Optimization of Earth Friendly Quantum Dots for Luminescent Down Shifting in Si Photovoltaic Encapsulants
Abstract
“Available energy is the main object at stake in the struggle for existence and the evolution of the world.”
- Ludwig Boltzmann - 1917
As both high income and especially developing countries continue to demand more and more access to energy, traditional fossil fuel power sources have struggled to maintain supply with demand. Additionally, environmental pressures resulting in climate change and poor air quality have led to increased challenges on governments and stakeholders to adopt utility scale alternative energy sources. Apart from nuclear fission, solar energy has the greatest potential to meet this increased demand and as such has garnered intense research interest over the past few decades. However, while PV module costs have begun to decrease, reducing the cost per Watt of solar installations through increasing power conversion efficiency of photovoltaic systems is an attractive strategy to increase the attractiveness of solar installations.
This work optimizes the transparency, stability, and quantum yield of ZnS, ZnO and carbon quantum dots, along with some fluorescent dyes, for integration into nanocomposite materials for luminescence down shifting (LDS). LDS is a process whereby a host polymer is infused with a luminescent species that absorbs UV photons that are not efficiently absorbed by the PV device. The device reemits photons (shifting the wavelength of the photon) to an energy state more readily absorbed by the Si PV cell.
Solar simulation experiments were performed to determine the increase of power conversion efficiency of prepared LDS layers with different luminescent species. Carbon dots prepared in diethylene glycol at a temperature of 230 oC loaded at 0.5% into polyethylene vinyl acetate (PEVA) showed an absolute PCE increase of increase of 0.15% which corresponds to a 0.89% increase over the blank device.
Of the explored organic dyes; BODIPY, pyrene, and rhodamine B, all showed very little PCE increase with BODIPY showing the greatest PCE increase of only 0.13% absolute corresponding to a relative increase of 0.49%.
Optimized Mn2+ ZnS QDs capped with 3-mercaptoprpyl methyl dimethoxysilane show a much higher improvement of PCE achieving relative 1.7% increase as compared to blank devices. Additionally, these Mn2+ ZnS QDs were found to have an even greater PCE improvement effect on previous generation monocrystalline PVs and were able to show an absolute PCE increase of 0.94% corresponding to a 6.88% relative increase on these devices. This combined with the tunability of the QD and CQD materials, their increased design potential related to coloring of the device allows for future integration into building integrated PV technologies.
The results of this work have shown that LDS is a potentially attractive method for the increase of power conversion efficiency (PCE) in a wide range PV technologies.