Bifunctional CO₂ Plasma Treatment at the i/p Interface Enhancing the Performance of Planar Silicon Heterojunction Solar Cells

Abstract

Passivation techniques are regarded as an effective strategy to obtain high-efficiency crystalline silicon (c-Si) solar cells. These include field-effect passivation and chemical passivation. [1] Among high-performance devices, intrinsic hydrogenated amorphous silicon (i-a-Si:H) provides outstanding chemical passivation on the surface of c-Si by saturating the dangling bonds and thus reducing the density of interface. Then, with the addition of thin doped silicon films, transparent conducting oxides (TCOs), and metal contacts, silicon heterojunction (SHJ) solar cells with an open-circuit voltage (V oc ) of 750 mV have been achieved. [2] To achieve high V oc , many efforts have been endeavored, such as pretreatment of silicon wafer surface before i-a-Si:H deposition, [3][4][5][6] controlling i-a-Si:H deposition, [7][8][9] suppressing epitaxial growth, [7] plasma treatment (PT) after i-a-Si:H deposition. [10][11][12][13][14] Hydrogen plasma treatment (HPT) is an effective approach to improve a-Si:H/c-Si interface passivation. However, etching [15] and crystallization of a-Si:H [16] induced by HPT, indicate that its application in passivation should be treated with care.For SHJ, the optical losses mainly lie in doped a-Si:H and TCO due to reflection and parasitic absorption, especially at the doped layer on the illuminated side of SHJ. [17,18] Compared with doped a-Si:H or hydrogenated amorphous silicon oxide (a-SiO x :H) layers, hydrogenated nanocrystalline silicon (nc-Si:H) or hydrogenated nanocrystalline silicon oxide (nc-SiO x :H) as window layers have a better potential to improve the short-circuit current density ( J SC ) of SHJ solar cells due to its

Achievement of 25.54% power conversion efficiency by optimization of current losses at the front side of silicon heterojunction solar cells

“…Generally, it is quite reasonable to expect that RF nc‐SiO x :H film with high E g would lead to a higher transparency, and hence a higher J sc . However, it has also been reported that RF nc‐SiO x :H films result in a higher series resistance and deteriorated surface passivation, causing both FF and J sc loss 3,12,16,63 . Therefore, it is necessary to explore the different contributions of RF and VHF nc‐SiO x :H films on SHJ solar cell performance more closely.…”

“…Generally, SHJ solar cells comprise a crystalline silicon (c‐Si) absorber sandwiched by intrinsic and doped hydrogenated amorphous silicon (a‐Si:H) thin layers, transparent conductive oxide (TCO) layers, and screen‐printed metallic finger grid contacts. All the a‐Si:H layers are usually grown by plasma‐enhanced chemical vapor deposition (PECVD), in which the intrinsic hydrogenated amorphous silicon (a‐Si:H(i)) layers passivate the surface of c‐Si by saturating the dangling bonds and thus reducing the density of surface states 3 . The boron‐doped p‐type hydrogenated amorphous silicon (a‐Si:H(p)) and phosphorous‐doped n‐type amorphous silicon (a‐Si:H(n)) layers create back‐surface field (BSF) and front‐surface field (FSF) playing a role of charge carrier selective collection 4 .…”

“…Parasitic absorption also occurs in the carrier selective a‐Si:H layers due to the presence of hydrogen related defects and high absorption coefficient 3,12,16–18 . Recently, many reports showed that nc‐SiO x :H has the advantages in improving the J sc due to its low absorption coefficient 1,3,4,12,16–18,42–46 .…”

“…Parasitic absorption also occurs in the carrier selective a‐Si:H layers due to the presence of hydrogen related defects and high absorption coefficient 3,12,16–18 . Recently, many reports showed that nc‐SiO x :H has the advantages in improving the J sc due to its low absorption coefficient 1,3,4,12,16–18,42–46 . Its higher doping efficiency and improved transport properties result from nanometer‐sized hydrogenated nanocrystalline silicon (nc‐Si:H) embedded in hydrogenated amorphous silicon oxide (a‐SiO x :H) matrix 16,17 .…”

Application of interface treatment at different position p-nc-Si:H hole collector of silicon heterojunction cells

“…Another effective method for reducing the incubation layer thickness is the CO 2 plasma treatment. The atomic oxygen can penetrate into the i-a-Si:H, forming highly stressed Si-O-Si bonds on the surface of the film, which facilitates the aggregation of Si clusters and the formation of nanocrystalline phases [22,23]. The CO 2 plasma treatment proves more effective than the seed layer in improving the crystallinity of p-nc-Si:H, but the material σ dark is ~0.2 S cm −1 , which does not meet the needs of carrier transport in high efficiency SHJ solar cells.…”