Polymer Solar Cell Based On Small Molecule Cathode Interlayer

Solar energy is a renewable, cleanly and inexhaustible resource in the world. Theresearch and development of solar cells is of great significance for the reason that thetraditional resource is facing dry-up and the environment was contaminated severely. Organicsolar cells have attracted considerable attention in recent years for their advantages over theirinorganic counterparts, such as simple preparation, light weight and flexibility. However, upto now, the power conversion efficiency of organic solar cells is still not high enough to fillthe requirements of commercialization. Therefore, further efforts still need to be done to thestrategies of improving the performance of organic solar cells. In this work, we introducedtwo new small molecules T0-Br and T1-Br as cathode interface layer to improve theperformance of polymer solar cells. The details of experiment are shown as follow：First, we fabricated devices in the structure of ITO/interlayer/PCDTBT:PC₇₁BM/MoO₃/Al. Small molecules T0-Br and T1-Br are used as the cathodebuffer layer to modify the surface of ITO.We used methol:ethonal=1:1as the solvent, and spincoated at2000r/min. Under AM1.5G,100mW/cm²irradiation, the devices with Tl-Br andT0-Br interlayer exhibited PCEs of5.30%and5.50%, which are of104%and111%highercompared to the solar cell without the interlayer respectively. Furthermore, the performanceof the devices with T0-Br and T1-Br interlayer is even better than their PFN counterpart,implying that the small molecules T0-Br and T1-Br can enhance the performance of polymersolar cell effectively. We investigated the dark currents, shunt resistance （Rsh） and seriesresistance （RS） to expolre the origin of enhancement. The results indicated that theenhancement can be attributed to the appearance of interface dipole（pointing away from ITOto the activelayer） between ITO and active layer which could shift the Fermi level of ITOcloser to vacuum level. The decreased work function of the ITO could well matches theLUMO level of PC71BM and increase the built-in potential, forming Ohmic contacts betweenITO and activelayer, which provided better tansportation and collection for electron.In order to further explore the application of T0-Br and T1-Br in polymer solar cells, wefabricated devices in the configuration of ITO/ZnO/interlayer/PCDTBT:PC₇₁BM/MoO₃/Al.The results demonstrate that with the insertion of T0-Br and T1-Br, the PCE were enhancedfrom5.07%to5.43%and5.75%respectively. We attributed this improvement to theformation of interface dipole as well, which could be verified from the dark current.For the ITO/PEDOT:PSS/PCDTBT:PC61BM/interlayer/Al device, the incorporation ofT0-Br and T1-Br improved PCE from3.29%to4.00%, which evidenced the performance-optimizing property of small molecule. We consider that there is a dipole pointing away from the activelayer to Al electrode, which could shift the Fermi level of Al upward and decreasethe cathod barrier, forming Ohmic contacts for charges.