INORGANIC – ORGANIC TANDEM SOLAR CELLS: Investigation and Design

Abstract

Organic and inorganic solar cells with the advantages of low cost and easy fabrication process, can be the great prospects of commercial applications. In order to overcome the Shockley-Queisser limit of the single junction solar cells, their tandem structure becomes an emerging technology. One way to achieve very high efficiencies is to make a tandem solar cell, in which multiple absorbing layers are used to divide the solar spectrum into parts. This allows tandem cells to reach efficiencies far beyond those achievable by single junction technologies, with the current record efficiency at 43.5%. We believe there is a great opportunity for a low-cost tandem device: one that combines the high efficiencies of multi-junction cells with the lower cost, higher throughput and relative defect tolerance of thin film technologies. Here we argue for the potential of a hybrid tandem photovoltaic, in which the top cell is an emerging photovoltaic technology that can be deposited at low temperatures and with rapid throughput, such as an organic or dye-sensitized cell, and the bottom cell is one of a variety of traditional inorganic photovoltaics. Low temperature processability is critical because most inorganic photovoltaic technologies have a highly optimized thermal processing flow and adding a top cell at high temperatures can damage the layers already there. Depositing the top cell layers near room temperature make hybrid tandem photovoltaic highly versatile in that it can be applied to nearly any inorganic cell to convert it to a tandem device. In this thesis, we designed the structure of hybrid tandem solar cell where the top cell is an organic bulk heterojunction solar cell, and the bottom cell is inorganic CIGS solar cell. In chapter 2, we introduced the manufacturing of CIGS solar cells, which includes the deposited conditions of each unit layer and the experimental procedures. Soda-lime glass of about of 1 ~ 3 mm thickness is commonly used as a substrate. A molybdenum (Mo) metal layer is deposited (commonly by sputtering) which serves as the back contact and reflects most unabsorbed light back into the CIGS absorber. Following molybdenum deposition, a p-type CIGS absorber layer is grown by 3-stage process method. A thin n-type buffer layer is added on top of the absorber. The buffer layer is typically cadmium sulfide (CdS) or zinc sulphide (ZnS) deposited via chemical bath deposition. The i-ZnO layer is used to protect the CdS (ZnS) and the absorber layer from sputtering damage while depositing the ZnO:Al window layer. The Al doped ZnO serves as a transparent conducting oxide to collect and move electrons out of the cell while absorbing as little light as possible. In chapter 3, we used a transparent silver nanowire (AgNW) electrode on organic solar cells to achieve a semi-transparent device. We placed the semi-transparent cell in a mechanically-stacked tandem configuration onto CIGS thin film in order to achieve solid-state polycrystalline tandem solar cells with a net improvement in efficiency over the bottom cell. In this chapter, we demonstrated a solution processable top electrode for conventional organic photovoltaic (OPV) devices. All solution-processed solar cells, with the device structure glass/ ITO/PEDOT:PSS/P3HT:PC61BM/ZnO/Ag NW, reached a PCE of 1.28%. Also, we found that the cell efficiency was changed by AgNW of different thickness and annealing conditions. In chapter 4, a tandem solar cell is constructed by series connection of a semi-transparent organic solar cell as a top cell and CIGS solar as a bottom cell, where the isolated organics. This result is hoped to make a start on the use of inorganic-organic hybrid tandem solar cells.