Exploitation Of High Performance Cross-linkable Organic Photovoltaic Materials

Due to their advantages of lightweight,flexibility,low cost and ease of processing,organic solar cells（OSCs）have been widely studied and considered as an alternative to commercial monocrystalline silicon solar cell.In the last two decades,especially as a result of numerous high performance donor-accetor（D-A）polymer photovoltaic materials invented,power conversion efficiencies（PCEs）of OSCs have achieved a remarkable progress,reaching a level of 9¹0%,which is closing to basic requirements for their real use.However,the stability issue of organic solar cells has not yet been well resolved and remained as a challenge task in the field.One of important reasons for device instability is that high performance devices for OSCs generally adopt a bulk heterojunction（BHJ）active layer,which is composed of a physical blend of electron donor and acceptor materials.During their use,these two kinds of materials in the active layer tend to form a large phase separation structure,thus greatly lowering exciton dissociation efficiency and resulting in a degraded device performance.To address this problem,a strategy has been proposed and proved to be an effective method.The strategy uses crosslinkable donor and/or acceptor materials in fabrication of active layer.After ideal microstructure and morphology is obtained,crosslinking reaction is triggered and performed to “lock in” the active layer morphology.However up to date,such strategy has mainly carried out on poly（3-hexylthiophene）（P3HT）,a representative photovoltaic donor material in the past decades.Only a few works focused on the present D-A photovoltaic polymers.And it is still lack of polymer materials that possess both crosslinkable ability and high photovoltaic performance at the same time.In this thesis work,we were endeavored to exploit such crosslinkable and high performance polymer photovoltaic materials by introducing additional crosslinkable moieties into side chains of current high performance D-A polymers.By this approach,we hope to invert certain key photovoltaic materials and device fabrication techonologies for OSCs with both high efficiency and long-term stability.In the first part,we foscused on high performance benzo[1,2-b:4,5-b′]-dithiophene（BDT）and thieno[3,4-c]pyrrole-4,6-dione)（TPD）-based D-A polymer and synthesized a BDT monomer with vinyle in the end of side chains.This monomer was used as a third component and polymerized with normal BDT and TPD monomers,affording a new series of crosslinkable polymers named PBDTTPD-V_x（x = 0,0.0125,0.025,0.05,and 0.10）,which possess different molar contents（x）of terminal vinyl unit on their side chains.It was found that the larger vinyl content the polymer had,the stronger thermal curing property it had,however,the lower performance its-based OSC device displayed.The results indicated that a vinyl content as small as 2.5% is enough for this polymer family to achieve an effective crosslinking,while at the same time keep a high device efficiency.The optimized PBDTTPD-V0.025-based device showed a PCE of 6.06% after thermal crosslinking,which represents the highest record for the OSC devices having a crosslinked active layer to date.Furthermore,such crosslinked device exhibited higher thermal stability than those based on non-crosslinkable PBDTTPD-V₀ and kept 91% initical PCE after thermal treatment at 150 oC for 40 h.In the second part,the researches focused on another high performance D-A polymer PTB7-Th based on alkylthienyl-functionalied two-dimensional conjugated BDT and thieno[3,4-b]thiophene（TT）units polymer,and synthesized its crosslinkable polymers named PTB7-Th-V_0.05 possessing 5% molar contents（x）of terminal vinyl unit on their side chains.It was found that terminal vinyl-appended polymers PTB7-Th-V_0.05 can be crosslinked upon UV-light.However,since photo-sensitive 1,8-diiodooctane（DIO）was needed as additive in device fabrication for tuning the active layer morphology,the PTB7-Th-V_0.05-based device subjected photo-crosslinking exhibited poor performance.Therefore,we tried compounds having multiple thiol units as photo-crosslinking agents and also additives in place of DIO.We found that the use of such multithiol compound can improve crosslinking effect.Preliminary device studies showed that PTB7-Th-V_0.05-based cells with 5% of multithiol crosslinking agent enhanced its efficiency to 2.12% after photo-crosslinking.