DOPED GLASSES

Review of use of Doped Glasses for Down Conversion in Photovoltaic System

Review of use of Doped Glasses for Down Conversion in Photovoltaic System

Introduction

As single-junction solar cells optimally perform under monochromatic light at wavelength ? opt ~ 1240/E g (with ? opt in nm and E g in eV), an approach “squeezing” the wide solar spectrum (300-2500 nm) to a single small band spectrum without too many losses would greatly enhance solar cell conversion efficiency. Such a quasi-monochromatic solar cell could in principle reach efficiencies over 80%, which is slightly dependent on band gap ( Luque and Martí 2003 ). For (multi)crystalline silicon ((m)c-Si) solar cells ? opt = 1100 nm (with E g = 1.12 eV); for amorphous silicon (a-Si:H) the optimum wavelength is ? opt = 700 nm (with E g = 1.77 eV). However, as these cells only contain a thin absorber layer, the optimum spectrum response occurs at about 550 nm (NREL 2002,47-49 ).

Modification of the spectrum by means of so-called down- and/or upconversion or -shifting is presently being pursued for single junction cells (NREL 2008,35-39), as illustrated in Fig. 1, as a relatively easy and cost-effective means to enhance conversion efficiency. In addition, so-called luminescent solar converters (LSC) employ spectrum modification as well (NREL 2008,35-39). Downconverters or -shifters are located on top of solar cells, as they are designed to modify the spectrum such that UV and visible photons are converted leading to a more red-rich spectrum that is converted at higher efficiency by the solar cell. Upconverters modify the spectrum of photons that are not absorbed by the solar cell to effectively shift the IR part of the transmitted spectrum to the NIR or visible part; a back reflector usually is applied as well.

In case of downconversion (DC) an incident high-energy photon is converted into two or more lower energy photons which can lead to quantum efficiency of more than 100%, therefore it is also termed 'quantum cutting' (Thornton 2000,15-20); for upconversion (UC) two or more low energy photons (sub band gap) are converted into one high-energy photon (Herig 2000,115-120). Downshifting (DS) is similar to downconversion where an important difference is that only one photon is emitted and that the quantum efficiency of the conversion process is lower than unity (NREL 2008,35-39), although close to unity is preferred to minimize losses. Downshifting is also termed photoluminescence (Herig 2000,115-120). DC, UC and DS layers only influence solar cell performance optically. As DC and DS both involve one incident photon per conversion, the intensity of converted or shifted emitted photons linearly scales with incident light intensity. UC involves two photons; therefore the intensity of converted light scales quadratically with incident light intensity.

The possibility to tune chemical and physical properties in nanosized materials has a strong impact on a variety of technologies, including photovoltaics. One of the prominent research areas of nanomaterials for photovoltaics involves spectral conversion. Conventional single-junction semiconductor solar cells only effectively convert photons of energy close to the semiconductor band gap (E ...