• 3 sets of diffusion/oxidation furnaces (up to 4 inch wafers processing)
• ALD (Thermal and plasma assisted, PICOSUN)
• RTP (AnnealSys, AS-One 150, France)
• Laser Scriber (Yuhan Sonic, China)

• I-V measurement system with Class AAA Sun Simulator (21x21 cm2, AM1 & AM1.5 solar spectrum) (Newport/Oriel)
• SR and I-V tester (CEP-25HS-50SR Japan) (5x5 cm2)
• Non-contact measurements (m-PCD, SPV, LBIC) (Semilab, WT 2000 P) ,
• Sinton’s Lifetime tester and Sun Voc measurement set up
• Four Point Probe (Quad Pro, Lucas USA)

Silicon Thin Film Photovoltaic (Si-TFPV) activity of CSIR-National Physical Laboratory is one of major laboratory of India for the development of silicon thin film photovoltaic research since many years.  Recently, the R&D activity of group took place in the frame of CSIR solar mission project called “TAPSUN” (Technology & Product for Solar energy Utilization through Networks) funded by MNRE, Govt. of India under “Jawaharlal Nehru National Solar Mission” program for the development of cost effective and efficient thin film silicon [(mainly micromorph silicon; two distinct microstructures of silicon: amorphous (a-Si:H) & microcrystalline (µc-Si:H)] based solar cells and module. 

Today’s main goal of photovoltaic (PV) industry is the reduction of production costs. Solutions to accomplish this requirement are the increase of the cell efficiency and the improvement of the throughputs. This is also true for silicon thin film PV. The deposition rate of the different layers of silicon thin film solar cells is a key parameter to improve the throughput. The Si-TFPV group at NPL in its recent research uses the very high frequency PECVD process for the high rate deposition of amorphous and micro/nano crystalline silicon material. In addition to increasing deposition rates, reducing non-uniformity and minimizing initial light induced degradation of a-Si:H & µc-Si:H thin film layers is also aimed, which will help in improving the efficiency and throughputs to reduce the cost of the production.  Efforts are being attempted to use advance process concepts such as up and down conversion, plasmonic, graded structures etc. for the improvement of efficiency in silicon thin film solar cells.  Below flow chart indicate our approach:

Fig. 1: Approach to achieve cost effective & efficient thin film silicon solar cells.

The group is also interested to make HIT solar cells.  In this direction our efforts are in progress.

• Single Chamber PECVD system

This system works at a very high frequency of 60 MHz and is also coupled with 2.45 GHz microwave frequency (Fig. 2). The system is indigenously designed and developed at CSIR-NPL.  The system is being used for deposition of amorphous and micro/nano crystalline silicon thin films and recently showed the capability to make the p-i-n solar cells. The cells are not having a fair efficiency due to cross contaminations in single chamber, however the improvements in the process is in progress and the same is being utilized to develop the expertise in making the high efficiency silicon solar cells.

Fig 2: Single chamber VHF (60 MHz) PECVD system designed and fabricated by CSIR-NPL.

The system consists of three process chambers for deposition of device quality p-type, intrinsic and n-type amorphous silicon based semiconductor materials and a load-lock chamber.  The system was procured by M/S GSI Inc, USA about 25 years ago and it was down for last many years. However, in the past the system has shown capability to fabricate efficient single junction amorphous silicon p-i-n structured solar cells & module (area: 10 x 10 cm2). Now the group has refurbished the system and given a fresh life after putting a lot of efforts (Fig. 3). The system is being used to deposit the doped and undoped a-Si:H and as well µc- Si:H films in order to fabricate efficient (>10%) tandem junction micromorph solar cells.

Fig 3: Multi-chamber PECVD system for deposition of  doped & undoped layers of amorphous and micro/nano-crystalline silicon on 10 x 10 cm2 area.
Sputtering and thermal evaporation systems for are available for deposition of metallic & other layers for solar cells (Fig. 4).

Fig. 4: Sputtering & Thermal evaporation systems.

• I-V & C-V and solar simulator characteristics set up

The lab is equipped with a commercially available semiconductor characterization system M/s Keithley 4200SCS and Precision LCR meter Agilent E4980A (Fig. 5, Left), which is being used with a Keithley switch matrix 708A for the current-voltage and capacitance-voltage characteristics for the a-Si:H and µc/nc- Si:H thin films and solar cells. Based on these characteristics, various parameters like doping profile, threshold voltage, resistance or conductance, metal-semiconductor work function, etc. are extracted for the deposited films and cells. Group has also an assembled/fabricated solar simulator for characterization of solar cell characteristics (Fig. 5, Right).  In addition, in-house temperature dependent dark & photoconductivity setup is also being used for the estimation of various optoelectronic properties of silicon thin films.

