Research Projects

Embedded Dielectric Nanoparticles for Enhanced Light Absorption in Thin Film Si Solar Cells

Silicon (Si) is nearly ideal for large-scale photovoltaics as itmakes up over 1/4 of the earth's crust and has superb electrical properties. However, it has an indirect bandgap which means that it absorbs light less efficiently than other semiconductors like CdTe, CZTS, and CIGSe that have direct bandgaps. All four of these semicondcutors can be used for thin film photovoltaics by depositing layers a few microns thick onto some cheaper substrate. The thin layers require less amterial and processing and thus make the cells less expensive. However at such thicknesses, Si can not absorb all of the red and infrared photons that make up a significant fraction of the solar spectrum. Thus the efficiency of thin film Si solar cells can be improved through optical designs that force the light either to have multiple reflections, to follow diagonal paths, or both - this general class of light manipulation is known as light trapping. Light trapping gives the photons more chance of being absorbed and converted into electricity and thus increases the current and power conversion efficiency of the cell.

Before light trapping can occur, the light has to pass into the cell. Any material having a different dielectric constant than air (i.e. Si) will reflect a large fraction of the incident light. Simple antireflective coatings (ARCs) can reduce this reflection resulting in about 30-40% more light entering a crystalline Si cell and other more advanced schemes involving nanopatterning are also very effective.

Many schemes have been developed for coupling more light into cells and for achieving light trapping. We have been theoretically studying a scheme that uses nanoparticles of a dielectric like SiO2 embedded inside the Si resulting in light trapping via lateral scattering of sunlight over a wide range of wavelengths. This scheme is compatible with a traditional ARC and importantly can be optimized nearly independently of it making deign and implementation much easier.

What:Physical model of a slab of Si covered by a Si3N4 antireflective coating and having a 200 nm diameter SiO2 nanoparticle embedded in it used for finite difference time domain electromagnetic simulations. Who:James Nagel & Mike Scarpulla.More: Our recent paper

What:Absorption enhancement calcualted in top 1 micron of Si for model pictured above. At an optimal spacing of 250-300 nm for this particular diameter (200 nm) of particle spaced 150 below the surface, we predict that 10% more photons would be absorbed from the AM1.5 spectrum. Who:James Nagel & Mike Scarpulla.More: Our recent paper

We recently published our results predicting roughly 10% gains in current realtive to a cell with an ARC:

J.R. Nagel and M.A. Scarpulla, Enhanced absorption in optically thin solar cells by scattering from embedded dielectric nanoparticles, Optics Express 18 (102) A139 (2010)

James presented his work at the TMS Electronic Materials Confernece in Notre Dame, IN recently and at the the NanoUtah Confernece in Oct. 2009.

Animations of lightwaves being scattered by such nanoparticles are on our scarpullagroup YouTube channel.

We are currently studying this concept further theoretically, but the real proof will come from actual cell fabrication incorporating these dielectric nanoparticles. The biggest challenge will be that the nanoparticles' interfaces with the Si would introduce additional recombination sites which might ruin the cell's electrical operation. We look forward to experimentally determining if this concept can be implemented effectively.

Copper Zinc Tin Sulfide (CZTS) for Thin Film Solar Cells

This project is funded by the US Department of Energy under a SISGR grant. We are synthesizing thin films of Cu2ZnSnS4 (CZTS) for use in thin film solar cells analogous to CIGSe cells. CZTS is made entirely from commodity elements, which means that scaling up a large PV industry based on this material would not have as many bottlenecks in terms of raw material supply or price that CIGSe might face. Thus it could be used to produce multi-gigawatt and even terawatt scales of PV panels to supply an appreciable fraction of our primary electricity.

CIGSe is used in polycrystalline form for 20% efficient PV cells, which is remarkable because polycrystalline semiconductors typically exhibit high recombination. The secret is in self-passivating defect reactions which convert multiple deep traps into a charge-neutral complex with no states near mid-gap. In this project, we are investigating the fundamental defect chemistry and physics of CZTS in order to see if this lesser-investigated material, which has been demonstrated to have cell efficiencies up to nearly 7%, is capable of reaching the 20% seen for CIGSe cells.

What: CIGSe is typically grown on soda-lime glass which contains Na that enhances grain growth however soda lime glass softens below the temperatures needed for CZTS phase formation. This experiment shows that Na may also be added through sinple means such as coating a CZTS film on Na-free glass with an aqueous solution of Na2S also leads to grain growth. Who: Jeff Johnson, Liz Lund, Mike Scarpulla. More: Our MRS Spring 2010 papers

What:CIGSe used in cells with efficiencies near 20% have been shown to have strong (112) crystallographic texture in its polycrystalline grain structure. We used grazing incidence focused ion beam (FIB) sample preparation and electron backscatter diffraction (EBSD) to map the grain structure through a film of our CZTS. From this large scan, we saw that the grain size was uniform with depth and that the film has statistically-significant (112) texture. Who: Jeff Johnson, Haritha Nukala, Ashish Bhatia, Matt Nowell (EDAX), Mike Scarpulla. More: Our MRS Spring 2010 papers

What: (112) planes in the tetragonal chalcopyrite lattice are equivalent to the (111) planes in the cubic zincblend lattice. Therefore grain boundaries with 0 and 60 degree rotations have special coincident site lattice relations across them resulting in lower recombination velocity. Grain boundary analysis from the EBSD data above shows that our CZTS films contain significant fractions of these special grain boundariesWho:Jeff Johnson, Haritha Nukala, Ashish Bhatia, Matt Nowell, Mike Scarpulla. More: Our MRS Spring 2010 papers

We recently presented some of our preliminary work at the 2010 Spring MRS Meeting in the following papers:

H. Nukala, et al. Synthesis of optimized CZTS thin films for photovoltaic absorber layers by sputtering from sulfide targets and sulfurization, Mat. Res. Symp. Proc. 1268 EE3.4 (2010)

J.L. Johnson, et al., Effects of 2nd phases, stress, and Na at the Mo/Cu2ZnSnS4 interface, Mat. Res. Symp. Proc. 1268 EE3.3 (2010)

and additionally at the TMS Electronic Materials Confernece in Notre Dame, IN.

Sponsored by DOE BES SISGR Program (DE-SCOOO1630)

Laser Annealing of CIGSe and Related Materials for Thin Film PV (Collaboration with Dr. Phillip Dale, University of Luxembourg)

We have recently been funded by the NSF to investigate how annealing thin films of CIGSe and related semicondcutors deposited by electrodeposition affects the microstructure and especailly the defects in these films. More information to be added soon.

Preliminary results from this work were reported at the Spring MRS 2010 in:

A. Bhatia, et al., Pulsed laser processing of electrodeposited CuInSe2 photovoltaic absorber thin films, Mat. Res. Symp. Proc. 1268 EE4.10 (2010)

Sponsored by NSF MPS DMR Materials World Network Program (DMR-1008302)

Binary Sulfides (Collaboration with Prof. Tonio Buonassisi at MIT)

Binary compound sulfides offer several promising earth-abundant material candidates for photovoltaics. We aim to improve the efficiency of binary compound sulfide thin films grown by RF magnetron sputtering. After the controlled growth of these films,  we will investigate impact of defects and surface properties on the films. Eventually, we aim to manipulate these properties via defect engineering, to create a more efficient binary sulfide thin film solar cell.

This work was presented at the 2010 EMRS Spring Meeting:

K. Hartman, et al., RF Sputtering of Tin Sulfide (SnS) at Room Temperature for Thin-Film Solar Cell Applications, Submitted to Thin Solid Films [Proceedings of European MRS Spring Meeting 2010]