Effi cient bulk heterojunction (BHJ) solar cells are characterized by a large interface area between donor and acceptor materials that ensures effi cient photogenerated exciton dissociation into free charge. The optimal scale of the phase separation between these consistuents is that of the exciton diffusion length ( L D ), and the separated phases must be contiguous to allow for low-resistance charge transport pathways from the photosensitive region to the electrodes. [1][2][3][4][5][6] To realize such a BHJ nanostructure, techniques such as thermal [ 7 ] and solvent-vapor annealing [ 8 ] have been demonstrated. The most successful processing protocols affect the aggregation and morphology in a predictable and and controlled manner. In past work, we have shown that solution-processed squaraine (SQ), followed by vacuum thermally evaporated C 60 donor/acceptor solar cells can have power conversion effi ciencies of η p = 4.6 ± 0.1% when they are fabricated into a lamellar device that is subsequently annealed at high temperature (110 ° C). [ 9 ] It was found that the annealing roughens the SQ surface, thereby creating a highly folded BHJ interface with the C 60 and thus compensating for the very short (1.Although the L D of SQ is very small, this defi ciency is partially compensated by its high absorption coeffi cient compared to that of C 60 . This motivates the use of SQ:fullerene blends, whereby the ratio of materials strongly favors that of the fullerene to take advantage of its large L D and low absorption. In previous work this approach has been partially successful, with the highest external quantum effi ciencies ( EQE ) under low intensity illumination of SQ:PC 70 BM (1:6) blends approaching 50% across the visible spectrum. Unfortunately, devices fabricated using such blends exhibited exceptionally low fi ll factors ( FF ∼ 0.35) due to a large internal series resistance to charge extraction from the low density of SQ in the mixture. Hence, under standard simulated solar illumination conditions (100 mW/cm 2 , AM1.5G spectrum), the effi ciency was limited to only ∼ 3%. [ 10 ] In this work, we explore annealing of these SQ:PC 70 BM (1:6) blends in solvent vapor to create continuous crystalline (and hence low resistance) pathways for hole conduction through the rareifi ed SQ environment. We note that, while spin-casting of these mixtures provides a simple means to prepare homogeneous thin fi lms, rapid solvent evaporation does not allow for suffi cient molecular reorganization, which is needed to achieve an equilibrium, crystalline, and uniformly phase-separated mixture. [11][12][13][14] We fi nd that post-annealing through additional extended exposure of the blend to dichloromethane (DCM) can lead to a more optimized morphology that reduces series resistance, and hence increases the FF to 0.50 ± 0.01 and a power conversion effi ciency of η p = 5.2 ± 0.3% of the resulting cells under AM1.5G, 1 sun simulated solar emission (corrected for spectral mismatch). Indeed, our best cells measured reached effi ciencies of...

The intermetallic phases in AA7075-T6, including the nm-scale precipitate MgZnCu and 10-μm size Mg 2 Si, S-phase, Al 7 Fe 2 Cu(Mn), and Al 15 (FeMn) 3 (SiCu) 2, were identified and evaluated with respect to chemical composition, structure, and relative nobility. Evaluation of each major intermetallic particle (IMP) after exposure of the alloy to electrolyte allowed assessment of their roles in the localized corrosion. Different corrosion scenarios, including dealloying, trenching, particle etching out/dissolution, localized anodic and cathodic behavior of the IMPs, and corrosion sequences were studied. Combined with previous studies, the results advance the understanding of the electrochemical properties and the associated mechanisms of localized corrosion in aluminum alloys.

