Si Quantum Dots for Solar Cell Applications

Transcription

1 IRCC Award Talk Si Quantum Dots for Solar Cell Applications 18th Aug Chetan S. Solanki Department of Energy Science and Engineering Indian Institute of Technology

2 Acknowledgements Dr. Ashish Panchal Dharmendra Rai Paresh Kale Dr. Pravin Narwankar, Applied Materials Department of Energy Science and Engineering Applied Materials Nano-electronics Project My family

3 Contents Solar PV scenario: World and India Why Si nanomaterials? - Solar PV potential, Cost and efficiency Si quantum dots for solar cells - All-Si multi-junction cells - Obtaining Si QDs: HWCVD and Porous Si - Results and Analysis Conclusion and future work 3

4 Generation of Power with Solar Cell A Solar cell is a device that convert light into electricity Metal contact It requires - Absorption of a photon - Separation of a electron-hole pair P-N Jn separation force Metal contact - Collection of the charges at electrodes Sun light as input energy No moving parts, long life, moderate efficiency Solar cell are being manufactured since 1954

5 Solar Energy Potential in India 300 days of clear sky average 2000 kwh/m2/year Our capita energy consumption is about 600 kwh/capita/year India s annual energy consumption is about 4 trillion kwh/year India s solar energy resource is 5000 trillion kwh/year Characteristics of Solar PV technologies are: - Clean, maintenance free - Modular - Distributed generation

6 Worldwide PV Module Production PV market is growing with over 35% rate since last decade Learning curve for PV is -18%

7 Solar PV in India Current cell and module manufacturing capacity is about 400 and 750 MWp respectively Jawaharlal Nehru National Solar Mission (JNNSM) has been announced to promote solar PV electricity generation in India SIPS scheme for promoting solar PV manufacturing in India Target is to install 20,000 MW of Solar power in India by 2022 stirred lot of activities in the country The Mission document mentions NCPRE at IIT Bombay: setting up of a National Centre for Photovoltaic Research & Education at IIT Bombay, drawing upon its Department of Energy Science & Engineering and its Centre of Excellence in Nanoelectronics 7

8 Solar PV Module Prices Si Shortage Cost of conventional power is ~1$/Watt Solar PV technology is expensive Source: Price fluctuation due to demand-supply balance, increase in production volume

9 Challenges to PV Technologies Technology Attributes Features High price per unit watt (high 1.5 to 4 $ / Watt, should be cost of material) about 1$/Watt Moderate efficiencies 14 to 16% for c-si, 6 to 9% for thin film, higher is better Availability of material Should be abundant Long term stability Minimum acceptable life is 25 years Long energy pay back period 2 to 3 years for c-si, < 1 year (high processing cost) for thin-film, should be low Long money pay back period Depends on region, 5 to 12 years

10 Efficiency and Cost I Im Isc Pm X r e w o P Vm Voc $ Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. Voc I sc FF Efficiency η = Pin 2 $ Production cost m Cost = = = Efficiency Watt Watt 2 m Raw material cost, cell and module processing Quality of material, technology understanding, cell size 10

11 Photovoltaic Generations: 1st, 2nd & 3rd US$0.10/W US$0.20/W US$0.50/W 100 Efficiency, % US$1.0/W III II US$3.50/W I Cost, US$/m Ref: M.A Green, Progress In Photovoltaics, 9 (2000) st generation: Si wafer based technologies 2nd generation: Thin-film technologies 3rd generation: Advanced nanostructure based concepts 11

12 Si is good but expensive How to minimize Si consumption? 12

13 Si is Expensive Solid Solid Liquid Metallurgic al grade Si (MGS) Melting Coal + Quarzite H2 Reaction Cholorosilanes Separation and purification Gas Pure SiHCl3 Deposit solid Si HCl Solid Grow single crystal EGS ingot Solid Liquid Si wafers 13 Purepoly poly-egs -EGS Pure Initial Gas

