• Energy Research

BiVO4 films were fabricated by radio frequency (RF) sputtering from a single target. The deposited BiVO4 films were found to be rich in Bi, and the reason for the Bi-richness was investigated. It was inferred from the Monte Carlo simulation that, during sputtering, the transfer process of target atoms through argon gas played a major role in this phenomenon. The transfer process resulted in an imbalanced ratio of Bi and V, arising from the difference in atom mass and interaction radius. The high RF power was found to be effective in adjusting the Bi/V ratio, influencing the sputtering yield. This type of preferential sputtering was maintained by the diffusion of target atoms from the bulk to the surface. BiVO4 films with monoclinic scheelite crystal structures were obtained at high RF power values and found to exhibit photocatalytic performances beneficial for photoanodic applications.

Abstract Deposition conditions of cuprous oxide (Cu2O) thin films on glass substrates and nitrogen doping into Cu2O were studied by using reactive radio-frequency magnetron sputtering method. The effects of defect passivation by crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, were also studied. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu2O at around 680 nm was enhanced, and the hole density was increased from 1016 to 1017 cm−3. Finally, polycrystalline p-Cu2O/n-ZnO heterojunctions were grown for use in solar cells. Two deposition sequences were studied, ZnO deposited on Cu2O and Cu2O deposited on ZnO. It was found that the crystallographic orientation and current–voltage characteristics of the heterojunction were significantly influenced by the deposition sequence, both being far superior for the heterojunction with structure Cu2O on ZnO than for the inverse structure. We successfully obtained a photoresponse for the first time in the deposited thin film of Cu2O/ZnO.

Defect study of the CuGaSe2 thin-films deposited with three-stage evaporation process under different Se-flux (PSe) conditions were performed using admittance spectroscopy (AS) technique. A dominant defect, A2 has been identified around 230~350 meV above the valance band (Ev) of the CuGaSe2. In general, density of defects was found to decrease with an increase in the PSe condition during deposition of the CuGaSe2 layer. Effect of the Se-flux on the performance of the fabricated solar-cells were discussed in relation to the defect-properties of the corresponding CuGaSe2 absorber-layer identified by admittance spectroscopy.

Abstract The influence of Se beam pressure on defect properties in Cu(In 1– x ,Ga x )Se 2 (CIGS)-based solar cells fabricated by the three-stage process was investigated by admittance spectroscopy. The Se beam pressure was varied from 1.3×10 −3 to 4.4×10 −3 Pa, at the position of the samples, by varying the Se cell temperature. The spectra for all samples show three distinct peaks denoted as α , β , and ζ with activation energies of 20, 150, and 300 meV, respectively. The trap density of ζ increased from 5×10 14 to 4×10 15 cm −3 with decrease in Se beam pressure; the trap densities of α and β did not change. These results suggest that the origin of ζ is related to the Se deficiency. The density of ζ seems to correlate with the cell efficiency. Therefore, it is important to control the Se beam pressure in order to obtain highly efficient solar cells.

Abstract Nonmixed planar heterojunction (PHJ) small-molecule organic photovoltaics (OPVs) with 96% internal quantum efficiency (at 595 nm) and 4.77% power conversion efficiency (PCE) have been demonstrated. In addition to boron subphthalocyanine chloride (SubPc) and C60 as electron donor and acceptor materials, respectively, PHJ OPVs contain an ultrathin (2 nm) buffer layer of bis-(naphthylphenylaminophenyl)fumaronitrile (NPAFN) between the indium tin oxide (ITO) anode and the donor layer (SubPc). Compared with copper phthalocyanine (CuPc) or α-naphthylphenylbiphenyl diamine (NPB) buffer layers, the NPAFN buffer layer blocks the exciton diffusion from the SubPc electron donor layer to the ITO anode more effectively and considerably improves the short circuit current (JSC) from 5.96 (without an NPAFN layer) to 7.70 mA/cm2 (with a 4-mm-thick NPAFN layer ). In addition, experimental results indicated that the NPAFN buffer layer reduces the crystallization, or stacking, of the SubPc electron donor, thereby limiting the reverse saturation current and elevating the open circuit voltage (VOC) from 1.01 (without an NPAFN layer) to 1.08 V (with a-2-nm thick NPAFN layer). However, series resistance (RS) of the OPV monotonically increases with increasing NPAFN layer thickness. The performance of the OPV is optimized when the NPAFN buffer layer thickness is 2 nm. Compared with a SubPc–C60 PHJ OPV without an NPAFN buffer layer, the PCE of a OPV with a buffer layer increases by 22% from 3.96% to 4.77%, with a concurrent increase in JSC (from 5.96 to 7.02 mA/cm2) and VOC (from 1.01 to 1.08 V). However, a decrease in RS (from 10.21 to 14.95 Ω cm2) and in fill factor (from 65% to 63%) is also observed.

