Positron Annihilation Studies on Thin Film Solar Cells: CdSe and PbSe Quantum Dot Thin Films and Cu(In1-xGax)Se2 Layered Systems

High efficiency, low cost, and long stability are three key factors for the wide application of photovoltaics (PV) which are currently intensively studied in order to meet the increasing global renewable energy demand. Currently, the PV market is mainly based on silicon. However, solar cells based on silicon may not be capable to meet the long-term global energy demand due to their relatively high costs and high energy required for the synthesis of silicon wafers, opening the door to conventional thin films (such as Cu(In1-xGax)Se2 (CIGS)) and innovative thin films (e.g. semiconductor quantum dots (QDs), based on PbS, PbSe, CdSe QDs). The advantages of QDs as solar cell materials are the low-temperature synthesis process, the tunable band gap via control of the composition and size, and the promise of physical mechanisms that may increase efficiency above the Shockley-Queisser limit, such as multiple exciton generation (MEG), in which more than one exciton is created from a single photon. However, the low efficiency, with a current laboratory record just above 10%, and the durability are still limitations on their widespread application in the PV market. It is very important to understand the surface structure and surface-ligand interactions in order to improve the efficiency and stability of QD solar cells. For CIGS solar cells, research-cell efficiencies have reached 22.6%, which is just below the efficiencies of Si-based solar cells. Besides, various deposition approaches have been developed that can supply high-efficiency, low-cost and large-area solar cell devices. However, it is still a challenge to guarantee long-term stability of CIGS modules. CIGS solar cells can be well protected by sealing into glass plates, but this in turn increases the manufacturing cost. Therefore, understanding of the degradation mechanism is necessary.
Positron techniques are powerful tools to study the surface composition of QDs and to determine the types of open space deficiencies in thin film materials. For QDs, previous studies provided indications that positrons can trap and annihilate at the surfaces of semiconductor QDs and can effectively probe the surface composition and electronic structure of colloidal semiconductor QDs. For CIGS, previous depth-sensitive positron experiments indicated the sensitivity of positrons to probe the types of vacancy-related defects in CIGS.