Shashi Poddar

Bio: Shashi Poddar is an academic researcher from University of Nebraska–Lincoln. The author has contributed to research in topics: Ferroelectricity & Piezoresponse force microscopy. The author has an hindex of 10, co-authored 26 publications receiving 919 citations. Previous affiliations of Shashi Poddar include Université catholique de Louvain.

Efficiency enhancement in organic solar cells with ferroelectric polymers

Abstract: The recombination of electrons and holes in semiconducting polymer–fullerene blends has been identified as a main cause of energy loss in organic photovoltaic devices. Generally, an external bias voltage is required to efficiently separate the electrons and holes and thus prevent their recombination. Here we show that a large, permanent, internal electric field can be ensured by incorporating a ferroelectric polymer layer into the device, which eliminates the need for an external bias. The electric field, of the order of 50 V μm−1, potentially induced by the ferroelectric layer is tens of times larger than that achievable by the use of electrodes with different work functions. We show that ferroelectric polymer layers enhanced the efficiency of several types of organic photovoltaic device from 1–2% without layers to 4–5% with layers. These enhanced efficiencies are 10–20% higher than those achieved by other methods, such as morphology and electrode work-function optimization. The devices show the unique characteristics of ferroelectric photovoltaic devices with switchable diode polarity and tunable efficiency. One of the key loss mechanisms in the operation of organic solar cells is the separation and extraction of the generated charge carriers from the active region. The use of a ferroelectric layer is now shown to create large internal electric fields, resulting in an enhanced carrier extraction and increased device efficiency.

Tuning the Energy Level Offset between Donor and Acceptor with Ferroelectric Dipole Layers for Increased Efficiency in Bilayer Organic Photovoltaic Cells

Abstract: Ultrathin ferroelectric polyvinylidene fluoride (70%)-tetrafluoroethylene (30%) copolymer film is inserted between the poly3(hexylthiophene) (P3HT) donor and [6,6]-phenyl-C61-butyric acid methylester (PCBM) acceptor layers as the dipole layer to tune the relative energy levels, which can potentially maximize the open circuit voltage of bilayer organic solar cells. In this work, the power conversion efficiency of P3HT/PCBM bilayer solar cells is demonstrated to be doubled with the inserted dipoles.

Understanding the effect of ferroelectric polarization on power conversion efficiency of organic photovoltaic devices

Abstract: It is demonstrated that the power conversion efficiency (PCE) of organic photovoltaic devices can be increased by inserting an ultrathin film of a ferroelectric co-polymer, poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)), at the metal–organic interface, due to an enhancement of the charge extraction efficiency Specifically, the effect of P(VDF-TrFE) crystallinity on its function in ferroelectric organic photovoltaic (FE-OPV) devices has been studied by several methods Highly crystalline and amorphous P(VDF-TrFE) films have been prepared by the Langmuir–Blodgett method and spin-coating from acetone solution, respectively The polymer solar cell devices with a crystalline P(VDF-TrFE) interfacial layer at the cathode have larger PCE than the structures with amorphous P(VDF-TrFE) and have the unique feature of switchable diode polarity and photovoltaic performance controlled by external applied voltage pulses The obtained results confirm that the spontaneous polarization of the ferroelectric P(VDF-TrFE) layer is responsible for the enhancement of PCE in FE-OPV devices and that a highly crystalline ferroelectric polymer film is required to observe the enhancement of PCE Amorphous P(VDF-TrFE) films act as regular dielectric layers with a little poling effect on device PCE The polarization of P(VDF-TrFE) is shown to be stable, and the photogenerated charges could be collected efficiently by the cathode rather than being compensated

Statics and Dynamics of Ferroelectric Domains in Diisopropylammonium Bromide.

Abstract: An electrically written domain structure formed by a biased tip, and visualized in the piezoresponse force microscopy mode, shows stable charged domain walls in the organic ferroelectric diisopropylammonium chloride microcrystal.

