Studies on monolithic tandem structure for low cost and high efficiency dye-sensitized solar cells

Abstract

Multi junction solar cells are devices fabricated from two or more solar absorbers that absorb different parts of the solar spectrum. Typically this is done to yield a device with a superior efficiency to single junction devices derived from those absorbers and to surpass the efficiency limit of a single junction device (approx 30 %). The highest performing solar cells currently available are multi junction, but they are also the most expensive, typically restricting their use to applications that require a high power output from a small area. Another possibility for multi junction solar cells is to combine absorbers with a low production cost and simplicity to produce. This could potentially realise a solar cell with an efficiency greater than existing single junction technologies, but with a lower cost. Absorbers such as the dye-sensitized solar cell (DSC or DSSC) are relatively simple and cheap to produce but are not currently being mass produced. By comparison the thin lm technology, Cu(In,Ga)Se2 or CIGS, is currently being manufactured for use commercially at a competitive price and performance to the current market leader, silicon. A tandem solar cell comprised of the DSC and CIGS absorbers has shown promise of an efficiency suitable for commercial application. Initially these tandem devices were demonstrated as a physical stack (one above the other) of the two separate solar cells connected electrically in series. The design has progressed to a monolithic design, highlighting several crucial areas requiring further development if the tandem is to prove successful. Typical components of the DSC are sub-optimal for use in a tandem cell and require development of alternative approaches when combined into the proposed tandem cell. DSCs suffer drawbacks such as a lower efficiency and long term stability issues which has so far limited their commercial use. Optical losses from the transparent conducting oxide (TCO) used in both the DSC and CIGS absorb a small amount of light that is required by the CIGS. These parasitic losses ultimately reduce the overall performance of the tandem device. The work presented in this thesis makes use of pulsed DC sputtering to deposit titanium-doped indium oxide (ITiO), a material that is highly transparent across all the wavelengths absorbed in the DSC/CIGS. Pulsed DC sputtering reduces the time taken to deposit layers of ITiO whilst also exhibiting excellent electrical and optical performance, potentially reducing the overall cost of the DSC/CIGS tandem. The monolithic configuration of the tandem leads to the electrolyte of the DSC being brought into contact with the CIGS cell. The electrolyte is crucial to the operation of the DSC (and tandem) as an efficient hole conductor between the two absorbers. This electrolyte also corrodes the CIGS layer and complete device failure occurs within a matter of hours of device assembly. Use of another TCO, zinc oxidedoped indium oxide (IZO) is examined in this work, deposited in the amorphous phase to act as a barrier between the DSC electrolyte and the CIGS surface by preventing the electrolyte from reaching the CIGS through pinholes in the TCOs typically used as a top contact. Finally the current-voltage (I-V) measurement of a solar cell determines critical parameters, of which includes the efficiency. The use of a mask across the DSC to accurately define the cell area is crucial for the measurement of its efficiency. This work demonstrates that applying this method to the tandem cell causes shading of the CIGS layer, resulting in a reduced electrical performance. A solution is proposed by modifying the device architecture to better match both absorber areas whilst preventing a short circuit between the DSC electrolyte and the CIGS back contact through the use of insulating SiO2 layers.