AVS2013 Session EN+SE+SS+TF-WeM: Thin Films for Energy

Atomic layer deposition (ALD) is well known for its ability to deposit ultrathin films with sub-nanometer growth control and with an excellent conformality over demanding 3D surface topologies. These films can be deposited in regular cycles in an ABAB-like fashion but also more complex cycles can be employed such as (AB)_nC(AB)_nC (n ≥ 1) etc. In addition to binary thin film materials, such more complex (super)cycles also allow for the preparation of complex compound films and doped films with a high level of control of the film properties. Moreover, besides thin films, ALD can also be used to deposit nanoparticles by exploiting the poor wettability of several thin film materials, in particularly metals, on certain substrate materials. By carrying out only a limited number of cycles in the initial growth phase of the films nanoparticles can be deposited with accurate size control and a relatively narrow size distribution. These features make ALD a very attractive method to synthesize nanomaterials for clean energy technologies such as solar cells, batteries, and fuel cells. In this presentation, I will build on our previous work with respect to the deposition of nanolayers for surface passivation of silicon solar cells [1], the preparation of active films for Li-ion batteries [2], and the synthesis of noble metal nanoparticles for catalysis applications [3]. More particularly, I will report on Al₂O₃/ZnO:Al stacks for passivating (tunnel) contacts in silicon solar cells, LiCoO₂ as cathode material in all-solid state Li-ion batteries, and Pt nanoparticles as counter-electrode in flexible dye-sensitized solar cells. Most ALD processes employed in this work go beyond the regularly employed ABAB-like cycles and this will be used to demonstrate the unique features of ALD.

[1] Status and prospects of Al₂O₃-based surface passivation schemes for silicon solar cells, G. Dingemans and W.M.M. Kessels, J. Vac. Sci. Technol. A 30, 040802 (2012).

[2] Atomic layer deposition for nanostructured Li-ion batteries, H.C.M. Knoops et al., J. Vac. Sci. Technol. A 30, 010801-1 (2012).

[3] Supported Core/Shell Bimetallic Nanoparticles Synthesis by Atomic Layer Deposition, M.J. Weber et al., Chem. Mater. 24, 2973 (2012).

Tungsten oxide (WO₃) is a fascinating material, which has potential for integration into a wide range of technological applications. Most recently, it has been considered for use in photoelectrochemical cells due to the associated photocatalytic properties. However, the optical, photochemical and electrical properties of W-oxide thin films grown from either chemical or physical vapor deposition methods are sensitive to the physical and chemical characteristics, which in turn depend on the processing conditions and precursor materials. In this work, titanium (Ti) doping into WO₃ has been considered to fabricate transparent conducting oxides (TCO) for photovoltaic devices. Ti-doped WO₃ (W-Ti-O) films were grown by sputter-deposition onto silicon, Si (100), and optical grade quartz wafers. Co-sputtering of Ti and W metal targets was performed in a wide growth temperature range (room temperature (RT)-500 ⁰C). The thin films were deposited for 1 hour, resulting in a thickness ranging from 80-90 nanometers. The structure and optical properties were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and the spectrophotometry measurements. The films are optically transparent and a correlation between the growth conditions and optical properties is derived. The XRD results show Ti-doped WO₃ films grown are amorphous and crystalline. A decrease in the peak intensity implies that the crystallinity decreases with an increase in titanium (Ti) along with a phase change at higher substrate growth temperatures. The optical results show the transparency of the films is well above 80% and the energy band gap is ~3 eV, which meet the criteria TCO parameters. The effect of Ti concentration on the structure and optical properties of W-Ti-O films grown at various temperatures is presented and discussed.

We demonstrate the reversible lithiation of SiO₂ up to 2/3 Li per Si, and propose a mechanism for it based on molecular dynamics and density functional theory simulations. Our calculations show that neither interstitial Li (no reduction), nor the formation of Li₂O clusters and Si–Si bonds (full reduction) are energetically favorable. Rather, two Li effectively break a Si–O bond and become stabilized by oxygen, thus partially reducing the SiO₂ anode: this leads to increased anode capacity when the reduction occurs at the Si/SiO₂ interface. The resulting Li_xSiO₂ (x < 2/3) compounds have electronic band gaps in the range of 2.0–3.4 eV. (Published in Applied Physics Letters 100 (2012) 243905.)

It has been shown over the last two decades that conversion efficiencies up to 12% can be achieved using TiO₂ based solar cells, such as in the dye sensitized solar cell. Recently, there has been a push to replace these hazardous electrolytes with organic materials to create environmentally friendly devices with extended lifetimes. However, one of the limitation of these hybrid solar cells is electron-hole interaction across the metal-oxide and organic hole transporter interface.

