Solar Photovoltaic Technologies: Solar Photovoltaic Technologies Prof. C.S. Solanki Energy Systems Engineering IIT Bombay Lecture-33


Contents: Contents Brief summary of the previous lecture Various Thin film solar cell technologies a-Si CdTe, CIGS Thin film crystalline Si Choice of material for thin film solar cell L33- Lecture-33


Thin-film technologies: Thin-film technologies L33- Lecture-33


Thin-film technologies: Thin-film technologies Material direct band gap tunable band gap, 1.3 to 2.0 eV deposition techniques: PECVD,VHF-PECVD, HWCVD Light induced degradation Device structure Defects introduces the mid gap states  short life time p-i-n/n-i-p structure thin i layers are advantageous  good light trapping is required Tandem cells band gap profiling double junction, triple junction L33- Lecture-33


Thin-film technologies: Thin-film technologies direct band gap 1.45eV(CdTe), 1.1eV(CIGS) heterojunction with n-CdS Solar cells are stable and technology is relatively cost effective material availability Toxicity of Cd L33- Lecture-33


Thin-film technologies: Thin-film technologies L33- Lecture-33


Thin film solar cells: Choice of material: Thin film solar cells: Choice of material GaAs  efficiencies above 30%, But very expensive, mainly suited for space application, As is toxic CdTe  Cd is toxic, covering large surface area with toxic material is not desirable CIGS  Availability of In, 0.08 ppm, cost can go high in case of increased demand a-Si  low-cost, but low stable efficiency C-Si  abundant raw-material (227000 ppm) high efficiency, stable efficiency, thin-film technology provides potential to reduce the cost of wafer based cells, very attractive option to explore Candidates for thin-film solar cells: ✰ Crystalline Si(c-Si), ✰ amorphous Si (a-Si), ✰ Cadmium teluride (CdTe), Copper-indium-gallium-arsenide (CIGS), Gallium-arsenide ( GaAs) L33- Lecture-33


Thin-film technologies: Case for Si: Thin-film technologies: Case for Si Calculated AM1.5 efficiencies (dashed line) and AM0 efficiencies (solid line), comparing achieved cell efficiencies (laboratory-best, confirmed) for various technologies Studies shows that if cell are made with thin-film crystalline Si such that they trap enough light, cell efficiencies greater than 20% can be obtained even in just few micron thick layer C-Si is near the maximum of this curve L33- Lecture-33


Slide9: Techniques for Si usage reduction L33-


Slide10: Reduction of Si-usage / pro’s – con’s L33- Lecture-33


Crystalline Si films: challenges & potentials: Crystalline Si films: challenges & potentials Challenges Light trapping Surface passivation Grain boundary passivation Supporting substrate Potentials Cost-effectiveness Material quality and efficiency L33-


Slide12: Contents Motivation Different thin-film solar cell technologies Why crystalline Si films? Classification based on grain size Thin-film solar cell structures Deposition techniques low temperature High temperature approaches Mono-crystalline Si thin films Other concepts L33- Lecture-33


Slide13: Classification of different approaches A large number of different technologies are under parallel development A classification can be made based on different criteria: According to Tmax during layer formation According to grain size According to cell structure The R&D on the high-temperature routes is mainly driven by considerations from classical bulk Si cells  Proven high efficiency and stability The R&D on the low-temperature routes is mainly driven by considerations from a-Si:H solar cells  low thermal budget processing L33- Lecture-33