Mirjam Theelen

On the environmental stability of ZnO thin films by spatial atomic layer deposition

Undoped and indium-doped ZnO films have been deposited by atmospheric spatial atomic-layer-deposition (spatial-ALD). The stability of their electrical, optical, and structural properties has been investigated by a damp-heat test in an environment with 85% relative humidity at 85 °C. The resistivity of the ZnO films increased during damp-heat exposure mainly due to a sharp decrease in the carrier mobility, while the carrier density and transparency degraded only partially. The increase in resistivity can be ascribed to a degradation of the structural properties of ZnO films, resulting in a higher level of tensile stress, as indicated by x-ray diffraction analysis, and in a reduced near-ultravoilet emission level in their photoluminescence spectra. Al2O3 thin (25–75 nm) films grown by spatial-ALD at 0.2 nm/s are used as moisture barrier to effectively enhance the stability of the electrical and structural properties of the films.

Accelerated performance degradation of CIGS solar cell determined by in-situ monitoring

An ‘hybrid’ degradation setup, in which humidity, temperature and illumination are used in order to accelerate degradation of CIGS, has been developed. This setup consists of a climate chamber, which can vary the temperature and humidity. Furthermore, an area of 80x80 cm2 is illuminated, which allows both the study of light induced behvaior and the in-situ measurement of the IV curve of the cell during the test, so the degradation behavior can be observed in time. The IV output is automatically logged and characteristic parameters like efficiency, currents, voltages, ideality factor and resistance can be extracted. The 40x40 cm2 in the center of the illuminated area was calibrated BAA according to IEC norm 60904-9. Twelve cells or minimodules can be degraded and measured in-situ at the same time. The continuous in-situ IV measurements allowed us to do various studies, including the determination of the temperature dependency and of the impact of light and dark exposure on the performance of CIGS solar cells. Furthermore, the impact of different Na and K concentrations in the CIGS absorber layer on the ininial as well as long term performance of CIGS solar cells was studied.

Degradation of CIGS solar cells due to the migration of alkali-elements

Non-encapsulated CIGS solar cells with different contents of sodium (Na) and potassium (K) were simultaneously exposed to damp heat and illumination. The solar cells with higher alkali (Na, K) content exhibited higher initial conversion efficiencies, but degraded severely within 100 hours, while samples with a lower alkali content retained their efficiency longer. The degradation was likely caused by the migration of alkali atoms, leading to shunting of the solar cells and the formation of alkali rich spots.

Highly improved transparent conductors by combination of TCOs and metallic grids

Transparent conductive oxides (TCOs) are widely used in optoelectronic applications and are a vital part in thin film solar cells. However, these materials have some drawback, such as brittleness and a maximum transmittance for a certain sheet resistance. Alternative materials, such as metal foils, nanowires and organic conductors are discussed and ranked for their suitability in solar cells. At this moment, organic conductors have a performance inferior to inorganic conductors. Recent results obtained at TNO demonstrate that the combination of TCOs and metal grids are the most viable option for improvement of TCO performance beyond the limits of a single material. For this demonstration commercial ITO on PET was used, on which metal grid was applied by easily upscalable wet chemical deposition techniques. The sheet resistivity was increased by a factor of over 400, with less than 5% loss additional in transmittance. A sheet resistance of 0.1 Ω/sq was obtained with a transparency above 83%.

Determination of the temperature dependency of the electrical parameters of CIGS solar cells

Two types of Cu(In,Ga)Se2 (CIGS) solar cells, both designed for implementation in CIGS modules, were subjected to temperatures between 25 oC and 105 oC. Simultaneous exposure to AM1.5 illumination allowed the measurement of their electrical parameters at these temperatures. These two types of solar cells, produced with different deposition routes on soda lime glass (SLG) and polyimide (PI) substrates, showed large variations in the temperature dependency of their electrical parameters. It was shown that the temperature dependency of the open circuit voltage (Voc) was dependent on its room temperature value: a high Voc at 25 °C led to a slower loss of Voc when the temperature was increased. For the Voc, the normalised temperature dependency varied between −0.28%/°C and −0.47%/°C, which is in agreement with the literature. The temperature dependency of the short circuit current density (Jsc) showed more surprising results: while the PI samples had the expected positive temperature dependency (0.03 to 0.32...

