized using techniques like I-V, C-V, Spectral Response and EDS/SEM. Cells with open circuit voltages of 420-450 mV, short circuit currents of 33-38 mA/cm², fill factors of 58-62% and efficiencies of 9-11% were routinely fabricated.

In the first chapter of this thesis, we will present the principle of PV solar cells with a special emphasis on the CIS absorber. In the second and third chapter, we will describe the methods we used to treat the many body problem. Finally, in the last chapter, we will apply methods presented in chapter 2 and 3 to CIS and pay a particular attention to the dependence of the bandgap on the anion displacement and the concentration of defects.

The use of solar cells for energy production has indeed a bright future. Reduction of cost for fabrication along with increased efficiency are key features for a market boom, both achieved as a result of increased knowledge of the technology. Especially the thin film solar cell technology with absorbers made of Cu(In,Ga)Se2 (CIGS) is promising since it has proven high power conversion efficiency in combination with a true potential for low cost fabrication. In this thesis different recipes for fabrication of the Cu(In,Ga)Se2 absorber layer have been studied. The deposition technique used has been co-evaporation from elemental sources. For all depositions the substrate has been heated to a constant temperature of 500 ºC in order for the growing absorber to form a chalcopyrite phase, necessary for the photovoltaic functionality. The selenium has been evaporated such to always be in excess during depositions whereas the metal ratio Cu/(In+Ga) has been varied according to different recipes but always to be less than one at the end of the process. In the work emphasis has been on the radiative properties of the CIGS film during growth. The substrate heater has been temperature controlled to maintain the constant set temperature of the substrate, regardless of varying emitted power caused by changing surface emissivity. Depending on the growth conditions the emissivity of the growing film is changing, leading to a readable variation in the electrical power to the substrate heater. Since the thermal radiation from the substrate during growth has been of central focus, this has been studied in detail. For this reason the substrate has been treated as an optical stack composed of glass/Mo/Cu(In,Ga)Se2/CuxSe which determine the thermally radiated power by its emissivity. An optical model has been adopted to simulate the emissivity of the stack. In order to use the model, the optical constants for Cu(In,Ga)Se2 and CuxSe have been derived for the wavelength interval 2 μm to 20 μm. The simulation of the emissivity of the stack during CIGS growth agreed well with what has been seen for actual growth. Features of the OP-signal could hereby be explained as a result of film thickness of Cu(In,Ga)Se2 and CuxSe respectively. This is an important knowledge for an efficient fabrication in large scale.

Cu(In,Ga)Se2 (CIGS) is one of the most promising semiconductor compounds for large-scale production of efficient, low-cost thin film solar cells, and several research institutes have announced their plans for CIGS production lines. But for the CIGS technology to become a commercial success, a number of issues concerning manufacturability, product definition, and long-term stability require further attention. Several studies indicate that CIGS-based modules are stable over many years in field operation. At the same time, it is shown in the present work that they may have difficulties in passing standard accelerated lifetime test procedures like the IEC 1646 damp heat test. In particular, CIGS modules are sensitive to humidity penetrating through the module encapsulation, which will increase the resistive losses in the front contact and cause severe corrosion of the back contact. It is also shown that cells experience degradation in both voltage and fill factor, and the causes of these effects are addressed. By concentrating the light falling onto a solar cell, the device will deliver a higher power output per illuminated absorber area, which can lower the electricity production costs. For CIGS-based solar cells, low-concentrated illumination could be an economically viable approach. In this work it is shown that the yearly performance of a photovoltaic system with CIGS modules can be significantly improved at a moderate cost by using parabolic aluminum mirrors as concentrating elements. However, in order to avoid detrimental power losses due to high temperatures and current densities, the modules need to be designed for the higher light intensity and to be sufficiently cooled during operation. A design where the front contact of the module is assisted by a metal grid has shown promising results, not only for concentrated illumination but also for normal operation. The benefits are enhanced window processing tolerance and throughput, as well as improved degrees of freedom of the module geometry.

Since the phase stability region of CuInSe2 (CIS) extends as wide as a few atomic percent, composition variation in a microscopic scale is nature to this material and can be detected by EPMA or TEM-EDS. As the detection volume is kept as small as possible (e.g. we used an electron probe with a diameter of 3nm to measure a TEM specimen thinned by a focused ion beam to a 80 nm thickness), the composition data fluctuate rather significantly. For a near-stoichiometric CIS film prepared by co-evaporation or a selenized film using binary selenides as precursor, the composition variations in a nanometer scale were quite distinct. Due to the tedious procedures for making TEM specimens and doing measurements, we normally used EPMA for the composition analysis. Although the composition was measured in a micrometer scale, its variation still can be detected and expressed by the standard deviation. Our results showed that the selenized films prepared by using binary selenides as precursors (they were used to make the device in this work) had much better composition uniformity as compared with the films selenized from the elemental precursors. We also found that even the time period for the selenization process was short (rapid thermal selenization) or long (conventional selenization), the composition variation did not make any changes. Since there still has problems for making devices by using rapid thermal selenization, we successfully fabricated the CIS thin-film solar cells through the conventional selenization processes. The I-V characteristics of the best CIS cell is in the following: Voc=0.398 V, Jsc=41.14 mA/cm2, fill factor (FF)=54.58%, efficiency= 9.29%. We also made a CIGS cell and found that the open circuit voltage was increased to 0.461 V. However, the efficiency was 4.42%. It still needs more effort to boost its short circuit current and fill factor.

