Field of inserted charges during Scanning Electron Microscopy of non-conducting samples

Three different approaches to calculating the electric potential in an inhomogeneous dielectric next to vacuum due to a charge distribution built up by the electron beam are investigated. An analytical solution for the electric potential cannot be found by means of the image charge method or Fourier analysis, both of which do work for a homogenous dielectric with a planar interface to vacuum. A Born approximation gives a good approach to the real electric potential in a homogenous dielectric up to a relative dielectric constant of 5. With this knowledge the electric potential of an inhomogenous dielectric is calculated. Also the electric field is calculated by means of a particle-mesh method. Some inhomogeneous dielectric configurations are calculated and their bound charges are studied. Such a method can yield accurate calculations of the electric potential and can give quantitative insight in the charging process. A capacitor model is described to estimate the potential due to the charge build up. It describes the potential build up in the first microseconds of the charging. Thereafter, it seems that more processes have to be taken into account to describe the potential well. This potential can further be used in a macroscopic approach to the collective motion of the electrons described by the Boltzmann transport equations or a derived density model, which can be a feasible alternative approximation to the more commonly used Monte-Carlo simulation of individual trajectories

SnS nanoparticles to boost CuInS2 solar cells

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 the open circuit voltage, presumably the SnS QDs acting as a charge diffusion barrier

Synthesis and Conductivity Mapping of SnS Quantum Dots for Photovoltaic applications

Quantumdots (QDs) are considered a possible solution to overcome the Shockley–Queisser efficiency limit of 31% for single junction solar cells by efficiently absorbing above band gap energy photons through Multiple Exciton Generation (MEG) or sub band gap energy photons using an Intermediate Band Solar Cell structure (IBSC). For the latter absorption process, we consider tin sulphide (SnS) as a promising candidate, having several advantages compared to the other nanoparticles studied extensively so far, such as CdS, CdSe, PbS, and PbSe; namely it is non-toxic and environmentally benign and thus will be most suitable in consumer products such as solar panels. In this work we propose a new colloidal synthesis method for SnS QDs. We have obtained mono-dispersive SnS and SnS/In2S3 core–shell nanoparticles with a size of ∼4 nm. Energy dispersive X-ray spectroscopy (EDX) elemental analysis revealed that the particles are indeed SnS and not SnS2. Furthermore, the conductive nature of the nanoparticles has been inferred by conductivitymapping using a relatively new contactless technique, Torsional Resonance Tunneling AFM (TR-TUNA). These results confirm that the SnS QDs possess all the requirements to be applied as photoactive layers in photovoltaic devices. -------------------------------------------------------------------------------

SnS nanoparticles to boost CuInS2 solar cells

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 the open circuit voltage, presumably the SnS QDs acting as a charge diffusion barrier

Synthesis and Conductivity Mapping of SnS Quantum Dots for Photovoltaic applications

Quantumdots (QDs) are considered a possible solution to overcome the Shockley–Queisser efficiency limit of 31% for single junction solar cells by efficiently absorbing above band gap energy photons through Multiple Exciton Generation (MEG) or sub band gap energy photons using an Intermediate Band Solar Cell structure (IBSC). For the latter absorption process, we consider tin sulphide (SnS) as a promising candidate, having several advantages compared to the other nanoparticles studied extensively so far, such as CdS, CdSe, PbS, and PbSe; namely it is non-toxic and environmentally benign and thus will be most suitable in consumer products such as solar panels. In this work we propose a new colloidal synthesis method for SnS QDs. We have obtained mono-dispersive SnS and SnS/In2S3 core–shell nanoparticles with a size of ∼4 nm. Energy dispersive X-ray spectroscopy (EDX) elemental analysis revealed that the particles are indeed SnS and not SnS2. Furthermore, the conductive nature of the nanoparticles has been inferred by conductivitymapping using a relatively new contactless technique, Torsional Resonance Tunneling AFM (TR-TUNA). These results confirm that the SnS QDs possess all the requirements to be applied as photoactive layers in photovoltaic devices. -------------------------------------------------------------------------------

