Inna Kozinsky

Design and Non‐Lithographic Fabrication of Light Trapping Structures for Thin Film Silicon Solar Cells

Along with increasing fossil-fuel consumption and greenhouse gas emissions, sustainable energy research has been a global topic for many years. Among all the renewable energy sources, solar energy has been attracting widespread attention because of its abundance, stability and environmental friendliness. Nowadays, more than 90% of the photovoltaic market is dominated by wafer-based silicon solar cells. The cost of this technique, which is dominated by the starting material, is diffi cult to reduce. [ 1 ] Thin fi lm silicon solar cells have an active layer of only several micrometers thick and are believed to be a promising candidate for further cost reduction while maintaining the advantages of bulk silicon. [ 2 ] However, the effi ciency of thin fi lm silicon solar cells critically depends on optical absorption in the silicon layer since silicon has low absorption coeffi cient in the red and near-infrared (IR) wavelength ranges due to its indirect bandgap nature. Therefore, an effective light trapping design is indispensable to achieve high effi ciency modules. To address this problem, several methods are used in current technology, for example, traditional schemes such as textured transparent conductive oxide (TCO) and metal refl ector. [ 3 ]

Textured conducting glass by nanosphere lithography for increased light absorption in thin-film solar cells

Nanoscale surface texturing in thin-film solar cells has been shown to enhance device efficiency by increasing light absorption through reduced reflectance and increased light scattering across a broad range of wavelengths and angles. However, light trapping in the industrial thin-film cells is still sub-optimal and creating optimized nanoscale texture over a large area remains challenging. In this article, we present a well-controlled low-cost process to fabricate a periodic nanocone texture optimized for maximum light absorption in thin-film microcrystalline silicon solar cells. The texture is fabricated using nanosphere lithography with the period controlled by the nanosphere diameter and the texture shape and aspect ratio controlled by the reactive ion etching conditions. Finite-difference time-domain optical simulations are used to optimize the texture in the state-of-the-art microcrystalline cells, and optical absorption measurements show that the same cells fabricated on the optimized nanocone-textured substrates exhibit a relative short-circuit current increase of close to 30% compared to a reference state-of-the-art cell with a randomly textured zinc oxide layer. This nanocone texturing technique is compatible with standard thin-film cell fabrication processes and can also be used for other thin-film cells (CIGS, CdTe, CZTS, etc) to maximize light absorption and minimize layer thickness enabling more efficient carrier collection and lower overall cost.

Efficient light trapping structure in thin film silicon solar cells

Thin film silicon solar cells are believed to be promising candidates for continuing cost reduction in photovoltaic panels because silicon usage could be greatly reduced. Since silicon is an indirect bandgap semiconductor, its absorption coefficient is low for photons in the wavelength ranges between 600nm and 1100nm. For high efficiency thin film modules, effective light trapping is essential. Traditional schemes such as textured transparent conductive oxide (TCO) and metal reflector have several disadvantages such as enhanced surface recombination, parasitic losses at the TCO/metal interface, and the lack of ability to control and optimize the textured surface. We have previously proposed to employ a light trapping structure, which combines a self-assembled submicron grating and a distributed Bragg reflector (DBR) on the backside of thin film silicon solar cells. The DBR works as a one-dimensional photonic crystal to obtain almost 100% reflectivity. The grating scatters the incident light into oblique angles to significantly enhance the optical path length. Numerical calculations predict that by optimizing the feature sizes of the grating and DBR, up to 31% relative efficiency increase can be obtained, compared to the bare thin film Si. By using self-assembly, the organized grating structure can be formed spontaneously at a much lower cost. Current-voltage relations and quantum efficiency measurements were taken to verify the performance of our designed back structure. In the wavelength range of 600-900nm, photon absorption is greatly enhanced. As a result, more than 20% relative efficiency enhancement is achieved for 1.5um thin film silicon cells. These numerical and experimental results show that a light trapping design can be low-cost and increase efficiencies for high performance thin film Si solar cells.

Research priorities in CdTe and CIGS photovoltaics

The development of cost-effective thin film cells and modules has been a centerpiece of the Department of Energys national photovoltaics research for several decades. Here we will present the SunShot Initiatives current perspective on research priorities in CIGS and CdTe photovoltaics, based upon various analyses of what challenges need to be overcome to continue lowering costs towards 2020 and beyond.