Light management through up-conversion and scattering mechanism of rare earth nanoparticle in polymer photovoltaics

Abstract Primary loss mechanisms in a single junction solar cell are below Eg loss and thermalization loss which limits the maximum achievable cell efficiency. In order to overcome the fundamental loss mechanisms called transmission loss due to the mismatch of the solar spectrum with the photoactive layer absorption spectrum, the up-conversion mechanism was adopted to improve the performance of organic photovoltaics. This mechanism was adopted using rare-earth nanoparticle β-NaYF4:Yb3+/Er3+ in P3HT and PTB7 conducting polymer-based n-i-p organic photovoltaic device. β-NaYF4:Yb3+/Er3+ nanoparticles absorb infrared (IR) photons whose wavelength approximately 980 nm and emit photons in the green wavelength region with the emission intensity maximum occurs at 520 nm and 541 nm. This non-linear optical process (up-conversion mechanism) was used in the polymer solar cell. The polymers used in this study exhibits the absorption range from ~300 nm to 680 nm and 300 nm-800 nm respectively for P3HT and PTB7. Using β-NaYF4:Yb3+/Er3+ in polymer solar cell can reduce the transmission loss through harvesting IR photons. In this study, up-conversion nanoparticles (UCNPs) were embedded between the photoactive layer and anode interface to harvest IR light. Presence of these UCNPs assists in visible photons generation process in the vicinity of the photoactive layer which leads to the increased photo collection. Optimum loading of β-NaYF4:Yb3+/Er3+ nanoparticles in the device interface for improvement in the photocurrent and efficiency were investigated and correlated with the device optical and electrical processes. The optical process suggests that improvement in photocurrent is a synergistic effect of the up-conversion process and light scattering by β-NaYF4:Yb3+/Er3+. The device with UCNPs showed absorbance improvement which suggests that backscattered light at the anode/active layer interface improve the active layer absorption. Detailed investigation of the device properties allows interpreting that the photocurrent improvement is an up-conversion process as well as the scattering. In the optimized up-conversion based device, the photo-current due to up-conversion process and light scattering are of the same order. However, this can vary with respect to photoactive layer absorption properties.