Jo Gjessing

2D back-side diffraction grating for improved light trapping in thin silicon solar cells

Light-trapping techniques can be used to improve the efficiency of thin silicon solar cells. We report on numerical investigation of a light trapping design consisting of a 2D back-side diffraction grating in combination with an aluminum mirror and a spacing layer of low permittivity to minimize parasitic absorption in the aluminum. The light-trapping design was compared to a planar reference design with antireflection coating and back-side aluminum mirror. Both normally and obliquely incident light was investigated. For normal incidence, the light trapping structure increases the short circuit current density with 17% from 30.4 mA/cm(2) to 35.5 mA/cm(2) for a 20 microm thick silicon solar cell. Our design also increases the current density in thinner cells, and yields higher current density than two recently published designs for cell thickness of 2 and 5 microm, respectively. The increase in current may be attributed to two factors; increased path length due to in-coupling of light, and decreased parasitic absorption in the aluminum due to the spacing layer.

Comparison of periodic light-trapping structures in thin crystalline silicon solar cells

Material costs may be reduced and electrical properties improved by utilizing thinner solar cells. Light trapping makes it possible to reduce wafer thickness without compromising optical absorption in a silicon solar cell. In this work we present a comprehensive comparison of the light-trapping properties of various bi-periodic structures with a square lattice. The geometries that we have investigated are cylinders, cones, inverted pyramids, dimples (half-spheres), and three more advanced structures, which we have called the roof mosaic, rose, and zigzag structure. Through simulations performed with a 20 μm thick Si cell, we have optimized the geometry of each structure for light trapping, investigated the performance at oblique angles of incidence, and computed efficiencies for the different diffraction orders for the optimized structures. We find that the lattice periods that give optimal light trapping are comparable for all structures, but that the light-trapping ability varies considerably between th...

Light-Trapping Properties of a Diffractive Honeycomb Structure in Silicon

Thinner solar cells will reduce material costs, but require light trapping for efficient optical absorption. We have already reported development of a method for fabrication of diffractive structures on solar cells. In this paper, we create these structures on wafers with a thickness between 21 and 115 μm, and present measurements on the light-trapping properties of these structures. These properties are compared with those of random pyramid textures, isotropic textures, and a polished sample. We divide optical loss contributions into front-surface reflectance, escape light, and parasitic absorption in the rear reflector. We find that the light-trapping performance of our diffractive structure lies between that of the planar and the random pyramid-textured reference samples. Our processing method, however, causes virtually no thinning of the wafer, is independent of crystal orientation, and does not require seeding from, e.g., saw damage, making it well suited for application to thin silicon wafers.

An Optical Model for Predicting the Quantum Efficiency of Solar Modules

The fact that a solar cell will be encapsulated in a solar module has a strong impact on how its optical properties should be characterized, designed, and optimized. In this paper, we present a ray-tracing model that is capable of calculating the optical properties of a solar cell with a random pyramidal texture both with and without encapsulation and for the light at any angle of incidence. We introduce a simple method wherein we combine ray tracing and external quantum efficiency (EQE) measurements of a solar cell to predict the EQE after encapsulation for any choice of encapsulation materials and antireflection coatings. We verify the quality of our EQE-prediction toward experimental data and find good agreement. We apply this approach and compare the influence of different encapsulation materials on the module performance and perform a full loss analysis at a few selected angles of incidence. Finally, we use the ray-tracing model to map generation current for all angles of incidence relevant for a pyramidal texture both with and without encapsulation.

Miniature, low-cost, 200 mW, infrared thermal emitter sealed by wafer-level bonding

Infrared (IR) thermal emitters are widely used in monitoring applications. For autonomous systems, miniaturized devices with low power consumption are needed. We have designed, fabricated and tested a novel device design, packaged on the wafer level by Al-Al thermo-compression bonding. 80 μm wide Aluminium frames on device and cap wafers were bonded in vacuum at 550°C, applying a force of 25 kN for 1 hour. The bond force translated to a bond pressure of 39 MPa. Subsequent device operation showed that the seals were hermetic, and that the emitters were encapsulated in an inert atmosphere. The emitters were optimized for radiation at λ=3.5 μm. Emission spectra by Fourier Transform Infrared Spectroscopy showed high emissivity in the wavelength range 3 – 10 μm at 35 mA driving current and 5.7 V bias, i.e. 200 mW power consumption. The emitter temperature was around 700 °C. The rise and fall times of the emitters were below 8 and 3 ms, respectively. The low thermal mass indicates that pulsed operation at frequencies around 100 Hz could be realized with about 90 % modulation depth. The measured characteristics were in good agreement with COMSOL simulations. Thus, the presented devices have lower power consumption, an order of magnitude higher modulation frequency, and a production cost reduced by 40 – 60%1-4 compared to available, individually packaged devices. The patented device sealing provides through-silicon conductors and enables direct surface mounting of the components.