10.1117/2.1201212.004585 Zinc oxide nanoparticles as luminescent down-shifting layer for solar cells Yao Zhu, Aleksandra Apostoluk, Bruno Masenelli, and Patrice Melinon The efficiency of commercial solar cells can be significantly improved by using an energy down-shifting material on their front surfaces. The manufacturing of photovoltaic (PV) devices has developed considerably in recent decades, spurred by continuous growth in the demand for renewable energy sources. About 90% of cur- rently fabricated solar arrays are made of crystalline silicon (Si). Considerable research effort has been applied to increasing the efficiency of Si PV devices. However, the spectral response of a Si PV device does not match the solar emission spectrum ow- ing to the limited absorption of the Si constituting the PV solar cell active layer (see Figure 1). A single-junction Si solar cell is transparent to photons with energies below the bandgap energy, and additional sunlight is lost because of thermalization induced by higher-energy (UV) photons. This is the origin of the largest proportion of losses in commercially available Si solar cells. One way to increase solar cell efficiency is to transform the solar emis- sion spectrum so that it overlaps better with the Si absorption spectrum. Up-conversion 2 and down-conversion techniques 3 are among those used to convert the incident solar light into a spectrum that matches the absorption of the active layer in solar cells. Up-conversion permits the conversion of IR light to visible light by simultaneous absorption of two photons. This is a nonlinear process, so the probability of such a transition is quite low. In down-conversion, one photon with higher energy (UV or blue) can be converted into two identical photons of equal energy, two times lower than the initial one. This concept is interest- ing but limited, as it requires the existence of an intermediate energy level exactly in the middle of the bandgap of the down- converting material. Our research focuses on the down-shifting technique. 4, 5 Like down-conversion, down-shifting also permits the conversion Figure 1. Global solar spectrum at air mass 1.5 showing the frac- tion that is currently absorbed by a thick silicon (Si) device and the additional regions of the spectrum that can contribute toward up- and down-conversion (UC and DC, respectively). : Wavelength. (Reprinted with permission from Elsevier. 1 ) of high-energy photons into one or more lower-energy ones, contributing to the generation of additional electron-hole pairs and thus to a potential increase in the overall solar cell effi- ciency. It also involves fewer constraints than down-conversion because it does not require the presence of an intermediate energy level in the bandgap. Figure 2 illustrates the principle of the down-shifting process. Efficient down-shifting requires materials that are absorbent in the UV. Therefore, we chose zinc oxide (ZnO) for its wide bandgap (3.37eV at room temperature 6 ), larger absorption co- efficient compared to other wide bandgap materials such as gal- lium nitride, low cost, abundance, good chemical stability, and non-toxicity. Because Si solar cells have the best spectral re- sponse in the green-red spectral range, our idea is to enhance the luminescence of the ZnO nanoparticles (NPs) in this spec- tral domain, to the detriment of their UV excitonic emission, and apply them as a down-shifting layer optically coupled to the front surface of a Si solar cell. This enhancement of the green-red Continued on next page