Study Of Highly-efficient Etching Techniques And Hole Transport Layer Design For Organicsilicon Hybrid Solar Cells

Organic-silicon hybrid heterojunction solar cells are one of the developing photovoltaic technologies.Recently they have reported promising efficiencies and that of high efficiency has been realized within past few years.The organic/Si solar cell devices are composed of n-type crystalline-Si substrate and an organic polymer called poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)(PEDOT:PSS)used as the emitter with ptype electrical conductivity.These solar cell devices provide an exclusive possibility to combine the sharp energy conversion efficiency of monocrystalline-Si with the potentially low fabrication cost of organic materials.The idea of this Ph.D.dissertation is to formulate a novel,highly efficient and low-cost anisotropic etching technique for photovoltaics and experimentally study the hole conducting layer of PEDOT:PSS by fabricating organicsilicon hybrid solar cell devices.Hole transport is an important parameter in photovoltaics with respect to their implementation in device applications.Organic(hole)transport materials are differentiated by low-cost synthesis,tunable frontier molecular orbital,effortless formation of thin layer and stable in ambient and air.However,the instability of pristine PEDOT:PSS encourage scientists and researchers to introduce a stable and ecofriendly surfactants into the PEDOT:PSS solution to enhance its hydrophilicity,improve the charge carrier lifetime and work function in the hybrid organic solar cell device.This project was designed with two important aims,in which first one is to develop novel and highly efficient anisotropic wet chemical etching solution for silicon wafers texturing to reduce the front surface reflection of silicon wafers(substrates).The purpose of developing a novel anisotropic etching solution is to reduce the cost of etching materials itself.Secondly,to reduce the time of etching to meet the requirements of commercial industry manufacturing standards for etching of silicon wafers for commercial photovoltaic applications.With number of experimental analysis,we used different anisotropic etching materials and mixtures from which we have achieved the lowest surface reflection of 9.94%with etching time of 25 minutes.We employed Na OH + KOH + Na2 Si O3 and IPA in specific concentration to prepare this novel etching solution which results in micropyramidal structure on the surface of silicon wafers.Besides lowest surface reflection our novel etching solution was effective in improving surface morphology,periodicity of pyramids and hydrophilicity of the etched silicon wafers.In another experimental study for fabrication of rapid texturing with specific structure for silicon wafers,I exercised the following alkaline novel compound of Na OH,KOH,Na2 Si O3,IPA and HF with explicit concentration to achieve fastest etching results in lesser time.This novel etching solution was prepared with an aim to employ low-cost materials with simplest processing and utilize less sophisticated experimental equipment's and apparatus.I use HF in the etching solution and introduce stirring during the whole experimental reaction to achieve a specific structure of octagonal pyramids from texturing process.After performing the analysis for experimental results,I found out significant improvement in the surface morphology,hydrophilicity,periodicity and a novel structure of octagonal micro-pyramids on the surface of silicon wafers for the first time with the anisotropic etching ever achieved.Our experimental result shows the lowest surface reflectance of 8.87% with a record short time of 15 minutes for etching.We are competent about these etching results it could be employed as a low-cost and highly efficient etching technique in commercial industry of photovoltaics and selected MEMS applications.It will certainly help in improve production capacity and reduce the cost of etching as compared to current commercial and complex etching processes.The second and important aim of my research project and experimental analysis was to design and fabricate an organic-silicon hybrid solar cells with focus on hole transporting layer(PEDOT:PSS).This is an important part of hybrid photovoltaic device that play key role in improving the overall performance of a hybrid solar cell.Hole conducting layer can enhance the absorption,improve carrier collection and stability of hybrid PV device,eventually leading to high-power conversion efficiency for hybrid organic-silicon solar cells.In our experimental analysis firstly we fabricate planar organic-silicon solar cell focusing on improving the electrical and optical properties of hole conducing layer by engineering and varying the thickness of PEDOT:PSS layer.Secondly,we introduce different surfactant such as Dimethyl Sulfoxide(DMSO)and Ethylene Glycol(EG)to enhance the conductivity as well as improve charge collection of the PEDOT:PSS layer.We also introduce 1wt.% Triton T100 to enhance the hydrophilicity of PEDOT:PSS for spin coating on the planar silicon wafers.We prepared a novel structure for hybrid solar cell device with a thickness of 150 nm for hole conducting layer of PEDOT: PSS(PH1000by Clevios,Heraeus,Hanau,Germany)mixed with 7 wt % ethylene glycol(EG)and 0.5wt % Triton-100 which was spin coated on planar silicon substrate.Results shows significance improvement in open circuit voltage(Voc)and Fill Factor(FF).The whole fabrication process was completed in less than three hours.A Power Conversion Efficiency(PCE)of 5.1%,an open circuit voltage(Voc)of 598 m V,short circuit current(Jsc)of 15.7m A/cm2 and a fill factor(FF)of 58% were achieved.Conclusively,summary was provided with the endorsements for existing and future work is also presented in the last chapter.