Exciton Mobility in Organic Photovoltaic Heterojunctions from Femtosecond Stimulated Raman

Exciton mobility is crucial to organic photovoltaic (OPV) efficiency, but accurate, quantitative measures and therefore precise understanding of this process are currently lacking. Here, we exploit the unique capabilities of femtosecond stimulated Raman spectroscopy (FSRS) to disentangle the signatures of the bulk and interfacial donor response in a bulk heterojunction composed of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and phenyl-C61-butyric acid methyl ester (PCBM). Surprisingly, we find that donor excitons are very mobile for the first ∼300 fs following excitation (before thermalization) even though their overall lifetime is significantly longer (170 ps). A sharp decrease in mobility occurs after the system relaxes out of the Franck-Condon (FC) region. From this observation we predict that any polymer lacking a significant resonance Raman effect and fluorescence Stokes shift, indicating slow FC relaxation and small reorganization energy, will make an efficient OPV material.

Reducing exciton binding energy by increasing thin film permittivity: an effective approach to enhance exciton separation efficiency in organic solar cells

Photocurrent generation in organic solar cells requires that excitons, which are formed upon light absorption, dissociate into free carriers at the interface of electron acceptor and donor materials. The high exciton binding energy, arising from the low permittivity of organic semiconductor films, generally causes low exciton separation efficiency and subsequently low power conversion efficiency. We demonstrate here, for the first time, that the exciton binding energy in B,O-chelated azadipyrromethene (BO-ADPM) donor films is reduced by increasing the film permittivity by blending the BO-ADPM donor with a high dielectric constant small molecule, camphoric anhydride (CA). Various spectroscopic techniques, including impedance spectroscopy, photon absorption and emission spectroscopies, as well as X-ray spectroscopies, are applied to characterize the thin film electronic and photophysical properties. Planar heterojunction solar cells are fabricated with a BO-ADPM:CA film as the electron donor and C60 as the acceptor. With an increase in the dielectric constant of the donor film from ∼4.5 to ∼11, the exciton binding energy is reduced and the internal quantum efficiency of the photovoltaic cells improves across the entire spectrum, with an ∼30% improvement in the BO-ADPM photoactive region.

Near-infrared azadipyrromethenes as electron donor for efficient planar heterojunction organic solar cells

We report the use of three solution processable azadipyrromethene (ADPM) based compounds, i.e., ADPM, B,F(2)-chelated ADPM (BF2-ADPM), and B,O-chelated ADPM (BO-ADPM), as electron donors in planar heterojunction solar cells. These small molecules possess exceptional light harvesting capability with high extinction coefficients (~1 × 10(5) M(-1) cm(-1) in solutions) and broad absorption spectra up into the near-infrared region. Planar heterojunction organic solar cells, consisting of a solution processed electron donor layer and a vapor deposited C(60) acceptor layer, have been constructed to give power conversion efficiencies of 0.56, 0.69, and 2.63% for ADPM, BF2-ADPM, and BO-ADPM, respectively, under AM 1.5G simulated 1 sun solar illumination. A high open circuit voltage (V(oc)) of ~0.8 V was achieved for the BO-ADPM/C(60) device, which is among the highest reported values for organic solar cells with photocurrent generation in the near-infrared region beyond 1.5 eV.