Development of SnS/In2S3 core-shell nanoparticles for solar cell application

SnS nanoparticles (NPs) were synthesized by a colloidal route at low temperatures and analyzed by several characterization techniques. Whereas transmission electron microscopy (TEM) and atomic force microscopy (AFM) imaging conclusively proved quantum dot sized particles (∼4 nm) with a narrow size distribution, Torsional Resonance-Tunneling AFM and Peak Force AFM proved the conductive nature of the particles. The chemical composition, studied by energy dispersive X-ray spectroscopy (EDX) showed the ratio S:Sn of 1:1, confirming that the semiconductor is SnS and not any other compound. To passivate the QD surface and protect it from reaction to ambient, core-shell structures were made. The SnS nanoparticles (NPs) were immersed in a CBD bath for deposition of In2S3 layers. The formed core/shell SnS/In 2S3 nanoparticles were separated by centrifugation and washed with ethanol. The structure of the core-shell SnS/In2S 3 NPs has been studied by High Resolution TEM which showed the lattice fringes of the SnS core, surrounded by amorphous matrix, tentatively attributed to In2S3. The EDX confirms the presence of the elements expected. The absorption spectra of SnS/In2S3 nanoparticles with increasing time of CBD In2S3 clearly showed increasing band gap, attributed to thicker In2S3 shell. Research on SnS QDs embedded in CIS solar cells is in progress

Development of SnS/In<inf>2</inf>S<inf>3</inf> core-shell nanoparticles for solar cell application

SnS nanoparticles (NPs) were synthesized by a colloidal route at low temperatures and analyzed by several characterization techniques. Whereas transmission electron microscopy (TEM) and atomic force microscopy (AFM) imaging conclusively proved quantum dot sized particles (∼4 nm) with a narrow size distribution, Torsional Resonance-Tunneling AFM and Peak Force AFM proved the conductive nature of the particles. The chemical composition, studied by energy dispersive X-ray spectroscopy (EDX) showed the ratio S:Sn of 1:1, confirming that the semiconductor is SnS and not any other compound. To passivate the QD surface and protect it from reaction to ambient, core-shell structures were made. The SnS nanoparticles (NPs) were immersed in a CBD bath for deposition of In2S3 layers. The formed core/shell SnS/In 2S3 nanoparticles were separated by centrifugation and washed with ethanol. The structure of the core-shell SnS/In2S 3 NPs has been studied by High Resolution TEM which showed the lattice fringes of the SnS core, surrounded by amorphous matrix, tentatively attributed to In2S3. The EDX confirms the presence of the elements expected. The absorption spectra of SnS/In2S3 nanoparticles with increasing time of CBD In2S3 clearly showed increasing band gap, attributed to thicker In2S3 shell. Research on SnS QDs embedded in CIS solar cells is in progress

Synthesis and conductivity mapping of SnS quantum dots for photovoltaic applications

Quantum dots (QDs) are considered a possible solution to overcome the Shockley–Queisser efficiency limit of 31% for single junction solar cells by efficiently absorbing above band gap energy photons through Multiple Exciton Generation (MEG) or sub band gap energy photons using an Intermediate Band Solar Cell structure (IBSC). For the latter absorption process, we consider tin sulphide (SnS) as a promising candidate, having several advantages compared to the other nanoparticles studied extensively so far, such as CdS, CdSe, PbS, and PbSe; namely it is non-toxic and environmentally benign and thus will be most suitable in consumer products such as solar panels. In this work we propose a new colloidal synthesis method for SnS QDs. We have obtained mono-dispersive SnS and SnS/In2S3 core–shell nanoparticles with a size of ~4 nm. Energy dispersive X-ray spectroscopy (EDX) elemental analysis revealed that the particles are indeed SnS and not SnS2. Furthermore, the conductive nature of the nanoparticles has been inferred by conductivity mapping using a relatively new contactless technique, Torsional Resonance Tunneling AFM (TR-TUNA). These results confirm that the SnS QDs possess all the requirements to be applied as photoactive layers in photovoltaic devices