Advanced Solar Cells Research Articles

Photoelectric engineering of all-weather bifacial solar cells in the dark

A promising but challenging problem for state-of-the-art solar cells is to persistently generate power in the daytime and dark. To address this profound issue, all-weather bifacial solar cells are built by combining long persistence phosphor (LPP) tailored fluorescent film with bifacial dye-sensitized solar cells. On behalf of the solar energy storage and fluorescent excitation behaviors of LPP phosphors, the so-called all-weather solar cells yield a maximized photoelectric conversion efficiency of 10.04% and a dark efficiency as high as 21%. Moreover, the newly launched all-weather solar cells with 32 modules can lighten a lamp, demonstrating a predictable application in future energy harvesting. The current work could also extend our knowledge to more advanced all-weather solar cells.

CsI Pre-Intercalation in the Inorganic Framework for Efficient and Stable FA1-x Csx PbI3 (Cl) Perovskite Solar Cells.

Engineering the chemical composition of organic and inorganic hybrid perovskite materials is one of the most feasible methods to boost the efficiency of perovskite solar cells with improved device stability. Among the diverse hybrid perovskite family of ABX3 , formamidinium (FA)-based mixed perovskite (e.g., FA1-x Csx PbI3 ) possesses optimum bandgaps, superior optoelectronic property, as well as thermal- and photostability, which is proven to be the most promising candidate for advanced solar cell. Here, FA0.9 Cs0.1 PbI3 (Cl) is implemented as the light-harvesting layer in planar devices, whereas a low temperature, two-step solution deposition method is employed for the first time in this materials system. This paper comprehensively exploits the role of Cs+ in the FA0.9 Cs0.1 PbI3 (Cl) perovskite that affects the precursor chemistry, film nucleation and grain growth, and defect property via pre-intercalation of CsI in the inorganic framework. In addition, the resultant FA0.9 Cs0.1 PbI3 (Cl) films are demonstrated to exhibit an improved optoelectronic property with an elevated device power conversion efficiency (PCE) of 18.6%, as well as a stable phase with substantial enhancement in humidity and thermal stability, as compared to that of FAPbI3 (Cl). The present method is able to be further extended to a more complicated (FA,MA,Cs)PbX3 material system by delivering a PCE of 19.8%.

Passivation properties of tunnel oxide layer in passivated contact silicon solar cells

Passivated contact in advanced high-efficiency silicon solar cells based on the full back surface field (BSF) is reported here in based on the application of a tunnel oxide layer that is less than 2nm thick. The open-circuit voltage (Voc) was significantly improved via interface passivation due to insertion of the tunnel oxide layer. During oxide layer growth, a transition region, such as a sub-oxide, was observed at a depth of about 0.75nm in the growth interface between the silicon oxide layer and silicon substrate. The properties of the less than 2nm thick tunnel oxide layer were primarily affected by the characteristics of the transition region. The passivation characteristics of tunnel oxide layer should depend on the physical properties of the oxide. The interface trap density Dit is an important parameter in passivation and is influenced by the stoichiometry of the oxide which in turn strongly affected by the fabrication and the post annealing conditions. During heat treatment of a-Si:H thin films (for the purpose of crystallization to form doped layers), thin film blistering occurs due to hydrogen effusion on flat substrate surfaces. To minimize this behavior, we seek to control the surface morphology and annealing profile. Also, the passivation quality of passivataed contact structure declined for the sample annealed above 900°C. This decline was attributed not only to local disruption of the tunnel oxide layer, but also to phosphorus diffusion. The resistivity of the tunnel oxide layer declined precipitously for the sample annealed above 900°C. On the basis of these, implied Voc over 740mV was achieved in n-type Si wafer through the control of the oxide stoichiometry via optimizing the annealing conditions.

Nondestructive Raman Scattering Assessment of Solution-Processed ZnO-Doped Layers for Photovoltaic Applications

This work reports a Raman scattering analysis of doped ZnO layers grown by different solution-based processes (including Al-doped ZnO grown by chemical bath deposition (CBD) and Cl-doped ZnO grown by electrodeposition) that have been developed for the transparent conductive oxide (TCO) window layers in advanced chalcogenide thin film solar cells. The use of different excitation conditions corresponding to resonant and off-resonance measurements is investigated for the Raman scattering assessment of the processed layers. The analysis of the Raman spectra indicates that off-resonance excitation with a wavelength of 442 nm provides the most useful information. The spectra measured under these conditions are characterized by the existence of two distinct spectral regions that are sensitive to electrical and structural characteristics of the layers: (a) the spectral contribution located in the 500 cm–1 region is mainly sensitive to the charge carriers density in the layers, and (b) the relative intensity of th...

