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Electronic Supplementary Information (ESI) available: Evidence of morphological evolution of CuO ellipsoids from Cu(OH)2. XRD pattern and SEM images of samples taken at the intermediate stage of experiment is presented. Crystal structure of CuO is shown in S2. SeeWe explore the finite size effect on magnetic properties of CuO mesocrystals synthesized by a surfactant-free solution phase method. Significant evidence on the evolution of 3D oval, elliptical and 2D sheet-like CuO mesocrystals driven by oriented aggregation of CuO nano subunits is presented. Dimension of CuO nano-subunits constituting the mesocrystals as well as its final morphology is tailored by NaOH concentration in precursor solution. Changes in mesocrystal morphology reflect on the position and intensity of Raman vibrational modes. DC magnetization curves reveal distinct features of antiferromagnetic transitions where the magnetization behaviour of mesocrystal is influenced by the dimension of CuO nano subunits. The ordering temperature (TN) is reduced from bulk value of 230 K to 58 K as the size of CuO nano subunit decreases to 7 nm. Moreover, all nanostructures exhibit linear field dependent magnetization below the transition temperature similar to antiferromagnetic system. However, below TN field cooled isothermal magnetization display anomalous behaviour as a shift along the vertical axis whose origin is related to surface uncompensated spins. The vertical shift decreases as the CuO nano-subunit dimension increases.

Density functional calculations are performed to study the magnetic order of the severely distorted square planar cupric oxide (CuO) and local spin disorder in it in the presence of the transition metal impurities M (=Cr, Mn, Fe, Co and Ni). The distortion in the crystal structure, arisen to reduce the band energy by minimizing the covalent interaction, creates two crisscrossing zigzag spin-1/2 chains. From the spin dimer analysis we find that while the spin chain along [Formula: see text] has strong Heisenberg type antiferromagnetic coupling (J ~ 127 meV), along [Formula: see text] it exhibits weak, but robust, ferromagnetic coupling (J ~ 9 meV) mediated by reminiscent p-d covalent interactions. The impurity effect on the magnetic ordering is independent of M and purely orbital driven. If the given spin-state of M is such that the [Formula: see text] orbital is spin-polarized, then the original long-range ordering is maintained. However, if [Formula: see text] orbital is unoccupied, the absence of corresponding covalent interaction breaks the weak ferromagnetic coupling and a spin-flip takes place at the impurity site leading to breakdown of the long range magnetic ordering.

p‐type copper oxide (CuO) nanocrystals were prepared by a wet chemical method and are used to make thin films by spin casting. CuO nanocrystals were characterised using XRD, Raman, TEM and XPS. Low temperature solution processed thin films of CuO crystals showed a hole mobility of 4.57 × 10−4 cm2 V−1 s−1 perpendicular to the plane of the film and is measured using space charge limited current method. Due to the difficulty in making solution processed homo p–n junction and n‐doping of CuO, a planar hybrid heterojunction solar cell is fabricated using solution processed CuO and a n‐type organic material, [6,6]‐phenyl‐C70 butyric acid methyl ester (PCBM). A power conversion efficiency of 0.23% is obtained for hybrid solar cell comprising CuO as active layer where all the device fabrications were carried out at temperature ≤100 °C. Analysis of the current density–voltage (J–V) characteristics at different light intensities is carried out to unravel the recombination mechanism occurring at the interface. Further, effect of annealing the active layer on the photovoltaic performance is investigated.

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