CZTS-based photovoltaic devices by non-vacuum techniques

The primary benefit of thin-film photovoltaic devices is that they require less active material, a secondary benefit is that they are ideally suited for the creation of large modules. Last but not least they allow, if a flexible substrate is provided, to fabricate light-weight, flexible and foldable modules. These qualities provide them a guaranteed path to lowering the €/W cost of PV modules, and if they attain high efficiencies, they will be able to obtain a sizable market share. The copper-indium-gallium-selenium alloy CuInGaSe2 (CIGS) thin-film technology has the highest conversion efficiency (above 23%), but its industrial development could be restricted by the current rate of global indium production. A possible rival to current solar cell technology is the Cu2ZnSnS4 (CZTS) solar cell. The CZTS’s absorption coefficient is on the order of 104 cm-1: the straight band gap is within the 1.0-1.5 eV region which is ideal for solar cells. One micrometre thick film may effectively function as photocurrent generator and absorb nearly all of the photons in the solar spectrum. As a result, the cost of a solar cell's material can be significantly decreased by utilizing elements that are abundant on Earth. Making a CZTS film involves several different processes, including spray deposition of the precursor and subsequent sulfurization, or reactive co-evaporation, or precursor evaporation and subsequent sulfurization, or spray pyrolysis, or co-sputtering, or PLD (pulsed laser deposition), or sol-gel, or spin coating, or and electrodeposition and so on. This thesis details several investigations into the effects of CZTS manufacturing settings (by non-vacuum processes), such as drying time and annealing temperatures and, on the structure and optical characteristics. In the first chapter the project is presented, with also a brief introduction to renewable energy, the physical principles of how a photovoltaic cell work and the structure of kesterite. Chapter II introduces different strategies for the preparation of CZTS based solar cells. The third chapter illustrates the several characterisation techniques applied in this work. The subsequent chapters present the results of extensive research on the synthesis, characterization, and optimization of kesterite thin films and their application as absorber layers in photovoltaic devices. The study covers various aspects such as deposition techniques, post-deposition treatments, device fabrication and optimization, and analyses the impact of these factors on the photovoltaic performance of finished solar cells. Drying time and annealing temperature was shown to affect the stoichiometry and the morphology of the compound, generating secondary phases. This strongly influences the final efficiency of the devices and the behaviour under accelerated stability test. The present thesis provides a contribution in the understanding of low-cost non-vacuum preparation of kesterite absorbers. We obtained a final efficiency of 7.1% by doping with germanium, and over 8% with a doping solution of cadmium (this result is under patent approval).