14 Approaches to Si PV cost reduction High cost due to large volume consumption of high purity material Si Concent rators Thin film C-Si Nanostructures of Si 14

15 Contents Why Si nanomaterials - Solar PV potential - Cost and efficiency Si nanomaterials for solar cells - All-Si multi-junction cells - Obtaining Si QDs: HWCVD and Porous Si - Results and Analysis Conclusion and future work 15

16 Concept of multi-junction cells How to obtain higher efficiency? 16

17 Single junction cells are inefficient Required work 200 J 100 J 50 J 100 J 17

18 Single junction cells are inefficient Electrons Photon Eff. Work done = Photon energy Eg Holes Photon efficiency is different for different energy Losses in energy conversion inefficient utilization of solar spectrum 23% 1-transmission, 4- contact losses 33% 2-thermalization, 5. recombination losses 3-junctionn loss 18 18

19 Strategies for higher efficiency Approach- I Band gap matching with spectrum (splitting spectrum over several materials/cells) tandem cells intermediate band cells (Adopting Solar spectrum for one host material) up- and downconversion 1.5 Multi-junction cell approach Sunlight intensity (kw/m2/µm) Approach- III: Reshaping the solar spectrum Approach- II: Reduced thermalization losses (adopting one host material for solar spectrum) hot electron cells Wavelength (µm)

20 All Si Multi-junction solar cells How to change band gap? 20

21 Quantum confinement and Band gap Quantum confinement occurs when the crystal size becomes less than the Bhor exciton radius (4.9 nm for Si) Effective band gap D Si D Si D Si SiO2, SiN, SiC could be used as possible dielectric matrix Band gap of the Si QD depends on size of the dots and quantum confinement parameter 21

22 Si-QDs for PV applications Wide band gap material 8m Te = 16 exp E d 2 h Si-QD Tunneling Probability Top cell Middle cell c-si cell k E g (ev ) = E g (bulk ) + 2 d Band gap variation 2 ev 1.5 ev 1.1 ev Multi-junction solar cell of Si with control over Si-QD size is possible Theoretical efficiency of triple junction cell is about 64% 22

23 Two approaches for obtaining Si QDs: - Porous Si Thin film deposition (HWCVD) Si QDs using porous Si Top down approach

24 Porosity, HF conc. & Current Density Porosity (p) as a function of HF conc. and current density Provides control over size of nanoparticles HF 10% H2 HF solution x=0 Porosity (%) 75 HF 12.5% 16.6% HF 55 HF 35 ft(t) HF 25% 35% 15 x Silicon 0 50 Current Density (ma/cm2) Larger porosity results in smaller Si particle size

25 Porous Silicon (PS) Thin Films Porous Silicon films between 5 to 15 micrometers are obtained Current J Density 2 (ma / cm2) Pore diameter can be from nano-meter to micro-meter range J1 Use of heavily doped P-type Si results in nano-porous Si t1 t2 Time (min or sec) Pores

26 TEM results Plan is: Dispersion of Si particle in suitable dielectric Spinning on substrate to deposit Si-QD layer 2 Hr Sonication. Particle size in range of 10nm. Control over particle size Distribution of particles 100 nm

27 Si Quantum dots using HWCVD Bottom up approach (Structural and optical characterization) 27

28 Hot Wire CVD silane ammonia hot filament If 5 cm substrate Principle of HWCVD SiH4 & NH3 cracked at hot filament Gas utilization: ~10% in PECVD ~80% in HWCVD Deposition rate as high as 2 nm/sec Wafer size 2 inch Substrate temperature Room temperature to 800oC Filament temperature Up to 2000oC Gases SiH4, NH3, H2, B2H6, N2 Chamber pressure Up to 10-7 mbar Distance between filament and 3-5 cm substrate

