High Efficiency Solution Processed Sintered CdTe Nanocrystal Solar Cells: The Role of Interfaces

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1 SUPPORTING INFORMATION High Efficiency Solution Processed Sintered CdTe Nanocrystal Solar Cells: The Role of Interfaces Matthew G. Panthani, J. Matt Kurley, Ryan W. Crisp,, Travis C. Dietz, Taha Ezzyat Joseph M. Luther,, * Dmitri V. Talapin,, * Department of Chemistry and James Franck Institute, University of Chicago, Chicago, IL , National Renewable Energy Laboratory Golden, CO 80401, Colorado School of Mines Golden, CO 80401, Center for Nanoscale Materials, Argonne National Lab, Argonne, Illinois 60439, United States *Corresponding authors: (T) , (F): , dvtalapin@uchicago.edu (D.V.T.); joey.luther@nrel.gov (J.M.L.) Experimental Synthesis of CdTe nanocrystals All chemicals and powders were purchased from Sigma Aldrich and used as received. CdTe NCs were synthesized by modification of the synthesis described by Jasieniak et al g of CdO (99.99+%) and 21 g oleic acid (OA; technical grade, 90%) were dispersed in 20 g octadecene (technical grade, 90%) and stirred in a 150 ml 3-neck flask and degassed at 100 C under vacuum for 1 hour. The flask is filled with nitrogen and the solution temperature is increased to 270 C, where 12 ml of 1M tellurium (shot, % trace metals basis) in tributylphosphine (97%) is injected and the flask is immediately taken off the heating mantle and cooled naturally to room temperature. The nanocrystals are precipitated with ethanol and redispersed in toluene. This process is repeated twice to remove excess ligands and unreacted precursors. All washing steps for OA-capped CdTe nanocrystals are done in ambient air. The purified nanocrystals are S-1

2 precipitated once more with methanol, transferred into a N2-filled glovebox, and redispersed in anhydrous toluene at a concentration of ~100 mg/ml, where they can be stored for several months. Ligand exchange with pyridine The CdTe solution is precipitated with ethanol and redispersed in anhydrous pyridine (99.8%) at a concentration of ~20 mg/ml. The CdTe NCs in pyridine are stirred under nitrogen overnight on a hotplate set to 105 C. This solution is precipitated with hexane, redispersed in pyridine, and precipitated once again using hexane. This is then redispersed in a 1:2 mixture of pyridine and 1-propanol (anhydrous, 99.7%) at a concentration of ~40 mg/ml. The CdTe nanocrystals are sonicated for 10 minutes then passed through a 0.45µm PTFE filter immediately before use. After the ligand exchange there is a blueshift in the excitonic peak position of ~10 nm, possibly due to partial etching of the surface during ligand exchange. This could be due to removal of surface Cd-atoms along with the oleate ligand. Absorbance spectra of CdTe nanocrystal solutions before and after ligand exchange are shown in Figure S1. pyridine Oleic acid Absorbance Wavelength (nm) Figure S1. Absorbance spectra of ~6.0 nm CdTe nanocrystals after ligand exchange with pyridine (black) and the as-synthesized nanocrystals capped with oleic acid (red dashed). S-2

3 Layer-by-layer deposition of CdTe NCs CdTe nanocrystals are deposited onto 25 mm x 25 mm substrates by spincoating between at 600 and 800 rpm for 30 seconds followed by 1000 rpm for 10 seconds. The substrates are placed onto a hotplate held at 150 C for 2 minutes to remove any solvent. This is followed by a dip in a saturated solution of CdCl 2 in methanol at 60 C. The substrate is rinsed with either isopropanol and dried with a stream of nitrogen. The substrate is then placed on a second hotplate held at 350 C for 20 seconds to promote grain growth. This process is repeated multiple times (typically 12) to build a nm thick film. Contact layer processing and device completion. ZnO NCs were synthesized by adding 0.44 g of zinc acetate dihydrate to 40 ml of ethanol in a flask and heating at 60 C. After 30 min of heating, 2 ml of tetramethylammonium hydroxide (20% in MeOH) in 10 ml of ethanol was added dropwise to the solution over 5 min. The ZnO nanoparticle solution was heated at 60 C for 30 min to attain the ZnO NCs of 5 nm in size. ZnO NCs dispersed in their growth solution were precipitated with hexane and centrifuged. The supernatant was discarded, and the precipitated nanoparticles were redispersed in 1-propanol at a concentration of 40 mg/ml. The ZnO NCs were then spincoated onto the substrate prior to a 2 minute heat treatment on a hotplate set to 200 C to dry residual solvent. Sol gel ZnO was prepared by mixing 1 g zinc acetate dihydrate with 10 ml 2-methoxyethanol and 0.28 ml ethanolamine. The mixture was stirred overnight then sonicated for 1 h. The solution was then filtered through a 0.2 µm filter. ZnO layers were made by depositing 300 µl of solution (either NCs or sol gel) onto the CdTe film and spincoating at 3000 rpm for 1 min. The substrate was then placed on a Al block heated to 300 C for 2 min, then placed onto a room temperature Al block to cool. Top Al contacts were deposited by thermal evaporation through a shadow mask, which defined a 8 mm 2 circular contact. 100 nm Ag was sometimes deposited on top of the Al to prevent oxidation of the contact. The Ag layer was not found to have any effect on the device performance other than increasing temporal stability. S-3

