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Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE

Date 20-25 June 2010

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Displaying Results 1 - 25 of 804
  • [Front cover]

    Page(s): c1
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  • [Copyright notice]

    Page(s): 1
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  • Author index

    Page(s): 1 - 18
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  • Chairman's message

    Page(s): 2 - 56
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  • Area 1 program summary

    Page(s): 57 - 204
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  • Experimental measurement of restricted radiative emission in quantum well solar cells

    Page(s): 000001 - 000005
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    We present the first experimental data showing a restriction of the radiative recombination in strain-balanced quantum well solar cells. This arises due to the combined effects of quantum confinement and strain splitting the valence band in the quantum wells into heavy and light hole sub-bands. Under increasing compressive strain, the light hole sub-band moves further in energy from the conduction band, suppressing the conduction-band-to-light-hole recombination which couples primarily to photons emitted in the plane of the quantum wells. As a solar cell fundamentally need only emit in the angular range of absorption, the strain-balanced quantum well solar cell operates more closely to this optimal regime than a bulk solar cell. View full abstract»

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  • Correlations of photo-electro-thermal-luminescent imaging of Cu(In,Ga)Se2 with device performance, defects, and micro-structural properties

    Page(s): 000006 - 000011
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    Several camera imaging techniques have been applied to the characterization of Cu(In,Ga)Se2 (CIGS) solar cells having a range of efficiencies. Photoluminescence (PL) imaging shows brightness variations after the deposition of the CIGS layer that persist through CdS deposition and subsequent processing steps to finish the devices. PL and electroluminescence imaging on finished cells show a correlation to the devices' corresponding efficiency and open-circuit voltage (VOC), and dark defect-related spots correspond to bright spots on images from illuminated lock-in thermography (LIT) and forward-bias dark LIT. These image-detected defect areas are weak diodes and shunts. Imaging provides locations of defects detrimental to solar cell performance. Some of these defects are analyzed in more detail by scanning electron microscopy techniques using top and cross-sectional views. View full abstract»

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  • Monolithically integrated CIGS submodules fabricated on flexible substrates

    Page(s): 000012 - 000015
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    Monolithically integrated Cu(In,Ga)Se2 (CIGS) submodules with energy conversion efficiencies over 15% have been demonstrated on two kinds of substrates. One of them is flexible zirconia-based ceramic sheet, which contains no alkali elements and needs an extrinsic Na-doping control to obtain high solar cell efficiencies. We have demonstrated a technique to allow fine control of the Na doping level in CIGS absorber layers grown on alkali-free substrates using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to the sputtering of the Mo back contact layer. Using this technique, a submodule efficiency of 15.9% (17 cells, aperture area 75.7 cm2) has been demonstrated on flexible ceramic substrates. Another substrate used in this study was 250-μm thick soda-lime glass (SLG). Thin glass sheets are also potential flexible and lightweight substrates. Such very thin SLG substrates, however, often distorted due to the high temperatures used for CIGS growth and therefore lower growth temperatures or thinner Mo back contact layers are necessary to reduce the thermal expansion induced stress from the Mo back contact layers. Substrate distortion can lead to difficulties in the subsequent mechanical scribing processes for cell integration. In this study, CIGS absorber layers were grown at the relatively low growth temperature of 500°C on thin SLG substrates using a rf-plasma cracked Se radical beam (R-Se) source and 15%-submodule efficiencies have been demonstrated. View full abstract»

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  • Latest results of the German joint project “flexible CIGSe thin film solar cells for space applications”

    Page(s): 000016 - 000019
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    This German joint project is directed towards the development of a flexible Cu(In,Ga)Se2 (CIGSe) thin film solar cell technology on a polyimide (PI) substrate for space applications. A group of partners with an academic and/or industrial background in the field of chalcopyrite based thin film solar cells work together to ingrain space technology in production facilities for terrestrial PV. The three production technologies batch type multi-stage co-evaporation, in-line co-evaporation and roll-to-roll co-evaporation are investigated. So far, a maximum total area solar cell efficiency of 15.5 % has been achieved for lab scale devices (0.5 cm2, AM 1.5, no AR). On a large area, standardized device an active area efficiency of 12.7 % has been achieved (25.9 cm2, AM 1.5, no AR) based on an industrial roll-to-roll production process. View full abstract»

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  • Dual electrochemical and photochemical aniline treatment for CdTe solar cells

