Design Of Donor Molecule And Voltage Loss In Organic Solar Cells

Organic photovoltaics(OPVs)have developed rapidly in recent years,due to the design of novel high-performance active layer materials and device optimization.Compared with other forms of photovoltaic device,OPVs has adjustability of light absorption range and flexible preparation and other characterastics.Furthermore,OPVs exhibits excellent utilization of light absorption and worse energy loss.Thus,it is necessary to investigate the reasons for the loss of energy of organic solar cells.This gradute thesis mainly focused on the relationship between donor molecular design and device performances.We designed and synthesized a series of conjugated copolymers for donor based on D-A copolymerization method and molecular side chain optimization to improve the morphology and energy level aligment,exploring the mechanism of energy loss in OPVs device.These works is a reference for the high-performance molecular design with low energy loss in the future.In addition,we also studied the stability-of-bendability interlayer composite material,which provides a certain foundation for the large-scale flexible preparation of OPVs in the future.This research work and results include:1.A novel D-A conjugated copolymer(PBDS-TZ)by introducing a excellent planar thiazolo[5,4-d]thiazole(TZ)unit into alkylsilyl-fused benzodithiophene(BDT)was designed and synthesized.Due to the σ*(Si)-π*(C)bond interaction and electron-affinity of TZ effectively downshifted the energy levels of the PBDS-TZ,ensuring it to well-match with the deep energy levels acceptor IT-4F.Furthermore,the better coplanarity of TZ unit is conducive to the efficient transport and charge collection.More importantly,without extra post-treatment,a desirable micromorphology with a vertical phase separation was readily achieved in as-cast active layer,thanks to the intrinsic feature of the PBDS-TZ donor materials.Eventually,a notably PCE of 12.01%,with a high FF of 73.1%and Jsc of 20.45mA cm-2 was obtained for the devices based on the PBDS-TZ:IT-4F.These results demonstrated that rational design of conductive materials to achieve intrinsically favorable morphology is more critical than the post-treatment for high performance OPVs.2.Two novel copolymers based on alkylsilyl-and chloride-functionalized benzodithiophene(BDT)were designed and synthesized for wide bandgap copolymer donor materials in OPVs.It was found that the two copolymers exhibited distinctly different properties in active layer when blended with fullerene-free acceptor IT-4F.The chloride-functionalized copolymer PBDTCl-TZ with deeper molecular energy level and better coplanar structure induced more ordered aggregation in blend film.Thus,the device based on PBDTCl-TZ exhibits better energy alignment with IT-4F and smaller radiative recombination.Furthermore,the non-radiative recombination of PBDTCI-TZ:IT-4F based device is about 45 mV lower than the PBDTSi-TZ:IT-4F based device,contributing to a lower energy loss(Eloss),and a higher open-circut voltage(VOC).As a result,the devices based on the blend of PBDTCl-TZ:IT-4F exhibit a high power conversion efficiency(PCE)as high as 12.2%with a high VOC of 0.837 V.Thus,the molecular geometry engineering is an effective strategy to control the micromorphology and molecular energy level in OPVs.3.Two deep HOMO energy level donor materials(PBDS-TPD and PBDTTPD)were designed and synthesized.This is mainly because of the strong electron-affinity and excellent co-planarity of the thiophene pyrrolidone(TPD)unit that ensures the electrons with a bigger delocalization space and down-shift the HOMO energy level of the molecule.Finally,PCE based on PBDS-TPD:IT-4F is 9.91%with VOC of 0.876 V,JSC of 18.84 mA cm-2,and FF of 61.30%.PCE based on PBDTCl-TPD:IT-4F is 12.67%,VOC of 0.915 V,Jsc of mA cm-2,and FF of 67.34%.In addition,we also found that this two similar structures molecules show different sensitivity of the third addcitive.The open-circut voltage of PBDS-TPD:IT-4F system is non-senstive to third additives.However,the open-circut voltage of PBDTCl-TPD:IT-4F is obviously changed with different third addictive.The VOC based on PBDTCl-TPD:IT-4F of device changed from 0.915 V to 0.965 V,which is one of the highest VOC of based IT-4F device.The difference of luminous efficiency is close to an order of magnitude based on PBDTCl-TPD:IT-4F with two kind of third addictive.Therefore,we can find that the voltage loss of the device is not only related to the luminescence efficiency of the narrow-bandgap acceptor,but also related to the characteristics of the donor.This work provides a reference for us to design high-performance OPVs with low energy loss in the future.4.The sability of bendability composite interlayer materials based on organic conjugated polyelectrolyte(PNDIT-F3N)and alcohol-soluble high molecular weight organic elastomer polyvinylpyrrolidone(PVP)are designed and prepared via making full use of the advantages of both.It is found that added an appropriate amount of PVP not only did not damage the morphology of the interface layer,but also improve the surface morphology of the active layer and obtain considerable PCE,significantly improving its mechanical bending performance.The PCE based on PM6:IT-4F is 13.42%,with VOC of 0.851V.Moreover,this composite material exhibits good universality.An excellent device performances based on narrow-bandgap accceptor materials was over 15.10%.This composite material is beneficial for large-scale and flexural preparation of OPVs in the future.