Stability of amorphous/crystalline silicon heterojunctions

Abstract

Silicon heterojunction solar cells have demonstrated high efficiencies through excellent surface passivation. However, the use of amorphous silicon (a-Si) in the structure raises the question of long term stability as a-Si solar cells suffer degradation due to the Staebler-Wronski effect. The stability of a-Si in terms of its ability to passivate silicon wafers is evaluated by measuring the minority carrier lifetime and it is found to be unstable over time. The instability is most evident in thin, -10 nm, intrinsic a-Si only layers where lifetime falls from over 1ms immediately after annealing, to 300JJs one month later. The change in lifetime occurs even in the dark at room temperature with the largest fall in the first few days after deposition. The lifetime can be recovered close to its initial level by annealing the sample in air, indicating that the lifetime change is not due to a simple oxidation of the outer layer of a-Si. However, device measurements where an intrinsic/doped a-Si structure is used for the emitter and contact show no change in open circuit voltage or fill factor after one year. The changes in the passivation during processing has important implications for the sequence of device fabrication, the use of lifetime testing as a diagnostic tool and the passivation of all back contact devices.

Low temperature front surface passivation of interdigitated back contact silicon heterojunction solar cell

“…However, the structure was not fully optimal and resulted in a low J SC due to extra optical absorption loss in a-Si:H, which means the a-Si:H film should be as thin as possible. The a-Si:H/crystalline silicon (c-Si) structure has demonstrated a low surface recombination, but it is often seen that the passivation quality degrades over time [3], especially for a thin (<20 nm) a-Si:H layer. Additionally, the growth of ITO AR coating, degrades the front surface passivation most likely due to ion damage during sputtering.…”

The role of back contact patterning on stability and performance of Si IBC heterojunction solar cells

“…As shown in the structure schematic in Fig. 1(a), the cell with PR/LFC has the 25 µm gap passivated by only 8 nm thick i-layer, which we have shown leads to unstable passivation quality as exhibited by a decrease in minority carrier lifetime over time [7]. Contamination at the i/doped aSi:H interface in this process may occur due to the presence of PR during deposition in the PECVD chamber [8].…”

Influence of a-Si:H deposition power on surface passivation property and thermal stability of a-Si:H/SiNx:H stacks

“…The samples were prepared and processed in the following way: firstly, the p-type, Ga-doped, CZ wafers with thickness of 180μm and resistivity of 4.5 cm were RCAcleaned: 6min in NH 4 OH/H 2 O 2 /H 2 O, followed by DI water rinsing, and then 6min in HCl/H 2 O 2 /H 2 O followed by dipping in diluted HF (1%) for 1min to remove the native oxide.…”

“…The appeal of the a-Si:H is due primarily to its extremely low SRV and absence of parasitic shunting caused by positive-charge-induced inversion layers often encountered in silicon nitride passivated rear surfaces a E-mail: hua.li@student.unsw.edu.au when non-ideal metal contacts are used. 3 However the instability of a-Si:H passivating layers under certain thermal treatments such as required for metal firing processes for industrial solar cells has hindered the industrial application of a-Si:H. 4,5 The surface passivation by an a-Si:H layer has been reported to suffer significant degradation at temperatures higher than 300 • C, attributed to the out-effusion of the hydrogen from the a-Si:H passivating layer 6 to the ambient. Thus the critical issue for achieving and preserving excellent surface passivation on the rear surface with a-Si:H is to improve the thermal stability of the passivating layer when exposed to certain thermal processes such as required for annealing or firing processes which are necessary for cell manufacturing to form front or rear localized metal contacts through such a passivating layer.…”