Acid-Assisted Ligand Exchange Enhances. Coupling in Colloidal Quantum Dot Solids

Transcription

1 Supporting Information for Acid-Assisted Ligand Exchange Enhances Coupling in Colloidal Quantum Dot Solids Jea Woong Jo,,, Jongmin Choi,, F. Pelayo García de Arquer,, Ali Seifitokaldani, Bin Sun, Younghoon Kim,,# Hyungju Ahn, James Fan, Rafael Quintero-Bermudez, Junghwan Kim, Min-Jae Choi, Se-Woong Baek, Andrew H. Proppe,, Grant Walters, Dae Hyun Nam, Shana Kelley,, Sjoerd Hoogland, Oleksandr Voznyy, and Edward H. Sargent*, * Department of Electrical and Computer Engineering, University of Toronto, 10 King s College Road, Toronto, Ontario, M5S 3G4, Canada. Pohang Accelerator Laboratory, Kyungbuk, Pohang 37673, Republic of Korea. Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3G4, Canada. Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Ontario, M5S 3M2, Canada. Present address: Department of Energy and Materials Engineering, Dongguk University- Seoul, Seoul, Republic of Korea. # Present address: Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute

2 of Science and Technology, Daegu 42988, Republic of Korea. EXPERIMENTAL SECTION Synthesis of IR-CQD. PbS CQDs were synthesized using a method described in the literature. S1,2 As an example, for a typical synthesis of CQDs with an exciton peak at 1170 nm, 1.35g of PbO, 4.5mL of oleic acid, and 15 ml of 1-octadecene were mixed in three-neck flask and degassed at 100 C for 2 hours to form clear lead-oleate solution. Then, the flask filled with N 2 and temperature increased to 115 C. The 210 ul of bis(trimethylsilyl)sulfide was dissolved in 8 ml of 1-octadecene in the glovebox and swiftly injected into the mixed solution. The color of the solution changed to brown rapidly and slowly cooled down to room temperature. The as-synthesized solution moved to the glove box and precipitated by adding acetone. This washing step repeated three times and final CQD powder was dissolved in octane with concentration of 50 mg/ml. Lead halide solution-phase ligand exchange of PbS IR-CQDs. Solution-phase ligand exchange was carried out by following previously reported method for IR-CQD inks. S2 4 To lead iodide (0.50 mmol, 0.23 mg), lead bromide (0.11 mmol, mg) and sodium acetate (0.21 mmol, mg) in DMF (5 ml), 35 mg of OA-capped PbS IR-CQDs (7 mg/ml in octane) were added. After adding HI (57 wt. % in H 2 O), the mixture was vortexed vigorously for transferring CQDs from octance to DMF, and then PbS IR-CQDs in DMF solution was washed three times with hexane. The optimized amount of HI additive was found to be around 10 µl (75.8 µmol) (Figure S7a). Although large amount of acid could etch the surface of CQD, the significant degradation of CQDs was not been observed during ligand exchange progress using the small amount of HI additive. When large amount (100 µl) of acidic

3 chemical compounds was used, large degradation of PCE (< 0.3 % with E g = 1.08 ev CQD) was shown, which may be attributed to the etching of the CQD surface. By precipitating with addition of acetone (2 3 ml), centrifuging, and drying under vacuum for 15 min, the lead halide-passivated PbS IR-CQDs were obtained. Characterization. To acquire 1 H NMR (Bruker Avance III 400) spectra, a mixture of n- butylamine:dmf-d7 (1:100 v/v) and tetramethylsilane were used as a solvent and an internal reference material, respectively. FT-IR spectra were obtained by performing FT-IR spectrometer (Bruker Tensor 27). XPS analysis was carried out using X-ray photoelectron spectrometer (Thermo Scientific K-Alpha system) with an Al Kα source. Optical absorption and photoluminescence spectra were obtained using an UV Vis IR spectrophotometer (Lambda 950) and PL spectrophotometer (Ocean Optics NIR-512), respectively. GIXRD were characterized at PLS-II 9A U-SAXS beamline of the Pohang Accelerator Laboratory in Korea. Density Functional Theory (DFT) Computations. DFT simulations were performed using the CP2K electronic structure code S6 using a generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) S7 for exchange correlation and a dual basis set of localized Gaussians molecular orbitals and plane waves. S8 Goedecker Teter Hutter pseudopotentials S9 were employed with an appropriate 500 Ry grid cutoff. To reduce the basis set superposition errors in molecules, a localized basis set of double-ζ plus polarization (DZVP) was used. S10 Stoichiometry was selected such that the charge neutrality of the dot is always preserved, which is necessary to position the Fermi level in the right place in the mid gap. The QD and the exchanging ligand far away from surface were placed in the same box and thus the calculated energies for the following reactions represent the full charge balanced