Advanced Photovoltaic Power System Development at the U.S. Air Force Research Laboratory

Photovoltaics continue to be the primary source of electrical power for most near-Sun space missions. The desire to enhance or enable new space missions through higher efficiency, increased specific power (W/kg), increased volumetric power density (W/m3) and improved radiation resistance, along with decreased costs, continues to push the development of novel solar cell and array technologies. To meet present and future space power requirements, advanced multijunction solar cells and novel cell technologies are being pursued. These efforts have resulted in a continual advancement in performance, but new paradigms will be required to continue that performance trend. Similarly, new array technologies are being investigated and developed to meet the ever increasing power system performance requirements.

Direct Comparison of Ground and In-Flight Measurements of New Multi-Junction Solar Cell Technologies

The pace of development for new photovoltaic structures such as the inverted meta-morphic (IMM) IIIV cells, exceeds the pace at which spectrally matched, calibrated, solar cells can be created due to the infrequent opportunities of balloon or space based calibration. As such, laboratories must rely on previous generation AM0 calibrated cells and make appropriate corrections. Equally challenging is measuring the same cells in space without a controlled laboratory environment. It is within this context, the 3rd Forward Technology Solar Cell Experiment (FTSCE III), measured three, four and five junction advanced technology solar cells in space. This report compares the beginning-of-life in-flight measurements with the laboratory measurements made before flight.

Metal Matrix Composite Solar Cell Metallization

Advanced solar cells are moving to ever thinner formats in order to save mass and in some cases improve performance. As cells are thinned, the possibility that they may fracture or cleave due to mechanical stresses is increased. Fractures of the cell can degrade the overall device performance if the fracture propagates through the contact metallization, which frequently occurs. To address this problem, a novel semiconductor metallization system based on multi-walled carbon nanotube (CNT) reinforcement, termed metal matrix composite (MMC) metallization is under investigation. Electro-mechanical characterization of MMC films demonstrate their ability to provide electrical conductivity over >40 micron wide cracks in the underlying semiconductor, with the carbon nanotubes bridging the gap. In addition, these materials show a “self-healing” behaviour, electrically reconnecting at ~30 microns when strained past failure. Triple junction (TJ) space cells with MMC metallization demonstrated no loss in J sc after intentional fracture, whereas TJ cells with conventional metallization suffer up to 50% J sc loss.

Nanostructured ZnO - its challenging properties and potential for device applications

Nanostructured ZnO possessing interesting structural and optical properties offers challenging opportunities for innovative applications. In this lecture the review of the optical and structural properties of ZnO nanostructured layers is presented. It is shown that they have a direct impact on the parameters of devices involving ZnO. An analysis of current trends in the photovoltaic (PV) field shows that improved light harvesting and efficiency of solar cells can be obtained by implementing nanostructured ZnO layers to process advanced solar cell structures. Because of amenability to doping, high chemical stability, sensitivity to different adsorbed gases, nontoxicity and low cost ZnO attracted much attention for application as gas sensors. The sensitivity of nano-grain ZnO gas elements is comparatively high because of the grain-size effect. Application of nanostructured ZnO for gas sensors and for increasing of light harvesting in solar cells is demonstrated.

Is conversion efficiency still relevant to qualify advanced multi-junction solar cells?

For better conversion of sunlight into electricity, advanced architectures of multi-junction (MJ) solar cells include increasing numbers of subcells. The Achilles’ heel of these cells lies in their increased sensitivity to the spectral distribution of sunlight, which is likely to significantly alter their performance during real working operation. This study investigates the capacity of MJ solar cells comprising up to 10 subcells to accommodate a wide range of spectral characteristics of the incident radiation. A systematic study is performed, aimed at a realistic estimation of the energy output of MJ-based concentrating photovoltaic systems at characteristic locations selected to represent a large range of climatic conditions. We show that optimal MJ architectures could have between 4 and 7 subcells. Beyond seven subcells, the slight gains in peak efficiency are likely outweighed by detrimental increases in dependence on local conditions and in annual yield variability. The relevance of considering either conversion efficiency or modeled energy output as the most appropriate indicator of the cell performance, when considering advanced architectures of MJ solar cells, is also discussed.

