Progress in Printed TEGs Thermoelectric Network, Manchester, 15/02/2017. Thomas Fletcher

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1 Progress in Printed TEGs Thermoelectric Network, Manchester, 15/02/2017 Thomas Fletcher

2 Introduction to CDT CDT is a spin-out company from the Cavendish Laboratory, University of Cambridge (1992), part of Sumitomo Chemical Company Group (SCC) since 2007 Interdisciplinary team with strong expertise in physics, chemistry, materials and life sciences State of the art chemistry laboratories, cleanrooms, analysis laboratories and device prototyping capabilities 2

3 Current Research Areas Biosensors and FlexOLED Bio-sensor Platform New systems powered by printed semiconductors Abingdon Health OLED/OPD transducer pilot production FlexOLED Flexible OLED for simple signage applications Process development towards manufacturing Technology demonstration Image and gas sensors Organic Photo Diode and Photo-Detector Arrays Printed & Low Cost Visible and near Infra-red Gas Sensors based on Organic Semiconductors Near Infra-red for medical and imaging applications Lighting and power Synthesis of novel materials for P-OLED lighting Low-cost large-area devices for Lighting Optical and device engineering Battery/Supercapacitor Hybrid Technology Printed Thermoelectric Generators Materials and Device research Demonstrator systems with Open Innovation Partners New Technologies Develop the next generation of ideas that support SCC business themes Feasibility projects to validate research proposals Eg. Active release of active ingredients, Biosensor Research, Rapid ES/iPS cell sorting Build Open Innovation relationships Facilities New Capabilities: & capabilities: Synthetic Flow chemistry for laboratories organic synthesis & deepening Chemical relationship & physical with materials University analysis of Cambridge Clean Chemistry rooms Laboratory (14 & 6 line) Formulation Utilise Open and Innovation printing laboratory broaden knowledge Electrical base, and widen optical capabilities device measurement and accelerate evaluation Fast laser cycle spectroscopy for new technologies Highly Strengthen skilled network teams (chemists, to enhance physicists, footprint engineers and visibility & post-doc of Sumitomo specialists) Chemical s corporate research Close links Europe with Universities

4 Printed TEG: Concept and Motivation CDT is developing devices to harvest low grade heat Battery free operation of low powered electronics. Wireless sensors networks - IoT Low power wearable electronics Printing offers manufacturing and application advantages compared to existing TEGs Existing TEG Rigid Brittle Costly to Manufacture Small size only <40cm 2 Thick (~5mm) Harmful Materials CDT Printed TEG Flexible Durable Printable Large size possible, custom size and shape. Thin (~200μm) Safe 4

5 Printed TEG: Concept and Motivation Existing printed TEG research focuses on lateral device structure CDTs development focus is on a flexible module with traditional throughplane geometry. Applicable to a wider range of applications Better suited to lower electrical conductivities of organic / printable materials Lateral Geometry Through-plane Geometry 5

6 Energy Harvesting Competes with Battery Replacement Energy harvesting TEGs compete with battery replacement cycles TEGs can compete with AA cell on 1 year replacement cycle with T >5K Li AA cell (1 year replacement) Industrial IoT Li coin cell (1 year replacement) Space for wearables Body Domestic IoT 6

7 Application Driven Development ~0.5-1mW /node Module and materials requirements driven by target application specifications: IoT will require μwcm -2 of output power from a heat source of C. DC-DC convertors typically require >100 mv, <50 Ω device. Printed material electrical performance target: S = 100 μv/k σ = 10 S/cm To achieve passive cooling from a printed device need low thermal conductivity material. CDT/SCC materials typically W/m.K so well suited to thin devices. 200 μm thick printed device would have similar potential for passive cooling to mm scale bulk materials with 1-10 W/m.K DC-DC convertor (40% Efficient, >100mV) 50cm 2 Module 25μWcm -2 T 10 C n,p ±100μV/K, 10S/cm 200μm thick k~0.2w/mk Heat source (e.g. heating pipe) ~75 C 7

8 Material Characterisation Lateral, in-plane, // S Vertical, out-of-plane, through-plane, cold plate S // hot plate cold plate hot plate DUT σ DUT σ // Useful for characterising intrinsic material properties Representative of material in printed device 8

