Tuning the performance of hybrid organic/inorganic quantum dot light-emitting devices

Publication Type:

Journal Article

Source:

Org. Electron., Elsevier, Volume 4, Issue 2–3, p.123-130 (2003)

ISBN:

1566-1199

Keywords:

2003, 2013 and earlier

Abstract:

The luminescence of inorganic core-shell semiconductor nanocrystal quantum dots (QDs) can be tuned across much of the visible spectrum by changing
the size of the QDs while preserving a spectral full width at half maximum
(FWHM) as narrow as 30 nm and photoluminescence efficiency of 50% [Journal
of Physical Chemistry B 101 (46) (1997) 9463] [1]. Organic capping groups,
surrounding the QD lumophores, facilitate processing in organic solvents
and their incorporation into organic thin film light-emitting device (LED)
structures [Nature 370 (6488) (1994) 354] [2]. A recent study has shown
that hybrid organic/inorganic QD-LEDs can indeed be fabricated with high
brightness and small spectral FWHM, utilizing a phase segregation process
which self-assembles CdSe(ZnS) core(shell) QDs onto an organic thin film
surface [Nature 420 (6917) (2002) 800] [3]. We now demonstrate that the
phase segregation process can be generally applied to the fabrication of
QD-LEDs containing a wide range of CdSe particle sizes and ZnS overcoating
thicknesses. By varying the QD core diameter from 32 Å to 58 Å, we show
that peak electroluminescence is tuned from 540 nm to 635 nm. Increase in
the QD shell thickness to 2.5 monolayers (∼0.5 nm) improves the LED
external quantum efficiency, consistent with a Förster energy transfer
mechanism of generating QD excited states. In this work we also identify
the challenges in designing devices with very thin (∼5 nm thick) emissive
layers [Chemical Physics Letters 178 (5–6) (1991) 488] [4], emphasizing
the increased need for precise exciton confinement. In both QD-LEDs and
archetypical all-organic LEDs with thin emissive layers, we show that
there is an increase in the exciton recombination region width as the
drive current density is increased. Overall, our study demonstrates that
integration of QDs into organic LEDs has the potential to enhance the
performance of thin film light emitters, and promises to be a rich field
of scientific endeavor.