AVS2015 Session EN-TuP: Energy Frontiers Poster Session

Organic solar cells (OSCs) offer many desirable properties such as flexibility, low cost, and easy fabrication. The lifetime is, however, still short. The degradation may be driven by light illumination, exposure to external air, and high temperature.

The degradation phenomenon under repetitive light illumination and high-vacuum is experimentally investigated in small molecular OSCs. The OSC has a structure of an indium tin oxide (anode)/copper phthalocyanine (donor, 20 nm)/fullerene (acceptor, 40 nm)/bathocuproine (buffer, 10 nm)/Ag (cathode, 100 nm). Xenon lamp illumination with an intensity of 100 mW/cm² is performed, and an air mass 1.5 global spectrum is obtained using a solar simulator. The repetitive illumination with the period of 30 s is composed of 100 repetitions of 3 s illumination followed by 27 s in the dark. The effect of light wavelength on the degradation is studied with the use of a long-pass filter of cut-wavelength λ_c. The filter allows the transmission of light into the OSC only with its wavelength longer than λ_c. Six kinds of filter with λ_c=0, 320, 370, 400, 550 and 665 nm are used.

It is found that the OSC degradation is dominated by the UV (ultra-violet) light with wavelength less than 400 nm. The short-circuit current density J_sc, open-circuit voltage V_oc, and fill factor FF in the case of λ_c=0 nm (without the filter) are decreased approximately 20%, 40%, and 60% compared with the case of λ_c greater than 400 nm at the 100th illumination, respectively. Resultantly, power conversion efficiency η_p is decreased 80%. The η_p value is decreased 60% with the filter of λ_c =320 nm, whereas initial η_p is almost kept even at 100th illumination with the filter of λ_c =400 nm. An S-shaped kink appears near the V_oc region in the current-voltage (J-V) characteristics at 100th illumination in the case of λ_c shorter than 400 nm. This shows the increase of series resistance in the OSC. The origin may be due to the disorder of crystal structure at the anode/donor interface. There are no appreciable changes of J-V characteristics between at 1st and 100th illuminations in the case of λ_c longer than 400 nm.

On the other hand, the initial η_p value at the first illumination is much smaller as λ_c is increased in the range of λ_c longer than 400 nm, accompanied by the large decrease of J_sc. This is because the total amount of light energy transmitted into active organic layers is decreased as λ_c is longer.

There are many researches on PEDOT:PSS treatment for application to flexible device electrode. Almost PEDOT:PSS treatment is consisted with step adding little bit surfactant to enhancement of adhesion between PEDOT:PSS and substrate or TCO materials. But basic research about surfactant effect is lacking. We study on the effects of sodium dodecyl sulfate (SDS) with controlling the concentration in aqueous PEDOT:PSS solution, and we confirm the conductivity enhancement of the mixture thin films with surfactant and PEDOT:PSS. Thin films are firstly prepared by spin coating method and then fabricated organic solar cells. To study the structural effects on the resulted electrical properties, thin films are mainly investigated by FE-SEM (Field Emission Scanning Electron Microscopy), AFM (Atomic Force Microscopy), respectively. At the same time, electric properties are also investigated by both 4-point probe and solar simulator.

Keywords: PEDOT, SDS, Surfactant, Organic solar cell, Electrical conductivity

Cu₂ZnSnS₄(CZTS) has attracted significant attention for thin film solar cells because it is composed on earth abundant and non-toxic elements and, has an optimal band gap of 1.5 eV. The control of the film stoichiometry is critical during the growth of CZTS thin films. We previously demonstrated the possibility to grow close to stoichiometric and phase pure crystallized CZTS thin films by reactive magnetron co-sputtering [1]. The films were close to stoichiometric (Zn/Sn=1.1-1.4, Cu/[Zn+Sn]=0.9-1.1). Nevertheless, it has been suggested that Cu-poor Zn-rich CZTS have would present the best solar cell performances [2].

