Recombination at textured silicon surfaces passivated with silicon dioxide

Abstract

The surfaces of solar cells are often textured to increase their capacity to absorb light. This optical benefit is partially offset, however, by an increase in carrier recombination at or near the textured surface. A review of past work shows that the additional recombination invoked by a textured surface varies greatly from one experiment to another. For example, in the most commonly investigated structure-pyramidal textured silicon diffused with phosphorus and passivated with a hydrogenated oxide-recombination ranges from being 1-12 times more than in an equivalently prepared planar ͕100͖ surface. Examination of these experiments reveals consistent trends: small increases in recombination occur when the surface is very heavily diffused and dominated by Auger recombination, while larger increases in recombination occur when the surface is lightly diffused and dominated by Shockley-Read-Hall recombination at the surface, making the latter depend critically on surface area and the density of surface states. Comparisons of pyramidal and planar ͕100͖ surfaces indicate that when lightly diffused, the difference in recombination is substantially greater than the difference in surface area ͑1.73͒ and it is regularly attributed to the pyramid facets having ͕111͖ orientations-well known for their higher density of dangling bonds than ͕100͖ orientations. This high dangling-bond density makes recombination at pyramidal facets strongly dependent on the passivation scheme, and it is variations in these schemes that led to the wide range of results observed in experimental studies. In addition to surface area and crystal orientation, some experiments suggest a third mechanism that enhances recombination on oxide-passivated pyramids. With capacitance-voltage and photoconductance measurements, we confirm this speculation, showing that oxide-passivated pyramidal textured silicon has a higher density of interface states than can be accounted for by surface area and orientation, and that the additional defects are predominantly acceptorlike when above, or donorlike when below, an energy of 0.3 eV higher than the valence band.

Progress and Design Concerns of Nanostructured Solar Energy Harvesting Devices

“…It is worth mentioning that an increase of surface recombination will particularly affect the performance of devices based on active materials with high surface recombination velocity such as single crystalline silicon, GaAs, etc. [ 34,[40][41][42] Furthermore, while nanostructures can increase the overall optical absorption of solar energy harvesting devices, it is important to evaluate how much light absorption enhancement is contributed by the active material as opposed to the passive materials in the device, such as electrodes, since only photocarriers generated in the active light absorber can contribute to the photo-current. Parasitic absorption occurring in the electrodes can cancel out the optical absorption enhancement introduced by nanostructure photon management.…”

Nanoimprint lithography for solar cell texturisation

“…To sum up the results of the review [105], a 2 -12 times higher recombination is found for lightly doped or non-diffused samples passivated by silicon oxide. This is by far higher than the surface enlargement alone.…”

“…A review on this topic was conducted by McIntosh and Johnson [105]. In terms of surface recombination velocities on non-diffused passivated samples, they report on a wide range of measured values by different groups.…”

The effect of microscopic texture on the direct plasma surface passivation of Si solar cells

“…The wet-chemistry-based approach where solar cells are textured with tetrahedral pyramids having four (111) planes is presently the state-of-the-art in surface texturing. 7,9 This process heavily relies on toxic alkaline solutions. However, in the near future, dry, plasma-based approaches are expected to become more favorable for thinner silicon wafers.…”

“…The Scanning Electron Microscopy (SEM) images of the wet-and drytextured Si surfaces are shown in Figure 1. However, due to the fact that the textured surface area is 1.73 times larger than the flat surface and the exposed surfaces are (111) oriented, [7][8][9] the surface passivation becomes more challenging. The reason is the existence of a large number of dangling bonds at the surface.…”