Nanostructured solar cells

Conversion into electrical power of even a small fraction of the solar radiation
incident on the Earth’s surface has the potential to satisfy the world’s energy demands without generating CO2 emissions. Current photovoltaic technology is
not yet fulfilling this promise, largely due to the high cost of the electricity produced. Although the challenges of storage and distribution should not be
underestimated, a major bottleneck lies in the photovoltaic devices themselves.
Improving efficiency is part of the solution, but diminishing returns in that area
mean that reducing the manufacturing cost is absolutely vital, whilst still retaining good efficiencies and device lifetimes.

Solution-processible materials, e.g. organic molecules, conjugated polymers
and semiconductor nanoparticles, offer new routes to the low-cost production of
solar cells. The challenge here is that absorbing light in an organic material
produces a coulombically bound exciton that requires dissociation at a
donor–acceptor heterojunction. A thickness of at least 100 nm is required to
absorb the incident light, but excitons only diffuse a few nanometres before
decaying. The problem is therefore intrinsically at the nano-scale: we need
composite devices with a large area of internal donor–acceptor interface, but
where each carrier has a pathway to the respective electrode. Dye-sensitized and
bulk heterojunction cells have nanostructures which approach this challenge in
different ways, and leading research in this area is described in many of the
articles in this special issue.

This issue is not restricted to organic or dye-sensitized photovoltaics, since
nanotechnology can also play an important role in devices based on more conventional inorganic materials. In these materials, the electronic properties can be controlled, tuned and in some cases completely changed by nanoscale confinement. Also, the techniques of nanoscience are the natural ones for investigating the localized states, particularly at surfaces and interfaces, which are often the limiting factor in device performance.

This issue provides concrete examples of how the techniques of nanoscience and nanotechnology can be used to understand, control and optimize the performance of novel photovoltaic devices.

Nanostructure Fabrication Processes

Progress in understanding and optimizing materials processing in wet chemical environments requires the use of in situ measurements of the structure and dynamics of the metal-electrolyte interface under realistic conditions. These measurements can provide insight into the mechanisms of relevant atomic, molecular, and mesoscale film growth and dissolution processes.

This project has several thrust areas ranging from measurements and modeling of surfactant mediated growth to the investigation of both surface and thin film growth stress. Particular attention has been given to the role of electrolyte additives in the formation and performance of advanced nanoscale and mesoscale interconnects as used in state of the art microelectronic devices. Measurements are also underway that detail the use of underpotential deposition (upd) reactions to precisely control the composition and structure of 2-D and 3-D alloys. This is complemented by measurements of stress changes that accompany alloy formation as well as the inverse dealloying process. An integral part of the program is the development of mechanistic linkage between atomic and molecular phenomena and rational design of the desired nanostructures.

Additional Technical Details:
This year we demonstrated the first example of void-free trench filling with ferromagnetic materials. Feature filling involves a new mechanism of superconformal growth that uses a single inhibitor whose consumption during deposition gives rise to positive feedback. Coupling of the non-linear dynamics with the non-planar substrate geometry gives rise to void-free nickel deposition in the recessed surface features as shown in the cross section TEM images given below. Two types of molecules have been shown to yield this effect; cationic Nbearing polymers and more recently certain benzimidazole derivatives. The latter provide feature filling dynamics that offer seamless integration with conventional Damascene processing and thereby the prospect of introducing ferromagnetic materials into 3-D metallization for ULSICMOS and MEMS applications.

Measurements developed for upd processes of alloy deposition have received significant attention in the past year. Practical interest in the production of Pt-transition metal alloys for use as either hard magnetic materials for memory applications and /or as potential fuel cell electrocatalyst has motivated much of this work. The Pt-Cu system has been examined as a model upd-codeposition system due to the absence of parasitic reactions. As shown below, co-deposition of Cu with Pt occurs at potentials well positive of that required to deposit pure Cu. The figure also demonstrates the use of in situ quartz crystal gravimetry for the determination of alloy composition along with a direct comparison to post deposition ex-situ methods.

The upd process has also been applied to Pt-Ni and Pt-Co alloys and preliminary studies indicate that these alloy films are more catalytic than pure Pt for the oxygen reduction reaction; the latter being a central impediment to improved fuel cell performance.

In order to gain a deeper insight into upd and molecular adsorption processes relevant to a wide range of electrochemical processing issues, a variety of in situ scanning tunneling microscope (STM), atomic force microscope (AFM), stress and gravimetric measurements are underway.

MSEL has recently constructed an optical bench for in situ measurement of surface stress during electrochemical processing using the wafer curvature method. Forces on the order of 0.008 N/m (23 km radius of curvature) can be resolved, sufficient to study the adsorption of upd and molecular monolayers. This powerful method is capable of monitoring the surface stress associated with reversible upd reactions such as Pb onto the (111)-textured Au surface as shown below.

The stress transient shows four regimes of behavior from ClO4- desorption, Pb-Au bond formation, stress relaxation due to hcp-Pb island coalescence, and electrocompression of the monolayer at potentials just positive of bulk Pb deposition. Interestingly, these measurements show that the complete Pb monolayer behaves as a free-standing elastic film where the stress - strain proportionality has a value equal to the biaxial modulus for Pb (111) in the bulk. Similar work has examined the upd of Bi on Au. Further work is underway exploring these timely and exciting issues.