Design,Synthesis,and Photovoltaic Application Of Cyano-Substituted Halogen-Free Hole-Transporting Materials

Bulk-phase heterojunction organic solar cells（BHJ OSCs）have attracted great attention for their advantages of low cost,light weight,and flexibility.With the rapid development of semiconductor materials and device fabrication,the power conversion efficiency（PCE）of OSCs has exceeded 18%.The high-performance acceptor materials for OSCs are mainly Y6 and its derivatives.These materials have low lowest occupied orbital（LUMO）energy levels（between-3.90--4.15 e V）,resulting in low open-circuit voltage(V_OC)of OSCs.To further improve the PCE of OSCs,it is important to increase the V_OC.Halogenation of organic semiconductors is an effective strategy to improve the V_OC,while the introduction of halogens usually involves complex synthesis processes and serious environmental pollution.We investigate how to develop halogen-free donor polymers with low highest occupied molecular orbital（HOMO）energy levels,and how to achieve high-performance of OSCs in this work.The specific of the research are as following:1.Two halogen-free donor polymers PCN1（zigzag-shape backbone）and PCN2（linear-shape backbone）were constructed with dicyanobenzotriazole as the acceptor（A）block.Their absorption,energy levels,charge transport properties,and applications as donors in OSCs were thoroughly investigated.PCN2 with strong absorption at 400-700nm,maximum extinction coefficient of 8.67×10⁴ M^-1 cm^-1,low HOMO energy level（-5.50 e V）,and the high hole mobility(1.42×10^-3 cm² V^-1 s^-1)is an excellent donor polymer.The PCN2:Y6-based OSCs achieved a PCE of 15.20%without the addition of any additives and any post-treatment,with a V_OC of 0.862 V,which is by far the highest PCE among OSCs with donor polymers based on benzotriazole derivatives.It is also one of the highest V_OC reported for binary OSCs matched to Y6 in halogen-free donor polymers.The measurements of atomic force microscopy（AFM）,transmission electron microscopy（TEM）,and grazing-incidence wide-angle X-ray scattering（GIWAXS）show that PCN2:Y6 blends have a more orderly arrangement and strongerπ-πstacking compared to PCN1:Y6 blends.In addition,the PCN2:Y6 device exhibits better charge transport,higher and more balanced carrier mobility,less exciton recombination loss,and suitable phase separation size,which lead to its higher photovoltaic performance.2.Herein,we reported three halogen-free ternary donor polymers（PCNx,x=3,4,5）based on dicyanobenzotriazole.When blended with Y6,PCN3 with strong interchain interactions leads to proper crystallinity and thermodynamic miscibility of the blended films.The GIWAXS tests show that PCN3 has a more ordered arrangement and strongerπ-πstacking compared to PCN2.Fourier-transform photocurrent spectroscopy and external quantum efficiency of electroluminescence show that PCN3-based OSCs have lower energy loss（0.529 vs 0.562 e V）than PCN2,which leads to its higher V_OC（0.873 V）.Meanwhile,PCN3-based devices achieve a PCE of 15.33%,which is one of the highest values for OSCs based on halogen-free donor polymers.3.Ternary strategy is an effective method for enhancing performance of OSCs.For organic photovoltaics,the stability of device performance is also an important issue.Polymer PCN3 with strong crystallinity and interchain interaction were selected as the third component of PM6:Y6 binary system,and its morphological behavior and photovoltaic performance were studied.The strong interchain interaction can effectively inhibit molecular interdiffusion,thus stabilize the phase separation morphology of the film.PCN3 was favor for the charge transport and separation in the device as well.The PCEs of 17.80%and 18.10%were obtained in PM6:PCN3:Y6 and PM6:PCN3:BTP-e C9 ternary devices,respectively,which were much higher than those of PM6:Y6（16.31%）and PM6:BTP-e C9（17.33%）binary devices.In addition,this ternary OSCs showed higher stability compared to the binary system,with a PCE loss of only 2%after 1200 h,while the PM6:Y6 binary OSCs showed a PCE loss of 9%.The above results suggest that PCN3 is a promising donor polymer in improving the performance and stability of OSCs.4.All-polymer solar cells（all-PSCs）have attracted increasing research interest due to their good solution processing properties,excellent stretchability,mechanical durability,and morphological stability.We selected PCN3 as the third component of PM6:PY-IT binary all-PSCs to finely tune the active layer morphology of binary blends to achieve high-performance of all-PSCs.Due to the ordered molecular arrangement and strong interchain interactions of PCN3,a denser molecular stacking of the ternary blends was obtained,as demonstrated by the GIWAXS study.In addition,ternary all-PSCs have more optimized exciton separation and charge transport than binary all-PSCs.When PCN3 was introduced as a third component into PM6:PY-IT based binary all-PSCs,it resulted in an increase of PCE from 15.05%to 16.29%,which is one of the highest PCEs reported so far for all-PSCs.The PCE reached 13.61%when the active layer thickness was about 300 nm,which is attributed to the higher and more balanced carrier mobility of the ternary blends,as well as the stronger crystallinity and more ordered molecular arrangement in the vertical direction.These results indicate the importance of the ternary blending strategy to improve the photovoltaic performance of all-PSCs.5.We constructed the halogen-free donor polymer PCN6 using 2-ethylhexyl-4,6-dibromo-3-cyano-thieno[3,4-b]thiophene as A block and compared it with the commercially available PTB7-Th.It was found that PCN6 has a wider film absorption and lower HOMO energy level than PTB7-Th,suggesting a greater advantage of the cyano-substitution strategy in terms of molecular absorption and energy level tuning.The performance difference between PCN6:Y6-and PTB7-Th:Y6-based OSCs were compared by a series of studies including light intensity dependence,carrier mobility,AFM,TEM,and GAWIXS.The results show that PCN6:Y6-based OSCs have stronger crystallinity,better charge transport,higher and more balanced carrier mobility,and less exciton complex loss.Therefore,the PCE of PCN6:Y6-based OSCs reached 11.34%,while the PCE of PTB7-Th:Y6-based OSCs was only 9.02%.These results suggest that2-ethylhexyl-4,6-dibromo-3-cyano-thieno[3,4-b]thiophene is an important A block for the construction of halogen-free donor polymers with low HOMO energy levels,and also demonstrate that the introduction of cyano in the conjugated backbone of polymers is a good strategy to achieve high-performance OSCs.