Over the past several years, CdTe-based photovoltaics have undergone a revolution. CdTe is a mature PV technology, spanning over 60 years. For a decade, CdTe record cell efficiencies remained dormant at 16.7%, far behind multi-crystalline Silicon (mc-Si) as well as the competing thin film PV technology, CuIn_xGa_1-xSe₂ (CIGS). Recently, the CdTe field reawakened as multiple fundamental breakthroughs pushed the record cell efficiencies from 16.7% in 2011 to 21.0%, announced recently¹. CdTe now stands as the most efficient, polycrystalline, thin-film PV system with ample room for further optimization.

CIGS thin film solar modules, despite their high efficiency, may contain three different kinds of macroscopic defects referred to as bulk defects, interface defects and interconnect defects. These occur due to the film’s sensitivity to inhomogeneities during the manufacturing process and decreasing the electrical power output from a cell or module. In this study, we present infrared (IR) imaging and contactless loss analyses of defects contained in commercially manufactured thin film solar modules. We investigated different relations between the emitted IR-signal (using illuminated lock-in thermography ILIT) and the respective open circuit cell voltage (Voc) as well as the maximum power point (Pmpp). A simulation study, using the 2D finite element method (FEM), provides a deeper understanding as to the impact on electrical performance when defects are present on the cell or module.

A study was conducted to investigate the performance of thin film flexible PV panels. The experimental study was conducted to simulate the performance of the panels for the conditions found during the unmanned aerial vehicle (UAV) flight in the city of Sharjah. Two 2 m and a single 1 m WaveSol Light PV panels were tested for the study. The 2 m panels are for each wing while the 1 m panel is for the horizontal stabilizer. WaveSol Light PV panels were considered for this research because of their relative light weight, high current and compatible voltage output. These PV panels also have a convenient width which can easily be mounted on the UAV wings and stabilizer. The panels were tested to measure the voltage and current over the test period of 19 minutes. A detailed parametric study was conducted to evaluate the flight duration and the performance of the UAV for regular operations. The study predicted the operational range and flight performance based on the motor power requirement, PV panel system type and fuel cell capacity. The best case scenario achieved the endurance increase of 4.5 hours while the worst case achieved an endurance of 0.4 hours.

Organic solar cells and organic LEDs are typically made of conductive and semi-conductive thin films. The uniformity requirement for these films is exceptionally high. In the case of multi-layer structures, surface characterization based methods (e.g. profilometer, atomic force microscope, scanning electron microscope) encounter certain challenges when attempting to detect the defects inside the structure. One way to overcome this drawback is by using synchronized thermography (ST). In this work ST is used to study multi-layered thin film structures. Indium Tin Oxide (ITO) was used as an example of conductive thin film and poly(3,4-ethylenedioxy-thiopene):poly(styrene-sulfonate) (PEDOT:PSS) was used as an example of a hole transporting layer. Uniformity differences were generated in these layers and ST was used to detect them. The results show that ST is capable of localizing small defects in the stack using a single infrared (IR) image. It can often be deduced from the same image in which layer the defect is located. This shows that ST is capable of profiling the structures of multi-layer thin films.

While Cu(In,Ga)Se₂ (CIGS) has established itself as the thin film photovoltaic material of choice with current record efficiencies in excess of 20%, current high-efficiency laboratory-scale fabrication techniques, such as multi-stage evaporation, are ill suited to mass production. Quaternary-sputtering is a promising alternative technique for CIGS deposition, where a single sputtering target made from CIGS itself in the desired stoichiometry is used as the sole deposition source. Devices made using this technique do not require any additional post deposition selenium treatment and have demonstrated peak efficiency in our laboratory in excess of 10%, showing the potential of quaternary sputtering. In an effort to reduce deposition times, we have fabricated films using pulsed DC sputtering, which substantially reduces the substrate time-at-temperature during absorber formation. DC-sputtered films are observed to have reduced surface roughness and different internal morphology from RF-sputtered films, but show increased crystallographic alignment along the (112) plane. DC-sputtered CIGS is thickness-limited to less than 600 nm due to excessive target damage and exhibits power conversion efficiencies of 5-6%.

