Kaushik Roy Choudhury

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.

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.

Charge carrier mobility in an organic-inorganic hybrid nanocomposite

This study was supported by a NSF, DMR Solid State and Polymer Chemistry Grant No. DMR0075867. Partial support by a Defense Research Initiative on Nanotechnology (DURINT), Contract No. F496200110358, through the Directorate of Chemistry and Life Sciences of the Air Force Office of Scientific Research is also acknowledged.

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.

Solution-processed pentacene quantum-dot polymeric nanocomposite for infrared photodetection

An organic/inorganic polymeric nanocomposite thin film device, consisting of poly-N-vinyl carbazole as host matrix, lead selenide quantum dots as photosensitizer, and the organic semiconductor pentacene as a conductivity booster, is fabricated. Because of the inherent insolubility of pentacene, it is incorporated in the form of a soluble precursor which is made to undergo thermal conversion into pentacene. The device exhibits dramatic enhancement of infrared photocurrent due to pentacene. Efficient photogeneration of carriers coupled with enhanced conductance results in high photoconductive quantum efficiency.

Polymeric nanocomposite infrared photovoltaics enhanced by pentacene

An infrared active thin film polymeric photovoltaic device is fabricated from regioregular poly(3-hexylthiophene), PbSe quantum dots, and the organic semiconductor pentacene. The PbSe quantum dots are infrared photosensitizers. Pentacene is incorporated into the formulation in a soluble precursor form. The current-voltage measurements of the device show that the photovoltaic performance is significantly increased by the introduction of pentacene, with both short-circuit current density and open-circuit voltage increased by a factor of 2. The improved performance of the device is attributed to the high mobility of charge carriers in pentacene probably due to conducting domains provided by it.

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.

Characterization of CZTSSe photovoltaic device with an atomic layer-deposited passivation layer

We describe a CZTSSe (Cu2ZnSn(S1−x,Sex)4) photovoltaic (PV) device with an ALD (atomic layer deposition) coated buffer dielectric layer for CZTSSe surface passivation. An ALD buffer layer, such as TiO2, can be applied in order to reduce the interface recombination and improve the devices open-circuit voltage. Detailed characterization data including current-voltage, admittance spectroscopy, and capacitance profiling are presented in order to compare the performance of PV devices with and without the ALD layer.

Employing time-resolved terahertz spectroscopy to analyze carrier dynamics in thin-film Cu2ZnSn(S,Se)4 absorber layers

We report the application of time-resolved terahertz spectroscopy (TRTS) to measure photoexcited carrier lifetimes and mobility, and to determine recombination mechanisms in Cu2ZnSn(S,Se)4 (CZTSSe) thin films fabricated from nanocrystal inks. Ultrafast time resolution permits tracking the evolution of carrier density to determine recombination rates and mechanisms. The carrier generation profile was manipulated by varying the photoexcitation wavelength and fluence to distinguish between surface, Shockley-Read-Hall (SRH), radiative, and Auger recombination mechanisms and determine rate constants. Surface and SRH recombination are the dominant mechanisms for the air/CZTSSe/SiO2/Si film stack. Diffusion to, and then recombination at, the air-CZTSSe interface occurred on the order of 100 picoseconds, while SRH recombination lifetimes were 1–2 nanoseconds. TRTS measurements can provide information that is complementary to conventional time-resolved photoluminescence measurements and can direct the design of efficient thin film photovoltaics.

Organic/inorganic nanocomposites for high-dielectric-constant materials

A different approach to fabricate metallic nanostructures in an insulating organic matrix in situ using thermal coevaporation is demonstrated. The method is used to fabricate metal-organic percolative nanocomposite capacitors. Thermal annealing of the nanocomposite films results in a dramatic enhancement (>50 times at 10kHz) of the dielectric constant accompanied by significant lowering (approximately four times) of conductivity and dissipation loss, especially at low frequencies. The improved performance is attributed to structural modifications in the films resulting from enhanced phase segregation of constituents, induced by annealing at optimized temperatures.