Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

Summary

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

Abstract

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 °C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH₄Cl:methanol to replace long-chain native ligands with small inorganic Cl^- anions. NH₄Cl_(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl_2(s) treatment) leading to grain growth (136±39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

Introduction

Due to their unique emerging properties, inorganic nanocrystal inks have found applications in a wide range of electronic devices including photovoltaics,^1-6 light emitting diodes,^7,8 capacitors⁹ and transistors.¹⁰ This is due to the combination of the excellent electronic and optical properties of inorganic materials and their solution compatibility on the nanoscale. Bulk inorganic materials are typically not soluble and are therefore limited to high temperature, low pressure vacuum depositions. However, when prepared on the nanoscale with an organic ligand shell, these materials can be dispersed in organic solvents and deposited from solution (drop-, dip-, spin-, spray- coating). This freedom to coat large and irregular surfaces with electronic devices reduces the cost of these technologies while also expanding possible niche applications.^6,11,12

Solution processing of cadmium(II) telluride (CdTe), cadmium(II) selenide (CdSe), cadmium(II) sulfide (CdS) and zinc oxide (ZnO) inorganic semiconductor active layers has led to photovoltaic devices reaching efficiencies (ƞ) for metal-CdTe Schottky junction CdTe/Al (ƞ = 5.15%)^13,14 and heterojunction CdS/CdTe (ƞ = 5.73%),¹⁵ CdSe/CdTe (ƞ = 3.02%),^16,17 ZnO/CdTe (ƞ = 7.1%, 12%).^18,19 In contrast to vacuum deposition of bulk CdTe devices, these nanocrystal films must undergo ligand exchange following deposition to remove native and insulating long-chain organic ligands which prohibit efficient electron transport through the film. Additionally, sintering Cd- (S, Se, Te) must occur during heating in the presence of a suitable salt catalyst. Recently, it was found that non-toxic ammonium chloride (NH₄Cl) can be used for this purpose as a replacement for the commonly used cadmium(II) chloride (CdCl₂).²⁰ By dipping the deposited nanocrystal film in NH₄Cl:methanol solutions, the ligand exchange reaction occurs simultaneously with exposure to the heat-activated NH₄Cl sintering catalyst. These prepared films are heated layer-by-layer to build the desired thickness of the photo-active layers.²¹

Recent advances in transparent conductive films (metal nanowires, graphene, carbon nanotubes, combustion processed indium tin oxide) and conductive metal nanocrystal inks have led to the fabrication of flexible or curved electronics built on arbitrary non-conductive surfaces.^22,23 In this presentation, we demonstrate the preparation of each precursor ink solution including the active layers (CdTe and CdSe nanocrystals), the transparent conducting oxide electrode (i.e., indium doped tin oxide, ITO) and the back metal contact to construct a completed inorganic solar cell entirely from a solution process.²⁴ Here, we highlight the spray process and the device layer patterning architectures on non-conductive glass. This detailed video protocol is intended to aid researchers who are designing and building solution processed solar cells; however, the same techniques described here are applicable to a wide range of electronic devices.

Protocol

Note: Please consult all relevant materials safety data sheets (MSDS) before use. Many of the precursor solutions and products are hazardous or carcinogenic. Special consideration should be directed to nanomaterials due to unique safety concerns that arise compared to their bulk counterparts. Proper protective equipment should be worn (safety goggles, face shield, gloves, lab coat, long pants and closed-toed shoes) at all times during this procedure.

