Energy Harvesting in Flexible and Semi-transparent Hydrogenated Amorphous Silicon Solar Cells
The goal of this study is to design, characterize, and fabricate cheap and efficient hydrogenated amorphous silicon (a-Si:H) photovoltaic (PV) modules, and semi-transparent solar cells on transparent plastic substrates for energy harvesting applications. The cells are deposited on thin flexible plastics at low temperature (120-150 °C).
In the first part of study, a-Si:H n-i-p solar cells have been fabricated using a plasma enhanced chemical vapor deposition (PECVD) in order to investigate how the deposition conditions affect their performance. To improve light absorption into the solar cells, we engineer the front window layer such as p-layer thickness and high energy bandgap (Eg) p-layer. The best a-Si:H n-i-p solar cell shows an open circuit voltage (Voc) of 0.67 V, a short circuit current density (Jsc) of 7.92 mA/cm2, a fill factor (FF) of 53.73 %, and an energy conversion efficiency (n) of 2.86 %. Using the developed deposition recipes, the a-Si:H PV module was demonstrated on a 10x10 cm2 polyethylene-naphthalate (PEN) plastic consists of 72 rectangular cells. The sub-cells are connected in series forming eight rows with connection pads, where the sections of 18 cells are connected in parallel using the external blocking diodes. The typical a-Si:H PV module shows a Voc of 12.78 V, a Jsc of 8 mA/cm2, a FF of 53.8 %, and an average n of 3.05 %. The PV module performance is similar to that of individual solar cells, which means good effectiveness of our solar cell processes.
In the second part, a-Si:H n-i-p solar cells were inverted to fabricate a-Si:H p-i-n solar cells. In this device structure, a p-type buffer-layer were introduced to improve cathode part between AZO/p-layer. The optimum device performance is achieved to a Voc of 0.885 V, a Jsc of 8.88 mA/cm2, a FF of 52.01 %, and an n of 4.09 %.
In the last part of this study, semi-transparent solar cells were fabricated both on glass and plastic substrates to demonstrate feasibility of build integrated photovoltaics (BIPV), based on the characterized and fabricated a-Si:H p-i-n cells on glass substrate in the second part. To overcome the stress balancing issue inside films between AZO and plastic, the barrier-layer coating was used to prevent the adhesion problems which frequently encounter between plastic substrate and TCO layer. Our semi-transparent a-Si:H solar cells show the n of 4.98 % and 4.77 % for the cells fabricated on glass and plastic substrates, respectively. In addition, the semi-transparent a-Si:H p-i-n solar cell is also used as a-Si detector with the response in the visible part of the spectrum. From the Ne spectral lines, the microplasma spectral from a-Si detector obtained similar response comparable with fiber optic detector.
In this thesis, we study an efficiency of a-Si:H solar cells for low-cost and large-area application. We believe that our experimental results have a potential for advanced functional solar cells such as energy harvesting in advanced BIPV applications.