Marina Foti IMS R&D. Convegno su Tecnologie, tecniche impiantistiche e mercato del fotovoltaico. 15 Ottobre2012 Mondello(PA)

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1 R&D sul fotovoltaico in STM Marina Foti IMS R&D STMicroelctronics Convegno su Tecnologie, tecniche impiantistiche e mercato del fotovoltaico 15 Ottobre2012 Mondello(PA)

2 Outline Thin film module technology Amorphous silicon (a-si:h) and microcrystalline (µc-si) Tandem and multiple junction solar cells Enhancement of light absorption in thin film Si Development of TCO front and back electrodes Next steps on light trapping Thin film silicon outlook TF PV flexible application for smart systems

3 A New PV Joint Venture: 3SUN ENEL GREEN POWER, ENEL Group Company, dedicated to the development and management of activities related to energy production from renewable sources at an international level, which operates in Europe and the American Continent. It is a leading Company in this sector at global level. SHARP CORPORATION, a Japanese Company, which operates at global level in the manufacturing and distribution of consumerproducts(lcdtv, LED TV,ecc).A leading company at global level in the photovoltaic sector (Solar Cells, and Electronic Devices). STMICROELECTRONICS, is one of the largest manufacturers of semiconductors in the world with customers in all electronics segments. The Corporate headquarter is in Geneva, advanced research and development centers in 10 countries, 14 main manufacturing sites and sales offices all around the world.

4 3SUN Thin Film Multi-Junction Modules Fab The biggest PV Italian fabdestined to compete with the most important players of the sector Thin film multi-junctions modules are manufactured in the innovative plant M6 built in Catania Large area modules: 1m 1.4 m Numbers: m 2 surface area m 2 Fab area 300 employees 160 MW/y (2011) 240MW/y, possible extension

5 Why TF Solar Cells? Solar cell Si raw material Efficiency Peak power Peak power c-si g/m 2 16% 160W/m W/g TF-Si 5 g/m 2 10% 100W/m 2 20W/g Amorphous / glass Large area multi-junction / glass Amorphous or tandem / flex

6 Technology options: thin film vs Si wafers Processing of wafers BULK Si SOLAR CELLS Series connection of individual solar cells Mature technology but needs a lotofsi THIN FILMS Monolithic integration (series connection by lasering) CVD on verylargeareas Potential for ultra low costs

7 Large area modules on glass Altomonte(CS -Italy): 8,2MW. 11 Millions of kwh. It can satisfy the needs of families

8 Other than Ground PV Plants Various Applications Innovative Future Solutions Parking area Roof Easy installation and no particular maintenance or cleaning required. No specific accurate angle to the sun. Perfect integration with the environment. Residential, Commercial and Industrial Roof Installation following the roof profile and good performance at any slope of the roof. Nice appearance integrated to the building design. Integration on Building Design Building front designed with Glass/Glass Frameless Thin-Film PV Modules Car, Truck and Trailer PV Roof Deserts and hot climate Countries Supplying high performance even at 50~60 C thanks to the low temperature coefficient (-0,24%/ C). Good performance even when the panels would be partially covered by dust and sand thanks to the feature to produce energy with diffuse light. Stand-alone Applications powered by PV panels E.g. Water sweetening kit

9 Thin film PV on flexible substrate

10 Substrate and superstrate configurations Thin film deposition at low temperatures on large area substrates Opaque sealing Back electrode (Ag, Al, white pvb) BC TCO a-si:h, uc-si:h or multij Front TCO glass Transparent sealing Front TCO a-si:h, uc-si:h or multij BC TCO Back electrode Metal, plastic.. Superstrate Substrate

11 Glass with TCO Edge seaming Laser scribe P1 TCO Cleaning PECVD deposition Laser Scribe cell P2 PVD deposition Back contact Laser scribe back contact P3 11 Laser scribe Isolation P4 Laser edge deletion In line solar simulator (IV) Bus bars and wires connection Lamination with PVB and back glass J-box connection 2 nd in line solar simulator (IV) Packaging Thin Film Module process flow The modules are fabricated monolithically on a glass substrate during front end process The back end is dedicated to add electrical connection, protection layers, frame and junction box Typical process flow tandem modules 1 x 1.4 m 2