Orientational imaging in polar polymers by piezoresponse force microscopy

Abstract: We report orientational imaging of the polarization distribution in nanostructured ferroelectric copolymer of polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) and collagen fibrils using vertical and lateral modes of piezoresponse force microscopy (PFM) In PVDF-TrFE, detection of azimuthal variations in the lateral PFM signal is attributed to the alignment of the molecular chains along different directions Local switching in PVDF-TrFE is shown to proceed via 120° or 180° rotation of dipoles around the molecular chain, depending upon the strength of the applied electric field Analysis of the vertical and lateral PFM signals in collagen reveals polar anisotropy of the electromechanical properties along the axes of the fibrils The surface plots of the piezoelectric response are constructed for both materials based on their piezoelectric tensors and are shown to be consistent with the observed vertical and lateral PFM maps

25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research

Abstract: Organic photovoltaic (OPV) technology has been developed and improved from a fancy concept with less than 1% power conversion efficiency (PCE) to over 10% PCE, particularly through the efforts in the last decade. The significant progress is the result of multidisciplinary research ranging from chemistry, material science, physics, and engineering. These efforts include the design and synthesis of novel compounds, understanding and controlling the film morphology, elucidating the device mechanisms, developing new device architectures, and improving large-scale manufacture. All of these achievements catalyzed the rapid growth of the OPV technology. This review article takes a retrospective look at the research and development of OPV, and focuses on recent advances of solution-processed materials and devices during the last decade, particular the polymer version of the materials and devices. The work in this field is exciting and OPV technology is a promising candidate for future thin film solar cells.

Advances in magnetoelectric multiferroics.

Abstract: The manipulation of magnetic properties by an electric field in magnetoelectric multiferroic materials has driven significant research activity, with the goal of realizing their transformative technological potential. Here, we review progress in the fundamental understanding and design of new multiferroic materials, advances in characterization and modelling tools to describe them, and the exploration of devices and applications. Focusing on the translation of the many scientific breakthroughs into technological innovations, we identify the key open questions in the field where targeted research activities could have maximum impact in transitioning scientific discoveries into real applications. Magnetoelectric multiferroics, where magnetic properties are manipulated by electric field and vice versa, could lead to improved electronic devices. Here, advances in materials, characterisation and modelling, and usage in applications are reviewed.

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion.

Abstract: An insight into the analogies, state-of-the-art technologies, concepts, and prospects under the umbrella of perovskite materials (both inorganic-organic hybrid halide perovskites and ferroelectric perovskites) for future multifunctional energy conversion and storage devices is provided. Often, these are considered entirely different branches of research; however, considering them simultaneously and holistically can provide several new opportunities. Recent advancements have highlighted the potential of hybrid perovskites for high-efficiency solar cells. The intrinsic polar properties of these materials, including the potential for ferroelectricity, provide additional possibilities for simultaneously exploiting several energy conversion mechanisms such as the piezoelectric, pyroelectric, and thermoelectric effect and electrical energy storage. The presence of these phenomena can support the performance of perovskite solar cells. The energy conversion using these effects (piezo-, pyro-, and thermoelectric effect) can also be enhanced by a change in the light intensity. Thus, there lies a range of possibilities for tuning the structural, electronic, optical, and magnetic properties of perovskites to simultaneously harvest energy using more than one mechanism to realize an improved efficiency. This requires a basic understanding of concepts, mechanisms, corresponding material properties, and the underlying physics involved with these effects.

Recent advances in solution-processed interfacial materials for efficient and stable polymer solar cells

Abstract: This article provides an overview on the recent development of solution processed organic, inorganic, and hybrid interfacial materials for bulk-heterojunction polymer solar cells. The introduction of proper interfacial materials to optimize the electronic and electrical properties between the interfaces of the light-harvesting active layer and the charge-collecting electrode has become an important criterion to improve the performance of polymer solar cells. The electronic processes at these interfaces play a critical role in determining the efficiency for photon-to-electricity conversion. An ideal interface requires the formation of Ohmic contact with minimum resistance and high charge selectivity to prevent charge carriers from reaching the opposite electrodes. For long-term stability of polymer solar cells, interfaces with matched surface energy are required to prevent interfacial dewetting and delamination. Several classes of interfacial materials including inorganic metal oxides, crosslinkable charge-transporting materials, conjugated polymer electrolytes, self-assembled functional molecules, and graphene-based materials are highlighted and the integration of these interfacial materials with new low bandgap polymers and fullerene derivatives as active materials in different device architectures is also discussed.

Stability of organic solar cells: challenges and strategies

Abstract: Organic solar cells (OSCs) present some advantages, such as simple preparation, light weight, low cost and large-area flexible fabrication, and have attracted much attention in recent years. Although the power conversion efficiencies have exceeded 10%, the inferior device stability still remains a great challenge. In this review, we summarize the factors limiting the stability of OSCs, such as metastable morphology, diffusion of electrodes and buffer layers, oxygen and water, irradiation, heating and mechanical stress, and survey recent progress in strategies to increase the stability of OSCs, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation. Some research areas of device stability that may deserve further attention are also discussed to help readers understand the challenges and opportunities in achieving high efficiency and high stability of OSCs towards future industrial manufacture.