Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

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 CdCl2(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.

Room-Temperature Postannealing Reduction via Aqueous Sodium Borohydride and Composition Optimization of Fully Solution-Processed Indium Tin Oxide Films

“…Ink-based deposition of electronic devices offers the versatility of printing roll-to-roll at low cost and on large substrates. − Transparent conductive materials are central to these devices and are currently used in touch screens, solar cells, light-emitting devices, and transparent thin film transistors, − which can be deposited at low temperatures. , Recent progress has been made with ink-based nanostructures including two-dimensional (2D) metal nanowires, reduced graphene oxide, and carbon nanotubes as flexible alternatives to indium tin oxide (ITO), where 26 Ω/□ and 90–94% T are possible while retaining high flexibility. ,,− However, despite these exceptional advancements, ITO and transparent conducting oxides remain the hallmark and current standard for transparent electronics due to their long-term stability and exceptional optical and electronic properties and are ideal for nonflexible applications. , …”