Optimizing ionic strength of interfacial electric double layer for ultrahigh external quantum efficiency of photomultiplication-type organic photodetectors

Abstract

A synthetic approach for engineering an electric double layer (EDL)–photoactive layer interface in photomultiplication-type organic photodetectors (PM-OPD), whereby the EDL is strategically embedded between a transparent cathode and the photoactive...

Boosting the Performance of Photomultiplication‐Type Organic Photodiodes by Embedding CsPbBr3 Perovskite Nanocrystals

“…where J is the current density, q is the elementary charge, and V is the applied voltage. [6,15,19,48] This equation can be further simplified for a sufficiently high applied positive bias as follows:…”

“…To date, most PM-OPDs reported in the literature exhibit high external quantum efficiencies (EQEs) of more than 10000%, which are related to photoconductive gain. [5,6,[15][16][17][18][19][20][21][22][23] The term "gain" is defined as an index that describes the number of conducting charges generated by a single incident photon. Recent reports regarding the generation of intrinsic and extrinsic gains show that the gain of a photoconductor with a majority of hole carriers can be expressed as follows:…”

“…where 𝜏 r is the minority carrier recombination lifetime, 𝜏 transit is the carrier transit time, Δp and Δn are the excess hole and electron densities, respectively, and μ p and μ n are the hole and electron mobilities, respectively. [5,6,24] This implies that intentionally created energetically and spatially efficient minority carrier (in this case, electrons) traps can lead to a longer carrier recombination lifetime (intrinsic gain), higher excess hole-toelectron concentration ratio (extrinsic gain), and higher hole-toelectron mobility ratio (extrinsic gain), which results in a high device gain and EQE. In conventional PM-OPDs with holes as the majority carriers, [15,[17][18][19][20][21][22][23] the photoactive layers primarily consist of an electron donor polymer, poly(3-hexylthiophene-diyl) (P3HT), which controls photon absorption, exciton formation, and charge carrier transportation, with a small amount of an electron acceptor molecule, [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM).…”

“…[5,6,24] This implies that intentionally created energetically and spatially efficient minority carrier (in this case, electrons) traps can lead to a longer carrier recombination lifetime (intrinsic gain), higher excess hole-toelectron concentration ratio (extrinsic gain), and higher hole-toelectron mobility ratio (extrinsic gain), which results in a high device gain and EQE. In conventional PM-OPDs with holes as the majority carriers, [15,[17][18][19][20][21][22][23] the photoactive layers primarily consist of an electron donor polymer, poly(3-hexylthiophene-diyl) (P3HT), which controls photon absorption, exciton formation, and charge carrier transportation, with a small amount of an electron acceptor molecule, [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM). Semiconductor-Al Schottky junctions are typically employed in the conventional device architecture of indium tin oxide (ITO)/hole transport layer/photoactive layer/Al.…”

“…However, the effective electron traps are located at the far end of the optical propagation pathway (semiconductor-Al interface); therefore, this structure is limited because the spectral shapes of the PM-OPD photoresponse are reversed compared to the absorption spectrum of the matrix donor polymer, and the EQE is restricted to ≈100000%. [15,[17][18][19][20][21][22][23] To address the challenges encountered by conventional PM-OPDs, we recently used the concept of "electrostatic interaction" by introducing a polyelectrolyte as an electric double layer (EDL) on top of the ITO cathode layer, [6] which enabled the formation of an ITO-semiconductor Schottky junction at an optically favorable position and induced electrostatic interactions between the exposed cations of the EDL and trapped electrons in the photoactive acceptor material domains at the EDL-photoactive layer interface. The resulting PM-OPD exhibited green-selective photoresponses, which resembled the absorption spectrum of the photoactive donor P3HT, and exceptionally high EQEs with an average value of ≈2000000% was achieved.…”