Decoding the Polymer p–n Junction: Controlled Dedoping and Reverse Bias Electroluminescence
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The polymer light-emitting electrochemical cell (PLEC) is a unique solid-state device possessing attractive attributes for low-cost applications, but also a junction structure that is still poorly understood. In a PLEC, the applied voltage causes in situ electrochemical p- and n-doping of the semiconducting polymer and the formation of a dynamic light-emitting p–n junction. Once the junction is fixed by cooling or chemical manipulation, the “frozen-junction” PLEC exhibits a unipolar electroluminescence (EL) and photovoltaic response. Repeated thermal cycling, however, can cause the frozen-junction PLEC to experience drastically enhanced EL under forward bias and the emergence of reverse bias EL. In this study, a combination of transport measurements and direct imaging is used to elucidate the origin of the mysterious reverse bias EL. A model is developed that explains the reverse bias EL as caused by the tunnel injection of electrons and holes from bandgap states into a dedoped “intrinsic” region between the p- and n-doped regions. The model explains the location, relative intensity, and evolution of EL under both forward and reverse bias. The results hint at a junction that is much narrower than previously resolved.