9 Printable Thermoelectric Materials CDT s thermoelectric material programme focuses on developing a high performance printable n-type Internal material is close to final requirements Ink formulation and film morphology has significant impact on thermoelectric properties and module performance. Printable Material Conductivity (S cm -1 ) Seebeck (μv K -1 ) PF (μw m -1 K -2 ) PEDOT:PSS (in-plane) PEDOT:PSS (out-of-plane) P(NDIO2-T2) Self-doped PDI CDT n-type CDT Target 10 ± G. H. Kim et al. Nat. Mater. 12, (2013) 2 Q. Wei et al. ACS Macro Lett. 3, (2014) 3 R. A. Schlitz et al. Adv. Mater. 26, 2825 (2014) 4 B. Russ et al. Adv. Mater. 26, 3473 (2014) 9

10 Flexible Substrates Development Typical flexible substrates (PEN) have high thermal resistance reducing T across TE materials Developed thermally conductive Al/Al 2 O 3 foil substrates in collaboration with Cambridge Nanotherm Ltd T conversion efficacy (experiment) PEN T Applied x = 0.15 T TE material PEN Al Al 2 O 3 x = 0.74 Al 2 O 3 Al T = T HOT T COLD T TE material = x T Applied Through open innovation we have increased temperature gradient across TE materials by a factor of 5 10

11 Fabrication Apply thermally conductive dielectric to foil substrate Add metal tracking and patterned photoresist bank Print p-type and n-type active materials from ink 5 4 Encapsulate with 2 nd substrate Deposit top electrode by evaporation

12 Printing Pitfalls Perfect Printing Continuous electrical contact. Good thermal contact Active Bank Active Doming Continuous electrical contact OK thermal contact Active Bank Active Slight Underfilling Poor electrical contact Poor thermal contact Poor Wetting Shorted electrical contact Poor thermal contact Active Bank Active Edge Thickening Poor electrical contact Poor thermal contact CDT 2017 Cambridge Display Technology Limited (Company 12 Number )

13 Case 1: Electrical Shorts Electrical shorts reduce TEG output voltage Potential causes: Poor wetting Material overspill Encapsulation misalignment Solved by: Ink formulation changes Substrate design Surface energy engineering Module Seebeck Response with Shorted Legs Voc / mv T / K -10 Actual -15 Expected -20 Poor wetting inside well Evaporated Electrode Overspill to next row of legs

14 Case 2: Electrode Breaks Cracks in printed material and electrode lead to high leg resistance & therefore reduced power output Caused by: Drying & baking processes Flexing of substrate Solved by changes to ink formulation Histogram of Leg Resistances Legs with R < X Ω 60% 50% 40% 30% 20% 10% 0% Electrode Breaks Original Formulation Improved Formulation >50 Leg Resistance (X) / Ω

15 Improved Ink Formulation Changes in ink formulation have solved major printing issues Old Formulation Module Resistance Breakdown Device resistance is no longer dominated by materials New Formulation Device Resistance / Ω Leadouts Top Electrode Bottom Electrode Material Contribution 15

16 Module Performance Modules fabricated with PEDOT:PSS and CDT n-type Tested at constant T with variable load resistance, maximum power delivery at matched load. Next Steps: Reduce electrode resistance Incorporate high performance p-type T=20K PEN Module Foil Module Voc 13 mv 44 mv Resistance 108 Ω 75 Ω Total Power 0.4 μw 6.5 μw Power Density 0.08 μwcm μwcm -2 16

17 Printed TEG Applications Roadmap Estimation of Relative Market for TEG Powered Device Watch (not smart) Temperature Monitor Medical Temperature Monitor Ozone Monitor Cochlear Implant Medical liquid flow sensor EEG ECG CO monitor Smart Nicotine Patch Differential Pressure Sensor Drug Pump Gas / Liquid Flow Sensor Smoke Detector Pulse-Ox (I-LED) TEG Power Requirement 1 µwcm µwcm µwcm -2 17

18 Conclusion CDT has entered the field of TEGs to create self sustaining low power electronic devices We have developed a flexible printed platform capable of achieving significant power from a thin device Ongoing materials research Materials development driven by application requirements Ink formulation is critical to achieving high power output from printed TEGs 18

19 Acknowledgments You for your attention The organisers for the invitation Colleagues at CDT Our collaborators Sumitomo Chemical Corporation Ltd Cambridge Nanotherm Ltd If you are interested in working with CDT on TEGs or other research areas please come and speak to us after this session Thomas Fletcher Simon King 19

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21 Appendix Passive cooling modelling: Based on hot side temp of 75 C. Heatsink is 1cm of 10W/mK material. 1m/s airflow. Data shown with 25% fill factor, for active material. Achieve 10 C dt with 200μm of organic / hybrid type material, would require 1mm of bulk type material. 21