How tilting and cavity-mode-resonant absorption contribute to light harvesting in 3D radial junction solar cells.

Radial junction (RJ) architecture has proven beneficial in boosting light harvesting and fast carrier separation in thin film solar cells. While a comprehensive understanding of the detailed absorption distribution and light incoupling mechanism within such a 3D RJ configuration remains largely unexplored. Taking hydrogenated amorphous Si (a-Si:H) RJ solar cells as an example, we here address in both experimental and theoretical manners the impacts of tilting and spacing configuration on the light absorption and external quantum efficiency (EQE) responses. A nice agreement between the calculated and experimental EQE responses indicates that the light harvesting realized within RJ thin film solar cells is quite robust against geometric variations and shadowing effects. Following the concepts of optical fiber injection, we have been able to single out the contribution arising solely from a resonant-mode-incoupling into the RJ cavities against a sidewall scattering incidence scenario. These results provide insightful viewpoints as well as practical guides in developing a new generation of high performance RJ thin film solar cells.

Boosting light emission from Si-based thin film over Si and SiO(2) nanowires architecture.

Silicon (Si)-based light emitting thin film has been a key ingredient for all-Si-based optoelectronics. Besides material engineering, adopting a novel 3D photonic architecture represents an effective strategy to boost light excitation and extraction from Si-based thin film material. We here explore the use of a nanowires (NW) framework, grown via vapor-liquid-solid mode, to achieve strongly enhanced yellow-green luminescence from SiN(x)O(y)/NW core-shell structure, with an order of magnitude enhancement compared to co-deposited planar references. We found that choosing geometrically-identical but different NW cores (Si or SiO(2)) can lead to profound influence on the overall light emission performance. Combining parametric investigation and theoretical modeling, we have been able to evaluate the key contributions arising from different mechanisms that include near-field enhancement, 3D light trapping and enhanced light extraction. These new findings indicate a new and effective strategy for strong Si-based thin film light emitting source, while being generic enough to be applicable in a wide variety of other thin film materials.

Operating principles of in-plane silicon nanowires at simple step-edges.

Growing silicon nanowires (SiNWs) into precise locations is a key enabling technology for SiNW-based device applications. This can be achieved via in-plane growth of SiNWs along a simple step-edge, where metal catalyst droplets absorb an amorphous Si matrix to produce c-SiNWs. However, a comprehensive understanding of this phenomenon is still lacking. We here establish an analytical model to address the driving force that dictates the growth dynamics under various droplet-step contact configurations, and to identify the key control parameters for effective guided growth. These new principles were verified against a series of experimental observations and proved to be powerful in designing a stable guided growth configuration. Furthermore, we propose and demonstrate a unique ability to achieve in situ capturing, guiding and release of the in-plane SiNWs along curved step-edges. We suggest that such a new understanding and results provide a fundamental basis and a practical guide for positioning and integrating self-assembled nanowires in a wide variety of material systems.

Understanding light harvesting in radial junction amorphous silicon thin film solar cells.

The radial junction (RJ) architecture has proven beneficial for the design of a new generation of high performance thin film photovoltaics. We herein carry out a comprehensive modeling of the light in-coupling, propagation and absorption profile within RJ thin film cells based on an accurate set of material properties extracted from spectroscopic ellipsometry measurements. This has enabled us to understand and evaluate the impact of varying several key parameters on the light harvesting in radially formed thin film solar cells. We found that the resonance mode absorption and antenna-like light in-coupling behavior in the RJ cell cavity can lead to a unique absorption distribution in the absorber that is very different from the situation expected in a planar thin film cell, and that has to be taken into account in the design of high performance RJ thin film solar cells. When compared to the experimental EQE response of real RJ solar cells, this modeling also provides an insightful and powerful tool to resolve the wavelength-dependent contributions arising from individual RJ units and/or from strong light trapping due to the presence of the RJ cell array.