Properties Of Solar Cell Devices Research Articles

Aluminum‐Based Chalcopyrite Thin Films and Solar Cells

AbstractAluminum‐based chalcopyrite materials have attracted attention because of the wide controllability range of their material properties and potential for use in energy‐conversion devices. Herein, CuAlSe2‐based thin‐film growth and solar cell device properties are discussed. Ternary CuAlSe2 thin films are relatively unstable and decompose weeks after film growth, even when preserved in a dry box. However, alloying with elemental In led to significant improvements in stability. Cu(In,Al)Se2 (CIAS) solar cells yield better photovoltaic performance than CuInSe2 cells, although the effective range of Al concentration that can improve device performance is narrower than that of elemental Ga in Cu(In,Ga)Se2 (CIGS). A decrease in the alkali metal concentration in CIAS films with increasing Al concentration is observed, indicating that the formation energy of alkali‐metal substitutional defects on the Cu site is high, and/or Al‐related complex defects formed are kinetically stable and difficult to replace with alkali metals once they form. Although the direct observation of alkali metals in the bulk (grain interior) of chalcopyrite CuInSe2 and CIGS films has been difficult to date, this result can serve as indirect evidence of the presence of alkali metals in the bulk of CuInSe2 films.

Temperature-dependent electronic, optical, and solar cell device properties of AlAs and AlSb semiconductors and their p-n homojunctions

AlSb and AlAs have attracted renewed interest in recent years as promising photovoltaic (PV) materials due to significant light absorption, eco-friendliness, and low cost. Here, density functional theory (DFT) in combination with the non-equilibrium Green's function method (NEGF) has been used to compute the temperature-dependent PV parameters of nanoscale AlSb and AlAs p-n junction solar cell devices. In particular, the influence of electron-phonon coupling (EPC) and spin-orbit coupling (SOC) on the PV properties of these devices is investigated. Our results highlight the importance of inclusion of EPC in evaluating the ab initio PV properties of p-n junctions at finite temperatures. At non-zero temperatures, the PV parameters can differ significantly compared to those at zero temperature due to phonon-assisted tunnelling of charge carriers across the p-n junction. In particular, the mobility of low-effective-mass carriers at the p-n junction is found to be enhanced by phonon scattering, which in turn results in improved PV transport properties. The short-circuit current (Jsc) at 300 K proved to be 21.7% (16.2%) higher for AlSb (AlAs) than that at 0 K. On the other hand, the open-circuit voltage (Voc) at 300 K proved to be 19.1% (16.7%) lower for AlSb (AlAs) than that at 0 K. Jsc is higher in AlSb than in AlAs due to higher phonon-influenced mobility of low-effective-mass carriers. Overall, our study strongly suggests the importance of inclusion of EPC and hence phonon scattering in the first-principles quantitative determination of PV parameters of solar cells. The estimates obtained in this study may be utilized as input ab initio parameters in continuum-model-based multi-scale simulations of AlSb and AlAs p-n homo-junction solar cells.

Fullerene-Functionalized Halogen-Bonding Heteroditopic Hosts for Ion-Pair Recognition.

Despite their hydrophobic surfaces with localized π-holes and rigid well-defined architectures providing a scaffold for preorganizing binding motifs, fullerenes remain unexplored as potential supramolecular host platforms for the recognition of anions. Herein, we present the first example of the rational design, synthesis, and unique recognition properties of novel fullerene-functionalized halogen-bonding (XB) heteroditopic ion-pair receptors containing cation and anion binding domains spatially separated by C60. Fullerene spatial separation of the XB donors and the crown ether complexed potassium cation resulted in a rare example of an artificial receptor containing two anion binding sites with opposing preferences for hard and soft halides. Importantly, the incorporation of the C60 motif into the heteroditopic receptor structure has a significant effect on the halide binding selectivity, which is further amplified upon K+ cation binding. The potassium cation complexed fullerene-based receptors exhibit enhanced selectivity for the soft polarizable iodide ion which is assisted by the C60 scaffold preorganizing the potent XB-based binding domains, anion-π interactions, and the exceptional polarizability of the fullerene moiety, as evidenced from DFT calculations. These observations serve to highlight the unique properties of fullerene surfaces for proximal charged guest binding with potential applications in construction of selective molecular sensors and modulating the properties of solar cell devices.