Spin-cast 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (SQ) thin films only 62 A thick are converted from amorphous to polycrystalline via postannealing at elevated temperatures. The surface roughness of the SQ films increases by a factor of 2, while selected area electron diffraction spectra indicate an increase in the extent of postannealed film crystallinity. Dichloromethane solvent annealing is also demonstrated to increase the exciton diffusion length of SQ by a factor of 3 over thermally annealed SQ films as a result of further enhancement in crystalline order. We find that the roughened surface features have a length scale on the order of the exciton diffusion length. Hence, coating the donor SQ with the acceptor, C(60), results in a nearly optimum controlled bulk heterojunction solar cell structure. Optimized SQ/C(60) photovoltaic cells have a power conversion efficiency of eta(p) = 4.6 +/- 0.1% (correcting for solar mismatch) at 1 sun (AM1.5G) simulated solar intensity, and a corresponding peak external quantum efficiency of EQE = 43 +/- 1% even for the very thin SQ layers employed.

The power conversion efficiencies of organic photovoltaic (PV) cells have steadily increased since the introduction of the donor/acceptor (DA) heterojunction.[1] Further improvements have been reported in entangled or "bulk-heterojunction" (BHJ) structures, where the DA interface is within an exciton diffusion length (∼10 nm) of the site for photon absorption.[2] However, the high series resistance [3,4] of these amorphous blends limits the active layer thickness, leading to low fill factor and reduced light absorption, and hence a low solar energy conversion efficiency. [5] One means to circumvent the low mobility of charge in disordered organic films has been the use of inorganic semiconductor "quantum dots". [6] These nanocrystals serve as charge generation sites that, when loaded into a polymer film at high density, can form a high conductivity percolating path to extract photogenerated charge from the device active region. Unfortunately, the mismatch in optical and excitonic properties between the quantum dots and the polymer matrix has limited PV cell efficiencies of photovoltaic cells based on these materials. In this work, we demonstrate that controlled crystallization of organic molecules results in a PV cell in which the active layer comprises a nanocrystalline organic region that forms high conductivity networks for charge extraction. This cell shares many of the benefits of organic/inorganic quantum dot cells as well as all-organic bulk heterojunctions without many of their disadvantages. Structural analysis confirms the existence of crystalline phases of the constituent donor molecule, copper phthalocyanine (CuPc), and the acceptor, C 60 . The new device architecture results in a three-fold increase of power conversion efficiency over that of an efficient planar HJ solar cell control.To reduce cell series resistance in the organic BHJ it is necessary to create morphological order that leads to a low resistance to charge conduction, lacking bottlenecks or islands that impede carrier extraction. Indeed, spatial ordering induced by vertical phase separation led to increased charge collection in organic/inorganic quantum dot hybrid cells from 1.7 % [6] for a disordered cell to 2.8 %. [7] For this reason, we recently reported organic solar cells with an ordered, interdigitated DA interface formed by crystalline donor protrusions and a planarizing acceptor layer, grown by the process of organic vapor phase deposition (OVPD). Control of organic film crystallization and morphology resulted in a low resistance, ordered, interdigitated interface that, when employed in solar cell structures, led to significantly improved efficiency over otherwise identical planar HJs. [8] Such an interface, however, does not increase efficiency for the materials with large exciton diffusion lengths, e.g., C 60 , where the finite protrusion size and density do not lead to increased exciton dissociation. [9] In the current work, we have expanded the DA crystalline interface concept into an extended bulk, highly interconnected and...

Among the diverse nonfullerene acceptors, perylene bisimides (PBIs) have been attracting much attention due to their excellent electron mobility and tunable molecular and electronic properties by simply engineering the bay and head linkages. Herein, guided by two efficient small molecular acceptors, we designed, synthesized, and characterized a new nonfullerene small molecule PPDI with fine-tailored alkyl chains. Notably, a certificated PCE of 5.40% is realized in a simple structured fullerene-free polymer solar cell comprising PPDI as the electron acceptor and a fine-tailored 2D-conjugated polymer PBDT-TS1 as the electron donor. Moreover, the device behavior, morphological feature, and origin of high efficiency in PBDT-TS1/PPDI-based fullerene-free PSC were investigated. The synchronous selection and design of donor and acceptor materials reported here offer a feasible strategy for realizing highly efficient fullerene-free organic photovoltaics.