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  • 1
    Language: English
    In: Journal of Applied Physics, 15 May 2012, Vol.111(10)
    Description: It has recently been suggested that the charge recombination rate in amorphous polymers could be affected by the energy distribution of electrons and holes as well as that of the resulting excitons. To test this hypothesis, we developed a new method for measuring charge recombination under highly imbalanced conditions. We find that if the electron density is higher than that of the holes, increasing the electron density further results in reduction of the recombination coefficient. We attribute this to the very different energy distribution between low and high carrier densities, which is not accounted for in the Langevin recombination model.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 2
    Language: English
    In: Applied Physics Letters, 02 May 2016, Vol.108(18)
    Description: We critically examine the recently suggested structure that was postulated to potentially add 50% to the photo-conversion efficiency of organic solar cells. We find that the structure could be realized using stepwise increase in the gap as long as the steps are not above 0.1 eV. We also show that the charge extraction is not compromised due to an interplay between the contact's space charge and the energy level modification, which result in a flat energy band at the extracting contact.
    Keywords: Device Physics
    ISSN: 0003-6951
    E-ISSN: 1077-3118
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  • 3
    Language: English
    In: Journal of Applied Physics, 15 March 2011, Vol.109(6)
    Description: We present light intensity dependent measurements of the quantum efficiency of P3HT:PCBM photovoltaic devices. Unlike previous studies we focus on ultralow light intensities down to 10 −3 mW/cm 2 . We find that although when the devices are excited at intensities close to 1 Sun they exhibit very little bias or light intensity dependence, this is clearly not the case for light intensities below 1 mW/cm 2 , where the cell’s efficiency becomes highly dependent on the bias and light intensity. Using a simple model for the device efficiency we can fit the experimental data across a wide range of parameters and thus separate the effects of generation efficiency (geminate recombination) and charge recombination. Our finding suggests that recombination through trap (charge transfer) states is an important loss mechanism and we are able to quantify the density and depth of these states.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 4
    Language: English
    In: Journal of Applied Physics, 07 December 2015, Vol.118(21)
    Description: We present a systematic device model that reproduces the important features of bulk heterojunction organic solar cells. While examining the model outputs we find that one of the limiting factors in organic solar cells is the reduced built-in potential due to effective pinning of the electrodes relative to the energy gap at the bulk of the device. Having identified this as a problem we suggest a device structure that can enhance the open circuit voltage. Our detailed modeling shows that such a structure can enhance the open circuit voltage as well as the short circuit current leading to above 40% improvement in power conversion efficiency of state of the art organic solar cells.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 5
    Language: English
    In: Journal of Applied Physics, 07 August 2015, Vol.118(5)
    Description: We investigate changes in the properties of 8 nm thin metal oxide (Nb-doped SrTiO 3 ) films in response to relatively high voltage or light soaking under three Sun excitation. We measure the current–voltage behavior of metal|insulator|metal type diodes and use a device model to relate changes in device behavior to the metal oxide film properties. We find that the device's resistive switching is mainly associated with shifts (switching) of the metal oxide work function between high and low injection barrier states. The method presented here can be used for in situ monitoring of the contact work function and for quantifying the uniformity of this value across the device. We also discuss the effect of non-uniform work function on the apparent diode's ideality factor.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 6
    Language: English
    In: Journal of Applied Physics, 15 August 2011, Vol.110(4)
    Description: We present a theoretical and experimental investigation of the recently reported new architecture of a patterned electrode vertical field effect transistor (PE-VFET). The investigation focuses on the role of the embedded source electrode architecture in the device behavior. Current-voltage characteristics was unraveled through the use of a self-consistent numerical simulation resulting in guidelines for the PE-VFET architecture regarding the On/Off current ratio, output current density, and apparent threshold voltage. Current modulation characteristics are obtained through the formation of virtual contacts at the PE nano-features (i.e., perforations) under gate bias, which lead to the formation of vertical channels under drain bias. As the vertical channel is formed the device characteristics change from contact-limited to space-charge-limited. The analytical model strength is shown with the parameter extraction procedure applied to a measured PE-VFET device fabricated using block copolymer lithography and with the appropriate simulation results.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 7
    Language: English
    In: Journal of Applied Physics, 14 March 2015, Vol.117(10)
    Description: By implementing Monte Carlo simulations and employing the concept of effective temperature, we explore the effects of an applied field bias on the charge carrier statistics and Peltier coefficient in hopping systems subject to the parameter range applicable to disordered organic semiconductors. Distinct differences are found between the observed field dependences as obtained from systems in which energetic disorder is spatially correlated and those in which it is not. Considerable differences are also found between the charge carrier statistics and the Peltier coefficient's field dependence in systems in which charge is transported by bare charge carriers and systems in which it is propagated by polarons. Peltier coefficient field dependence investigations are, hence, proposed as a new tool for studying charge transport and thermoelectricity in disordered organic semiconductors and systems which exhibit thermally activated transport in general.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 8
    Language: English
    In: Journal of Applied Physics, 21 May 2017, Vol.121(19)
    Description: We use device modelling for studying the losses introduced by metallic electrodes in organic solar cells' device structure. We first discuss the inclusion of pinning at the integer charge transfer state in device models, with and without using the image charge potential. In the presence of disorder, the space charge introduced due to the image potential enhances the pinning by more than 0.2 eV. The explicit introduction of the image potential creates band-gap narrowing at the contact, thus affecting both dark leakage current and photo conversion efficiency. We find that there are two regimes in which the contacts may limit the performance. For low (moderate) barriers, the contacts introduce minority carrier recombination at the contacts that adds to the bulk recombination channels. Only for high barriers, the contacts directly limit the open circuit voltage and impose a value that is equal to the contact's energy difference. Examining the device structures with blocking layers, we find that these are mainly useful for the low to moderate contacts' barriers and that for the high barrier case, the enhancement of open circuit voltage may be accompanied by the introduction of serial resistance or S shape.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 9
    Language: English
    In: Journal of Applied Physics, 15 July 2012, Vol.112(2)
    Description: There is no abstract available for this article.
    Keywords: Errata
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 10
    Language: English
    In: Journal of Applied Physics, 01 January 2011, Vol.109(1)
    Description: The exciton formation (direct charge recombination) is studied and quantified as a function of material physical-properties such as the exciton binding energy, the exciton lifetime, and the mechanism causing the electronic disorder. By using a model that is an extension of a charge transport model [ Y. Preezant and N. Tessler, Phys. Rev. B 74 , 235202 ( 2006 ) ] we are able to compare the direct exciton formation rate with the one predicted by the Langevin model. Using reasonable material parameters we find that in many cases the overall balance between free charge carrier and excitons is significantly affected by the exciton formation rate with its values being significantly low compared to the Langevin rate. We also find that in order to describe the complete recombination process it is important to introduce an intermediate state which we term exciton-precursor. This is in contrast to the common practice of using the Langevin model which embeds the assumption that the exciton formation rate is negligibly fast. The relations found between the physical-properties and the recombination rate can explain why certain materials exhibit Langevin rate while others exhibit significantly suppressed rates. This would eventually lead to the design of new materials better suited for either photocells or light-emitting diodes.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
    Source: © 2011 American Institute of Physics (AIP)〈img src=http://exlibris-pub.s3.amazonaws.com/AIP_edited.gif style="vertical-align:middle;margin-left:7px"〉
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