AbstractsPhysics

SnS nanoparticles to boost CuInS2 solar cells

by C. Prastani




Institution: Universiteit Utrecht
Department:
Year: 2015
Keywords: SnS quantum dots; core-shell nanoparticles; CIGS; solar cells; spray deposition; AFM
Record ID: 1250604
Full text PDF: http://dspace.library.uu.nl:8080/handle/1874/309387


Abstract

To make photovoltaics a competitive energy source it is essential to increase its energy conversion efficiency. Quantum dots can play an important role to overcome the Shockley-Queisser efficiency limit. Theoretically, a quantum dot based Intermediate Band solar cell can reach an efficiency of 63.2% at maximum light concentration. This thesis is focused on chalcogenide quantum dots, SnS and SnS/In2S3 core-shell structure, to boost the efficiency of CuInS2 solar cells. SnS nanoparticles were synthesized and characterized in order to study their structure and optical properties. These SnS nanoparticles are crystalline spherical dots with a diameter of 4 ± 2 nm and zincblende structure with a band gap of ~1.6 eV. Moreover, the presence of two absorption peaks in the infrared region and the detection of a paramagnetic center with free electron type g-value led us to suspect that an electron was transferred into the SnS QDs. Scanning Tunneling Spectroscopy measurements showed that nanoparticles with a size of 2 nm behave as perfect semiconductor quantum dots with a clear band gap, whereas 4 nm sized particles showed a different behavior. Afterwards, SnS nanoparticles were capped with a shell made of In2S3 material by chemical bath deposition, making a core-shell structure. These core-shell nanoparticles have a size in the range of 5-15 nm and they showed a crystalline core and an amorphous shell. Moreover, the effect of the deposition time and temperature were studied to verify their effect on the optical absorption and to optimize the deposition parameters. For electronic device applications, it is important to study the electrical properties of individual particles. For this purpose two different types of Atomic Force Microscopy techniques, torsional resonance tunneling AFM (TR-TUNA) and peak force AFM (PF-AFM), were used to investigate the topography and local conductivity of SnS and SnS/In2S3 core-shell nanoparticles. By means of TR-TUNA it was possible to obtain the topography and conductivity maps of the core-shell nanoparticles, but it was not possible to study the conductivity of SnS nanoparticles. This is due to the presence of TOPO ligands, which does not allow the tip to approach the particles properly. To overcome this problem PF-AFM was used. We found that this mode allows one to study SnS capped with TOPO, mapping both the size and the current of single SnS nanoparticles with a single measurement. These two studies combined confirmed that both the core and the shell are conductive and that the charge transport across the core/shell interface can take place. In the end SnS and SnS/In2S3 core-shell nanoparticles were embedded into a CuInS2 solar cell, made by spraying technique, and in a CIGS-type solar cells made by co-evaporation. I-V measurements showed that the CIS solar cell with embedded core-shell nanoparticles has the best performance among all the above combinations. The measurements carried out on CIGS solar cells proved that the SnS nanoparticles embedded in the cell do not improve the current, but…