IMPROVED HIGH INSOLATION AND TEMPERATURE SOLAR CELLS AND CELL ASSEMBLY TECHNOLOGY. ESTEC Noordwijk, The Netherlands

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Prosper Miller
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1 IMPROVED HIGH INSOLATION AND TEMPERATURE SOLAR CELLS AND CELL ASSEMBLY TECHNOLOGY ESTEC Noordwijk, The Netherlands

2 Agenda Objective of TDA Technology requirements Scope of TDA Estimate of achieved TRL Deliverables Critical technology requirements not met Critical technology items not covered Lessons learnt Planning to flight model

3 Objective of TDA Purpose of this work was the selection and testing of materials and manufacturing processes able to withstand the high solar intensity and temperature conditions (HIT) typical of interplanetary missions towards the Sun and Mercury The development approach was aimed to use as much as possible already existing technologies and materials

4 Technology requirement HIT overall requirements Solar Radiation 1. SEPM, 10.7 SC close to Mercury (SAA 70 ) 2. MPO, 4.6 SC close to Mercury (90 SAA) 3. SOLO, 25 SC PVA Maximum Temperature 1. SEPM <200 C 2. MPO <300 C 3. SOLO <250 C (TBC) Solar Aspect Angle From 0 to 75 Radiation Damage 3.0E+15 e - /cm 2 Power Needs Sun Sensor Requirements 1. SEPM > 15 kw AM0 2. SOLO > 500 W at 0.89 AU 3. MPO covered by SOLO module Develop a solar cell based low precision SS

5 Definition and preliminary testing of solar cells and PVA related parts and materials (including substrate) Development of a PVA 3D mathematical model Testing and calibration to measurements of the models Final assessment on the HIT technology readiness level

6 A finite element analysis (p-method), using PRO-MECHANICA software, was performed with the aim to: generate 3-D and 2-D models for behavioural simulations provide indications / recommendations identify critical areas Example of mathematical model concept (courtesy of Pretech)

Temperature distributions Displacements of PVA parts Eq.

7 After the identification of a representative PVA geometry, all the enveloping mission cases have been analysed Temperature distributions Displacements of PVA parts Eq. von Mises stresses Example of analysis output (courtesy of PreTech)

8 SCA level Coupon level BOL and EOL EP TC BOL and EOL SAA influence at different T Influence of HT baking INTC adherence after ageing test (4000 CY) Thermal gradient High Temperature Temperature baking Thermal cycling (4000 CY) Destructive investigation on pre aged hardware

9 Test house identification and HIT set up modifications SCA level sample stage for HT measurement under different SAA (INTA Spasolab) Coupon level thermal chamber for temperature gradient monitoring (DLR) thermal chamber capable to reproduce, under controlled atmosphere, HIT worst case cycling profile (QinetiQ)

10 Solar cell high temperature bake out Cell performance after more than 1000 hours at +300 C Type Time Isc Voc FF Pm (hours) % % % % GaAs/Ge single junction InGaP/GaAs/Ge single junction GaAs/Ge SJ with InGaP filter InGaP/GaAs/Ge DJ-plus TJ using TECSTAR wafer SCA with RWE cell

11 Coverglasses sandwich temperature bake out Transmission and reflectivity after about 1000 hours at +300 C Reflectivity of CMO Coverglasses with RTV - S695 in between Transmissivity of CMO Coverglasses with RTV - S695 in between , sample (reference) 2 sample 2,5 T (%) sample after 300 C 2 sample after 300 C 2 sample after 330 C 2 sample after 300 C 2 sample after 300 C R (%) 2 1,5 1 sample WF1 (reference) 2 sample after 300 C(WF2) sample after 300 C(WF2) 2 sample after 300 C(WF2) 20 0,5 2 sample after 300 C(WF2) nm nm