29 Variation in HWCVD parameters Deposition of Si-nS with variation in parameters to : Substrate temperature SiH4 & NH3 gas flow rate Deposition time to see the effects in Si-nS size With increasing above parameters, the Si-nS size expected to increase Depo. parameters SiNx deposition Si-nS deposition Basic Pressure ~ 10-6 mbar ~ 10-6 mbar Gas pressure ~ 10-2 mbar ~ 10-3 mbar Filament Temp. 1900oC 1900oC Substrate Temp. 250oC oC Gas Flow 1:20 sccm (SiH4:NH3) sccm (SiH4) Dep. Time 75 sec sec

30 Superlattice of SiNx/a-Si Using the optimized conditions 40 alternate layers of SiNx and a-si are deposited 1 deposition takes about 5 hr A novel deposition technique is developed where the flow of NH3 is interrupted for pre-determined time a-si SiNx a-si SiNx 40 layers a-si SiNx a-si SiNx Substrate Annealing 40 layers a-si a-si SiNx SiNx Substrate

31 Effect of annealing Temp.: 1st order Raman spectra Annealing temperature of sample (oc) a-si peak position (cm-1) Si-QD peak position (cm-1) Intensity ratio for 2nd order Raman spectra As-deposited disappeared making shoulder With increasing annealing temp. - a-si phase reduces - Asymmetric shoulder appears 0.88

32 2nd order Raman spectra With increasing annealing temp. the 2nd order Raman peak for Si increases, shows the growth of Si-QD Regain c-si shape & increased intensity Asymmetric shift & reduced intensity

33 PL intensity (a.u.) PL Analysis With PL presence of quantum dots can be established Room temp. PL performed with He-Cd laser 325 nm PL peak shifts from blue to red wavelength as the Si-QD size increases as annealing temp. increases Estimated band gap between 2.3 to 2.6 ev

34 TEM analysis a-si/sinx multilayer as-deposited at 250oC and annealed at 850oC for 30 min. 40 alternate layers are deposited Thickness of layers in 4 to 6 nm range Optimized deposition and annealing conditions were used SIMS analysis is done for Si/N variation in layers

35 TEM: as deposited multilayer No Si-QD formation as-deposited a-si/sinx ML at 250oC

36 Annealing of No Si precipitation in SiNx layer after annealing Si-QD with av. Inter-dot distance < 5 nm annealed a-si/sinx multilayer at 850oC Optimized temp. for Si-QD growth

37 Annealing of Inter-dot distance < 1 nm making continuous nc Si film annealed a-si/sinx multilayer at 900oC

38 Effect of deposition time of a-si layer PL HRTEM No ML A20AS A20AS A40AS 2.67 ev a-si ~3.0 nm At smaller deposition time, there is no continuity in a-si layers A40AS A60AS 2.44 ev a-si ~3.47 nm A60AS 38

39 Solar cell with Si-QD/SiNx ML Solar cell device structure with Si quantum dots FrontAl Al Front p-type a-si layer (30 nm) QD QD QD Whole device fabricated in HWCVD i-type Si-QD/SiNx QD QD SiNx layer n-si substrate Back Al 39

40 Cell with A60AS and A60AN ML Hydrogenation for 40 min at 420oC after Al contact evaporation 40 Si-QD/SiNx ML 40 Si-QD/SiNx ML V I oc (mv) 80 sc (pa) as-deposited on annealed gives no different devices I-V characteristics illuminated with illumination Best cell 40

41 Conclusion and Future Work Successful formation of less than 10 nm Si dots using PS Successful development of alternate 40 layers of a-si/sinx in single HWCVD Formation of Si-QD 3-5 nm with optimized annealing temp. 850 oc No Si-QD formed in as-deposited samples Charge transport increased due to light shining on the device Best cell with 340 mv of open circuit voltage Effect of band gap enhancement on open circuit voltage is yet to be demonstrated Current transport through quantum dot structure is under investigation

42 It isn't new energy that will make such a difference in the next millennium. The power to run... It's new technology that will bring proven ways of generating power.. Thank you for your 42