4 reaction energy difference, without the need of calculating the binding energies of charged ligands. Device fabrication and testing. For IR-CQD solar cells, a device configuration of ITO/ZnO/PbI 2 -passivated PbS IR-CQD/EDT-treated PbS CQD/Au was used. The ZnO nanoparticle solution was prepared by following previous literature and was spin-coated two times onto ITO-coated substrate at 3000 rpm for 30 s. And then, lead halide passivated-pbs IR-CQDs were dispersed in n-butylamine:dmf (4:1 v/v) mixture ( mg/ml) and then were spin-coated on the top of ZnO layer at 2500 rpm for 30 s. The optimized thickness of IR-CQD layers was observed to be around 330 nm (Figure S7b). 1,2-Ethanedithiol-treated CQD layer was formed using the process described in previously reported paper. As a top electrode, 120 nm of Au was thermally evaporated under vacuum (<10 6 Torr). The effective device area was cm 2. The Si-filtered J V characteristics of IR-CQD solar cells were measured under AM1.5G illumination at 100 mw cm 2 using a Keithley 2400 source-meter and an 1100 nm long wave pass filter. The simulated AM1.5G illumination (Sciencetech class 3A,) used a Xe lamp with filters (Solar Light Company Inc.). We corrected for spectral mismatch between our lamp and AM 1.5G. S11 EQE measurement was carried out under monochromatic illumination (400 W Xe lamp equipped with a monochromator and cut-off filters) after the calibration of power with Newport 818-UV and Newport 838-IR photodetectors. The current response was obtained with a Lakeshore pre-amplifier connected to a Stanford Research 830 lock-in amplifier. FET measurements of IR-CQDs (Keithley 2400 source meters) were carried out by fabricating bottom-gate top-contact configurations. On glass substrate, 70 nm of Ti was thermally evaporated and then 10 nm of ZrO 2 was coated using atomic layer deposition. After thermal annealing at 300 C for 1 h, IR-CQD layer deposited using a spin-coater. Then, to form source/drain electrodes, 70 nm of Au were

5 thermally evaporated under vacuum (<10 6 Torr). The carrier mobility of FET was extracted by using the following equation: I DS = µ C i (V GS V TH ) V DS W/L, where I DS is the drain current, µ is the carrier mobility in the linear regime, C i is the gate dielectric layer capacitance per unit area, V GS is the gate voltage, V TH is the threshold voltage, L is the channel length (50 µm), and W is channel width (2500 µm).

6 Figure S1. Comparison of solution-phase ligand exchanges before and after adding acid additive. Figure S2. FT-IR spectra of IR-CQD films for E g of (a,e,d,h) 1.08, (b,f) 0.99, and (c,g) 0.82 ev prepared with and without (a-c and e-g) HI or (d,h) AcOH additive in the wavenumber range of (a-d) and (e-h) cm 1.

7 Figure S3. Distribution of inter-dot spacing measured from GIXRD for IR-CQD films with E g of (a,d) 1.08, (b) 0.99, and (c) 0.82 ev prepared without and with (a-c) HI or (d) AcOH additive

8 Figure S4. (a d) O 1s XPS spectra and (e) O/Pb atomic ratio measured from XPS of IR- CQD films prepared without and with (a-c) HI or (d) AcOH additive for E g of (a,d) 1.08, (b) 0.99, and (c) 0.82 ev. (f) Photoluminescence spectra of IR-CQD films (E g = 1.08 ev) before and after ligand exchange using different additives.

9 Figure S5. (a) V OC, (b) J SC, (c) FF, and (d) filtered-pce values of IR-CQD solar cells without and with HI additive measured with 1100 nm long wave pass filter.

10 Figure S6. (a c) Si-filtered J V curves of IR-CQD solar cells prepared without and with (a,b) HI or (c) AcOH additive for E g of (c) 1.08, (a) 0.99, and (b) 0.82 ev. (d) Filtered-PCE changes of the IR-CQD solar cells prepared with AcOH additive as a function of exposure time to air at room temperature. Despite more compact packing of IR-CQD films, adding AcOH did not lead to the improved performances of IR-CQD solar cells. IR-CQD devices (E g = 1.08 ev) with AcOH provided a higher J SC of 3.23 ma cm 2 but yielded a lower filtered-pce of 0.73% with a significantly reduced open-circuit voltage (V OC ) of 0.37 V compared with control devices (filtered-pce = 0.84%, J SC = 3.06 ma cm 2, and V OC = 0.43 V) (Figure S6c). This suggests that adding AcOH does not provide high quality passivation of IR-CQD, resulting in increased V OC deficient by CQD fusing. The insufficient passivation of IR-CQD with AcOH additive was further supported by device stability test, where devices using AcOH rapidly lost their performance on air exposure (Figure S6d).

11 Figure S7. Filtered-PCE values of IR-CQD solar cells (E g = 1.08 ev) depending on (a) the amount of HI additive and (b) thickness of IR-CQD layer. Figure S8. (a c) EQE of IR-CQD solar cells prepared without and with HI additive for E g of (a,d) 1.08, (b,e) 0.99, and (c,f) 0.82 ev.

12 Table S1. Device properties of IR-CQD solar cells prepared without and with HI additives under standard AM 1.5G illumination Additive Bandgaps of CQD (ev) V OC (V) J SC (ma/cm 2 ) FF (%) PCE avg a (%) ± ± ± ±0.1 HI ± ± ± ± ± ± ± ±0.5 HI ± ± ± ± ± ± ± ±0.1 HI ± ± ± ±0.3 a The averaged values calculated from at least 4 devices. Table S2. Device properties of IR-CQD solar cells (E g = 1.08 ev) prepared using different hydrohalic acids under Si-filtered AM 1.5G illumination Additive V OC (V) J SC (ma/cm 2 ) FF (%) PCE avg a (%) ± ± ± ±0.08 HCl 0.41± ± ± ±0.03 HBr 0.42± ± ± ±0.02 HI 0.42± ± ± ±0.05 a The averaged values calculated from at least 4 devices.

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