Nanoimprint-Transfer-Patterned Solids Enhance Light Absorption in Colloidal Quantum Dot Solar Cells

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1 Supporting Information Nanoimprint-Transfer-Patterned Solids Enhance Light Absorption in Colloidal Quantum Dot Solar Cells Younghoon Kim, Kristopher Bicanic, Hairen Tan, Olivier Ouellette, Brandon R. Sutherland, F. Pelayo García de Arquer, Jea Woong Jo, Mengxia Liu, Bin Sun, Min Liu, Sjoerd Hoogland, and Edward H. Sargent* Department of Electrical and Computer Engineering, University of Toronto, 10 King s College Road, Toronto, Ontario, M5S 3G4, Canada. * ted.sargent@utoronto.ca Experimental Section Preparation of PbS CQDs Oleic-acid PbS CQDs (OA-CQDs) were synthesized using a previously published method. 1 Lead iodide (PbI 2 )-passivated PbS CQDs (PbI 2 -CQDs) were prepared through a solution ligand exchange process of OA-CQDs in a test tube under atmospheric conditions as follows. First, PbI 2 (1.50 mmol, mg), lead bromide (PbBr 2 ) (0.33 mmol, mg) and ammonium acetate (NH 4 Ac) (0.62 mmol, mg) were completely dissolved in dimethylformamide (DMF) (5 ml). 5 ml of OA-CQDs dispersed in octane (6 mg/ml) were added to the prepared DMF solution, and transferred to DMF phase by vortexing vigorously at room temperature for 2 min. The CQDs dispersed in DMF phase were then washed three times using octane in order to remove residual original ligands (i.e., oleic acid) from the CQD solution. After being washed completely, the PbI 2 -CQDs were precipitated by adding toluene (2 ml) as an anti-solvent, and 1

2 dried under vacuum for 20 min, and finally dispersed in butylamine (BTA) at the desired concentrations. 2 Fabrication of the patterned PDMS mold The mixtures of poly(dimethylsiloxane) (PDMS) precursors and cross-linking agent were spincoated onto glass substrates at 3000 rpm for 30 sec, and cured at 80 C for 2 hours. The dried powder of polystyrene (PS) microspheres with 500 nm diameter were mechanically rubbed onto the cured PDMS films to acquire the hexagonally-assembled PS microspheres. The mixtures of PDMS precursors and cross-linking agent were poured onto the PS-assembled PDMS film, and cured at 120 C for 2 hours again. After then, the cured PDMS was carefully peeled off from the PS-assembled bottom PDMS film, and soaked in acetone overnight in order to completely remove the remaining PS microspheres from the cured PDMS. Fabrication of flat control and nanostructured CQD solar cell devices Patterned-ITO glass substrates were cleaned by soaking and sonicating sequentially in acetone, isopropyl alcohol, and deionized water. ZnO nanoparticles were synthesized as previously reported. 3 The ZnO nanoparticle solution was spin-coated onto the patterned and cleaned ITO glass two times at 3000 rpm for 30 sec, and subsequently PbI 2 -CQDs in BTA (200 mg/ml) were spin-coated at 3000 rpm for 30 sec. We used this film as a starting substrate for the fabrication of flat control and nanostructured CQD films. The flat control film was fabricated by additionally spin-coating the PbI 2 -CQDs in BTA (120 mg/ml) onto this film. In the case of the nanostructured film, the OA-CQDs in octane (10 mg/ml) were first spin-coated onto the prepared PDMS mold, and subsequently PbI 2 -CQDs in BTA (120 mg/ml) were spin-coated onto this layer. The CQD layers formed onto the PDMS mold were transferred to other PbI 2 -CQD layer, which was pre-spin-coated onto ITO glass substrates with a ZnO electron accepting layer. After transferring completely, we removed the OA-CQDs remaining onto the PbI 2 -CQD layer by octane washing. 2

