DEFECT CHARACTERIZATION AND PHASE ANALYSES OF COPPER ZINC TIN SULFIDE/SELENIDE by

This work characterizes defects and phases within Cu2ZnSn(S,Se)4 (CZTS(Se)), an earth-abundant material used to make thin film photovoltaic solar cells. Overall research efforts focus on improving the solar cell device efficiency with the hope that it can be produced at the terawatt energy scale and circumvent material supply bottlenecks of current thin film photovoltaic technology. In this work, deep defects, composition-dependent crystalline disorder and secondary phase formation, and polymorph variation are all explored to determine the effects on the CZTS(Se) absorber layer within a solar cell device. Chapter 1 introduces how thin film photovoltaics fit into the global energy perspective and gives background into the ideal CZTSSe material characteristics, solar cell function, and current knowledge about why CZTS(Se) device performance still lags other current thin film photovoltaics. Chapter 2 explains the temperature admittance and deep level transient capacitance spectroscopy methods used in Chapter 3. Chapter 3 reports the observation of a deep defect state 590 meV from the conduction band edge with an attractive capture cross section of 2 x 10-14 cm behaving as an electron trap within nanoparticle-ink deposited CZTSSe. This is the first report of minority carrier trapping within CZTS(Se). Chapter 4 reports the coherent (Cu2SnS3, CZTS) and incoherent (CuxS, SnxSy) phases formed in a compositionally-varied coevaporated CZTS film. Raman spectroscopy experiments show Cu/Sn composition-dependent differences within the Cu2SnS3 crystalline structure of films deposited at low temperatures and CZTS crystalline disorder in films deposited at higher temperatures. This work demonstrates the deleterious effect of Sn-rich growth on the iv overall crystalline quality and possible defect concentration within CZTS. Chapter 5 reports the results of modeling the effects of varying the polymorph of the absorber layer within a CZTS solar cell. The kesterite and stannite polymorph variation do not significantly negatively impact the absorber layer in the bulk, however, the presence of kesterite at the interface and stannite in the bulk is shown to have the highest solar cell efficiencies. Chapter 6 summarizes each project and outlines future work.