Towards the Development and Fabrication of a Low-Cost Photovoltaic CellPhil Glover and Ian Braly

School of Chemical, Biological and Environmental Engineering, Oregon State University

Background:�A scalable source of sustainable energy is needed to meet increasing energy demands with minimal environmental impact.

�The cost of solar energy must be decreased 15-20 times before it will be competitive with current energy costs.1

�Cu2ZnSnS4 (CZTS) thin films offer a low-cost alternative as an absorber material for photovoltaics.

Methods:

Future Work:�Perform microreactor syntheses investigating the effect of

-Increased temperature-Varied reactant stoichiometry�10% efficient CZTS devices have been reported utilizing

Photovoltaic Cell Construction

�Sulfur and metal precursors prepared in separate ethylene glycol solutions.

�Precursors pumped into separate preheating coils.

�Tee mixes precursors at desired temperature for CZTS nanoparticle formation.

�Reaction conditions designed to investigate the effect of precursor concentration and stoichiometry on the size, composition, and band-gap of CZTS nanoparticles

Accomplishments:

�Designed microreactor for small scale continuous production of solar absorber layer.

�Developed and demonstrated effectiveness of air as segmentation fluid in continuous reaction.

�Synthesized CZTS with low temperature, low residence time reaction.

�Performed Design of Experiments to investigate effects of precursor concentration and stoichiometricratios.

�Produced nanoparticles with desired 1.5 eV band-gap.

Air Pump Reactant Pump

Sulfur precursor Metal precursors

Mixing tee Segmenting tee

CZTS ink

Segmenting Air

“Ink blobs”

170°C Glycerol Heating Bath

Microreactor Flow Diagram

Metal Precursor Solution:�CuCl�ZnCl2

Sulphur Precursor Solution:�Thioacetemide

Solvent:

Kesterite Unit Cell3

Project Focus

Results:

-Varied reactant stoichiometry

�Increase reactor coil length to increase residence time.

�Investigate bulk crystalline and optical properties of synthesized material.

�Construct a solar cell with CZTS continuously synthesized by the microreactor.

� Test photovoltaic device efficiency.

Acknowledgments:�Sharp Labs of America for financial support�Dr. Herman for lab access and resources�Brendan Flynn for XRD and project guidance�Dr. Harding for project supervision�Dr. Yi and Teresa Sawyer for SEM training�Wei Wang for UV-Vis training�Bill Donnithorne for project documentation

References:1. Basic Needs for Solar Energy Utilization. Department of Energy.

2005.2. Todorov et. Al. High-Efficiency Solar Cell with Earth-Abundant

Liquid-Processed Absorber. Advanced Energy Materials. 2010. 3. Fischereder et. Al. Investigation of Cu2ZnSnS4 Formation from

Metal Salts and Thioacetamide. Chem Mater. 2010.4. Chang, C.H. et. al. Synthesis and post-processing of

nanomaterials using microreaction technology. J Nanopart Res. 2008.

�10% efficient CZTS devices have been reported utilizing batch liquid processing techniques.2

�Current synthesis techniques are expensive and energy intensive.

-Multi-target sputtering-Hot-injection batch synthesis with harsh solvents-Vacuum techniques

�Continuous, uniform synthesis of nanoparticles is possible with microreactor technology.

�Microreactors utilize high surface area to volume ratios.4

-Reduces heat and mass transfer limitations-Allows two-phase segmented flow

Objectives:�Characterize CZTS particles from batch reactions to verify experimental parameters

�Develop microreactor process for continuous CZTS production using inexpensive, non-toxic solvents.

�Integrate continuously synthesized CZTS into photovoltaic cells.

�Report efficiency of photovoltaic cells with CZTS absorber layer.

�ZnCl2�SnCl4•H2O

Solvent:�Ethylene glycol

XRD Characterization

UV-Vis Spectra�The low concentration, adjusted stoichiometry run yielded the desired copper-poor, zinc rich composition.

�The high concentration run resulted lower incorporations of zinc. This indicates that the order of reaction of zinc is lower than that of the other metal reactants.

�Nanoparticles form and agglomerate in the microreactor in order to reduce surface energy during their growth. The “ideal” runs yielded a zinc-poor stoichiometry. �All samples match expected peak locations, confirming

the formation of particles with the desired lattice structure.

�Extrapolation from linear region of the high concentration sample shows desired 1.5 eV band-gap.

�Lack of a linear region in the other samples indicates the presence of multiple compounds.