Search

Solar cell degradation can occur through many pathways and at the different stages of the fabrication process, notably because of the water condensation. In the case of Cu(In,Ga)Se2 (CIGS) solar cells, the impact of water ingress on Mo back contact can be substantial for not only the film itself but also the device performance and reliability. Microstructural modification, change in film morphology, loss in reflectance, and increased resistivity because of the moisture ingress were observed via X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, and four-point probe measurements, respectively. Secondary ion mass spectrometry measurements revealed drastic changes in the alkali (Na, K) profiles in both the CIGS and Mo layers, likely because of the modification of their diffusion coefficient through Mo. This, in turn, negatively impacts the solar cell efficiency by decreasing both fill factor and open-circuit voltage. Additional experiments, modifying the substrate and utilizing a NaF postdeposition treatment, highlight the mechanisms of degradation as being due to both a modification of the Mo/CIGS interface and the lack of alkali diffusion after water ingress.

CIGS solar cell degradation due to the impact of moisture and heat treatment was studied as a function of sputtering pressure of the transparent conductive oxide (TCO) deposition. The devices were immersed in deionized water for up to 72 hours. The results were compared and showed decrease in efficiency with time. The performance loss was mainly driven by change in shunt resistance, impacting the FF. Decrease in current density and increase in series resistance were also noted. Interestingly, performance loss was found to be more drastic for the devices with TCO deposited at higher pressure than at lower pressure.

International audience; Cu(In,Ga)Se-2 samples were fabricated using a 3stage thermal co-evaporation process on molybdenum back contact at low temperature. The process of recrystallization was carried out in between the 2nd and 3rd stages by flashing 25 mg of AgBr for 2 minutes. A change in morphological structure was observed as small grains transformed into large grains, as confirmed by XRD and SEM measurements. The decrease of the Ga gradient, seen in the SIMS depth profile, suggests Ga interdiffusion due to AgBr treatment. Overall, the AgBr treatment contributes to a general improvement in device performance as compared to the as-deposited devices.

CuIn (1−x) Ga x Se 2 (CIGS) photovoltaics have limited market success due to high capital costs largely attributed to a slow, high-temperature deposition procedure. To reduce such costs we show that a high-rate, low-temperature approach including a short recrystallization step using metal halides can dramatically increase grain size in CIGS. We have demonstrated this both with high-rate evaporated films and amorphous films sputtered at room temperature. Both approaches can greatly increase throughput and reduce the costs of CIGS deposition. We review previous results for a number of metal halides and specifically focus on InCl 3 and AgBr as the most promising. We show that using high-temperature x-ray diffraction that crystallization begins at 270°C and ends at 380°C and is greatly accelerated by the metal halides, resulting in much larger grains over the same temperature range.

CIGS solar cells were submitted to damp heat treatment of the window layer by immerging the full devices into deionized water at 50°C for 24 hours. An overall loss in device performance was observed by current-voltage (J-V) measurements after such exposure, mostly due to a decrease in short-circuit current density, open-circuit voltage, and fill factor. To determine the underlying cause of device degradation, the devices parameters were numerically simulated using SCAPS-1D, in correlation with other materials characterization such as secondary ion mass spectrometry. The device modifications were best simulated by a change in transmission coefficient, total trap density and trap density peak position.

International audience; Deposition of CIGS semiconductor thin films was performed at low temperature and high rate by three-stage co-evaporation process on molybdenum coated glass substrate. A vapor treatment was done in between the second and third stage by flashing CuCl2 for 5 mins at 400 degrees C. A large change in morphology and crystal structure was observed after the treatment. XRD and SEM showed that small grains transformed into large grains. A smoother Ga profile was observed by SIMS measurements for the treated films as compared to as-deposited films. Furthermore, the Na profile was also modified in the recrystallized samples, with a lower content after recrystallization.

CuInSe 2 semiconductor thin films were fabricated using a single-stage thermal co-evaporation process. The as-deposited samples were prepared at 400 °C with both Cu-poor and Cu-rich composition before being recrystallized. The process of recrystallization involved a post-deposition treatment by InCl 3 for two different durations (30 min or 60 min) and two different temperature (450 °C or 500 °C). The post deposition treatment resulted in increased grain size. The Cu-rich thin films showed the more effective recrystallization as compared to the Cu-poor films. Similarly, higher temperature and longer time seemed to be more successful at enhancing grain size.

Cu(In,Ga)Se 2 samples were deposited onto molybdenum back contact at low temperature using thermal evaporation. Recrystallization was then performed at 400 °C using indium bromide vapor treatment for one hour. The treatment developed change in morphology with improved grain size and increased crystallinity. Variation in composition profile was observed, enhancing the In content while decreasing the Ga content. No apparent phase separation was observed from XRD but preferential orientation along (204) were induced.

