Improving the performance of solar cells requires, among other things, the development of efficient surface passivation techniques. Passivation consists in neutralizing the defects present on the surface of the semiconductor which act as recombination centers of photo-generated charge carriers. This passivation, which can be ensured by a chemical effect (saturation of the pendant bonds at the surface) or by a field effect (repulsion of minority charge carriers by an electrostatic potential), is achieved through the deposition of a thin layer on the surface of the semiconductor. The quality of the passivation is evaluated by a measurable quantity which is the lifetime of the minority carriers.

Conventionally, studies on passivation are carried out sequentially, that is to say, a measurement before, then an ex-situ measurement after deposition. Everything that may have happened during the deposition or surface treatment operation was until now inaccessible to us!

It is to study this dynamics of surface transformation during the semiconductor passivation process that we have developed a diagnostic tool allowing the determination of the life time in-situ, in a PECVD deposition reactor.

PECVD reactor equipped with the MPL device allowing in-situ determination of the lifetime of minority carriers

The lifetime is generally given for a well-defined injection level (typically 10¹⁵ cm^-3) in order to be able to compare the different results with each other. In order to be able to carry out this measurement for a given injection, a method of laser modulation (scanning laser power and modulation frequency), data acquisition and processing has been specifically developed. As shown in the figure below, this allows us to successfully reproduce the lifetime measurements carried out on dedicated ex-situ instruments, such as the Sinton

Minority carrier lifetime as a function of minority carrier density (MCD). The black squares (filled) is the carrier lifetime profile provided by Sinton WCT-120. The black squares (unfilled) and the red circles (unfilled) are the carrier lifetime profile obtained from MPL measurement before and after applying the lifetime correction respectively.

The first studies focused on the passivation of crystalline silicon by a layer of aluminum oxide deposited by ALD. This type of layer requires annealing after deposition by ALD in order to give the material its optimum passivation properties. The evolution of the life time during the annealing treatment of the alumina passivation layers revealed a behavior of astonishing complexity as can be seen in the figure below. Studies are underway to try to account for these phenomena [1].

Evolution of carrier lifetime profiles during annealing of a c-Si wafer, passivated by Al₂O₃, at 350°C for 30 minutes.

 

[1] Anatole Desthieux, Mengkoing Sreng, Pavel Bulkin, Ileana Florea, Etienne Drahi, Barbara Bazer-Bachi, Jean-Charles Vanel, François Silva, Jorge Posada, Pere Roca i Cabarrocas : « Stable Positive Fixed Charges in AlO_x Activated During Annealing with In Situ Modulated PhotoLuminescence”. Solar Energy Materials and Solar Cells. 230 (2021) 111172. https://doi.org/10.1016/j.solmat.2021.111172