Thermodynamic considerations for epitaxial growth of III/V alloys

III/V semiconductor alloys have been extensively studied because of their usefulness for electronic and photonic devices. Nevertheless, the search for new alloys for specific applications continues. Often, thermodynamic factors restrict the compositional range accessible by epitaxial growth processes, particularly when the size difference between atoms mixing on a particular sublattice is large. This causes solid phase immiscibility, leading to important effects on the epitaxial growth, the resultant alloy properties, and, consequently, device performance. Stringent thermodynamic limits exist for a number of alloys being considered for advanced LED, laser, and solar cell applications where the atomic sizes are very dissimilar, such as GaInN, GaAsN and GaAsBi. This paper will review the basic thermodynamics of the epitaxial growth processes and mixing in semiconductor alloys, as well as the causes and consequences of the resultant complex microstructures.

Boron Emitter Formation by Plasma Immersion Ion Implantation in n-type PERT Silicon Solar Cells

The use of plasma immersion ion implantation (PIII) is a relevant approach for the development of advanced solar cells technologies at lower cost (€/Wp). In this paper, we report on the development of homogeneous boron (B) doping of n-type crystalline silicon (cSi) substrate by the PIII technique. Using diborane (B2H6) as gas precursor, various doping profiles were identified fitting the requirements for boron-doped emitters in n-type PERT solar cells. Particularly, saturation current density (J0e) of 50 fA/cm2 were achieved on symmetrical samples for a 94 Ω/sg textured B-emitter passivated with SiO2/SiN stack. Bifacial n-type Passivated Emitter Rear Totally-diffused (n-PERT) solar cells were fabricated using the PIII technology and conversion efficiencies up to 19.8% on 239 cm2 Cz-Si wafers were obtained. As a consequence, these results indicate that PIII can compete with beamline technology but will have lower running cost. It is therefore a promising technology to create high efficiency cSi solar cells.

Dislocation-limited performance of advanced solar cells determined by TCAD modeling

In recent years, low-cost silicon for photovoltaic applications has trended toward smaller grain sizes with lower dislocation densities. These changes, coupled with advanced device architectures like heterojunction and passivated emitter and rear cells, have enabled higher efficiencies, including new world records. To establish material requirements for further improvement, we model dislocations with different recombination strengths in Sentaurus Device using mid-gap defects spatially localized at the nanoscale in three dimensions. This approach accounts for the charging of dislocations and differentiates between dislocations and grain boundaries or metal impurities. Simulations that model dislocations as macroscopic variations in effective bulk lifetime or diode parameters do not address these issues. We validate our model by simulating the world record multicrystalline silicon solar cell. We find that three parameters influence cell efficiency: the area fraction of the cell containing dislocations, the dislocation density within defect clusters, and the concentration of recombination centers at these dislocations. We provide targets for these parameters and show how our results can be used to determine the potential gains from reducing dislocations in experimental devices and materials. We further predict that device architectures that lead to higher injection, such as silicon heterojunction cells, are more robust to the presence of dislocations.

Design of micropillar wicks for thin-film evaporation

The generation of concentrated heat loads in advanced microprocessors, GaN electronics, and solar cells present significant thermal management challenges in defense, space and commercial applications. Liquid to vapor phase-change strategies are promising due to the high latent heat of vaporization of the working fluid. In particular, thin-film evaporation has received increased interest owing to advances in micro/nanofabrication and the potential to dissipate high heat fluxes by increasing the evaporative meniscus area. Yet, predictive tools to design various wicking structures are limited due to the complexity of the thermal–fluidic transport. In this work, we performed systematic experiments to characterize capillary-limited thin-film evaporation from silicon micropillar wicks in the absence of nucleate boiling. The insights gained from experiments were used to model the capillary pressure, permeability, and thermal resistance. Accordingly, we developed a semi-analytical model to determine the capillary-limited dryout heat flux and wall temperature with ±20% accuracy, compared to our experiments. The model provides a versatile platform to design and optimize micropillar wicks for next generation thermal management devices.