This research describes an investigation on the fabrication of CuInSe2-based thin-film solar cells with the device structure of Al/ZnO:Al/ZnO/CdS/CIS/Mo/SLG at the substrate temperature of 450oC, which is at least 100oC below the temperature currently used for depositing CIS thin films. A great advantage for the low temperature process is that the polymer material can be used as substrate and it is feasible to make lightweight and flexible thin-film solar cells. In this work, we used a co-evaporation technique with an introduction of Sb during the film deposition process to modify the film growth mechanisms and produce the CIS film with compact grain structure and smooth surface morphology. In most cases, there was only tiny amount of Sb existed in the film as a p-type dopant. In some cases, second phases of Sb compounds could be detected in the film as the Sb flux was kept too high during the film deposition stage. The I-V characteristics measured under the AM1.5 condition for the solar cell using a CIS:Sb film as the absorber showed that the open circuit voltage (Voc) was 0.364 V, short circuit current (Jsc) was 48.16 mA/cm2, fill factor (FF) was 44.5%, and energy conversion efficiency (£b) was 8%. The device with the same layer structure except the use of CIS film prepared without the addition of Sb and at a higher substrate temperature of 550oC had a comparable device performance but a slightly lower efficiency, i.e. Voc=0.325 V, Jsc=48.54 mA/cm2, FF=45.1%, £b=7.4%. It is clear that a lower temperature process using Sb to modify the growth process can be successful to obtain a device quality CIS layer. In addition, a CIGS thin-film solar cell was also fabricated and its device properties were Voc=0.392 V, Jsc=37.28 mA/cm2, FF=46.2%, and £b=7.0%. We see that the addition of Ga to increase the bandgap do increase the Voc and decrease the Jsc. However, a low efficiency of this cell indicates that further improvement in fill factor of the cell is a necessary.

In this study, we add an additional layer above and under the CIGS absorber layer as a secondary absorption layer respectively. We made the conventional structure of ZnO/CdS/CIGS/Mo becomes the structure of ZnO/CdS/CIGS/InGaP/Mo and ZnO/CdS/InGaP/CIGS/Mo which can improve the conversion efficiency. And we translate the thickness proportion of Ga and the doping concentration to find out the best parameter. According to the simulation, the wavelength of EQE in 600 nm ~ 1200 nm for our proposed CIGS solar cell which the additional layer under CIGS layer has been improved when compared to the conventional CIGS solar cell. The short-circuit current density has been increased about 9 %. And the conversion efficiency has also been increased about 9 %.When the additional layer above the CIGS absorber layer, according to the simulation, the wavelength of EQE in 300 nm ~ 600 nm for our proposed CIGS solar cell is improved when compared with the conventional CIGS solar cell. The short-circuit current density has been improved about 7.7 %, the open-circuit voltage about 7.1 %, and the conversion efficiency about 20.6 %. The main reason is that when the InGaP absorption layer under the CIGS layer which can catch the light which can¡¦t be absorbed by CIGS layer. The InGaP absorption layer above the CIGS layer which can catch the light immediately.

The study focus on low temperature process with doping antimony to refine the quality of the CI(G)S thin film, and doping gallium to increase energy band gap in two-stage co-evaporation process. Furthermore, we discuss about the variety of crystal structure, and recognize the value of energy band gap in transmission spectra. It has been achieved to increase the energy band gap of material with doping gallium. Recognizing the shift of XRD pattern and research result from papers, I estimate the content ratio of gallium in ¢»A atoms is 0.28~0.29, near my establishment ratio 0.3. By tuning the molecular beam flux of antimony effusion cell from 1.1¡Ñ1013 atoms/cm2second to 2.2¡Ñ1014 atoms/cm2second , to find out the property content of antimony involving of co-evaporation to optimize the quality of the CI(G)S polycrystalline thin film. We just observed that the thin film with antimony involving make effect of smoother and denser surface morphology. In our study, we also try discontinue supplying the antimony vapor to reduce the amount of antimony which involves the reaction process, and make low content of antimony leaved in the CI(G)S thin film. Here, We found out a special effect of the grain- growth of the CI(G)S thin film supplying antimony continually or not in the process. It should be strong (112) prefer orientation when we deposit the thin film using SLG substrate. However, we found out that antimony enhance the (220/204) .

