Improved Hole Injection in Hybrid Light‐Emitting Transistors Incorporating Lithium and Copper(II) Poly(Styrene Sulfonate)

Light‐emitting transistors (LETs) are optoelectronic devices that perform switching and light‐emitting functions in a single device. Hybrid LETS (HLETs) using inorganic metal oxide semiconductors as the transport layer with organic emissive layers and hole‐injection layers (HILs) combine the excellent switching performance of metal oxides with the flexibility and tunability of organic semiconductors. However, the efficiency of n‐HLETs typically suffers from unbalanced electron and hole injection. To overcome this issue, two hybrid polyelectrolytes—lithium poly(styrene sulfonate) (Li:PSS) and copper(II) poly(styrene sulfonate) (Cu:PSS)—are investigated as HILs in HLETs. HLETs employing Cu:PSS interlayers exhibit significantly enhanced brightness values of up to 4.89 × 10 3 cd m −2 and an external quantum efficiency (EQE) of 0.45%, compared to HLETs without HIL (no emission) and pristine poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (2.17 × 10 2 cd m −2 with an EQE of 0.01%). To understand how the HILs influence the performance, ultraviolet photoelectron spectroscopy (UPS) analysis and photoluminescence (PL) quenching studies are performed, which reveal improved energy band structure and reduced quenching using metal:PSS HILs. This work provides useful information about the function that polyelectrolyte HILs perform in HLET devices which may be exploited to develop new materials and applied in other types of optoelectronic devices.