12 Thin film on glass: FEOL process Glass with TCO Layer Cleaning Laser Scribe P1 PECVD Deposition a-si:h -pin SOIR µc-si:h- pin Laser Scribe P2 TCO Deposition Laser Scribe Back Contact P3 Cleaning

13 Scheme of thin film module load back contact - + cell TCO glass

14 Glass Size Matters for Thin Film High automation PECVD deposition large area and high throughput is needed to achieve low cost/wp TCO deposition

15 TF Silicon Costs breakdown 14% 6% 15% 6% 5% TCO Gas/Chem Target 46% Back glass Encapsulant Terminal Box Silver Paste/ Bus Bar/ Packing/Other Lead Wire / MultiFrame J 5% 3%

16 Amorphous silicon Amorphous Si: a-si:h layers were first deposited by R. Chittick (1969) experimenting with SiH4 in a plasma reactor. First systematic study by Spear et al Phil Magaz, 33, 935 (1976) Tetrahedrally bonded c-si structure Amorphous Si: absence of Long range order

17 Distribution of density of allowed energy states for electrons due to the disorder direct optical transitions are not forbidden in amorphous Si 17 Eg= ~1.8 ev better light absorption than c-si

18 Deposited Amorphous by plasma-enhanced Si for CVD thin of SiH4 film at 150- PV 300 C. Low gas utilization (10-30%). Heavily hydrogenated 1-10 at.% H. PN (PIN) junctions formed through boron or phosphorous containing gases. Total thicknesses in some cases below 1 µm (100 times thinner than c-si). Multiple junction devices with two or three junctions grown one upon the other and current matched. The BIG three challenges Improve efficiency from 6-8% up to 12-15%; Minimize or eliminate the self-limited degradation Increase deposition rate

19 Staebler-Wronski Effect Exposure to light induced degradation, which stabilizes with time Typically after 1000h of continuous light soaking at 1 Sun AM1.5G New dangling bonds (from 1e15 to 1E17 cm-3) are created under light exposure Degradation is recovered after annealing at T<150C L Typically % of degradation For a-si:h of nm Limitation on the thickness

20 Amorphous a-si:h: p-i-n Drift charge transport p-i-n junction p i n Carriers are photogenerated in the intrinsic region and collected by drift 20

21 Amorphous and Microcrystalline silicon Two materials with the same process PECVD 21 Columnar microstructure a-si:h Eg=1.8eV µc-si:h Eg=1.1 ev

22 Enhanced absorption: double junction/tandem Light intens sity (kw/m 2 µm) spectrum splitting. Wavelength (nm) Amorphous Eg=1.8eV «High» absorption in the green-blue Microcrystalline Eg=1.1eV «High» absorption in the red-near I.R. Micromorph cell efficiency 11-14% Micromorph module efficiency %

23 Tandem configuration: Top a-si:h, Bottom µc-si:h TCO EXTERNAL QUANTUM EFFICIENCY a-si:h µc-si:h Wavelength (nm) Multiple junction devices with two junctions grown one upon the other and current matched spectrum splitting enables higher absorption and higher efficiency 23

24 From Single to Multiple junctions Single Junction asi:h cellwithenhancedlight trapping TCO and Texturing Double Junction / Tandem cell highest efficiency: combination of absorber materials having band gap 1.8 evand 1.1 evfor the top and bottom cell.. Triple junction asige:h middle absorber more than 12% on large areas glass textured TCO a-si:h top absorber a-sige:h middle absorber Best stabilized efficiencies above 12% Higherefficiencies(from12 to20%) are possible with additional junctions Butso far : Reduced throughput:~ 30% lower for triple Junction µc-si:h bottom absorber ZnO Ag Costs ~ 20% higher than tandem Despite the lower efficiency of tandem technology higher throughput in MW/years

25 Issues limiting a-si:h and µc-si:h efficiencies 25 a-si:h : Voc too low 0.9V instead of 1.4V (bandtails contacts) µc-si:h: Low Jsc. Improve absorption, light trapping

26 Light trapping increases the absorption because increases the optical thickness Light can be captured in the desired parts of a solar cell (absorber layers) and can be confined in it. 300nm a-si:h The cell current can be enhanced by increasing the effective optical path in the absorber layer(a-si:h or µcsi:h) M. Zeman, J ELECTRICAL ENG, VOL. 61, NO. 5, 2010,

27 Light trapping Asahi VU (SnO 2 :F) Asahi W light glass TCO ~700nm p-i-n a-si:h ~250nm p-i-n uc-si:h ZnO:B-MOCVD ~1.6µmµ W text ZnO TCO ~50nm Back reflector Improvement: about 50 % reduction of the deposition time (today limiting process step).

