Close-packed arrays of semiconductor nanocrystals (NCs), or quantum dots, are ideal systems for fundamental investigations of photo-induced charge and energy transfer in interacting quantum-confined materials. The materials, diameters, and arrangement of the NCs can be used to tune the inter-NC coupling to exploit both the properties of the individual NCs and the long-range effects of the solid. The emergent optical, electronic, and thermal properties of NC superlattices may lead to transformational improvements in applications including photovoltaics, photonics, and thermoelectrics.
The broad objectives of this proposal are (1) to understand ultrafast charge carrier generation, separation, recombination, and transport phenomena in semiconductor nanocrystal superlattices, and (2) to control these fundamental photophysical processes to improve solar cell performance. Specifically, we will investigate films of CdSe, CdTe, and Cu2ZnSnS4 (CZTS) NCs. CdSe and CdTe NCs are excellent model systems because their synthesis and optical properties are well-understood, enabling fundamental ultrafast studies of carrier dynamics in glassy arrays and ordered superlattices of a single monodisperse NC species, as well as binary NC superlattices. CZTS NCs provide an exciting new direction for high efficiency photovoltaics made from non-toxic, earth-abundant elements. The PIs will refine the synthesis of monodisperse CZTS NCs to enable meaningful ultrafast spectroscopic characterization.
This approach centers on time-resolved terahertz spectroscopy (TRTS) and femtosecond visible/infrared transient absorption (TA) to probe intraband and interband transitions, respectively. THz spectroscopy is an ideal, non-contact probe of electronic materials because the THz frequency regime (0.1 - 3 THz) brackets typical carrier scattering rates in semiconductors. THz spectroscopy is unique in its abilities to distinguish between excitons and free carriers and to measure their dynamics on sub-picosecond to nanosecond time scales, providing an excellent complement to our steady-state field effect transistor (FET) measurements. Pump-probe TRTS and TA are ideal techniques to investigate the dynamics of interfacial charge transfer, recombination, and inter-NC transport of photoexcited carriers on their natural time and energy scales.
This work will advance our understanding of the physical phenomena that govern ultrafast exciton and free carrier dynamics in NCs and NC superlattices. Specific studies will include: (1) Determining mechanisms of charge transport in NC superlattices, e.g. by extended states or by activated hopping; (2) Measuring dynamics of inter-NC coupling, interfacial charge transfer, and long-range charge transport in superlattices of a single monodisperse NC species; (3) Determining the dependence of dynamics and transport mechanisms on NC size, capping ligand, inter-NC spacing, and long range order; (4) Understanding charge separation and transport in binary NC superlattices; and (5) Incorporating good candidate materials into solar cells to demonstrate improvements in efficiency that result from carefully designed NC architectures. This work will address the challenge of maintaining quantum-confined NC photophysics while also enabling long range charge transport necessary for devices. PI Baxter?s expertise in ultrafast spectroscopy and solar cells and PI Murray?s expertise in synthesis of NCs and superlattices make the team well-equipped to carry out this work.
The understanding of fundamental photophysical processes such as interfacial charge transfer, recombination, and inter-NC transport in NC superlattices developed here can be applied to create high-efficiency NC solar cells. Availability of efficient, low-cost, clean, and sustainable solar cells made from earth-abundant, non-toxic materials would transform the US energy portfolio. This project will result in the education and training of two Ph.D. students and multiple undergraduates. Additionally, PI Baxter is developing new courses on "Fundamentals of Solar Cells" and lab-based "Nanomanufacturing for Energy Applications" for students from both universities. Outreach will extend to K-12 students by the PIs? continued participation in NanoDay@Penn, Philly Materials Day at Drexel, and mentoring local high school teachers through NSF RET and university programs. These programs are particularly beneficial for underrepresented groups since they target students and teachers from the School District of Philadelphia, whose student body is over 80% minorities.
