Silicon Solar Cells

1. Fabrication of a novel solar cell with 3-D junction

Students: Som Mondal

Faculty member: Chetan Singh Solanki

Introduction: In this work, a new structure for crystalline silicon solar cell has been proposed. The basicmotivation behind this structure of the cell is to reduce the optical losses and to increase charge collectionprobability even in low quality material. A novel cell architecture with through the wafer 3D junctions wasproposed and simulations of this structure was reported in the previous year. The progress made towards therealization of this cell structure in the year under review is reported below. Fig. 1.1 shows the proposed cellstructure.

Fig. 1.1: Schematic of the novel solar cell with 3-D junction

Simulation studies at earlier stage showed that for low quality substrate, the proposed structure gives a highrelative improvement in performance over conventional structure. This can be attributed to zero shadow loss andhigh carrier separation probability in the bulk (Fig. 1.2). Laser assisted diffusion (LAD) was identified as thekey challenge for fabrication of such doped vertical channel. To address this issue, planar junctions werefabricated by spin-on-dopant deposition followed by laser anneal. Fig. 1.3 shows the illuminated I-V responsesat 1000 W/m2 radiation and confirm the formation of a junction by LAD. The laser scan speed was varied andvery low sheet resistance (Rsh) values were obtained (Fig. 1.4). The Rsh values were measured using twodifferent four probe set-ups. The junction depth measurements (Fig. 1.5) were done with sheet resistanceprofiling using HNA etching. Maximum value of junction depth obtained was 5.7 µm.

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Ni/Cu contact Ag contact

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Fig. 1.2: Current paths in proposed cell structure.Simulated using Sentaurus TCAD tools.

Fig. 1.3: I-V curves of cells prepared with Laser assisted diffusion processes with two different kinds of contacts under one sun illumination.

Fig. 1.4: Sheet resistance of emitter doped withphosphorous by Laser assisted doping process.

Fig. 1.5: Junction depth measured by spreadingresistance probe.

2. Silicon Concentrator Cells

Students: Mehul Raval and Vishnu Kant Bajpai.

Faculty members: Chetan Singh Solanki.

Introduction: Concentrator solar PV systems offer possibility of cost reduction but offer challenges of suntracking, heat removal from solar cells, and special design of solar cells. In this work fabrication of Si solar cellsfor concentrator applications is proposed. One of the main features of concentrator solar cells is to have lowseries resistance. The low series resistance can be obtained by lower metal-semiconductor contact resistance andlower metal resistance itself. For this Ni/Cu plated contact for solar cell application is being developed. Theprogress made during the year under review is summarized below.

Two aspects of Ni/Cu contacts by electro(less) plating were investigated. Contact areas should be etched in theantireflective layer. A laser based process was investigated for this purpose. During the electroplating process,metal can be unintentionally plated on the ARC layer. Partially processed solar cells obtained from industrywere used for these investigations.

Due to reduced metal finger obtained by laser patterning, the shading loss is reduced by ~50%. As observed inFig. 1.6, compared to screen printed solar cell there is improvement in short circuit current density (J sc). Thereduction in open circuit voltage Voc and fill factor are due to laser induced junction defects, which would beaddressed in subsequent work.

Fig. 1.6: Comparison of the I-V curve for screen-printed cell(green) and Ni-Cu metallized cell using laser patterning(blue).

For electroless plating of Ni, the samples are treated in an ativation solution of PdCl 2. Prominentbackground Ni deposition was observed for bath temperature of 90°C, with a higher concentration forsamples treated with 1%HF before exposure to activation solution as indicated in Fig. 1.7. Thedeposition can be due to weak etching of PECVD SiNx which promotes Pd nucleation on the surface.The increase in reflectance to around 7.5% in broadband region for cells exposed to bath at 90°C canbe attributed to the Ni deposition as was observed by EDAX analysis. The process is being optimizedto reduce the background plating.

Fig. 1.7: Background plating for Ni bath at T = 90°C exposed to activation solution with dip in 1% HF andplating time of 30seconds.

3. Novel technologies for contact formation using temperature sensitive paste

Students: Akella Sastry, Mehul RavelFaculty members: Chetan Singh Solanki

Introduction: Commercially available solar cells are patterned using screen-printing technique to makecontacts at front side with silver containing metal paste. Screen printed solar cells have average finger widths of100 - 125 µm which result in significant losses due to shading. We propose the use of a temperature sensitivepaste for the realization metal contacts with narrow width. The progress made during the year under review issummarized below. In this work, the temperature sensitive paste is applied on a wire and then transferred to the wafer surface.Subsequently the wafer is heated to result in etching of the ARC and formation of the contact. Most of the cellsare patterned using a 50μm Tungsten wire and finger openings obtained are varied from 85-125 μm as indicatedin Fig. 1.8.

Fig. 1.8: Contacts made using temperature sensitive paste. The wire used for the pattern on the left was 50 μmresulting in contact width of 86 μm was obtained. Using a wire of 30 μm width, contacts with 47 μm width wereobtained.

4. Plasmonics for Photovoltaic Applications

Students: Hemant Kumar SinghFaculty members: Chetan Singh SolankiThe relatively new field of plasmonics seeks to control light at the nanoscale by coupling it to charge densityoscillations at the interface between a metal and a dielectric. Implementing plasmonics (i.e. enhanced scatteringof incident light in photoactive absorbing material using metal nano particles) we can make photovoltaicabsorbers “optically thick” which enable us nearly complete light absorption even in thin films and apparentlycomplete photo carrier current collection. As increased scattering at longer wavelengths using metal nanoparticles would enable good light trapping, it can offer the possibility of reducing the physical thickness of thephotovoltaic absorber layers while keeping their ‘optical thickness’ constant. Clearly, plasmonics - enhancementcould be applied to all kinds of solar cell technology as an add-on. However Si solar cells would be used as aninitial test vehicle to study the effectiveness of the technology. The progress made during the year under reviewis summarized below.

