Search

In light of the intense recent interest in the methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via 2H, 14N, and 207Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ2, for the C–N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, 2H NMR line shapes indicate that the C–N axis is essentially frozen.

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor–acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C60-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC71BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

Diaryl and dialkyl chalcogenide molecules serve as convenient precursors to silicon–chalcogenide bonds, ≡Si–E–R groups, on silicon surfaces, where E = S, Se, and Te. The 254 nm light, coupled with gentle heating to melt and liquefy the chalcogenide precursors for 15 min, enables formation of the resulting silicon–chalcogenide bonds. R groups analyzed comprise a long alkyl chain, octadecyl, and a phenyl group. Quantification of substitution levels of the silicon-hydride on the starting ≡Si(111)–H surface by an organochalcogen was determined by XPS, using the chalcogenide linker atom as the atomic label, where average substitution levels of ∼15% were found for all ≡Si–E–Ph groups. These measured substitution levels were found to agree with 2-dimensional stochastic simulations assuming kinetically irreversible silicon–chalcogen bond formation. Due to the small bond angle about the chalcogen atom, the phenyl rings in the case of ≡Si–E–Ph effectively block otherwise reactive Si–H bonds, leading to the observed lower substitution levels. The linear aliphatic dialkyl disulfide version, ≡Si–S–n-octadecyl, is less limited by steric blocking of surface Si–H groups as is the case with a phenyl group and has a much higher substitution level of ∼29%. The series, ≡Si–S–Ph, ≡Si–Se–Ph, and ≡Si–Te–Ph, was prepared to determine the effect of chalcogenide substitution on the electronics of the silicon, including surface dipoles and work function. The electronics did not change significantly from the starting ≡Si–H surface, which may be due to the low level of substitution that is believed to be caused by steric blocking by the phenyl groups, as well as the relatively similar electronegativities of these elements relative to silicon.

Directed assembly for nanopatterning on semiconductor surfaces is of interest as a cost-effective approach for lithography on silicon, which is complementary to photolithography. In this work, self-assembly of block copolymers is used to produce nanoscale hexagonal arrays of gold hemispheroids, which are then incorporated into an optically transparent, flexible PDMS stamp. These “plasmonic stamps” can then be used to drive hydrosilylation of alkenes and alkynes on hydride-terminated silicon surfaces upon illumination with low-intensity green light [which corresponds with the absorption of the localized surface plasmon resonance (LSPR) of the gold nanostructures]. The resulting hexagonal arrays of nanoscale alkyl or alkenyl patches mirror the spacing of gold nanoparticles in the parent plasmonic stamp. Close examination of the hydrosilylated patches reveals that they are not continuous across the 20–30 nm diameter patches but instead display an annular motif, which closely resembles the plasmonic electric ...

Silver nanowire mesh electrodes represent a possible mass-manufacturable route toward transparent and flexible electrodes for plastic-based electronics such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), and others. Here we describe a route that is based upon spray-coated silver nanowire meshes on polyethylene terephthalate (PET) sheets that are treated with a straightforward combination of heat and pressure to generate electrodes that have low sheet resistance, good optical transmission, that are topologically flat, and adhere well to the PET substrate. The silver nanowire meshes were prepared by spray-coating a solution of silver nanowires onto PET, in air at slightly elevated temperatures. The as-prepared silver nanowire electrodes are highly resistive due to the poor contact between the individual silver nanowires. Light pressure applied with a stainless steel rod, rolled over the as-sprayed silver nanowire meshes on PET with a speed of 10 cm s-1 and a pressure of 50 psi, results in silver nanowire mesh arrays with sheet resistances of less than 20 Ω/□. Bending of these rolled nanowire meshes on PET with different radii of curvature, from 50 to 0.625 mm, showed no degradation of the conductivity of the electrodes, as shown by the constant sheet resistance before and after bending. Repeated bending (100 times) around a rod with a radius of curvature of 1 mm also showed no increase in the sheet resistance, demonstrating good adherence and no signs of delamination of the nanowire mesh array. The diffuse and direct transmittance of the silver nanowires (both rolled and as-sprayed) was measured for wavelengths from 350 to 1200 nm, and the diffuse transmission was similar to that of the PET substrate; the direct transmission decreases by about 7-8%. The silver nanowires were then incorporated into OPV devices with the following architecture: transparent electrode/PEDOT:PSS/P3HT:PC61BM/LiF/Al. While slightly lower in efficiency than the standard indium tin oxide substrate (ITO), the rolled silver nanowire electrodes had a very good device yield, showing that short circuits resulting from the silver nanowire electrodes can be successfully avoided by this rolling approach. © 2013 American Chemical Society.

