Vertically aligned ZnO nanorods doped with lithium for polymer solar cells: defect related photovoltaic properties

Abstract

The nanorod arrays of ZnO incorporated with lithium atoms show specific crystallinity, photoluminescence and absorption properties, which are promising for the improvement of photovoltaic performance of hybrid solar cells based on ZnO/poly(3-hexylthiophene). Li ions can be incorporated into ZnO crystals during the hydrothermal growth of the nanorods. The presence of Li in ZnO crystal was confirmed through X-ray diffraction analysis and by the photoluminescence spectra obtained. The difference in photovoltaic properties brought about by Li doping were determined from concentrations of the precursor solution. It was determined that appropriate Li doping improves both the short circuit current density (J sc) and open circuit voltage (V oc). The quenching of photoluminescence of Li-doped ZnO nanorods/P3HT films indicates effective charge transfer at the interface due to oxygen-enrichment surface, corresponding to the enhancement of J sc. The improvement of V oc was due to the suppression of 65 (LiNO 3), monoethanolamine (H 2 NCH 2 CH 2 OH), and chlorobenzene (C 6 H 5 Cl) were purchased from Wako Pure Chemical Industries, Japan. Hexamethylenetetramine (C 6 H 12 N 4) and 2-methoxyethanol (C 2 H 8 O 2) were bought from Aldrich.

Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions

“…The yellow emission is commonly attributed to oxygen interstitial defects, although some impurities such as Li may also play a role. 45,46 In addition to this common hypothesis, the possible presence of Zn(OH) 2 at the surface was identified as a possible reason for the weak UV and the strong visible (broad yellow and green) emission. 50,51 The orange-red emission centered at $640-650 nm is also commonly attributed to the presence of excess oxygen in the samples, such as oxygen interstitial 5,47,48 defects.…”

“…5,11 The yellow emission (550-650 nm) for undoped ZnO can be due to the Zn(OH) 2 or OH group. 46,52 The peak "f" observed at 615 nm in our ZnO sample corresponds to yellow emission. The yellow luminescence in doped nanoparticles can be due to both Li Zn and Li i defects.…”

“…The yellow luminescence in doped nanoparticles can be due to both Li Zn and Li i defects. 46 The peak "f" observed at 574 6 4.84 nm (2.16 6 0.04 eV) in doped samples can correspond to the transition from donor levels (V o , Zn i , and Li i ) in the crystal to Li Zn acceptor levels, releasing yellow luminescence. 7 Pipat et al 46 have reported yellow luminescence in undoped ZnO due to OH ions and in doped ZnO nanorods due to Li ions.…”

“…46 The peak "f" observed at 574 6 4.84 nm (2.16 6 0.04 eV) in doped samples can correspond to the transition from donor levels (V o , Zn i , and Li i ) in the crystal to Li Zn acceptor levels, releasing yellow luminescence. 7 Pipat et al 46 have reported yellow luminescence in undoped ZnO due to OH ions and in doped ZnO nanorods due to Li ions. Rauch et al 53 observed experimentally that the deep Li Zn acceptor defect has been made responsible for yellow emission in Li doped ZnO nanocrystal.…”

Lithium doping and photoluminescence properties of ZnO nanorods

“…22 However, they can diffuse into the space between the lattices as interstitials (Li i ). 23 As a result Li ions can form substitutional as well as interstitial point defects in ZnO crystal lattice, therefore to maintain the charge balance of the lattice, intrinsic defects of O i and V O are created and coexist in ZnO NRs. 24 From the figure 5, it can be speculated that Li in ZnO NRs acts as a defect mediator.…”