Thickness dependent effects of an intermediate molecular blocking layer on the optoelectronic characteristics of organic bilayer photovoltaic cells

Abstract

In this work, we address the microscopic effects related to the implementation of a bathophenanthroline (BPhen) exciton blocking layer (EBL) sandwiched between Ag cathode and molecular diindenoperylene (DIP)/C60 bilayer of a photovoltaic cell. Complementary studies of current density, external quantum efficiency, and photoluminescence quenching for EBL thicknesses up to 50 nm indicate that Ag atoms are able to penetrate through the whole 35 nm thick C60 film into the polycrystalline DIP layer underneath, thereby enhancing exciton quenching if no blocking layer is applied. In contrast, an optimal trade-off between exciton blocking, suppression of metal penetration, and electron transport is achieved for a 5 nm thick BPhen layer yielding an improvement of power conversion efficiency by more than a factor of 2.

Excited-State Dynamics of Diindenoperylene in Liquid Solution and in Solid Films

“…However, external quantum yield spectra for DIP− C 60 bilayer OPVs suggest that photoexcitation of the electron acceptor C 60 , plays an important role in generating photocurrent in these devices. 75,76 This is consistent with our spectroscopic results showing that most photoexcited DIP molecules relax back to the ground state within a few hundred picoseconds. Photoexcited C 60 molecules, on the other hand, can undergo rapid intersystem crossing to form long-lived triplet excitons with diffusion lengths exceeding 30 nm.…”

Exciton interaction with a spatially defined charge accumulation layer in the organic semiconductor diindenoperylene

“…Besides its exceptional exciton diffusion properties in correlation with a long-range ordered thin film structure, which are necessary for the excitons to reach the accumulated charges, this compound has already proven successfully in OTFT applications as well as in photovoltaic cells. 10,11,12,13 Furthermore, the low optical absorption of DIP proves beneficial for the intended studies on exciton-charge carrier interaction since it avoids pronounced quenching contributions by bimolecular recombination.Thin film transistors were prepared via a five-step shadow mask process in bottom-gate topcontact geometry as described by Klauk et al14 Illustrated in Fig. 1 a), a 60 nm thick aluminium film comply with the condition d DIP < /4 and therefore, interference effects can be disregarded.…”

Understanding the Role of Phenanthroline as Interlayer in Bulk Heterojunction Organic Photovoltaic Cells

“…Organic interlayers consisting of n‐type materials, such as 4,7‐diphenyl‐1,10‐phenanthroline (bathophenanthroline or BPhen), and 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (bathocuproine or BCP), thermally deposited between the electron acceptor fullerene and the metallic electron collecting electrode (low workfunction electrode), have often demonstrated to be effective at improving the performance of small molecules‐based bilayer heterojunction solar cells . BCP and BPhen were also used as electron transporting layers (ETL) in organic light‐emitting devices (OLEDs) because they combine transparency in the visible spectrum (energy gaps of about 3.5 eV) with high electron mobilities (∼10 −4 cm 2 V −1 s −1 in BPhen and ∼10 −6 cm 2 V −1 s −1 in BCP).…”

“…The performance improvements achieved in bilayer heterojunction PV devices upon introducing BPhen or BCP layers were attributed to the suppression of metal‐induced trap states caused by the penetration of the atomic metal into the organic layers (i. e. buffer effect) combined with exciton and hole blocking, resulting from the mismatch between the frontier energy levels of the phenanthroline layer, the C 60 acceptor, and the low workfunction metallic electrode . Typically, the optimization of bilayer heterojunction cells performance is achieved with BPhen and BCP layers few nanometers thick ( ca .…”