4 Cross-sectional SEM images of completed devices are shown in Figure S2. Figure S2. Cross-sectional SEM of CdTe devices at different magnification. CdS Chemical Surface Deposition CdS was deposited using a chemical surface deposition technique developed by McCandless and Shafarman which was used to deposit a thin (~20 nm) and uniform layer of CdS. 2, 3 The substrates were placed on a hotplate set to 100 C and covered with a glass petri dish where they were allowed to heat for 10 minutes. Aqueous stock solutions of 0.015M CdSO 4, 1.5M thiourea, and ~18 M NH 4 OH were used. To prepare the solution for chemical surface deposition, 0.22 ml CdSO4 solution, 0.22 ml thiourea solution, 0.28 ml NH 4 OH, and 1.5 ml H 2 O were combined and chilled in an ice bath if storing for more than 10 minutes to prevent homogenous nucleation of CdS particles. The dish was removed and 700 microliters of the solution was dispensed on each of the substrates (25 mm x 25mm). The petri dish was reapplied and the deposition was allowed to proceed for 2 min at which the substrates were removed from the hotplate, thoroughly rinsed with DI water, and blown dry with a stream of dry N 2. S-4

5 Device Characterization Devices were tested by illuminating the device using a Xe lamp with a AM1.5G filter (Newport) calibrated with a Si solar cell with a KG5 filter (Hamamatsu Inc, S ), which has a good spectral match to CdTe. The cell was calibrated by the National Renewable Energy Laboratory s Measurements and Characterizations facility. A self-aligning stainless steel aperture mask with nominally 6 mm 2 circular holes (measured 5.94 mm 2 ) was used to control the illumination area. JV characteristics were acquired using a Keithley 2400 sourcemeter controlled by a Labview program. To mitigate heating during measurements, the perimeter of the cell was in direct contact to an Al heatsink. External quantum efficiency measurements were taken using either a homemade system or an Oriel IQE-200 quantum efficiency analyzer. The homemade setup used a chopped monochromated Xe light source (Princeton Instruments monochromator) and lock-in amplifier (Stanford Research Systems SR830). The instruments were controlled and data collected using a homemade Labview program. A calibrated Si photodetector (Thorlabs, FDS100-CAL) was used as a reference. Current/Light soaking Current/Light soaking was done by applying 2-3V (forward bias) to the device under illumination for 10 minutes. Typically, this generated a current density of ~2.5 A cm -2. The current was monitored carefully to not exceed a 3 A cm -2, as current densities greater than this generally caused performance degradation. Holding the devices in reverse bias generally caused a transient decrease in performance (due to reduced V OC ). Capacitance-voltage measurements Capacitance voltage data was acquired using a Gamry Reference 600 potentiostat. Data were acquires using a frequency of 500 Hz, with an amplitude of 10 mv. S-5

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7 Supplemental Figures Figure S3. Expanded JV characteristics of Fig 2a, showing rollover in forward bias before current/light soaking p 3/2 Counts / s p 1/ Binding Energy (ev) Figure S4. XPS Cu2p Spectrum within the In:ZnO layer. The concentration of Cu in the ZnO was determined to be ~0.2 at %. S-7

8 Figure S5. Intensity-dependent IV curves used to gather data in Figure 4. Figure S6. IV characteristics taken by the NREL Measurement and Certification group using an X25 solar simulator under standard test configuration. A stainless steel aperture with a cm 2 area was placed over the device (active device area ~ 0.08 cm 2 ). An overall PCE of 8.54% was achieved with a V OC of 626 mv and J SC of ma cm -2 S-8

9 References 1. Jasieniak, J.; MacDonald, B. I.; Watkins, S. E.; Mulvaney, P. Nano Letters 2011, 11, (7), McCandless, B. E.; Shafarman, W. N. Chemical surface Depostion of Ultra-thin Semiconductors. 6,537,845, McCandless, B. E.; Shafarman, W. N. In Chemical surface deposition of ultra-thin cadmium sulfide films for high performance and high cadmium utilization, Photovoltaic Energy Conversion, Proceedings of 3rd World Conference on, May 2003, 2003; pp Vol.1. S-9