2. Articole
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Item THIN LAYER PHOTOVOLTAIC MODULES BASED ON CdS-CdTe HETEROJUNCTIONS(2011) Fiodorov, Vladimir; Gașin, Petru; Gagara, Ludmila; Chetruș, PetruA technology of thin layer photovoltaic modules with dimensions of 65 x 80 mm2 based on CdS-CdTe heterojunctions prepared via the quasi-closed volume technique was developed. The modules were made on a single glass plate; each element was positioned along the given plate so that the resulting module contains nine solar cells with an area of 3 cm2. The design of the module made it possible to study the energy parameters of each solar cell as well as the entire module. The study of the photovoltaic module based on CdS-CdTe heterojunctions showed the following parameters: short circuit current Isc = 270 mA; open circuit voltage Ucd = 0.786 V; FF =0.46, power discharged in the load Pmax ≈ 100 mW, and efficiency η ≈ 8.2% at the radiation power of 53 mW/cm2.Item PHOTOELECTRICAL PARAMETERS OF nCdS–pCdTe THIN-FILM SOLAR CELLS WITH A CdO BUFFER LAYER(2016) Gașin, Petru; Gargara, Ludmila; Chetruș, Petru; Inculeț, Ion; Fiodorov, Vladimir; Qassem, Amjad-AlThe results of the studies of the effect of a CdO buffer layer on the photoelectrical parameters of CdS–CdTe solar cells have been described. It has been found that the formation of a 5–8 nm thick CdO buffer layer leads to an increase in the CdS–CdTe solar cell open circuit voltage (Voc) by 90–140 mV, short circuit current (Isc) by 1.4–3.2 mA/cm2, and efficiency by 2.8–4.1% at 300 K and an illumination of 100 mW/cm2.Item ELECTRICAL AND PHOTOELECTRICAL PROPERTIES OF HETEROJUNCTIONS ON THE BASE OF Cu(InGa)Se2(2005) Chetruș, Petru; Gașin, Petru; Nicorici, Valentina; Suman, VictorCdS films deposited by hot wall technique on mica were used as substrates for Cu(InGa)Se2 deposition. Two methods were used for the deposition of Cu(InGa)Se2 films: a) vacuum thermal evaporation from a single source and b) “flash” evaporation. The obtained films were of p-type conductivity with hole concentration varied from 2×1018 cm-3 to 6×1020 cm-3 depending on the fabrication method. The structures Cu(InGa)Se2–CdS were divided into two groups: the structures of type I having the CdS film thickness from 1.6 m to 2.8 m and the structures of type II having the CdS film thickness from 0.6 m to 0.8 m. It was established that the direct/reverse current ratio is 8-16. For the first type heterostructures the diffusion potential is 1.2–1.8 V and for the second type is 0.2–0.34 V. The Cu(InGa)Se2–CdS fotosensitivity is situated in the wavelength region from 0.51 m to 1.1 m and is determined by the electron-hole pair generation in both materials.Item MECANISMUL DE TRANSPORT AL CURENTULUI ŞI LUMINESCENŢA STRUCTURILOR Pd-ZnSe(UTM, 2009-10-01) Scurtu, Roman; Gașin, Petru; Covali, AndreiAu fost analizate proprietăţile electrice şi de luminescenţă ale structurilor Schottky ZnSe-Pd obţinute pe baza monocristalelor de ZnSe cu rezistivitatea de 0.78 Ω•cm şi concentraţia electronilor 1.82 10 16 cm-3 . S-a determinat mecanismul de transport al purtătorilor de sarcină prin structură. La polarizări directe curentul depinde exponenţial de tensiunea aplicată şi este determinat de emisia electronilor. La polarizări inverse dependenţa curentului de tensiune este descrisă de o funcţie de putere, cu factorul m=3÷6. Cu mărirea temperaturii curentul invers creşte exponenţial, ceea ce indică faptul că în mecanismul de transport predomină procesele de tunelare. La polarizări directe şi temperaturi ~80 K structurile Pd-ZnSe iradiază lumină în regiunea albastră a spectrului cu maximul de luminescenţă la 2.7 eV.Item PHOTOLUMINESCENCE STUDIES OF THE INTERFACE OF CdS/CdTe HETEROJUNCTIONS(2009) Vatavu, Sergiu; Zhao, Hehong; Caraman, Iuliana; Gașin, Petru; Morel, Don; Ferekides, ChrisThe photoluminescence analysis of CdS/CdTe interface layer of CdS/CdTe heterojunction has been carried out in the 15- 100 K temperature range. An attempt to correlate observed PL features with the CdSxTe1–x layer presence at the interface of the heterojunctions was made. It is assumed the 1.525 eV band has an excitonic origin and 1.37X PL band is a characteristic impurity band due to CdSTe layer. Comparative plots showing SnO2/CdTe PL spectra are given as well.Item CURRENT FLOW MECHANISM IN ZnSe-ZnO-Pd STRUCTURES(CEP USM, 2010) Scurtu, Roman; Gașin, Petru; Chetruș, PetruStructurile MOS au fost obţinute folosind ca suporturi plăcuţe monocristaline de ZnSe orientate în direcţia [110] cu grosimea 3-5mm. Monocristalele de ZnSe au fost tratate în zinc +0,1%Al la 950ºC timp de 100 ore, în vid. Concentraţia electronilor la 300 K este de 1,85x1016cm-3, iar mobilitatea – 466 cm2 /VS. În calitate de oxid s-au folosit straturi subţiri de ZnO, obţinute prin tratarea cristalelor de