Daniel L. Meier

Efficient black silicon solar cell with a density-graded nanoporous surface: Optical properties, performance limitations, and design rules

We study optical effects and factors limiting performance of our confirmed 16.8% efficiency “black silicon” solar cells. The cells incorporate density-graded nanoporous surface layers made by a one-step nanoparticle-catalyzed etch and reflect less than 3% of the solar spectrum, with no conventional antireflection coating. The cells are limited by recombination in the nanoporous layer which decreases short-wavelength spectral response. The optimum density-graded layer depth is then a compromise between reflectance reduction and recombination loss. Finally, we propose universal design rules for high-efficiency solar cells based on density-graded surfaces.

Efficient heterojunction solar cells on p-type crystal silicon wafers

Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.

Self-doping contacts to silicon using silver coated with a dopant source [for solar cells]

A contact system for silicon solar cells is described in which silver is coated with a layer of dopant and alloyed with silicon, thereby simultaneously doping the silicon substrate and forming a low-resistance ohmic contact to it. The concept has been demonstrated using evaporated silver with commercially available phosphorus and boron liquid dopants. Silicon surface topography, along with I-V and SIMS analyses all indicate that the Ag-Si eutectic temperature (835/spl deg/C) must be exceeded and the dopant coating must be present for the contact to be self-doping. The concept has also been implemented in the form of screen-printable silver pastes with phosphorus. A fritless version of the paste exhibited only 3 m/spl Omega/-cm/sup 2/ contact resistance directly to 7 /spl Omega/-cm n-type dendritic web silicon. For contacting lightly-doped n/sup +/ layers (<100 /spl Omega///spl square/) through SiN/sub x/, a fritted version is used. The resultant contact metal is highly conductive (3 /spl mu//spl Omega/-cm) and solderable. No carrier lifetime degradation associated with the alloying process has been observed in dendritic web silicon solar cells.

Efficient black silicon solar cells with nanoporous anti-reflection made in a single-step liquid etch

We fabricated black silicon solar cells with conversion efficiency of 16.8% on p-type single crystal Si wafers with a conventional diffused emitter and Al back-surface field (BSF). We replaced the anti-reflection coating step with a single 3-minute ‘black-silicon’ etch of the bare wafer before processing. The nanoporous black-silicon layer, about 300-nm thick is produced in a 3-minute single-step liquid etch based upon catalysis by Au nano-particles formed in a solution containing HF and H2O2. Solar cell reflectance is well below 5% at incident wavelengths from 350 to 1000 nm. We present reflectance versus time data during this simple single-step etching. We also characterize cell performance and find that recombination in the black silicon surface layer must still be reduced.

Photoconductive decay lifetime and Suns-V oc diagnostics of efficient heterojunction solar cells

Minority carrier lifetime and Suns-Voc measurements are well-accepted methods for characterization of solar cell devices. We use these methods, with an instrument from Sinton Consulting, as we fabricate and optimize state-of-the-art all hot-wire chemical vapor deposition (HWCVD) silicon heterojunction (SHJ) devices. For double-sided SHJ devices, lifetime measurements were performed immediately after hydrogenated amorphous silicon (a-Si:H) deposition of the front emitter and back base contacts on a Silicon wafer, and also after indium tin oxide (ITO) deposition of transparent conducting oxide contacts. We report results of minority carrier lifetime measurements for double-sided p-type Si heterojunction devices and compare Suns-Voc results to Light I–V measurements on 1-cm2 solar cell devices measured on an AM1.5 calibrated XT-10 solar simulator.

Cu backside busbar tape: Eliminating Ag and enabling full al coverage in crystalline silicon solar cells and modules

A Cu tape was developed and tested for use as a backside busbar for any silicon solar cell having a screen-printed and fired Al backside, particularly the Al BSF cell and the PERC cell. A key element of the tape is the adhesive which penetrates through the full Al thickness and bonds to the underlying Si surface under short-duration heat and pressure. Such tape eliminates the need for backside silver (reducing cost), and allows the Al layer to remain continuous (increasing efficiency). Measured 180° peel strength of standard interconnect ribbon soldered to Cu tape exceeded 100 g-force/mm. Cu/Al contact resistivity measured 2.6 mΩ-cm2. Five 60-cell modules were fabricated with commercially-produced 156 mm Al BSF cells having full Al backside and Cu tape busbars. Tape was applied using prototype semi-automated production equipment, designed for production-scale throughput. Cells were interconnected with standard production stringing equipment. Module power exceeded 260 W, as fabricated. Module power degradation was well within the limits allowed by IEC 61215, even after doubling the IEC test requirements to 2000 hours of damp heat and 400 thermal cycles. EL imaging showed no cell cracking in the modules as-fabricated or after IEC stress testing.

20% n-Type silicon solar cell fabricated by a simple process with an aluminum alloy rear junction and extended emitter

A simple process is defined and executed to achieve 20% efficiency for cells fabricated on 156 mm n-Cz Si wafers. The cell structure is n+np+ with the p+ emitter being formed over most of the rear surface by Al alloying, but extended to the wafer edges by a light doping with B. The n+ FSF is doped with P which makes the front surface easy to passivate and contact. B and P dopants are introduced into the wafer by ion implantation and are co-annealed in a single high temperature step, during which a passivating thermal oxide is also grown on the front and back surfaces. The back surface remains textured, and the entire process is additive as the cell structure is built layer-by-layer, with no subtractive steps needed for planarization, stripping diffusion glasses, or creating vias in dielectrics. A key benefit of the extended emitter is to improve p-n junction quality (since the depletion region is essentially confined within the interior of the silicon and does not intersect the surface), as indicated by the pseudo FF regularly exceeding 0.83. In addition, Jsc increases because of the reclaimed Si border area (Al to wafer edge). Cell parameters were found to be tightly distributed. The highest efficiency measured was 20.04% for a 239 cm2 cell with a selective FSF and five busbars. Interconnecting cells in a module could be done with 3M Cu tape or Schmid Tin Pad applied directly to backside Al.

Stand-alone solar generator with LED floodlights for outdoor sign illumination

Overall, the solar lighting system is aesthetically pleasing and it performed well during its first 200 days of operation. The illuminated sign was easily visible at night and the outdoor system withstood severe weather conditions: temperatures down to -18°C, snow, ice, rain, and wind. However, limitations were revealed in a failure to provide 8 hours of lighting every night. These limitations can be addressed by doubling the ratio of Battery Capacity (Ah) to LED Demand (Ah). This can be done by adding a second battery or by reducing the number of floodlights from 4 to 2.

Silicon solar cells with front hetero-contact and aluminum alloy back junction

We prototype an alternative n-type monocrystalline silicon (c-Si) solar cell structure that utilizes an n/i-type hydrogenated amorphous silicon (a-Si:H) front hetero-contact and a back p-n junction formed by alloying aluminum (Al) with the n-type Si wafer. Such a structure combines a conventional high-throughput Al-Si alloying process with excellent front surface passivation provided by a-Si:H. A key process consideration is to preserve the clean c-Si front surface through the high-temperature alloying, so there will be effective a-Si:H passivation. From cell simulations, we estimate a front SRV of 10–50 cm/sec has been achieved in our process. The best prototype 1×1 cm2 cell with planar front surface and single anti-reflection (AR) coating layer has demonstrated a confirmed conversion efficiency of 13.5%, Voc of 604.7 mV, and fill factor (FF) of 79.9%. Processes for further efficiency improvements are described.