Impact of Laser-Induced Oxidation on Silicon Wafer Solar Cells’ Performance

Lasers are frequently used to locally open dielectric layers for contacting the front or rear side of advanced silicon wafer solar cell architectures. For reasons of cost, laser processes are usually carried out in ambient environment, which inevitably leads to the oxidation of silicon. In order to achieve maximum solar cell efficiency, the laser processing has to be optimized thoroughly. The current research only focuses on minimizing laser-induced defects in silicon. However, our investigation shows another side effect of laser ablation, i.e., laser-induced oxidation, which is often neglected and undesirable for the subsequent metallization process. In this paper, we investigated the impact of laser-induced oxidation, which is caused by laser ablation in air, on the performance of silicon wafer solar cells. We show that by applying a postlaser etching step, laser-induced defects and oxide film can be removed effectively, providing improved conditions for the subsequent metallization process.

A Solar Cell That Is Triggered by Sun and Rain.

All-weather solar cells are promising in solving the energy crisis. A flexible solar cell is presented that is triggered by combining an electron-enriched graphene electrode with a dye-sensitized solar cell. The new solar cell can be excited by incident light on sunny days and raindrops on rainy days, yielding an optimal solar-to-electric conversion efficiency of 6.53 % under AM 1.5 irradiation and current over microamps as well as a voltage of hundreds of microvolts by simulated raindrops. The formation of π-electron|cation electrical double-layer pseudocapacitors at graphene/raindrop interface is contributable to current and voltage outputs at switchable charging-discharging process. The new concept can guide the design of advanced all-weather solar cells.

Counter electrodes from conducting polymer intercalated graphene for dye-sensitized solar cells

Creation of cost-effective and platinum-free counter electrodes (CEs) is persistent for developing advanced dye-sensitized solar cells (DSSCs). We present here the fabrication of conducting polymers such as polyaniline (PANi), polypyrole (PPy), or poly(3,4-ethylenedioxythiophene) (PEDOT) intercalated reduced graphene oxide (rGO) CEs on flexible Ti foil or polyethylene-terephthalate substrate for liquid-junction DSSC applications. The ration architecture integrates the high electron-conducting ability of graphene and good electrocatalytic activity of a conducting polymer into a single CE material. The preliminary results demonstrate that the resultant CEs follow an order of rGO/PPy > rGO/PANi > rGO/PEDOT > rGO. A maximal cell efficiency of 6.23% is determined on the optimized solar cell device, yielding 104.9% enhancement in comparison to rGO based device.

Raman scattering quantitative assessment of the anion composition ratio in Zn(O,S) layers for Cd-free chalcogenide-based solar cells

This work reports the use of Raman scattering for the chemical characterization of Zn(O,S) layers that are being developed as a Cd-free alternative for the buffer layer in advanced chalcogenide solar cells.

Trends, Challenges and Opportunities in Advanced Solar Cells Technologies and PV Market

Trends, Challenges and Opportunities in Advanced Solar Cells Technologies and PV Market

Trends, Challenges and Opportunities in Advanced Solar Cells Technologies and PV Market

Trends, Challenges and Opportunities in Advanced Solar Cells Technologies and PV Market

Dissolution-resistant platinum alloy counter electrodes for stable dye-sensitized solar cells

Creation of dissolution-resistant counter electrodes (CEs) is a long-term objective for searching advanced dye-sensitized solar cell (DSSC) platforms. We present here the electrochemical deposition of PtM0.1 (M=Fe, Co, Ni) alloy CEs and understanding of the dissolution-resistance by thermodynamical calculation. On behalf of interaction between host Pt and guest M atoms, the electrocatalytic activity of alloyed PtM0.1 CEs is markedly enhanced in comparison with pristine Pt electrode. The competitive dissolution reactions from guest M atoms and liquid electrolyte contribute to the long-term stability of PtM0.1 alloy CEs. Compared with the liquid-junction DSSC from pristine Pt CE, the solar cells with PtNi0.1, PtCo0.1, and PtFe0.1 alloy CEs represent remarkable power conversion efficiencies of 8.77%, 8.26% and 7.84%, respectively. These PtM0.1 CEs show great promise for commercial application as they allow for a maximum 21.8% improvement in cell efficiency in comparison to the Pt CE system without compromising device stability.