Enhanced exciton diffusion in an organic photovoltaic cell by energy transfer using a phosphorescent sensitizer

Abstract

We demonstrate enhanced exciton diffusion in an organic photovoltaic cell through the incorporation of a phosphorescent sensitizer. The increase in exciton diffusion length (LD) is realized using a composite electron donor layer consisting of a N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPD) host doped with the phosphorescent guest fac-tris(2-phenylpyridine) iridium [Ir(ppy)3]. The presence of the phosphor at low concentration allows for the population of the long-lived NPD triplet state and an increase in LD. An increase in the NPD LD from 6.5±0.3 to 11.8±0.6 nm is extracted from measurements of the external quantum efficiency for donor layers containing 5 wt % Ir(ppy)3. This enhancement leads to a ∼80% improvement in the power conversion efficiency relative to devices containing an undoped donor layer.

Generalized Kinetic Monte Carlo Framework for Organic Electronics

“…Doping of a BHJ with nanoparticles can increase the effective exciton lifetime and thus improve the power conversion efficiency [24,25,65,66]. Furthermore, in phosphorescent OLEDs, organometallic complexes are used to improve the efficiency.…”

“…Most kMC models either account for singlet [10,[17][18][19] or triplet exciton dynamics [20][21][22][23] for the use in OSCs or OLEDs, respectively. However, the interplay between singlets and triplets can be used to modify exciton lifetimes and diffusion lengths, which is a promising way to tailor efficiencies in organic devices and therefore requires a better understanding [24][25][26].…”

Role of electronic localization in the phosphorescence of iridium sensitizing dyes

“…More recently, the strong intersystem-crossing mechanism that take place in these complexes have been exploited to design organic solar cells (OSCs) of improved power conversion efficiency [3,4]. In fact, in organic heterojunctions, sensitizing transition-metal complexes are utilized as promoters in converting photogenerated singlet excitons into long-lived triplet excitons, thereby increasing the probability for an exciton to diffuse to a donor-acceptor interface and dissociate into collectible charges [3,4].…”

“…In fact, in organic heterojunctions, sensitizing transition-metal complexes are utilized as promoters in converting photogenerated singlet excitons into long-lived triplet excitons, thereby increasing the probability for an exciton to diffuse to a donor-acceptor interface and dissociate into collectible charges [3,4]. In a recent study [3] it was reported that incorporating Ir(ppy) 3 (in concentrations as low as few mass percents) into electron-donating polymers nearly doubled the photovoltaic efficiency of the considered organic heterojunctions.…”

“…In fact, in organic heterojunctions, sensitizing transition-metal complexes are utilized as promoters in converting photogenerated singlet excitons into long-lived triplet excitons, thereby increasing the probability for an exciton to diffuse to a donor-acceptor interface and dissociate into collectible charges [3,4]. In a recent study [3] it was reported that incorporating Ir(ppy) 3 (in concentrations as low as few mass percents) into electron-donating polymers nearly doubled the photovoltaic efficiency of the considered organic heterojunctions. * present adress: Nuclear Engineering and Materials Science, Universitat Politècnica de Catalunya, Barcelona, Spain These remarkable experimental achievements motivate further theoretical effort to understand the optical properties of transition-metal complexes and ultimately optimize their sensitizing functions taking advantage of their almost unlimited chemical versatility, i.e., modifying the transition-metal center and attached chromophores.…”