International audience; The recrystallization by indium chloride of Cu(In,Ga)Se-2 thin films deposited by co-evaporation at 350 degrees C was studied. The process of recrystallization consists of the post-deposition treatment of CIGS sample by InCI3 vapor for 30 minutes with emphasis on grain growth. The treatment resulted in uniform increased grain size and improved crystallinity. XRD measurements suggests the formation of indium rich phase. Dynamic SIMS were also performed to further understand the process, phase separation and the extent of gallium depletion.

Cu(In,Ga)Se2 (CIGS) thin films were deposited at low temperature (350 °C) and high rate (10 µm/h) by a single stage process. The effect of post-deposition treatments at 400 °C and 500 °C by indium bromide vapor were studied and compared to the effect of a simple annealing under selenium. Structural, electrical, and chemical analyses demonstrate that there is a drastic difference between the different types of annealing, with the ones under indium bromide leading to much larger grains and higher conductivity. These properties are associated with a modification of the elemental profiles, specifically for gallium and sodium.

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se2 films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities ( NT ) for both the EV + 0.65 eV and EV + 0.98 eV traps increase after water exposure, while the majority carrier concentration ( NA ) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage ( V OC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both NA and NT yield most of the change in V OC. A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.

In this paper, we studied the effect of rubidium fluoride (RbF) post-deposition treatment (PDT) on the properties of Cu(In,Ga)Se2 (CIGS) solar cells. Specifically, the recombination mechanisms were analyzed by a series of characterizations including thermal and optical defect spectroscopies, temperature dependent current density–voltage measurements, and time resolved photoluminescence. It was found that the main effect of RbF PDT on the solar cell was an increase of the open circuit-voltage, $V_{{\text{oc}}}$ , by 30 mV due to a decrease of the values of the diode quality factor and reverse saturation current. Recombination mechanisms were identified as being in the CIGS space charge region, likely at the grain boundaries and near the CIGS surface. Breakdown of contributions to the $V_{{\text{oc}}}$ increase showed that part of it is due to an increase of the majority carrier concentration (16 mV) and another to the increase in the minority carrier lifetime (1 mV). The latest is mostly due to a reduction in the EV+0.99 eV deep-level trap density. An additional CIGS surface modification (contributing 13 mV), observed by the secondary ion mass spectrometry, is essential to explain the full change in $V_{{\text{oc}}}$ .

The effect of water ingress on the surface of the buffer layer of a Cu(In, Ga)Se2 (CIGS) solar cell was studied. Such degradation can occur either during the fabrication process, if it involves a chemical bath as is often the case for CdS, or while the modules are in the field and encapsulants degrade. To simulate the impact of this moisture ingress, devices with a structure sodalime glass/Mo/CIGS/CdS were immersed in deionized water. The thin films were then analyzed both pre and post water soaking. Dynamic secondary ion mass spectroscopy (SIMS) was performed on completed devices to analyze impurity diffusion (predominantly sodium and potassium) and to assess potential degradation mechanisms. The results were compared to device measurements, which indicate a degradation of all device parameters due to an increase in the total and peak trap densities, as shown by simulation. This is potentially due to a modification of the sodium profile in the bulk CIGS, with a decrease content after water soaking or because the oxygen profile increased in the bulk CIGS after water soaking.

The impact of moisture ingress on the surface of copper indium gallium diselenide (CIGS) solar cells was studied. While industry-scale modules are encapsulated in specialized polymers and glass, over time, the glass can break and the encapsulant can degrade. During such conditions, water can potentially degrade the interior layers and decrease performance. The first layer the water will come in contact with is the transparent conductive oxide (TCO) layer. To simulate the impact of this moisture ingress, complete devices were immersed in deionized water. To identify the potential sources of degradation, a common window layer for CIGS devices&mdash, a bilayer of intrinsic zinc oxide (i-ZnO) and conductive indium tin oxide (ITO)&mdash, was deposited. The thin films were then analyzed both pre and post water soaking. To determine the extent of ingress, dynamic secondary ion mass spectroscopy (SIMS) was performed on completed devices to analyze impurity diffusion (predominantly sodium and potassium) in the devices. The results were compared to device measurements, and indicated a degradation of device efficiency (mostly fill factor, contrary to previous studies), potentially due to a modification of the alkali profile.

One of the key challenges for furthering the economic viability of thermally evaporated CIGS is the thermal energy required for higher efficiencies. While several techniques have pushed the efficiency of CIGS cells higher, including alkali post deposition treatment, high temperature is often necessary for high efficiency. By using a modified 3-stage process, with low stage 1 and stage 2 deposition temperatures, it is possible to create CIGS absorber layers with high current collection capabilities. By judiciously introducing group I B metal halides it may be possible to further improve device quality while decreasing deposition temperature thus lowering costs.

