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UID:6a086e08a421a
DTSTART:20260529T080000Z
SEQUENCE:0
TRANSP:OPAQUE
DTEND:20260529T100000Z
LOCATION:ICFO Auditorium
SUMMARY:ICFO | ADITYA MALLA
CLASS:PUBLIC
DESCRIPTION:Shortwave infrared (SWIR) light sources are indispensable for a
 pplications including advanced imaging\, spectroscopy\, and sensing\; howe
 ver\, their widespread adoption is hindered by the high cost and limited s
 calability of epitaxial semiconductor technologies such as InGaAs. Colloid
 al quantum dots (QDs) offer an attractive alternative owing to their high 
 photoluminescence quantum yield\, size-tunable emission\, large-area proce
 ssability\, and compatibility with low-cost solution-based fabrication. Am
 ong various QD-based emitters employing lead sulphide (PbS)\, this thesis 
 focuses on two complementary technologies: electrically driven quantum-dot
  light-emitting diodes (QLEDs) and optically pumped downconverters (DCs). 
 The first part of this thesis addresses performance enhancement in QLEDs (
 emitting at 1380 nm) through systematic device engineering. Charge imbalan
 ce is identified as a key factor limiting QLED efficiency and radiance. By
  optimising the ZnO electron transport layer via controlled annealing-temp
 erature tuning\, electron injection was modulated\, leading to a maximum e
 xternal quantum efficiency (EQE) of 20%. Furthermore\, the charge balance 
 within the emissive layer was optimised by controlling its thickness\, res
 ulting in an increase in maximum radiance from 5 W.sr-1.m-2 to 17.5 W.sr-1
 .m-2. Building upon this\, a dual electron transport layer architecture wa
 s implemented to decouple interfacial quality from bulk electron transport
 \, enabling a further enhancement in maximum radiance to 30 W.sr-1.m-2 whi
 le maintaining comparable EQE. Light extraction and Joule heating constitu
 te an additional bottleneck in achieving high-performance QLEDs. To overco
 me substantial optical losses into substrate modes inherent in conventiona
 l bottom-emission devices\, top-emission QLED (TQLED) architectures were i
 nvestigated. These offer improved light extraction and allow for the use o
 f opaque\, high-thermal-conductivity silicon substrates to manage Joule he
 ating. A high-performance sputtered indium tin oxide (ITO) electrode was d
 eveloped\, exhibiting optical transmission exceeding 85% at 1400 nm and a 
 low sheet resistance of 33 Ω/□. By utilising optimised architecture wi
 th integrated ITO optical spacers and a dielectric/metal/dielectric top el
 ectrode\, a low-Q microcavity was established. This modified the far-field
  radiation pattern to a forward-directed profile and narrowed the emission
  linewidth. The synergy between this resonant optical design and superior 
 thermal dissipation enabled a record radiance exceeding 100 W.sr-1.m-2\, a
 nd allowed for the first demonstration of active see-through SWIR imaging 
 illuminated solely by QLEDs. The second part of the thesis is focused on l
 ead sulphide QD-based DCs. The QD-DCs suffer from performance degradation 
 under high excitation power densities due to the significant heat generati
 on in the process of light absorption. We have developed high-power\, stab
 le\, and spectrally tunable narrowband and broadband SWIR DCs (1000 nm - 1
 600 nm). By mixing two different-sized QDs\, we exploit F&ouml\;rster reso
 nance energy transfer and photon reabsorption to realise a binary system w
 ith a high photoluminescence quantum yield of 35 %. Embedding the QDs in a
  poly(methyl methacrylate) host mitigates local thermal stress on the QDs\
 , enabling standalone DCs with a high emission power density (EmPD) of 110
  mW.cm-2 at 1380 nm. Further optimisation with a spectrally selective dist
 ributed Bragg reflector for enhanced light extraction and a sapphire subst
 rate for efficient heat dissipation\, we achieved a record EmPD of 385 mW.
 cm-2 at 1380 nm with optical power conversion efficiency of 10% and operat
 ional stability above 230 hours at an EmPD of 190 mW.cm-2. This demonstrat
 es a scalable route to low-cost SWIR light sources\, narrowing the perform
 ance gap between solution-processed DCs and conventional epitaxial semicon
 ductors.\n&nbsp\;\nFriday May 29\, 10.00 h. ICFO Auditorium\nThesis Direct
 or: Prof. Dr. Gerasimos Konstantatos
DTSTAMP:20260516T131552Z
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