Interface Effect on Efficiency Loss in Organic Light Emitting Diodes with Solution Processed Emitting Layers

Abstract

easily achieved with sequential deposition, the redissolution of the preceding layer by solvents used in the subsequent layers is a challenge for solution processed OLEDs during the multilayer fabrication process. [ 4 ] To get around this problem, cross-linkable hole transporting layers (HTLs) are typically used such that subsequent deposition of the emitting layer (EML) would not damage the underlying HTL during processing. [5][6][7][8][9][10][11] To fi nish the device fabrication, evaporated electron transport layers (ETLs) are used in OLED fabrication to avoid solvent damages to the EML.Even with the same multilayer architecture, most OLEDs with a solution processed EML have lower effi ciencies than their thermal-evaporated counterparts. [12][13][14][15][16][17] One fundamental difference between evaporated and solution processed fi lms is the molecular packing. As summarized in Table S1 of the Supporting Information, [16][17][18][19][20][21][22][23][24][25][26] it is generally accepted that solution processed organic fi lms have a lower packing density than the vacuum deposited fi lms. Previous studies comparing solution processed and evaporated OLEDs were mostly focused on HTLs, where the packing density and hence carrier transport plays a critical role. [ 18,20,25,26 ] On the other hand, there are only a few reports directly comparing the performance of devices with solution processed and evaporated phosphorescent EMLs, [ 17,19 ] and the root cause for the difference in device performance is not well understood. It is, therefore, important to identify and study the factors determining the effi ciency loss mechanism in devices with a solution processed EML.In this work, we study the differences in high effi ciency OLEDs having solution processed and thermal-evaporated EMLs. Similar to other fi ndings, we found that the EQE of OLEDs with a solution processed EML is 22% lower than that of the devices with an evaporated EML using 1,3,5tris (N-phenylbenzimidazol-2,yl) benzene (TPBi) as an ETL. Interestingly, this difference in effi ciency became signifi cantly smaller as TPBi was replaced with bis -4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM) as an alternative ETL. Because of the deep highest occupied molecular orbital (HOMO) energy of B3PYMPM, we attribute the lower effi ciency in OLEDs with solution processed EMLs to the ineffi cient hole blocking properties of the TPBi ETL as revealed by the single carrier device results. A subsequent study on interfacial exciplex formation and energetic disorder revealed that for devices with a solution processed EML, band tail states broadening along with an energy level shift at the EML/ETL interfaces results in a higherThe performance of multilayered OLEDs with a solution processed emitting layer (EML) is compared to that of counterparts with an evaporated EML and it is found that the interfacial energy changes at the EML and electron transport layer (ETL) interface is a key factor determining the device effi ciency. From the results of exciplex photoluminesc...

Conversions from Normal to Abnormal Current‐Dependent ISC and from Abnormal to Normal Current‐Dependent RISC Processes in Exciplex‐Based OLEDs

“…Intersystem crossing (ISC) from singlet to triplet spin‐pair states, including polaron pairs (PP 1 and PP 3 ) and charge‐transfer states (CT 1 and CT 3 ), and their reverse ISC (RISC) in exciplex‐based organic light‐emitting diodes (EB‐OLEDs) can efficiently tune the singlet/triplet ratio and thus are important spin‐mixing processes for attaining high‐efficiency device performance. [ 1,2 ] However, few tool can directly probe these ISC and RISC processes previously. This is because the optoelectronic properties of OLEDs are usually affected by various physical microscopic processes of spin‐pair states in the device, such as ISC, RISC, singlet fission (S 1 + S 0 → T 1 + T 1 ), triplet fusion (T 1 + T 1 → S 1 + S 0 ), and triplet‐charge annihilation (TQA) processes.…”

Highly Efficient Organic Light‐Emitting Diode Using A Low Refractive Index Electron Transport Layer

“…As it can be seen in Figure D, we observe a noticeably smaller efficiency roll‐off at higher luminance levels for Device B, implying a better charge balance resulting from a more effective hole blocking under higher biases . We have previously shown that a deeper HOMO for an effective hole‐blocking ETL is required for solution processed EMLs due to a wider band‐broadening and the energetic disorder at the EML/ETL interface . As we have shown in Figure D, the EQE of Device A reduces to 5% at a high luminance of 2000 cd m −2 from a maximum EQE value of 12%, showing a large efficiency roll‐off of 58%.…”

Improved thin film stability of differently formulated, printed, and crosslinked polymer layers against successive solvent printing

“…However, some challenges related to the IJP OLED technology have to be overcome. For example, intermixing of successive solution processed layers resulting in the damage of previously deposited layers was shown to happen and to be hard to prevent . Such undefined interfaces are critical for OLED performance .…”