Metal halide perovskite quantum wells (QWs) have been used to fabricate efficient optoelectronic devices, and exhibit stability superior to that of their bulk 3D counterparts. The perovskite QWs are tuned in synthesis so that they possess different bandgaps and exciton binding energies owing to variable quantum confinement as a function of QW thickness. Accordingly, the device performance of these materials depends on the efficiency of various interwell carrier dynamical processes, principally exciton and charge transfer. I will discuss the use of transient absorption and ultrafast two-dimensional electronic spectroscopy to probe interwell exciton transfer on timescales of 100s of femtoseconds, and show that interwell charge transfer occurs on timescales of 10s to 100s of picoseconds. These results, in addition to photoelectron spectroscopy experiments, are used to reconcile conflicting observations of type-I and type-II band alignment amongst perovskite QWs.
KEYWORDS: Polymers, Solar cells, Heterojunctions, Composites, Electron transport, Energy efficiency, Photovoltaics, Nanoparticles, Absorption, Solar energy
We report photovoltaic devices based on composites of a branched nanoheterostructure containing a CdTe core and CdSe arms, CdTe(c)-CdSe(a), combined with either poly(3-hexylthiophene), P3HT, or poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, with solar power conversion efficiencies of 1.2% and 1.8%, respectively. A comparison with previously reported composite devices of a related branched nanoheterostructure: CdSe(c)-CdTe(a) reveals an improved device performance that is attributed to a better electron percolation pathway provided by the dominant, higher electron affinity CdSe arms of the nanoheterostructures.
The diverse optical properties of colloidal semiconductor nanocrystals, including the linear and non-linear aspects of systems ranging from bulk-like to quantum confined, have been a topic of intense investigation for more than twenty years. However, many of the fundamental issues in these materials remain unresolved. Understanding these issues is key to their future use since the direct interrelation between their synthetically controllable size and shape and their physical properties make these systems ideal candidates for the development of designed materials.
The mechanism of resonance energy transfer between quantum dots is investigated theoretically. In order to incorporate explicit account of the selection rules for absorption of circularly polarized light, a quantum electrodynamical treatment of the electronic coupling is derived. The electronic coupling is mediated by the exchange of a virtual photon, which in the far zone limit acquires real character and is circularly polarized. The conditions by which quantum information, in terms of exciton spin orientation (total angular momentum quantum number), can be exchanged or switched through resonance energy transfer are discussed. Intrinsic exciton spin flip processes are shown experimentally to compete with typical energy transfer rates. Exciton spin flip times correspondingly range from <100 fs to 1.2 ps are reported.
Conference Committee Involvement (1)
Physical Chemistry of Interfaces and Nanomaterials IV
2 August 2005 | San Diego, California, United States
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