In-situ analysis of the degradation of Cu(In, Ga)Se 2 solar cells

A unique method has been developed to in-situ monitor the accelerated degradation in Cu(In, Ga)Se2 solar cells, in which a degradation setup uses humidity, high temperatures and illumination as loads. IV characteristics of the solar cells are collected during the degradation. Cu(In, Ga)Se2 solar cells and minimodules were degraded in the set-up and analyzed. We report on the formation of spots on the cell surface and indicate modifications in the i-ZnO/CdS region that result in changes in the external quantum efficiency. Furthermore, an increase in series resistance and a decrease current density was measured.

Thin film surface processing by UltraShort Laser Pulses (USLP)

In this work, we studied the feasibility of surface texturing of thin molybdenum layers on a borosilicate glass substrate with Ultra-Short Laser Pulses (USLP). Large areas of regular diffraction gratings were produced consisting of Laserinduced periodic surface structures (LIPSS). A short pulsed laser source (230 fs-10 ps) was applied using a focused Gaussian beam profile (15-30 μm). Laser parameters such as fluence, overlap (OL) and Overscans (OS), repetition frequency (100-200 kHz), wavelength (1030 nm, 515 nm and 343 nm) and polarization were varied to study the effect on periodicity, height and especially regularity of LIPSS obtained in layers of different thicknesses (150-400 nm). The aim was to produce these structures without cracking the metal layer and with as little ablation as possible. It was found that USLP are suitable to reach high power densities at the surface of the thin layers, avoiding mechanical stresses, cracking and delamination. A possible photovoltaic (PV) application could be found in texturing of thin film cells to enhance light trapping mechanisms.

The influence of atmospheric species on the degradation of aluminum doped zinc oxide and Cu(In,Ga)Se2 solar cells

Aluminum doped zinc oxide (ZnO:Al) layers were exposed to the atmospheric gases carbondioxide (CO2), oxygen (O2), nitrogen (N2) and air as well as liquid H2O purged with these gases, in order to investigate the chemical degradation behavior of these layers. The samples were analyzed by electrical, compositional and optical measurements before, during and after exposure to these conditions in order to follow the degradation behavior of these layers in time. We have shown that ZnO:Al layers degraded in the presence of a mixture of H2O and CO2. Individually, CO2 does not impact the degradation at all during the tested period, while the individual impact of H2O is small. However, when CO2 is also present, the concentration of OH increases greatly in the bulk and even more at the air/ZnO:Al and the ZnO:Al/glass interfaces. Carbon based species are then also present, indicating that Zn5(OH)6(CO3)2 is also formed at the grain boundaries. The degradation of ZnO:Al was accompanied by the occurrence of holes in the ZnO:Al layer near the ZnO:Al/glass interface. The impact of gaseous O2 as well as water purged with N2 and O2 on ZnO:Al degradation is very small. Complete Cu(In,Ga)Se2 solar cells were also exposed to unpurged liquid H2O and H2O purged with CO2, O2, N2 and air. The samples exposed to H2O purged with air and CO2 showed a rapid decrease in efficiency after approximately 180 hours of exposure. This efficiency decrease is mainly driven by a very rapid decrease in current density and an increase in series resistance.

Interlaboratory comparison of photovoltaic performance measurements using CIGS solar cells

An interlaboratory test involving four PV laboratories was carried out to compare performance measurements using nonencapsulated CIGS solar cells. 15 samples were sorted according to their electrical parameters measured in the first lab and distributed to the three remaining ones, where the cells were remeasured. The comparison of the cells performance showed significant differences between the tested institutions, with variances in Voc, Jsc and FF. The method for contacting the cells, as well as the hardware and software used, were identified as the probable causes for the differences, suggesting that improvements in methodology and equipment, as well as constant monitoring, are necessary to assure the reliability of the measurements.

Stability of unpackaged CIGS solar cells under illumination with damp heat, dry heat and dry cold followed by cycling

Unpackaged CIGS solar cells were simultaneously exposed to illumination and either dry heat, damp heat or dry cold followed by thermal cycling. In-situ monitoring of their electrical parameters demonstrated a rapid decrease of the efficiency for the solar cells exposed to damp heat plus illumination. This decrease was mainly driven by changes in the shunt resistances, affecting also the open circuit voltage and the fill factor, while a minor increase in series resistance was also observed. All solar cells exposed to dry heat plus illumination were quite stable during the exposure in both their electrical and material parameters. The dry cold/cycling solar cells similarly barely showed any degradation, although one sample became shunted, likely due to structural changes in the solar cell. From these experiments, it was concluded that unpackaged CIGS solar cells can degrade rapidly in the presence of humidity, but in the absence of water, these solar cells were mostly stable. This indicates that these unpackaged CIGS solar cells can majorly stand combined illumination, and the accompanying electrical field, and various temperatures.