Jessica D. Douglas

Synthetic Control of Structural Order in N-Alkylthieno[3,4-C]pyrrole-4,6-Dione-Based Polymers for Efficient Solar Cells

The correlation between the nature of alkyl substituents on N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD)-based polymers and solar cell device performance has been investigated. After adjusting device parameters, these TPD-based polymers used with PC(61)BM provided photovoltaic responses ranging from 4.0% to 6.8%, depending on the size and shape of the alkyl solubilizing groups. Further, we have correlated the effect of the alkyl groups on the structural order and orientation of the polymer backbone using grazing incidence X-ray scattering analysis, and we have demonstrated how fine-tuning of these parameters can improve the power conversion efficiency.

Efficient charge generation by relaxed charge-transfer states at organic interfaces

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Efficient small molecule bulk heterojunction solar cells with high fill factors via pyrene-directed molecular self-assembly

Efficient organic photovoltaic (OPV) materials are constructed by attaching completely planar, symmetric end-groups to donor-acceptor electroactive small molecules. Appending C2-pyrene as the small molecule end-group to a diketopyrrolopyrrole core leads to materials with a tight, aligned crystal packing and favorable morphology dictated by π-π interactions, resulting in high power conversion efficiencies and high fill factors. The use of end-groups to direct molecular self-assembly is an effective strategy for designing high-performance small molecule OPV devices.

Long-Term Thermal Stability of High-Efficiency Polymer Solar Cells Based on Photocrosslinkable Donor-Acceptor Conjugated Polymers

Solution-processable polymer-based organic photovoltaics (OPVs) have attracted considerable attention over the past two decades because of the many advantages they can provide: lowcost fabrication, fl exible devices, and light-weight construction. [ 1 ] In the most successful OPV device architectures, the photoactive layer is composed of a blend of a p-type conjugated polymer and an n-type fullerene derivative, forming the socalled donor–acceptor bulk heterojunction (BHJ). [ 2 ]

On the Efficiency of Charge Transfer State Splitting in Polymer:Fullerene Solar Cells

The field dependence and yield of free charge carrier generation in polymer:fullerene blends with varying energetic offsets is not affected when the excitation energy is varied from above band-gap to direct CT state excitation. Instead, the ability of the CT state to split is dictated by the energetic offset between the relaxed CT state and the charge separated (CS) state.

Solution‐Processed, Molecular Photovoltaics that Exploit Hole Transfer from Non‐Fullerene, n‐Type Materials

J. D. Douglas, M. S. Chen, J. R. Niskala, O. P. Lee, A. T. Yiu, E. P. Young, J. M. J. Fréchet, Departments of Chemistry and Chemical and Biomolecular Engineering University of California, Berkeley Berkeley, CA, 94720–1460, USA E-mail: mschen@berkeley.edu J. D. Douglas, M. S. Chen, J. R. Niskala, O. P. Lee, A. T. Yiu, J. M. J. Fréchet, Materials Science Division Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA Dr. M. S. Chen Departments of Chemistry and Chemical and Biomolecular Engineering University of California Berkeley, Berkeley, CA 94720–1460, USA Prof. J. M. J. Fréchet King Abdullah University of Science and Technology (KAUST) Thuwal 23955–6900, Saudi Arabia E-mail: jean.frechet@kaust.edu.sa

Improving the long-term stability of PBDTTPD polymer solar cells through material purification aimed at removing organic impurities

While bulk heterojunction (BHJ) solar cells fabricated from high Mn PBDTTPD achieve power conversion efficiencies (PCE) as high as 7.3%, the short-circuit current density (JSC) of these devices can drop by 20% after seven days of storage in the dark and under inert conditions. This degradation is characterized by the appearance of S-shape features in the reverse bias region of current–voltage (J–V) curves that increase in amplitude over time. Conversely, BHJ solar cells fabricated from low Mn PBDTTPD do not develop S-shaped J–V curves. However, S-shapes identical to those observed in high Mn PBDTTPD solar cells can be induced in low Mn devices through intentional contamination with the TPD monomer. Furthermore, when high Mn PBDTTPD is purified via size exclusion chromatography (SEC) to reduce the content of low molecular weight species, the JSC of polymer devices is significantly more stable over time. After 111 days of storage in the dark under inert conditions, the J–V curves do not develop S-shapes and the JSC degrades by only 6%. The S-shape degradation feature, symptomatic of low device lifetimes, appears to be linked to the presence of low molecular weight contaminants, which may be trapped within samples of high Mn polymer that have not been purified by SEC. Although these impurities do not affect initial device PCE, they significantly reduce device lifetime, and solar cell stability is improved by increasing the purity of the polymer materials.