Bilayer Photovoltaic Devices Research Articles

Diketopyrrolopyrrole latent pigment-based bilayer solar cells

Abstract Two Diketopyrrolopyrrole based latent pigment donor materials were fabricated into thin film bilayer photovoltaic devices featuring PCBM as the acceptor. Thermal deprotection of the thin film, carried out at 200∘ C, returns the dye-like small molecule to the corresponding pristine pigment quantitatively. The connected evolution of electrical and morphological features of pure thin films and blends are examined. A significant decrease in extinction coefficient was noted and correlated both to intrinsic changes of the electronic structure upon cleavage and to an increase in internal scattering due to extensive crystallization. Power conversion efficiencies of 0.33% were achieved for bilayer devices, nearly doubling previous results with latent pigment DPP devices, under comparable experimental conditions.

A Robust, High-Temperature Organic Semiconductor

We introduce a new pentacene-based organic semiconductor, 5,6,7-trithiapentacene-13-one (TTPO). TTPO is a small-molecule organic semiconductor that is simple to synthesize and purify, readily crystallizes, melts in air from 386–388 °C without decomposition, and is indefinitely stable against degradation in acid-free solution. TTPO has a high molar absorptivity, optical and electrochemical HOMO–LUMO gaps of 1.90 and 1.71 eV, respectively, and can be thermally evaporated to produce highly uniform thin films. Its cyclic voltammogram reveals one reversible oxidation and two reversible reductions between +1.5 and −1.5 V. The crystal structure for TTPO has been solved and its unique parallel displaced, and head-to-tail packing arrangement has been examined and explained using high-level density functional theory. High-resolution scanning tunneling microscopy (STM) was used to image individual TTPO molecules upon assembly on a pristine Au(111) surface in ultrahigh vacuum. STM images reveal that vapor-deposited TTPO molecules nucleate in a unique stacked geometry with a small acute angle with respect to Au(111) surface. Preliminary TTPO-based bilayer photovoltaic devices show increases in short circuit current density upon heating from 25 to 80 °C with a concomitant 4–160-fold increase in power conversion efficiencies. TTPO has the potential to be used in thin-film electronic devices that require operation over a wide range of temperatures such as thin-film transistors, sensors, switches, and solar cells.

Template role of polyhexylthiophene nanowires on efficient bilayer photovoltaic cells

Nanowires of the conjugated polymer poly(3-hexylthiophene) (P3HT) were produced by solution self-assembly and incorporated into bilayer photovoltaic devices with PCBM as the electron-acceptor material. Atomic Force Microscopy enabled a full assessment of the self-assembled P3HT fibrils network. It also revealed the effect the pre-existing nanowires have on the evaporated PCBM overlayer, strongly suggesting a master role. The power conversion efficiency of the bilayer with P3HT nanowires dramatically increases, when compared to the non-structured bilayer photovoltaic devices of P3HT. Photovoltaic devices characterization ensured a comparative overview of the performance parameters for both structured and non-structured P3HT:PCBM bilayers and blend devices.

Acridine orange as a biosensitive photovoltaic material

Acridine orange (AO), a biosensitive molecule that is customarily used for labeling nucleic acids including DNA and RNA, is here investigated as a cost effective, water soluble, and photoactive material for the fabrication of potentially biosensitive organic photovoltaics. The electronic energy levels of AO are determined using Kelvin Probe Force Microscopy (KPFM) and UV-Visible spectroscopy. The effect of anticrystallization agents, as well as low-temperature annealing, on the work function of AO is investigated: amorphous AO films are shown to possess a significantly higher work function than microcrystalline AO films and the work function also increases by annealing. Photo-induced processes in AO films are investigated by considering the changes of the KPFM signal under illumination. We demonstrate that acridine orange is able to photogenerate electron-hole pairs at rates comparable to the most commonly used solar-grade photovoltaic materials, including polythiophenes. In addition, the effect of the morphology of different types of AO thin films spun from different solvents is studied in bilayer photovoltaic devices fabricated from stacks of AO and phenyl-C61-butyric acid methyl ester thin films.

Solution-processed bilayer photovoltaic devices with nematic liquid crystals

The cross-linking of polymerisable liquid crystalline semiconductors is a promising approach to solution-processable, multilayer, organic photovoltaics. Here we demonstrate an organic bilayer photovoltaic with an insoluble electron-donating layer formed by cross-linking a nematic reactive mesogen. We investigate a range of perylene diimide (PDI) materials, some of which are liquid crystalline, as the overlying electron acceptor layer. We find that carrier mobility of the acceptor materials is enhanced by liquid crystallinity and that mobility limits the performance of photovoltaic devices.

