Marina Foti

Carrier transport mechanism in the SnO2:F/p-type a-Si:H heterojunction

We characterize SnO2:F/p-type a-Si:H/Mo structures by current-voltage (I-V) and capacitance-voltage (C-V) measurements at different temperatures to determine the transport mechanism in the SnO2:F/p-type a-Si:H heterojunction. The experimental I-V curves of these structures, almost symmetric around the origin, are ohmic for |V|<0.1V and have a super-linear behavior (power law) for |V|<0.1V. The structure can be modeled as two diodes back to back connected so that the main current transport mechanisms are due to the reverse current of the diodes. To explain the measured C-V curves, the capacitance of the heterostructure is modeled as the series connection of the depletion capacitances of the two back to back connected SnO2:F/p-type a-Si:H and Mo/p-type a-Si:H junctions. We simulated the reverse I-V curves of the SnO2:F/p-type a-Si:H heterojunction at different temperatures by using the simulation software SCAPS 2.9.03. In the model the main transport mechanism is generation of holes enhanced by tunneling by...

Plasmonic effects of ultra-thin Mo films on hydrogenated amorphous Si photovoltaic cells

We report on the improvement of short circuit current (JSC), fill factor (FF), and open circuit resistance (ROC) in hydrogenated amorphous silicon (a-Si:H) photovoltaic cells with a p-type/intrinsic/n-type structure, achieved by the addition of an ultra-thin molybdenum film between the p-type film and the transparent conductive oxide/glass substrate. For suitable conditions, improvements of ≈10% in average internal quantum efficiency and up to 5%–10% under standard illumination in JSC, FF, and ROC are observed. These are attributed to the excitation of surface plasmon polariton modes of the a-Si:H/Mo interface.

Evolution of SiHx hydrides during the phase transition from amorphous to nanocrystalline silicon films

This paper investigates the morphological evolution of hydrogenated amorphous silicon layers obtained by plasma enhanced chemical vapor deposition at different H dilutions in the regime close to the formation of the nanocrystalline phase. The role of hydrogen in the transition from the amorphous to the crystalline phase is investigated by accurate structural and chemical characterization, from the early stages of nucleation, where the nuclei present size slightly larger than the critical nucleus, i.e., about 0.8 nm in radius, up to the formation of crystalline grains larger than 30 nm in radius. A correlation between the structural characteristics of such crystalline phase and the bonding mechanism of Si with H through multiple hydrides, such as Si-H2 and Si-H3 is found, particularly the tri-hydrides are found to be directly correlated to the shape and the size of the nanocrystallites present in the films. The multiple hydrides are found to play a role also in the electrical characteristics of p-i-n a-Si:...

Data supporting the role of electric field and electrode material on the improvement of the ageing effects in hydrogenated amorphous silicon solar cells.

Hydrogenated amorphous Si (aSi:H) solar cells are strongly affected by the well known Staebler–Wronski effect. This is a worsening of solar cell performances under light soaking which results in a substantial loss of cell power conversion efficiency compared to time zero performance. It is believed not to be an extrinsic effect, but rather a basic phenomenon related to the nature of aSi:H and to the stability and motion of Hrelated species in the aSi:H lattice. This work has been designed in support of the research article entitled “Role of electric field and electrode material on the improvement of the ageing effects in hydrogenated amorphous silicon solar cells” in Solar Energy Materials & Solar Cells (Scuto et al. [1]), which discusses an electrical method based on reverse bias stress to improve the solar cell parameters, and in particular the effect of temperature, electric field intensity and illumination level as a function of the stress time. Here we provide a further set of the obtained experimental data results.

Improvement of solar cell performance and reversibility of ageing effects in hydrogenated amorphous silicon solar cells under illumination and electric field stress: Role of TCO and substrate

The ageing effects of hydrogenated amorphous Si (a-Si:H) under illumination have been investigated. These are strongly affected by the well-known Staebler-Wronski effect, occurring during light soaking of a-Si:H, producing a substantial loss of cell power conversion efficiency. In this work the light soaking effect was investigated and it was shown that the application of a strong electric field in reverse bias not only slows down the solar cell ageing kinetics but also even produces an improvement of the cells parameters. Such improvements have been investigated discussing the impact of temperature, electric field intensity, illumination level, substrate type and reversibility properties. Further information on the possible improvement causes are provided by the analysis of the behavior of single thin films of doped a-Si:H, and by comparing to the case of micromorph solar cells.

Case study of failure analysis in thin film silicon solar cell

Abstract Thin-film silicon modules are commonly produced by an alternating sequence of layer deposition and layer patterning steps, which lead to a monolithic series connected device. Most used process is laser scribing process that offers a high throughput and a small area loss. Tin oxide (SnO2) or zinc oxide (ZnO) are the most used front contact TCO in the superstrate configuration. ZnO presents better optical properties with respect to SnO2 and can be realized by low thermal and cost effective deposition processes. Electrical performance of our tandem thin film silicon cell deposited on ZnO front contact has shown higher shunt with respect with our reference process using SnO2 front contact, not explained only as difference between the two materials. In this work, a failure analysis process was followed in order to explain the origin of the difference. SEM, FIB and Auger electron spectroscopy were used in order to characterize the laser scribe that is known to be a possible cause of electrical deviation. We found residuals either on the bottom either on the later wall of P3 scribe that can explain the lowering shunt resistance and open circuit voltage observed into the electrical performances of the module.

Measurements and Simulations on the Mechanisms of Efficiency Losses in HIT Solar Cells

We study the electrical and the optical behavior of HIT solar cell by means of measurements and optoelectrical simulations by TCAD simulations. We compare the HIT solar cell with a conventional crystalline silicon solar cell to identify the strengths and weaknesses of the HIT technology. Results highlight different mechanisms of electrical and optical efficiency losses caused by the presence of the amorphous silicon layer. The higher resistivity of the a-Si layers implies a smaller distance between the metal lines that causes a higher shadowing. The worst optical coupling between the amorphous silicon and the antireflective coating implies a slight increase of reflectivity around the 600 nm wavelength.