In this paper, a systematic analysis of native oxides within a Metal-Insulator-Metal (MIM) diode is carried out, with the goal of determining their practicality for incorporation into a nanoscale Rectenna (Rectifying Antenna). The requirement of having a sub-10nm oxide scale is met by using the native oxide, which forms on most metals exposed to an oxygen containing environment. This, therefore, provides a simplified MIM fabrication process as the complex, controlled oxide deposition step is omitted. We shall present the results of an investigation into the current-voltage characteristics of various MIM combinations that incorporate a native oxide, in order to establish whether the native oxide is of sufficient quality for good diode operation. The thin native oxide layers are formed by room temperature oxidation of the first metal layer, deposited by magnetron sputtering. This is done in-situ, within the deposition chamber before depositing the second metal electrode. Using these structures, we study the established trend where the bigger the difference in metal workfunctions, the better the rectification properties of MIM structures, and hence the selection of the second metal is key to controlling the device's rectifying properties. We show how leakage current paths through the non-optimised native oxide control the net current-voltage response of the MIM devices. Furthermore, we will present the so-called diode figures of merit (asymmetry, non-linearity and responsivity) for each of the best performing structures.

Hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) layers are one alternative approach to ensure sufficient interlayer charge transport while maintaining high transparency and good passivation in Si-based solar cells. We have used a combination of complementary x-ray and electron spectroscopies to study the chemical and electronic structure of the (μc-SiO_x:H) material system. With these techniques, we monitor the transition from a purely Si-based crystalline bonding network to a silicon oxide dominated environment, coinciding with a significant decrease of the material’s conductivity. Most Si-based solar cell structures contain emitter/contact/passivation layers. Ideally, these layers fulfill their desired task (i.e., induce a sufficiently high internal electric field, ensure a good electric contact, and passivate the interfaces of the absorber) without absorbing light. Usually this leads to a trade-off in which a higher transparency can only be realized at the expense of the layer’s ability to properly fulfill its task. One alternative approach is to use hydrogenated microcrystalline silicon oxide (μc-SiO_x:H), a mixture of microcrystalline silicon and amorphous silicon (sub)oxide. The crystalline Si regions allow charge transport, while the oxide matrix maintains a high transparency. To date, it is still unclear how in detail the oxygen content influences the electronic structure of the μc-SiO_x:H mixed phase material. To address this question, we have studied the chemical and electronic structure of the μc-SiO_x:H (0 ≤ x = O/Si ≤1) system with a combination of complementary x-ray and electron spectroscopies. The different surface sensitivities of the employed techniques help to reduce the impact of surface oxides on the spectral interpretation. For all samples, we find the valence band maximum to be located at a similar energy with respect to the Fermi energy. However, for x > 0.5, we observe a pronounced decrease of Si 3s – Si 3p hybridization in favor of Si 3p – O 2p hybridization in the upper valence band. This coincides with a significant increase of the material’s resistivity, possibly indicating the breakdown of the conducting crystalline Si network. Silicon oxide layers with a thickness of several hundred nanometres were deposited in a PECVD (plasma-enhanced chemical vapor deposition) multi chamber system using an excitation frequency of 13.56 MHz with a plasma power density of 0.3 W/cm². Glass (Corning type Eagle) and mono-crystalline silicon wafer substrates were coated in the same run at a substrate temperature of 185°C. The deposition pressure was 4 mbar and the substrate-electrode distance 20 mm. Mixtures of silane (SiH₄), 1% TMB (B(CH₃)₃) diluted in helium, hydrogen (H₂), and carbon dioxide (CO₂) gases were used at flow rates of 1.25 - 0.18/0.32/500/0 – 1.07) sccm (standard cubic centimeters per minute) for the deposition of μc-SiO_x:H(B) layers. By changing the CO₂/SiH₄ gas flow rate ratio from 0 to 6, μc-SiO_x:H(B) layers with a composition of 0 ≤ x = O/Si ≤ 1 were prepared using a constant sum of SiH₄ and CO₂. The TMB flow and the H₂ flow were kept constant within the series. For more details see Ref. [1]. The oxygen content in the films was determined using Rutherford Backscattering Spectroscopy (RBS). With RBS, the area-related atomic density of oxygen and silicon can be determined (± 2% [2]), and thus x can be calculated. This quantity considers only the number of silicon / oxygen atoms and not the number of atoms of other elements, such as hydrogen, which is also incorporated to a considerable extent: up to 20% in μc-SiO_x:H (measured using the hydrogen effusion method). To avoid charging effects, the measurements were performed on films deposited on a substrate of mono-crystalline silicon wafers. The electrical conductivity was measured in the planar direction of the film in a vacuum cryostat, using voltages from - 100 V to + 100 V. For that two co-planar Ag contacts were evaporated on the film with a gap of 0.5 mm  5 mm. In the present study, the optical band E04 is arbitrarily used as a measure for the optical band gap. E₀₄ is defined by the photon energy E for which an optical absorption coefficient of α of 10⁴cm^-1 is obtained. The absorption coefficient α(λ) versus the wavelength λ of the films was determined by measuring the transmittance T(λ) and reflectance R(λ), using the Beer-Lambert law, as suggested by Ref. [3]. The film thickness d was measured using the step profiler close to the measurement spot of the spectrophotometer. It is important to measure the transmittance T(λ) and the reflectance R(λ) at the same spot on the sample, to avoid inaccuracies in the calculated absorption spectra that arise from non-uniformity of the film thickness and different positions of the reflectance and transmittance minima and maxima in the spectrum [4]. Hard X-ray photoelectron spectroscopy (HAXPES) experiments were conducted at the HiKE end-station [5] on the KMC-1 beamline [6] of the BESSY-II electron storage ring. This end-station is equipped with a Scienta R4000 electron energy analyzer capable of measuring photoelectron kinetic energies up to 10 keV. A pass energy of 200 eV was used for all measurements. Spectra were recorded with a photon energy of 2003 eV using the first and fourth order supplied by a Si(111) double crystal monochromator. The combined analyzer plus beamline resolution is approx. 0.25 eV for spectra taken at both photon energies. The top surface of the sample was electrically grounded for all measurements. The binding energy was calibrated by measuring the 4f spectrum of a grounded Au foil and setting the Au 4f_7/2 binding energy equal to 84.00 eV. In SiO₂, the inelastic mean free path of electrons was estimated to be approx. 5 and 13-16 nm for the core levels and valence band measurements performed with 2003 and 8012 eV [7].