12 SCA temperature coefficients test conditions: T= 25 C, 150 C and 200 C BOL & EOL results: Demonstration of TJ GaAs/Ge solar cell suitability for HIT missions BOL Efficiency vs temperature EOL Efficiency vs temperature 30,00 25,00 Efficiency [%] 25,00 20,00 15,00 10,00 y = -0,054x + 24,344 y = -0,0346x + 16,742 y = -0,0714x + 28,955 CESI SJ CESI TJ RWE TJ Lineare (RWE TJ) Lineare (CESI TJ) Lineare (CESI SJ) Efficiency [%] 20,00 15,00 10,00 y = -0,0456x + 19,461 y = -0,0273x + 11,761 y = -0,055x + 22,295 CESI SJ CESI TJ RWE TJ Lineare (RWE TJ) Lineare (CESI TJ) Lineare (CESI SJ) 5,00 5,00 0, Temperature [ C] 0, Temperature [ C]

13 Extrapolation of temperature coefficient results up to +300 C BOL Efficiency vs Temperature EOL Efficiency vs Temperature 35,00 30,00 30,00 25,00 Efficiency [%] 25,00 20,00 15,00 10,00 CESI SJ CESI TJ RWE TJ Efficiency [%] 20,00 15,00 10,00 CESI SJ CESI TJ RWE TJ 5,00 5,00 0, , Temperature [ C] Temperature [ C]

14 SCA Solar Aspect Angle Test test conditions: SAA = 0, 60, 70 and 85 T= 25 C, 150 C and 200 C BOL & EOL results: EOL 25 C Isc vs SAA BOL 25 C Isc vs SAA 1,200 1,200 1,000 1,000 Normalised Current [#] 0,800 0,600 0,400 CESI SJ CESI TJ RWE TJ Cosine Law Modified Cosine Law Normalised Current [#] 0,800 0,600 0,400 CESI SJ CESI TJ RWE TJ Cosine Law Modified Cosine Law 0,200 0,200 0, , SAA [ ] SAA [ ]

15 Concentration test conditions: SC= from 1 to 10,7 T= from 25 C to 200 C results: TJ cells are suitable for medium concentration applications with a dedicated grid design BOL Efficiency vs Concentration BOL Efficiency vs Concentration 1,200 1,16 1,150 1,14 1,100 1,12 Normalised efficiency [#] 1,050 1,000 0,950 RWE TJ 25 C RWE TJ 80 C RWE TJ 100 C RWE TJ 200 C Standard grid Spectrolab QR Normalised Efficiency [#] 1,10 1,08 1,06 1,04 CESI SJ 25 C CESI SJ 100 C CESI SJ 200 C CESI TJ 25 C 0,900 1,02 0,850 1,00 0, Concentration [SC] 0, Concentration [SC]

16 Coupons photos: Al orbiter Al orbiter Carbon cruise C-C orbiter

Thermal Gradient test @ coupon level (DLR test facility) Test conditions Simulation of mission environment through solar irradiation of samples in a vacuum

inclinations: 0,45,70 Irradiance Coupon 5, 0 TS_14 [ C] 300 12 TS_15 [ C] Temperature [ C] 250 200 150 100 50 0-50 -100 10 8 6 4 Irradiance [kw/m²] TS_16 [ C]

17 Thermal Gradient coupon level (DLR test facility) Test conditions Simulation of mission environment through solar irradiation of samples in a vacuum chamber with increasing temperatures and different angles Backside temperature: below -100 C - Front side temperature:from 50 C to 300 C 3 different inclinations: 0,45,70 Irradiance Coupon 5, 0 TS_14 [ C] TS_15 [ C] Temperature [ C] Irradiance [kw/m²] TS_16 [ C] TS_24 [ C] TS_25 [ C] TS_26 [ C] TS_33 [ C] TS_34 [ C] TS_43 [ C] Time [h:m] 2 0 TS_44 [ C] TSS av. [ C] Irradiance [KW/m²]

