Douglas S. Ruby

ZnO Nanostructures as Efficient Antireflection Layers in Solar Cells

An efficient antireflection coating (ARC) can enhance solar cell performance through increased light coupling. Here, we investigate solution-grown ZnO nanostructures as ARCs for Si solar cells and compare them to conventional single layer ARCs. We find that nanoscale morphology, controlled through synthetic chemistry, has a great effect on the macroscopic ARC performance. Compared with a silicon nitride (SiN) single layer ARC, ZnO nanorod arrays display a broadband reflection suppression from 400 to 1200 nm. For a tapered nanorod array with average tip diameter of 10 nm, we achieve a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC. Calculations using rigorous coupled wave analysis suggest that the tapered nanorod arrays behave like modified single layer ARCs, where the tapering leads to impedance matching between Si and air through a gradual reduction of the effective refractive index away from the surface, resulting in low reflection particularly at longer wavelengths and eliminating interference fringes through roughening of the air-ZnO interface. According to the calculations, we may further improve ARC performance by tailoring the thickness of the bottom fused ZnO layer and through better control of tip tapering.

Characterization of random reactive ion etched-textured silicon solar cells

Hemispherical reflectance and internal quantum efficiency (IQE) measurements have been employed to evaluate the response of Si nanostructured surfaces formed by using reactive ion etching (RIE) random texturing techniques. Random RIE-textured surfaces typically exhibit broadband anti-reflection behavior with solar-weighted-reflectance (SWR) of /spl ap/3% over 300-1200-nm spectral range. RIE-texturing has been demonstrated over large areas (/spl sim/180 cm/sup 2/) of both single and multicrystalline Si substrates. Due to the surface contamination and plasma-induced damage, as formed RIE-textured solar cells do not provide enhanced short-circuit current. However, improved surface cleaning combined with controlled wet-chemical damage removal etches provide a significant improvement in the short-circuit current. For such textures, the internal quantum efficiencies are comparable to the random, wet-chemically-textured solar cells. In both the UV and near-IR wavelength regions, the RIE-textured subwavelength surfaces exhibit superior performance in comparison with the wet-chemically-textured surfaces. Due to their large area, low-reflection capability, random, RIE-texturing techniques are expected to find widespread commercial applicability in low-cost, large-area multicrystalline Si solar cells.

Rie-texturing of multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.

HIGH-EFFICIENCY SILICON SOLAR CELLS BY RAPID THERMAL PROCESSING

Silicon solar cell efficiencies of 16.9% have been achieved on 0.2 Ω cm float zone silicon, using a simplified cost effective rapid thermal process (RTP). Although the individual processing steps are not fully optimized yet, this represents the highest reported efficiency for solar cells processed with simultaneous front and back diffusion with no conventional high‐temperature furnace steps. A diffusion temperature schedule coupled with an added short in situ slow cooling during RTP resulted in greater than 200 μm diffusion length and appropriate diffusion profiles for high efficiency cells. Plasma enhanced chemical vapor deposition (PECVD) of SiN/SiO2 was used for surface passivation and antireflection coating. Conventional cells fabricated by furnace diffusions and oxidations gave an efficiency of 18.8%. Process optimization can further reduce the gap between the conventional and RTP/PECVD cells.

Diffraction grating structures in solar cells

Sub-wavelength periodic texturing (gratings) of crystalline-silicon (c-Si) surfaces for solar cell applications can be designed for maximizing optical absorption in thin c-Si films. The authors have investigated c-Si grating structures using rigorous modeling, hemispherical reflectance and internal quantum efficiency measurements. Model calculations predict almost /spl sim/100% energy coupling into obliquely propagating diffraction orders. By fabrication and optical characterization of a wide range of 1D and 2D c-Si grating structures, they have achieved broadband, low (/spl sim/5%) reflectance without an antireflection film. By integrating grating structures into conventional solar cell designs, they have demonstrated short-circuit current density enhancements of 3.4 and 4.1 mA/cm/sup 2/ for rectangular and triangular 1D grating structures compared to planar controls. The effective path length enhancements due to these gratings were 2.2 and 1.7, respectively. Optimized 2D gratings are expected to have even better performance.

