Bridging the voltage gap

Publieke samenvatting / Public summary

In recent years, n-type polycrystalline silicon (polySi) has emerged as novel electron-contact for crystalline silicon (c-Si) solar cells, enabling extremely high efficiencies (25.8%, FhG ISE, Aug. 2017) on laboratory (4 cm2) solar cells. To get to even higher efficiencies, also the opposite (hole) polarity will need to feature a carrier selective contact: p-type polySi. On short term, the p-type polySi counterpart is attractive from an industrial point of view, as p-type polySi could form an incremental upgrade to mainstream industrially produced cell architectures, such as PERC and Al-BSF, while at the same time it can also be used in high-efficiency bifacial n-PERT and IBC solar cells. Nevertheless, the performance of p-type polySi contacts is still lagging behind considerably compared to its n-type polySi counterpart. Particularly, a strong reduction in output voltage is observed after industrially screen-printed metallization of these contacts. This voltage loss is attributed to a lower surface passivation, a lack of carrier selectivity and metallization-induced damage, and prevents a successful introduction of p-type polySi in industry.

The goal of this project is to strongly improve p-type polySi passivating contacts to make its performance on par with its n-type counterpart and creating a key building block to enable >24% efficiency for industrial single junction Si solar cells. Moreover p-type polySi will be introduced to the photovoltaic industry as a high-end hole-selective contact for application into PERC cells on the short term and more advanced, n-type solar cell architectures, including IBC, on the longer term.

Korte omschrijving
Mitigation of the voltage loss of p-type polySi contacts upon metallization will be addressed through three routes: a) innovations in the tunnel oxide employed, b) improvement of the polySi doping and structuring, and c) through the application of novel metallization pastes to specifically tailor the p-type polySi-to-metallization contacts. TU/e will work on understanding of the role of the SiO2 tunnel oxide in current p-type polySi contacts. Novel tunnel oxides (e.g. Al2O3, Ga2O3, …) prepared by atomic layer deposition (ALD) will be compared to the currently used SiO2-based tunnel oxide, in collaboration with Levitech. Research on the employed tunnel oxides aims to i) reduce the density of interface states, ii) to improve the hole-selectivity, e.g. through favorable (additional) band offsets and fixed charge and iii) to improve the diffusion barrier properties for boron, which is used to dope the p-type polySi. Tempress and ECN will further optimize the hole selectivity of polySi and improve its resilience to industrial metallization technology by implementing novel doping schemes. Together with paste manufacturer Heraeus (the paste and the process conditions will be tailored to c

In this project, the root cause(s) of the voltage loss of p-type polySi upon metallization will be identified and tackled and a lean process flow for the preparation of p-type polySi contacts that enable a high voltage output after metallization will be delivered.
With a combination of enhanced atomic layer deposition (ALD) tunnel oxide properties, improved p-type polySi layers and dedicated metal contacting pastes, the following targets that are compatible with industrial >24% efficient solar cells are set:
• Implied Voc >730 mV on textured solar cells before metallization
• External Voc > 720 mV on textured solar cells after metallization of p-poly, using industrial processes
Besides enabling high efficiency solar cells, a success of this project will already on a shorter term enable the introduction of p-type polySi into the PV industry e.g. as incremental upgrade as the rear contact for PERC or Al-BSF cells. In this way, the project contributes to a faster development of highly efficient solar cells and will further strengthen the position of Dutch equipment manufacturers and research institutes.