Structural Modification And Device Performance Characterization Of Highly Efficient Polymer Donor Photovoltaic Materials

Polymer solar cells（PSCs）as an emerging clean energy technology attrated great attention in recent years due to their advantages such as semitransparency,flexibility and compatibility with large-scale manufacturing,and wide application prospect in building integration and wearable devices.The power coneversion efficiency（PCE）is one of the main parameters to evaluate the performance of PSCs,and has exceeded 13%in PSCs,reaching the lowest threshold for practical application.In addition to high PCE,the commercial application of PSCs still needs to meet the conditions of high device stability and low preparation cost,which is mainly dependent on the design and matching of high-performance donor and acceptor photovoltaic materials.In this work,different molecular design strategies,such as side-chain engineering of alkyl side chains and halogenations,are used to accurately regulate the absorption,energy levels and crystallization of polymer photovoltaic materials,and a series of highly efficient conjugated polymer donor materials were synthesized and characterized systematically.The main achievements and conclusions of this paper are as follows:（1）A 1,1’-vinylene-fused indacenodithiophene（IDTV）unit with 22π-conjugated electrons was synthesized.Ladder-type D-A polymer PIDTV-ffBT using IDTV as donor unit and 5,6-difluorobenzothiadiazole as acceptor unit was developed for photovoltaic applications.Compared to other analogue polymers,PIDTV-ffBT has multi-electrons fused-ring and excellent planarity,leading to the higher light harvesting coefficient and enhanced hole mobility of 0.032 cm² V^-1 s^-1.The PIDTV-ffBT:PC₇₁BM-based PSCs achieved a promising PCE of 7.3%with a high short-circuit current density(J_sc)of 17.1mA cm^-2.These results indicate that the introduction of 1,1’-vinylene-fused system onto ladder-type polymers is an effective strategy to enhance light absorption coefficient and improve charge carrier mobility for high efficiency PSCs.（2）D-A structured fluorinated polythiophene derivative,named PBDD-ff4T,was synthesized for photovoltaic applications.Compared to the fluorine-free PBT1,PBDD-ff4T exhibited a smaller bandgap,deeper HOMO level,and higher crystallinity.The as-cast PSCs based on PBDD-ff4T:PC₇₁BM yielded a PCE of up to 9.2%with an open-circuit voltage(V_oc)of 0.95 V and fill factor（FF）of 73%.Notably,the photovoltaic performance of devices is insensitive to the variations of actively layer thickness.Moreover,the flexible PSCs exhibited a PCE of 6.6%.The results indicate that fluorinated PBDD-ff4T is a promising candidate for the practical application of highly efficient PSCs.（3）The side-chain engineering of thienyl benzodithiophene-alt-bezotriazole（BDT-T-alt-BTz）-structured polymer donor materials was performed to improve photovoltaic performance of the polymer donors including:i)Polymer PSBZ was synthesized by alkyl side-chain engineering to increase the steric hindrance of donor unit and decrease the steric hindrance of acceptor unit.Compared to the analogous polymer J61,the acceptor unit of PSBZ can be more effectively exposed to the acceptor ITIC,thereby the charge separation and transport were improved in PSCs.Compared to J61 with a PCE of 9.53%and J_sc of 17.43 mA cm^-2,the PSCs based on PSBZ:ITIC yielded a higher PCE of10.5%with a higher J_sc of 19.0 mA cm^-2.The results show that the side chain engineering can effectively improve the photovoltaic performance of the polymers.ii)Polymer PFBZ based on fluorinated-thienyl benzodithiophene（BDT-2F）unit was synthesized.Compared with the analogues polymer PBZ,PFBZ shows a lower HOMO level,higher extinction coefficient,and improved hole mobility.The as-cast PSCs based on PFBZ:ITIC exhibits a higher PCE of 10.4%compared to the PBZ:ITIC-based PSCs（8.0%）.Notably,the devices based on PFBZ with an active layer thickness of 200 nm still remain a PCE of 8.9%.And,the devices based on PFBZ:ITIC processed by annealing at 150 ^oC for 1 hour also show a high PCE of 9.3%.The results indicate that the fluorination in the thienyl conjugated side chains can effectively improve the photovoltaic performance of the polymer donor materials.