Investigations on RF-magnetron sputtered Co₃O₄thin films regarding the solar energy conversion properties

Rapid Thermal Treatment of Reactive Sputtering Grown Nanocrystalline Co3 O4 for Enhanced All-Oxide Photovoltaics

“…The estimated efficiency ( η ) with this configuration was around 0.1% (Table ). A thicker layer of metal oxides is not suitable due to the short diffusion length of the photogenerated charge carriers, and we can see that the 200 nm thick TiO 2 layer caused a significantly low η value of 0.0039%, due to the dominant recombination transport in the Co 3 O 4 solar cell . Even the obtained η value of a mere ≈0.1% for the nanocrystalline Co 3 O 4 solar cell was much higher than the pulsed laser deposited Co 3 O 4 solar cell with a TiO 2 layer, or the reactively grown Co 3 O 4 solar cell with a ZnO layer…”

“…The band diagram of this structure in a flat band condition is shown in Figure b, and this device architecture depicts the “all‐oxide photovoltaics.” Light exposure from the FTO induces prominent absorption near the junction (TiO 2 /Co 3 O 4 ), and photogenerated electron–hole pairs can be separated out so that electrons and holes can be collected at the FTO and NiO/AgNWs/NiO contact, respectively. In particular, NiO is an effective hole transporting layer to compensate the low carrier mobility and short diffusion length of the Co 3 O 4 layer . The transmittance spectra of the Co 3 O 4 device is shown in Figure c, to represent the near infrared semitransparent characteristic, with an average transmittance of 51% and 2% which suggests the high potential of tandem photovoltaics …”

“…All‐oxide photovoltaics devices employing CuO, Cu 2 O, Fe 2 O 3 , Co 3 O 4 , and BiFeO 3 in a heterojunction configuration have been examined, with Cu 2 O/Ga 2 O 3 and BiFeO 3 /ZnO heterojunction showing the maximum conversion efficiency of ≈3.9% . In particular, the spinel Co 3 O 4 structure is very interesting because of its dual band gap of 1.5 and 2.2 eV, p‐type carriers, and excellent chemical stability, which satisfy the requirements of efficient and reliable single junction solar cells . The spinel structure of Co 3 O 4 is composed of two cation coordinates, a fourfold tetrahedral site (Co, 2 + ) and a sixfold octahedral site (Co, 3 + ) giving an overall formula of CoCo 2 O 4 .…”

“…Octahedral and tetrahedral cobalt vacancies are the dominant sources of the p‐type conductivity of Co 3 O 4 under oxygen‐rich conditions. Ab initio studies suggest there are challenges associated with fabricating a quality p‐type Co 3 O 4 material with fewer defects and high mobility, and these properties are essential to enhancing the performance of the all‐oxide photovoltaic . The spinel‐type Co 3 O 4 nanocrystals are also a potential alternative for the renewable‐energy technologies such as fuel cell and energy storage …”

“…Studies on back metal contacts of Ag, Cu, Ni, and Co for Co 3 O 4 photovoltaics have shown a consistent V OC of 0.4 V . Wang et al studied hydrothermal Co 3 O 4 nanoparticles on a Pt electrode for a V OC of 0.7 V. Lohaus et al studied Co 3 O 4 film fabricated by sputtering Co 3 O 4 to obtain a V OC of 0.15 V, and concluded that the efficiency of Co 3 O 4 devices is limited by poor transport properties, combined with defect states in the forbidden gap of Co 3 O 4 , and Fermi level pinning at heterointerfaces. Moreover, the surface recombination and diffusion length of the minority carrier of the Co 3 O 4 material are the critical parameters to enhance the general power conversion efficiency …”