Dhritiman Gupta

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

Large-area soft-imprinted nanowire networks as light trapping transparent conductors

Using soft-imprint nanolithography, we demonstrate large-area application of engineered two-dimensional polarization-independent networks of silver nanowires as transparent conducting electrodes. These networks have high optical transmittance, low electrical sheet resistance, and at the same time function as a photonic light-trapping structure enhancing optical absorption in the absorber layer of thin-film solar cells. We study the influence of nanowire width and pitch on the network transmittance and sheet resistance, and demonstrate improved performance compared to ITO. Next, we use P3HT-PCBM organic solar cells as a model system to show the realization of nanowire network based functional devices. Using angle-resolved external quantum efficiency measurements, we demonstrate engineered light trapping by coupling to guided modes in the thin absorber layer of the solar cell. Concurrent to the direct observation of controlled light trapping we observe a reduction in photocurrent as a result of increased reflection and parasitic absorption losses; such losses can be minimized by re-optimization of the NW network geometry. Together, these results demonstrate how engineered 2D NW networks can serve as multifunctional structures that unify the functions of a transparent conductor and a light trapping structure. These results are generic and can be applied to any type of optoelectronic device.

Transport of Photogenerated Charge Carriers in Polymer Semiconductors

The electrical transport of photogenerated charge carriers in disordered polymer semiconductors is reviewed. We emphasize that the mobility parameter in these disordered semiconducting systems is not a well-defined quantity. We highlight the utility of scanning probe photocurrent technique on variety of polymers in an asymmetric-electrode patterned configuration. The multiple length and time scales present in carrier transport processes are indicated by the large observed decay length scales in these systems.

Deformation of metallic liquid drop by electric field for contacts in molecular–organic electronics

We study and use the behaviour of a metallic liquid drop in the presence of an external electric field (EF). The droplet profile is governed by the stabilizing surface energy and the destabilizing electrostatic energy, with a critical voltage beyond which the droplet becomes unstable. We explore the EF-induced behaviour of low melting temperature alloy in the liquid state and observe that the droplet modifications in the linear response regime can be retained upon cooling the drop to the solid state. We demonstrate that this procedure can be used as an electrode with precise dimensions for applications in molecular and polymer electronics.

Semiconducting polymer coated single wall nanotube field-effect transistors discriminate holes from electrons

Single wall carbon nanotube (SWNT) based field effect transistors (FET) coated with semiconducting polymers respond to photoexcitation revealing characteristic features which depend on the electronic structure of the polymer. The authors observe a decrease in the drain source current of the SWNTFET in the accumulation mode in an environment of acceptor type polymer network, and a significant increase in the current in the depletion mode for a donor type polymer network around the nanotube.

Photocurrent imaging of phase segregation in a ternary polymer blend induced via a non-solvent route

Control of phase separation length scales of polymers in a solution-processed film of binary (two-component) polymer blends is the most important step to realize significant photocharge generation and transport efficiencies leading to efficient solar cells. The dynamics of phase segregation becomes more complex in ternary (three-component) polymer blends. In the three-component blend, one component serves as sensitizer, and other two act as donor and acceptor polymer. The sensitizer absorbs photons and the energy is quickly transferred to the host donor. Subsequently, efficient charge transfer occurs at the host donor/acceptor interface. Depending on the relative amount of phase segregation between sensitizer/donor, sensitizer/acceptor and donor/acceptor, the different photophysical mechanisms (viz. energy transfer and charge transfer) are either boosted or suppressed. The correct kind of morphology should allow efficient energy transfer at the sensitizer/donor interface followed by efficient charge transfer at the donor/acceptor interface. We attempt to address these requirements by modulating the nanomorphology of a ternary blend via tailoring the non-solvent concentration in the ternary polymer blend solution. Here, we study the effect of non-solvent mixing and resultant film morphology is characterized by wide-field local photocurrent imaging and photoluminescence imaging in a confocal microscope set-up.

Scanning probe photocurrent microscopy of polymer based devices

Summary form only given. Semiconducting polymer films when photoexcited with defined beam profiles generate excess charge carriers from the electronically excited state, which eventually spread over a volume exceeding the beam-sample cross-section. Our studies focuses on the length scales in model polymer systems, which are of fundamental importance in understanding the transport mechanisms and are crucial parameters for device development. A modified scanning photocurrent contrast microscopy technique was designed and implemented for these studies. The electron/hole transport processes can be independently evaluated by positioning the narrow-light beam selectively over the anode/cathode regions, enabling the estimation of the efficacy of hole transport vis-a-vis electron transport. Sizable photocurrent-signals between the electrodes are observed even at lateral distances several microns away from the counter electrode. These length scales in the first approximation corresponds to the mobility-lifetime product (mutau).

Photogenerated charge carrier length scales and optimization for efficient plastic solar cells

Performance of efficient bulk heterojunction solar cell relies on the mobility-lifetime product of the photogenerated charge carriers. A recent technique based on scanning photocurrent microscopy has been useful in estimating diffusion length scales of the photogenerated charge carriers in pristine polymer systems. The method provides an estimate of the decay length scales of both positively and negatively charged carriers and also the degree of ambipolarity. The estimates from these measurements of solar cells consisting of pristine polymers as well as blend systems can be utilized to construct pixilated solar cells of appropriate dimensions. We demonstrate the importance of the geometrical factors of the device and correlate it to the microscopic transport characteristics in order to maximize the efficiency of the system. We also discuss the dependence of the open circuit voltage and short circuit current density on the geometrical factors and the effect of local illumination on the geometry of the cathode.