Exciton Lifetime in PbS Quantum Dots in Glass
- PDF / 190,514 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 90 Downloads / 271 Views
0974-CC06-01
Exciton Lifetime in PbS Quantum Dots in Glass Peter D. Persans, A. Filin, N. E. Berry, F. Huang, and E. Chan Physics, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY, 12180
ABSTRACT We report experimental measurements and analysis of HOMO-LUMO transition photoluminescence and photoinduced bleaching magnitude and decay kinetics in lead sulfide quantum dots in glass. We find that the radiative lifetime is independent of temperature from 77K to 500K. The lifetime of the HOMO-LUMO state decreases dramatically for T>300K, indicating thermally activated nonradiative recombination. The excitation decay time is much greater than the radiative lifetime and increases monotonically with decreasing temperature with decay times of 2 µs at room temperature and 4-10 µs at 77K. INTRODUCTION A single gap solar cell has an optimal unconcentrated solar energy conversion efficiency of ~32% for a band gap of ~1.5 eV. The discovery of efficient multiexciton generation by high energy photons in quantum dots [2, 3] may lead to significant improvements in efficiency with a concomitant decrease in the optimal band gap. When multiexciton generation is considered, quantum dots with their lowest excited state near 0.5-0.8 eV are particularly interesting for solar photovoltaic applications [4]. PbS and PbSe are prototypical materials that satisfy the desired properties of narrow gap (to access low energy photons), large exciton radius (to permit use of larger particles thereby ameliorating surface effects), and nearly equal electron and hole effective masses (for high efficiency inverse Auger effect) [5]. Although efficient multiexciton generation is important, it is not useful if the excited carriers cannot be extracted or converted into useful photons. The lifetime and radiative recombination efficiency of excitons in PbS is an important probe of carrier recombination pathways. The decay lifetime for excitons has been observed to be unusually long in similar PbS particles [1, 6] and the presence of a "dark exciton" state has been recently proposed [7]. The term "dark exciton" is used to refer to an intrinsic quantum dot state that is optically forbidden [8]. If a dark exciton state lies at lower energy than the allowed HOMO-LUMO transition, the decay lifetime for an excited exciton can be extended by orders of magnitude. To address the question of the nature of the lowest excited state, we report here on the temperature dependence of the photoluminescence and photoinduced bleaching magnitudes and decay lifetimes.
THEORETICAL BACKGROUND The intensity of steady state HOMO-LUMO photoluminescence depends only on the occupation and radiative lifetime τ r of the allowed HOMO -LUMO transition: I PL ∝ N Lτ r−1 1 where NL is the number of dots with an exciton in the HOMO-LUMO state. (We use the term "HOMO-LUMO exciton state" to indicate a state with an electron in the LUMO state and a hole in the HOMO state.) The occupation of the HOMO-LUMO exciton state can be measured directly using the steady state photoinduced bleachi
Data Loading...
