Optimization of multilayer antireflection coatings for bulk heterojunction organic solar cells using an improved analytical optoelectronic model

Bulk heterojunction organic solar cells made of poly(3-hexylthiophene) and 6,6-phenyl C61-butyric acid methyl ester and covered by multilayer antireflection coatings are investigated using an improved analytical optoelectronic model. The model utilizes both optical transfer matrix method and drift-diffusion equations, and it incorporates the effect of incoherent light transmission through the thick glass substrate. The influence of the multilayer antireflection coating on electromagnetic radiation propagating throughout the layered structure is studied by calculating the position-dependent charge-carrier generation rates. Dark and photocurrents are calculated by considering the first-order carrier recombination and by solving the continuity equations for electrons and holes. Justification of the improved model is achieved by comparing the obtained results with published experimental datasets from independent studies. A modified expression for exciton generation rate that includes an empirical term of sinusoidal functions is used, thus better simulating the published data. Using our improved model, good agreements are obtained between the modeled current–voltage characteristics and the measurements for different light intensities (A.M1.5G) while having an electron-to-hole mobility ratio below the limit of 50 as reported in the literature. An optimization method for plotting figures-of-merits characteristics of coated devices, which are useful to design efficient multilayer antireflection coatings and to estimate the device performance improvements, is also proposed. This optimization was successfully performed to material films adequate to antireflective applications. An improvement in the power conversion efficiency about 4.5% for active layer thickness of 90 nm is reached by using MgF2/ZrO2/MgO material films in quarter-half-quarter configuration as a multilayer antireflection coating. This coating is more efficient for active layer of 215 nm thickness showing an improvement of 5.4% in the power conversion efficiency.