Study On Photovoltaic Materials And Devices For Low-Cost Organic Solar Cells

The lab-scale power conversion efficiencies(PCEs)of organic solar cells(OSCs)have exceeded 19%for single-junction devices and 20%for tandem devices due to the enormous efforts on the rational molecular design,fine morphology control,indepth mechanism study and sophisticated device structure construction,and the industrialization of OSCs has become the common goal of academia and industry.However,almost all the high-performance photoactive materials have complicated fused-ring structures,requiring tedious synthesis and purification processes,thereby resulting in the high costs and poor scalability,which is contrary to the industrialization of OSCs.Simplifying molecular structure is one of the most effective ways to reduce material cost.Therefore,non-fused-ring photoactive materials attracted more and more attention in recent years,and have begun to show the potential of catching up with the fused-ring ones in terms of photovoltaic performance.In this paper,we have carried out a series of studies on the optimization of miscibility between the acceptor and the donor,the phase separation morphology of the non-fused-ring photoactive material system,and pointed out the effect of the planar conformation and intramolecular energy disorder on the photovoltaic performance of the non-fused-ring materials.The details are as follows:(1)P3HT has always been regarded as a promising material for commercial OSCs due to its low cost,scalable production and good stability.However,the PCE of P3HT-based OSCs is currently too low to meet the requirement of practice applications,and how to improve it remains a challenge.Here,a new non-fullerene acceptor,ZY-4Cl,was designed and synthesized by tuning the cyano-substituted end groups of the acceptor BTP-4Cl.Such a modification enables ZY-4Cl with matched energy levels with P3HT and,more importantly,reduces the miscibility between P3HT and ZY-4Cl,which optimizes the phase separation morphology of the P3HT:ZY-4Cl-based active layer.As a result,the corresponding device achieves a record PCE of 9.46%,which is approximately nine times higher than that of the P3HT:BTP-4Cl-based device.Encouragingly,this molecular design strategy is also effective for other NFAs.(2)To further improve the performance of P3HT-based OSCs,a volatilizable solid additive SA4 was employed to process P3HT:ZY-4Cl-based device.In comparison with the non-additive and DIO-processed devices,the SA4-processed one exhibited a more ordered molecular packing and more favorable phase separation,leading to enhanced charge transport and reduced carrier recombination.As a result,the SA4-processed device delivered a PCE of 10.24%,which was much higher than that of the DIO-processed counterpart.This work reported a PCE over 10%in P3HT-based OSCs for the first time.(3)Developing polythiophene(PT)derivatives should be another feasible plan to solve the cost issue.In this work,two new alkylthio substituted PT donors,P301 and its fluorinated derivative P302,were synthesized.In comparison,the introduction of fluorine atoms on the backbone can not only downshift the energy levels,but also enhance the molecular planarity and the aggregation effect of P302,resulting in the reduced miscibility between donor and acceptor and thus the improved phase separation morphology of the active layer.Consequently,the P302:Y5-based OSC exhibits a significantly improved PCE of 9.65%when compared with the P301:Y5-based one,indicating the important role of fluorination in the construction of efficient PT derivative donors.(4)Different from the rigid coplanar structure of fused-ring molecules,the nonfused-ring compounds usually have more rotation sites on the conjugated backbones,leading to the unstable conformations and different intramolecular energetic disorder,which could have great influence on photovoltaic performance of the corresponding devices.In this work,the 2-ethylhexyl on A4T-16 was replaced with the 3-ethylheptyl to obtain a novel acceptor A4T-3.The out-shifted branching position of 3-ethylheptyl reduces the steric hindrance effect,endowing A4T-3 with a more coplanar structure.As a result,A4T-3 exhibits a lower intramolecular energetic disorder and a more uniform surface electrostatic potential(ESP)distribution.Therefore,A4T-3 exhibits a smaller barrier for intramolecular electron transport and a higher electron mobility.Meanwhile,the lower ESP endows A4T-3 with reduced non-radiative energy loss when blending with the donor.When using PTVT-T as the donor,the A4T-3-based OSC exhibited comprehensively improved photovoltaic properties in comparison with the A4T-16-based one,delivering a high PCE of 14.26%.According to the material-only cost(MOC)evaluation,the cost of PTVT-T:A4T-3-based device is much lower than that of other high-performance OSCs,revealing the great potential of completely non-fused-ring photoactive materials for application-oriented OSCs.