We explore the dependence of the dark current of C(60)-based organic photovoltaic (OPV) cells on molecular composition and the degree of intermolecular interaction of several molecular donor materials. The saturation dark current density, J(S), is an important factor in determining the open circuit voltage, V(oc). The V(oc) values of OPVs show a strong inverse correlation with J(S). Donor materials that show evidence for aggregation in their thin-film absorption spectra and polycrystallinity in thin film X-ray diffraction result in a high dark current, and thus a low V(oc). In contrast, donor materials with structures that hinder intermolecular pi-interaction give amorphous thin films and reduced values of J(S), relative to donors with strong intermolecular pi-interactions, leading to a high V(oc). This work provides guidance for the design of materials and device architectures that maximize OPV cell power conversion efficiency.

Organic light-emitting diodes (OLEDs) have been the subject of a significant research effort for the past two decades with a focus on devices that emit almost exclusively in the visible part of the electromagnetic spectrum.[1] Recently, there has been a growing interest in OLEDs that emit in the nearinfrared (NIR) region (700-2500 nm). [2][3][4] Applications for these NIR OLEDs are particularly interesting for nightvision-readable displays [5] and sensors.[6] The efficiency of OLEDs are markedly improved when fluorescent emissive dopants are replaced with phosphorescent heavy-metal complexes that can effectively harvest both the singlet and triplet excitons formed in electroluminescence, with wavelengths (l) ranging from the near-ultraviolet into the red (with peak emission at l max = 380-650 nm).[7-10] Herein, we report on an efficient NIR OLED that utilizes a phosphorescent Ptmetalloporphyrin dopant, with an external quantum efficiency (EQE) greater than 6 % at l max = 765 nm and a full width at half maximum of 31 nm (500 cm À1 ). Previously, two classes of phosphorescent complexes have been employed as dopants in NIR-emitting OLEDs. The first utilizes trivalent lanthanide cations (Ln 3+ ) as the emitting centers, for example, Er 3+ or Nd 3+ , chelated with chromophoric ligands to sensitize excitation-energy transfer to the lanthanide ion.[11] Schanze et al. have reported an NIR OLED utilizing Ln 3+ in conjunction with a porphyrin/polystyrene matrix, with EQE ranging from 8.0 10 À4 to 2.0 10 À4 % at approximately 1 mA cm À2 . [5] Similarly, a Nd(phenalenone) 3 -based OLED had an EQE of 0.007 % at l max = 1065 nm.[4] The second class of NIR OLEDs is transition-metal complexes, similar to those used in the visible region. A recent report of an electrophosphorescent device that used a cyclometalated [(pyrenyl-quinolyl) 2 Ir(acac)] complex as the phosphor gave l max = 720 nm and an EQE of 0.1 %. [6] A family of complexes that have shown intense absorption and emission in the red-to-NIR region of the spectrum are the metalloporphyrins. [12,13] There are a number of reports of OLEDs fabricated with [Pt(oep)], [Pt(tpp)] (oep = 2,3,7,8,12,13,17,18-ocatethylporphryin, tpp = 5,10,15,20-tetraphenylporphyrin), or analogues of these compounds as phosphorescent emitters, with emission maxima between 630 and 650 nm, [7,[14][15][16][17][18][19][20][21][22] however, there has been no apparent effort to shift the Pt-porphyrin-based OLED emission into the NIR region. Porphyrin chromophores with fused aromatic moieties at the b-pyrrole positions, for example, tetrabenzoporphyrin (bp), exhibit a bathochromic shift (relative to unsubstituted porphyrin) of the absorption and emission energy, owing to the expansion of the p-electronic system of the porphyrin core.[23] The addition of bulky groups to the meso positions of the porphyrin macrocycles with b-substituted pyrroles leads to the formation of nonplanar porphyrins, and further red-shifts the absorption spectra.[24] Coordination of a heavy-metal atom increases the rate of the int...

A new thermally cross-linkable copolymer containing reactive benzocyclobutene (BCB) units and the well-known hole-transporting moiety N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine) (TPD) was synthesized and characterized. Thermal annealing of spun-cast films of the copolymer, followed by crosslinking at 200 °C, led to insoluble polymer films with a smooth surface. Green emitting fluorescent OLEDs were fabricated using the new cross-linkable polymer and compared with conventionally prepared devices. Using the thermally cross-linked copolymer as a hole-transporting layer, solution processed multilayer light emitting diodes were prepared that exhibited high performance with 10.4% external quantum efficiency at a brightness of 350 cd/m 2 .

The triplet annihilation dynamics of near infrared organic light-emitting devices are studied with peak electrophosphorescence at a wavelength of 772nm using a platinum-porphyrin derivative Pt(II)-tetraphenyltetrabenzoporphyrin as dopant. Both the photoluminescent decay transients of the thin films and the quantum efficiency versus current density characteristics of devices using tris(8-hydroxyquinoline) aluminum or 4,4′-bis(N-carbazolyl)biphenyl (CBP) as hosts are fitted by a model based on triplet-triplet annihilation. When the phosphor is codoped with Ir(III) bis(2-phenyl quinolyl-N,C2′) acetylacetonate in CBP, the quantum efficiency is enhanced, and the observed decrease of efficiency at high current densities is explained by field-induced charge pair dissociation. The external quantum efficiency has a maximum of (8.5±0.3)%, decreasing to (5.0±0.3)% at 1mA∕cm2.