Adherent and Conformal Zn(S,O,OH) Thin Films by Rapid Chemical Bath Deposition with Hexamethylenetetramine Additive.

ZnS is a wide band gap semiconductor whose many applications, such as photovoltaic buffer layers, require uniform and continuous films down to several nanometers thick. Chemical bath deposition (CBD) is a simple, low-cost, and scalable technique to deposit such inorganic films. However, previous attempts at CBD of ZnS have often resulted in nodular noncontinuous films, slow growth rates at low pH, and high ratio of oxygen impurities at high pH. In this work, ZnS thin films were grown by adding hexamethylenetetramine (HMTA) to a conventional recipe that uses zinc sulfate, nitrilotriacetic acid trisodium salt, and thioacetamide. Dynamic bath characterization showed that HMTA helps the bath to maintain near-neutral pH and also acts as a catalyst, which leads to fast nucleation and deposition rates, continuous films, and less oxygen impurities in the films. Films deposited on glass from HMTA-containing bath were uniform, continuous, and 90 nm thick after 1 h, as opposed to films grown without HMTA that were ∼3 times thinner and more nodular. On Cu2(Zn,Sn)Se4, films grown with HMTA were continuous within 10 min. The films have comparatively few oxygen impurities, with S/(S+O) atomic ratio of 88%, and high optical transmission of 98% at 360 nm. The Zn(S,O,OH) films exhibit excellent adhesion to glass and high resistivity, which make them ideal nucleation layers for other metal sulfides. Their promise as a nucleation layer was demonstrated with the deposition of thin, continuous Sb2S3 overlayers. This novel HMTA chemistry enables rapid deposition of Zn(S,O,OH) thin films to serve as a nucleation layer, a photovoltaic buffer layer, or an extremely thin continuous coating for thin film applications. HMTA may also be applied in a similar manner for solution deposition of other metal chalcogenide and oxide thin films with superior properties.

High-efficiency solution-processed Cu2ZnSn(S,Se)4 thin-film solar cells prepared from binary and ternary nanoparticles.

A new solution-based method to fabricate Cu(2)ZnSn(S,Se)(4) (CZTSSe) thin films is presented. Binary and ternary chalcogenide nanoparticles were synthesized and used as precursors to form CZTSSe thin films. The composition of the CZTSSe films can be easily controlled by adjusting the ratio of the nanoparticles used. The effect of compositional adjustment on device performance is illustrated. Laboratory-scale photovoltaic cells with 8.5% total-area efficiency (or 9.6% active-area efficiency) were demonstrated without anti-reflective coatings. Material characterization data revealed the formation of a bilayer microstructure during thermal processing and suggested a path forward on device improvement.

Synthetic principles directing charge transport in low-band-gap dithienosilole-benzothiadiazole copolymers.

Given the fundamental differences in carrier generation and device operation in organic thin-film transistors (OTFTs) and organic photovoltaic (OPV) devices, the material design principles to apply may be expected to differ. In this respect, designing organic semiconductors that perform effectively in multiple device configurations remains a challenge. Following "donor-acceptor" principles, we designed and synthesized an analogous series of solution-processable π-conjugated polymers that combine the electron-rich dithienosilole (DTS) moiety, unsubstituted thiophene spacers, and the electron-deficient core 2,1,3-benzothiadiazole (BTD). Insights into backbone geometry and wave function delocalization as a function of molecular structure are provided by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level. Using a combination of X-ray techniques (2D-WAXS and XRD) supported by solid-state NMR (SS-NMR) and atomic force microscopy (AFM), we demonstrate fundamental correlations between the polymer repeat-unit structure, molecular weight distribution, nature of the solubilizing side-chains appended to the backbones, and extent of structural order attainable in p-channel OTFTs. In particular, it is shown that the degree of microstructural order achievable in the self-assembled organic semiconductors increases largely with (i) increasing molecular weight and (ii) appropriate solubilizing-group substitution. The corresponding field-effect hole mobilities are enhanced by several orders of magnitude, reaching up to 0.1 cm(2) V(-1) s(-1) with the highest molecular weight fraction of the branched alkyl-substituted polymer derivative in this series. This trend is reflected in conventional bulk-heterojunction OPV devices using PC(71)BM, whereby the active layers exhibit space-charge-limited (SCL) hole mobilities approaching 10(-3) cm(2) V(-1) s(-1), and yield improved power conversion efficiencies on the order of 4.6% under AM1.5G solar illumination. Beyond structure-performance correlations, we observe a large dependence of the ionization potentials of the polymers estimated by electrochemical methods on polymer packing, and expect that these empirical results may have important consequences on future material study and device applications.

