Alexander N. Sprafke

Cloaked contact grids on solar cells by coordinate transformations: designs and prototypes

Nontransparent contact fingers on the sun-facing side of solar cells represent optically dead regions which reduce the energy conversion per area. We consider two approaches for guiding the incident light around the contacts onto the active area. The first approach uses graded-index metamaterials designed by two-dimensional Schwarz–Christoffel conformal maps, and the second uses freeform surfaces designed by one-dimensional coordinate transformations of a point to an interval. We provide proof-of-principle demonstrators using direct laser writing of polymer structures on silicon wafers with opaque contacts. Freeform surfaces are amenable to mass fabrication and allow for complete recovery of the shadowing effect for all relevant incidence angles.

Properties of black silicon obtained at room-temperature by different plasma modes

Black silicon plasma technology begins to be integrated into the process flow of silicon solar cells. However, most of the current technology is used at cryogenic or very low substrate temperatures. Here, the authors investigate the temperature-dependent properties of black silicon prepared by two different plasma etching techniques for black silicon, a pure capacitively coupled process (CCP), and an inductively and capacitively coupled process (ICP + CCP). It turns out that the ICP + CCP process at room-temperature yields black silicon samples with 93% absorption and minority carrier lifetime above 1 ms. The authors show that these optoelectronic properties are comparable to samples obtained at low temperatures.

Influence of black silicon surfaces on the performance of back-contacted back silicon heterojunction solar cells

The influence of different black silicon (b-Si) front side textures prepared by inductively coupled reactive ion etching (ICP-RIE) on the performance of back-contacted back silicon heterojunction (BCB-SHJ) solar cells is investigated in detail regarding their optical performance, black silicon surface passivation and internal quantum efficiency. Under optimized conditions the effective minority carrier lifetime measured on black silicon surfaces passivated with Al(2)O(3) can be higher than lifetimes measured for the SiO(2)/SiN(x) passivation stack used in the reference cells with standard KOH textures. However, to outperform the electrical current of silicon back-contact cells, the black silicon back-contact cell process needs to be optimized with aspect to chemical and thermal stability of the used dielectric layer combination on the cell.

Localized surface plasmon resonance in the IR regime

Arrays of differently sized disk shaped gold nanoantennas are prepared on glass, which show localized surface plasmon resonance and Rayleigh anomalies in the near infrared and telecom range between 1000 and 1500 nm wavelength. The spectral position of these grating resonances depends critically on the period of the array and the size of the nanoantennas. When PbS quantum dots embedded in PMMA surround the nanoantennas, an up to four fold enhancement of the photoluminescence is observed at the grating resonances due to the constructive diffractive feedback among neighboring antennas. In accordance with the grating resonances a shift of the emission towards smaller wavelengths with decreasing disk diameter is demonstrated.

Automated spray coating process for the fabrication of large-area artificial opals on textured substrates

3D photonic crystals, such as opals, have been shown to have a high potential to increase the efficiency of solar cells by enabling advanced light management concepts. However, methods which comply with the demands of the photovoltaic industry for integration of these structures, i. e. the fabrication in a low-cost, fast, and large-scale manner, are missing up to now. In this work, we present the spray coating of a colloidal suspension on textured substrates and subsequent drying. We fabricated opaline films of much larger lateral dimensions and in much shorter times than what is possible using conventional opal fabrication methods.

Femtosecond laser-written high-efficiency blazed phase gratings in the volume of soda lime glass for light management in solar modules

Highly efficient volume phase gratings have been fabricated in low-iron soda lime glass using femtosecond (fs) laser pulses with 1030 nm wavelength and 270 fs pulse duration. Optical simulations based on rigorous coupled-wave analysis theory were performed to determine optimal grating parameters and designs for the application of the gratings for light management in solar modules, suggesting a very effective blazed-like design. Several of such blazed phase gratings have been fabricated and analyzed by measuring their diffraction efficiencies into first and higher orders. Up to 77% of the incoming light in the wavelength region relevant for silicon-based photovoltaics were diffracted by these gratings. Typical induced refractive index changes between 0.002 and 0.006 were derived by comparing the experimental efficiencies with the simulation results.

Strategy for tailoring the size distribution of nanospheres to optimize rough backreflectors of solar cells

We study the light-trapping properties of surface textures generated by a bottom-up approach, which utilizes monolayers of densely deposited nanospheres as a template. We demonstrate that just allowing placement disorder in monolayers from identical nanospheres can already lead to a significant boost in light-trapping capabilities. Further absorption enhancement can be obtained by involving an additional nanosphere size species. We show that the Power Spectral Density provides limited correspondence to the diffraction pattern and in turn to the short-circuit current density enhancement for large texture modulations. However, in predicting the optimal nanosphere size distribution, we demonstrate that full-wave simulations of just a c-Si semi-infinite halfspace at a single wavelength in the range where light trapping is of main importance is sufficient to provide an excellent estimate. The envisioned bottom-up approach can thus reliably provide good light-trapping surface textures even with simple nanosphere monolayer templates defined by a limited number of control parameters: two nanosphere radii and their occurrence probability.

Tailored substrate topographies by self-organized colloidal particles

We exploit two dimensional arrangements of nanoparticles that serve as templates to fabricate substrates for light-management in optoelectronic devices. Strategies to tailor substrate topographies through self-organization of particles are described.

Enhanced density of negative fixed charges in Al2O3 layers on Si through a subsequent deposition of TiO2

The passivation of silicon surfaces play an important role for achieving high-efficiency crystalline silicon solar cells. In this work, a stack system comprising of 20nm Al2O3 with a 22nm TiO2 topping layer was deposited on p-type Si using thermal atomic layer deposition (ALD) and was investigated regarding its passivation quality. Quasi-steady-state photo conductance (QSSPC) measurements reveal that the minority carrier lifetime at an injection density of 1015cm−3 increased from 1.10ms to 1.96ms after the deposition of TiO2, which shows that the deposition of TiO2 onto Al2O3 is capable of enhancing its passivation quality. Capacity voltage (CV) measurements show that the amount of negative charges in the dielectric layer has increased from -2.4·1012cm−2 to -6.3·1012cm−2 due to the deposition of TiO2. The location of the additional charges was analyzed in this work by etching the dielectric layer stack in several steps. After each step CV measurements were performed. It is found that the additional negative charges are created within the Al2O3 layer. Additionally, ToF-SIMS measurements were performed to check for diffusion processes within the Al2O3 layer.

Increasing the Efficiency of Solar Modules by Femtosecond Laser Written Blazed Phase Gratings in the Volume of Soda Lime Glass

We inscribed phase gratings into the cover glass of a solar module using femtosecond laser pulses to guide light around the front side metallization. Photocurrent and efficiency of the module increased by 1 % (relative).