NANO-SCALE CHARGE SEPARATION IN PdTeI AND PEROVSKITE PHOTOVOLTAIC DEVICES OBSERVED BY CRYSTALLOGRAPHY AND TEMPERATURE-DEPENDENT PHOTOCURRENT SPECTROSCOPY
Cottingham, Patrick Landon
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Charge separation in the solid state is a process that is fundamental to technologies such as heterogenous catalysts, batteries, and photovoltaics. Additionally, spontaneous charge separation occurs as part of certain phase transitions, such as charge ordering and the formation of charge density waves. A variety of well-developed techniques exist for studying bulk charge separation and phases in which charge separation is coherent over long lengthscales. However, many interesting charge separation processes are ordered on the nano-scale or are affected by nano-scale structure (for materials) or architecture (for devices). This thesis represents progress towards combining and improving existing methodologies in order to investigate charge separation on the nano-scale. In Chapter 2, a combination of physical properties measurements, traditional diffraction measurements, and total scattering measurements are used to investigate a charge density wave in the material PdTeI. PdTeI features quasi-1D -Pd-Te-Pd-Te- chains with palladium formally in the 3+ oxidation state. Using pair distribution function analysis of x-ray and neutron total scattering data, we find that there is a local charge-density wave arising from the disproportionation of Pd3+ towards Pd2+ in pseudo-square planar, and Pd4+ in pseudo-octahedral, coordination. The magnitude and coherence length for this distortion is small, such that the average structure determined by Bragg diffraction techniques possesses higher symmetry than the local structure at all temperatures. Temperature-dependent resistivity measurements show a transport anomaly at TCDW = 50 K, corresponding to the reduced fluctuations of the charge separated Pd2+ and Pd4+ sites. At higher temperatures, the charge separation is dynamic, with local Pd2+/Pd4+ pairs persisting up to room temperature. Non-spontaneous charge separation may be driven by the absorption of a photon to generate one of more free carriers. A storied technique for studying this process is the measurement of photoconductivity. Contemporary technology allows for photoconductivity to be measured over a far greater continuous parameter space than previously possible. However, in order to make reproducible measurements that are comparable between instruments, it is crucially important to consider the effects of temperature, non-linearity, and the spectral width and intensity of lightsources used. Chaper 3 describes instrumentation for photovoltage and photocurrent spectroscopy over a larger continuous range of wavelengths and temperatures than other instruments described in the literature. This instrument uses a monochromated light source with total power < 30 μW incident on the device under test to maintain low temperatures, avoid thermal artifacts, and probe different regions of non-linear responses when used in conjunction with a second light source. The instrument may also be used to measure a related property, the photomagnetoresistance. The importance of normalizing measured responses for variations in light power is discussed and a rigorous process for performing these normalizations is detailed. Several circuits suited to measuring different types of samples are described and analysis for converting measured values into physically relevant properties is provided. Chapter 3 also discusses the role of non-linearity in photoconductivity measurements. Chapter 4 discusses results obtained by measuring the photocurrent of perovskite photovoltaic cells using the instrument described in chapter 3. Photocurrent measurements on devices containing the perovskite (CH3NH3)PbI3 show two distinct spectral responses when deposited in a mesoporous oxide matrix, compared with one response for planar perovskite alone. These two responses likely correspond to the medium-range ordered component and the nanocrystalline component that are known to form when the perovskite is deposited in a matrix. With a TiO2 matrix, the shorter wavelength response has an inverted temperature dependence with increasing performance on cooling. The trend is well described by an Arrhenius-type model, suggesting that it results from thermally-activated recombination. This work demonstrates how physical properties measurements can be used to characterize the energy- and time-scales of charge separation. It also demonstrates how crystallographic techniques can be used to examine changes in charge distribution at the nano-scale, both locally and in terms of average structure. Taken together, these results suggest a strategy for elucidating unknown mechanisms of charge separation in nano-scale systems.