International audience; One of the key challenges to promote the economic viability of Cu(In,Ga)Se-2 (CIGS) solar cells is the multi-stage co-evaporation process required to make a high quality absorber layer. One phenomenon of interest is dynamic recrystallization using a fluxing agent. While these techniques have significantly improved material systems like CdTe, their impact on other nonmetallic systems are relatively unexplored. In this paper, we demonstrate that InCl3 can be used effectively to recrystallize CIGS for temperature as low as 450 degrees C, but that it also induces a modification of the surface composition. Analyses, notably via glancing incidence X-ray diffraction and secondary ion mass spectrometry, show an indium enriched surface as well as a modified alkali profile.

Deposited CIGS layers typically undergo a NaF post deposition treatment (PDT) to improve the electrical properties of the deposited films. These NaF annealing processes can also be used to recrystallize CIGS films. To determine the effect of NaF as a fluxing agent, CIGS was deposited at varying substrate temperatures. In-situ annealing with NaF was then done to determine how the initial grain structure energy impacts recrystallization. Gallium appears to be a significant deterrent to recrystallization; the most crystallized film experienced a phase segregation between CIS and gallium containing compounds. By investigating these phase separations more, it might be feasible that a process of rapidly depositing CIGS at extremely low temperatures could create films of sufficient quality to be economically viable.

In this work, the diffusion process of sodium (Na) in molybdenum (Mo) thin films while it was deposited on soda lime glass (SLG) was studied. A small amount of oxygen was present in the chamber while the direct-current (DC) magnetron sputtering was used for the deposition. The substrate temperatures were varied to observe its effect. Such molybdenum films, with or without oxidations, are often used in thin film solar cells, either as back contact or as hole transport layers. Secondary ion mass spectrometry (SIMS) was used to quantify the concentration of the species. A grain diffusion mechanistic model incorporating the effect of grain and grain boundary geometrical shape and size was developed. The model was used to provide an in-depth theoretical analysis of the sodium diffusion in molybdenum thin films that lead to the measured SIMS data. It was observed that not only diffusion coefficients should be considered when analyzing diffusion processes in thin films but also the ratio of grain boundary size to grain size. Both depend on substrate temperature and directly affect the amount of diffused species in the film. The data were analyzed under the light of the film growth speed versus diffusion front speed, the effect of oxygen content, and the effect of substrate temperature on the overall diffusion process. The temperature inversely affects the ratio of grain boundary size and grain size and directly affects the diffusion coefficient, which leads to a preferable temperature at which the highest amount of alkali can be found in the film.

Preliminary studies of instabilities and degradation due to moisture ingress into the molybdenum (Mo) back contact and its impact on the performance of Cu(In,Ga)Se 2 solar cells are presented. The samples along with reference devices were characterized to obtain an outline of the deteriorating effect. Overall, the resistivity and surface roughness of the Mo layer increased with exposure to humidity and has a significant impact on the device performance.

The prospective cross-sectional study of human brain was conducted to establish the range of size of ventricular system in normal human brain of Nepalese population (irrespective of race) by using magnetic resonance imaging (MRI) at Institute of Medicine, Department of Radiology and Imaging, Tribhuvan University Teaching Hospital (TUTH) on 150 subjects (75 male and 75 female) of age between 10 - 80 years with 0.3 Tesla static magnetic field. The bifrontal diameter, bihemispheric diameter, frontal horn ratio, transverse dimension of third ventricle, anterior - posterior dimension and width of fourth ventricle were observed to be 32.7 mm and 31.0 mm, 105.1 mm and 101.2 mm, 0.31 and 0.31, 4.7 mm and 4.4 mm, 9.9 mm and 9.4 mm, 12.3 mm and 11.7 mm in case of male and female respectively with overall average of 31.9 mm, 103.1 mm, 0.31, 4.6 mm, 9.7 mm, and 12.0 mm respectively. The measurement of ventricular dimension was observed to be statistically significant between male and female except transverse dimension of third ventricle and was greater in male than in female in all cases.Journal of Institute of Science and Technology, 2015, 20(1): 6-14

Using thermal evaporation, Cu(In,Ga)Se2 (CIGS) layers were deposited at low temperature (350 °C) and high rate (10 μm/h) using a single stage process. They were then recrystallized using a variety of alkali fluorides: NaF, KF, RbF and CsF. To ensure that the substrate would not influence the study (via alkali diffusion), the samples were deposited on silicon wafers. The chemical, physical and electrical properties of the films were then characterized, demonstrating that all alkali fluorides behave as fluxing agents to enhance recrystallization and conductivity, and induce a (112) preferential orientation. Secondary ion mass spectrometry analysis showed that no modification of the elements' distribution occurs because of recrystallization. Solar cells were also fabricated and characterized, indicating that NaF can double the efficiency of solar cells compared to the as-deposited layers. This enhancement is accompanied by the disappearance of a rollover, voltage dependent current collection and shunt from the current density-voltage curves. However, even for the best recrystallization, the current is still limited to 28 mA/cm2, indicating that only a portion (0.75 μm) of the full device (2 μm) is activated.