Synthesis of large CZTSe nanoparticles through a two-step hot-injection method

Grain boundaries in Cu2ZnSn(SxSe1x)4 (CZTSSe) thin films act as a defect that reduces the mobility of the charges. Hence one way to improve the performance of these thin film solar cells is to increase the grain size in the films. Most of the synthesis methods published so far for CZTSSe colloidal nanoparticles can achieve a general size distribution range from 5–20 nm. This is where the particle size will saturate for most recipes used today. The assumption is that uniform size distribution is good for grain growth in a thin film but based on packing considerations, an optimal mixture of large and small nanoparticles that can easily be dispersed in non-polar solvents could be better. Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanoparticles are synthesized using the hot-injection method with oleylamine, trioctylphosphine, and hexadecane as the solvents. Selenium (Se) is introduced in the liquid phase to encourage grain growth – liquid selenization. This eliminates the need to anneal the film in a Secontaining atmosphere and allows for a more environmentally friendly process with lower temperatures and shorter annealing times. We show that a good dispersion can be achieved by choosing suitable surfactant molecules, solvents and precursors, and by controlling the initial monomer concentration. Additionally, we show how our new synthesis route can be utilized to achieve targeted ratios of CZTS and CZTSe nanoparticles to be used for mixed-phase CZTSSe thin films

Picosecond dynamics of internal exciton transitions in CdSe nanorods

The picosecond dynamics of excitons in colloidal CdSe nanorods are directly measured via their 1s to 2p-like internal transitions by ultra-broadband terahertz spectroscopy. Broadened absorption peaks from both the longitudinal and transverse states are observed at 8.5 and 11 THz, respectively. The onset of exciton-LO phonon coupling appears as a bleach in the optical conductivity spectra at the LO phonon energy for times > 1 ps after excitation. Simulations show a suppressed exciton temperature due to thermally excited hole states being rapidly captured onto ligands or unpassivated surface states. The relaxation kinetics are manipulated and the longitudinal transition is quenched by surface ligand exchange with hole capturing pyridine

Influence of interfacial chemistry on conjugated polymer : cadmium selenide nanorods hybrid photovoltaics

Organic-inorganic hybrid solar cell (HSC) combines both the unique properties of the conjugated polymers and inorganic nanocrystals. It has shown great potential in realizing low-cost solar cells. One of the key challenges in the advancement for the performance of hybrid photovoltaics is the incompatible interfaces between the organic and inorganic components. Post-synthesis ligand exchange of the nanocrystals is one of the viable approaches to improve the donor-acceptor interfaces. The selection of the replacing surface ligands are based on several considerations such as the size, structure and end groups of the ligand molecules and its affinity for nanocrystals surface and polymers. In this thesis, surface chemistry of cadmium selenide (CdSe) nanocrystals is investigated to improve the performance of the solar cells. CdSe nanorods are synthesized using hot-coordinating solvent method and are used as the electron acceptor in the HSCs. The ligand used to control growth during the synthesis is phosphonic acid with relatively long alkyl chain. To improve the electrical properties of the blend films, ligand exchange of CdSe nanorods using thiophenealkylamines is first demonstrated. Surface analysis of the CdSe nanorods showed that the original bulky surface ligands are partially replaced by the replacing ligands. The short-circuit current density (JSC) of the HSCs made of ligand exchanged CdSe nanorods is found to be improved by at least three times due to the reduced distance between polymer and nanocrystals and higher conductivity across the new ligands. Atomic force microscopy (AFM) is used to determine the surface morphology of the films. It is found that a combination of surface ligands is a better option to obtain optimal film morphology for photovoltaics applications. Carboxylic acid-based molecules have the potential for both controlling the anisotropic growth and providing the surface chemistry for good electrical transport of the CdSe nanocrystals. Therefore, the effect of several thiophene-carboxylic acid molecules as surface ligands for CdSe nanorods are studied and the HSCs made of these nanorods was able to achieve reasonably good performance. Pyridine is added into the host solvent as solvent additive to aid the dispersion of the nanorods. The effect of solvent additive is observed to be two-fold. It aids in dispersing the nanocrystals but at the same time induces severe aggregation of the semi-crystalline poly(3-hexylthiophene) (P3HT).Doctor of Philosophy (MSE