1. Synthesis of Nanocrystal Precursor Inks

1. CdSe and CdTe Inks^18,25
   1. In an inert atmosphere glove box, combine 0.24 g (0.0019 mol) tellurium (Te) for CdTe (or 0.1527 g (0.0019 mol) selenium (Se) for CdSe) powder with 4.39 g (0.012 mol) trioctylphosphine (TOP) into a 5 ml round bottom flask (RBF).
   2. Seal this flask with a rubber septum and remove from glove box for sonication (40 kHz) in a heated (60 °C) water bath until all of the solid Te or Se has dissolved (about 20 min). Set aside 5 ml of 1-octadecene (1-ODE).
   3. Separately, in a clean and dry 3-neck 250 ml RBF with a magnetic stir bar, combine 0.48 g (0.0037 mol) cadmium(II) oxide (CdO) powder with 4.29 g (0.015 mol) Oleic Acid (OA) and 76 ml of 1-octadecene. Inspect glassware for defects prior to use, and assemble all glass-to-glass joints with high temperature vacuum grease.
   4. Connect a vacuum pump and an inert gas (argon, Ar or nitrogen, N₂) source on low flow to the flask through a Schlenk line glassware leaving at least one neck of the RBF free to inject the TOP-chalcogenide precursor. Insert temperature probe directly into the solution from one of the necks and seal.
   5. Set to stir at the highest speed and set temperature to 110 °C under vacuum for 30 min.
      Note: Exceeding 250 °C may degrade the oleic acid component indicated by a color change from colorless to yellow.
   6. Switch from vacuum to inert gas to build a slight positive pressure in the flask. Adjust the flow of gas to a low pressure (~1 psi). Bubbles should be forming at a frequency of 1-5 Hz in the oil bubbler.
      1. Separately prepare a glass neck extension topped with a rubber septum. Attach a syringe needle to tubing on the Schlenk line.
      2. Pierce the syringe needle into the septum to allow pressure to release. Remember to lightly grease the joint with vacuum grease.
      3. At this time, quickly remove the top glass stopper from the reaction flask and replace it with a glass extension. Excess inert gas will flow through the flask, and this will be indicated with bubbles emerging from the oil bubbler.
   7. Close the original inert gas source and open the second vent to allow a slow controlled stream of inert gas into the top of the flask during the rest of the synthesis.
   8. Increase the temperature of the solution to 260 °C for CdTe (250 °C for CdSe) and wait until the solution turns from a slight brown to completely colorless and transparent.
   9. Once the desired reaction temperature is reached, prepare a syringe for injection by extracting the TOP-chalcogenide precursor and the additional 5 ml 1-ODE.
   10. In one step, remove the heating mantle while continuing to stir and rapidly inject the TOP-chalcogenide/1-ODE mixture.
   11. Allow solution to cool to RT (~30 min) and monitor color changes as quantum confined particle seeds form and grow into larger nanocrystals. CdSe is a deep red color and CdTe is a dark brown.
   12. Directly to the flask, add 25 ml heptane and 100 ml ethanol to precipitate the product. Transfer 40 ml aliquots to a 50 ml centrifuge tube and add 5 ml toluene and 5 ml ethanol to complete precipitation.
   13. Centrifuge the product at 1,722 x g for 2 min or until the supernatant is transparent. Decant supernatant and combine solid product into a 5 ml RBF by adding 0.5 ml toluene and 5 ml distilled pyridine to disperse the nanocrystals. CAUTION: Conduct all pyridine experiments under the fume hood.
   14. Flush the RBF with inert gas and then seal with rubber septum. Attach heating mantle and bring to 85 °C. Relieve any pressure using a needle inserted briefly into the rubber septum. Continue heating and stirring gently for 18 hr.
   15. Following pyridine exchange, combine CdTe or CdSe product and 40 ml hexanes and centrifuge at 1,722 x g for 2 min or until supernatant is colorless. Decant supernatant and add 5 ml distilled pyridine and 5 ml 1-propanol. Flush flask with inert gas and sonicate (40 kHz) this mixture for 30 min. Collect the supernatant and discard any solid product.
   16. Filter the ink through a 1 μm Polytetrafluoroethylene (PTFE) syringe filter to remove large or aggregated particles. Measure the concentration of the ink by drying and weighing 1 ml. Typical concentrations are 40 mg ml^-1 for CdTe and 16 mg ml^-1 for CdSe.
   17. Dilute ink with pyridine / 1-propanol as needed. Store ink under inert gas while not in use.
2. Au Ink²⁶
   1. In a 500 ml Erlenmeyer flask while stirring, combine 1.518 g (0.00385 mol) of gold(III) chloride trihydrate, HAuCl₄^.3H₂O and 126 ml H₂O to produce a yellow solution.
   2. Add a pre-mixed solution of 9.52 g (0.0174 mol) tetraoctylammonium bromide in 334 ml toluene.
   3. Next add the ligand, 0.452 g (0.00382 mol) hexanethiol in 2 ml toluene.
   4. Finally, separately combine 1.58 g (0.0418 mol) sodium borohydride (NaBH₄) with 105 ml H₂O and immediately add this bubbling reducing solution drop-wise to the reaction flask.
   5. After stirring at RT in air for 3 hr, separate the organic phase with a separatory funnel.
   6. Use a rotary evaporator to reduce the volume to 20 ml and wash this ink with 50 ml hexanes and 200 ml methanol. Precipitate solid with centrifugation at 1,722 x g for 2 min and decant the colorless supernatant.
   7. Dry the solid in air and re-disperse in chloroform with a concentration of 70 mg ml^-1.
3. ITO Inks²³
   1. Combine solid salts of indium(III) nitrate hydrate (In(NO₃)₃^.2.85H₂O, 2.93 g, 0.00974 mol) and tin(II) chloride dihydrate, (SnCl₂^.2H₂O, 0.357 g, 0.00158 mol) with 10 ml 2-methoxyethanol into a 50 ml polypropylene centrifuge tube.
   2. To this, add 167 μl of 14.5 M ammonium hydroxide (NH₄OH, 0.0024 mol) as a pH stabilizer and 0.83 g (0.0104 mol) ammonium nitrate (NH₄NO₃) as an oxidizer.
   3. Sonicate at 40 kHz for 20 min with heating (60 °C) or until the ink changes from hazy white to colorless and transparent.