A water-soluble cationic polythiophene derivative, poly[3-(6-{4-tert-butylpyridiniumyl}-hexyl)thiophene-2,5-diyl] [P3(TBP)HT], is combined with anionic poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (PEDOT:PSS) on indium tin oxide (ITO) substrates via electrostatic layer-by-layer (eLbL) assembly. By varying the number of eLbL layers, the electrode's work function is precisely controlled from 4.6 to 3.8 eV. These polymeric coatings are used as cathodic interfacial modifiers for inverted-mode organic photovoltaics that incorporate a photoactive layer composed of either poly[(3-hexylthiophene)-2,5-diyl] (P3HT) and the fullerene acceptor [6,6-phenyl-C61-butyric acid methyl ester (PC61BM) or the low bandgap polymer [poly({4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithiophene}-2,6-diyl)-alt-({5-octylthieno[3,4-c]pyrrole-4,6-dione}-1,3-diyl) (PBDTTPD)] and the electron acceptor [6,6-phenyl-C71-butyric acid methyl ester (PC71BM)]. The power conversion efficiency (PCE) of the resulting photovoltaic device is dependent on the composition of the eLbL-assembled interface and permits the fabrication of devices with efficiencies of 3.8% and 5.6% for P3HT and PBDTTPD donor polymers, respectively. Notably, these devices demonstrate significant stability with a P3HT:PC61BM system maintaining 83% of its original PCE after 1 year of storage and a PBDTTPD:PC71BM system maintaining 97% of its original PCE after over 1000 h of storage in air, according to the ISOS-D-1 shelf protocol.

Two isostructural low-band-gap small molecules that contain a one-atom substitution, S for Se, were designed and synthesized. The molecule 7,7′-[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene]bis[6-fluoro-4-(5′-hexyl-2,2′-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole] (1) and its selenium analogue 7,7′-[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene]bis[6-fluoro-4-(5′-hexyl-2,2′-bithiophen-5-yl)benzo[c][1,2,5]selenodiazole] (2) are both based on the electron-rich central unit benzo[1,2-b:4,5-b′]dithiophene. The aim of this work was to investigate the effect of one-atom substitution on the optoelectronic properties and photovoltaic performance of devices. Theoretical calculations revealed that this one-atom variation has a small but measurable effect on the energy of frontier molecular orbital (HOMO and LUMO), which, in turn, can affect the absorption profile of the molecules, both neat and when mixed in a bulk heterojunction (BHJ) with PC71BM. The Se-containing variant 2 led to higher efficiencies [highest power conversion efficiency (PCE) of 2.6%] in a standard organic photovoltaic architecture, when combined with PC71BM after a brief thermal annealing, than the S-containing molecule 1 (highest PCE of 1.0%). Studies of the resulting morphologies of BHJs based on 1 and 2 showed that one-atom substitution could engender important differences in the solubilities, which then influenced the crystal orientations of the small molecules within this thin layer. Brief thermal annealing resulted in rotation of the crystalline grains of both molecules to more energetically favorable configurations.

