Multi‐junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large‐scale electricity generation. However, suppressing optical reflection in the UV regime is particularly challenging due to the lack of adequate dielectric materials. In this work, bio‐inspired antireflective structures are demonstrated on a monolithically grown Ga0.5In0.5P/In0.01Ga0.99As/Ge triple‐junction solar cell, which overcome the limited optical response of reference devices. The fabricated device also exhibits omni‐directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. A comprehensive design scheme is further developed to tailor the reflectance spectrum for maximum photocurrent output of tandem cells. Copyright © 2012 John Wiley & Sons, Ltd.

high experimental effi ciencies are typically achieved through advanced front-surface optimization of the solar cells, to maximize the collection effi ciency of the highenergy photons absorbed at the surface of the devices. Such optimization techniques prove in many cases too costly for largescale industrial production, and typical mass-produced solar modules only reach conversion effi ciencies of around 20%. [ 2 ] Better use of high-energy photons thus remains an important limiting factor for commercial solar cells.GaAs-based semiconductor solar cells can exhibit very high PCEs, with demonstrated values in excess of 40% [3][4][5] and even reaching 44.7%. [ 6 ] These solar cells are typically triple-junction devices, with a Ge bottom cell absorbing mostly nearinfrared photons between 0.65 and 1.4 eV, a middle InGaAs cell principally absorbing red photons in the 1.4-1.86 eV range, and an InGaP top cell harvesting blue and UV photons above 1.86 eV. A thin window layer of AlInP is typically deposited above the top cell to act as a passivation layer. This layer minimizes non-radiative surface recombination of the excitons created near the surface of the top cell by creating an energy barrier for the minority carriers (see Figure 1 b). While improving the extraction effi ciency in the top cell, the window layer also acts as an absorber for highenergy photons, since AlInP is an indirect semiconductor with a bandgap around 2.2 eV. [ 7 ] The carriers created in the window layer tend to recombine through surface states, which reduces their extraction effi ciency. [ 8 ] This effect lowers the quantum High-effi ciency III-V solar cells typically incorporate an indirect wide-bandgap semiconductor as a passivation layer to limit surface recombination at higher photon energies. The poor extraction effi ciency of the carriers photogenerated in this window layer limits the performance of the devices in the high-energy region of the spectrum. To address this problem, a resonance energy transfer (RET)-mediated luminescent down-shifting (LDS) layer is engineered by depositing an epilayer of colloidal quantum dots (QDs) on an InGaP solar cell. In this confi guration, while the QDs act as a standard LDS layer, excitons are also funneled from the window layer to the QD epilayer using near-fi eld RET. The luminescence energy of the QDs is tuned below the bandgap of the window layer and the emitted light is absorbed in the p-n junction, where carriers are generated and effi ciently extracted. The overall performance of the solar cell is found to be signifi cantly improved after hybridization, with a large 14.6% relative and 2% absolute enhancement of the photon conversion effi ciency.

Biomimetic nanostructures have shown to enhance the optical absorption of Ga(0.5)In(0.5)P/GaAs/Ge triple junction solar cells due to excellent antireflective (AR) properties that, however, are highly dependent on their geometric dimensions. In practice, it is challenging to control fabrication conditions which produce nanostructures in ideal periodic arrangements and with tapered side-wall profiles, leading to sacrificed AR properties and solar cell performance. In this work, we introduce compound biomimetic nanostructures created by depositing a layer of silicon dioxide (SiO(2)) on top of titanium dioxide (TiO(2)) nanostructures for triple junction solar cells. The device exhibits photogenerated current and power conversion efficiency that are enhanced by ~8.9% and ~6.4%, respectively, after deposition due to their improved antireflection characteristics. We further investigate and verify the optical properties of compound structures via a rigorous coupled wave analysis model. The additional SiO(2) layer not only improves the geometric profile, but also serves as a double-layer dielectric coating. It is concluded that the compound biomimetic nanostructures exhibit superior AR properties that are relatively insensitive to fabrication constraints. Therefore, the compound approach can be widely adopted for versatile optoelectronic devices and applications.