Photoelectrical properties, energy level spectra, and photogeneration mechanisms of pentacene

Abstract

The main optical, dark and photoconductivity, photo‐emf, and photoemission characteristics are studied for thin‐layers of pentacene (Pc) crystals. It is shown that in the h = = 2.2 to 4.0 eV spectral region intrinsic photogeneration is dominant in Pc crystals, presumably proceeding via an autoionization mechanisms with a threshold value Ec = = (2.20 ± 0.05) eV. Whereas in the hvv = 1.4 to 2.2 eV spectral region an exciton photogeneration mechanism, namely, generation of excess charge carriers through triplet exciton‐trapped hole interaction, seems most probably. The energy level spectra of neutral and ionized states of the Pc crystal are determined. The Pc crystal ionization energy Ic equals to (5.07 ± 0.05) eV; the forbidden energy gap ΔE0 = (2.20 ± 0.05) eV; the intrinsic dark conductivity activation energy E a0 = ΔE0/2 = (1.15 ± 0.15) eV. The electron affinity Ac of the Pc crystal, determined by three independent methods equals to (2.87 ± 0.10) eV, (2.70 ± 0.15) eV, or (2.73 ± 0.2) eV, respectively. The characteristic energies of the quasi‐continuous trap distribution for vacuum evaporated thin Pc crystals layers are kTc = 0.12 eV in an exponential approximation and ± = 0.28 eV in a Gaussian one as determined from SCLC measurements.

Amorphouslike Density of Gap States in Single-Crystal Pentacene

“…Therefore, we must compare a variety of optical measurements in this spectral region to distinguish the exciton effects from the absorption due to the localized band-tail states. Figure 1 shows our photocurrent yield spectrum along with that of Silinsh et al [9], VOLUME 93, NUMBER 8 P H Y S I C A L R E V I E W L E T T E R S which agrees remarkable well above the 2.2 eV HOMO-LUMO gap (no adjustable parameters), but is dramatically different in the range 1.5 to 2.2 eV. Both samples were purified by two-or threefold vacuum sublimation before the final growth, so it is unlikely that the spectral differences are due to impurities.…”

“…In pentacene the singlet excitons at 1.82 and 1.94 eV dominate the optical absorption spectrum [9,11,12] and rapidly decay with a rate of 1:3 10 13 sec ÿ1 into pairs of 0:8 eV triplet excitons [13]. The optical decay of triplet excitons is forbidden by selection rules, leaving nonradiative decay paths such as interacting with paramagnetic molecules [14], free carriers [14], or trapped carriers [9,10]. The photocurrent yield therefore depends on the relative strengths of the triplet decay paths that emit trapped carriers versus those that emit only vibrational energy.…”

“…To explain these effects we need to briefly review some pertinent facts about excitons in organic materials. Excitons in organic materials play a complex role in photoconductivity [8][9][10] compared to inorganic materials where they typically contribute directly at room temperature as electrons and holes separated by thermal excitation and electric fields. The large exciton binding energies in organics effectively eliminate direct dissociation under most conditions so that excitons must follow a more complex series of decay paths to create photoconductivity.…”

Bias-Dependent Generation and Quenching of Defects in Pentacene

“…We observe a defect in pentacene single crystals that can be generated by bias-stress and exhibits metastable effects in space-charge-limited current (SCLC) IV curves. The defect concentration recovers to zero-field equilibrium at room temperature in the dark in more than an hour, but in less than 1 s if illuminated with photons above the band gap (2.2 eV [9]). Defect reactions of this type have been observed in inorganic semiconductors [10], but at significantly higher temperatures or with optical excitation.…”

Performance Characteristics of Organic Photovoltaic Cells using Pentacene as a Hole Conducting Layer Material

“…Various researchers reported on organic photovoltaic applications of pentacene as a dopant into a hole transport layer [10], an interlayer for polymer bulk-heterojunction photovoltaic cells [11], and a donor material for p-n junction photovoltaic cells [12][13][14][15][16][17] because of a high mobility, long exciton diffusion length and well suited absorption spectrum in the solar spectrum [18][19][20][21]. Moreover, it was reported that the performance of polymer schottky diode was improved by poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)-pentacenelayer [22,23].…”