Fig 5: IV/CV & solar simulator characterization set up

• Photothermal Deflection Spectroscopy (PDS) set up

The PDS system (Fig. 6) is being re-established to characterize a-Si:H and µc/nc- Si:H thin films for estimation of Urbach energy & midgap density of states (DOS).  PDS is based on the absorption of the optically exciting beam by the liquid medium (Carbon tetrachloride, CCl4), which causes a corresponding change in the index of refraction of the optically heated region. The absorption also causes an index of refraction gradient in a thin layer adjacent to the sample film surface. This produces a deflection of the probe beam i.e. He-Ne laser (l=633 nm), which is measured by means of a quadrant photo sensitive detector (PSD) and a lock in amplifier. Further, this evaluated measured DOS and absorption coefficients against photon energy are used to determine the Urbach tail energy (Eu) of the deposited semiconducting film.  

Fig. 6: Photothermal deflection spectroscopy (PDS) measurement setup

• Thin Film Stress Measurement System

This system has been procured from M/s Frontier Semiconductor (FSM), USA.  (Fig. 7) The system has capability to measure the stress of thin film as a function of temperature from ambient to 5000C up to 200 mm substrate size.  The equipment is useful to evaluate the thermal properties and stability of thin film materials subjected to thermal cycles.  Information obtained from this characterization is useful in detection of problems such as film cracking, voiding and hillock formation, which could lead to critical reliability issues during the device process.

Fig 7: Stress Measurement System.

Amorphous & Microcrystalline silicon thin films usually deposited by variation of pressure, power, frequency, dilution (Ar, He, H2 etc.) silane flow rate etc. using PECVD technique.  The films deposited under such conditions are characterized by Raman spectroscopy, low angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM) and UV-visible (UV-Vis) spectroscopy etc. to see the effects of process parameters on deposition rate, crystalline volume fraction, hydrogen bonding and morphology of Si:H  thin films. Growth rate of amorphous & microcrystalline silicon films above 10 A0/sec is obtained at certain process conditions. A transition region from amorphous to micro/nanocrystalline structure has been found by the variation of silane flow, power, pressure & dilution. Different crystalline volume fraction (20 % to 60 %) and band gap (1.50 eV to 1.90 eV) were achieved by variation of silane flow rate (5-30 sccm). 
The structures of these films were found to be mixed-phase consisting of amorphous and micro/nano-crystalline as revealed from TEM (Fig. 8) and Raman spectra (Fig. 9).

Fig 8: HRTEM images of three typical silicon thin films deposited at various process conditions: (a) primarily a-Si:H film (b) mixed phase structures of a-Si:H and µc-Si:H having low crystalline fraction and (c) mixed phase structures of a-Si:H and µc-Si:H having high crystalline fraction. The diffraction pattern is shown in the inset.

Fig 9: Raman spectra of mixed phase structure (a-Si:H & µc-Si:H) of silicon thin films grown using VHF (60 MHz) PECVD process as a function of silane flow rate (Ar flow rate fixed).  Crystalline fraction (?) estimated using Raman spectra is also indicated.

The photo-degradation study of these films deposited at high rate using 60 MHz VHF PECVD process was performed. It was found that these films were quite stable compared to amorphous silicon.  It was observed that for a typical µc-Si:H films deposited under certain set of chosen process parameters were of good quality having photo-sensitivity (>103) and photo induced degradation (< 5%).  It was also observed that microcrystalline films having high crystallinity showed less photo degradation at low rate (Fig. 10). More investigations in this direction are in progress.

Fig. 10: Light induced degradation of µc-Si:H films deposited at (a) 23 A0/sec (Left) and (b) 12 A0/sec (Right).

Doping of these materials was performed and single junction microcrystalline silicon p-i-n solar cells were made. Now efforts are in progress to make device quality amorphous and microcrystalline silicon layers in mutli-chamber PECVD system for efficient solar cells.

