(52c) Solution-Processed Energy Harvesting Electronic Devices Using Amine-Thiol Solvent Media

Solution-processed
energy harvesting electronic devices using amine-thiol solvent media

Caleb Miskin, Kevin Bock, Robert Boyne, and Rakesh Agrawal

School of
Chemical Engineering, Purdue University, West Lafayette, IN

On February 15, 2008 the National Academy of
Engineering unveiled their fourteen grand challenges of engineering for the 21^st
century. At the top of the list and voted by the public as the most important
challenge was the thrust to make solar energy economical. My research is
dedicated to solving this millennial challenge by developing routes to
high-efficiency, solution-processed photovoltaics (PV) for low-cost and
low-energy manufacturing. Traditional PV manufacturing has emphasized
high-vacuum techniques which are both costly, slow, and energy intensive.
Solution-processed techniques performed at ambient temperatures and pressures
relieve much of the cost and energy burden of thin-film manufacturing.

Figure 1. Cross section of solution-processed CdTe film after annealing at 500 °C (a). Plane view of same CdTe film (b). Scale same for both.
" src="https://www.aiche.org/sites/default/files/aiche-proceedings/conferences/..." height="404" class="documentimage">My research has primarily advanced two methods for
solution processed PV. In one method, semiconducting nanocrystals are
synthesized and then suspended in an appropriate solvent to form an ink. The
ink is then applied to a substrate by a variety of high-throughput methods such
as spray coating or doctor blading and then annealed to form a polycrystalline
absorber layer for solar energy. I have applied this method with great success
to Cu₂ZnSnS₄, a promising earth-abundant, non-toxic
semiconductor. A challenge with this material is its propensity to form binary
and ternary undesired phases. Using advanced nano-characterization techniques,
my colleagues and I have been able to determine the spatially resolved
composition of these nanoparticles and have found them to be highly
non-uniform.^1,2 As a result, I have focused on synthesis techniques
aimed at controlling the nucleation and growth of this material to improve
nanocrystal compositional homogeneity. Though particles produced through my
work still show some non-uniformities, they are greatly improved. I have combined
these improved synthetic techniques with optimized conditions for grain growth
of the absorber layer to achieve micron-sized densely packed grains, while
minimizing the unwanted ?fine-grain? layer characteristic of most
solution-processed CZTSSe devices. As a result, I have been able to advance the
published record efficiency for nanocrystal ink based solar cells of CZTS from
7.2 to 9.0 percent.³

Another promising route to solution-processed PV is by
directly coating molecular precursor solutions (rather than first forming
nanocrystals) and annealing the coating to form the polycrystalline solar absorber
layer. While processing in this manner has long been employed by the organic
electronic community, ultimately it is
desirable to combine the superior performance of inorganic electronic materials
with the facile processing of organic solutions. A major challenge is that many salts and metals that would be useful
precursors to such films have poor solubility in organic solvents compatible
with roll-to-roll manufacturing techniques. Our
group and others have overcome this challenge by developing a mixture of
commonly available thiols and amines to dissolve a host of materials that are
otherwise insoluble in either solvent by itself.^4?6 The solvent system has found significant use in processing photovoltaic absorber
layers,^7?9
luminescent quantum dot films,⁷ and
other thin films¹⁰
making it a very general solvent system for processing of inorganic thin films.

In my work, I have focused on CdTe?which has been by
far the most successful technology in terms of production cost ($/peak watt)
and energy payback time for thin-film solar cells. The solution-processing of
this material may provide another step change in cost for this material. In
this research thrust I have demonstrated for the first time the fabrication of
CdTe thin films via a solution-processed molecular precursor approach by
dissolving CdCl₂ and Te metal in ethylenediamine and propanethiol.
The films are formed by spin-coating ultra-thin layers of the solution and then
annealing each layer until a ~1 μm thick film is achieved. A plane view
and cross sectional image of an annealed CdTe flim is shown in Figure 1. By
optimizing the annealing and deposition conditions I am actively increasing the
efficiency of these devices with a goal to reach those currently achieved in
vacuum-based production (15-20%).

While amine-thiol mixtures are truly fantastic
solvents for molecular precursors, I have found they are highly useful for
nanoparticle synthesis as well. The phase of Cu₂ZnSnS₄
nanoparticles can be changed from tetragonal kesterite to hexagonal wurtzite
simply by changing the solvent and sulfur source from dissolved sulfur in
oleylamine to a mixture of oleylamine and dodecanethiol.¹¹ The
increased solvating power of amine-thiol mixtures also provides a method to
tailor the solubility of monomer in solution, thereby altering the temperatures
at which species nucleate. I am using this to pursue routes to increasingly
monodispersed CZTS nanoparticles both in terms of size and composition by
suppressing the formation of certain phases and favoring the nucleation of
others. In addition, I have also been successful in exploiting the alkahest
capabilities of amine-thiol mixtures for the controlled synthesis of several other
useful binary and multinary nanocrystals.

To better understand this solvent system's properties
and chemistry so that it might be improved for current uses and tailored for
others, I am collaborating with an analytical chemistry group to perform
advanced tandem mass spectrometry (ESI and APCI) on the solvent mixtures and
solutions to determine the species present. Based on the compositions found, we
are employing quantum chemical
calculations and modeling to determine
the structure of the species. This insight will allow the further development
of this already incredibly versatile solvent system for not only PV but other
electronic devices as well. 

References

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