Thin film solar cells with Copper Indium Gallium Diselenide (Cu(In,Ga)Se2) absorber layers is one of the most promising candidates to emerge as an efficient solar cell technology in the near future. CIGS cells with efficiencies of 19.2 % have already been reported [1]. In this study, CIGS absorber layers are fabricated by a two-stage all-solid-state manufacture-friendly process. In the first stage, designated as precursor deposition, Copper and Gallium are sequentially deposited followed by co-deposition of Indium and Selenium. In the second stage, designated as selenization, the substrate is annealed at high temperatures in a selenium environment during which a thin layer of copper is also deposited. The typical thickness of the absorber layers fabricated by this process is around 2um. The ZnO transparent front contact of these cells is a bi-layer with a thin intrinsic layer and a thicker Al doped n-type layer. These layers have been fabricated by different methods using Al-doped and undoped ZnO targets. The effect of the intrinsic layer thickness on the device performance was studied. Best performances were obtained when the intrinsic layer was around 350° thick and fabricated from an Al-doped ZnO target with excess oxygen partial pressure during deposition. The main focus of this work is to reduce the thickness of the CIGS absorber layers with no or minor loss in efficiency as this would translate directly into reduction in production costs and the amount of material being used. Reducing the thickness can be done either by reducing the deposition rates or duration of deposition. Due to the complex time-temperature profile during fabrication, reducing the thickness by reducing the deposition time would also affect the duration for which the substrates will be at high temperatures. To understand what effect this would have in film formation and performance of the device, and if any post-deposition annealing would be required to compensate for the reduced time at temperatures, experiments were carried out with the cells being annealed at different stages before and after completion of the device itself. Annealing was done at 250°C in both air and vacuum. Although annealing the finished devices always yielded poorer performance, it was certainly helpful in understanding which aspects of the device were affected. Devices with reduced absorber layer thicknesses of 1.5um, 1.0um and 0.65um were fabricated. The devices showed improved Voc's when the absorber layer thickness was reduced to 1.5um and 1.0um but the Jsc's dropped by 2-3 mA/cm2. The 1.0um thick devices also showed an increase in band gap. The thickness of the Molybdenum back contact layer was increased to see if the amount of Sodium from the substrate had any effect on the device performance. The Ga/In ratio was altered and its effect was also studied. The 0.65um thick devices showed a large reduction in Voc's and Jsc's. The effect of Selenization time and Selenium flux during Selenization were studied at each of the different thicknesses.

The aim of this thesis is to provide an in-depth investigation of zinc tin oxide, Zn1-xSnxOy or ZTO, grown by atomic layer deposition (ALD) as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. The thesis analyzes how changes in the ALD process influence the material properties of ZTO, and how these in turn affect the performance of CIGS solar cells. It is shown that ZTO grows uniformly and conformably on CIGS and that the interface between ZTO and CIGS is sharp with little or no interdiffusion between the layers. The band gap and conduction band energy level of ZTO are dependent both on the [Sn]/([Zn]+[Sn]) composition and on the deposition temperature. The influence by changes in composition is non-trivial, and the highest band gap and conduction band energy level are obtained at a [Sn]/([Zn]+[Sn]) composition of 0.2 at 120  °C. An increase in optical band gap is observed at decreasing deposition temperatures and is associated with quantum confinement effects caused by a decrease in crystallite size. The ability to change the conduction band energy level of ZTO enables the formation of suitable conduction band offsets between ZTO and CIGS with varying Ga-content. It is found that 15 nm thin ZTO buffer layers are sufficient to fabricate CIGS solar cells with conversion efficiencies up to 18.2 %. The JSC is in general 2 mA/cm2 higher, and the VOC 30 mV lower, for cells with the ZTO buffer layer as compared to cells with the traditional CdS buffer layer. In the end comparable efficiencies are obtained for the two different buffer layers. The gain in JSC for the ZTO buffer layer is associated with lower parasitic absorption in the UV-blue region of the solar spectrum and it is shown that the JSC can be increased further by making changes to the other layers in the traditional CdS/i-ZnO/ZnO:Al window layer structure. The ZTO is highly resistive, and it is found that the shunt preventing i-ZnO layer can be omitted, which further increases the JSC. Moreover, an additional increase in JSC is obtained by replacing the sputtered ZnO:Al front contact with In2O3 deposited by ALD. The large gain in JSC for the ZTO/In2O3 window layer stack compensates for the lower VOC related to the ZTO buffer layer, and it is demonstrated that the ZTO/In2O3 window layer structure yields 0.6 % (absolute) higher conversion efficiency than the CdS/i-ZnO/ZnO:Al window layer structure.