Page 285 - A Comprehensive Guide to Solar Energy Systems

Chapter 13 • Upconversion and Downconversion Processes for Photovoltaics 289

13.2.3 Approaches to Increase Upconversion Performance
Enhancement

In contrast to the promising potential of upconversion and the steady progress that has
been made in terms of application to PV over the last few years, the overall efficiency en-
hancement achieved upon application of upconversion are still low. This necessitates the
important means to be explored to increase upconverting solar cell device performance.
There are two approaches that could potentially help in addressing this issue: (1) enhanc-
ing the upconverter properties and (2) by modifying the upconverter environment.

13.2.3.1 Material Optimization
3+
13.2.3.1.1 LN -BASED UPCONVERTERS
For enhancing the upconverter properties especially in case of lanthanum-based upcon-
verters, the choice and concentration of dopant material and the properties of the host
are important and need to be optimized for highest device performance. In the context of
photovoltaics, the additional desirable properties for the host material and upconverter
are chemical and optical stability, strong crystal fields at the position of the upconverting
material for high radiative transition probabilities and broad absorption, high transparen-
cy outside the active region and low scattering to avoid parasitic optical losses and finally,
a high refractive index to reduce optical losses.

13.2.3.1.2 ORGANIC UPCONVERTERS
In addition to synthesis of even more efficient TTA upconverter molecules, optimizing the
concentration of these materials in their surroundings is an important factor to be con-
sidered. Increasing the concentration of active species has been mentioned as the major
strategy to increase the UCQy of TTA-based upconverter materials—especially increasing
the steady-state concentration of emitter molecules in their excited state [28]. As simply
increasing the concentration of emitter molecules in the solution is limited by the solution
limit, increasing the concentration of sensitizer molecules as well as the lifetime of the
excited triplet state of the emitter is considered a promising approach [6].

13.2.3.2 Material Environment
13.2.3.2.1 PLASMONICS AND PHOTONICS
Dielectric and metal photonic nanostructures are powerful tools to enhance upconver-
sion performance, because they can enhance both the absorption and emission process-
es. Photonic nanostructures can increase the local irradiance on the upconverter, thus
increasing absorption and, due to the nonlinearity of upconversion, also the UCQy. The
photonic nanostructures on the other hand change the local photonic density of states
(lDOs), which can be used to amplify desired radiative transitions and suppress others
[31–34].
It is important to note that both the plasmonic and the dielectric photonic concepts to
enhance upconversion performance are not compatible with mono- and microcrystalline