In this paper, we describe an apparatus capable of measuring electrical resistance and light transmission through a flexible transparent conductor (TC) as it is actively deformed. The measurements are performed in situ with time resolution on the order of ~100 ms, and the resistance/transmission data is further used to calculate TC figures-of-merit in real-time. With this instrument, we are able to track and compare the evolution of figures-of-merit for different TCs as they are actively deformed. To demonstrate the tool, we evaluate several common TCs including indium tin oxide, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and silver nanowires, as well as a more unconventional nanowire/PEDOT:PSS hybrid. As expected, indium tin oxide degrades rapidly with strain, while the more flexible PEDOT:PSS is far more strain tolerant. The nanowire/PEDOT:PSS composite is found to produce an advantageous effect, as the silver nanowires generate large figures-of-merit at low strain, and the addition of a PEDOT:PSS component vastly improves strain tolerance.

High conductivity poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) was spray cast to form highly flexible transparent electrodes for forward- and inverted-mode organic solar cells (OSCs). A multiple solvent ink containing ethylene glycol was developed, and a post-deposition annealing step contributed to a high conductivity of 1070±50 S cm-¹. Sheet resistance and transmission at a wavelength of 550 nm were controlled within 24-259 Ω □-¹ and 71-95%, respectively, which are amongst the best-reported combined characteristics. Forward-mode OSCs with spray coated PEDOT:PSS anodes yielded a power conversion efficiency of 3.2%. Mechanical bending and stretching tests demonstrated that the flexibility of these PEDOT:PSS layers were far superior to that of ITO: elastic moduli were reduced by more than an order of magnitude, and the resistance increased far more slowly under both uniaxial stretching and bending to progressively smaller radii of curvature. With these experiments, the minimum radii of curvature and maximum uniaxial strains at which acceptable performance is maintained were investigated. Collectively, our results illustrate a promising future for the scalable printing of low-cost PEDOT:PSS-based flexible transparent electrodes. © 2012 Crown All rights reserved.

Control over interfacial properties in organic photovoltaics (OPVs) is critical for many aspects of their performance. Functionalization of the transparent conducting electrode, in this case, indium tin oxide (ITO), through an electrostatic layer by layer (eLbL) approach with cationic N,N-bis[2-(trimethylammonium)ethylene] perylene-3,4,9,10-tetracarboxyldiimide (PTCDI +) and anionic poly(3,4-ethylenedioxythiophene):poly(p- styrenesulfonate) (PEDOT:PSS -), led to high control over the surface properties. The films were studied through a variety of surface and spectroscopic techniques, including X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy, atomic force microscopy (AFM), and ellipsometry. The work function of modified ITO was measured by UV photoelectron spectroscopy (UPS) and showed oscillating values with respect to odd-even layer numbers; the strong odd-even effect is due to the differing electronic characteristics of the top layer, either PTCDI + or PEDOT:PSS -. The modified ITO electrodes were then used as the cathode in a series of inverted organic photovoltaic architectures. The performance of inverted OPVs was, in parallel to the UPS results, found to be highly dependent on the layer number of coated films and showed an obvious oscillation based on layer number. Inverted OPVs were retested after 128 days of storage in air, and almost all devices maintained over 70% of original power conversion efficiency (PCE). © 2011 American Chemical Society.

Conjugated regioregular poly[3-(5-carboxypentyl) thiophene-2,5-diyl] (P3CPenT), a derivative of poly(3-hexylthiophene) (P3HT), is introduced as a hole transport layer (HTL) for P3HT:PCBM plastic solar cells (PSCs). P3CPenT, a carboxyl-functionalized polythiophene, is insoluble in solvents typically used in the preparation of photoactive layers such as chloroform, chlorobenzene and dichlorobenzene, allowing successive processing of the photoactive layer without dissolution of the HTL. P3CPenT self-assembles into nanowires in DMSO solution and when cast as a film, reduces concentration gradients of P3HT:PCBM photoactive layers. P3HT:PCBM PSCs incorporating P3CPenT nanowires as the HTL have a higher fill factor (FF, 0.67) and power conversion efficiency (PCE, 3.7%) than devices with conventional PEDOT:PSS HTLs.