28 Figures of merit for TCO 28 A good TCO must have a high figure of merit (conductivity/absorbtion coefficient) Rs is the sheet resistivity R is the reflectance R.G. Gordon MRS Bulletin 2000

29 BAND GAP Engineering: Impact of work function on solar cell conductivity TCO / asi:h(n doped) TCO / asi:h(p doped) barrier barrier barrier electrons rich area TCO objective: WF < 4,3eV Hole rich area barrier TCO objective: WF > 5eV

30 Index grading at the TCO/p interface (whole spectral range) Light trapping and index Grading at the back reflector (red spectral range) Haze impact H=T diffused /T total λ/n Apart from the transmittance and the low sheet resistance (~7-10 Ω/ ) a TCO must have: Cell reflectivity (%) low haze high haze Wavelength (nm) reduced reflection due to refractive index grading (1<n<3); this effect applies to the whole wavelength range of the spectral response light scattering and subsequent trapping in the silicon absorber; this applies more to the weakly absorbed light that penetrates up to the back contact

31 Impact of texturing and TCO material 100 TCO a-si:h µc-si:h BR low haze TCO high haze TCO Haze: ratio between diffusely scattered and total intensity T H = T diffused total 2π = 1 exp n1 cosϑ1 n2 cosϑ2 λ θ 1 direction incidentbeam θ 2 direction scatteringbeam exponent: 2<β<3 Texturing (increases optical path) can improve the currents generated in the top and bottom cells β Transmittance (%) EXTERNAL QUANTUM EFFICIENCY Total T Diffused T ZnO:B SnO 2 :F a-si:h Wavelength (nm) UV-type SnO2:F ANX10 OE_B_TD ZnO:B OE_C_TD ANX Wavelength (nm) µc-si:h ZnO -H=20% ZnO -H=20% ZnO -H=20% SnO2-H=10% SnO2-H=10% SnO2-H=10%

32 Impact of TCO on the cell performances 32 Current Densisty (m ma/cm2) (1) η=12.5% (2) η=11.5% Voltage (V) (1) ZnO:B 20% Haze higher Jsc (2) SnO2:F 10% Haze Φ ZnO < Φ SnO2 Difference of Workfunctions differences in the Voc

33 Increase the efficiency: intermediate reflecting layer Currents (Jsc) matching (top cell bottom cell IRL refractive index between 1 and 3 silicon oxide based intermediate reflector layer (SOIR) Filters low energy photons Reflects high energy photons SOIR is obtained in the same PECVD chamber used for a-si and µc-si A. Feltrin et al, MRS 2009

34 Texturing of ZnO on front and on backside Use of LPCVD or MOCVD ZnO controlling texturing on front and backside Increased light path in a-si and µc-si Reduced absorber thickness of~ 50% Glass Front TCO a-si:h Increased efficiency µc-si:h EQE Back TCO White pvb Wavelength (nm)

35 Thin cell /Effect of white sheet reflection on microcrystalline White polymer can be used instead of expensive Ag for reflection Textured thick TCO as back contact 1.00E+00 TOP (ZnO no white paper) 9.00E-01 BOTTOM (ZnO no white paper) 8.00E-01 SUM (ZnO no white paper) 7.00E-01 TOP(ZnO with white paper) Glass a-si:h EQE (%) 6.00E E E-01 BOTTOM(ZnO with white paper) SUM(ZnO with white paper) µc-si:h 3.00E E-01 White pvb Back TCO 1.00E E wavelength (nm) White PVB contributes significantly to the reflection especially in the bottom cell (µc-si:h)