18 Cells Adhesives Cover glasses Substrates interconnector Wiring TJ InGaP/GaAs/Ge with integrated by pass diode (CESI / RWE Solar) CVG adh. RTV-S695 CVG adh. DC Aluminium plate (thickness: 2.0 mm) with insulating layer (0.05 mm thick Upilex-S / Kapton). Silver plated Invar 0.03 mm (Parallel Gap Resistance Welding) Candidates SJ GaAs/Ge with integrated by pass diode (CESI) Lay down adh. RTV-S691 Lay down adh. RTV 566 CMO (Thales Space Technology) C-C facesheet, insulating layer (0.05 mm thick Upilex-S / Kapton) with C- C honeycomb core (about 30.0 mm thick) Silver 0.02 mm (Parallel Gap Resistance Welding) Gore type SCC 3901/019 (*) locally shielded with protection tape (Dunmore) Lay down adh. CV 2568 Candidates Miscellaneous Adhesives CV (Nusil Technology) RTV-S691 (Wacker Chemie) RTV 566 (GE Silicones) Bleed Resistor RWR (Dale) Crimped Blocking and external bypass diodes Planar GaAs diode (CESI) Crimped Thermistor Insulation plates Feed thru wiring protection inserts 118 MF (manufactured by Rosemount) Crimped Kapton (Du Pont) Teflon inserts (Du Pont)

Estimate of achieved TRL TRL 9 : Actual system flight proven through successful mission operations TRL 8 : Actual system completed and flight qualified through test and demonstration (ground or

19 Estimate of achieved TRL TRL 9 : Actual system flight proven through successful mission operations TRL 8 : Actual system completed and flight qualified through test and demonstration (ground or space) TRL 7 : System prototype demonstration in a space environment TRL 6 : System/subsystem model or prototype demonstration in a relevant environment (ground or space) TRL 5 : Component and/or elegant breadboard validation in relevant environment TRL 4 : Component and/or breadboard validation in laboratory environment TRL 3 : Analytical and experimental critical function and/or characteristic proof-of concept TRL 2 : Technology concept and/or application formulated TRL 1 : Basic principles observed and reported

20 Deliverables Solar cell assemblies dedicated to environmental and destructive testing Adhesive samples dedicate to characterization at HT and destructive testing PVA coupons dedicated to thermal gradient / shock test and rapid cycling Substrate materials were Al and C-C

21 Critical technology requirements not met To test all the +300 C was impossible in combination with illumination and electrical biasing Extrapolation by analysis and superposition of effects for deriving the 300 C data Combination of HT and sun concentration was not possible at component level Performed only at coupon level but Set up interference to be solved To find TC test facilities 100% compliant to the HIT test levels Reduction of the temperature extreme leaving unchanged the maximum delta T Implicit requirement to achieve TRL 4

22 Critical technology items not covered Component level UV on SCA Test rate, acceleration factor for qual purposes, lifetime prediction Mercury environment induced degradation Protons and non ionising particles in general Component ageing Solar Cell Electrical biasing Illumination and HT effects, acceleration factors for qual purposes, lifetime behaviour Diode (by-pass and blocking) Technology definition Acceleration factors for qual purposes, lifetime behaviour Coupon level Shunt Diode Integration External diode solution not explored Development of OSR s suitable for high temperature applications

23 Lessons learnt Bare Solar Cells and SCA s Existing TJ solar cell structure may be adequate for the HIT environment but the influence of electrical biasing (with or without illumination) and high temperature should be further investigated Testing of bare cell is extremely difficult as far as the set up is concerned especially at high temperature Whenever possible comparison of bare cell and SCA test results is extremely meaningful (understanding of degradation mechanisms, if any) Integral diode behaviour at high temperature is doubtful and so external solutions should be considered as mandatory alternatives

24 Photovoltaic Assembly Lessons learnt Standard integration processes and methods could be compatible with the HIT environment There are few EEE parts to be further developed in terms of technology, housing and integration process Si-C diode, flat case Thermistor bonding process Reflective tape selection and bonding process Test set up correct tuning Dry run on representative hardware is mandatory Interference with the test environment should be avoided EMC issues Good control of imposed test levels

25 Planning to flight model past High Insolation and Temperature TDA future BepiColombo TDA 18 months Characterization of critical items: Diodes (shunt plus blocking) Solar cells OSR s Other PVA components BepiColombo Formal PVA Qualification 20 months Flight

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