Cascade Organic Solar Cells

Abstract

We demonstrate planar organic solar cells consisting of a series of complementary donor materials with cascading exciton energies, incorporated in the following structure: glass/indium-tin-oxide/donor cascade/C 60 /bathocuproine/Al. Using a tetracene layer grown in a descending energy cascade on 5,6-diphenyl-tetracene and capped with 5,6,11,12-tetraphenyl-tetracene, where the accessibility of the π-system in each material is expected to influence the rate of parasitic carrier leakage and charge recombination at the donor/acceptor interface, we observe an increase in open circuit voltage (V oc ) of approximately 40% (corresponding to a change of +200 mV) compared to that of a single tetracene donor. Little change is observed in other parameters such as fill factor and short circuit current density (FF = 0.50 ( 0.02 and J sc = 2.55 ( 0.23 mA/cm 2 ) compared to those of the control tetraceneÀC 60 solar cells (FF = 0.54 ( 0.02 and J sc = 2.86 ( 0.23 mA/cm 2 ). We demonstrate that this cascade architecture is effective in reducing losses due to polaron pair recombination at donorÀacceptor interfaces, while enhancing spectral coverage, resulting in a substantial increase in the power conversion efficiency for cascade organic photovoltaic cells compared to tetracene and pentacene based devices with a single donor layer.

8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer

“…2). As mentioned above, because SubNc and SubPc have similar LUMO energy levels, a decrease in V OC is circumvented, contrary to previously reported cascade structures 21,27 . This results in an improved performance for the three-layer energy cascade structure, with the best performing device generating a PCE of 8.4%, surpassing the current state-of-the-art evaporated single-junction OPV devices 37,38 .…”

“…However, this is at the expense of the voltage generated by the solar cell, as the V OC is ultimately limited by the energy levels of the outer layers in such a cascade structure. Alternatively, in an energy-relay cascade structure, excitons in a large-bandgap donor material are transferred by interlayer exciton energy transfer to a smallerbandgap donor, and subsequently dissociated at the interface with the acceptor 27,28 . In this device architecture, a broad part of the solar spectrum can be covered by combining materials with complementary optical properties, and, significantly, a decrease in V OC can be avoided by aligning the highest occupied molecular orbital (HOMO) energy levels of the multiple donor materials.…”

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

“…The performances of the CPB and CTB based cells are significantly higher than previously published using these dipolar donors. 13 This difference is attributed to the higher purity of CTB and CPB source materials resulting additional purification by train sublimation, and the use of the BPhen:C 60 cathode buffer layer that improves the electron collection at the active layer/cathode interface 11 .…”

“…Using these dipolar donors, we obtain a maximum power conversion efficiency of 9.6 ± 0.5 % under 1 sun, AM1.5G simulated illumination for an OPV comprised of an active layer containing a dipolar donor mixed with C 70 . INTRODUCTION Paths to improving energy generation in organic photovoltaic (OPV) cells include blending donor (D) and acceptor (A) molecules in the active region to promote efficient exciton dissociation, 1-3 incorporating exciton blocking layers adjacent to the anode and/or cathode to prevent exciton quenching at the electrodes, 4-9 and using donor 10,11 or acceptor 12 energy cascades to drive exciton transfer toward a dissociating interface. These strategies are designed to increase exciton harvesting by reducing exciton recombination while also increasing charge conduction away from the D-A heterojunctions.…”

Recent Developments in Quantum Dots/CNT Co-Sensitized Organic Solar Cells

“…Various sensitizers based on low band gap polymers [55][56][57][58][59][60][61][62][63], small molecules [64][65][66], dyes [67][68][69] and nanoparticles [13][14][15][16][17][18][19][20][21] are employed in the structure of the photoactive layer in combination with donor material which is usually consists of a large bandgap polymer and fullerene derivative as acceptor. Recently, the record PCEs over 12% have been reported for the PBDB-T:IT-M: BisPC 70 BM (1:1:0.2) ternary solar cells with a Jsc=17.3 mA/cm², Voc=0.95 V and FF=74% [42], which demonstrate the potential of ternary sensitization concept for OPVs' upscaling.…”