36 3D Structures 3D TCO 3D architectures by using TCO 3D patterning Higher efficiency 3D structures obtained by using TCO 3D templates To increase light trapping and orthogonalizelight absorption and photocarriercollection W. Soppe eta al 26 th PVSEC 2011 Planar waveguides with disordered pores to enhance the absorption of the light (Anderson localization effects ) Riboli et al Optics Letter, 36, 127, 2011

37 Plasmonics Plasmonic enhancement effects by metal layers and nanoparticles Scattering Near-field enhancement Waveguide modes V.E. Ferry, et.al., APL (2009) 37

38 Silicon wires and quantum dots Developing cell architectures with silicon wires in order to orthogonalizelight absorption and photocarriercollection 1/α n-type ħω 1/α p-type ~L Traditional planar, single junction solar cell Idealized radial junction wire solar cell ~L B.M. Kayes, et.al., J. APPL PHYS (2005). Quantum dot based heterojunction solar cells c:si synthesis host precursor + Colloidal TCO nanocrystals precursor Liquid injection head Applied on Si Thin Film for efficiency > 20% Substrate chuck in atmospheric pressure Substrate spin chuck 38

39 Si PV Thin film outlook Thin film technology addresses low cost/wp by using large area high throughput (e.g. PECVD with high dep, rate low T)equipments with very high level of automation To achieve the target material costs, especially the front glass with TCO, need to be low. Improving light trapping is fundamental to increase the efficiency (12% on single junction) or reduce costs because lower Si absorber thickness is necessary Multiple-junctions solar cells are necessary to increase the efficiency but to date they are still characterized by low throughput and higher costs Silicon TF solar cells are expected to achieve a much higher conversion efficiency (up to ~20%) than other TF technologies (CdTe, CIGS,..), which today are strong rival to Si, by exploiting new materials and by applying multi-junction structures. Bring together the experience and know-how of researchers in applied physics to speed up the development of materials and devices.

40 Electronic device integrated energy harvesting with flexible thin film PV

41 Harvesting in Smart Systems An example in Wireless Sensor Nodes for Automation Integrating Harvesting in Smart Systems Energy Harvesting Device (PV, Piezo, etc) Energy Conversion Battery Storage Sensors Ultra Low Power Microcontroller Low Power RF Transceiver Autonomous Wireless Sensor Node Enabling wireless sensors for energy autonomy

42 Harvesting system with flexible foils Platform features PV module collects energy from indoor light (300lux minimum) Harvested energy stored in an microbattery (ST-TF) Managing of the system energy Sensing ambient temperature Powering an STM8L15 microprocessor Supplying an RF transceiver Processed data transmitted to a BST

43 Flexible PV Modules Modules of 30 cm 2 Thin film solar cells are monolithically series connected (13 cells) 250µW TCO pin a-si:h Back contact Polymeric substrate 4 patents of ST on the subject 43

44 13 cells 300 lux F Lux: 8% -9% 3.00E+03 Spectral density (a.u.) 2.50E E E E E E+00 AM1.5G F wavelenght (nm) Current (A) 5.0E E E E E E E E-04 Power (W W) Fluorescence lamp spectrum 300 lux~ 1W/m 2 1.0E-05 Jsc (µa) Voc (V) 8.42 Pmax (µw) eff (%) E E E Voltage (V)

45 Implementation: contact layer Sequence SnO 2 :F/p-type a- Si:H/Mo : to study the interface between the contact layers and the p-type a-si:h Plays an important role on the PV cell performances By C-V and I-V data coupled with modeling we find that the Mo provides a better Schottky contact with p-type a-si:h compared to SnO 2 :F. (a) Strong synergy with CNR-IMM (S. Lombardo) (b) M. Fotiet al, ECS 2011 G. Cannella et al. JAP 2011

46 Flex PV: Benchmark with indoor light Efficiency comparison at indoor ST Flex Module 30cm 2, high robustness, less leakage max power Voc Isc PV module is very robust Mechanical stress test: Module is bent with very small radius, r = 2cm No significant changes in the electrical characteristics (Voc, Isc, Max Power) v a riatio n (% ) cm 1.2cm 1.5cm 1.9cm 2.9cm 3.9cm Number of bending 46

47 Roll to roll Roll to roll technology polyethylene-naphtalate (PEN) Si TF development at low deposition T from 150C to RT using new deposition techniques IC PECVD

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