Role of Surface Chemistry on Charge Carrier Transport in Quantum Dot Solids

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1 Role of Surface Chemistry on Charge Carrier Transport in Quantum Dot Solids Cherie R. Kagan, University of Pennsylvania in collaboration with the Murray group

2 Density of Electronic States in Quantum Dot Solids ligand exchange E o Energy E o E o doping chemically electrochemically electrostatically photo-carrier recombination carrier transport E o Density of States Kagan, Murray, Nat. Nano 10, 1013 (2015)

3 Examples of Ligand Exchange in Pb-Chalcogenide Quantum Dot Solids BDT NH 4 Cl SCN OA EDT TBAB organic inorganic SCN+PbCl 2 MPA TBAI

4 Field-Effect Transistor Measurements V D + metalsemiconductor Energy - Density of States semiconductorgate dielectric n or p ( V V ) G q T C OX metal-semiconductor and semiconductor-gate oxide interfaces affect measurements induced carrier concentration depends on the gate voltage shifting the quasi-fermi level

5 Charge Injection and Transport in PbSe Quantum Dot FETs workfunction electron affinity charge injection and transport control the polarity and magnitude of the current I D [A] EDT V DS =50V Al Ag Pd Cr Au I D [A] Al Ag Pd Cr Au V DS SCN =50V I D [A] TBAI V DS =50V coupling ligand Oh, Wang, Berry, Choi, Zhao, Gaulding, Paik, Lai, Murray, Kagan, Nano Lett 14, 6210 (2014).

6 Stoichiometric Control of Pb-Chalcogenide Thin-Film QD Solids NH 4 SCN RT substrate Excess Pb deposition CB Pb rich n-type device E Ei VB PbX NCs device Excess Se deposition N Pb or N Se U. SCHLICHTING and K. H. GOBRECHT, J. Phys, Chem. Solids, 1973, 34, 753 Capacitance [nf] Pb deposition 0 Å 0.3 Å C max 2 Å 1.2 Å 0.9 Å 0.6 Å Capacitance [nf] Se deposition 1.2 Å Å 0.3 Å 0 Å Se rich p-type device C max controlled carrier concentration over the range of /cm /cm 3 Oh, Berry, Choi, Gaulding, Paik, Hong, Murray, Kagan, ACS Nano 7, 2413 (2013)

7 Case Study: Stoichiometric Control of Carrier Statistics in Pbchalcogenide Quantum Dot Thin Films I D [A] V DS =50V 1 Å Pb 0.1 Å Se I D [A] Excess Pb pristine 1A 2A 3A 4A I D [A] Excess Se pristine 0.3A 0.1A 0.4A 0.2A 0.5A V DS =-50V V DS =50V Pb Se At 2-3 Å Pb, µ e ~10 cm 2 /Vs At Å Se, µ h ~0.5 cm 2 /Vs

8 Solution-based Stoichiometric Control of PbE (E=S, Se) Quantum Dot Thin Films Insulating Film P-type Device Na2Se (Na2S, KHS) N-type Device PbCl nm 10nm 10nm hexagonal to square ordered assemblies driven by high chalcogen surface energy akin to ligand stripping by Vanmaekelbergh, Hanrath.. Oh, Berry, Choi, Gaulding, Lin, Paik, Diroll, Muramoto, Murray, Kagan, Nano Lett 14, 1559 (2013).

9 As-synthesized NCs Stoichiometry Change Chalcogen-rich NCs Lead-rich NCs Na 2 Se (Na 2 S, KHS) PbCl 2 No. of Pb atoms : 2190 No. of Se atoms :1925 = < +620 No. of Pb atoms : 2190 No. of Se atoms : < = No. of Pb atoms : 2754 No. of Se atoms : 2558 Pb:Se (model)=1.137:1 Pb:Se (model)=0.847:1 Pb:Se (model)=1.065:1 Pb:Se (by ICP)=1.145:1 Pb:Se (by ICP)=0.841:1 Pb:Se (by ICP)=1.073:1

