Abstract
In carrier multiplication, the absorption of a single photon results in two or more electron–hole pairs. Quantum dots are promising materials for implementing carrier multiplication principles in real-life technologies. So far, however, most of research in this area has focused on optical studies of solution samples with yet to be proven relevance to practical devices. Here we report ultrafast electro-optical studies of device-grade films of electronically coupled quantum dots that allow us to observe multiplication directly in the photocurrent. Our studies help rationalize previous results from both optical spectroscopy and steady-state photocurrent measurements and also provide new insights into effects of electric field and ligand treatments on multiexciton yields. Importantly, we demonstrate that using appropriate chemical treatments of the films, extra charges produced by carrier multiplication can be extracted from the quantum dots before they are lost to Auger recombination and hence can contribute to photocurrent of practical devices.
Highlights
In carrier multiplication, the absorption of a single photon results in two or more electron–hole pairs
In the present report we demonstrate that the above challenges can be successfully addressed by using an ultrafast transient photocurrent (TPC) technique for studies of early time electronic dynamics, and carrier multiplication’ (CM), in device-grade films of coupled quantum dots (QDs)
The dots are incorporated into a photoconductive switch (Fig. 2c), which comprises a B200-nm thick QD layer assembled on a glass substrate with a 100 nm-thick gold ground plane on its back side
Summary
If the changes in mobility due to, for example, charge trapping at intraband defects are slow compared with recombination time scales, the photocurrent directly reports on carrier population dynamics. In this regime, TPC is not sensitive to exciton dissociation or charge migration between the dots as these processes do not affect the average occupancy of the QDs. Equation (1) suggests that the contribution from a given charge to photocurrent does not dependent on the total number of other charges residing in the QD, meaning in particular that the contribution from a biexciton to j is twice that of a single exciton, as confirmed below based on the analysis of TPC data. As we show below, some of the methods developed for the analysis of transient absorption data can be directly applied to TPC measurements
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