PhD defence - Mohsen Mahmoudysepehr

Wednesday, January 18, 2017 1:30 pm - 1:30 pm EST (GMT -05:00)


Mohsen Mahmoudysepehr


Enhanced Ultra-Thin Film Nanocrystalline Silicon Photovoltaic Device Architectures


Siva Sivoththaman


Semi-transparent photovoltaics (STPV) is an important component in building integrated PV (BIPV), which is a rapidly growing PV sector. Thin film based transparent solar panels have strongest potential for window and skylight applications, and the main challenge is to incorporate good transparency with acceptable conversion efficiency.

This PhD thesis presents the intended research on nanoplasmonic architecture for enhanced ultrathin film solar cells for application in BIPV and mobile devices. The first chapter briefly explains the recent status of solar thin-film. The second chapter concisely introduces the role of nanoplasmonics in solar cells and discusses theoretical treatment to incorporate them in photovoltaic energy conversion. With this background information and by taking into account the application of these architectures for enhanced photovoltaic energy conversion, the main aspect of experimental works of this doctoral thesis are stated in chapter three. In this chapter development process for the fabrication of AZO as a TCO layer with enhanced transparency and conductivity explained. The influence of chamber pressure, RF power, and deposition temperature has been systematically studied and the electrical parameters such as film resistivity, carrier concentration, carrier mobility as well as optical transmission have been analyzed. Film deposition at 250°C and a low chamber pressure of 0.5mT resulted in a very low resistivity of 2.94 x 10-4 ohm-cm. In developing ultra-thin film solar cells, doped layers with high conductivity, highly efficient yet thin absorber layers, and high conductive transparent conductive oxides are key issues in developing large area, transparent devices. Ultrathin nanocrystalline silicon (nc-Si), due to its higher electron mobility and acceptable levels of transparency and optical absorption, is a potential candidate for transparent PV devices. Additionally, incorporating nc-Si material as the absorber layer in single or tandem thin film PV cells greatly enhances the spectral efficiency and device stability.

In chapter four, a new transparent nc-Si solar cell with p-i-n configuration is developed on glass substrates incorporating highly conductive aluminum doped zinc oxide (AZO) films, developed in chapter 3, as top and bottom electrodes. The main experimental processes involved, (i) development of a high deposition rate process in a modified PECVD system, (ii) optimization of doped and undoped micro and nanocrystalline Si layers at temperature 300?C, (iii) process development for AZO films by sputter deposition at low pressure and moderate power with enhanced conductivity and transparency, and (iv) process integration for single junction p-i-n device process on glass substrate. Additionally, highly conductive (17S/cm) n-type µc-Si:H (SiH4, PH3, 98% H2 dilution) and p type µc-Si:H (SiH4, B2H6 , 98.5% H2 dilution) films have been developed at 300?C. The optimum photovoltaic performance is achieved with an absorber layer right at the transition from amorphous to nanocrystalline silicon. Conversion efficiencies exceeding 5-6% have been obtained.

The novel contribution of this research: nanoplasmonic architectures for ultrathin film solar cells, is presented further more in chapter five by investigating the key design criteria and analyzing device optical performance accordingly. In addition, re-establishment of certain design parameters for nanoplasmonics in ultrathin film solar cells is further propounded. The main focus is to evaluate design criteria for enhanced absorption in a-Si thin and ultrathin film through designing a proper nanoplasmonic metallic structure. For this approach, the shape effect (size and type), type of dielectric, and type of metal are optimized to achieve maximum optical absorption.

In chapter 6 different fabrication method for nanoplasmonic structures further elaborated and experimental work for random Ag nanoisland formation through NSTS method explained. Different process parameters such as thin film thickness, annealing temperature are investigated to reach to the optimized deposition condition based on chapter 5 simulation results. At the end, 2 nanoplasmonic device architecture (superstrate and substrate) developed by incorporating Ag nanoisland in front and back of fabricated ultra-thin film devices. Comparing the performance of the fabricated cells shows that in general using nanoplasmonic structure on the back of the device has better overall performance. In substrate configuration around 10% enhancement in efficiency was observed. Strong light scattering of Ag nano-island and near-field enhancement of incoming light are the main driving force of the performance improvement in the nanoplasmonic thin film solar cells. Although, for the superstrate device configuration the performance enhancement is offset by introducing metallic recombination sites and also increasing the metallic backward scattering. In general, using nanoplasmonic architects has more effect when they are used in the back of the device to surpass the effect of reflection and parasitic absorption of final devices.