Helio Moutinho

Growth and characterization of radio frequency magnetron sputter-deposited zinc stannate, Zn2SnO4, thin films

Single-phase, spinel zinc stannate (Zn2SnO4) thin films were grown by rf magnetron sputtering onto glass substrates. Uniaxially oriented films with resistivities of 10−2–10−3 Ω cm, mobilities of 16–26 cm2/V s, and n-type carrier concentrations in the low 1019 cm−3 range were achieved. X-ray diffraction peak intensity studies established the films to be in the inverse spinel configuration. 119Sn Mossbauer studies identified two octahedral Sn sites, each with a unique quadrupole splitting, but with a common isomer shift consistent with Sn+4. A pronounced Burstein–Moss shift moved the optical band gap from 3.35 to as high as 3.89 eV. Density-of-states effective mass, relaxation time, mobility, Fermi energy level, and a scattering parameter were calculated from resistivity, Hall, Seebeck, and Nernst coefficient transport data. Effective-mass values increased with carrier concentration from 0.16 to 0.26 me as the Fermi energy increased from 0.2 to 0.9 eV above the conduction-band minimum. A bottom-of-the-band ...

Effects of CdCl2 treatment on the recrystallization and electro-optical properties of CdTe thin films

The effects of CdCl2 processing on the physical and electro-optical properties of CdTe were evaluated for thin films produced by physical vapor deposition and close-spaced sublimation (CSS). Two substrates (CdS and Indium–tin–oxide) were used with the physical vapor deposition (PVD) films specifically to isolate the effects of the Cd(SxTe1−x) alloy formed during the treatment of films deposited on CdS. The samples were analyzed by x-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence. The observed changes in microstructure were caused by recrystallization, which consisted of the nucleation and development of a new CdTe structure and subsequent grain growth. Nevertheless, for these processes to take place, it was necessary that enough lattice-strain energy was available in the films. For this reason, PVD films did recrystallize, while CSS films did not. For the first time, recrystallization was observed directly in AFM images of CdTe films and confirmed by XRD analysis, which indica...

Local Built-in Potential on Grain Boundary of Cu(In,Ga)Se2 Thin Films

We report on a direct measurement of two-dimensional potential distribution on the surface of photovoltaic Cu(In,Ga)Se2 thin films using a nanoscale electrical characterization of scanning Kelvin probe microscopy. The potential measurement reveals a higher surface potential or a smaller work function on grain boundaries of the film than on the grain surfaces. This demonstrates the existence of a local built-in potential on grain boundaries, and the grain boundary is positively charged. The local built-in potential on the grain boundary is expected to increase the minority-carrier collection area from one to three dimensional. In addition, a work function decrease induced by Na on the film surface was observed.

Does the local built-in potential on grain boundaries of Cu(In,Ga)Se2 thin films benefit photovoltaic performance of the device?

In a previous paper [C.-S. Jiang et al., Appl. Phys. Lett. 84, 3477 (2004)], we reported the existence of a local built-in potential on grain boundaries (GBs) of photovoltaic Cu(In,Ga)Se2 (CIGS) thin films. However, whether the built-in potential benefits photovoltaic properties of the device has not been proven. Using a scanning Kelvin probe microscope, we found that, with increasing Ga content in the CIGS film, the built-in potential on the GB drops sharply in a Ga range of 28%–38%. Comparing the changes in the built-in potential, the device efficiency, and the CIGS band gap, we conclude that the built-in potential on the GB plays a significant role in the device conversion efficiency of NREL’s three-stage CIGS device.

Investigation of polycrystalline CdTe thin films deposited by physical vapor deposition, close‐spaced sublimation, and sputtering

CdTe thin films, deposited on different substrate structures by physical vapor deposition, sputtering, and close‐spaced sublimation, have been treated with CdCl2 at several temperatures. The morphology of the films has been studied by atomic force microscopy, and the observations were correlated to results obtained from x‐ray diffraction, cathodoluminescence, and minority‐carrier lifetime measurements. The samples treated at 400 °C resulted in the best device‐quality films, independent of deposition method and underlying substrate structure. For the first time, a nanograin structure was observed in CdTe sputtered samples.CdTe thin films, deposited on different substrate structures by physical vapor deposition, sputtering, and close‐spaced sublimation, have been treated with CdCl2 at several temperatures. The morphology of the films has been studied by atomic force microscopy, and the observations were correlated to results obtained from x‐ray diffraction, cathodoluminescence, and minority‐carrier lifetime measurements. The samples treated at 400 °C resulted in the best device‐quality films, independent of deposition method and underlying substrate structure. For the first time, a nanograin structure was observed in CdTe sputtered samples.

