The Study Of Organic Heterojunction's Modification And Working Mechanism

Organic semiconductors are regarded as the next generation semiconductor,due to their outstanding advantages in terms of low cost,various molecular structures and the potential of light weight,thinness and flexibility.As the active layer,organic semiconductor has been widely employed in organic lignt-emitting diode(OLED),photovoltaic(OPV),detector,memorizer and field-effect transistor etc.Organic heterojunction is the contact region of two different organic semiconductors.Thanks to the unique energy level structure of the heterojunction,special functions and better performance can be realized in the devices compared to the single organic semiconductor.Therefore,organic semiconductor heterojunctions are essential to achieve high performance OLED and OPV.The OLED with multi-layers structure and the OPV with appropriate bulk heterojunction structure show better efficiency and lifetime in general.However,processing high quality organic heterojunction is very challenging.Interface heterojunction is constructed from two different organic semiconductor layers.Since most organic semiconductors have similar solubility in common organic solvents,such as toluene,xylene,chloroform,chlorobenzene,etc.,the deposition of multilayers could mix layers together and damage each other.As for the bulk heterojunction,the size and purity of phase separation are affected by the molecular structures,solvents and processing conditions.In our contribution,a novel semi-orthogonal solubility relationship in OLED is proposed and demonstrated for the first time by using an alkane soluble dendrimer(G0)as electron transport layer.The combination of the alkane solubility and the compatible optoelectronic properties makes it possible to sequentially solution-process G0 as electron transport layer in OLED.With G0 as ETL in a dithienyl-benzothiadiazole(DBT,variously abbreviated as DTBT and TBT)based PLED,the maximum luminous efficiency(LEmax)is increased by more than 300%.It should be pointed out that the alkane solution processed electron transport layers are not competing with the more popular water/alcohol solution processed electron transport layers.In fact,they are also orthogonal to each other,the three-dimension “solvent orthogonality” relationship is developed.As a result,more exquisite multilayer devices could be fabricated when combining the alkane with water/alcohol solution processed electron transport layers.A BN-embedded polycyclic aromatic compound BN-dibenzotetrathienocoronene(BN-DBTTC),is utilized in the application of organic photovoltaic devices for the first time.The BN-DBTTC with a saddle-shape ? system is added to PTB7:PC71BM device to control the bulk heterojunction morphology and promote stability by suppressing the crystallization of PCBM.After BN-DBTTC is introduced,the domain size is reduced from ~ 200 nm to well below 20 nm.At the weight ratio of PTB7:PC71BM:BN-DBTTC = 8:3:12 wt,the PCE of PTB7:PC71BM device is enhanced from 3.91% to 4.75%,and the PCE degradation is reduced from 47.8% to 26.7% after 596 days storage.BN-DBTTC is better than 1,8-diiodooctane as the PCBM's aggregation inhibitor in terms of process simplicity,because as photoactive material,BN-DBTTC doesn't need to be removed after processing.In addition,we insert a thin layer of aliphatic amine based conjugated polyelectrolyte at the interface of organic heterojunction,to reveal the electron injection machanism of aliphatic amine groups.In such structure,the interface dipoles resulting from the aliphatic amine groups of both sides cancel each other.Our experiments on the organic heterojunction devices with neutralized dipole reveal that the Interface Dipole Model can't fully explain the electron injection ability of the aliphatic amine based conjugated polyelectrolyte.During the operation of the OLEDs,the aliphatic amine groups trap a huge amount of holes,bending the energy level and decreasing the electron tunneling distance.As a result,the reduced electron tunneling distance facilitates the electron injection.