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SUMMARY:Chemical Modifications and Passivation Approaches in Metal Halide 
 Perovskite Solar Cells - Dr. Mojtaba Abdi-Jalebi\, Wolfson College Researc
 h Fellow\, University of Cambridge
DTSTART:20190522T130000Z
DTEND:20190522T140000Z
UID:TALK124717@talks.cam.ac.uk
CONTACT:Dr Kaveh Delfanazari
DESCRIPTION:Metal halide perovskite materials have shown a relatively fast
  evolution in the power conversion efficiency (PCE) reaching the level exc
 eeding those of CIGS and CdTe and approaching those of crystalline silicon
  solar cells. However\, low luminescence efficiency of metal halide perovs
 kite in a complete device and non-radiative losses originating from sub ga
 p charge carrier trap states on the grain surfaces (e.g. halide vacancies)
  are the main barriers against reaching the efficiency limit in solar cell
 s. In addition\, the long-term stability of perovskite solar cells (PSCs) 
 remains a pressing challenge that hinders their commercialisation. Here\, 
 I will detail several new and promising passivation approaches through com
 positional modification and interface engineering aimed at eliminating the
  sources of instability and loss processes in metal halide perovskites. We
  demonstrate substantial mitigation of both non-radiative losses and photo
 -induced ion migration in perovskite films and interfaces by decorating th
 e surfaces and grain boundaries with passivating potassium halide layers. 
 We find significant enhancement in both micro-photoluminescence and photol
 uminescence quantum efficiency (e.g. internal yields exceeding 95%) while 
 maintaining high mobilities\, giving the elusive combination of both high 
 luminescence and excellent charge transport translating into over 21% PCE 
 of the PSCs with the entire elimination of hysteresis. We found that the m
 ain source of instability in PSCs is interfacial defects\, in particular\,
  those that exist between the perovskite and the hole transport layer (HTL
 ). We then demonstrate that thermally evaporated dopant-free tetracene on 
 top of the perovskite layer\, capped with a doped Spiro-OMeTAD layer and t
 op gold electrode offers an excellent hole-extracting stack with minimal i
 nterfacial defect levels. However\, we and others find that dopant-free or
 ganic semiconductor HTLs introduce undesirable injection barriers to the m
 etal electrode. By capping 120 nm of tetracene with 200 nm solution proces
 sed lithium TFSI doped Spiro OMeTAD\, we demonstrate a graded hole injecti
 on interface to the top gold layer with enhanced ohmic extraction. For a p
 erovskite layer interfaced between this graded HTLs structure and a mesopo
 rous TiO2 electron extracting layer\, its external photoluminescence yield
  reaches 15%\, compared to 5% for the perovskite layer interfaced between 
 TiO2 and Spiro-OMeTAD alone. For complete solar cell devices containing te
 tracene/Spiro- OMeTAD as the HTL with graded doping profile\, we demonstra
 te PCEs of up to 21.5% and extended power output over 550 hours continuous
  illumination at AM1.5 retaining more than 90% of the initial performance\
 , validating our approach. Our findings represent a breakthrough in the co
 nstruction of stable PSCs with minimized non-radiative losses.\n\nReferenc
 es:\n\n[1] Mojtaba Abdi-Jalebi\, et. al\; Richard H Friend\, Samuel D Stra
 nks\, "Maximising and Stabilising Luminescence in Halide Perovskite Device
  Structures Using Potassium-Halide Passivating Layers"\, Nature 555\, 497-
 501\, (2018). https://www.nature.com/articles/nature25989\n\n[2] Mojtaba A
 bdi-Jalebi\, et. al\; Richard H Friend\, "Charge extraction via graded dop
 ing of hole transport layers gives highly luminescent and stable metal hal
 ide perovskite devices" Science Advances\, 5\, eaav2012 (2019). https://ad
 vances.sciencemag.org/content/5/2/eaav2012
LOCATION:Mott Seminar Room (531)\, Cavendish Laboratory\, Department of Ph
 ysics
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