10 Flash-Photolysis, Time-Resolved Microwave Conductivity Energy Gτ n Gτ p generation rate, G Density of States Δn,p = Gτ n,p 0 at low intensity contactless Fermi energy unaltered by measurement, particularly at low intensity time-resolution limited by cavity response and microwave frequency in the ns-µs Goodwin, Straus, Gaulding, Murray, Kagan, Chemical Physics, (2016)

11 TRMC Measurements of Doped PbSe Quantum Dot Thin Films I D (A) (V) carrier mobility increases as Fermi level approaches the band edges

12 Quantum Dot Surface Repair in IV-VI Materials solvents and ligand exchange processes strip atoms from the quantum dot surface Owen, Hens, Sargent, Pb:Se ratio OA SCN SCN+PbCl 2 Oh, Wang, Berry, Choi, Zhao, Gaulding, Paik, Lai, Murray, Kagan, Nano Lett 14, 6210 (2014) Goodwin, Diroll, Oh, Paik, Murray, Kagan, J. Phys. Chem C 118, (2014).

13 Coupling and Carrier Concentration Dependent Transport in PbSe Quantum Dot FETs Al Au Energy Density of States I D [A] V DS =-50V I D [A] V DS =50V doping PbCl 2 as electron concentration increases through chemical or electrostatic doping or charge injection, transport transitions from hopping to more extended state σ [S/cm] SCN - + PbCl 2 SCN - EDT MPA T [K] I -

14 Integrated Quantum Dot CMOS Inverter V DD =-50V 12 V OUT V IN V OUT 8 4 Gain V IN 0

15 Can Solar Cells Be Fabricated from Strongly-Coupled NCs? Current density [ma/cm 2 ] PCE: 0 % PCE: 1.3% PCE: 2.2% Voltage SCN BDT Hybrid Exchange Normalized Φ (µ e + µ h ) [a.u.] BDT SCN SCN/ BDT BDT SCN SCN/ BDT Lifetime [µs] Oh, Straus, Zhao, Choi, Lee, Gaulding, Murray, Kagan, Chem Comm, (2017).

16 Photoconductors and Photodiodes V = 10 V Current [A] Hybrid R=167 A/W SCN R=1.1 A/W BDT R=0.58 A/W Time [s] Current density [ma/cm 2 ] PCE: 0 % 0 PCE: 2.1% -10 PCE: 3.5% Voltage Avenue to realize high carrier mobility (>1 cm 2 /Vs), long carrier lifetime QD materials for optoelectronics?

17 Surface Engineering of PbSe Nanowires to Construct Electronic and Optoelectronic Devices V DD V OUT GND N P Au N P Au V OUT V IN Gain I [µa] Time [s] I [A] V Oh, Uswachoke, Zhao, Choi, Diroll, Murray, Kagan, ACS Nano, 9, 7536 (2015).

18 Advanced Architecture for Colloidal PbS Quantum Dot Solar Cells Sargent, Alivisatos, Bawendi/Bulovic, Luther/Beard, Nozik. MoO 3 /Au ZnO NP/PbS QD PbS QDs CdSe QDs X 100 nm ZnO ITO Glass ZnO NP/CdSe QD/PbS QD N=20 Voc (V) Jsc (ma/cm 2 ) FF PCE (%) ZnO NP/PbS QD 0.586± ± ± ±0.5 ZnO NP/CdSe QD/PbS QD 0.600± ± ± ±0.4 Zhao, Goodwin, Guo, Wang, Diroll, Murray, Kagan, ACS Nano, 10, 9267 (2016).

19 Band Gap Engineering of QD Solar Cells 8-nm CdSe QDs 4-nm CdSe QDs PbS ZnO PbS ZnO J (ma/cm 2 ) V (V) J (ma/cm 2 ) V (V) dark light>800 nm white light 15-min light soaking

20 PbS Quantum Dot Solar Cell Performance Enhancement EQE Wavelength (nm) EQE ΦΣµ (x10-3 cm 2 /Vs) Photoexcitation/QD (10-2 ) reduce interface recombination and increase photogeneration

21 Conclusions 1 synthesis of monodisperse nanocrystals 2 exchange long surface ligands used in synthesis for compact ligands 3 passivate surface traps and remotely dope NC solid 4 engineer device interfaces to design the carrier mobility and lifetime of NC solids E o E o Energy E o E o Density of States