Recent work has shown that perovskite BaSnO₃, when doped n-type by rare earth or Sb substitution, has potential as a transparent conducting oxide (TCO), replacement for In₂O₃:Sn (ITO). Here we discuss the properties of this material and the related SrSnO₃, as well as strain dependence and epitaxy using first principles calculations. The compounds show remarkably strong strain tunability, but in a manner very different from well-studied perovskites such as SrTiO₃. The differences are explained in terms of the s-electron nature of the conduction bands in these stannates.

Al doped Zn_1-xMg_xO (AZMO) thin film was developed by sol-gel process with two-step annealing. Effect of annealing temperature on structural, optical and electrical properties was investigated. It was found that first annealing mainly determines the film crystallinity and grain size, while the second annealing plays a critical role in activating dopant. All of AZMO thin films show a (002) preferential orientation and a high transmittance of over 85% in wavelength region from 400 nm to 1500 nm. A resistivity of 2.1×10^-2 Ω∙cm was achieved for a film with an optical bandgap of 3.6 eV.

Silver nanowires with 40 nm diameter and copper nanowires with 150 nm diameter were synthesized using low-temperature routes, and deposited in combination with ultrathin graphene sheets for use as transparent conductors. A systematic and detailed analysis involving nature of capping agent for the metal nanowires, annealing of deposited films, and pre-treatment of substrates revealed critical conditions necessary for preparing high performance transparent conducting electrodes. The best electrodes show ~90% optical transmissivity and sheet resistance of ~10 Ω/□, already comparable to the best available transparent electrodes. The metal nanowire-graphene composite electrodes are therefore well suited for fabrication of opto-electronic and electronic devices.

