Sanjeev Singh

Bio: Sanjeev Singh is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Organic solar cell & Solar cell. The author has an hindex of 14, co-authored 19 publications receiving 795 citations. Previous affiliations of Sanjeev Singh include University of Utah & Los Alamos National Laboratory.

Experimental determination of the charge/neutral branching ratio η in the photoexcitation of π -conjugated polymers by broadband ultrafast spectroscopy

Abstract: We demonstrate a long-sought reliable method for determining the important branching ratio $\ensuremath{\eta}$ between photogenerated charged polarons and neutral excitons in $\ensuremath{\pi}$-conjugated polymer films and solutions, using femtosecond transient photomodulation spectroscopy with broad spectral range from 0.14 to $2.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We found that both excitons and polarons are instantaneously photogenerated, but $\ensuremath{\eta}$ critically depends on the film nanomorphology, which ultimately controls the interchain coupling strength. We also found that a correlation exists within each polymer family between the obtained $\ensuremath{\eta}$ value, photoluminescence quantum efficiency, and the transient polarization memory lifetime; where the interchain coupling strength in the film determines them all. We show that $\ensuremath{\eta}$ varies from less than 1% in solutions and glassy films of poly(p-phenylene-vinylene) derivatives, where the polymer chains are relatively isolated; to more than 30% in ordered films that contain lamellae, such as regio-regular poly(3-hexyl-thiophene). Our results may serve for matching polymers to specific device applications, where polymers with large $\ensuremath{\eta}$ values are good candidates for photodetector and photovoltaic applications, whereas those with small $\ensuremath{\eta}$ values are more suitable for active layers in organic light emitting devices.

Below-Gap Excitation of π-Conjugated Polymer-Fullerene Blends: Implications for Bulk Organic Heterojunction Solar Cells

Abstract: We used a variety of optoelectronic techniques such as broadband fs transient and cw photomodulation spectroscopies, electroabsorption, and short-circuit photocurrent in bulk heterojunctions organic solar cells for studying the photophysics in $\ensuremath{\pi}$-conjugated polymer-fullerene blends with below-gap excitation. In contrast to the traditional view, we found that below-gap excitation, which is incapable of generating intrachain excitons, nevertheless efficiently generates polarons on the polymer chains and fullerene molecules. The polaron action spectrum extends deep inside the gap as a result of a charge-transfer complex state formed between the polymer chain and fullerene molecule. With appropriate design engineering the long-lived polarons might be harvested in solar cell devices.

Passivation of trap states in unpurified and purified C60 and the influence on organic field-effect transistor performance

Abstract: We investigate trap-state passivation by addition of ultra-low amounts of n-dopants in organic field-effect transistors (OFET) made of as-received and purified fullerene C60. We find a strong dependence of the OFET threshold voltage (VT) on the density of traps present in the layer. In the case of the unpurified material, VT is reduced from 17.9 V to 4.7 V upon trap passivation by a dopant:C60 ratio of ∼10−3, while the Ion/off current ratio remains high. This suggests that ultra-low doping can be used to effectively compensate impurity and defect-related traps.

Highly Efficient Charge Separation and Collection across in Situ Doped Axial VLS-Grown Si Nanowire p–n Junctions

Abstract: VLS-grown semiconductor nanowires have emerged as a viable prospect for future solar-based energy applications. In this paper, we report highly efficient charge separation and collection across in situ doped Si p–n junction nanowires with a diameter <100 nm grown in a cold wall CVD reactor. Our photoexcitation measurements indicate an internal quantum efficiency of ∼50%, whereas scanning photocurrent microscopy measurements reveal effective minority carrier diffusion lengths of ∼1.0 μm for electrons and 0.66 μm for holes for as-grown Si nanowires (dNW ≈ 65–80 nm), which are an order of magnitude larger than those previously reported for nanowires of similar diameter. Further analysis reveals that the strong suppression of surface recombination is mainly responsible for these relatively long diffusion lengths, with surface recombination velocities (S) calculated to be 2 orders of magnitude lower than found previously for as-grown nanowires, all of which used hot wall reactors. The degree of surface passiva...