Therefore, in the present work, aiming to better control the film stoichiometry and to deeper understand the relationship between the chemical composition of the deposited film and its phase constitution and ultimately to reach the best performances of the cell, two Cu poor CuSn targets presenting Cu/Sn ratio of 1.5 (CuSn1.5) and 1 (CuSn1) have been utilized. In both case, the effect of the sputtering power (P_CuSn) on the material properties has been studied. The films were characterized by multiwavelength Raman spectroscopy (785 and 325 nm), by X-Ray Diffraction (XRD), and by Energy-dispersive X-ray spectroscopy (EDX).

As expected, the phase constitution is affected by increasing P_CuSn. Using the CuSn1.5 target, its evolution is similar than the one observed using the CuSn2 target [1]: at low P_CuSn, the films mainly contain CZTS, with a low content of ZnS. Increasing the power, CZTS disappears to the profit of SnS, Cu₄Zn clusters, S₆ and ZnS. On the contrary, using the CuSn1 target does not allow the formation of CZTS dominated films. Indeed, even for the low P_CuSn conditions, ZnS dominates the Raman spectra. For medium power (P_CuSn = 45W), the film is composed by SnS, S₆, ZnS and CZTS while for higher powers, the films are similar than the one obtained with the CuSn1.5 and CuSn2 targets and the CZTS is replaced by Cu₄Zn clusters. For all sets of data, the concentration in Zn is stable. Therefore, the decomposition of the CZTS material seems to be related to the increase of the Cu and Sn concentrations and to the decrease of the S one, which deviates from the theoretical values in CZTS. Based on these data, a phase diagram can be built.

Finally, one film of each region was synthesized and used in solar cell devices in order to investigate the influence of the phase constitution on the cell performance with efficiency up to 1.8%.

[1] Cormier et al., Acta Materiallia, accepted for publication subjected to minor revisions (2015)

[2] Mitzi et al., Solar Energy Materials & Solar Cells 95 (2011)

Nowadays, Cu₂In_xGa_1-xSe₄ (CIGS) and CdTe are the leading commercially available compounds in thin film photovoltaics technology. Nevertheless, the scarcity of In and Te, and the toxicity of Cd, are considerable threats that may hinder the production and increase the cost of these materials in the near future. Replacement of In, Ga, and Se in CIGS with Zn, Sn, and S yields the promising quaternary compound Cu₂ZnSnS₄ (CZTS). This material has a favorable direct band gap of 1.5 ev. Further, each of its constituent elements is earth abundant and non-toxic. These two attractive characteristics make it plausible to launch CZTS as the wave of the future in thin film PV. We now present the synthesis process of CZTS by electron beam evaporation and a subsequent sulfurization step¹. A thin film of Zn, Cu, and Sn stacked layers was obtained upon localized heating of their respective metallic sources. The as-deposited layers were subsequently sulfurized under a vacuum inside a sealed quartz tube at temperatures ranging from 500°C to 600°C. One of the main difficulties that has been reported for CZTS synthesis is the stoichiometric control of the material. Secondary, unwanted phases such as CuS, SnS₂, ZnS, and Cu₂SnS₃ may nucleate if the initial atomic percentage of the layers or the sulfurization parameters are off a small window in which CZTS can be produced^2,3. Extreme care was taken to prevent this issue. Characterization techniques such as Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), and Raman Spectroscopy were heavily employed to confirm the presence of CZTS crystals. We are also presenting preliminary results regarding the synthesis of a Cu₂ZnSn_1-xSi_xS₄ structure. To the best of our knowledge, synthesis of this compound by the E-beam evaporation and sulfurization process is yet to be reported. Previous theoretical and experimental data regarding its wide band gap add up to the interest of engineering this material for optoelectronic applications⁴.

1. Cheng, a.-J. et al. Imaging and phase identification of Cu2ZnSnS4 thin films using confocal Raman spectroscopy. J. Vac. Sci. Technol. A Vacuum, Surfaces, Film.29, 051203 (2011).

2. Nagoya, A., Asahi, R., Wahl, R. & Kresse, G. Defect formation and phase stability of Cu2ZnSnS4 photovoltaic material. Phys. Rev. B81, 113202 (2010).

3. Polizzotti, A., Repins, I. L., Noufi, R., Wei, S.-H. & Mitzi, D. B. The state and future prospects of kesterite photovoltaics. Energy Environ. Sci.6, 3171 (2013).

