Thin film solar cells: Thin film deposition, solar cell fabrication and characterization.

Thin film solar cells have the potential to lower the solar cell cost. The global market share of the thin film solar cells have been increased from 5% to 17.7% in just 6 years (2003--2009) compared to the crystal silicon solar cells.

In this work, a Chemical Vapor Deposition (CVD) system was converted into an Atomic Layer Deposition system by hardware modification, including the installation of metal-organic precursor bubbler and in-line H2S generator, and computer automation realization by Labwindows programming. ALD CuxS film, a good absorber layer, was deposited on both corning glass and nanostructured TiO2 layer. SEM image of ALD CuS coated 100nm nanostructured TiO2 showed perfect penetration of ALD film, which resulted from self-saturated surface reaction of ALD. Another absorber layer, SnS was also deposited by the same ALD system using dibutyltin diacetate (DBTDA) metal-organic precursor as the tin source. It is the first time DBTDA was used as tin source in ALD SnS deposition. ALD CuS and SnS films are proposed to be used in extremely thin absorber (ETA) solar cells.

Chemical Bath Deposition (CBD) of Indium sulfide was also demonstrated in this thesis. It was the first time that CBD indium sulfide was used in P3HT:PCBM bulk heterojunction organic solar cell as the buffer layer between ITO and P3HT:PCBM to replace the problematic PEDOT:PSS. By using different front contact metal (Al and Au), it was found that In2S3 can transport both electrons and holes, which makes it a really good buffer layer in the tandem cells serving both hole transporter and electron transporter, where usually two buffer layers are needed. The efficiency of the un-optimized single junction device using In2S3 as hole transporter is 1.5%.

The last part of the thesis shows thermoreflectance (TR) imaging and Electroluminescence (EL) imaging of poly-crystalline silicon solar cells. These two imaging techniques in principle can be used in thin film solar cell characterization also. The local I-V curve on a diode shunt was obtained using high spatial resolution TR imaging for the first time. From local I-V curve on the defects, we can tell if a shunt is linear or diode behavior. The shunt we looked was a weak diode; by fitting local I-V curve with the diode equation, we got the ideality factor of that particular shunt much larger than 2, which indicated that the defect is probably caused by high concentration dislocation. Thermal diffusivity and thermal conductivity were also obtained from the TR phase image and amplitude image of the hot spot. Both TR image and EL image were used to detect the cracks. Analyzing the reverse biased EL image of the solar cell the cause of the pre-breakdown was discussed.