We report on our investigation of the microstructure and chemical composition at the CdTe/Te-rich interfaces generated by NP-etching polycrystalline and single-crystalline CdTe films and followed with HgTe-graphite pasting and thermal annealing. We find that after this process, a thin layer of CdxHg1-xTe forms between CdTe and Te-rich layers, giving a structure like CdTe/CdxHg1-xTe/Te. High-resolution electron microscopy reveals that the CdxHg1-xTe layer has an epitaxial relationship with the CdTe. No CdxHg1-xTe layer has been observed in bromine/methanol-etched samples or samples with intentionally deposited Te layers.

The electronic properties of polycrystalline CuIn1-xAlxSe2 (CIAS) films, which are incorporated as the absorber layer in photovoltaic devices, have been studied to better understand limitations on device performance. These studies have shown that compared to lower Al content films and to CuIn1-yGaySe2films, films with x ≥ 0.29 are relatively intrinsic, spatially nonuniform, and have broader bandtails characterized by much higher Urbach energies. This indicates that the CIAS films with x ≥ 0.29 are significantly more disordered than lower Al content CIAS or corresponding CIGS films, which likely negatively impacts the resulting photovoltaic device performance.

Gallium-doped zinc oxide films were prepared by rf magnetron sputtering at room temperature as a function of the substrate-target distance. The best results were obtained for a distance of 10 cm, where a resistivity as low as 2.7×10-4 Ωcm, a Hall mobility of 18 cm2/Vs and a carrier concentration of 1.3×1021 cm-3 were achieved. The films are polycrystalline presenting a strong crystallographic c-axis orientation (002) perpendicular to the substrate. The films present an overall transmittance in the visible part of the spectra of about 85 %, in average. The low resistivity, accomplished with a high growth rate deposited at RT, enables the deposition of these films onto polymeric substrates for flexible applications.

CdTe etching was investigated using variable angle spectroscopic ellipsometry and glancing angle x-ray diffraction. Treatment with HNO3:H3PO4 (NP) based etches was shown to form amorphous-Te surface films which spontaneously crystallize following etching. Br2/methanol (BM) etching forms very thin amorphous-Te films. NP-etched surfaces are stable in ambient air for ∼1 hr before beginning to oxidize, while BM etched films oxidize immediately following treatment. CdTe grain boundary etching by NP was minimized using more acidic etches. Device analysis suggests that a higher Te content produces more stable back contacts by attenuating Cu diffusion. Mechanistic details of NP etching are discussed.

Zinc-based buffer layers like ZnSe, ZnS, or wet-chemically deposited ZnO on Cu(In, Ga)(S, Se)2 absorber materials (CIGSSe) have yielded thin film solar cell efficiencies comparable to or even higher than standard CdS/CIGSSe cells. However, little is known about surface and interface properties of these novel buffer layers. In this contribution we characterize the specific chemical environment at the absorber/buffer-interface using X-ray Emission Spectroscopy (XES) and Photoelectron Spectroscopy (PES) in a complementary way. Evidence of intermixing and chemical reactions is found for different buffer materials and deposition methods.

The built-in electrical potential on cross sections of Cu(In, Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. In the conditions of open and short circuits, no significant potential variation on the p-n junction was probed due to the surface Fermi-level pinning. With an external reverse-bias voltage applied to the device, we were able to probe the potential on the junction; the potential profiles demonstrate that the p-n junction is a buried homojunction, located 30-80 nm from the CIGS/CdS interface in the CIGS film. The potential measurement over the CdS and ZnO layers, which is consistent with the band diagram calculations, indicates that the CdS and ZnO layers are inactive for the collection of photoexcited carriers.