4. Nakamura, S., Maeda, T. & Wada, T. Phase Stability and Electronic Structure of In-Free Photovoltaic Materials: Cu2ZnSiSe4 , Cu2ZnGeSe4 , and Cu2ZnSnSe4. Jpn. J. Appl. Phys.49, 121203 (2010).

The use of nano-crystalline TiO₂ is a promising strategy for photocatalytic remediation of wastewater due to its broad ability to remove various pollutants in aqueous media. In order to use a major part of the spectral solar distribution it would be necessary to reduce the bandgap of TiO₂ based materials. In this study, a series of nanocrystalline CeO₂-TiO₂ powders were successfully synthesized by sol-gel at room temperature, employing titanium isopropoxide and cerium nitrate as precursors, with nominal 2, 5, 10, 20 and 50 % of CeO₂:TiO₂ molar ratio. The crystalline structure, surface morphology and optical properties of synthesized samples were characterized using powder XRD, TEM, SEM, XPS. The optical band gap was determined by DR-UV-Vis spectroscopy and the emission spectrum by photoluminescence spectroscopy. Photocatalytic activity was evaluated by the photodegradation of Orange II azo dye, employing two different sources of irradiation 365 and 425 nm. Structural characterization showed that the Ce-TiO₂ particles crystallized into the anatase phase. TEM micrographies shown clearly the formation of nanocrystals. The optical bandgap energy shows a red shift for all synthesized samples compared with pure TiO₂. The results shown that the incorporation of 5 % CeO₂ effectively improves the photocatalytic activity comnpared wit pure TiO₂ at UV; while Ce incorporation activates it for 425 nm excitation still the nominal 5% shows the best performance. The results are discussed in terms of the electronic structure modification by Ce incorporation in the TiO2 matrix.

Ferroelectric thin films of perovskite-type structure are of practical interest because of their excellent ferroelectric, dielectric and optical properties. Recently these materials have generated lot of research interest for photovoltaic applications (PV) . Photovoltaic properties of Pb_0.95La_0.05Zr_0.54Ti_0.46O₃ (PLZT) thin film capacitors prepared using solution based method with various top electrodes having different work functions are investigated. The results demonstrate that ITO (Sn doped In₂O₃), a transparent conducting oxide, as top electrode, enhances the magnitude of photo voltage and photo current as compared to metal electrodes. The photovoltaic efficiency is enhanced by orders of magnitude with ITO as top electrode when compared to metal electrode devices. Thus the choice of transparent conducting oxide as an electrode holds potential for improving the photovoltaic response of ferroelectric thin film capacitors.

A self-aligned nanostructured battery entirely confined within a single nanopore offers a powerful platform to investigate the rate performance and cyclability limits of nanostructured storage devices. Atomic layer deposition (ALD) has enabled such a structure that embeds coaxial nanotubular electrodes and electrolyte confined inside a single anodic aluminum oxide (AAO) nanopore, realizing an ultrasmall full cell battery with ~1μm³ volume (~1fL). These nanopore batteries display exceptional power-energy performance and cyclability when tested as massively parallel devices (~2billion/cm²). The extraordinary thickness and conformality control of ALD and the highly self-aligned nanoporous structure of AAO are crucial to fabrication of precise, self-aligned, regular nanopore batteries. Using controlled-conformality ALD processes, we optimized metal nanotube current collector (Ru or Pt) length at two ends of AAO nanopores to provide fast electron transport to overlying anode and cathode materials, while keeping them spatially and electrically isolated. Crystalline V₂O₅ was deposited as lithium ion storage material inside the metal nanotubes using O₃ as the oxidant. . Subsequently, the V₂O₅ was electrochemically prelithiated at one end to serve as anode while pristine V₂O₅ without Li at the other end served as cathode, enabling the battery to be cycled between 0.2V and 1.8V. Capacity retention of this full cell is 95% at 5C rate and 46% at 150C, with more than 1000 charge/discharge cycles. Further increase of full cell output potential is also demonstrated for SnO₂ and TiO₂ anodes in asymmetric full cells with V₂O₅ cathodes. These results reveal the potential of ultrasmall, self-aligned/regular, densely packed nanobattery structures as a building block for high performance energy storage systems and as a test bed to study ionics and electrodics at nanoscale with a variety of geometrical modifications.