    Page(s): 000020 - 000023
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    Large-area electronic devices such as thin-film solar cells are well-known to develop nonuniformities and even pinholes in the semiconducting layers. These nonuniformities lead to the formation of weak diodes that can seriously degrade the device performance and pinholes that can shunt the back electrode to the front electrode of the cell. Since it is essentially impossible to avoid such non-uniformities, one solution is to block conduction through these pinholes with a resistive material such as polyaniline, using an electro-polymerization technique. After the polymer film deposition, this new layer acts as an insulating layer, reducing current flow from the back contact (e.g. Cu/Au) to the transparent conductive oxide and/or the n-type CdS layer, thus limiting the spatial extent of the current sink. Less dramatic than pinholes that lead to shunts, nonuniformities in the semiconducting layers are also detrimental to the CdTe device, leading to weak (low VOC) diodes that will conduct forward current when surrounding material is producing reverse, photogenerated current. Under strong illumination, the effect can be similar to a shunt. These weak diodes rob current from the neighboring cells thereby decreasing the device-generated power. Roussillon et al. showed that blocking such weak diodes can be done with a photochemical polymerization treatment. Using the device stack, glass/SnO2:F/CdS/CdTe/Cu/Au (CdS and CdTe are sputtered), we will illustrate in this paper the difference between the two types of shunt passivation treatments and demonstrate the advantage of using both treatments sequentially for shunt passivation of polycrystalline, thin-film solar cells. View full abstract»

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  • Chemical structure of buried interfaces in CdTe thin film solar cells

    Page(s): 000024 - 000027
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    We have used a lift-off technique and X-ray photoelectron spectroscopy to probe initially buried interfaces of a CdTe solar cell after CdCl2 and contact treatments. We find that the cleavage takes place at or near the CdTe/CdS interface. On both surfaces, Cl is present, most likely due to diffusion through the CdTe layer during the high-temperature CdCl2 activation step. Te is present on both cleavage-exposed surfaces, as well as on the external back contact surface before cleavage. We find that the Te atoms are in (at least) two different chemical environments. From our data we are able to paint a comprehensive picture of the chemical structure of the CdTe/CdS interface. View full abstract»

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  • Enabling dielectric rear side passivation for industrial mass production by developing lean printing-based solar cell processes

    Page(s): 000028 - 000033
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    AI2O3 rear-passivated large-area silicon solar cells with screen-printed metallization are demonstrated for the first time. An industrially feasible solar cell process is described that is based on printing steps to contact base and emitter of large area solar cells with dielectric rear side passivation. The base of the cell is contacted at the rear by a full area screen-printed aluminum layer on an inkjet-structured Al2O3/SiNx-layer stack. The Al rear contacts are co-fired with the screen-printed silver front contacts. The firing temperature is reduced to limit deterioration of the passivation ability of the aluminum oxide layer. Synergies are exploited by combining the structuring steps for the formation of openings in the rear side dielectric by hydrofluoric acid with the selective emitter formation on the front side. Investigations on lifetime samples show a 2.5-fold increase in effective lifetime for surfaces passivated by an Al2O3/SiNx stack compared to fully metalized AI-BSF rear sides. This low surface recombination velocity is combined with a low contact resistance. On 125 × 125 mm2 boron-doped Czochralski wafers with resistivity of 3 Ωcm an efficiency of 18.6% is achieved, that is a gain of 0.7% absolute compared to the efficiency of 17.9% of the best reference cells with a full area AI-BSF. An increase in the infrared spectrum of the internal quantum efficiency is determined as the source of this gain. Also, a higher reflectance at the rear side is measured that originates most probably from the Si/Al2O3 interface. The quality of the rear side passivation is assessed for the metalized and non-metalized area qualitatively and quantitatively. The local rear contacts are examined via scanning electron microscopy (SEM). A contact passivation mechanism based on a local BSF formation is found that is dependent on firing condi- ions. View full abstract»

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  • Towards 19% efficient industrial PERC devices using simultaneous front emitter and rear surface passivation by thermal oxidation