Performance of fluorene and terthiophene copolymer in bilayer photovoltaic devices: The role of the polymer conformations

We report experiments using fluorene and terthiophene copolymer as the active layer in bilayer devices with C60. The highest short circuit current, open circuit voltage and power conversion efficiency upon AM1.5 illumination were 6.8mA/cm2, 0.68V and 2.33%, respectively. Density functional theory analysis was used to identify the most stable configurations of the terthiophene moieties in the polymer: the most stable form has the thiophene rings in the alternate configuration (anti) and the second conformation has the thiophene rings pointing to the same direction (syn). Comparing theoretical results with measurements of absorbance, X-ray diffraction, and X-ray reflectometry experiments, we conclude that the annealing treatment produces conformational anti to syn transition along the backbone of poly[9,9′-n-dihexyl-2,7-fluorene-alt-2,5-terthiophene] (LaPPS45). The syn segments of the chain condensed then in a lamellar ordered structure which increases the degree of crystallinity of the annealed samples and improve the light harvest at long wavelengths. From absorption measurements of films submitted to different annealing temperatures and with the help of theoretical calculations we propose a “wave-like” aggregation pattern to the syn segments in those lamellas.

Achieving high open-circuit voltage in the PPV-CdHgTe bilayer photovoltaic devices on the basis of the heterojunction interfacial modification

On the basis of the surface modification of CdHgTe nanocrystals (NCs), a bilayer photovoltaic device composed of poly(p-phenylene-vinylene) (PPV) and CdHgTe NC heterojunction is prepared. The energy bands bend to the vacuum level through tuning the dipole moment of the CdHgTe NCs at the heterojunction interface and 0.3 eV energy band shifting is detected by the ultraviolet photoelectron spectrometer (UPS) measurement. As a result, open-circuit voltage (Voc) of the photodiode can reach 1.4 V under AM 1.5G illumination (100 mW cm−2).

Synthesis and optoelectronic behavior of conjugated polymer poly(3‐hexylthiophene) grafted on multiwalled carbon nanotubes

AbstractA nanocomposite of multiwalled carbon nanotubes (CNTs) and poly(3‐hexylthiophene) (P3HT) was prepared by grafting P3HT uniformly on the surface of CNTs (P3HT/P3HT‐g‐CNT) via a “grafting from” method with the coating thicknesses controlled. It was found that as the coating thickness decreased, the crystallinity of the P3HT decreased, along with significant red‐shifting of Raman that signified alterations of chain conformation. Furthermore, although the photoluminescence (PL) peak remained unchanged when grafted on CNTs, modifications of P3HT energy gap was observed, indicating variations of vibronic levels arising from the grafting. Moreover, broadening of the PL emission took place that suggested decreasing of lifetimes of the photo‐generated species when grafted on CNTs. Bilayer photovoltaic devices with the (6,6)‐phenyl C61‐butyric acid methyl ester as the electron acceptor have shown that the nanocomposite P3HT/P3HT‐g‐CNT performed much more efficiently as the electron donor, in both photocurrent density and power conversion efficiency, compared with the neat P3HT. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Soluble P3HT-Grafted Graphene for Efficient Bilayer−Heterojunction Photovoltaic Devices

CH(2)OH-terminated regioregular poly(3-hexylthiophene) (P3HT) was chemically grafted onto carboxylic groups of graphene oxide (GO) via esterification reaction. The resultant P3HT-grafted GO sheets (G-P3HT) are soluble in common organic solvents, facilitating the structure/property characterization and the device fabrication by solution processing. The covalent linkage and the strong electronic interaction between the P3HT and graphene moieties in G-P3HT were confirmed by spectroscopic analyses and electrochemical measurements. A bilayer photovoltaic device based on the solution-cast G-P3HT/C(60) heterostructures showed a 200% increase of the power conversion efficiency (η = 0.61%) with respect to the P3HT/C(60) counterpart under AM 1.5 illumination (100 mW/cm(2)).