Reduction of contact resistance by selective contact doping in fullerene n-channel organic field-effect transistors

Abstract: We have investigated the contact-doping effect on high performance n-channel C60 organic field-effect transistors (OFETs) using the air-stable rhodocene dimer as an n-type dopant The average charge mobility improved from a value of 048 cm2/(Vs) in a reference device to 165 cm2/(Vs) for contact-doped devices with a channel length of 25 μm The operational stability of contact-doped OFETs under continuous stress bias was found similar to the reference devices

Charge Photogeneration in Organic Solar Cells

Abstract: In recent years, organic solar cells utilizing π-conjugated polymers have attracted widespread interest in both the academic and, increasingly, the commercial communities. These polymers are promising in terms of their electronic properties, low cost, versatility of functionalization, thin film flexibility, and ease of processing. These factors indicate that organic solar cells, although currently producing relatively low power conversion efficiencies (∼5-7%),1–3 compared to inorganic solar cells, have the potential to compete effectively with alternative solar cell technologies. However, in order for this to be feasible, the efficiencies of organic solar cells need further improvement. This is the focus of extensive studies worldwide. The backbone of a π-conjugated polymer is comprised of a linear series of overlapping pz orbitals that have formed via sp2 hybridization, thereby creating a conjugated chain of delocalized electron density. It is the interaction of these π electrons that dictates the electronic characteristics of the polymer. The energy levels become closely spaced as the delocalization length increases, resulting in a ‘band’ structure somewhat similar to that observed in inorganic solid-state semiconductors. In contrast to the latter, however, the primary photoexcitations in conjugated polymers are bound electron-hole pairs (excitons) rather than free charge carriers; this is largely due to their low dielectric constant and the presence of significant electron-lattice interactions and electron correlation effects.4 In the absence of a mechanism to dissociate the excitons into free charge carriers, the exciton will undergo radiative and nonradiative decay, with a typical exciton lifetime in the range from 100 ps to 1 ns. Achieving efficient charge photogeneration has long been recognized as a vital challenge for molecular-based solar cells. For example, the first organic solar cells were simple single-layer devices based on the pristine polymer and two electrodes of different work function. These devices, based on a Schottky diode structure, resulted in poor photocurrent efficiency.5–7 Relatively efficient photocurrent generation in an organic device was first reported by Tang in 1986,8 employing a vacuum-deposited CuPc/ perylene derivative donor/acceptor bilayer device. The differing electron affinities (and/or ionization potentials) between these two materials created an energy offset at their interface, thereby driving exciton dissociation. However, the efficiency of such bilayer devices is limited by the requirement of exciton diffusion to the donor/acceptor interface, typically requiring film thicknesses less than the optical absorption depth. Organic materials usually exhibit exciton diffusion lengths of ∼10 nm and optical absorption depths of 100 nm, although we note significant progress is now being made with organic materials with exciton diffusion lengths comparable to or exceeding their optical absorption depth.9–12 The observation of ultrafast photoinduced electron transfer13,14 from a conjugated polymer to C60 and the * To whom correspondence should be addressed. E-mail: j.durrant@ imperial.ac.uk. Chem. Rev. 2010, 110, 6736–6767 6736

Molecular Understanding of Organic Solar Cells: The Challenges

Abstract: Our objective in this Account is 3-fold. First, we provide an overview of the optical and electronic processes that take place in a solid-state organic solar cell, which we define as a cell in which the semiconducting materials between the electrodes are organic, be them polymers, oligomers, or small molecules; this discussion is also meant to set the conceptual framework in which many of the contributions to this Special Issue on Photovoltaics can be viewed. We successively turn our attention to (i) optical absorption and exciton formation, (ii) exciton migration to the donor−acceptor interface, (iii) exciton dissociation into charge carriers, resulting in the appearance of holes in the donor and electrons in the acceptor, (iv) charge-carrier mobility, and (v) charge collection at the electrodes. For each of these processes, we also describe the theoretical challenges that need to be overcome to gain a comprehensive understanding at the molecular level. Finally, we highlight recent theoretical advances, ...

The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors

Abstract: The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (

Organic Optoelectronic Materials: Mechanisms and Applications

Abstract: Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

The energy of charge-transfer states in electron donor-acceptor blends : insight into the energy losses in organic solar cells

Abstract: Here, a general experimental method to determine the energy ECT of intermolecular charge-transfer (CT) states in electron donor–acceptor (D–A) blends from ground state absorption and electrochemical measurements is proposed. This CT energy is calibrated against the photon energy of maximum CT luminescence from selected D–A blends to correct for a constant Coulombic term. It is shown that ECT correlates linearly with the open-circuit voltage (Voc) of photovoltaic devices in D–A blends via eVoc = ECT − 0.5 eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S1 with energy Eg) in the blend to the CT state (S1 → CT) occurs when Eg − ECT > 0.1 eV. Additionally, it is shown that subsequent charge recombination from the CT state to the lowest triplet excited state (ET) of D or A (CT → T1) can occur when ECT − ET > 0.1 eV. From these relations, it is concluded that in D–A blends optimized for photovoltaic action: i) the maximum attainable Voc is ultimately set by the optical band gap (eVoc = Eg − 0.6 eV) and ii) the singlet–triplet energy gap should be ΔEST < 0.2 eV to prevent recombination to the triplet state. These favorable conditions have not yet been met in conjugated materials and set the stage for further developments in this area.