Photomultiplication in Disordered Unipolar Organic Materials

Abstract

Photomultiplication in conventional inorganic semiconductors has been known and used for decades, the underlying mechanism being multiplication by impact ionization triggered by hot carriers. Since neither carrier heating by an electric field nor avalanche multiplication are possible in strongly disordered organic solids, charge multiplication seems to be highly unlikely in these materials. However, here the photomultiplication observed in the bulk of a unipolar disordered organic semiconductor is reported. The proportion of extracted carriers to incident photons is experimentally determined to be in excess of 3000 % in a single‐layer device of the air‐stable, n‐type organic semiconductor F16CuPc (Pc: phthalocyanine). This effect is explained in terms of exciton quenching by localized charges, the subsequent promotion of these detrapped charges to the high‐mobility energy band of the density‐of‐states (DOS) distribution, and subsequent slow equilibration within this broad intrinsic DOS. Such a mechanism allows multiple replenishment of the optically released charge by mobile carriers injected from an Ohmic electrode. Also shown is photomultiplication in double‐layer devices composed of layers of donor and acceptor small‐molecule materials. This result implies that, apart from exciton dissociation at a donor/acceptor interface, exciton energy transfer to trapped carriers is a complementary photoconductivity process in organic solar cells. This new insight paves the way to cheap, highly efficient organic photodetectors on flexible substrates for numerous applications.

A Novel Gas Sensor Transducer Based on Phthalocyanine Heterojunction Devices

“…In SCLC regime, the bulk properties of material are dominating the contact effects. In relation to the perfluorinated phthalocyanine, and considering that Al forms Ohmic contacts with Ni(F 16 Pc), the lack of rectification currents suggests that Au also forms an Ohmic contact with Ni(F 16 Pc), in non-oxygenated atmospheres, as previously stated by different authors [21, 22, 34]. Thus, the dependence of activation energy does not follow terms of SCLC regime, as a possible result of the interaction of the molecule with Al atoms, as demonstrated in the case of Cu(F 16 Pc) [35].…”

Influence of injected charge carriers on photocurrents in polymer solar cells

“…Such a photoenhanced injection was observed in some organic devices [18][19][20][21] and is known as photomultiplication.…”

Solar cells utilizing small molecular weight organic semiconductors

“…Apart from non-zero R s and finite R p , some common deviations from an ideal diode observed for organic heterojunctions are the following: R p dependent on light intensity [28][29][30] as R p ∝ P −1 0 , R s sometimes decreases with increased light intensity causing a crossing of the light and dark curves at positive current densities, 31 n > 2, a "kink" in the J-V curve near open-circuit, [31][32][33] and a voltagedependent photocurrent. 28,29,32 The inverse and linear dependence of R p on P 0 suggests an increase in free charge carriers in the bulk produced either as a result of exciton annihilation transferring energy to trapped charges, 28,34 or from photoconductive gain, as these processes are dependent on exciton density which is directly proportional to P 0 . The reduced R s which has been observed under light illumination likely comes from photoconductive gain in the organic film which decreases the resistance to carrier flow through the bulk film.…”