Triplet Exciton and Polaron Dynamics in Phosphorescent Dye Blended Polymer Photovoltaic Devices

AbstractThe triplet exciton and polaron dynamics in phosphorescent dye (PtOEP) blended polymer (MEH‐PPV) photovoltaic devices are investigated by quasi‐steady‐state photo‐induced absorption (PIA) spectroscopy. According to the low‐temperature PIA and photoluminescence (PL) results, the increase in strength of the triplet‐triplet (T1‐Tn) absorption of MEH‐PPV in the blend system originates from the triplet‐triplet energy transfer from PtOEP to MEH‐PPV. The PtOEP blended MEH‐PPV/C60 bilayer photovoltaic device shows a roughly 30%–40% enhancement in photocurrent and power‐conversion efficiency compared to the device without PtOEP. However, in contrast to the bilayer device results, the bulk heterojunction photovoltaic devices do not show a noticeable change in photocurrent and power‐conversion efficiency in the presence of PtOEP. The PIA intensity, originating from the polaron state, is only slightly higher (within the experimental error), indicating that carrier generation in the bulk heterojunction is not enhanced in the presence of PtOEP. The rate and probability of the exciton dissociation between PtOEP and PCBM is much faster and higher than that of the triplet‐triplet energy transfer between PtOEP and MEH‐PPV.

Soluble P3HT-Grafted Carbon Nanotubes: Synthesis and Photovoltaic Application

CH2OH-terminated regioregular poly(3-hexylthiophene) (P3HT) was grafted onto carboxylic groups of acid-oxidized carbon nanotubes (CNTs) via esterification reaction. The P3HT-attached CNTs (P3CNTs) are soluble in common organic solvents, facilitating an intimate mixing with free P3HT chains for strong electronic interactions. The optical and electrochemical properties of the resultant nanocomposite were found to be different from the conventional composite, in which the pristine CNT and P3HT were physically mixed together (P3HT/CNT). Electrochemical measurements on the onset oxidation and reduction potentials of the P3CNT showed positive shifts by 0.06 and 0.1 eV, respectively, with respect to the corresponding values of pure P3HT, indicating that P3CNT has a lower highest occupied molecular orbital (HOMO) energy level and a lower lowest unoccupied molecular orbital (LUMO) energy level than those of pure P3HT. Bilayer photovoltaic devices with a thin film of pure P3CNT as the electron-donor and C60 as the ...

Heterojunction Photovoltaics Using Printed Colloidal Quantum Dots as a Photosensitive Layer

We demonstrate a bilayer photovoltaic device consisting of a heterojunction between colloidal cadmium selenide (CdSe) quantum dots (QDs) and a wide band gap organic hole-transporting thin film of N,N'-diphenyl-N,N'-bis(3-methylphenyl)[1,1'-biphenyl]-4,4'-diamine (TPD) molecules. The active light-absorbing film of QDs is nondestructively printed onto TPD using microcontact stamping. Indium-tin-oxide (ITO) provides the top contact. The resulting device structure can accommodate different size QDs, produces an exceptionally large open circuit voltage (0.8 V) for an architecture with symmetric electrodes, and yields an internal quantum efficiency of 10% at the first QD absorption peak.

Donor‐Acceptor Oligothiophenes as Low Optical Gap Chromophores for Photovoltaic Applications

Extended donor-acceptor oligothiophenes exhibit a narrow optical energy gap in combination with strong light absorption and good intermolecular π–π stacking in the solid state for efficient charge transport. Bilayer photovoltaic devices utilizing these oligomers as p-type semiconducting photosensitizers afford high power conversion efficiencies of up to 2.7%.

Star-shaped conjugated systems derived from dithiafulvenyl-derivatized triphenylamines as active materials for organic solar cells

Hybrid conjugated systems consisting of a triphenylamine core substituted by three dithiafulvenyl moieties have been synthesized and tested as active materials in organic field-effect transistors and photovoltaic devices. UV–vis. spectroscopy studies demonstrate that, despite their amorphous character shown by X-ray diffraction and differential scanning calorimetry, strong interactions exist in these materials as an aggregative band is observed at low energies. A first evaluation of their potentialities as p semiconductor in organic field-effect transistors shows significant hole mobilities. For bilayer photovoltaic devices, a power conversion efficiency of 0.11% is observed and the external quantum efficiency of the cell under monochromatic irradiation shows a non-negligible contribution of the aggregative band.

Enhanced photovoltaic performance in nanoimprinted pentacene-PbS nanocrystal hybrid device

Pentacene and PbS nanocrystal bilayer photovoltaic devices are fabricated after the pentacene layer is subjected to nanoimprinting using a laser textured silicon stamp. Increased short circuit current densities are observed for the imprinted devices, which are attributed to increased charge mobility in the pentacene film caused by the decrease in the intermolecular distances during nanoimprinting. This work is consistent with previous reports where hydrostatic pressure induced mobility increases have been observed in polyacenes under gigapascal pressure regimes. It is believed that the pentacene film undergoes localized high pressures during nanoimprinting, giving rise to the increased hole mobilities.