2. ITO Patterning

1. Cut and clean a (25 mm x 25 mm x 1.1 mm) glass slide by sonicating in ethanol and acetone.
2. Soak glass substrate in concentrated (> 5 M) aqueous sodium hydroxide (NaOH) for 1 min and briefly rinse with water.
3. Place the glass substrate on the spin coater and fill slide with ITO ink. Spin at 3,228 x g for 20 sec.
4. Immediately place the substrate on a hotplate set to 400 °C and heat for 10 min. Cool slowly at RT on a ceramic plate.
5. Repeat this process (2.3 - 2.4) until the sheet resistance is below 1,000 Ohms per square (about 10 layers). Approximate the sheet resistance with a multimeter or measure with a four-point probe by placing the ITO/glass film on a stable surface and pressing down the multimeter probes approximately 0.5 cm apart to record the resistance. If a four-point probe is available, depress the probe tips onto the film to record the sheet resistance following established methods.²⁷
6. Finally, briefly dip (~2 sec) the film in dilute aqua regia and rinse with distilled water followed by drying to reduce the resistance below 500 Ohms per square.
7. Construct a device pattern by cutting strips of tape (i.e., polyamide tape for heat treatments or cellophane tape for acid etching) and adhering them along the pre-designed grid. For example, perpendicular strips with a width of 0.10 cm will produce 0.10 cm² device areas.
   1. Design grids with document editing software, print onto paper and position under the substrate to act as a guide for mounting tape onto the transparent glass slide.
      Note: Depending on the application and the properties of the inks, these grids can be used to produce devices with overlapping top and bottom electrodes in the shape of a square, a rectangle or any shape with measurable area. For example, by alternating two parallel strips of ITO that are each 0.10 cm wide, followed by depositing the active layers (CdSe and CdTe), the gold layer can be deposited using the same pattern only rotated by 90 degrees to form two 0.10 cm² devices.
8. Soak the glass/ITO film with adhered tape strips in dilute aqua regia at 60 °C until the exposed ITO dissolves, leaving behind bare glass substrate.
9. Remove the tape and wash film with acetone and ethanol to remove any residue from the tape adhesive.
10. Place small drops of silver epoxy on the ITO strips on one end of the glass substrate. Heat this on a hotplate at 150 °C for 2 min, followed by cooling to RT. These will serve as contact points for device measurement since it is difficult to remove the CdTe/CdSe active layers after annealing.