（4）The polymer PBPD-Th was synthesized by introducing the 2D-conjugated side-chains of meta-alkoxy phenyl into BDT unit.Compared to the polymer PBDB-T with the 2D-conjugated thienyl side-chains,PBPD-Th has slightly blue-shifted abosption spectrum and significantly down-shifted HOMO level（-5.42 eV）.The PSCs based on PBPD-Th:ITIC display a PCE of 10.8%with a V_oc of up to 1.01 V and energy loss(E_loss)as low as 0.54 eV.The results indicate that PBDP-Th is high performance polymer donor material in the ITIC-based PSCs with high PCE,high V_oc of greater than 1 V and low E_lossoss of less than 0.55 eV.（5）A series of high-performance wide bandgap polymer donor materials were synthesized by introducing chlorine atoms into the thienyl conjugated side chains of the2D-conjugated polymers based on BDT unit:i)Polymer PM7 based on chlorinated BDT-T（BDT-2Cl）donor unit and benzodithiophene-4,8-dione（BDD）acceptor unit was synthesized.Compared to the halogen-free PBDB-T,PM7 shows lower HOMO level,higher absorption coefficient,enhanced crystallinity,and higher carrier mobility.Moreover,the toluene-processed PSCs based on PM7:IT-4F gained a high PCE of 13.1%with a high V_oc of 0.88 V,J_sc of 20.9 mA cm^-2,and FF of 71.1%,while the PSC based on PBDB-T:IT-4F only exhibited a low PCE of 5.8%with a low V_oc of 0.67 V,J_sc of 15.0 mA cm^-2,and FF of 57.6%.ii)Polymers PBZ-Cl based on BDT-2Cl unit,and PBZ-ClSi based on chlorine and alkylsilyl substituted BDT-T（BDT-ClSi）unit were synthesized.From PBZ to PBZ-Cl and then to PBZ-ClSi,the polymers show gradually down-shifted HOMO levels,increased absorption coefficient,and improved charge carrier mobility.The toluene-processed PSCs based on PBZ-ClSi:IT-4F gained a high PCE of 12.8%with a high V_oc of 0.93 V,J_sc of 19.2 mA cm^-2,FF of 71.5%,while the PSCs based on PBZ or PBZ-Cl only obtain low PCEs of 6.4%(V_oc of 0.60 V,J_sc of 17.0 mA cm^-2,and FF of 63.2%)and9.7%(V_oc of 0.80 V,J_sc of 17.7 mA cm^-2,and FF of 68.3%),respectively.The results indicate that the chlorination is the effective strategy to design high-performance polymer photovoltaic donor materials.（6）The photovoltaic performances of the PSCs based on fluorinated polymer donor PM6 and different nonfullerene acceptors（such as IDIC and IT-4F）were studied.From IDIC to IT-4F,the acceptors show gradually decreased energy levels,enhanced photon capture ability.IDIC with alkyl side-chains shows higher crystallinity compared to IT-4F.The as-cast PSCs based on PM6:IDIC exhibit a high PCE of 11.9%.Notably,the PCEs of PSCs are insensitive to the active layer thicknesses（95²25 nm）and device area（0.20⁰.81 cm²）with the values of over 11%.Moreover,the flexible PSCs with a large device area of 1.25 cm² also exhibited a PCE of 6.54%.Using IT-4F as acceptor material,the PCE of PSCs was futher improved to 13.5%,while the PSCs with an ultra thick active layer of 285 nm obtained a high PCE of 12.2%and the PSCs with a large device area of 1 cm² also gained a high PCE of 11.4%.The results indicate that i)PM6 is very promising to be a polymer donor for commercial application of PSCs,and ii)the reasonable matching of high-perormance donor and acceptor materials is also beneficial for PSCs to obtain high PCE.