Inverted Solution-Processed Quantum Dot Light-Emitting Devices with Wide Band Gap Quantum Dot Interlayers

Abstract

Despite its benefits for facilitating device fabrication, utilization of a polymeric hole transport layer (HTL) in inverted quantum dots (QDs) light-emitting devices (IQLEDs) often leads to poor device performance. In this work, we find that the poor performance arises primarily from electron leakage, inefficient charge injection, and significant exciton quenching at the HTL interface in the inverted architecture and not due to solvent damage effects as is widely believed. We also find that using a layer of wider band gap QDs as an interlayer (IL) in between the HTL and the main QDs' emission material layer (EML) can facilitate hole injection, suppress electron leakage, and reduce exciton quenching, effectively mitigating the poor interface effects and resulting in high electroluminescence performance. Using an IL in IQLEDs with a solution-processed poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB), HTL improves the efficiency by 2.85× (from 3 to 8.56%) and prolongs the lifetime by 9.4× (from 1266 to 11,950 h at 100 cd/m 2 ), which, to the best of our knowledge, is the longest lifetime for an R-IQLED with a solution-coated HTL. Measurements on single-carrier devices reveal that while electron injection becomes easier as the band gap of the QDs decreases, hole injection surprisingly becomes more difficult, indicating that EMLs of QLEDs are more electron-rich in the case of red devices and more hole-rich in the case of blue devices. Ultraviolet photoelectron spectroscopy measurements verify that blue QDs have a shallower valence band energy than their red counterparts, corroborating these conclusions. The findings in this work, therefore, provide not only a simple approach for achieving high performance in IQLEDs with solution-coated HTLs but also novel insights into charge injection and its dependence on QDs' band gap as well as into different HTL interface properties of the inverted versus upright architecture.

Differences in Electron and Hole Injection and Auger Recombination between Red, Green, and Blue CdSe-Based Quantum Dot Light Emitting Devices

“…As can be seen from the J – V characteristics of these HODs in Figure S1d, the CBP-based HODs exhibit hole currents approximately 10 1 –10 2 times higher than their TCTA-based counterparts, showing that the use of CBP indeed allows for increasing the supply of holes to all QDs. Also, here again we see that the hole supply becomes progressively easier as the bandgap of the QDs increases, consistent with recent findings …”

“…Figure S1a and S1b show the general device structure and the J – V data, respectively. As can be seen from the results, increasing the annealing temperature from 100 to 350 °C results in an increase in electron currents at any given voltage by about 2 orders of magnitude in all EODs regardless of the QD material used (R-, G-, and B-QDs), which can be attributed to the possible role of the higher annealing temperature in increasing crystallinity as well as oxygen vacancies, which would increase the mobility and concentration of charge carriers. − It is also worth pointing out the progressive decrease in current at any given voltage as the bandgap of the QDs increases for each annealing temperature, pointing to electron injection becoming increasingly more difficult in the same direction, consistent with our recent findings . Similarly, the choice of CBP for the high hole supply scenario (+↑) HTL is based on its higher hole mobility, which is approximately 10 –3 cm 2 /v·s, about an order of magnitude higher than that of TCTA (∼10 –4 cm 2 /v·s), the material utilized for the (+↓) scenario HTL.…”

“…Proposed schematic diagram illustrating the charge supply into R-QLEDs (left side), G-QLEDs (middle), and B-QLEDs (right side) in (a) the (+↑)(−↓) scenario and (b) the (+↓)(−↑) scenario. The numbers represent the valence band energy of the QD shells obtained from ultraviolet photoelectron spectroscopy measurements …”

“…While it is possible that a similar effect occurs in the devices of the two other colors, especially in the G-QLEDs, the fact that this band is much stronger in the B-QLEDsmaking it possible for us to discern these variationsgives direct evidence of the very high concentrations of excess holes in the QDs-EML of B-QLEDs (i.e., e/h ≪1), allowing for a significant fraction of unrecombined holes to reach the ETL, as concluded previously. Knowing that the presence of holes in the ZnO-based ETLs can lead to luminescence degradation in QLEDs, one can expect the leakage of such a significant fraction of them to the ETL in the case of B-QLEDs to play a major role in the low stability of these devices …”

“…However, most investigations of the influence of excess charges have been conducted on single QDs or on ensembles of QDs in single-carrier devices and therefore do not accurately represent the conditions in QLEDs. In a recent work, we found out that significant differences in electron and hole injection efficiencies between red (R)-, green (G)-, and blue (B)-QDs exist due to differences in their conduction and valence band levels. It is therefore expected that R-, G-, and B-QLEDs will experience different extents and polarities of charging, resulting in differences in the level of interactions between excess charges and excitons and therefore in Auger recombination efficiency.…”