Renate Zapf-Gottwick

Laser-Doped Back-Contact Solar Cells

We present laser-doped interdigitated back-contacted solar cells with a record efficiency η = 22.0%. The high versatility and spatial resolution of our laser doping process enable local n-type and p-type doping with a precision below 30 μm and avoid any masking for doping. Nevertheless, the presented solar cells use photolithography (masking) to define the contact area and metallization layout. Quasi-steady-state photoconductance measurements prove the low-saturation current density of the laser doping. We process solar cells with varied pitch and emitter fraction and compare their measured current density-voltage characteristics with a 3-D solar cell simulation. The good agreement of the simulation and experimental data allows a reliable efficiency forecast when optimizations are applied. Furthermore, the influence of the base-busbar region on solar cell performance is discussed.

Long-term leaching of photovoltaic modules

Some photovoltaic module technologies use toxic materials. We report long-term leaching on photovoltaic module pieces of 5 × 5 cm2 size. The pieces are cut out from modules of the four major commercial photovoltaic technologies: crystalline and amorphous silicon, cadmium telluride as well as from copper indium gallium diselenide. To simulate different environmental conditions, leaching occurs at room temperature in three different water-based solutions with pH 3, 7, and 11. No agitation is performed to simulate more representative field conditions. After 360 days, about 1.4% of lead from crystalline silicon module pieces and 62% of cadmium from cadmium telluride module pieces are leached out in acidic solutions. The leaching depends heavily on the pH and the redox potential of the aqueous solutions and it increases with time. The leaching behavior is predictable by thermodynamic stability considerations. These predictions are in good agreement with the experimental results.

Use of Coated-Metal Particles in Rear Busbar Pastes to Reduce Silver Consumption

Reducing the amount of silver is one of the most important ways to reduce the cost of photovoltaic cells. The common way to reduce silver consumption on a cell is the reduction of the metal content in the paste. We present a new paste with silver-coated nickel particles, reducing the silver amount and still keeping the properties of silver related to oxidation and sintering. This paper shows the limits in conductivity due to porosity and oxidation of coated-metal particle pastes in comparison with silver pastes. Simulations and cell tests show that coated-metal particle pastes reduce silver consumption without decreasing the cell efficiency replacing busbar pastes. Coated-metal particle pastes are able to decrease silver consumption for rear-side busbars to cAg <; 1.4 mg/cm2, leading to a conductivity σBB = 1.1105 S/cm, without decreasing cell or module efficiency. The conductivity of coated-metal particle pastes is too low using pastes with coated-metal particles as a replacement for the metallization paste for grid fingers but good enough to replace the silver paste for busbars with a cheap alternative.

18.4% Efficient Grid Optimized Cells With 100-Ω/sq Emitter

An optimized front-grid design with fine-line fingers allows efficiency η ≈ 18.4%, for industrial c-Si solar cells with homogeneously doped emitter of sheet resistance <i>R</i>_sh = 100 Ω/sq. An additional gain Δ η_gain,bb ≈ 0.5% in efficiency is expected by using a grid of busbars instead of the standard H-pattern front grid. We apply a new method to optimize the front-grid design by minimizing the optical, electronic, and electrical losses of the metallization. The “remove, design, optimize” (ReDO) method theoretically removes the existing contact grid, calculates the ideal efficiency η* without any front-side losses as a goal, designs new front grids for the particular cell, and selects the optimum design for a maximum efficiency η. Starting from the experimentally measured performance of a real solar cell, the ReDO method predicts η of the cell, by considering the dependence of the current <i>J</i>_mpp and the voltage <i>V</i>_mpp at the maximum power point on the front-side losses, enabling a precise efficiency loss analysis.