This contribution investigates the diffusion of In and Ga in Cu(In1-xGax)S2 thin film absorbers for photovoltaic applications. Sequentially prepared Cu-rich CuGaS2/CuInS2thin film diffusion couples have been annealed at temperatures in the range of 500°C to 600°C. Annealed samples have been examined by XRD and SNMS depth profiling. It was found that despite significant interdiffusion of Ga and In the bilayer retains its two phase structure of two layers of almost homogeneous composition, where the composition of these layers depends on the annealing temperature. Furthermore the interdiffusion process is less effective if the binary copper-sulfide phase is removed before annealing. On the basis of a tentative two dimensional diffusion model, which includes rapid diffusion along path ways of high diffusivity, the Ga-depth distribution could be modeled by numerical fits of calculated XRD-spectra to the experimental data. CuS is proposed as a possible pathway for rapide In-Ga interdiffusion.

An Al-doped ZnO front contact was successfully used for the first time for the fabrication of high efficiency CdS/CdTe thin-film solar cells. The ZnO:Al films were deposited on aluminosilicate glass by RF sputtering from a ZnO:Al2O3 target. The ZnO:Al film has ∼95% average transmission in the visible spectrum with ∼3 ohm/square sheet resistance. The CdS and CdTe thin films were also deposited by RF sputtering and devices were completed with a vapor CdCl2 treatment and evaporated Cu/Au back contacts. The highest processing temperature was 387°C, reached during the vapor CdCl2 treatment. The devices were tested at NREL with efficiency of 14.0% which is a record for an all-sputtered CdS/CdTe solar cell. The ZnO-based cell had JSC of 23.6 mA/cm2 compared to 20.7 mA/cm2 for our recent NREL-tested 12.6% cell on a commercial soda-lime-glass/SnO2:F substrate. Other parameters of the 14% ZnO based cell are: FF = 73.25% and VOC = 814 mV. The improved performance is almost entirely due to higher current because of better optical and electrical properties of ZnO:Al TCO. We report also on relative stability between devices on SnO2:F and ZnO:Al TCO, under one-sun light soak at VOC.

We have used the fine structure in the Cu K-edge x-ray absorption spectrum to help elucidate the lattice location of Cu in polycrystalline, thin-film CdTe solar cells. In particular, we have studied how the typical CdCl2 vapor treatment in dry air changes the local environment of the Cu in CdTe. We find the Cu absorption spectrum to be similar to that of Cu2Te in the as-deposited CdTe film but to convert to a spectrum similar to Cu2O environment after the vapor CdCl2 treatment.

Grazing incidence X-ray diffraction (GIXRD) in conjunction with a layer absorption modeling algorithm is a powerful tool for studying the structural properties of polycrystaline thin films. A typical application is the refinement of compositional depth profiles. Of genera interest are the depth profile of the Ga/(In+Ga)-ratio over the entire thickness of the thin film, as well as the depth profile of th Cu/(In+Ga)-ratio near the surface. In this respect the three stage process is a particular interesting deposition technique. A remarkable recrystalization of the entire thin film at the end of the second stage has a strong impact on Ga-gradients as well as on the Cu-depletion close to the surface. In this contribution we use the GIXRD technique to refine composititional depth profiles obtained by secondary ion mass spectrometry (SIMS). We demonstrate that structura changes near the surface due to the recrystalization can be monitored. In addition we are able measure the depth of a Cu-depleted surface layer with high accuracy.

It is known that carriers photogenerated in the polycrystalline CdS layer of a CdS/CdTe cell are not collected. Thus, the short-circuit current (JSC) of CdS/CdTe devices should be improved if the bandgap of CdS is increased to permit better blue response. Wu, et al, showed that alloying of CdS with oxygen can increase the absorption edge of the layer. We report here on studies of this ‘oxygenated’ CdS and its use in sputtered cells. We find that at a deposition temperature of 250°C the addition of O2 to the sputter gas results in a red shift of the absorption edge (from 2.35 eV to 1.94 eV), but that room temperature deposition gives a blue shift (from 2.36 eV to 3.28 eV). The Raman spectra of the room temperature deposited films show a considerable broadening of LO phonon peaks suggesting a micro- to nano- to amorphous transition as the O2 fraction increases. XRD measurements of these films confirm the formation of an amorphous structure at high O2 fractions. The quantum efficiency measurements of CdS/CdTe device with room temperature deposited oxy-CdS show an improvement in blue response and hence increased JSC, but are accompanied by poorer junction quality so that the overall efficiency is not increased.

