Investigación cuantitativa

The focus of this research has been to customize and optimize CZTS and CIS

chalcogenide thin film PVs layer-by-layer. This was accomplished by beginning with the synthesis of the absorbing layers, followed by the optimization of each subsequent layer in the solar cell as well as modifying the interfaces between them. The physical,

chemical, and optical properties of CIS and CZTS were explored and identified utilizing a wide range of techniques and methods. This work has allowed for insight into the properties of the chalcogenide thin films in order to facilitate an effective absorbing layer and incorporate it into a PV device while maintaining low costs.

Chapter 2 deals with the synthesis of CZTS nanocrystals (NCs) via a facile and low-cost one-pot method. Using photoelectrochemical measurements (PECMs) as a basis for effective light absorbing layers, photovoltaic behaviour of NC films was assessed. The optimization of the CZTS through PECMs is a simple and powerful strategy. Non- stoichiometric copper rich and zinc poor starting molar ratios provided the best overall products although they are not concerted throughout the reaction. Other analytical and physical techniques were further used to identify NC composition, topography and

crystallinity and oxidation states of the elements. Analogous experiments were

performed to characterize CIS thin films in a similar fashion.34 _{This work generated the} basis for much of the following research and functioned as a base on which the following work was carried out.

Through the continuation of electrochemical probing, the interface of CIS nanocrystal films (NCF) were explored in Chapter 3. Intensity modulated photocurrent spectroscopy (IMPS) was performed at the nanocrystal film/solution interface. IMPS quantified the kinetic constants of the photoreaction. The rate ratio of the product separation to recombination increased as a function of applied potential, while the RC time constant decreased. Interfacial reaction kinetics of a solution-phase oxidant and the CIS film was revealed by means of PECMs and IMPS. The interaction of the light absorbing CIS NCFs at a hole-rich interface allow for an understanding of what might happen at an analogous n-type junction. CZTS/solution interfaces were also probed utilizing the IMPS

technique, giving insight into how CIS and CZTS would be able to act at an interface as well as which conditions were conducive to photocurrent generation.67-68

Chapter 4 delved into CZTS NCs based on their initial metal stoichiometry and photoresponse behavior. CZTS films were divided into groups of high-photoresponse (hp-CZTS) and low-photoresponse samples (lp-CZTS). An X-ray absorption near-edge structure (XANES) study was then performed to unravel the origin of the difference in their photovoltaic properties. The results demonstrated that, the local structures of the elements and their interaction with capping ligands are different and strongly affect the photovoltaic behavior, although both CZTS groups through one-pot synthesis were free of secondary phases. It was determined that the coordination of Zn and the capping

ligand-metal interaction are the two major factors for the production of higher

photocurrent. The correlations were used as a guide to produce viable CZTS films crucial for the development of solar energy conversion.

The CZTS/CdS heterojunction was examined using photoelectrochemical and synchrotron radiation (SR) spectroscopies in Chapter 5. The study provided physical insights into the interface that was formed by electrophoretic deposition (EPD) of CZTS NCs and chemical bath deposition (CBD) of CdS for the two respective films. Results showed that CBD induced a change in the local and long range environment of the Zn in the CZTS lattice that was detrimental to the photoresponse. XANES and extended X-ray absorption fine structures (EXAFS) of the junction showed that this change was at an atomic level and was associated with the coordination of oxygen to zinc, which was confirmed through FEFF fitting. It was discovered that this change can be reversed with the use of low temperature annealing in both photoresponse and in the Zn. Investigating CZTS through synchrotron radiation (SR) techniques provides detailed structural

information of minor changes from the zinc perspective.

Both Chapter 6 and Chapter 7 dealt with the conclusion of the construction of solar cell devices. The construction of the full device and the measurement of efficiency for CZTS solar cell constructed via a NC route and through galvanostatic electroplating of metal precursors. The EPD of CZTS NCs and formation of the solar cell followed closely with the information obtained from previous chapters on optimized solar cell layers. For the electroplated samples, galvanostatic electrodeposition precursors from environmentally friendly electrolytes and sulfurization were implemented. Optimized sequential

while the sulfurization of Mo/Cu/Sn/Zn precursors was performed. An analogous series of tests were performed to confirm if the findings from previous chapters could be applied to these new CZTS films. The produced CZTS films were characterized in detail by PECMs, scanning electron microscopy (SEM) with energy dispersive X-ray

spectroscopy (EDX), Raman spectroscopy, X-ray diffraction (XRD), and UV-Vis spectroscopy. In both cases the results of XRD and Raman depicted that all CZTS films possessed kesterite structure. A direct band gap of about 1.47 and 1.45 eV for the produced for the EPD and electroplated CZTS films respectively. All the measurements demonstrated that the CZTS films fabricated by EPD and galvanostatic electroplating meet the requirements for a light-absorbing layer and is a potential candidate in CZTS solar cells. The J-V measurement demonstrates that the conversion efficiencies of the CZTS solar cells are 1.28 and 2.21 % for EPD and electroplating respectively.

These chapters have formed a plethora of knowledge in areas of NC CIS and CZTS solar cells and serve as a stepping stone for those to continue to optimize and change the interfaces and reactions of the solar cell device. The CZTS route has not yet reached its full potential, but with increasing amounts of work and investigative studies into its properties, a penultimate solar cell is close at hand. Focusing on a cost-effective and environmentally-friendly approach not only enriches the sole drive for solar energy, but is forward thinking that will allow for a sustainable future in energy production.