In this work, we introduce core-shell nanostructured hybrid solar cells with a single crystal core in order to increase electron mobility and light scattering without additional recombination effects. Atomic layer deposition (ALD) was used to fabricate TiO₂ nanowires (NWs) in order to take advantage of the directed electric field within the structures. The TiO₂ NWs were grown in a template structure using titanium iso-propoxide and water as precursors, forming high aspect ratio arrays. During the growth process a Sn⁴⁺ dopant is introduced to create a doped Sn:TiO₂ nanostructured array with increased electron mobility. After the NW growth, an additional ALD step is used to deposit a TiO₂ layer, creating the core-shell structure. The dopant gradient within the core-shell structure causes the electrons to migrate toward the core of the nanowire due to the lower energy conduction band, potentially reducing electron-hole recombination at the TiO₂:P3HT interface. The conduction band engineering has a similar effect as that seen with dipole modification of the interface. Additionally, the surface can be further modified with dye molecules. This doped core-shell structure has resulted in a conversion efficiency of 2 % with a surface treatment of the squaraine dye SQ2. This increase in efficiency is due to the contribution of the P3HT in the photon conversion, which is limited in various dyes due injection of electrons caused by the conduction band offset. Furthermore, the engineered energy levels and interfacial modifiers have a significant effect on the external quantum efficiency and internal resistances, as determined using various characterization methods.

In this talk we will present results from a systematic study to understand charge and heat transport through polymer composites containing inorganic nanowires. Specifically we have synthesized tellurium and bismuth telluride nanowires of various diameter/length by controlling the synthesis conditions. We used the aqueous conducting polymer poly(3,4-ethylenedioxy-thiophene):polystyrene sulphonate, or PEDOT:PSS, due to its low thermal conductivity. Thin films of the nanocomposites were made by drop casting and annealing. The films were characterized by scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy and Raman spectroscopy. In general we find that increasing the weight fraction of nanowires with respect to PEDOT:PSS increases both the electrical conductivity and Seebeck coefficient of the films. In addition, the electrical conductivity can be further increased by adding a secondary dopants without significant degradation of the thermopower factor. The spectroscopic measurements suggest that chemical and structural changes at the poly(3,4-ethylenedioxy-thiophene) segment/inorganic nanowire interface are responsible for the improved thermoelectric performance of the nanocomposite.

Charge separation at small molecule organic donor/acceptor (D/A) interfaces is controlled, among other factors, by morphology and molecular packing. These buried interfaces are both crystalline and amorphous and exhibit a considerable degree of molecular intermixing. This combination prevents any single technique from solving the detailed picture of D/A interfaces. Here, a general methodology that combines Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and Atomic Force Microscopy (AFM) is shown. Since AFM is little sensitive to buried interfaces, it is employed here to investigate the surface after sequential profiling using TOF-SIMS. The unique combination of high (few nanometers) resolution depth profiling and ultra-high (parts-per-billion) chemical selectivity of TOF-SIMS and nanometer spatial resolution of AFM can assess the degree of molecular intermixing, local crystallinity and molecular packing at such interfaces. We will present results for two model interfaces: copper phthalocyanine/fullerene (CuPc/C₆₀) [1] and squaraine/fullerene (SQ/C₆₀) [2]. We find that both interfaces deviate considerably from the cartoon like picture with sharp interfaces. The CuPc/C₆₀ interfaces are characterized by an interdiffusion of the C₆₀ molecules as far as 6.5 nm into the CuPc amorphous regions. A similar interdiffusion scenario is found for SQ/C60 interfaces that can be correlated with the resulting 60% increase in the external quantum efficiency (EQE).

References:

[1] Na Sai, Raluca Gearba, Andrei Dolocan, John R. Tritsch, Wai-Lun Chan, James R. Chelikowsky, Kevin Leung, and Xiaoyang Zhu, “Understanding the Interface Dipole of Copper Phthalocyanine (CuPc)/C60: Theory and Experiment”, The Journal of Physical Chemistry Letters 3, 2173 (2013).

[2] Jeramy D. Zimmerman, Brian E. Lassiter, Kai Sun, Andrei Dolocan, Raluca Gearba, Keith J. Stevenson, David Vanden Bout, and Stephen R. Forrest, “Control of interface order by inverse quasi-epitaxial growth of squaraine/fullerene thin film photovoltaics”, submitted to Proceedings of the National Academy of Sciences of the USA (2013).