Investigation Of Organic Solar Cells Based On Polymer/Fullerene Heterojunction

Organic thin film devices are emerging technology with many different applications in optoelectronic field such as Organic Solar Cells(OSCs).Unlike inorganic devices,they offer considerable advantages such as wide variety,large area fabrication,flexibility,tuned color and low-cost fabrication open up new niche markets previously unexplored.However,theory of organic photovoltaic is still ambiguous.Abundant factors significantly affect the Power Conversion Efficiency(PCE)of OSCs.As a result,it is important to investigate the fabrication and charactertization processes of OSCs.In this dissertation,the emphasis is mainly focused on the study of polymer/Fullerene OSCs based on polymer(P3HT)and Fullerene(PCBM)materials.The fabrication and photophysics of polymer/Fullerene OSCs were deeply investigated in this dissertation,the major work includes:In order to improve the PCE,all layers of polymer/Fullerene OCSs were optimized.Contrasted with traditional wet etching technique,Zinc powder was applied on ITO surface before the ectching step.The phenomene of undercutting was significantly decreased because a galvanic reaction between SnOx and Zinc increased the etching speed.Meanwhile,the ITO edges canbe protected because Zinc has stronger reactivity with hydrochloric acid.In this study,the ITO etching step takes only around 20 seconds while traditional wet etching needs over 40 s.The thickness optimization of polymer/Fullerene(1: 1 in mass ratio)bulk heterojunction was investigated.The confliction between photo-absorption and exciton-diffusion needs to be balanced if active layers were based polymer/Fullerene heterojunctions.It was found in this dissertation that 90 nm thickness active layer was both benefit to photo-absorption and excitionseperation.Contrasted with thicker or thiner active layers,polymer/Fullerene OSCs based 90 nm active layer had a PCE improvement around 12%.Based on optimized active layer,Cathode and anode were investigated by applying an Electron Transportation Layer(ETL)and Hole Transportation Layer(HTL),respectively.It was found that Open-Circuit Voltage(Voc)was significantly increased by applying Calcium(Ca)as ETL.Contrasted with polymer/Fullerene OSCs without ETL,the PCE of polymer/Fullerene OSCs modified with 10 nm ETL was doubled,increased from 0.55% to 1.27%.Because a chemical bond between Ca and Sulfur(S)was formed which decreased the Highest Occupied Molecular Orbital(HOMO)energy of donor material.The potential difference between Donor and Acceptor was then enhanced after the formation of Ca-S bond,leading to a Voc improvement.Meanwhile was investigated through dopping technique.The conduvtivity of HTL was strongly related with dopping concentration.In experiments,PCE of OCSs was dramatically improved after applying Dimethyl Sulphoxide(3% in volume concentration)dopped PEDOT: PSS solution,increased to3.79%.Contrasted with prinstine polymer/Fullerene OSCs(1.27 % in PCE),the ETL was benefit to the PCE improvement of polymer/Fullerene OSCs.After all layers of polymer/Fullerene OSCs were optimized,the photocurrent generation process was then investigated by femto transient pump probe tenique.The evolution associated differential spectra results were analized through global fit for polymer/Fullerene heterojunction.It was found that photo generated electrons were rapidly generated after photo absorption.An exciton can be turned into a pair of free polaron within 10 picoseconds.The generation of free polarons was stable increased around 100 picoseconds after photo absorption.The recombination process was investigated through transient photovoltaic technique as well.In open circuit condition,geminate recombination was the main factor in active layers.However,the recombination rate was decreased after 10 microsecronds when the polymer/Fullerene OSC was connected with a resistor.The reason of the recombination reduction is that charge carriers were drifed by the built-in electric field.The non-geminate recombination was then considered into the recombination process.As a result,charge carrier diffusion and recombination were occurred in nanosecond to microsecond time scale.A position sensor was fabricated based on the the research as we mentioned above.The active layer was based on thickness gradient polymer/Fullerene heterojunction.The position sensing was successfully measured by applying dual-wavelength(550 nm and 620 nm)excitation.Im summary,the optimized polymer/Fullerene OSC in this dissertation was designed a strcucture as 30 nm DMSO dopped PEDOT: PSS film,90 nm polymer/Fullerene active layer,10 nm Calcium ETL.The PCE based on this structure achieved to 3.79% which is 6.89 times higher than pristine polymer/Fullerene solar cells.However,there is still a long way when the PCE of polymer/Fullerene OSCs compares with silicon solar cells.An important reason,as we found in this dissertation,is that the time difference between photocurrent generation(10 μs)and charge carrier diffusion(100 ps)cannot be balanced in polymer solar cells.In this dissertation,the fabrication process and photovoltaic properties of polymer/Fullerene OSCs were deeply investigated based on polymer/Fullerene heterojunction.The PCE of polymer/Fullerene OSC was steadily improved through optimization of each layer.Based on optimized structure,the photo-electronic process and carrier recombination process were then studied.The emphases are focused on improvements of surface morphology,influence of transportation layers,mechanisms of photo-generated electrons and recombination process.The study and analysis on the mechanisms and some basic issues of polymer solar cell are the emphases.It is hoped that some instructional suggestions may be given to the futural investigation.