THE EFFECT OF TEMPERATURE DEPENDENCE ON TIN PEROVSKITE SOLAR CELL USING SCAPS 1D

Perovskite solar cell (PSC) has become a force to reckon with in the renewable energy community because of its performance and low cost of production. Solar energy is one of the most demanding renewable sources of electricity. Electricity production using photovoltaic technology, not only helps meet the growing demand for energy, but also contributes to mitigate global climate change by reducing dependence on fossil fuels. Simulation is based on a mathematical design that describes the system. Numerical simulation technique of solar cells devices has over the years proven to be a viable tool for observing and understanding the properties of solar cell devices such as the optical, electrical and mechanical properties of complex solar cell devices. It also helps to reduce processing cost and time spent on solar cell device fabrication by providing useful information on how to vary the production parameters to improve the device performance. Solar cell capacitance simulator in one dimension (SCAPS-1D) was used in the modeling and simulation of sandwiched perovskite solar cells (PSCs) with a planar hetero-junction structure in the arrangement of the sandwiched model (FTO/CdS/CH3NH3SnI3/HTM). The energy band diagram, I-V characteristics and other parameters was obtained. The configuration for better performance was then determined, from which further simulations were carried out. When the operating temperature was varied the result shows an overall efficiency of 24.25%, FF of 82.80%, JSC of 30.73mA/cm2, VOC of 0.95V was obtained.

Optimization of Intrinsic ZnO Thickness in Cu(In,Ga)Se2-Based Thin Film Solar Cells.

The typical structure of high efficiency Cu(InGa)Se2 (CIGS)-based thin film solar cells is substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al(AZO) where the sun light comes through the transparent conducting oxide (i.e., i-ZnO/AZO) side. In this study, the thickness of an intrinsic zinc oxide (i-ZnO) layer was optimized by considering the surface roughness of CIGS light absorbers. The i-ZnO layers with different thicknesses from 30 to 170 nm were deposited via sputtering. The optical properties, microstructures, and morphologies of the i-ZnO thin films with different thicknesses were characterized, and their effects on the CIGS solar cell device properties were explored. Two types of CIGS absorbers prepared by three-stage co-evaporation and two-step sulfurization after the selenization (SAS) processes showed a difference in the preferred crystal orientation, morphology, and surface roughness. During the subsequent post-processing for the fabrication of the glass/Mo/CIGS/CdS/i-ZnO/AZO device, the change in the i-ZnO thickness influenced the performance of the CIGS devices. For the three-stage co-evaporated CIGS cell, the increase in the thickness of the i-ZnO layer from 30 to 90 nm improved the shunt resistance (RSH), open circuit voltage, and fill factor (FF), as well as the conversion efficiency (10.1% to 11.8%). A further increas of the i-ZnO thickness to 170 nm, deteriorated the device performance parameters, which suggests that 90 nm is close to the optimum thickness of i-ZnO. Conversely, the device with a two-step SAS processed CIGS absorber showed smaller values of the overall RSH (130–371 Ω cm2) than that of the device with a three-stage co-evaporated CIGS absorber (530–1127 Ω cm2) ranging from 30 nm to 170 nm of i-ZnO thickness. Therefore, the value of the shunt resistance was monotonically increased with the i-ZnO thickness ranging from 30 to 170 nm, which improved the FF and conversion efficiency (6.96% to 8.87%).

