Visualization of functionally different domains in bulk heterojunction (BHJ) solar cells is of paramount importance to understand the routes of optimization of their structure for best performance. In this work, a concept of detecting n-type and p-type semiconductor domains in BHJ structures by methods based on atomic force microscopy (AFM) is proposed. It assigns an active role to the semiconducting coating of the AFM probe tip which is able to form different junctions, i.e., p-n anisotype or p+-p, n+-n isotype, with the surfaces inspected. Here, we illustrate this concept on the example of BHJ structures composed of the n-type inorganic microcrystalline semiconductor CdS and mechanochemically prepared p-type kesterite nanopowder and two types of AFM probe tip coatings, i.e., p-type boron-doped diamond and n-type nitrogen-doped diamond coating, respectively. Conductive AFM (CAFM) measurements demonstrated unequivocally the different diode behavior when contacting n- or p-type semiconductor domains in the BHJ structures. Simulation of the energy level alignment at the probe-sample interfaces allowed us to explain the formation of anisotype or isotype junctions depending on the sample domain and probe used. Kelvin probe force microscopy measurements were consistent with the CAFM results and indicated the different contact potentials from the diverse types of domains in the BHJ structure.
A conductive pyrolytic carbon fiber (CF) has been found to serve as an alternative material to metal electrodes, since it forms an Ohmic contact to CdS crystals. The methods of preparation of polycrystalline layers and nanocrystalline arrays of CdS are described that allow formation of an ohmic or quasi-ohmic contact to CF. It is shown that the ohmic contact between the CF and polycrystalline CdS layer is stable for at least several months and its exploitation characteristics are not worse than the indium contact. Advantages of the CF electrode, such as thermostability to extremely high temperatures and low cost are discussed. CdS nanowire arrays grown on a carbon fiber.
Over the past decades, zinc oxide has attracted considerable attention for its possible application in optoelectronics due to simultaneous observation of intense ultraviolet and visible emission offering the development of white light-emitting devices. However, in the most cases the native defects responsible for visible emission are not stable upon materials processing. Moreover, for white phosphors, efficient and controllable emission in specific spectral range is often required. This can be achieved in particular via materials doping with rare-earth (RE) ions. In the present study the results on effect of doping of screen-printed ZnO films with Sm 3+ and/or Ho 3+ ions are presented. Photoluminescence (PL) and Raman scattering spectra as well as the X-ray diffraction patterns of undoped and doped films are examined in details versus sintering conditions and doping level. In the PL spectra of undoped ZnO films sintered at 400−700°C, the excitonic emission was observed only, whereas sintering at higher temperatures (up to 1200°C) resulted in the appearance of visible defect-related PL bands peaked at 540−600 nm. The most intensive defect-related emission was found in the films sintered at 1000°C and these latter were doped with rare-earth ions of different concentration in the range of 1·10 19 − 4·10 20 cm -3 . In the PL spectra of the RE-doped films, the corresponding RE emission was observed at low temperatures, but not at room temperature. Since the defect-related PL band caused by intrinsic defects in ZnO overlapped essentially with corresponding Ho and Sm PL bands, the PL bands peaked at about 700 nm due to 4 G 5/2 → 6 H 11/2 transitions in Sm 3+ ions were observed only. At the same time, simultaneous codoping with both Sm 3+ and Ho 3+ ions produced significant decrease in the intensity of the UV and visible defect-related PL bands as well as the increase in the intensity of Sm 3+ and Ho 3+ PL components. In the PL excitation spectra of the PL band peaked at about 720 nm in addition to excitation caused by ZnO band-to-band absorption and absorption involving intrinsic defects of zinc oxide, a new characteristic absorption band at about 410 nm appeared. The effect of RE doping on the PL and PL excitation spectra is discussed in terms of the formation of RE ions complexes as well as energy transfer from ZnO host to RE ions.
The effect of Sm 3+ and/or Ho 3+ doping on structural and luminescent properties of screen-printed ZnO films sintered at 1000°C was investigated by photoluminescence (PL), PL excitation and Raman scattering methods. For all the films, ultraviolet excitonic and visible defect-related PL bands of ZnO were detected. The doping with Ho 3+ ions produced an enhancement of PL in ZnO films, the excitonic PL intensity being increased prominently, while the co-doping with Sm 3+ and Ho 3+ ions resulted in PL decrease in ZnO films. Only for (Sm,Ho)- co-doped ZnO films, the rare-earth PL bands were detected. The reduction of Sm 3+ to Sm 2+ was observed demonstrating 5 D 0 → 7 F J radiative transitions. The mechanism of PL and PL excitation is discussed in terms of the formation of rare-earth complexes as well as energy transfer towards them from ZnO host.
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