The fabrication, characterization, and performance of wafer-bonded (WB) GaInAsSb thermophotovoltaic (TPV) devices for monolithically series-interconnected cells are reported. TPV epilayers were bonded to GaAs handle wafers with SiOx/Ti/Au. This dielectric/metal layer is multi-functional in that it is used as the adhesive to bond the epilayers; it provides electrical isolation; and it is an internal back surface reflector. Considerations were made to minimize residual stress and provide high reflectivity. Excellent structural and optical properties of WB TPV structures are observed. The external quantum efficiency of WB and unbonded TPV cells is comparable. TPV cells were monolithically series interconnected, and a 10-junction device exhibited linear voltage building with an open-circuit voltage of 1.8 V.

A technique for spatially resolved optical characterization of CdS/CdTe thin film solar cells has been developed using electroluminescence (EL). In EL, excess minority carriers are injected via forward biasing. Light produced in radiative carrier recombination is collected with a CCD camera. Because EL intensity depends upon radiative vs. non-radiative recombination lifetimes, EL provides insight into material quality.

Spatial resolution is a key benefit of EL as it provides insight into the non-uniformities of polycrystalline CdTe. At high magnification the resolution is diffraction limited, but coarser measurements of up to several millimeters in range may also be made. Non-uniformities in emission have been observed throughout this range.

Further benefits of EL as a characterization technique are as follows: EL probes the region of most interest, namely the CdTe near the main junction. Also, it is observable at room temperature and data acquisition is fast. Finally, EL is observable at very low carrier injection rates, comparable to short circuit current. (Though more structure is often revealed at higher injection rates.) This low injection means that EL can be a non-destructive probe. This fact, along with the aforementioned ease of observation, means that EL could possibly be used for quality control and in situ testing of modules.

Data gathered from CdS/CdTe cells from various institutions deposited using different methods such as close spaced sublimation, vapor transport, and sputtering are presented. In addition to changes in deposition technique, changes in processing parameters were observed to affect EL emission. Furthermore, overall EL emission decreased noticeably with stress at various biases and elevated temperature, with non-uniformity increasing in many cases. Changes in EL become apparent before changes in parameters acquired with standard current-voltage measurements, suggesting that this technique can be used as an early indicator for degrading cells. Finally, some dramatic changes in EL with stress suggest highly non-uniform degradation of the back contact.

Due to the high chemical reactivity of GaSb surfaces, many commonly used aqueous sulfide passivation techniques lead to the growth of surface oxides that degrade device performance. We have developed a non-aqueous passivation regime consisting of Na2S/benzene/15-crown-5/oxidant. The use of a non-polar, aprotic organic medium required the addition of a specific chelating agent, i.e. a 15-crown-5 ether, to solubilize sodium sulfide, and organic oxidizing agents, such as anthraquinone and benzophenone, to act as electron acceptors. The surface optical and chemical properties of GaSb surfaces after aqueous and non-aqueous sulfide treatments were compared. Non-aqueous passivation resulted in higher PL intensity, lower oxide content, and a less amount of elemental Sb than aqueous passivation.

This paper presents data showing a Meyer-Neldel rule (MNR) in InGaAsN alloys. It is shown that without this knowledge, significant errors will be made using Deep-Level Transient-Spectroscopy (DLTS) emission data to determine capture cross sections. By correctly accounting for the MNR in analyzing the DLTS data the correct value of the cross section is obtained.

Cu(In, Ga)Se2 (CIGS) solar cells have been fabricated on different metal foil substrates. Preferred foils with regard to costs and physical properties were stainless steel, titanium and Kovar® (Fe/Ni/Co alloy). SiOx films prepared by PECVD and sol/gel methods were deposited as barrier layers on these metal foil substrates. Both film types showed good suitability as diffusion barriers for substrate elements. In the SiOx:Na (sol/gel) barrier sodium could be incorporated to provide the Na-supply to the CIGS absorber. With relatively low sodium concentrations in these films (6% Na2O) large amounts of Na in the absorber have been found. Using a combination of SiOx (PECVD) and SiOx:Na (sol/gel) layers AM 1.5 cell efficiencies of 13.1% on titanium foil substrates have been achieved. With a newly implemented insulation test based on electrolysis measurements pinholes in the barrier layers could be localized and conclusions on their origin could be drawn.