The group also involved in simulation work for the improvement of the efficiency of p-i-n silicon solar cells.  The efficiency of hydrogenated amorphous silicon (a-Si:H) p-i-n solar cell strongly depend on p-layer band gap and its thickness.  i and n- layer band gap also play a key role in the conversion efficiency. Hence we optimized the p, i and n layer band gaps by computer aided one-dimensional AFORS-HET software. Such an optimized value of these band gaps would further helps to prepare efficient solar cells experimentally. In addition, we have used various types of diamond like carbon films as window layer to see its effect on the efficiency of the cells and compared the results with conventional silicon carbon alloy.  We are also trying to simulate the layers for Micromorph silicon tandem junction solar cells using ZnO as interlayer between amorphous & microcrystalline silicon solar cells.  In addition, simulation approach was also used for HIT solar cells to achieve ~ 27 % efficiency using the same AFORS-HET software. 
Plasma Diagnosis
The group is also involved in Plasma Diagnosis of species in different deposition conditions (feed gases, power, pressure operating frequencies, etc) by various diagnostic techniques like, plasma impedance analysis and optical emission. The Plasma diagnostic techniques help us for precise optimization of process parameters for deposition of amorphous & micro/nanocrystalline silicon material properties to desired specifications.

The group has capability to deposit variety of DLC films such as metal DLC, nano structured carbon, nanocomposite carbon etc. on variety of substrates.  Electrical, optical and mechanical properties of diamond-like carbon (DLC, a-C:H) and modified DLC (nitrogen incorporated DLC, a-C:N:H) thin films deposited using radio frequency-plasma enhanced chemical vapor deposition (PR-PECVD) have been explored.  a-C:N:H/Si and a-C:H/Si hetrojunction diodes (rectifying circuits), metal (Ti and Cu)/Si/a-C:H based multijunction devices etc. have also been explored. The possible application of DLC and modified DLC films as window layer for amorphous silicon (a-Si:H) based p-i-n solar cell has also been explored using theoretical simulation. 
In addition, efforts have also been made to prepare DLC and modified DLC based hard and super-hard (above 40 GPa) coatings . The super-hard and hard coatings realized by employing combination of various layers such as titanium/DLC bilayer and multilayer structure, nitrogenated DLC, oxygen modified DLC, copper/DLC bilayer structure and copper incorporated DLC films. The DLC deposition followed by oxygen plasma treatment was performed for the improvement of the properties of DLC coating deposited on large area (size ~15 cm x 13 cm) glass substrates.

Patents (Filed):

• Indian Patent Application No.: 2750 DEL 2007/IN, Dated: 28-12-2007. [“Process for the preparation of photo luminescent nanostructure silicon thin films” (0097NF2007)], International Patent Application No.: PCT/IN08/000371, Dated: 13-06-2008 (Pub. No.: WO/2009/084005, dated: 09-07-2009).
• Taiwan Patent Application No.: 097123735/TW, Dated: 25-01-2008. (TW200929337)
• US Patent Application No.: 12/810920/US, Dated: 28/06/2010 (Pub. No.:US2010/0285235A1, dated Nov. 11, 2010).
• Indian Patent Application No.: 0313 DEL 2009/IN, Dated: 18-02-2009. “Process to deposit diamond like carbon as protective coating on inner surface of a shaped object”, [0179NF2008], International Patent Application No.: PCT/IB2010/000133/WO, dated: 27-01-2010 (Pub. No.: WO/2010/095011, dated: 26-08-2010).
• US Patent Application No.: 13201210, dated 11-08-2011 (Pub. No.: 2012/0045592 A1, dated: 23-02-2012).
• European Patent Application No. 2010704976, dated: 18-08-2011 (Publication No. EP 2398933, dated 28-12-2011).
• Republic of Korea, Application No.: 1020117020190, date: 30-8-2011, Published on 14-12-2011.
• Japan Patent, Entry date: 17-08-2011, No.: 2011550662, Published on 14-12-2011.

Publications (in SCI Journals):

