Amorphous silicon carbide passivating layers for crystalline-silicon-based heterojunction solar cells

Abstract

Amorphous silicon enables the fabrication of very high-efficiency crystalline-silicon-based solar cells due to its combination of excellent passivation of the crystalline silicon surface and permeability to electrical charges. Yet, amongst other limitations, the passivation it provides degrades upon high-temperature processes, limiting possible post-deposition fabrication possibilities (e.g., forcing the use of low-temperature silver pastes). We investigate the potential use of intrinsic amorphous silicon carbide passivating layers to sidestep this issue. The passivation obtained using device-relevant stacks of intrinsic amorphous silicon carbide with various carbon contents and doped amorphous silicon are evaluated, and their stability upon annealing assessed, amorphous silicon carbide being shown to surpass amorphous silicon for temperatures above 300 C. We demonstrate open-circuit voltage values over 700 mV for complete cells, and an improved temperature stability for the open-circuit voltage. Transport of electrons and holes across the hetero-interface is studied with complete cells having amorphous silicon carbide either on the hole-extracting side or on the electron-extracting side, and a better transport of holes than of electrons is shown. Also, due to slightly improved transparency, complete solar cells using an amorphous silicon carbide passivation layer on the hole-collecting side are demonstrated to show slightly better performances even prior to annealing than obtained with a standard amorphous silicon layer. V

Toward Annealing‐Stable Molybdenum‐Oxide‐Based Hole‐Selective Contacts For Silicon Photovoltaics

“…Notably, the corresponding JV ‐curve does not exhibit an S‐shape around V OC and the determined FF is close to the value achieved with sister samples featuring our baseline a‐Si:H(i)/a‐Si:H(p) front hole‐selective contact. The slightly reduced V OC compared to values obtained with standard a‐Si:H(p) layers can be attributed to a slightly degraded a‐Si:H passivation from pre‐annealing, which may be resolved by designing passivation layers more resilient to thermal annealing at 250 °C, or releasing less hydrogen at this temperature, such as a‐SiC X :H films …”

“…The V OC is degraded by the pre‐MoOx‐deposition annealing, with a stronger drop when increasing the pre‐annealing temperature. This can be attributed to dehydrogenation and thus passivation degradation of the a‐Si:H passivation layer, both on the hole‐collecting side and electron‐collecting‐side for highest temperatures . Notably, a degradation is seen after metal curing at 190 °C for devices exposed to a pre‐MoOx‐deposition annealing at a temperature below 250 °C, whereas an improvement is observed for temperatures higher than 250 °C.…”

Light-induced performance increase of silicon heterojunction solar cells

“…2 In recent years, a-Si:H layers also garnered significant attention, thanks to their excellent crystalline silicon (c-Si) surface passivation properties, even when only a few nm thin. [3][4][5][6][7][8] This property is exploited with remarkable success for passivating-contact fabrication in silicon heterojunction (SHJ) solar cells, [9][10][11][12][13][14][15][16][17][18][19][20][21][22] with reported conversion cell efficiencies as high as 26.3%. 23 For any solar cell technology, an important criterion for ultimate device performance is its stability under prolonged light exposure.…”

Plasma-initiated rehydrogenation of amorphous silicon to increase the temperature processing window of silicon heterojunction solar cells

“…The annealing temperature strongly affects the rate at which hydrogen evolves from a-Si:H deposited by PECVD: for example, the evolution rate at 510 C was shown to be ten times higher than that at 300 C. 26 Hydrogen evolution has been reported to begin occurring at temperatures between 250 C 26 and 350 C, 18 but we observed prominent lifetime degradation only at 400 C and above, consistent with previous results on 20-nm-thick a-Si:H layers. 8 As shown in Figure 3, 450 C is the maximum temperature after which >1 ms lifetime can be recovered via hydrogen plasma treatment. At this temperature, the effect of rehydrogenation is dramatic: the post-annealing lifetime is only 64 ls, and thus the implied V oc jumps from 614 mV to 722 mV with a 30-s hydrogen plasma treatment.…”

“…8 However, the incorporation of carbon also increases the defect density at the interface, establishing a tradeoff between high-temperature stability (high carbon content) and excellent passivation (low carbon content). Another approach to obtain excellent and stable passivation is to replace the intrinsic aSi:H layer with a very thin silicon oxide layer, allowing charges to be collected by tunneling.…”