    Page(s): 000034 - 000038
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    Higher solar cell efficiencies enable a reduction of the cost per watt ratio, if production effort is maintained at an acceptable level. A proven high-efficiency concept is the passivated emitter and rear cell (PERC). However, the transfer of this solar cell structure from demonstrator level to industrial application is challenging. We present a simple approach for the industrial fabrication of PERC solar cells which utilizes the simultaneous passivation of the front emitter and the rear surface by a thin layer of thermally grown oxide. This Thermal Oxide Passivated All Sides (TOPAS) structure represents an industrially feasible implementation of the PERC concept. Instead of using masking or sacrificial layers to obtain a structure with a textured, diffused front surface and a plain non-diffused rear surface, side selective wet chemical etching is chosen in this work, since it features a higher cost reduction potential. The current cell design features a selective emitter structure, introduced by laser-doping in combination with conventional screen-printed front contacts. With the presented approach we achieve an initial efficiency of 18.9 % on large area (149 cm2) 180 μm thick, Czochralski grown, boron doped p-type wafers. The stabilized device reaches a high open circuit voltage of Voc = 641 mV. The comparison of the internal quantum efficiency of the TOPAS device and a full Al-back surface field (BSF) reference reveals a strong advantage in the blue and red response for the TOPAS concept. View full abstract»

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  • Towards high efficiency on full wafer a-Si:H/c-Si heterojunction solar cells: 19.6% on 148cm2

    Page(s): 000039 - 000043
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    In this work, we present our recent technological improvements realized on 148.5cm2 amorphous/crystalline double heterojunction solar cells. In particular, we have transferred our old solar cells fabrication process to an integrated JUSUNG large area cluster (PECVD/ MOCVD/ PVD). An optimization of all the fabrication steps separately, as well as a careful preparation of the silicon wafer's surface has allowed us to obtain efficiencies up to 19.6 % on large area solar cells (148.5cm2). The homogeneity and reproducibility of the process developed has been demonstrated on several successive batches with all efficiencies measured over 19.2%. All the fabrication process is performed at temperatures <;200°C. View full abstract»

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  • Excellent rear side passivation on multi-crystalline silicon solar cells with 20 nm uncapped Al2O3 layer: Industrialization of ALD for solar cell applications

    Page(s): 000044 - 000049
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    Current bottlenecks for industrialization of Al2O3 deposited by Atomic Layer Deposition (ALD) for crystalline silicon solar cell applications are low growth rate and stability of thin and uncapped layers during co-firing. First results on the performance of a high throughput ALD proto-type, the Levitrack, are presented. Excellent passivation properties have been obtained after firing, for 12 nm thick films deposited on p-Cz (2.3 Ω.cm) with Seff <; 15cm/s (Δn=3×1015 cm-3). These layers are compatible with solar cells that operate at a maximum open-circuit voltage of 720mV. Furthermore, we report on the passivation of 20nm uncapped aluminum oxide layers on the rear of p-type mc-Si bifacial cells. LBIC measurements unveiled excellent passivation properties on areas covered by 20nm of Al2O3 characterized by an IQE of 91% at 980nm. Remarkably, these lifetime and cell results were obtained without lengthy post-treatments like forming gas anneal. View full abstract»

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  • n-type silicon - enabling efficiencies > 20% in industrial production

    Page(s): 000050 - 000056
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    In the first part of this paper we estimate the efficiency potential of crystalline silicon solar cells on conventionally pulled p-type boron-doped Czochralski-grown silicon with typical oxygen concentrations. Taking into account an industrial high-efficiency cell structure featuring fine-line metallization, shallow and well-passivated emitter and a rear surface structure with dielectric passivation and local laser-fired point contacts, the maximum achievable efficiency is around 20%. The main limitation of such a cell is due to the rather low bulk lifetime after light-induced degradation. Even when avoiding the metastable boron-oxygen defect by using Gallium-doped or magnetic Cz-silicon, it has to be kept in mind that the detrimental impact of metal contaminations on p-type silicon is greater than on n-type silicon. A potential strategy to reduce this loss is the use of n-type silicon. Therefore, the second part of the paper discusses different architectures for solar cells on n-type silicon substrates and shows the latest results achieved at Fraunhofer ISE in this field. View full abstract»

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  • Hot carrier solar cells: Challenges and recent progress

    Page(s): 000057 - 000060
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    The limiting efficiency on the conversion efficiency of terrestrial global sunlight is not circa 31%, as commonly assumed, but 74%. To reach the lowest possible costs and hence to attain its intrinsic potential as a major source of future sustainable energy supplies, it would appear photovoltaics has to evolve to devices targeting the latter efficiency rather than the former. The hot carrier solar cell, although presenting substantial device challenges, is arguably the highest efficiency photovoltaic device concept yet suggested and hence worthy of efforts to investigate its practicality. Challenges in the implementation of hot carrier cells are identified and progress in overcoming these are discussed. View full abstract»

