The most effective semiconductors used as absorber layers for solar cells have concerns regarding earth abundance, toxicity, cost-volatility of the materials, or high capital expenditure (CapEx) for new manufacturing facilities. Solution processing is a low cost, low temperature development method leading to lower CapEx. The exploration of "new" photovoltaic materials seeks to develop an earth abundant, non-toxic semiconductor via solution processing with efficiencies comparable to market-leading materials like silicon or CdTe. Bismuth rudorffites (chemical formula A_aBi_bX_a+3b) are a category of new materials proven to be solution processable, to have high absorption, and to be capable of cell efficiencies over 5%. One of the limitations of bismuth rudorffites thus far is current flow; the electrons that are capable of providing electrical power are not being extracted from the absorber material effectively before they return to their stable low-energy state. A way in which this limitation can be explored is via photoconductivity, the difference between material conductivity under illumination versus in the dark. My project seeks to characterize the photoconductivity of silver bismuth iodide (Ag_aBi_bI_a+3b) as a function of the ratio a/b, identifying the composition(s) that best facilitate electron transport and the crystal phases to which they correspond. Results indicate that ion migration within the crystal lattice occurs when a/b is large, and that high a/b ratios introduce AgI impurities in the film that dramatically increase the photoconductivity, among other important phenomena. This presentation gives procedures, results, and analyses from this photoconductivity exploration, working toward a more advanced understanding of bismuth rudorffite material properties.

The human body does not fully metabolize a pharmaceutical dose. Consequently, these stable compounds are excreted through human waste, and many pass through wastewater treatment plants untreated, polluting aquatic ecosystems and drinking water supplies. Point-source electrochemical oxidation of fresh human urine can be a low cost, versatile option for eliminating these biologically active compounds before their discharge into the environment. To design an effective device to do this, it is important to understand the mechanisms of pharmaceutical degradation in a complex system containing solution-phase and interfacial chemistry as well as kinetic and mass transport limited degradation rates. We used the limiting current technique to characterize the mass transport conditions in a cylindrical electrochemical cell stirred by a magnetic stir bar. By conducting a steady state potential scan on electrolyte containing a kinetically facile redox couple, [Fe(CN)₆]^3-/[Fe(CN)₆]^4-, we measured the mass transport limiting current. The limiting current reflects the reaction rate of an electroactive compound with the electrode surface, and this rate is limited by the diffusion of the compound through a boundary layer with a thickness defined by the system’s convective conditions. We derived a Sherwood number correlation as a function of Reynolds number and Schmidt number by varying stir rate, stir bar dimensions and shape and measured the corresponding limiting current. Knowledge of the mass transport coefficient demonstrated that pharmaceutical degradation can be mass transport limited on boron-doped diamond (BDD) anodes in full synthetic urine matrixes. For the same stir conditions, observed rate constants on iridium(IV) oxide (IrO₂) anodes fell below the mass transport limiting rate indicating a kinetic limit for pharmaceutical degradation. The limiting current technique is an easy method to characterize and predict mass transport conditions for any geometry and helps differentiate between interfacial and solution-phase degradation pathways for environmental pollutants.