Differential thermal analysis (DTA) measurements were carried out in order to investigate thermal properties of model flux compositions of CdTe-CdCl2 with CdTeO3, the last recognized as a main oxidation product of CdTe. The composition of the flux was chosen to obtain larger quantity of residual flux phases for analyses and the presence of solid CdTe phase (about 20 mol%) at the fusion temperature 520 °C in a vacuum-closed isothermal quartz ampoule. Quantity of CdTeO3 constituted 20 mol % of the whole sample mass. DTA curves taken in the 300-700°C interval reveal considerable lowering of melting and solidification temperatures for the flux with addition of CdTeO3. The fused batches were vacuum annealed at 470 °C in a long narrow quartz tube for separation and deposition of volatile components CdCl2 and Te at the room temperature part of the tube. After vacuum annealing the residual CdTe phase on the batch with tellurite was coated with transparent platelike crystals morphologically typical to the CdTeO3, while oxide-free batch had no alien phases. In the flux, CdCl2 has a role of catalytic agent forming low melting point composition with native oxides and CdTe. After processing, CdCl2 can be separated by vacuum annealing.

Numerical simulations are used to investigate the role of the Cu-poor surface defect layer on Cu(In, Ga)Se2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In, Ga)Se2 heterojunction solar cells. We model the surface layer either as a material which is n-type doped, or as a material which is type-inverted due to Fermi-level pinning by donor-like defects at the interface with CdS. We further assume a band gap widening of this layer with respect to the Cu(In, Ga)Se2 bulk. This feature turns out to represent the key quality of the Cu(In, Ga)Se2 surface as it prevents recombination at the absorber/CdS buffer interface. Whether the type inversion results from n-type doping or from Fermi-level pinning is only of minor importance as long as the surface layer does not imply a too large number of excess defects in its bulk or at its interface with the normal absorber. With increasing number of those defects an n-type layer proofs to be less sensitive to material deterioration when compared to the type-inversion by Fermi-level pinning. For wide gap chalcopyrite solar cells the internal valence band offset between the surface layer and the chalcopyrite appears equally vital for the device efficiency. However, the unfavorable band-offsets of the ZnO/CdS/Cu(In, Ga)Se2 heterojunction limit the device efficiency because of the deterioration of the fill factor.

The microstructure and microchemistry of Cu(In, Ga)Se2 (CIGS) films have been analyzed by means of transmission electron microscopy (TEM). Specimens were obtained from a number of groups producing high-performance solar cells from these materials. Both plan-view and cross-sectional TEM samples were prepared by mechanical grinding and ion milling. Twins can be found easily within the films while dislocations are present only in a few grains and with low density. No extended structural defects such as stacking faults were discovered. X-ray energy dispersive spectroscopy was used to study the chemical composition of grains and grain boundaries. Experimental results showed no difference between the composition in the grain interiors and the grain boundary. In addition, there is no obvious enhancement of oxygen and sodium at grain boundaries. Structural depth dependences were also not found.

Low resistivity and transparent Al doped ZnMgO films were deposited on glass substrates by a pulsed laser deposition system. For up to 32 atm% of Mg content, segregation of a MgO phase was not observed. The bandgap of these films could be widened to about 4 eV with increasing Mg content. The relation between bandgap and resistivity was found to be a trade-off; i.e. the larger the bandgap, the higher the resistivity. The maximum bandgap among films with an electrical resistivity of less than 10-3 Ω cm was 3.94 eV. The average optical transmittance of these films was more than 90 % for wavelengths λ between 400 and 1100 nm. The transmittance around λ = 330 nm was still 50 %.