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(a) (i) The depleted heterojunction solar cell shows the transparent electrode and an active layer consisting of p-type PbS CQDs. Au provides the Ohmic contact on the PbS nanocrystal side. (ii) Spatial band alignment of , PbS, and Au at equilibrium. The quasi Fermi energy is shown by the dotted line. (b) Current-voltage (J-V) response of a representative device from FTO-porous device. The device area is . (c) EQE spectrum of the same device. (d) XPS data from pure Au and PbS–Au sample. The binding energies of and are located at 84.0 eV and 87.67 eV, respectively, for both the samples, indicating that no interfacial reaction takes place between PbS and Au.
(a) J-V response of devices made with and without LiF between PbS and Ni. (b) EQE spectrum of the devices made from LiF–Ni. The integrated compares well with the J-V data. (c) Spatial band diagram at equilibrium of (i) PbS–Ni while assuming no interfacial reaction, i.e., sulfur interdiffusion, (ii) PbS–Pb/NiS–Ni, when sulfur interdiffusion takes place, and (iii) PbS–LiF–Ni when LiF acts as a thin tunnelling barrier and suppresses sulfur diffusion.
XPS spectra from PbS–Ni (left) and PbS–LiF–Ni (right) interfaces. (a) and (b) spectra taken from pure Ni metal and compared with PbS–Ni and PbS–LiF–Ni samples. Without LiF, NiS is formed at the interface and peak moves to higher binding energy. A thin LiF layer prevents such a reaction. (c) and (d) PbS is reduced to Pb at the PbS–Ni interface, whereas LiF hinders such reduction for PbS–LiF–Ni devices. [(e) and (f)] Formation of NiS can also be seen from spectrum at the PbS–Ni interface. The existence of pure PbS is also confirmed for PbS–LiF–Ni devices.
Average performance of four representative devices made with each discussed metal contact. All data were obtained under illumination (AM1.5).
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