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  • Hot carrier solar cell efficiency simulation with carrier extraction through non ideal selective contacts

    Page(s): 000061 - 000064
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    The hot carrier solar cell is an innovative concept of high efficiency 3rd generation photovoltaic, where thermal losses are reduced by slowing the electron thermalization. Such cells offer maximal potential performance equivalent to an ideal multi-junction cell. Energy selective contacts are necessary to insulate the “hot” carriers in the absorber from the “cold” carriers in the outside world. These contacts allow only electrons with one specific energy to be collected. Here we propose a model of a cell with realistic contacts allowing transmission of electrons within a narrow energy range. The influence of the transmission range of the contact on the cell efficiency and on thermal losses is investigated. We show that, in some specific conditions, the acceptable energy width of the selective contacts can be large and semi-selective contacts allow significant efficiency enhancement. View full abstract»

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  • Advances in quantum dot intermediate band solar cells

    Page(s): 000065 - 000070
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    Several groups have reported on intermediate band solar cells (IBSC) fabricated with InAs/GaAs quantum dots (QD) which exhibit quantum efficiencies (QE) for sub-bandgap photon energies. However, this QE is produced by the absorption of photons only through valence band (VB) to intermediate band (IB) transitions. The absorption of photons of that energy in IB to conduction band (CB) transitions is weak and is usually replaced by carrier escape. This mechanism is incompatible with the preservation of the output voltage, and therefore, it cannot lead to the high efficiencies predicted by the IBSC model. In this work, we discuss the contribution of thermal and tunneling mechanisms to IB-CB carrier escape in current QD-IBSCs. It is experimentally demonstrated that in QD-IBSC prototypes where tunnel escape has been eliminated, the sub-bandgap QE is suppressed at sufficiently low temperatures, and when this occurs, the only limit for the open-circuit voltage (VOC) is the fundamental semiconductor bandgap, as stated by the IBSC theoretical model. View full abstract»

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  • Self-consistent drift-diffusion analysis of intermediate band solar cell (IBSC): Effect of energetic position of IB on conversion efficiency

    Page(s): 000071 - 000075
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    The intermediate band solar cell (IBSC) has been intensively investigated both experimentally and theoretically. Numerical analyses based on the detailed balance method are performed to search for the best suitable candidates of material combination for IBSC and the operation conditions. Analytical treatment of drift-diffusion equations has also been reported under limited approximations. However, to study the device characteristics, self-consistent treatments of both the carrier continuity equations and the Poisson equation are required. In this work, we report on the dependence of conversion efficiency on energetic position of IB and on the concentration by using 1-D self-consistent drift-diffusion simulator which we developed for GaAs based solar cell with InAs quantum dots. The dependence of the efficiency on energetic position of IB above the midgap of GaAs was calculated for 1, 10, 100 and 1000 suns conditions with and without doping in IB region. The optimal IB position shifted to lower energies with increase of concentration in the case of intrinsic IB region. While, in the case of doped IB region, the optimal IB position was almost fixed at 0.95eV. If, however, the IB was set in the middle of the energy gap of GaAs, efficiencies showed lower values with higher concentrations. This is because, according to our present model, very few photons contribute to the optical transition (generation) in CB-IB and most photons are absorbed in VB-IB transitions such that there is a large mismatch in the generation-recombination rates in these transition paths. View full abstract»

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  • Limit of nanophotonic light-trapping in solar cells

    Page(s): 000076 - 000078
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    Establishing the fundamental limit of nanophotonic light-trapping schemes is of paramount importance and is becoming increasingly urgent for current solar cell research. The standard theory of light trapping demonstrated that absorption enhancement in a medium cannot exceed a factor of 4n2 / sin2 θ, where n is the refractive index of the active layer, and θ is the angle of the emission cone in the medium surrounding the cell. This theory, however, is not applicable in the nanophotonic regime. Here we develop a statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach. Our theory reveals that the standard limit can be substantially surpassed when optical modes in the active layer are confined to deep-subwavelength scale, opening new avenues for highly efficient next-generation solar cells. View full abstract»

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  • Determination of recombination mechanisms in organic solar cells

