Atmospheric pressure Plasma Enhanced CVD for large area deposition of TiO 2-x electron transport layers for PV Heather M. Yates
Why the interest? Perovskite solar cells have shown considerable promise due to o Increasingly high efficiencies >22% (single), >30% (tandem) o High absorption coefficient, large diffusion length, high carrier mobility o Inexpensive, abundant raw materials Au HTL Perovskite absorber M-TiO 2 /perovskite TiO 2 FTO Glass Au HTL/ETL Perovskite absorber ETL/HTL ITO/FTO Glass The Challenge To provide cost effective large scale production. To link with existing in-line coating technology. o Typically spin coating / vacuum based technologies
How? Salford AP CVD optimise FTO / perovskite absorber/carrier transport layers In-line deposition of compact TiO 2-x ETL via AP PECVD Compatible with the online production of transparent conducting oxides Solar cells constructed to evaluate ETL performance compared to a reference sputtered ETL
Perovskite solar cells Au HTL Perovskite absorber M-TiO 2 /perovskite TiO 2 FTO Glass Perovskite absorber - CH 3 NH 3 PbI 3 Charge conducting mesoporous scaffold TiO 2 Electron Transport Layer (ETL) n-type material - compact TiO 2-x Block recombination between e - in the front electrode and h + in the perovskite Hole Transport Layer (HTL) p-type material - spiro-meotad Electrodes F-doped SnO 2 (front), evaporated Au (back)
Technical Approach APCVD is a cost effective industrial process Continuous film growth on large area substrates Deposit FTO on-line Advantage for ETL process CVD provides control of film properties Crystallinity Morphology Stoichiometry Activated processes to reduced substrate temperatures Plasma (PECVD), Flame etc PECVD typically operates under vacuum Stable low temperature glow discharge Diffusion driven processes give conformal high quality films Relatively low deposition rates
Atmospheric Pressure Plasma Enhanced CVD AP plasmas typically operate close to thermal equilibrium e.g. RF ICP, metal cutting, arc-jet plasma spray Barrier discharge systems provide non thermal plasma at AP Controlled reaction of precursors for film growth Deposition on thermally sensitive substrates Development can produce diffuse discharges and lead to CVD Typically used for surface treatment & polymers where plasma stability and film quality is less critical Optimised system can provide device quality films at AP and RT!
Process Demonstrator Dual flow AP PECVD roll to roll system Convey solid substrates up to 20 x 20 cm Continuous film growth with high uniformity TiO 2-x deposited from TTIP on 10 x 10 cm Solaronix TCO22-15, 10.7 m.hr -1 Room temperature deposition 4, 6 & 8 passes resulted in 40 nm, 55 nm & 85 nm films Argon or nitrogen plasma
Morphology PE CVD TiO 2-x AP PECVD SEM of masked sample to show coverage. Sputtered 50 mm High resolution TEM showed coverage comparable to the sputtered reference.
Control of Film Properties 20 18 O 1s 12 Ti 2p 16 14 Ti 2p CPS x 10-4 16 14 12 CPS x 10-4 10 8 CPS x 10-4 12 10 Sputtered reference 8 10 6 8 4 6 6 4 4 2 2 540 537 534 531 528 Binding Energy (ev) 468 465 462 459 456 Binding Energy (ev) 468 465 462 459 456 Binding Energy (ev) XPS shows sub-stoichiometric titania Depending on deposition conditions (Applied voltage) o AP CVD TiO 1.79 to TiO 1.91 o Sputtered reference sample TiO 1.73
Control of Film Properties Audio AC power supply DC pulsed power supply Strong correlation between voltage and Ti:O ratio Films could be soft, hard or powdery dependant on plasma conditions Conditions chosen to provide dense, adherent powder free films
Control of Film Properties - defects CPS x 10-4 12 10 8 Ti 2p Narrowing of Ti 2p 3/2 FWHM associated with a reduction in structural defects. 6 4 2 468 465 462 459 456 Binding Energy (ev) 4 kv to > 8 kv reduced the Ti 2p 3/2 FWHM from 1.36 ev to 1.28 ev But may increase homogeneous reaction so more powder. Reference sputtered TiO 2-x FWHM 1.12 ev
TiO 2-x on FTO/glass substrates Optical Properties Reflection increases due to increased specular component. Limited change in transmission after initial addition of TiO 2-x. Reduction in haze on addition of TiO 2-x due to smoother surface. 14 nm to 4 nm RMS
Annealing Behaviour TiO 2-x on FTO/glass substrates annealed at 500 o C over 80 mins. No change in sheet resistance of underlying FTO. Minimal %T loss for FTO. %T losses for annealed samples due to onset of crystal formation and associated grain boundaries within the TiO 2-x layer. In agreement with XPS observed reduction in Ti 2p 3/2 peak width (1.36 ev to 1.16 ev) which suggests a reduction in defects.
Dark Conductivity σ Dark & E a measurements for a range of TiO 2-x thicknesses on float glass Reduced conductivity compared to the reference Higher activation energy suggests less defective than the sputtered film Strong correlation with thickness
MPP - Maximum Power Point Often used to give a measure of the stability of the cell over time. Cell Results Efficiency depend on 3 main parameters - J sc Short circuit current density Relates to light absorption and photocurrent conversion. V oc Open circuit voltage Improved by reduction in non-radiative losses and better charge transport. FF Fill Factor Affected by charge carrier transport and the recombination rate.
Cell Results TiO 2-x / FTO 1 cm 2 mesoporous CH 3 NH 3 PbI 3 Increase in V oc,ff, J sc and hence efficiency with reduced thickness. Possibility to further improve efficiency with reduced thickness Efficiency greater than the sputtered reference 14.53%, 13.57% mpp vs 14.11% 13.15% mpp
Current Density Plots & MPP Tracking 40 nm 55 nm 85 nm Ref 23 nm Increased FF for 40 nm AP PECVD & Sputtered film Hysteresis reduced for thinner films due to improved interface contact and reduced interfacial charge traps MPP efficiency stable after five minutes
External Quantum Efficiency 40 nm AP PECVD film gave reduced EQE compared to sputtered film This deficiency could be completely offset by applying a 0.5 V negative bias Collection issue related to an extraction barrier at the interface
Summary TiO 2-x deposited via continuous AP PECVD process. Adherent film deposition can occur at room temperature. Film properties and uniformity suitable for use as an ETL in PV cells. Excellent cell performance demonstrated by non optimised films 14.53%, 13.57% mpp (AP PECVD) vs 14.11% 13.15% mpp (Optimised sputter coated) Issues generally related to film thickness ( 40 nm vs 23 nm in reference) Dark conductivity, series resistance, FF Significant potential for efficiency gains in conjunction with large area processing.
Acknowledgments This work was financed by Horizon 2020 - H2020-LCE-2015-16-53296 CHEOPS Highly Efficient photovoltaic Perovskite Solar cells Collaborators John Hodgkinson, University of Salford Arnaud Walter, D. Sacchetto, S.-J.Moon, S. Nicolay Centre Suisse d Electronique et de Microtechnique Tyndall National Institute, Ireland - HRTEM images XPS data was provided by NEXUS UK national facility Geoff Parr, Salford Analytical Services for SEM