Fabrication And Performance Of Solar Cells Based On ColloidalⅣ-ⅥQuantum Dots

The worldwide increase in energy demand and the concomitant rise of greenhouse gas emissions are challenging researchers to find clean and sustainable energy sources. Solar energy is thought to be the most promising candidate for replacing fossil-fuel power because of its availability, safety, sustainability and potential to reduce pollution. Photovoltaics are perhaps the most direct way of harvesting solar energy. While solar cells based on conventional inorganic materials exhibit high energy conversion efficiency, the current cost of electricity generated by these photovoltaics still cannot compete with the cost of fossil fuel-generated electricity. Photovoltaic devices based on colloidal inorganic nanoparticles have the potential for combining lower cost with higher efficiency via solution processing and possible multiple exciton generation. In this dissertation, I present research on fabrication and performance optimization of solution-processed solar cells based on colloidal IV-VI quantum dots. The exploration of multiple exciton generation in working quantum dot solar cells is also included.Lead selenide nanocrystals with different shapes including octahedral, spindle-like and cluster-like nanopartilces were synthesized via a hot-injection method by adjusting the reaction conditions. On the basis of experimental observations and analysis, we proposed possible growth mechanisms and shape evolution processes of these nanocrystals. Also, lead sulfide nanoparticles with different sizes were synthesized with a similar method, laying the foundation for fabricating photovoltaic devices based on the colloidal nanocrystals.The device structure of PbS quantum dot solar cells were optimized by studying the effects of various structure parameters on device performance. Power efficiencies above 3% under AM 1.5 were achieved from optimized TiO2/PbS quantum dot solar cells, with peak external quantum efficiencies of about 80% and significant responses in the near-infrared region.The short-term exposure of TiO2/PbS quantum dot photovoltaics to air is shown to increase the open circuit voltage and fill factor, resulting in an increase in power conversion efficiency to above 4% for～1.1 eV PbS quantum dots. For the～1.7 eV bandgap devices, open circuit voltages of up to 0.65V are obtained. Long-term air exposure results in much lower conductivities, an effect that can be partly reversed through soaking in 1,2-ethanedithiol. These results are caused by changes in the surface chemistry of the PbS quantum dots, and not by the surface trap states. The performance improvement of TiO2/PbS quantu dot solar cells is observed during the aging process in nitrogen, which is ascribed to the changes in the electrical properties of PbS quantum-dot films upon N2 exposure. Our results emphasize the importance of environmental conditions on understanding and optimizing QD solar cell performance.While it can improve their open circuit voltages, air annealing at 90℃results in performance degradation of quantum-dot solar cells due to drastically decreased short circuit currents. Nitrogen annealing at or below 90-100℃for 10 minutes is shown to increase devices' short circuit current and external quantum efficiency due to improved TiO2/PbS interface properties. However, nitrogen annealing at higher temperatures will degrade device performance. An external quantum efficiency of～95% is achieved from PbS quantum-dot solar cells annealed in nitrogen at 90℃for 6 minutes.Device performance improvement of photovoltaic cells based on ternary PbSxSe1-x quantum dots is achieved by utilizing a heterojunction type device configuration. The best device shows an AM 1.5 power conversion efficiency of 4.25%,30% higher than the previous efficiencies achieved for PbSxSe1-x photovoltaics based on Schottky junctions. Furthermore, this ternary PbSxSe1-x quantum dot heterojunction device has a peak external quantum efficiency above 100% at～2.76 eV, indicating the existence of multiple exciton generation in ternary quantum dots and their working devices.The limiting factors of our PbS quantum-dot devices are analysed by applying a physical model developed by Goodman and Mihailetchi for photoconductive layers. It is shown that our devices are limited by carrier mobility and lifetime. Also, trap-assisted recombination exists in our devices. These results shed light on new approaches for further optimization of quantum-dot solar cell performance by increasing carrier mobility and lifetime and decreasing traps in active layers.