Study of the Interface in a GaP/Si Heterojunction Solar Cell

Abstract

We have investigated the GaP/Si heterojunction interface for application in silicon heterojunction solar cells. We performed X-ray photoelectron spectroscopy (XPS) on thin layers of GaP grown on Si by metal organic chemical vapor deposition and molecular beam epitaxy. The conduction band offset was determined to be 0.9 ± 0.2 eV, which is significantly higher than predicted by Anderson's rule (0.3 eV). XPS also revealed the presence of Ga-Si bonds at the interface that are likely to be the cause of the observed interface dipole. Via cross-sectional Kelvin probe force microscopy (x-KPFM), we observed a charge transport barrier at the Si/GaP interface which is consistent with the high-conduction band offset determined by XPS and explains the low open-circuit voltage and low fill factor observed in GaP/Si heterojunction solar cells.

S-Shaped Current–Voltage Characteristics in Solar Cells: A Review

“…Full mitigation of s-shapes was achieved by adjusting the contacts, such as by using a different work function material [110] or by doping the contact layer [58]. Improvements of the FF without fully mitigating the s-shape was achieved by doping either absorber [44], [132] or selective contact materials [131]. Furthermore, decreasing the thickness of passivating [30], charge blocking [100], or buffer layers has been shown to mitigate s-shape I-V curves.…”

“…Annealing was also found to harm the FF of SHJ solar cells with metal oxide selective contacts in a study performed by Bivour et al [48]. In a study on GaP as selective contact material, s-shaped behavior was observed, which vanished after highly doping the crystalline silicon at the interface [44]. Via X-ray photoelectron spectroscopy and crosssectional scanning Kelvin probe microscopy, a high conduction band offset and subsequently a high charge transport barrier was identified as the culprit [44].…”

“…A more detailed overview of the history and device physics of SHJ solar cells has recently been published by Chavali et al [11]. S-shaped I-V curves have commonly occurred in SHJ solar cells with a-Si [4], [11], [41]- [43] as well as with alternative emitter materials [30], [44]. Examples will be presented in the following section.…”

“…In a study on GaP as selective contact material, s-shaped behavior was observed, which vanished after highly doping the crystalline silicon at the interface [44]. Via X-ray photoelectron spectroscopy and crosssectional scanning Kelvin probe microscopy, a high conduction band offset and subsequently a high charge transport barrier was identified as the culprit [44]. Seif et al replaced the intrinsic a-Si by intrinsic a-SiO x and found a thickness-dependent FF similar to the observation with intrinsic a-Si made by Lu et al [4] and Shu et al [41].…”

“…In conclusion, the observation of s-shaped I-V curves in SHJ solar cells has been linked to barriers in charge transport. These barriers were either created through a mismatch in band alignment with the selective contact [30], [36], [44] or with the passivation layer [4], [35], [41]. Fig.…”