Aesthetically pleasing conjugated polymer:fullerene blends for blue-green solar cells via roll-to-roll processing.

The practical application of organic photovoltaic (OPV) cells requires high throughput printing techniques in order to attain cells with an area large enough to provide useful amounts of power. However, in the laboratory screening of new materials for OPVs, spin-coating is used almost exclusively as a thin-film deposition technique due its convenience. We report on the significant differences between the spin-coating of laboratory solar cells and slot-die coating of a blue-green colored, low bandgap polymer (PGREEN). This is one of the first demonstrations of slot-die-coated polymer solar cells OPVs not utilizing poly(3-hexylthiophene):(6,6)-phenyl-C(61)-butyric acid methyl ester (PCBM) blends as a light absorbing layer. Through synthetic optimization, we show that strict protocols are necessary to yield polymers which achieve consistent photovoltaic behavior. We fabricated spin-coated laboratory scale OPV devices with PGREEN: PCBM blends as active light absorbing layers, and compare performance to slot die-coated individual solar cells, and slot-die-coated solar modules consisting of many cells connected in series. We find that the optimum ratio of polymer to PCBM varies significantly when changing from spin-coating of thinner active layer films to slot-die coating, which requires somewhat thicker films. We also demonstrate the detrimental impacts on power conversion efficiency of high series resistance imparted by large electrodes, illustrating the need for higher conductivity contacts, transparent electrodes, and high mobility active layer materials for large-area solar cell modules.

PbSe nanocrystal-based infrared-to-visible up-conversion device.

Low-cost hybrid up-conversion devices with infrared sensitivity to 1.5 µm were obtained by integrating a colloidal PbSe nanocrystal near-infrared sensitizing layer on a green phosphorescent organic light emitting diode. A ZnO nanocrystal hole blocking layer is incorporated in the devices for keeping the device off in the absence of IR excitation. The maximum photon (1.3 µm)-to-photon (0.52 µm) conversion efficiency is 1.3%. The extension (until 1.5 µm) of the near-infrared wavelengths, which can be converted to visible light, may be able to improve night vision.

Efficient green solar cells via a chemically polymerizable donor-acceptor heterocyclic pentamer.

In this contribution, we report on bulk-heterojunction solar cells using a solution-processable neutral green conjugated copolymer based on 3,4-dioxythiophene and 2,1,3-benzothiadiazole as the donor and [6,6]phenyl-C61 butyric acid methyl ester (PCBM) as the acceptor. We have found that the short-circuit current is very sensitive to the composition of the donor-acceptor blend and it increases with increasing acceptor concentration. The device with a donor-acceptor ratio of 1:8 gives the best performance with a short-circuit current of 5.56 mA/cm(2), an open-circuit voltage of 0.77 V, and a power conversion efficiency of 1.9% under AM 1.5 solar illumination. The incident photon-to-current efficiency (IPCE) of the green solar cells shows two bands, one with a maximum of 57% in the UV region corresponding to absorption of PCBM and a second one with a maximum of 42% at longer wavelengths corresponding to the absorption of the green polymer.

Shape control of PbSe nanocrystals using noble metal seed particles.

We demonstrate that the shape of PbSe nanocrystals can be controlled systematically by seeding their growth with noble metal nanoparticles (Au, Ag, or Pd) and varying the seed and precursor concentrations. Cylinders (quantum rods), cubes, crosses, stars, and branched structures were produced in high yield at 150 degrees C in reaction times of a few minutes. Although their absorption spectrum does not exhibit sharp features, the quantum rods exhibit significant photogeneration efficiency, enabling infrared sensitization of a polymeric photoconductive nanocomposite.