• “High pressure growth of nanocrystalline silicon films” by Sushil Kumar, Jhuma Gope, Aravind Kumar, A. Parashar, C.M.S.Rauthan, and P.N. Dixit, Journal of Nanosci. & Nanotechnol. Vol. 8, No. 8, pp 4211-4217 (2008).
• “High pressure condition of SiH4+Ar+H2 plasma for deposition of hydrogenated nanocrystalline silicon films” by A. Parashar, Sushil Kumar, P.N. Dixit, Jhuma Gope, C.M.S. Rauthan and S.A. Hashmi. Solar Energy Mater. & Solar Cells 92, 1199-1204 (2008).
• “Effect of power on the growth of nanocrystalline silicon films” by Sushil Kumar, P.N. Dixit, and C.M.S. Rauthan, J. Phys.: Condens. Matter. 20, 335215(2008).
• “Amorphous and nanocrystalline Silicon made by varying deposition pressure in PECVD process” by Jhuma Gope, Sushil Kumar, A. Parashar, P.N. Dixit, C.M.S. Rauthan, O.S. Panwar, D.N. Patel and S.C. Agarwal, J. Non-Crystalline Solids 335, 2228-2232 (2009).
• “Properties of nitrogen diluted hydrogenated amorphous carbon (n-type a-C:H)  films and their realization in n-type a-C:H/p-type crystalline silicon heterojunction diodes” by Sushil Kumar, Neeraj Dwivedi, C.M.S. Rauthan, and O.S. Panwar,  Vacuum 84, 882-889( 2010).
• “Influence of argon dilution on growth and properties of hydrogenated nanocrystalline silicon films” by A. Parashar , Sushil Kumar, Jhuma Gope , C.M.S. Rauthan , P.N. Dixit  and S.A. Hashmi, Solar Energy Mater. & Solar Cells  94, 892(2010).
• "RF power density dependence phase formation in hydrogenated silicon films" A. Parashar, Sushil Kumar, Jhuma Gope, C. M. S. Rauthan, S. A. Hashmi, P. N. Dixit, J. Non-Cryst. Solid 356, 1774-1778 (2010).
• “Nano indentation measurements on nitrogen incorporated diamond-like carbon coatings” by Neeraj Dwivedi, Sushil Kumar, CMS Rauthan and O.S. Panwar, Applied Physics A  102, 225-230 (2011).
• "Studies of nanostructured copper/hydrogenated amorphous carbon multilayer grown in low vacuum system" by Neeraj Dwivedi, Sushil Kumar, Ishpal, Saurabh Dayal, Govind, CMS Rauthan and O.S. Panwar, Journal of Alloys and Compounds 509, 1285-1293 (2011).
• “Effect of ambient gaseous environment on the properties of amorphous carbon thin films” by Ishpal, O.S. Panwar, Mahesh Kumar, Sushil Kumar, Materials Chemistry and Physics 125, 558-567 (2011).
• “Role of metallic Ni-Cr dots on the adhesion, electrical, optical and mechanical properties of diamond-like carbon thin films”’ by Neeraj  Dwivedi, Sushil Kumar, C.M.S. Rauthan, O.S. Panwar, Plasma Processes and Polymers 8 (2011).
• “Nanoindentation measurements on modified diamond-like carbon thin films " by Neeraj Dwivedi, Sushil Kumar and H.K. Mallik, Appl. Surf. Sci. 257, 9953-9959 (2011).
• "Influence of bonding environment on nano-mechanical properties of nitrogen containing hydrogenated amorphous carbon thin films" by Neeraj Dwivedi, Sushil Kumar, Hitendra K. Malik, C. M. S. Rauthan, O. S. Panwar, Materials Chemistry and Physics  130, 775-785 (2011).
• “Field emission, morphological and mechanical properties of variety of diamond-like carbon thin films by N. Dwivedi, Sushil Kumar, Hitendra K. Malik, Ravi K Tripati, and O. S. Panwar, Applied Physics A,  DOI 10.1007/s00339-011-6556-0 (2012).
• “Investigation of properties of Cu containing DLC films produced by PECVD process” by Neeraj Dwivedi, Sushil Kumar, H.K. Mallik, C. Sreekumar, Saurabh Dayal, CMS Rauthan and O.S. Panwar, Journal of Physics and Chemistry of Solids 73, 308-316 (2012).
• “Nanoindentation measurements on copper/diamond-like carbon bilayer films”, N. Dwivedi and Sushil Kumar, Current Applied Physics, 12, 247-253 (2012).
• “Studies of pure and nitrogen incorporated hydrogenated amorphous carbon films and their possible role in the development of efficient amorphous silicon solar cells”, Neeraj Dwivedi, Sushil Kumar and Hitendra Malik, J. Appl. Phys. 111, 014908 (2012).
• "Oxygen modified diamond-like carbon as a window layer for amorphous silicon solar cells" by Neeraj Dwivedi, Sushil Kumar, Sukhbir Singh and Hitendra Malik,   Solar Energy  86, 220-230 (2012).
•  “Effect of metallic interfacial layers on the properties of diamond-like carbon thin films”, by Neeraj Dwivedi, Sushil Kumar, C. Sreekumar, C.M.S. Rauthan, Saurabh Dayal, O.S. Panwar, Metals & Materials International 18, (2012).