High aspect-ratio ultra-long (> 70 μm) and thin (< 50 nm) copper nanowires (Cu-NW) were synthesized in large quantities using a solution-based approach. The nanowires, along with reduced graphene-oxide sheets, were coated onto glass as well as plastic substrates, thus producing transparent conducting electrodes. Our fabricated transparent electrodes achieved high optical transmittance and low sheet resistance, comparable to those of existing Indium Tin Oxide (ITO) electrodes. Furthermore, our electrodes show no notable loss of performance under high temperature and high humidity conditions. Adaptations of such nano-materials into smooth and ultrathin films lead to potential alternatives for the conventional tin-doped indium oxide, with applications in a wide range of solar cells, flexible displays, and other opto-electronic devices.

The antireflection structure (ARS) for solar cells is categorized to mainly two different techniques, i.e., the surface texturing and the single or multi-layer antireflection interference coating. In this study, we propose a novel hybrid ARS, which integrates moth eye texturing and multi-layer coat, for application to organic photovoltaics (OPVs). Using optical simulations based on the finite-difference time-domain (FDTD) method, we conduct nearly global optimization of the geometric parameters characterizing the hybrid ARS. The proposed optimization algorithm consists of two steps: in the first step, we optimize the period and height of moth eye array, in the absence of multi-layer coating. In the second step, we optimize the whole structure of hybrid ARS by using the solution obtained by the first step as the starting search point. The methods of the simple grid search and the Hooke and Jeeves pattern search are used for global and local searches, respectively. In addition, we study the effects of deviations in the geometric parameters of hybrid ARS from their optimized values. The design concept of hybrid ARS is highly beneficial for broadband light trapping in OPVs.

This paper studies theoretically light trapping in a solar cell configuration consisting of a 50-500 nanometer-thin planar silicon (aSi:H) film with a planar silver back-reflector, and scatterer(s) placed directly on the silicon surface. The usual picture for thicker films is that part of the light incident on the scatterer(s) can be coupled into the silicon film at a continuum of angles above the critical angle for the silicon-air interface, in which case light will be trapped and subsequently absorbed. However, for thin films a more appropriate picture is that of light being coupled into the guided modes of the air-silicon-silver geometry corresponding to discrete angles. The scattering of light into each guided mode, and out-of-plane scattering, will be quantified by the related scattering cross section. It will be shown that scatteringcross- section spectra have sharp resonances near cut-off wavelengths of guided modes, with more closely spaced resonances for thicker films. Total resonant cross sections can easily exceed physical cross sections by a factor 10. This study also includes light trapping due to coupling into the Surface-Plasmon-Polariton mode that exists due to the silver surface. It will be shown that peaks in scattering cross sections can be tuned via the geometry to the appropriate wavelength range where light trapping is advantageous due to weak absorption in the silicon, resulting in an optimum thickness around 250 nanometers. We consider both theoretical calculations with and without material losses, and both dielectric and metal scatterers are considered. The calculations were carried out with Green’s function integral equation methods.

We consider light trapping in photonic crystals. Using temporal coupled-mode theory and assuming that the active material is weakly absorbing, we show that the upper bound of the angle-integrated light trapping absorption enhancement is proportional to the photonic density of states. The tight bound can be reached if all the modes supported by the structure are coupled to external radiation. We discuss the roles of van Hove singularity, effective medium theory, and periodicity. By appropriate design, the angle-integrated absorption enhancement could surpass the conventional limit substantially in two dimension and marginally in three dimension.

The optimization of a silicon solar cell involves also the design of a proper antireflective coating (AR). We have considered different bilayer structures. The use of bilayers is oriented to have an antireflective effect on a broader range of wavelengths compared to single film AR. The materials considered include silicon oxide, magnesium fluoride, silicon nitride and titanium oxide. The thickness of each film in each structure has been optimized by theoretical calculations in order to minimize the weighted reflectivity, R_w. This is calculated taking into account the optical reflectivity, the internal quantum efficiency of the silicon solar cell and the solar flux on all the range of wavelengths of interest. Some of these optimized structures have been realized by e-beam vapor deposition as first tests. The improved optical performance of the samples have been verified at the UV-vis-NIR spectrophotometer.