Transparent-Conductive-Oxide-Free Front Contacts for High Efficiency Silicon Heterojunction Solar Cells

Abstract

In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both-sides contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for efficient carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design. A series resistance of 0.32 Ωcm2 and a fill factor of 80.7% were measured for a TCO-free back-junction SHJ solar cell with a conventional finger pitch of 1.8 mm, thereby proving that relying on lateral conduction in the c-Si bulk is compatible with low series resistances. Achieving high efficiencies in SHJ solar cells with TCO-free front contacts requires suppressing deterioration of the passivation quality induced by direct metal-a-Si:H contacts and in-diffusion of metal into the a-Si:H layer. We show that an ozone treatment at the a-Si:H/metal interface suppresses the metal diffusion and improves the passivation without increasing the contact resistivity. SHJ solar cells with TCO-free front contacts and ozone treatment achieve efficiencies of > 22%.

Towards bifacial silicon heterojunction solar cells with reduced TCO use

“…When the iMPP appears at the Δ n region of 1 sun illumination, the R sh, e in the wafer is lower than that of the standard 75‐nm‐thick TCO layers. This implies that the c‐Si absorber is able to provide efficient lateral electron transport towards metal contacts during operation, which has been proven in literature 12,13,27 . On the other hand, for the lateral hole transport, the R sh,h of the wafer is above 350 Ω/sq at 0.2 suns illumination, which is comparable with that the R sh of 25‐nm‐thick TCO layers.…”

“…This implies that the c-Si absorber is able to provide efficient lateral electron transport towards metal contacts during operation, which has been proven in literature. 12,13,27 On the other hand, for the lateral hole transport, the R sh,h of the wafer is above 350 Ω/sq at 0.2 suns illumination, which is comparable with that the R sh of 25-nm-thick TCO layers. The R sh,h of the wafer could be reduced to below 200 Ω/sq at 1 sun illumination.…”

“…For the AZO use in (i), challenges may lie in low carrier mobility and stability issues, [9][10][11] and the promise of nontraditional TE material use still needs to be tested and developed. For (iii), proof-of-concept TCO-free SHJ solar cells have been demonstrated, 12 but efforts have been hampered from the passivation deterioration and contacting problems. [12][13][14] For instance, a thick thinfilm silicon layer was reported to reduce the passivation deterioration, but it accompanies additional optical loss due to the parasitic absorption from the thin-film silicon layer 12 ; titanium (Ti) was reported to maintain a low contact resistance between silicon layer and metal, but Ti can easily react with thin-film silicon layer and depassivate the solar cell.…”

“…However, the functions of ARC and BR are weakened. As for the ARC purpose, an additional optical capping layer (OCL), such as MgF 2 , 19 SiO x , 9,20,21 SiN x , 12,17 and TiO x , 22 could be utilized to reach an optimal light in-coupling. It has been well proven that as compared with a SHJ device counterpart with only TCO layer acting as ARC at illumination side(s), the device with a double layer anti-reflection coating could produce an even higher current output 23 and meanwhile potentially improve the device stability.…”