Barrier‐Free Electron–Hole Capture in Polymer Blend Heterojunction Light‐Emitting Diodes

Abstract

Organic semiconductors fabricated as thin-film light-emitting diodes, LEDs, now provide a promising new display technology.[1] Solution-processed semiconductor polymers make possible direct printing (using ink-jet deposition) and allow high-resolution full-color displays to be conveniently manufactured. [2] Multiple-layer deposition, used in vacuum-sublimed molecular semiconductor LEDs, is difficult to achieve by solution processing. We have instead fabricated distributed heterojunction' structures that are formed by de-mixing of two polymers co-deposited from common solution. We have used hole-accepting and electron-accepting derivatives of polyfluorene, and have optimized these structures to achieve high-efficiency diodes (above 19 lm W ±1 for green emission) that operate at very low voltages (100 cd m ±2 at 2.1 V for green emission). This very low voltage operation is achieved because electron±hole capture across the heterojunction is arranged to be a barrier-free process to form an interface state (termed an exciplex) that has significant charge-transfer character and is lower in energy than the charge-separated state. With respect to the bulk exciton, the exciplex is red-shifted (here between 140 and 360 meV) and its radiative lifetime is strongly increased (between 68 and 118 ns at low temperatures). The barrier for thermal excitation of the exciplex to allow it to move away from the heterojunction is small (100± 250 meV), and this process can give efficient bulk exciton emission at room temperature. The heterojunction formed between dissimilar organic semiconductors is generally found to be remarkably free of gap-states and other defects that would otherwise compromise semiconductor device operation. Heterojunction LEDs are designed so that the offsets between conduction and between valence band edges are type II' and electrons and holes accumulate on opposite sides of the heterojunction (Fig. 1). In a non-interacting electron scheme, type II heterojunctions would destabilize an exciton present in either semiconductor, since the exciton state would be higher in energy than the charge-separated state. However, organic semiconductors are low dielectric constant materials (typically having values less than 4) so that the coulomb interaction between electron and hole gives a substantial exciton binding energy (of order 0.5 eV). When this binding energy is larger than the band-edge offsets, excitons are stable at the interface. By selecting semiconductors with larger band-edge offsets, charge separation at the heterojunction can be readily achieved, giving efficient photovoltaic behavior. [3] LEDs made using molecular semiconductors are generally fabricated as multiple-layer heterojunction structures by successive vacuum sublimation steps.[1] However, with solutionprocessed polymers it is possible to make distributed heterojunction' diodes by de-mixing of two polymers spin-coated from common solution. [4] This is an obviously desirable structure for photovoltaic diodes, because it allows excitons photogenerated...

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

“…Morteani et al have studied polymer:polymer blends and have shown with temperaturedependent measurements that energy transfer from the CT state dominates below 4V, with an endothermic activation energy of 200 meV ± 50 meV. [51][52][53] Since all of the presented EL spectra have been measured at voltages smaller than 4 V, this would suggest that the principal mechanism is energy transfer from the CT state to a component singlet. The activation energy of 200 meV corresponds to about twice the width of the density of states (DOS) of an organic semiconductor, 54 which may suggest that singlet emission is linked to an overlap of CT state emission and singlet absorption.…”

Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

“…This loss mechanism, however, can be reduced by reducing the electronic coupling matrix element V * by, for example, increasing the distance between donor and acceptor molecule. However, decreasing V * will unavoidably result in a decrease in electron transfer rate, which is proportional to (V * ) 2 . 32 The analysis presented above allows experimental determination of V * , which we find is ∼6 meV for the TQ1:PCBM system studied in this work.…”

“…32 The analysis presented above allows experimental determination of V * , which we find is ∼6 meV for the TQ1:PCBM system studied in this work. Assuming that this is a typical value for polymer:fullerene systems, a lower limit for the ratio between the molar extinction coefficient of CT absorption and pure polymer absorption can be calculated: Assuming intensity borrowing as the only cause for CT absorption, a ratio α CT /αD * = (V * ) 2 (E D * −E CT ) 2 between 10 −2 and 10 −4 can be calculated for E D * − E CT between 0.06 and 0.6 eV, typical energy offsets for organic photovoltaic applications. For finely intermixed systems, given a sufficiently high sensitive technique, this means that CT absorption bands will always be detectable if E CT is lower than E D * and V * is sufficiently large.…”

“…Some polymer-polymer interfaces are quite efficient light emitters, with the emitted light originating from the interfacial excited state, often termed exciplex emission. [1][2][3][4] On the other hand, the polymer-fullerene interface, exploited extensively in organic solar cells, can exhibit efficient charge carrier generation, [5][6][7] but is, generally, a poor light emitter. [8][9][10][11] Theoretical and experimental determination of the electronic structure of such organic-organic interfaces can elucidate on the coupling between the relevant states, rate constants, and efficiencies of charge carrier generation and recombination.…”

Charge‐Carrier Density Independent Mobility in Amorphous Fluorene‐Triarylamine Copolymers

“…Fluorene-triarylamine copolymers are specifi cally designed to be amorphous, with high glass transition temperatures, for use as interlayer and hole transport layer materials in polymer light emitting diodes. [31][32][33][34][35][36][37] They have also been used, along with the homopolymer poly(triarylamine), as p-type semiconductors in OFETs, allowing highly uniform charge transport over large-area substrates, an important requirement for applications such as display backplanes. [ 38 ] Their charge transport has been extensively studied in bulk diodes using the time-of-fl ight (TOF) and transient SCLC dark injection (DI) techniques, [39][40][41][42] and modeled in the low chargecarrier density regime using the GDM, correlated GDM, and polaronic correlated GDM.…”