Experimental investigation on plasmonics technology for effective light trapping is being explored to implementit in c-Si based solar cells. In this process, we studied the forward scattering based plasmonic technology usingsilver nanoparticles. We are currently able to do silver nanoparticle fabrication using RF sputtering followed byrapid thermal anealing where we have size variation from 50 - 150 nm with surface coverage of around 22 %and shape anisotropy (spherical shape) around 89 % at the best case for our c-Si based samples. These werefabricated in much less process time as up to now we used only one minute of RTP and reported same surfacecoverage and size distribution what other researcher did in annealing of more than 30 – 40 minutes. However tohave more controlled and efficient way for fabrication of Ag nano particles, we are looking into the techniqueswith aim of having less process time requirement, more repeatable results and fabricating the nanoparticles withdesired shape, size, and surface coverage. In addition, there are many problems which are to be addressed andexplored like Fano resonance effect, technology for reducing the high reflection in visible region which areobserved till now in our experimental investigation and testing other designs and structures and optimization fortesting the best feasible way for forward scattering based plasmonics technology in c-Si based cells. Belowsome best observed results are shown in Figures 1.9 to 1.12.

Fig.1.9: SEM image of the best-fabricated Agnanoparticles.

Fig. 1.10: Properties of the Ag nanoparticles extractedfrom the SEM image on the left.

Fig. 1.11: Design for Ag nanoparticles on PV cell (front surfaceview).

Fig. 1.12: Best reflectance profile observedfor plasmonic-based sample.

Description of Parameter Observed Results

Sample Si/ Ag NP (15s)

Size Distribution 50 nm -150 nm% Coverage of surface by NP’s 22.5 %

Shape Anisotropy 0.89

5. Slicing of silicon wafers for PV applications using Wire Electric Discharge Machining (WEDM)

Students: Dongre G. G. and Cyrus VesvikarFaculty members: Suhas S. Joshi and Ramesh Singh

The use of wire-EDM technology for slicing of silicon ingots reduces kerf loss by 50%, improvement in surfaceroughness from 3-5 µm to 2-3 µm and overall improvement in productivity by 40% has been demonstrated overand above the conventional methods of silicon ingot slicing. In addition, the wire-EDM technology has thepotential to produce ultra-thin silicon wafers. At the same time, the process is capable of producing very thinwafers of size 150 µm thick which after etching and polishing can reach to the size of 100-120 µm. A broadprocessing window has been arrived which gives above results. The wire-EDM process produces silicon waferswith-out any wire saw marks and the wire material contamination on the wafer surface, which is the majorproblem in the conventional methods. Thus, this process could be effectively employed for slicing of ingots.

Progress made during the period under reviewDuring the last year the project work has completed following objectives.1. Understanding the process mechanism of silicon ingot slicing by wire-EDM process2. To understand the effect of wire-EDM process on the physical, dimensional, micro-structural and

topographical aspects of sliced surface3. Numerical model for predication of erosion rate in silicon ingot slicing by wire-EDM

Understanding the process mechanism of silicon ingot slicing by wire-EDM processExperimental work has been carried out for parametric analysis of silicon ingot slicing by wire-EDM process.The Fig. 13(a) shows schematic of the experimental setup. The experimental studies helped to understand theeffect of process parameters (pulse on-time, Pulse off-time, voltage, current, wire feed rate and water pressure)on process responses like slicing speed, kerf loss and surface roughness. This study has helped to understand theprocess mechanics and effective utilization of the process for silicon ingot slicing. Fig. 1(b) shows 3 inch squareand 200 μm thick wafer sliced by wire-EDM process.

Fig. 1.13: (a) Schematic of silicon ingot slicing by wire-EDM (b) 3 inch wafer sliced by wire-EDM process.

Effect of wire-EDM process on the physical, dimensional, micro-structural and topographical aspects ofsliced surfaceThe experimental analysis shows that the surfaces generated by wire-EDM cutting process are free fromsubsurface damage and wire material contamination. A detail study has been carried out for characterization ofwafers sliced by wire-EDM process, which includes phase analysis by XRD, Raman spectrum analysis, residualstress analysis nano indentation hardness analysis. GIXRD analysis before and after wire-EDM did not showany amorphousation due to the process and EDS analysis shows no contamination of the wafers during ingotslicing by wire-EDM. The wafer surface sliced by wire-EDM does not wire marks and the surface cracks whichare detrimental to the quality of the wafer.

Numerical model for predication of erosion rate in silicon ingot slicing by wire-EDM process

In order to establish wire-EDM process for silicon ingot slicing, a theoretical model which describes the processin a comprehensive manner was developed. This work presents a thermal model for wire-EDM process whichpredicts the temperature distribution in the silicon work piece and erosion rate (MRR). The model has beendeveloped considering multiple sparks with varying energy values. The model studies effect of processingparameters on erosion rate. The results obtained from the model are being validated with the experimentalresults for both polycrystalline and mono-crystalline silicon ingots, which shows reasonable accuracy in thepredication of erosion rate.

Fig. 1.14: (a) Temperature contour plot after pulse on time, (b) experimental validation of the model for erosionrate.

2. Silicon Concentrator Cells3. Novel technologies for contact formation using temperature sensitive paste5. Slicing of silicon wafers for PV applications using Wire Electric Discharge Machining (WEDM)