Dynamical properties and charge accumulation in solar cells with embedded GaSb quantum dots

We studied the dynamical properties of quantum dot solar cells in which GaSb quantum dots (QDs) were embedded in the middle part of an undoped layer sandwiched by n- and p-layers. Three kinds of GaSb QDs with GaAs or AlGaAs as a barrier material for QDs were prepared for comparison of their properties. The capacitance C was measured as a function of the frequency f in the ranges between 5 kHz and 3 MHz under different illumination conditions. A rise in capacitance in response to illumination was observed in all the samples, showing that photogenerated carriers (or holes) contribute to the dynamical properties of solar cell devices. Differences in capacitance change among the three samples are observed in magnitude and its frequency dependence. We discuss them in terms of the difference in dynamical processes, specifically the thermal escape rate of holes in QDs.

Optimisation of pH of the CdCl2 + Ga2(SO4)3 activation step of CdS/CdTe based Thin-Film solar cells

In order to produce high efficiency solar cells based on CdTe, CdCl2 post-growth treatment is an essential processing step. This treatment can be further improved by adding elements such as Fluorine and Gallium into the CdCl2 solution. Through systematic experimentation, it has been found that the pH value of the treatment solution also affect the conversion efficiency of the solar cells. This work therefore focuses on the effect of pH value of CdCl2 + Ga2(SO4)3 aqueous solution on the device efficiencies. The graded bandgap device structure, glass/FTO/n-ZnS/n-CdS/n-CdTe/Au was used in this work. The pH values of 1.00, 2.00 and 3.00 for CdCl2 + Ga2(SO4)3 solutions were utilised for the activation of glass/FTO/n-ZnS/n-CdS/n-CdTe layers and its effects were explored for both the CdTe material and device properties. It has been found that both CdTe material properties and solar cell device properties are superior when the pH value of 2.00 is used for post-growth treatment. The best conversion efficiency observed in this work for the above graded bandgap device is 12.2%.

A comparative study of the effects of sputtering deposition conditions for ZnO surface electrode layers on Cu(In,Ga)Se2 and CuGaSe2 solar cells

The effects of post p-n junction formation processes, namely transparent conducting oxide (TCO) layer deposition conditions on Cu(In,Ga)Se2 (CIGS) and CuGaSe2 (CGS) solar cell device properties were comparatively studied. It was found that CIGS devices were relatively insensitive to the presence of oxygen and heat during TCO layer deposition, whereas CGS devices were extremely sensitive to oxygen and heat, resulting in a degradation of device performance in the presence of them. The use of the relatively low TCO deposition temperature (Ts) of 80°C led to significant improvements in the fill factor values of CGS devices compared to the case for Ts ~180°C, though the open circuit voltage was reduced. These results suggest that TCO deposition conditions can be an important parameter which critically affects ternary CGS solar cell device properties. It is, therefore, suggested that TCO layer deposition conditions for CGS devices should be optimized independently of that for conventional narrow-gap CIGS devices.

Study of photoexcitations in poly(3-hexylthiophene) for photovoltaic applications

We used a variety of steady state and transient optical techniques for studying the primary and steady state photoexcitations in pristine films of poly(3-hexylthiophene) [P3HT] with different molecular weight used for organic photovoltaic solar cells. The employed techniques include picosecond transient and steady state photoluminescence and photomodulation spectroscopies, laser action, and doping induced absorption. We show that solar cell device qualities based on donor-acceptor blends of the P3HT polymers with substituted fullerene molecules crucially depend on the polymer properties, which can be readily discerned by the steady state photomodulation technique. This technique can, therefore, be applied to specific polymer donors for estimating their solar cell device properties prior to actually fabricating an optimal device.

Fermi-level-dependent defect formation in Cu-chalcopyrite semiconductors

Valence-band photoelectron spectroscopy of CuInSe2, CuInS2, and CuGaSe2 surfaces and interfaces give evidence for the formation of Cu vacancies when the Fermi level moves upwards in the band gap due to contact formation. The effect might be a key issue in understanding basic properties of solar cell devices based on these materials.