The high efficiency of the HIT solar cell derives from a Type II heterojunction between wide-bandgap a-Si and c-Si [Tanaka, et. al., Jpn. J. Appl. Phys., 31, 3518 (1992)]. However, this heterojunction has both a conduction and a valence-band offset, which can lead to undesirable ‘S-shaped’ current-voltage characteristics [A. Kanevce, et. al., J. Appl. Phys. 105, 094507 (2009)]. We present a new class of heterojunctions formed between c-Si and the wide bandgap transition metal-oxides TiO and ZnO, which block the flow of holes but allow the flow of electrons. TiO/Si interfaces have reached a carrier recombination velocity of < 100 cm/s.
TiO layers were deposited by CVD from titanium(IV) tetra-(tert-butoxide) [S. Avasthi, et al., APL 102, 203901 (2013)], while ZnO layers were deposited by plasma-enhanced ALD using diethyl-zinc and CO [D.A. Mourey, et al., Elec. Dev., IEEE Trans., 57, 530 (2010 )]. The fabrication temperature is only 100°C for TiO/Si and 200°C for ZnO/Si heterojunction, making them especially well-suited for low-cost crystalline silicon solar cells.
Photoelectron spectroscopy shows a large valence-band barrier (ΔE) of 3.4 eV at the TiO/Si interface, but only a small conduction-band barrier (ΔE) of 0.1 eV [J. Jhaveri, et al., 39th IEEE PVSC (2013)]. Therefore the heterojunction blocks the flow of holes from silicon to TiO but allows transport of electrons from Si to TiO. Because the ZnO band offsets are similar [V. Srikant, et al., J. Appl. Phys., 83, 5447 (1998)], the ZnO/Si interface is also expected to selectively block the flow of only holes.
The hole-blocking property of the metal-oxide/Si interfaces was tested on Al/metal-oxide/c-Si/Ag devices made on p-type and n-type Si (100) with < 10 nm undoped metal-oxide films. Without the metal-oxide device characteristics were ohmic. With the metal-oxides, the Al/ZnO/p-Si and Al/TiO/p-Si devices, show diode-like characteristics with a J of 10- and 10- A/cm, respectively. Since the current from p-Si to Al is dominated by holes, the change in characteristics, from ohmic in Al/p-Si/Ag to diode-like in Al/metal-oxide/p-Si/Ag, shows that the holes are being blocked by the large ΔE at the metal-oxide/Si interface. I-V characteristics were also measured under light, by shining light through the semi-transparent Al electrode. Due to the Al-induced electric field in Si, the photogenerated holes in Si are swept towards the Ag electrode while electrons in the Si are swept towards the TiO (or ZnO). The separated electrons do not see any electron barrier (since ΔE ≈0), yielding a well-behaved solar cell I-V curve devoid of any ‘S-shaped’ character.
We also measured the interface recombination velocity (IRV) of the TiO/Si junction by photoconductance decay. IRV at as-deposited interfaces on n- and p-Si vary over a wide range, from 500 to 10- cm/s. However, annealing at only 250°C yields IRVs of <100 cm/s, which corresponds to an interfacial defect densitiy of only ~10- cm-.
Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University
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