• "Nanostructured Titanium/Diamond-like Carbon Multilayer Films: Deposition, Characterization and Applications" by Neeraj Dwivedi, Sushil Kumar and Hitendra Malik, Applied Materials & Interfaces Vol. 3, No. 11, pp 4268-4278 (2011).
•  “Investigation of radio frequency plasma for the growth of diamond like carbon films” by Ishpal, Sushil Kumar, Neeraj Dwivedi and C. M.S. Rauthan, Physics of Plasmas 19, 033515 (2012).
• "Growth of mixed- phase of amorphous and ultra nanocrystalline silicon thin films at low pressure regime by VHF PECVD process" by Jhuma Gope, Sushil Kumar, Sukhbir Singh, C.M.S. Rauthan and P.C. Srivastava, Silicon, Vol. 4, pp. 127-135 (2012).
• Band gap optimization of p-i-n layers of a-Si:H by computer aided simulation for development of efficient solar cell, Sukhbir Singh, Sushil Kumar and Neeraj Dwivedi, Solar Energy 86, 220-230 (2012).
•  “Role of ex-situ oxygen plasma treatments on the mechanical and optical properties of diamond-like carbon thin films” Neeraj Dwivedi, Sushil Kumar, Hitendra K. Malik, Materials Chemistry and Physics: Communication, Vol. 134, pp. 7-12 (2012).
• "Cost effective deposition system for nitrogen incorporated diamond-like carbon coatings" Sushil Kumar, Neeraj Dwivedi and M. K. Dalai, Plasma Processes and Polymers 9, 890-903 (2012).
• “Mechanical properties of nanocrystalline silicon thin films deposited by high frequency PECVD process”, Jhuma Gope, Sushil Kumar, A. Parashar, Saurabh Dayal, C.M.S. Rauthan and P.C. Srivastava, IRSN Nanomaterials, Vol. 2012, Article ID 429348 doi:10.5402/2012/429348 (2012).
• "Superhard Behaviour, Low Residual Stress and Unique Structure in Diamond-Like Carbon Films by Simple Bilayer Approach: Deposition and Characterizations" by Neeraj Dwivedi, Sushil Kumar, and Hitendra Malik, J. Appl. Phys 102, 023518 (2012).
• Structural and Electronic Characterization of Nanocrystalline Diamond-Like Carbon Thin Films" Author(s): Neeraj Dwivedi, Sushil Kumar, R. K Tripathi,. J David Carey, Hitendra K Malik, M. K Dalai., Applied Materials & Interfaces (dx.doi.org/10.1021/am301252e, 2012).
• "Photoconductivity and Characterization of Nitrogen Incorporated Hydrogenated Amorphous Carbon Thin Films", by Neeraj Dwivedi, Sushil Kumar, J. David Carey, Hitendra Malik, and Govind, J. Appl. Phys. 112, 113706 (2012).
• Strange hard characteristic of hydrogenated diamond like carbon thin films deposited by PECVD process”, by Neeraj Dwivedi, Sushil Kumar and Hitendra Malik, Appl. Phys. Lett. 102 (2013).
• "Simulation approach for optimization of device structure and thickness of HIT solar cells to achieve ~ 27 % efficiency" by Neeraj Dwivedi, Sushil Kumar, Atul Bisht, Kamlesh Patel and S. Sudhakar, Solar Energy 88, 31 (2013).
• “Investigation of structural and nano-mechanical properties of nanostructured diamond-like carbon thin films” by Saurabh Dayal, Sushil Kumar, Neeraj Dwivedi, Sreekumar Chockalingam, C.M.S Rauthan, and O.S. Panwar, Metals and Materials International (In Press, 2013).
• “Role of base pressure on the structural and nano-mechanical properties of metal/diamond-like carbon bilayers” Neeraj Dwivedi, Sushil Kumar, and Hitendra Malik, Applied Surface Science (In Press, 2013).
• Influence of Silver Incorporation on the Structural and Electrical Properties of Diamond-Like Carbon Thin Films by Neeraj Dwivedia, Sushil Kumara,, J. David Carey, R. K. Tripathia, Hitendra K. Malikb and M. K. Dalai, ACS Applied Materials & Interfaces (In Press 2013).

Polymer Solar Cell Developed CSIR-NPL under CSIR TAPSAN Program, PCE ? 6%

• Glove box integrated with thermal evaporator for device fabrication
• Laser Scrubbing system
• Thermal evaporator system
• Solar simulator
• UV-Vis-NIR spectrometer
• FT-IR spectrometer
• Photoluminescence set-up
• Spin coater
• Electrochemical work station
• Impedance spectrometer
• Atomic force microscope
• Ellipsometry

Glove box integrated with thermal evaporator

Laser scrubbing system

Ellipsometry

Photoluminescence set-up

Atomic force microscope