A dynamic calibration method for injection-dependent charge carrier lifetime measurements
A dynamic calibration method for injection-dependent charge carrier lifetime measurements
A dynamic calibration method for injection-dependent charge carrier lifetime measurements
Temperature coefficient of photovoltaic devices
Photovoltaic (PV) devices operate under a wide range of temperatures, however, they are often characterized and optimised only at standard testing conditions (STC; at 25 °C with an irradiance of 1000 W/m2 under the AM1.5G solar spectrum). Since the temperature sensitivity of various cell technologies is different, the temperature coefficient (TC) is an essential figure of merit to evaluate the cell performance at different operating temperatures and to allow a more in-depth comparison between various cell technologies. More importantly, in combination with the typical meteorological data, this parameter enables to accurately evaluate the energy yield of PV installations. Hence, it plays a crucial role in the selection of suitable cell technologies for a PV field at a specific site to maximise the annual energy yield. The performance of silicon (Si) solar cells is typically reduced with increasing temperature, which is mainly attributed to the reduction of the cell’s open-circuit voltage (Voc). In general, the higher the cell’s Voc, the better the open-circuit voltage TC (TCVoc), and hence the efficiency TC (TCη). To achieve a favourable TCη, cell structures enabling a high Voc are therefore desired.
Tunnel oxide passivated contact (TOPCon) and transition metal oxide (TMO)-based solar cells
Recently, passivating-contact-based solar cells have been developed. These structures simultaneously minimise recombination losses at the interfaces of the base, while effectively collecting only one type of charge carrier (electrons or holes). Solar cells based on this concept show a high potential to obtain excellent performance with a record Voc of up to 750 mV. Among them, solar cells with polysilicon (poly-Si) passivating contacts exhibit, not only outstanding cell performance, but may also be easily introduced into existing PV production lines. Note that different terms are used to name solar cells with poly-Si passivating contacts such as TOPCon (tunnel oxide passivated contact), POLO (poly-Si on oxide), and monoPolyTM. Besides poly-Si passivating contacts, passivating contacts based on transition metal oxide (TMO) films have also attracted much attention due to their excellent performance. Depending on the electrical properties (such as work function, conductivity, and band alignment), these contacts can be used as hole- or electron-selective collectors.
Figure 1. (a) TCVoc, (b) TCJsc, (c) TCFF and TCpFF, and (d) TCη of the MoOx-based, TiOx-based, standard SHJ, and TOPCon solar cells. TCs of other cell structures in the literature are also shown for comparison.
We investigate the temperature-dependent performance of advanced solar cell structures such as MoOx-, and TiOx-based, and TOPCon solar cells and compare them to that of other cell structures reported in the literature (see the figure below). We also examine the temperature-dependent behaviour of the surface saturation current density (J0s) and the contact resistivity (ρc) of those contacts to gain a deeper understanding regarding their impact on the TC of Voc and FF, respectively. More details about these studies can be found here [1,2].
Si heterojunction (SHJ) solar cells with high bulk resistivities >1,000 Ω.cm
The significant improvements in the surface and contact passivation of Si solar cells as well as their bulk quality recently have shifted their operating point to higher injections. Hence, they are less dependent on wafer doping. This shift opens an opportunity of using high-resistivity wafers for practical photovoltaic applications, introducing a promising approach to push the cell efficiency towards the intrinsic limit and to improve the module reliability by increasing the cell breakdown voltage. Therefore, insights into the performance of Si solar cells using high-resistivity wafers at various operating temperatures are of significant interest.
Figure 2. (a) TCVoc, (b) TCJsc, (c) TCFF and TCpFF, and (d) TCη of SHJ solar cells using a wide range of wafer resistivities (between 3 and 1,000 Ω.cm).
In this study, we investigate the temperature- and illumination-dependent performance of SHJ solar cells using a wide range of wafer resistivities (between 3 and 1,000 Ω.cm). The TCs of the main electrical cell parameters of these cells are almost independent of the wafer resistivity. Furthermore, we also found that the investigated cells are more sensitive to temperature variation at lower illumination intensities. The findings of this study demonstrate that there is no limitation in using very high-resistivity wafers for SHJ solar cells under field operating conditions. The study, therefore, set a premise for using such wafers for reducing the Auger recombination and to push SHJ solar cell efficiency towards their theoretical efficiency limit. More details about these studies can be found here [3].
Temperature-dependent performance and radiation stability of ultra-thin SHJ solar cells for space applications
Multi-junction solar cells based on III-V semiconductor alloys have dominated the market for space applications due to their high efficiency and resilience to radiation damage. Nevertheless, the growing demand for cost-effective satellites has boosted the market share of silicon-based solar cells due to their competitive pricing. Unfortunately, the performance of these cells is strongly affected by radiation, consequently limiting their deployment to short-duration space missions. The acknowledged influence of radiation-induced damage on carrier diffusion in silicon implies that employing ultra-thin wafers could provide a potential remedy. Recently, ultra-thin SHJ solar cells with a base thickness of 40-60 µm and an efficiency above 26% have been successfully developed. These cells are a promising candidate to power small, lightweight, and cost-effective satellites in the coming years. Therefore, insights into their performance under relevant operating conditions in space—specifically, a wide range of temperatures under the air mass zero (AM0) spectrum before and after electron irradiation—are of significant interest.
In this study, we investigate the temperature-dependent performance of ultra-thin (50 µm) SHJ solar cells under such conditions. Their temperature-dependent behaviour is then compared to that of 180 µm-thick SHJ cells and cell structures without heterojunctions. The established models successfully replicate the experimental trends, providing a more profound comprehension of the intriguing behaviour exhibited at these low temperatures. The findings of this study identify a threshold of SHJ solar cells at low temperatures, highlighting their critical importance for the evaluation and optimisation of SHJ cells for space applications. In particular, the unique self-curing capability of ultra-thin SHJ cells and the advantages of employing ultra-thin wafers in Si solar cells are emphasised, especially for space applications where minimising weight and maximising performance are critical. More details about these studies can be found here [4].
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Figure 3. (a) Voc, (b) Jsc, (c) FF, and (d) efficiency of the SHJ-1 and SHJ-2 cells with different thicknesses and homojunction cell structures under the AM0 spectrum as a function of temperature. Fits using Green’s model are also presented as dashed lines. The corresponding operating temperature ranges of solar cells in different space locations are presented in (d).