K. S. Narayan

Nonplanar Perylene Diimides as Potential Alternatives to Fullerenes in Organic Solar Cells.

Perylene diimides (PDIs) are attractive alternatives to fullerenes as electron transporters because of their optoelectronic properties, durability, and ease of synthesis. Belying this promise, devices that utilize PDIs as electron acceptors have low efficiencies. The primary deficiency in such cells is the low short circuit current density (JSC), which is traceable to the crystallinity of PDIs. Therefore, disrupting the crystallinity without adversely impacting the charge-transfer properties of PDIs is proposed as an important design principle. This has been achieved using a nonplanar perylene. In combination with a hole transporting polymer, a device efficiency of 2.77% has been achieved. A 10-fold increase in JSC is observed in comparison with a planar PDI, resulting in one of the highest JSC values for a solution processed device featuring a PDI. Indeed, this is one of the highest efficiencies for devices featuring a nonfullerene as the electron transporter.

Charge generation and transport in efficient organic bulk heterojunction solar cells with a perylene acceptor

The origin of high current density in efficient non-fullerene based bulk heterojunction (BHJ) organic solar cells employing a non-planar perylene dimer (TP) as an electron acceptor and a thiophene based donor polymer PBDTTT-CT is investigated using electrical and optical techniques. Photoluminescence measurements reveal almost complete quenching of both the donor and acceptor excitons, indicating efficient electron and hole transfer processes. The nanomorphology of the films shows fine mixing of the donor polymer and TP at 50 : 50% weight ratio with a photon to current conversion efficiency (IPCE) of 45% in the visible regime. At the donor–acceptor interface, both polymer and TP excitons undergo fast dissociation with similar time scales of a few picoseconds. The magnitude of the polaron yield of PBDTTT-CT:TP blends is observed to be comparable to that of PBDTTT-CT:PC70BM blends and exhibits similar μs-decay dynamics. A power conversion efficiency of 3.2% is achieved for devices with 50 : 50% by weight compositional ratio of polymer and TP.

Light responsive polymer field-effect transistor

We report the effect of light incident on a polymer-based field-effect transistor and demonstrate the utility of light as an additional controlling parameter of the transistor state. The transistor exhibits large photosensitivity indicated by the sizable changes in the drain–source current at low levels of light. The response here is considerably higher than that from existing organic/polymeric planar, two-terminal photodetectors due to an additional process contributing to the enhancement. The light-responsive polymer transistor opens up a device-architecture concept for polymer-based electronics.

Correlating reduced fill factor in polymer solar cells to contact effects

A probable limiting factor for efficiency and fill factors of organic solar cells originates from the cathode-polymer interface. We utilize various forms of cathode layer such as Al, Ca, oxidized Ca, and low melting point alloys in model systems to emphasize this aspect in our studies. The current-voltage (JV) response in the fourth quadrant indicates a general trend of convex shaped JV characteristics (d2J∕dV2>0) for illuminated devices with good cathode-polymer interfaces and linear or concave JV responses (d2J∕dV2<0) for inefficient cathode-polymer interfaces.

Area dependent efficiency of organic solar cells

Efficiency estimations of organic solar cells are observed to be dependent on the dimensions of electrode defining the active area. We address this issue and explore the manner in which efficiency scales in polymer solar cells by studying these devices as a function of electrode area and incident beam size. The increase in efficiency for smaller active areas can be explained by the reduced electrical resistive loss, the enhanced optical effects, and the finite additional fraction of photogenerated carriers in the vicinity of the perimeter defined by the metal electrode

Semiconducting-polymer-based position-sensitive detectors

We demonstrate the utility of the organic semiconducting polymers as active media for light-sensitive position-sensitive detectors (PSDs). The characteristics of these PSDs include reasonable linearity and photoresponsivity over a large range of distance, photovoltaic mode of operation, and other advantages such as nonrigid substrates and absence of any other transporting-conducting coating layer. These devices utilize the formation of Al-polymer Schottky-type photoactive interface as a common backcontact. We demonstrate results for poly(3-hexylthiophene)-based PSDs of different interelectrode spacings where the photovoltaic signals as a function of incident-beam position are linear over the entire range.

SIZABLE PHOTOCURRENT AND EMISSION FROM SOLID STATE DEVICES BASED ON CDS NANOPARTICLES

CdS nanoparticles exhibit size dependent optical and electrical properties. We report here the photocurrent and I-V characteristic studies of CdS nanoparticle devices. A sizable short circuit photocurrent was observed in the detection range governed by the size of the clusters. We speculate on the mechanisms leading to the photocurrent and emission in these nanometer scale systems.

Nonexponential relaxation of photoinduced conductance in organic field effect transistors

We report detailed studies of the slow relaxation of the photoinduced excess charge carriers in organic metal-insulator-semiconductor field effect transistors consisting of poly(3-hexylthiophene) as the active layer. The relaxation process cannot be physically explained by processes, which lead to a simple or a stretched-exponential decay behavior. Models based on serial relaxation dynamics due to a hierarchy of systems with increasing spatial separation of the photo-generated negative and positive charges are used to explain the results. In order to explain the observed trend, the model is further modified by introducing a gate voltage dependent coulombic distribution manifested by the trapped negative charge carriers.

Electric-Field-Induced Patterns in Soft Viscoelastic Films: From Long Waves of Viscous Liquids to Short Waves of Elastic Solids

We show that the electric field driven surface instability of viscoelastic films has two distinct regimes: (1) The viscoelastic films behaving like a liquid display long wavelengths governed by applied voltage and surface tension, independent of its elastic storage and viscous loss moduli, and (2) the films behaving like a solid require a threshold voltage for the instability whose wavelength always scales as approximately 4xfilm thickness, independent of its surface tension, applied voltage, loss and storage moduli. Wavelength in a narrow transition zone between these regimes depends on the storage modulus.

Nanocrystalline titanium dioxide-dispersed semiconducting polymer photodetectors

Photodetection properties of nanocrystalline titanium dioxide, TiO2, dispersed in poly-2-methoxy-5-(2′-ethyl-hexoxy)-1,4 paraphenylenevinylene MEHPPV are studied. Responsivity as high as 50 mA/W is observed in the single-layered composite device. The spectral response is sensitive to the magnitude of the bias in the low-voltage range and the crossover from a symbatic to antibatic response is closely followed. At higher reverse bias, the response is relatively uniform throughout the entire spectral range. Differences in the switching response in the forward bias and reverse bias are observed with a pronounced effect of the persistence photocurrent in the forward bias.