The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within t...

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied Solution sequential processing (SqP) was used to introduce 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into poly(3-hexylthiophene-2,5-diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity X-ray data suggest that F4TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π-stack within the polymer crystallites Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 A from the polymer backbone Electrical measurements show that the conductivity of P3HT films doped by F4TCNQ via SqP can be improved by increasing the polymer crystallinity AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 01 cm2 V−1 s−1 in the most crystalline P3HT samples Temperature-dependent conductivity measurements show that polaron mobility in SqP-doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature

The Effects of Crystallinity on Charge Transport and the Structure of Sequentially Processed F4TCNQ-Doped Conjugated Polymer Films

We demonstrate that solution-sequential processing (SqP) can yield heavily doped pristine-quality films when used to infiltrate the molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into pure poly(3-hexylthiophene) (P3HT) polymer layers. Profilometry measurements show that the SqP method produces doped films with essentially the same surface roughness as pristine films, and 2-D grazing-incidence wide-angle X-ray scattering (GIWAXS) confirms that SqP preserves both the size and orientation of the pristine polymer's crystallites. Unlike traditional blend-cast F4TCNQ/P3HT doped films, our sequentially processed layers have tunable and reproducible conductivities reaching as high as 5.5 S/cm even when measured over macroscopic (>1 cm) distances. The high conductivity and superb film quality allow for meaningful Hall effect measurements, which reveal p-type conduction and carrier concentrations tunable from 10(16) to 10(20) cm(-3) and hole mobilities ranging from ∼0.003 to 0.02 cm(2) V(-1) s(-1) at room temperature over the doping levels examined.

Overcoming Film Quality Issues for Conjugated Polymers Doped with F4TCNQ by Solution Sequential Processing: Hall Effect, Structural, and Optical Measurements

High-Performance Large-Area Organic Solar Cells Enabled by Sequential Bilayer Processing via Nonhalogenated Solvents

Polymer solar cells are conventionally processed by coating a multicomponent mixture containing polymer, fullerene, solvent, and cosolvent. The photovoltaic performance strongly depends on the nanoscale morphology of the blend, which is largely determined by the precise nature of the solvent composition and drying conditions. Here, an alternative processing route is investigated in which the two active layer components are deposited sequentially via spin coating or doctor blading. Spin coating the fullerene from o-dichlorobenzene on top of the polymer provides virtually identical morphologies and photovoltaic performance. Using blade coating, the influence of the second-layer solvent for the fullerene derivative is investigated in further detail. Different aromatic solvents are compared regarding swelling of the polymer layer, fullerene solubility, and evaporation rate. It is found that while swelling of the polymer by the second-layer solvent is a necessity for sequential processing, the solubility of the fullerene derivative in this solvent has the strongest influence on solar cell performance. Homogeneous layers in which a sufficient amount of fullerene can infiltrate the polymer film can only be achieved when solvents are used that have a very high solubility for the fullerene and swell the polymer layer.

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Depositing Fullerenes in Swollen Polymer Layers via Sequential Processing of Organic Solar Cells

Polymer:fullerene bulk heterojunction (BHJ) solar cell active layers can be created by traditional blend casting (BC), where the components are mixed together in solution before deposition, or by sequential processing (SqP), where the pure polymer and fullerene materials are cast sequentially from different solutions. Presently, however, the relative merits of SqP as compared to BC are not fully understood because there has yet to be an equivalent (composition- and thickness-matched layer) comparison between the two processing techniques. The main reason why matched SqP and BC devices have not been compared is because the composition of SqP active layers has not been accurately known. In this paper, we present a novel technique for accurately measuring the polymer:fullerene film composition in SqP active layers, which allows us to make the first comparisons between rigorously composition- and thickness-matched BHJ organic solar cells made by SqP and traditional BC. We discover that, in optimal photovoltaic devices, SqP active layers have a very similar composition as their optimized BC counterparts (≈44-50 mass % PCBM). We then present a thorough investigation of the morphological and device properties of thickness- and composition-matched P3HT:PCBM SqP and BC active layers in order to better understand the advantages and drawbacks of both processing approaches. For our matched devices, we find that small-area SqP cells perform better than BC cells due to both superior film quality and enhanced optical absorption from more crystalline P3HT. The enhanced film quality of SqP active layers also results in higher performance and significantly better reproducibility in larger-area devices, indicating that SqP is more amenable to scaling than the traditional BC approach. X-ray diffraction, UV-vis absorption, and energy-filtered transmission electron tomography collectively show that annealed SqP active layers have a finer-scale blend morphology and more crystalline polymer and fullerene domains when compared to equivalently processed BC active layers. Charge extraction by linearly increasing voltage (CELIV) measurements, combined with X-ray photoelectron spectroscopy, also show that the top (nonsubstrate) interface for SqP films is slightly richer in PCBM compared to matched BC active layers. Despite these clear differences in bulk and vertical morphology, transient photovoltage, transient photocurrent, and subgap external quantum efficiency measurements all indicate that the interfacial electronic processes occurring at P3HT:PCBM heterojunctions are essentially identical in matched-annealed SqP and BC active layers, suggesting that device physics are surprisingly robust with respect to the details of the BHJ morphology. © 2014 American Chemical Society.

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Comparing matched polymer:Fullerene solar cells made by solution-sequential processing and traditional blend casting: Nanoscale structure and device performance

Embodiments of a 3D micro-battery structure are disclosed. The 3D micro-battery structure includes an electrode micro-structure and a thin film electrolyte. The electrode micro-structure includes a base and a group of electrodes extending from the base. The thin film electrolyte conformally coats each of the group of electrodes to form a group of coated electrodes, such that the thin film electrolyte is ionically conducting and is electrically insulating.

Robert James Thompson

Papers

Comparing matched polymer:Fullerene solar cells made by solution-sequential processing and traditional blend casting: Nanoscale structure and device performance

Thin film electrolyte based 3d micro-batteries