Direct evidence of a buried homojunction in Cu(In,Ga)Se2 solar cells

The built-in electrical potential of Cu(In,Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. Profiles of the electrical potential along cross sections of the device demonstrate that the p–n junction is a buried homojunction, and the p/n boundary is located 30–80 nm from the CIGS/CdS interface in the CIGS film. The built-in electric field terminates at the CIGS/CdS interface, indicating that the CdS and ZnO layers of the device structure are inactive for the collection of photoexcited carriers.

Understanding the Formation and Temperature Dependence of Thick-Film Ag Contacts on High-Sheet-Resistance Si Emitters for Solar Cells

Physical and electrical properties of screen-printed Ag thick-film contacts were studied and correlated to understand and achieve good-quality ohmic contacts to high-sheet-resistance emitters for solar cells. Analytical microscopy and surface analysis techniques were used to study the Ag-Si contact interface of three different screen-printed Ag pastes (A, B, and PV168) subjected to high (∼835°C) and conventional (740-750°C) temperature firing conditions. At ∼750°C firing, all three pastes failed on a 100 Ω/□ emitter because of incomplete etching of the silicon nitride film (PV168), an irregular small distribution of regrown Ag crystallites (paste A), or an excessive diffusion of Ag into the p-n junction (paste B). At a firing temperature of ∼835°C, paste A gave a lower open-circuit voltage because of the diffusion of Al from the glass frit into the emitter region. Paste B failed because of the formation of very large (0.3-1 μm) Ag crystallites that shunted the p-n junction. Of the three pastes, the PV168 paste from DuPont gave the best contact quality on a 100 Ω/□ emitter with a solar cell fill factor of 0.782 only after annealing in a hydrogen atmosphere.

INVESTIGATION OF INDUCED RECRYSTALLIZATION AND STRESS IN CLOSE-SPACED SUBLIMATED AND RADIO-FREQUENCY MAGNETRON SPUTTERED CDTE THIN FILMS

We have induced recrystallization of small grain CdTe thin films deposited at low temperatures by close-spaced sublimation (CSS), using a standard CdCl2 annealing treatment. We also studied the changes in the physical properties of CdTe films deposited by radio-frequency magnetron sputtering after the same post-deposition processing. We demonstrated that the effects of CdCl2 on the physical properties of CdTe films are similar, and independent of the deposition method. The recrystallization process is linked directly to the grain size and stress in the films. These studies indicated the feasibility of using lower-temperature processes in fabricating efficient CSS CdTe solar cells. We believe that, after the optimization of the parameters of the chemical treatment, these films can attain a quality similar to CSS films grown using current standard conditions.

Measurement of built-in electrical potential in III–V solar cells by scanning Kelvin probe microscopy

We report on direct measurements of the built-in electrical potential in III–V semiconductor-based solar cell devices by using scanning Kelvin probe microscopy. Potential profiles on cross sections of the devices were measured quantitatively and spatially resolved in open and short circuits, under and without illuminations, with selective photon energies matching band gaps of the junctions. The measurements provide valuable information about the electrical properties of the devices, and are useful for understanding the performance of solar cells. On a GaInP2-single junction cell, two potential features were measured and were assigned to the p-n junction and the potential barrier at the interface between the GaInP2 base layer and the GaAs substrate. The potential on the p-n junction is photoactive, and that on the GaInP2/GaAs interface is photoinactive. On a GaInP2/GaAs tandem cell, two potential features were measured near the top and the bottom p-n junctions. When the sample was illuminated by light with...

Microstructural properties of Cu(In, Ga)Se2 thin films used in high-efficiency devices

Abstract Thin-film polycrystalline photovoltaic devices based on Cu(In,Ga)Se2 have a demonstrated efficiency approaching 19%. The best performance was achieved when the Ga/In+Ga ratio was in the 25–30% range. The short-circuit current density exhibited for the device containing CdS was almost at its expected maximum. The open-circuit voltage was relatively low considering the optical bandgap (Eg) of the above absorber (∼1.15 eV); at best, it is 0.6×Eg. In this work, we examined the microstructural properties, e.g. defects due to misorientation, micro-twinning, stacking faults, and dislocations, for films prepared by our ‘ three-stage’ process, including the CIGS and Mo back-contact. We also attempted to make a correlation between the above observations and device performance.