Understanding polycarbazole-based polymer : CdSe hybrid solar cells

We report for the first time the fabrication and characterization of organic–inorganic bulk heterojunction (BHJ) hybrid solar cells made of poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and pyridine-capped CdSe nanorods. By optimizing both CdSe loading and active layer film thickness, the power conversion efficiencies (PCEs) of PCDTBT:CdSe hybrid solar cells were able to reach 2%, with PCDTBT:CdSe devices displaying an open-circuit voltage (VOC ) that is 35% higher than P3HT:CdSe devices due to the deeper HOMO level of PCDTBT polymer. The performance of PCDTBT:CdSe devices is limited by its morphology and also its lower LUMO energy offset compared to P3HT:CdSe devices. Hence, the performance of PCDTBT:CdSe solar cells could be further improved by modifying the morphology of the films and also by including an interlayer to generate a built-in voltage to encourage exciton dissociation. Our results suggest that PCDTBT could be a viable alternative to P3HT as an electron donor in hybrid BHJ solar cells for high photovoltage application

Understanding polycarbazole-based polymer : CdSe hybrid solar cells

We report for the first time the fabrication and characterization of organic–inorganic bulk heterojunction (BHJ) hybrid solar cells made of poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and pyridine-capped CdSe nanorods. By optimizing both CdSe loading and active layer film thickness, the power conversion efficiencies (PCEs) of PCDTBT:CdSe hybrid solar cells were able to reach 2%, with PCDTBT:CdSe devices displaying an open-circuit voltage (VOC ) that is 35% higher than P3HT:CdSe devices due to the deeper HOMO level of PCDTBT polymer. The performance of PCDTBT:CdSe devices is limited by its morphology and also its lower LUMO energy offset compared to P3HT:CdSe devices. Hence, the performance of PCDTBT:CdSe solar cells could be further improved by modifying the morphology of the films and also by including an interlayer to generate a built-in voltage to encourage exciton dissociation. Our results suggest that PCDTBT could be a viable alternative to P3HT as an electron donor in hybrid BHJ solar cells for high photovoltage application

Electron transport limitation in P3HT : CdSe nanorods hybrid solar cells

Hybrid solar cells have the potential to be efficient solar-energy-harvesting devices that can combine the benefits of solution-processable organic materials and the extended absorption offered by inorganic materials. In this work, an understanding of the factors limiting the performance of hybrid solar cells is explored. Through photovoltaic-device characterization correlated with transient absorption spectroscopy measurements, it was found that the interfacial charge transfer between the organic (P3HT) and inorganic (CdSe nanorods) components is not the factor limiting the performance of these solar cells. The insulating original ligands retard the charge recombination between the charge-transfer states across the CdSe–P3HT interface, and this is actually beneficial for charge collection. These cells are, in fact, limited by the subsequent electron collection via CdSe nanoparticles to the electrodes. Hence, the design of a more continuous electron-transport pathway should greatly improve the performance of hybrid solar cells in the future.NRF (Natl Research Foundation, S’pore)Accepted versio

Correction: Polymer nanofibers: preserving nanomorphology in ternary blend organic photovoltaics

International audienceno abstrac

Electron Transport Limitation in P3HT:CdSe Nanorods Hybrid Solar Cells

Hybrid solar cells have the potential to be efficient solar-energy-harvesting devices that can combine the benefits of solution-processable organic materials and the extended absorption offered by inorganic materials. In this work, an understanding of the factors limiting the performance of hybrid solar cells is explored. Through photovoltaic-device characterization correlated with transient absorption spectroscopy measurements, it was found that the interfacial charge transfer between the organic (P3HT) and inorganic (CdSe nanorods) components is not the factor limiting the performance of these solar cells. The insulating original ligands retard the charge recombination between the charge-transfer states across the CdSe–P3HT interface, and this is actually beneficial for charge collection. These cells are, in fact, limited by the subsequent electron collection via CdSe nanoparticles to the electrodes. Hence, the design of a more continuous electron-transport pathway should greatly improve the performance of hybrid solar cells in the future