ZnSe în apă oxigenată. Grosimea stratului de ZnO depinde de timpul de tratare chimică şi alcătuieşte 4-13 nm. Contactul metalic, strat subţire din Pd, a fost obţinut prin evaporare termică în vid. Ca contact ohmic la cristalele de ZnSe s-a folosit In obţinut prin tratare în aer la 370ºC timp de 30-120 sec. La polarizări directe, curentul depinde exponenţial de tensiunea aplicată şi, la tensiuni de până la 0,7 V, e determinat de curentul de tunelare, iar mai mare de 0,7 V – de procesele de recombinare. Factorul de idealitate variază de la 2,86 –la 93 K la 1,47 – la 333 K. Potenţialul de difuzie variază între 0,8 V şi 1,28 V, corespunzător la 353 K şi 113 K. La polarizarea inversă este o funcţie de putere şi are factorul de putere 3,93 la 113 K şi de 2,42 la 353 K. La polarizări directe 2÷3 V şi T=77 K structurile Pd-ZnO-ZnSe radiază în regiunea albastră. La 77 K se observă două fâşii: prima mai intensivă, cu maximul 2,7 eV, şi alta – cu maximul 2,06 eV.Item THE ANALYSIS OF CURRENT FLOW MECHANISM IN CDS/CDTE HETEROJUNCTION(Elsevier, 2007) Vatavu, Sergiu; Gașin, PetruAn analysis of current–voltage dependencies of CdS/CdTe heterojunction in the 78–370 K temperature range has been carried out. According to this analysis the current flow mechanism is determined by the tunneling processes through dislocations, which penetrate the heterojunction space charge region. The concentration of dislocations has been estimated as 2 · 105 cm− 2. The number of steps necessary for tunneling varies: 2.5 · 102–1.7 · 103. The characteristic energy has a weak temperature dependence (− 0.2 meV/K) and its value vary 120–200 meV. The increase of the annealing duration results in the decrease of the characteristic energy. The multistep tunneling processes through local centres, determined by impurity centres, interface states and defects in the space charge region, predominate at reverse biases. The number of tunneling steps is 1–4 · 102. The concentration of local centres (traps) in the heterojunction have been estimated as 2.37 · 105–1.63 · 106 cm− 3. The thermal annealing in the presence of CdCl2 up to 60 min does not modify the current flow mechanism in CdS/CdTe heterojunctions.Item THE COPPER INFLUENCE ON THE PL SPECTRA OF CDTE THIN FILM AS A COMPONENT OF THE CDS/CDTE HETEROJUNCTION(Elsevier, 2009) Vatavu, Sergiu; Zhao, Hehong; Caraman, Iuliana; Gașin, Petru; Ferekides, ChrisThe influence of annealing in the presence of CdCl2 and a thin copper layer deposited onto CdTe on the photoluminescence spectra of CdTe, as a component of CdS/CdTe heterojunction, has been studied for two excitation wavelengths: 0.337 μm and 0.6328 μm. The behavior of the PL was studied as a function of the measurement temperature and excitation intensity. At 0.6328 μm excitation, the interface PL consists of a known 1.43X band, and the chloride annealing enhances radiative transitions at 1.536 eV. The intensity of the 1.536 eV transitions increases when Cu is present. The PL of as-deposited CdTe films prepared in the presence of oxygen has the 1.45X band attenuated when excited with 0.337 μm excitation wavelength.Item THE INVESTIGATION OF TCO/CdS/CdTe HETEROJUNCTIONS BY C-U and C-f MEASUREMENTS: EXPERIMENT AND MODELING(IEEE, 2013) Rotaru, Corneliu; Vatavu, Sergiu; Fedorov, Vladimir; Gașin, Petru; Lux-Steiner, Martha; Ferekides, Chris; Rusu, MarinThe paper presents the analysis of C-U and C-f data obtained in the 1 kHz-10 MHz range for CdS/CdTe heterojunctions prepared by CSS onto (ZnO:Al, ZnO:Al/i-ZnO)/Glass substrates. An attempt to model the physical properties of the heterojunctions is made. A small signal analysis has been carried out to establish the equivalent circuit of device. The unusual capacitance behavior of the ZnO based heterojunction is attributed to presence of the barrier at the back contact.Item GROWTH AND CHARACTERIZATION OF Eu DOPED GaSe SINGLE CRYSTALS BY X-RAY DIFFRACTION AND RAMAN SPECTROSCOPY(CEP USM, 2017) Untila, Dumitru; Evtodiev, Igor; Caraman, Iuliana; Kantser, Valeriu; Spalatu, Nicolae; Dmitroglo, Liliana; Evtodiev, Silvia; Spoială, Dorin; Rotaru, Irina; Gașin, PetruGaSe single crystals doped with Eu (0.025, 0.05, 0.5, 1.0 and 3.0 at%) were grown by Bridgman method using Ga, Se and Eu elementary components. The crystalline structure and vibration modes of the GaSe: Eu crystals lattice were studied by X-ray diffraction and Raman spectroscopy. Eu atoms arranged in the van der Waals space of GaSe: Eu crystals form Eu-Se valence bonds and restructure hexagonal lattice of GaSe leading to EuGa2Se4 crystallites formation. Defects generated by EuGa2Se4 crystallites lead to broadening and shifting of single phonon peaks present in Raman spectra towards shorter wavenumbers, and at the same time, activate the longitudinal optical vibrations of EuSe sublattice.