Optimized Design of Silicon Heterojunction Solar Cells for Field Operating Conditions

Solar modules are currently characterised at standard test conditions (STC), defined at 1000 W/m2 and 25 °C. However, solar modules in actual outdoor operating conditions typically operate at lower illumination and higher temperature than STC, which significantly affects their performance ratio (average harvesting efficiency over efficiency in STC). Silicon heterojunction (SHJ) technology displays both good temperature coefficient and good low-illumination performances, leading to outstanding performance ratios. We investigate here SHJ solar cells that use a-SiCx(n) layer as front doped layer with different carbon contents under different climates conditions. Adding carbon increases transparency but also resistive losses at room temperature (compared to carbon-free layers), leading to a significant decrease in efficiency at STC. We demonstrate that despite this difference at STC, the difference in energy harvesting efficiency is much smaller in all investigated climates. Furthermore, we show that a relative gain of 0.4% to 0.8% in harvesting efficiency is possible by adding a certain content of carbon in the front (n) layer, compared to carbon-free cells optimised for STC.

Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

Christophe Ballif, Stefaan De Wolf, Antoine Descoeudres, Johannes Peter Seif

In amorphous/crystalline silicon heterojunction solar cells, optical losses can be mitigated by replacing the amorphous silicon films by wider bandgap amorphous silicon oxide layers. In this article, we use stacks of intrinsic amorphous silicon and amorphous silicon oxide as front intrinsic buffer layers and show that this increases the short-circuit current density by up to 0.43 mA/cm2 due to less reflection and a higher transparency at short wavelengths. Additionally, high open-circuit voltages can be maintained, thanks to good interface passivation. However, we find that the gain in current is more than offset by losses in fill factor. Aided by device simulations, we link these losses to impeded carrier collection fundamentally caused by the increased valence band offset at the amorphous/crystalline interface. Despite this, carrier extraction can be improved by raising the temperature; we find that cells with amorphous silicon oxide window layers show an even lower temperature coefficient than reference heterojunction solar cells (-0.1%/K relative drop in efficiency, compared to -0.3%/K). Hence, even though cells with oxide layers do not outperform cells with the standard design at room temperature, at higher temperatures—which are closer to the real working conditions encountered in the field—they show superior performance in both experiment and simulation.

American Institute of Physics2014

Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions

Jean Cattin

The global photovoltaic market is mostly dominated by solar cells based on crystalline silicon (c-Si), which are covering 95% of the market. This thesis concerns silicon heterojunction (SHJ), a high-efficiency technology with a 2% market share in 2018, but showing a steady global production capacity growth and potential for industry-compatible modules with a conversion efficiency above 21%. This thesis focuses on improving the device efficiency in real operating conditions. More specifically, different effects affecting a cell used in non-standard conditions are assessed, including different temperatures and illuminations, during prolonged exposure to light and the effect of surface inhomogeneities. The main SHJ technology specificity consists of the use of thin hydrogenated amorphous silicon (a-Si) layers that provide excellent c-Si surface passivation. This enables very high operating voltages compared to other c-Si-based technologies, which directly reduces the loss of efficiency induced by higher operating temperatures. The cell's properties are measured at very low temperatures are studied, which amplifies charges transport phenomena. The voltage shows a deviation from its theoretical curve, which we explain to be related to a decrease of the positive contact holes selectivity due to the influence of the transparent conductive oxide layer. An increase in the resistive losses at low temperature is measured. These losses are caused by a resistivity increase in the different a-Si layers and interfaces because of a thermal energy decrease at low temperatures. These effects may also affect cell performances at room temperature and under operating conditions. This implies that optimisation at standard test conditions (STC), generally used to assess the performance of PV modules, does not necessarily result in the same optimum as in real climatic conditions, which generally correspond to a higher temperature and/or a lower illumination. We show that by adding carbon in the front doped layer, a transparency increase, and thus a current gain is observed at the expense of larger ohmic losses, which overall reduces the efficiency at STC. Using simulations and real climate data, we show that using such a layer leads to a relative energy production gain of 0.5 to 0.8%, both in a moderate and arid climate. Another specificity of SHJ cells is a slight efficiency increase when exposed to light for a few days. We show that in case of a too-small positive doped layer charges reserve, cell performance degradation is observed instead of an improvement. In order to avoid such degradation, the positively doped layer should be sufficiently thick and doped to screen the influence of the subsequent transparent conductive oxide layer. A treatment in which a forward bias voltage is applied to the cell allows triggering the gain in efficiency without causing the detrimental effects of light exposure. Finally, the influence of a non-homogeneous surface is studied. Parasitic electric currents are induced between the well-passivated area and poor passivation area through the metallisation. Using dedicated samples, correspondence between these currents, and an often measured drop of the low injection lifetime is described and linked to the detrimental effect of either the sample edges or by surface defects. Concretely, these effects can affect the performances of the cell at low illumination, reducing the performances in real outdoor conditions.

EPFL2020

Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions

Jean Cattin

EPFL2020