Abstract In this article, we look, from the carrier perspective, at the effect of the double grading in a CIGS absorber, in order to understand its benefit compared to a single grading. We focus on the double grading effects on the conduction band edge, generation rate and carrier collection. We first show that, when the minimum bandgap position (notch position) is outside of the absorber depletion region, a local maximum of the conduction band edge appears at the back of the depletion region, creating a hump, which hinders the collection of electrons. Interpreting the effect of the grading as an effective electric field, we show that the hump is caused by the competition between the absorber internal electric field and the grading created effective electric field. From this, we deduce that the notch should be positioned in the depletion region. As expected, the generation rate increases significantly at the notch position. However, studying carrier collection and the recombination rate, we show that, even if in the ideal case we should observe a total collection of carriers generated in the depletion region, the recombination rate in the SCR increases significantly when the notch is in it. We propose that this is caused by the increased carrier generation close to recombination centres. The ideal notch position is, therefore, a compromise between being close enough to the front to have a maximal carrier collection and avoiding the creation of a hump, while minimizing the carrier recombination.

Abstract Precise defect control is crucial for optimizing thermoelectric (TE) materials. However, thin film processes differ from bulk synthesis, necessitating distinct approaches to defect management. This study investigates the impact of varying Mg flux rates in the molecular beam epitaxy (MBE) growth of epitaxial Mg2Sn (Ge) thin films, with Mg: Sn (Ge) ratios from 3.9 to 9.1 while maintaining constant Sn and Ge flux rates. Our results indicate that while the films mainly consisted of the Mg2Sn phase due to excess Mg compensating evaporation at the growth temperature, the Mg flux rate significantly influenced film growth dynamics. X-ray diffraction analysis showed that higher Mg flux rates increased microstrain and decreased vertical grain sizes, suggesting increased planar defect density. However, the full-width half maximums of rocking curves tend to be reduced at higher flux rates, attributed to enhanced in-plane grain alignment and reduction of point defect density. Positron annihilation experiments revealed lower vacancy-type defects at higher Mg flux rates, aligning with the rocking curve measurements. The higher Mg flux rates enhanced surface migration and promoted larger horizontal grain growth. As these grains coalesce, slight misalignments between them introduce strain within the crystal lattice. To accommodate this strain, planar defects such as stacking faults form, as indicated by the x-ray pole figure measurements. Despite the higher crystal quality and reduction in vacancy-type defects, the total thermal conductivity of the films decreased with increasing Mg flux rates. This suggests that modulating Mg flux rates in MBE-grown Mg2Sn thin films, it is possible to achieve enhanced crystalline alignment and controlled formation of beneficial higher-dimensionality defects, which together contribute to the reduction in thermal conductivity and improve the film’s overall TE performance.

Abstract Cu(In0.52Ga0.48)Se2 solar cell performance with an MBE-grown ZnS buffer layer was studied. It was found that cell performance with the ZnS buffer layer was inferior compared to that with the CdS buffer layer; however, by insertion of a 50 nm CdS layer in addition to the ZnS buffer layer, the device performance was improved and was better than that with the CdS buffer layer. The higher device performance by using the ZnS/CdS double buffer layer may be due to the improved interface properties of the device.