3. Solution Processing of CdSe, CdTe and Au Films

1. Spin Coating²⁸
   1. Place patterned ITO-glass substrate on spin coater and fill the top surface by drop coating CdSe nanocrystals.
   2. Spin at 610 x g for 30 sec followed by drying on a hotplate at 150 °C for 2 min. Cool to 25 °C.
   3. Dip the film in a NH₄Cl:methanol (saturated at 25 °C) solution set to 60 °C. Hold for 15 sec and then dip film into a separate container of isopropanol.
   4. Dry under inert gas and then heat on a hotplate at 380 °C for 25 sec. Cool to RT and rinse off excess salt with distilled water before drying under inert gas.
   5. Repeat this process (3.1.1 - 3.1.4) until the desired thickness is reached. Typically, 3 layers of CdSe produce a 60 nm film and 6 layers of CdTe produces a 400 nm CdTe film.
2. Spray Coating^12,29
   1. Mount the ITO-glass substrate vertically with tape or clips onto a flat solid backing.
   2. Dilute the CdTe and CdSe ink to 4 mg ml^-1 with chloroform and load the gravity-fed airbrush (equipped with 0.5 mm needle) with 0.25 ml of the ink.
   3. Adjust the carrier gas pressure between 10 and 40 psi. Use higher pressures for thinner smoother films.
   4. Depress nozzle and spray nanocrystal ink next to the substrate followed by spraying uniformly over the substrate using a rapid perpendicular side-to-side motion where the air brush nozzle is kept approximately 60 mm from the substrate. Clean the airbrush by spraying ~1 ml pure chloroform away from the device.
   5. Remove substrate from the mount and treat the deposited CdTe or CdSe nanocrystal film with the same procedure as for spin coating (3.1.5) until the desired thickness is achieved.
   6. Likewise, spray the back metal contact nanocrystal film onto the active layers to complete the device. Using the same procedure employed for the ITO electrodes, pattern 0.01 cm thick strips using low-adhesive tape to the active layer perpendicular to the ITO strips.
   7. Load the airbrush with 2 ml of the gold (Au) nanocrystal ink (70 mg ml^-1) dispersed in chloroform.
   8. After a dark opaque film is deposited, dismount the substrate and carefully remove the tape before heating on a hotplate at 250 °C for 20 sec. The gold color will appear and the device can be cooled to RT and tested.

Results

Small angle X-ray Diffraction Patterns are used to verify the crystallinity and phase of the annealed nanocrystal film (Figure 1A). If crystallite sizes are below 100 nm, their crystal diameter can be estimated with the Scherrer equation (Eq. 1) and verified with Scanning Electron Microscopy (SEM),

Discussion

In summary, this protocol provides guidelines for the key steps involved with building a solution processed electronic device from a spray- or spin-coating deposition. Here, we highlight new methods for solution processing transparent conductive indium tin oxide (ITO) films onto non-conductive glass substrates. After a facile etching procedure, individual electrodes can be formed before spray-depositing the photo-active layers. Using a layer-by-layer technique, CdSe and CdTe nanocrystals can be deposited in air under amb...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Oleic acid, 90%	Sigma Aldrich	364525
1-octadecene, 90%	Sigma Aldrich	O806	Technical grade
Trioctylphosphine (TOP), 90%	Sigma Aldrich	117854	Air sensitive
Trimethylsilyl chloride, 99.9%	Sigma Aldrich	92360	Air and water sensitive
Se, 99.5+%	Sigma Aldrich	209651
NH4Cl, 99%	Sigma Aldrich	9718
CdCl2, 99.9%	Sigma Aldrich	202908	Highly toxic
CdO, 99.99%	Strem	202894	Highly toxic
Te, 99.8%	Strem	264865
In(NO3)3.2.85H2O, 99.99%	Sigma Aldrich	326127-50G
SnCl2.2H2O, 99.9%	Sigma Aldrich	431508
NH4OH	Sigma Aldrich	320145	Caustic
NH4NO3, 99%	Sigma Aldrich	A9642
HAuCl4.3H2O, 99.9%	Sigma Aldrich	520918
Tetraoctylammonium bromide (TMA-Br)	Sigma Aldrich	294136
Toluene, 99.8%	Sigma Aldrich	244511
Hexanethiol, 95%	Sigma Aldrich	234192
NaBH4, 96%	Sigma Aldrich	71320
Hexanes, 98.5%	Sigma Aldrich	650544
Ethanol, 99.5%	Sigma Aldrich	459844
Methanol, anhydrous, 99.8%	Sigma Aldrich	322415
1-propanol, 99.5%	Sigma Aldrich	402893
2-propanol, 99.5%	Sigma Aldrich	278475
Pyridine, > 99%	Sigma Aldrich	360570	Purified by distillation
Heptane	Sigma Aldrich	246654
chloroform > 99%	Sigma Aldrich	372978
Acetone	Sigma Aldrich	34850
Glass microscope slides	Fisher	12-544-4	Cut with glass cutter
Gravity Fed Airbrush	Paasche	VSR90#1
Syringe needle	Fisher	CAD4075
Solar Simulator Testing Station	Newport	PVIV-1A
Software	Oriel	PVIV 2.0
Round bottom flask	Sigma Aldrich	Z723134
Round bottom flask	Sigma Aldrich	Z418668
Polytetrafluoroethylene (PTFE) syringe filter	Sigma Aldrich	Z259926
Polyamide tape	Kapton	KPT-1/8
Cellophane tape	Scotch	810 Tape
Polypropylene centrifuge tube	Sigma Aldrich	CLS430290
Silver epoxy	MG Chemicals	8331-14G