Micro-Solar Cells By Electrodeposition into a Microelectrode Array – Effect of Dot Diameter

Abstract

Next generation concentrator micro-solar cells offer enhanced power conversion efficiencies whilst using less precious semiconductor material than normal photovoltaic modules, as less device area is required to harvest the same amount of light. Vacuum-based deposition processes are typically used to produce large-area thin film semiconductors. However, to make micro-sized semiconductors for solar cells, area selective electrodeposition (ASED) onto well-defined microelectrodes is an attractive strategy. Here, we investigate the electrodeposition of Cu, In, and Ga onto variable sized microelectrodes in order to produce working micro solar cells based on copper indium gallium diselenide, Cu(In,Ga)Se2. To template arrays of microelectrodes, soda lime glass/molybdenum (SLG/Mo) substrates were patterned by plasma enhanced chemical vapour deposition of a 2 µm thick SiO2 insulating layer. Direct laser lithography coupled with reactive ion etching was used to define circular wells – referred to as dots – with diameters ranging from 20 to 300 µm. The template enables area selective electrochemical growth of metallic layers – copper (Cu), indium (In) and gallium (Ga) – from a combination of aqueous and ionic liquid (IL) media, which are the precursor layers required to produce, the Cu(In,Ga)Se2 semiconductor by means of a subsequent reactive selenium annealing. Here we investigate the Cu electrodeposition in detail. The arrays’ electrochemical behaviour was characterized with cyclic voltammetry (CV) using the ruthenium hexamine (Ru2+/3+(NH3)6) redox couple at different scan rates, keeping the working microelectrodes stationary. Qualitatively, sigmoidal shaped CVs recorded at 150 mVs-1 were observed for dots with diameters up to 100 µm, indicating that a characteristic hemispherical diffusion layer is formed, leading to a steady-state limiting current that appears to depend on dot diameter. At a slower scan rate of 10 mVs-1, sigmoidal shape is observed on dot diameters up to 300 µm. Furthermore, the current observed for an array is equal to the sum of the currents of the individual dots, indicating that no overlapping of the individual diffusion layers is occurring and that each dot is behaving as an individual microelectrode. Using chronoamperometric techniques, potentiostatic continuous and pulsed electrodepositions of Cu were performed on all dot sizes, keeping a constant deposition charge per unit area, using an aqueous Cu2+ basic bath at room temperature. Characterization using scanning electron microscopy showed the deposited layers in the dots are continuous throughout the array and energy-dispersive X-ray spectroscopy data showed no difference in the amount of material deposited in each dot across the array. Surprisingly, the electrodeposition plating efficiency does appear to depend upon the dot size. Overall, large area samples prepared using a continuous potentiostatic method have shown RMS roughness of about 70 nm, while the pulsed method has produced layers with lower roughness of around 10 nm, as measured by profilometry for Cu layers with thicknesses between 150 < d (nm)< 350. Several micro solar cell proof-of-concept devices were successfully prepared from electrodeposited precursors and showed a maximum power conversion efficiency of 4.8% at 1 sun illumination by obtaining current-voltage curve from a single dot with a diameter of 200 µm. Figure 1