Postsynthetic Doping Control of Nanocrystal Thin Films: Balancing Space Charge to Improve Photovoltaic Efficiency

Engel, Jesse; Alivisatos, A. Paul

doi:10.1021/cm402383r

Cited by 36 publications

(47 citation statements)

References 115 publications

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“…Unpassivated bonds on the QD surface are known to form carrier traps with energy levels in the energy gap. [ 26 ] The observed reduction of the hole concentration in the graphene layer is fully consistent with the transfer of electronic charge to it from the adjacent QDs. Our measurements on the three devices under dark conditions also reveal a systematic variation of the shift, V Δ g dark , with respect to the value of V g dark at the conductance minimum of pristine graphene, see Figure 4 b.…”

Section: Lumosupporting

confidence: 64%

“…Ligand exchange has been reported [ 9 ] to enhance the performance of devices with a QD layer. However, this process induces structural disorder in the QD fi lms [ 26 ] and reduces the quantum yield due to oxidation of the QDs. [ 29 ] In our case, the confl uent layer of capping molecules provides a uniform separation of the QDs from the SLG, thus reducing electrostatic disorder and enhancing the carrier mobility.…”

Section: Lumomentioning

confidence: 99%

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Ligand‐Induced Control of Photoconductive Gain and Doping in a Hybrid Graphene–Quantum Dot Transistor

Turyanska

Makarovsky

Svatek

et al. 2015

Adv Elect Materials

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concentration can be modifi ed, and photoresponsivity of SLG can be signifi cantly enhanced by the choice of ligands. By reducing the length of capping ligands, hence the thickness of the dielectric barrier between the QDs and the SLG, and by preserving the integrity of the ligand layer, we achieve the efficient transfer of photogenerated carriers from the QDs to the graphene before recombining, resulting in enhanced photoresponsivities of up to ≈10 9 A W −1 .Single layer graphene grown by low-pressure chemical vapor deposition (LP-CVD) [ 14,15 ] was processed into a planar transistor and deposited onto a SiO 2 /n-Si substrate with oxide layer thickness t = 300 nm, as shown schematically in Figure 1 a (see also the Supporting Information, Figure S1). The length, L , and width, w , of the graphene channel are 6 and 10 µm, respectively. The n-Si layer serves as a gate electrode. Colloidal quantum dots with an average PbS core diameter of d = 4.5nm QD (Figure 1 b) were stabilized using the following capping ligands of different length, l , which determines the effective separation between the PbS nanocrystals and graphene: polyethylene glycol H (O CH 2 CH 2 ) n OH with n = 2000 for QD p2000 and n = 500 for QD p500 [ 16 ] and corresponding length of l ≈ 10 nm and l ≈ 5 nm, respectively; [ 17 ] a mixture of thioglycerol (TGL) and 2,3-dimercapto-1-propanol (DTG), l ≈ 0.5 nm for QD TGL , [ 18 ] see Table 1 . We note that the ligand size is estimated for extended molecules and does not include any effects that could arise from compression of the ligand layer upon drying under vacuum. The interparticle distances observed on TEM images are comparable with the theoretical estimate of the ligand length (see the Supporting Information, Figure S2). The QDs were dropcast from aqueous solution (5 mg mL −1 ) to produce continuous thick (>100 nm) coverage of the graphene layer, followed by drying for 12 h in vacuum at room temperature. The size of the QDs and their room temperature photoluminescence spectra (with a peak at photon energy ≈1.1eV) are similar in all three structures, i.e., they are not affected by the ligand. All measured devices demonstrate stable and reproducible behavior during the measurement period of at least 14 days. All samples were stored in dry nitrogen cabinet and all electrical measurements were performed at room temperature T = 300 K in vacuum (≈10 −6 mbar). The I V ( ) g characteristics were measured using slow sweeping rate, r , of the gate voltage, i.e., r ≤ 0.02 V s −1 .The pristine monolayer graphene devices show a linear dependence of current, I , on bias voltage, V s , and a minimum conductance at a gate voltage, V ≈ +30V g min corresponding to p-type doping with hole concentration, p ≈ 2 × 10 12 cm −2

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Section: Lumosupporting

confidence: 64%

Section: Lumomentioning

confidence: 99%

Ligand‐Induced Control of Photoconductive Gain and Doping in a Hybrid Graphene–Quantum Dot Transistor

Turyanska

Makarovsky

Svatek

et al. 2015

Adv Elect Materials

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“…[27][28][29][30][31] In CQD solids, the large surface-to-volume ratio results in added opportunities for electronic defect formation. Increased recombination losses risk compromising the charge collection if not suitably addressed.…”

mentioning

confidence: 99%

Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance

et al. 2015

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A solution-based passivation scheme is developed featuring the use of molecular iodine and PbS colloidal quantum dots (CQDs). The improved passivation translates into a longer carrier diffusion length in the solid film. This allows thicker solar-cell devices to be built while preserving efficient charge collection, leading to a certified power conversion efficiency of 9.9%, which is a new record in CQD solar cells.

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“…Therefore, by designing p-i-n photodiodes with PbS-HYD as the intrinsic layer, we may be able to achieve much higher efficiencies in solar cells and photodetectors. 38,39 ■ ASSOCIATED CONTENT * S Supporting Information…”

mentioning

confidence: 99%

Charge Percolation Pathways Guided by Defects in Quantum Dot Solids

et al. 2015

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Charge hopping and percolation in quantum dot (QD) solids has been widely studied, but the microscopic nature of the percolation process is not understood or determined. Here we present the first imaging of the charge percolation pathways in two-dimensional PbS QD arrays using Kelvin probe force microscopy (KPFM). We show that under dark conditions electrons percolate via in-gap states (IGS) instead of the conduction band, while holes percolate via valence band states. This novel transport behavior is explained by the electronic structure and energy level alignment of the individual QDs, which was measured by scanning tunneling spectroscopy (STS). Chemical treatments with hydrazine can remove the IGS, resulting in an intrinsic defect-free semiconductor, as revealed by STS and surface potential spectroscopy. The control over IGS can guide the design of novel electronic devices with impurity conduction, and photodiodes with controlled doping.

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Postsynthetic Doping Control of Nanocrystal Thin Films: Balancing Space Charge to Improve Photovoltaic Efficiency

Cited by 36 publications

References 115 publications

Ligand‐Induced Control of Photoconductive Gain and Doping in a Hybrid Graphene–Quantum Dot Transistor

Ligand‐Induced Control of Photoconductive Gain and Doping in a Hybrid Graphene–Quantum Dot Transistor

Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance

Charge Percolation Pathways Guided by Defects in Quantum Dot Solids

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