    Page(s): 000079 - 000084
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    Studies of the recombination mechanisms in bulk heterojunction solar cells are reported. DC and transient photoconductivity are used to measure the carrier mobility and recombination times and also to show that geminate recombination is not significant in P3HT/PCBM and PCDTBT/PCBM cells. Light intensity measurements indicate that recombination is mostly monomolecular up to 1-sun light intensity. We conclude that the recombination at interface states within the bulk heterojunction structure is the dominant recombination mechanism. View full abstract»

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  • Morphology dependent short circuit current in bulk heterojunction solar cell

    Page(s): 000085 - 000089
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    Polymer based bulk heterostructure (BH) solar cell offers a relatively inexpensive option for the future solar cell technology, provided its efficiency increases beyond the current limit of 4-6%. In this paper we propose a novel theoretical/computational process/device simulation model of organic solar cell and thereby quantitatively and explicitly relate (possibly for the first time) the process conditions to the solar cell performance. We find that the maximum limit of short-circuit current (JSC) follows a simple power law with the anneal time (JSC ∝ ta-n) for ta ≥ ta,opt, and we determine the optimum anneal time (ta,opt) considering exciton dissociation (thj) and percolating pathway formation for charge carriers (tprc). Our results anticipate experimentally observed trends and have obvious and significant implications for the determination of performance limit and the optimization of the BH solar cell. View full abstract»

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  • Efficient bulk heterojunction solar cells incorporating carbon nanotubes and with electron selective interlayers

    Page(s): 000090 - 000094
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    Here, we demonstrate incorporation of carbon nanotubes (CNTs) in the active layer of donor-acceptor based solar cells and thereby significantly enhanced device performance. In this work, bulk heterojunction solar cells were fabricated with P3HT and PCBM incorporating chemically modified CNTs. Enhanced device performance was achieved for P3HT:PCBM solar cells incorporating a certain amount of modified CNTs in the active layer. P3HT:PCBM bulk heterojunction solar cells were fabricated with a thin interlayer of TiOx between the active layer and top electrode as a electron selective layer. The TiOx layer also acts as a barrier for shorting and shunting in the device, caused by the presence of metallic CNTs. In the fabricated device, CNTs provide efficient charge transportation path and the TiOx layer acts as an electron selective layer. The enhanced device performance with introduction of CNTs is attributed to better charge transportation. Work function of MWNTs is in the range of 4.5~5.1 eV, which is close to valance band of polymer; it signifies possible hole transportation through MWNTs in the active layer. As well as, we have studied affect of MoO3 and C60 as a hole and electron selective interlayer respectively for P3HT:PCBM bulk heterojunction solar cells. Fabricated device with MoO3 layer compare to PEDOT:PSS device shows better fill factor and small improve in open circuit voltage thought the short circuit current density was less. The initial degradation of P3HT:PCBM solar cells with MoO3 and C60 interlayer at the respective electrode is reduced that of the device with PEDOT:PSS interlayer. View full abstract»

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  • Role of single walled carbon nanotubes in improving the efficiency of P3HT:PCBM solar cells - impedance spectroscopy and morphology studies

    Page(s): 000095 - 000101
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    Bulk heterojunction (BHJ) solar cells of P3HT:PCBM doped with SWNTs were fabricated which doubled the efficiency over the undoped devices. No surface modifications of SWNTs were done during fabrication. Absorption and photoluminescence spectra along with photocurrent and spectral response of the devices show that SWNTs do not result in any significant charge generation at the P3HT:SWNT interface indicating that possible type-II heterojunctions between s-SWNTs and P3HT were dominated by the effects due to metallic tubes. At an optimum concentration of 0.75 wt% SWNTs, a 10% improvement in effective mobility was observed. In the voltage range of solar cell operation, two orders increase in injected current density is observed which has an Ohmic behavior. From the peak voltage of the capacitance-voltage characteristics, it was inferred that SWNTs reduce the Vbi of the devices by only 60 mV. From the Cole-Cole in the diffusion transport regime, it was observed that the injected carrier life time gets lowered from 0.628 ms to 0.125 ms with SWNTs. A negative capacitance was observed in reverse bias in devices with SWNTs at low frequencies which has similar dependence on applied field as that in forward bias. This is attributed to the large reverse current injected through SWNT energy levels, making the effects of space charge, trapping, and recombination significant. The surface roughness and volume were more than doubled with SWNTs which resulted in increased cathode coverage area affecting the charge collection efficiency. Charge extraction efficiency is analyzed using the photocurrent loss normalized to the dark current where two orders of magnitude improvement is observed with SWNTs. View full abstract»

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