Study On The Photoexcitation Dynamics Of Polymers For Organic Solar Cells

In recent years,Organic polymer photovoltaic materials draw widely attention in energy conversion and storage as well as application in solar cells,rechargeable batteries and super capacitors.Versatility,flexibility and low cost are the features of the organic photovoltaic devices.In fact,bulk heterojunction OPV are the most efficient devices which is certificated to 13.2%by power conversion efficiency(PCE).Despite the obvious advantages in performance,the complexity of photo carriers in organic photovoltaic polymers and evolution of OPV application are still the research topics in terms of developing further photovoltaics and inceasing PCE.In this dissertation,optical measurements are utilized to investigate photo physics in two traditional organic photovoltaic materials regio-regular poly(3-hexylthiophene)(RR-P3HT)and Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV).The results are divided into 4 parts attached below.Part I Long lived photoexcitation dynamics in P3HT/PCBM blend films with above gap excitation.In millisecond time regime,delocalized polarons(DP)and localized polarons(LP)in RR-P3HT/PCBM as well as polarons in MEH-PPV/PCBM all exhibit dispersive bimolecular recombination process which was limited by the trap states,with the average lifetimes of those polarons inverse proportional to the square root of pump intensity(I).The recombination in RR-P3HT/PCBM was weak temperature dependence with small thermal activation energy.Furthermore,we proved that the values of ? for both of LP and DP increase,as well as the relatively intensity ratio of DP and LP decreases,in an intentionally degraded RR-P3HT/PCBM film.Overall,it is demonstrated that steady state photomodulation technique with thermal-activated-recombination analysis can be applied to evaluate polymer(dis)order in bulk heterojunction films.Part II Long lived photoexcitation dynamics in P3HT/PCBM blend films with below gap excitation.Compared to above-gap excitation,the polarons exhibited thermally activated and multi-trap limited bimolecular recombination too,but with narrower distribution profile in polaron states in millisecond time regime.However,magnetophoto-induced absorption(MPA)of polarons presented much weaker magnetic field effect in below-gap excitation(-0.5 mT)than in above-gap excitation(order of?10 mT),although the underline mechanism is same as spin mixing of polaron pairs by the hyperfine interaction at small field region.Part III Long lived photoexcitation dynamics in P3HT/PbS quantum dot blended films.The charge transfer from RR-P3HT to PbS as well as from PbS to RR-P3HT were observed after changing the capping ligand of PbS from a long chain molecular to a short one.Therefore,PbS could be used to extend the working spectral range in hybrid solar cells with a proper capping ligand.However,we found that the recombination mechanism in the millisecond time region is dominated by the trap/defects in blended films,while it improves to a bimolecular recombination partially after ligand exchange.Our results suggest that passivating traps of nanocrystals by improving surface ligands will be crucial for relevant solar cell applications.Part IV The aggregation effect and its influence in MEH-PPV films.we analyzed absorption and photoluminescence spectra to study the optical properties of MEH-PPV films,in which we found both intrachain(J-aggregates)and interchain(H-aggregates)played crucial roles,in particular,at low temperature lower than 200K.While the temperature is above 200K,the random coil of polymer chain could be thermally activated for emission.Our work experimentally proved that the aggregate of MEH-PPV films is characterized by HJ-aggregates.The photophysical properties from a competition between intrachain interaction(J-aggregate-like)and interchain interaction(H-aggregation like)can be tuned by solvent and annealing simply.