High-mobility Hydrogenated Indium Oxide without Introducing Water During Sputtering

AbstractThe key role of water to obtain high-mobility IO:H (hydrogenated indium oxide) layers has been well documented, but introducing the required tiny amount of water is technologically challenging. We first use simulations to evidence the key role of high mobility for the transparent conductive oxide for high-efficiency crystalline silicon solar cells. Then, we investigate an approach to fabricate high-mobility IO:H that circumvent the introduction of water vapor, relying on water vapor from ambient air. A sputtering tool equipped with a residual gas analyzer allows partial pressure monitoring of hydrogen and water in the system, and to link the gas composition to the properties of the deposited films. To vary the residual water pressure, we varied the pumping time after opening the chamber and before starting the deposition to reach different base pressures (1. 10-5 mbar to 3. 10-7 mbar), which are mostly composed of residual water. An optimum base pressure around 3. 10-6 mbar—and not lower pressures—was found to yield the highest mobility values after annealing. An alternative approach by introducing a small flow of hydrogen together with argon and oxygen is also shown to provide promising results

Light trapping in solar cells: When does a Lambertian scatterer scatter Lambertianly?

We derive scaling laws for the Rayleigh-Sommerfeld formulation we recently developed to describe light scattering from nanotextured interfaces. These scaling laws provide precious intuition on how to link scattering from different interfaces. In particular, we answer the question how to obtain a Lambertian scatterer into silicon, starting from a Lambertian scatterer into air relevant to the development of light trapping schemes in thin-film silicon solar cells. We also define a Lambertionality factor which measures how close an arbitrary scatterer approaches Lambertian scattering and extend the fundamental 4n2 light trapping limit to arbitrary scattering distributions

Influence of Light Soaking on Silicon Heterojunction Solar Cells With Various Architectures

In this article, we investigate the effect of prolonged light exposure on silicon heterojunction solar cells. We show that, although light exposure systematicallyimproves solar cell efficiency in the case of devices using intrinsic and p-type layers with optimal thickness, this treatment leads to performance degradation for devices with an insufficiently thick (p) layer on the light-incoming side. Our results indicate that this degradation is caused by a diminution of the (i/p)-layer stack hole-selectivity because of light exposure. Degradation is avoided when a sufficiently thick (p) layer is used, or when exposure of the (p) layer to UV light is avoided, as is the case of the rear-junction configuration, commonly used in the industry. Additionally, applying a forward bias current or an infrared light exposure results in an efficiency increase for all investigated solar cells, independently of the (p)-layer thickness, confirming the beneficial influence of recombination on the performance of silicon heterojunction solar cells

Unlinking absorption and haze in thin film silicon solar cells front electrodes

We study the respective influence of haze and free carrier absorption (FCA) of transparent front electrodes on the photogenerated current of micromorph thin film silicon solar cells. To decouple the haze and FCA we develop bi-layer front electrodes: a flat indium tin oxide layer assures conduction and allows us to tune FCA while the haze is adjusted by varying the thickness of a highly transparent rough ZnO layer. We show how a minimum amount of FCA leads only to a few percents absorption for a single light path but to a strong reduction of the cell current in the infrared part of the spectrum. Conversely, a current enhancement is shown with increasing front electrode haze up to a saturation of the current gain. This saturation correlates remarkably well with the haze of the front electrode calculated in silicon. This allows us to clarify the requirements for the front electrodes of micromorph cells

Reassessment of cell to module gains and losses: Accounting for the current boost specific to cells located on the edges

The power produced by a photovoltaic module is not simply the sum of the powers of its constituents cells. The difference stems from a number of so-called “cell-to-module” (CTM) gain or loss mechanisms. These are getting more and more attention as improvements in cell efficiency are becoming harder to achieve. This work focuses on two CTM mechanisms: the gain due to the recapture of light hitting the apparent backsheet in the “empty” spaces around the cells and the loss from the serial connection of “mismatched” cells i.e. with different maximum power points. In general, for insulation purposes, the spaces on the edges of modules are larger than the spacing between cells. This study reveals that, when reflective backsheets are used, these “edge spaces” provide an additional current boost to the cells placed at the edges that can lead to a 0.5% gain in the output power of modules (with 60 or 72 cells). This location-dependent current boost adds to the usual variations in cell characteristics dictated by the binning size and results in larger “cell-to-cell mismatch losses”. However, the simulations reveal that for short-circuit current bin size smaller than 5%, this additional mismatch loss is lower than 0.05%. All considered, this study demonstrates that the spaces at the edges of PV modules have a significant impact on the cell to module ratios (≈+0.5%abs or ≈16% of the CTM gains) when reflective backsheets are used

Advanced intermediate reflector layers for thin film silicon tandem solar cells

Tandem solar cells based on thin film silicon benefit from an intermediate reflector layer between the top and bottom cells since it enhances the absorption in the top cell. The top cell can thus be manufactured thinner and less prone to light induced degradation. Made from a thin layer of nanocrystalline silicon oxide, the interlayer provides a second functionality since it aids in the spatial separation of local shunts occurring in both sub-cells. Recently, the reflector morphology received attention since it can provide a third function; here, a substantial difference exists between the commonly used configurations, i.e. superstrate or substrate. In the former, the thin layer of nanocrystalline silicon oxide reproduces the morphology of the underlying top cell. Its surface may thus be too rough for the growth of the bottom cell. In the latter configuration, reflectors made from a thick layer of ZnO can yield an adequate texture for the top cell, but conductive ZnO loses the effect of shunt quenching. We present our recent progress with improved intermediate reflector layers in both cell types. For silicon oxide based interlayers, we introduce a smoothing lacquer layer with self-organized openings that allow current transport. For ZnO based interlayers, we demonstrate that a treatment in oxygen plasma is capable of tuning the in-plane resistivity