Rapid thermal processing of high-efficiency silicon solar cells with controlled in-situ annealing

Abstract Silicon solar cell efficiencies of 17.1%, 16.4%, 14.8%, and 14.9% have been achieved on FZ, Cz, multicrystalline (mc-Si), and dendritic web (DW) silicon, respectively, using simplified, cost-effective rapid thermal processing (RTP). These represent the highest reported efficiencies for solar cells processed with simultaneous front and back diffusion with no conventional high-temperature furnace steps. Appropriate diffusion temperature coupled with the added in-situ anneal resulted in suitable minority-carrier lifetime and diffusion profiles for high-efficiency cells. The cooling rate associated with the in-situ anneal can improve the lifetime and lower the reverse saturation current density ( J o ), however, this effect is material and base resistivity specific. PECVD antireflection (AR) coatings provided low reflectance and efficient front surface and bulk defect passivation. Conventional cells fabricated on FZ silicon by furnace diffusions and oxidations gave an efficiency of 18.8% due to greater short wavelength response and lower J o .

Recent progress on the self-aligned, selective-emitter silicon solar cell

We developed a self-aligned emitter etchback technique that requires only a single emitter diffusion and no alignments to form self-aligned, patterned-emitter profiles. Standard, commercial, screen-printed gridlines mask a plasma-etchback of the emitter. A subsequent PECVD-nitride deposition provides good surface and bulk passivation and an antireflection coating. We succeeded in finding a set of parameters which resulted in good emitter uniformity and improved cell performance. We used full-size multicrystalline silicon (mc-Si) cells processed in a commercial production line and performed a statistically designed, multiparameter experiment to optimize the use of a hydrogenation treatment to increase performance. Our initial results found a statistically significant improvement of half an absolute percentage point in cell efficiency when the self-aligned emitter etchback was combined with a 3-step PECVD-nitride surface passivation and hydrogenation treatment.

Simplified high-efficiency silicon cell processing

Abstract We developed an emitter diffusion process that yields a near-ideal doping profile with a passivating oxide in a single furnace step. Because this process subjects the material to only one high-temperature thermal excursion, bulk lifetime is better preserved. This is especially true for lower-cost silicon materials containing a high concentration of oxygen or carbon. Using this process, we routinely obtain one-sun cell efficiencies over 19% on float-zone material and over 18% on Czochralski material. Using solar-grade Czochralski material, we have demonstrated record efficiencies of 18.3% at one sun and 20.0% under concentration. Simple processes that yield high-performance diffusion profiles are expected to become increasingly important as manufacturers adopt improved techniques for ohmic contacts.

Development of RIE-textured silicon solar cells

A maskless plasma texturing technique using reactive ion etching (RIE) for silicon solar cells results in a very low reflectance of 5.4% before, and 3.9% after SiN deposition. A detailed study of surface recombination and emitter properties was made, then solar cells were fabricated using the DOSS solar cell process. Different plasma-damage removal treatments are tested to optimize low lifetime solar cell efficiencies. Highest efficiencies are observed for little or no plasma-damage removal etching on mc-Si. Increased J/sub sc/ due to the RIE texture proved superior to a single layer anti-reflection coating. This indicates that RIE texturing is a promising texturing technique, especially applicable on lower lifetime (multicrystalline) silicon. The use of nontoxic, noncorrosive SF/sub 6/ makes this process attractive for mass production.

Self-aligned selective-emitter plasma-etchback and passivation process for screen-printed silicon solar cells☆

Abstract We studied whether plasma-etching techniques can use standard screen-printed gridlines as etch masks to form self-aligned, patterned-emitter profiles on multicrystalline-silicon (mc-Si) cells from Solarex. We conducted an investigation of plasma deposition and etching processes on full-size mc-Si cells processed in commercial production lines, so that any improvements obtained would be immediately relevant to the PV industry. This investigation determined that reactive ion etching (RIE) is compatible with using standard, commercial, screen-printed gridlines as etch masks to form self-aligned, selectively doped emitter profiles. This process results in reduced gridline contact resistance when followed by plasma-enhanced chemical vapor deposition (PECVD) treatments, an undamaged emitter surface easily passivated by plasma-nitride, and a less heavily doped emitter between gridlines for reduced emitter recombination. This allows for heavier doping beneath the gridlines for even lower contact resistance, reduced contact recombination, and better bulk defect gettering. Our initial results found a statistically significant improvement of about half an absolute percentage point in cell efficiency when the self-aligned emitter etchback was combined with a PECVD-nitride surface passivation treatment. Some additional improvement in bulk diffusion length was observed when a hydrogen passivation treatment was used in the process. We attempted to gain additional benefits from using an extra-heavy phosphorus emitter diffusion before the gridlines were deposited. However, this required a higher plasma-etch power to etch back the deeper diffusion and keep the etch time reasonably short. The higher power etch may have damaged the surface and the gridlines so that improvement due to surface passivation and reduced gridline contact resistance was inhibited.