Ionic Space Charge Driven Organic Photovoltaic Devices

Most all-organic solar cells rely on undoped electron donor–acceptor heterojunctions. Power-conversion efficiencies depend critically on the photoinduced charge generation at these interfaces such as the charge transport through the layers and collection at the electrodes. Hence, the ability to regulate and control these processes would offer advanced device functionality. Mobile ions are able to create internal electric fields similar to conventional, electronic p-n junctions without having the inconvenience of doping, which often leads to carrier recombination and excited state quenching. We demonstrate that at organic heterointerfaces these ionic junctions can shift the electronic orbital energy level, which allows the direction of electron transfer processes to be controlled. Cationic cyanine dyes offer a convenient model system to study the effect of mobile ions systematically. In conjunction with usually strong electron acceptors such as the Buckminsterfullerene C60, and donors such as the poly(p-phenylenevinylene) derivative MEH-PPV, we fabricated bilayer photovoltaic devices to reveal exciting effects due to ionic interfacial space charge. For example, we show that C60 can be turned into an electron donor. Furthermore, oxidative or reductive electron transfer processes can simply be switched on and off with an applied bias, thereby drastically altering device performance and spectral sensitivity.

IV–VI Nanocrystal–polymer solar cells

Processes are investigated for the fabrication of hybrid bilayer photovoltaic (PV) devices consisting of IV–VI nanocrystals (NCs) having near-infrared optical gaps and an organic conductive polymer. Our design utilizes PbS NCs spin-cast onto an indium tin oxide (ITO) coated glass slide, covered with a poly(3-hexylthiophene-2,5-diyl) (P3HT) layer. We demonstrate here that the removal of the surface ligand, by pre-rinsing the NCs and subsequent annealing, generates a smoother film with a greater degree of cross-linking between the NCs. Additionally, a post-production treatment enhances the interfacial layer and improves charge carrier separation and mobility, rendering better performing solar cells. Further, results using PbSe NCs indicate the method can be adapted to similar systems. The method proposed here was designed to optimize more independently the polymer and nanocrystal components of the device and, to an extent, the interface between them.

Comparative Study of M3EH−PPV-Based Bilayer Photovoltaic Devices

We have recently shown that efficient polymer solar cells can be fabricated by using a weakly soluble derivative of poly-p-vinylene (M3EH−PPV) as the electron donor. Here we present studies on bilayer devices using organic electron acceptors with varying LUMO levels and M3EH−PPV. It is found that the open-circuit voltage scales linearly with the LUMO level of the acceptor, reaching values as high as 1.5 V when cyano-substituted poly(p-phenyleneethynylene)-alt-poly(p-phenylenevinylene) copolymers are used. Further, we discovered that for an increasing number of triple bonds in the repeat unit of the acceptor polymer the device performance decreases with increasing thickness of the acceptor layer. Also, the quantum efficiency was smaller when using polymers with higher LUMO levels. Thus, further effort is needed to design optimum acceptor polymers for devices exhibiting large open-circuit voltage and high quantum efficiency.

Bi-layer photovoltaic devices with PPQ as the electron acceptor layer

Abstract Photovoltaic cells made from bi-layer thin-film heterojunctions having poly(3-octylthiophene) (P3OT) as the electron donor and poly(phenylquinoxaline) (PPQ) as the electron acceptor have been investigated. The photovoltaic characteristics of the heterojunction device are considerably enhanced from those in a single-layer device. The photocurrent is improved by almost two orders of magnitude, and the open-circuit voltage corresponds to the estimated energy level difference of the two polymers. The observed enhancement of the heterojunction device has been discussed in terms of photoinduced charge transfer between P3OT and PPQ. The results showed that PPQ can be regarded as an efficient electron acceptor material.

Synthesis and properties of end-capped sexithiophenes incorporating the ethylene dithiothiophene unit

Two new end-capped sexithiophene derivatives (10a and 10b) have been synthesised and characterised by absorption spectroscopy, cyclic voltammetry and UV-vis spectroelectrochemistry. Both oligomers undergo two reversible, sequential oxidation processes (10a: E1½ = +0.61 V, E2½ = +0.97 V; 10b: E1½ = +0.70 V, E2½ = +0.91 V vs. Ag/AgCl) and the electronic band gaps of the materials are ca. 2.2 eV (solid state). Sexithiophene 10a has been incorporated as an electron donor material, with C60 as acceptor, in a bilayer photovoltaic device. The power conversion efficiency is a modest 0.12%, whilst the maximum IPCE at 350 nm gave an external quantum efficiency of ca. 12%.