3 For the final device fabrication, two thin layers of 1,2-ethanedithiol (EDT)-treated CQDs (EDT- CQDs) were deposited onto both the flat control and the nanostructured PbI 2 -CQD films, followed by gold deposition as a top metal electrode. Characterization of solar cell device The current density-voltage (J-V) curves of each device were obtained using a Keithley 2400 SourceMeter. The solar spectrum at AM1.5G was simulated to within class A specifications (< 25 % spectral mismatch) with a Xe lamp and filters (Solar Light Company Inc.) with measured intensity of 100 mw/cm 2. The illumination power was calibrated using a Melles Griot broadband power meter and a Thorlabs broadband power meter through a circular optical aperture (area cm 2 ) at the position of the device and confirmed with a calibrated reference Si solar cell (Newport, Inc.). The final accuracy of the solar-to-electricity measurement was estimated to be ±5%. Optical absorption measurements The total light absorption (A) was determined by A=1 R T, where R is the total reflectance measured from the glass side and T is the total transmittance through the back of solar cells (T=0 for devices with gold electrode). R and T were measured using a Perkin Elmer LAMBDA 950 spectrometer equipped with an integrating sphere. External quantum efficiency measurements External quantum efficiency (EQE) spectra were carried out by aligning the cell to monochromatic light (a 400 W Xe lamp passing through a monochromator and proper cut-off filters). The active area was defined by optical aperture, and the light power was calibrated with Newport 818-UV and Newport 838-IR photodetectors. The monochromatic light beam was chopped at 220 Hz and was collimated onto the device active layer using a solar simulator at 1 sun intensity to provide light bias. Pre-amplifier (Stanford Research Systems SR570) and lock-in 3

4 amplifier (Stanford Research 830) were used for collecting the current signals from the solar cell devices. Finite-difference time-domain (FDTD) simulations FDTD simulations were carried out with a commercial-grade simulator from Lumerical Solutions, Inc ( version The simulations were performed for a 2D structure, with both device structures described in Figure 1, both with equivalent total film thicknesses as a result, the flat control device has a larger active volume. The simulation volume was performed with periodic boundary conditions to simulate a large area pattern. The simulated absorption profiles of the patterned devices are averaged over multiple periodicities to replicate variations in real devices. Within the simulation area a staircase mesh grid of 10 nm was used to provide an accurate representation of the planar layers; however, at the QD dome this mesh size was insufficient, as such it was refined to 2 nm in the z-direction and 0.5 nm in the x-direction. A broadband plane wave source (λ = 300 nm to 1350 nm) was implemented to illuminate normal to the back side of the device structure from within the glass layer, and was polarized in-plane. Finally, the absorption of the QD layer was extracted using a mask based on the material selected within the grid region. Other measurements Atomic force microscopy (AFM) images were obtained by the Asylum Research Cypher S, operating in AC tapping-mode, using an ASYELEC-01 titanium/iridium coated silicon tip. Tilted and cross-sectional images were obtained from the field-emission scanning electron microscope (FE-SEM, Hitachi SU8230). 4

5 Figure S1. Optical images of PbI 2 -CQD films onto PDMS mold and the resulting films after transferring process. (a) PbI 2 -CQDs were not fully covered onto the PDMS mold without OA- CQD layer and were not transferred to the primary PbI 2 -CQD layer. (b) PbI 2 -CQD film was formed onto the PDMS mold with OA-CQD layer and transferred to the primary PbI 2 -CQD layer. 5

6 Figure S2. Optical microscope and atomic force microscope images of the nanostructured CQD film investigated over a range of regions on a representative sample. 6

7 Figure S3. Tilted and cross-sectional scanning electron microscopy images of the nanostructured CQD films prior to gold deposition. 7

8 Figure S4. Comparison of device histograms for flat and pattern CQD solar cells: (a) J SC, (b) V OC, (c) FF, and (d) PCE, respectively. 8

9 Figure S5. Current density-voltage (J-V) curves of two flat devices. The bilayer flat device was fabricated by transferring a flat CQD layer that has been spin-coated onto the flat PDMS mold on to the target device. 9

10 Figure S6. Cross-sectional scanning electron microscopy images of the flat control and nanostructured device. 10

11 References 1. Ning, Z.; Zhitomirsky, D.; Adinolfi, V.; Sutherland, B. R.; Xu, J.; Voznyy, O.; Maraghechi, P.; Lan, X.; Hoogland, S.; Ren, Y.; Sargent, E. H. Adv. Mater. 2013, 25, Liu, M.; Voznyy, O.; Sabatini, R.; García de Arquer, F. P.; Munir, R.; Balawi, A. H.; Lan, X.; Fan, F.; Walters, G.; Kirmani, A. R.; Hoogland, S.; Laquai, F.; Amassian, A.; Sargent, E. H. Nat. Mater. 2017, 16, Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Nat. Mater. 2014, 13,