Analysis of the Performance of Narrow-Bandgap Organic Solar Cells Based on a Diketopyrrolopyrrole Polymer and a Nonfullerene Acceptor.

The combination of narrow-bandgap diketopyrrolopyrrole (DPP) polymers and nonfullerene acceptors (NFAs) seems well-matched for solar cells that exclusively absorb in the near infrared but they rarely provide high efficiency. One reason is that processing of the active layer is complicated by the fact that DPP-based polymers are generally only sufficiently soluble in chloroform (CF), while NFAs are preferably processed from halogenated aromatic solvents. By using a ternary solvent system consisting of CF, 1,8-diiodooctane (DIO), and chlorobenzene (CB), the short-circuit current density is increased by 50% in solar cells based on a DPP polymer (PDPP5T) and a NFA (IEICO-4F) compared to the use of CF with DIO only. However, the open-circuit voltage and fill factor are reduced. As a result, the efficiency improves from 3.4 to 4.8% only. The use of CB results in stronger aggregation of IEICO-4F as inferred from two-dimensional grazing-incidence wide-angle X-ray diffraction. Photo- and electroluminescence and mobility measurements indicate that the changes in performance can be ascribed to a more aggregated blend film in which charge generation is increased but nonradiative recombination is enhanced because of reduced hole mobility. Hence, while CB is essential to obtain well-ordered domains of IEICO-4F in blends with PDPP5T, the morphology and resulting hole mobility of PDPP5T domains remain suboptimal. The results identify the challenges in processing organic solar cells based on DPP polymers and NFAs as near-infrared absorbing photoactive layers.

Efficient Thick-Film Polymer Solar Cells with Enhanced Fill Factors via Increased Fullerene Loading.

Developing effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5'-bis(2-butyloctyl)-(2,2'-bithiophene)-4,4'-dicarboxylate- alt-5,5'-2,2'-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.

The influence of siloxane side-chains on the photovoltaic performance of a conjugated polymer.

The effect of gradually replacing the branched alkyl side chains of a diketopyrrolopyrrole (DPP) conjugated polymer by linear side chains containing branched siloxane end groups on the photovoltaic performance of blends of these polymers with a common fullerene acceptor is investigated. With an increasing proportion of siloxane side chains, the molecular weight and solubility of the polymers decreases. While the siloxane containing polymers exhibit a higher hole mobility in field-effect transistors, their performance in solar cells is less than the polymer with only alkyl sides chains. Using grazing-incidence wide-angle X-ray scattering, transmission electron microscopy, and fluorescence spectroscopy we identify two main reasons for the reduced performance of siloxane containing polymers in solar cells. The first one is a somewhat coarser phase-separated morphology with slightly wider polymer fibers. This is unexpected as often the fiber width is inversely correlated with polymer solubility. The second one is stronger non-radiative decay of the pristine polymers containing siloxane side chains.

Subnaphthalocyanines as Electron Acceptors in Polymer Solar Cells: Improving Device Performance by Modifying Peripheral and Axial Substituents.

A new class of subnaphthalocyanines bearing various peripheral and axial substituents have been synthesized for use as electron acceptors in solution-processed bulk-heterojunction polymer solar cells. The resulting solar cells exhibit modest photovoltaic performance with contributions from both the polymer donor and subnaphthalocyanine acceptor to the photocurrent.

"Double-Cable" Conjugated Polymers with Linear Backbone toward High Quantum Efficiencies in Single-Component Polymer Solar Cells.

A series of "double-cable" conjugated polymers were developed for application in efficient single-component polymer solar cells, in which high quantum efficiencies could be achieved due to the optimized nanophase separation between donor and acceptor parts. The new double-cable polymers contain electron-donating poly(benzodithiophene) (BDT) as linear conjugated backbone for hole transport and pendant electron-deficient perylene bisimide (PBI) units for electron transport, connected via a dodecyl linker. Sulfur and fluorine substituents were introduced to tune the energy levels and crystallinity of the conjugated polymers. The double-cable polymers adopt a "face-on" orientation in which the conjugated BDT backbone and the pendant PBI units have a preferential π-π stacking direction perpendicular to the substrate, favorable for interchain charge transport normal to the plane. The linear conjugated backbone acts as a scaffold for the crystallization of the PBI groups, to provide a double-cable nanophase separation of donor and acceptor phases. The optimized nanophase separation enables efficient exciton dissociation as well as charge transport as evidenced from the high-up to 80%-internal quantum efficiency for photon-to-electron conversion. In single-component organic solar cells, the double-cable polymers provide power conversion efficiency up to 4.18%. This is one of the highest performances in single-component organic solar cells. The nanophase-separated design can likely be used to achieve high-performance single-component organic solar cells.

Aqueous Nanoparticle Polymer Solar Cells: Effects of Surfactant Concentration and Processing on Device Performance.

Polymer solar cells based on PDPP5T and PCBM as donor and acceptor materials, respectively, were processed from aqueous nanoparticle dispersions. Careful monitoring and optimization of the concentration of free and surface-bound surfactants in the dispersion, by measuring the conductivity and ζ-potential, is essential to avoid aggregation of nanoparticles at low concentration and dewetting of the film at high concentration. The surfactant concentration is crucial for creating reproducible processing conditions that aid in further developing aqueous nanoparticle processed solar cells. In addition, the effects of adding ethanol, of aging the dispersion, and of replacing [60]PCBM with [70]PCBM to enhance light absorption were studied. The highest power conversion efficiencies (PCEs) obtained are 2.0% for [60]PCBM and 2.4% for [70]PCBM-based devices. These PCEs are limited by bimolecular recombination of photogenerated charges. Cryo-TEM reveals that the two components phase separate in the nanoparticles, forming a PCBM-rich core and a PDPP5T-rich shell and causing a nonoptimal film morphology.

The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk-Heterojunction Solar Cells.

Four hexachlorosubphthalocyanines SubPcCl6 -X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution-processed bulk-heterojunction organic solar cells. Subphthalocyanines are aromatic chromophoric molecules with cone-shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptor and PTB7-Th as the electron donor exhibit a power conversion efficiency up to 4 % and an external quantum efficiency approaching 60 % due to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating a promising prospect of non-fullerene acceptors based on subphthalocyanines and structurally related systems.