High-efficiency Solar Cell Applications Research Articles

Numerical simulation of graphene/GaAs heterojunction solar cells

For a thorough understanding of graphene/GaAs heterojunction solar cells, the effect of the graphene work function, absorption loss, density of interface states, as well as the incorporation of an AlxGa1−xAs (0≤x≤1) buffer layer on the performance of graphene/GaAs heterojunction solar cells was investigated via AFORS-HET simulation software. It is demonstrated that graphene with a work function of approximately 5 eV favors the efficiency improvement. The absorption loss is also an unnegligible factor for the optimization of the graphene work function. The density of interface states can significantly affect the carrier recombination at the heterojunction interface between graphene and GaAs and the corresponding photovoltaic performance. In addition, the work function of a back electrode should also be taken into account for selecting a back electrode material due to its significant influence on the carrier transportation and collection. Furthermore, when an AlxGa1−xAs (x = 0.2) buffer layer was introduced to this heterojunction interface, an enhanced photovoltaic conversion efficiency of 24.78% can be achieved. Finally, a comparison among simulation and experimental data for the graphene/GaAs heterojunction solar cells is presented. This study indicates the great potential of graphene/GaAs heterojunction for the application in high-efficiency solar cells.

Dynamic Disorder Restriction of Methylammonium (MA) Groups in Chloride-Doped MAPbBr3 Hybrid Perovskites: A Neutron Powder Diffraction Study.

The hybrid methylammonium (MA) lead halide MAPbX3 perovskites present an appealing optoelectronic behavior with applications in high-efficiency solar cells. The orientation of the organic MA units may play an important role in the properties, given the degrees of freedom for internal motion of MA groups within the PbX6 network. The present neutron powder diffraction study reveals the dynamic features of the MA units in the hybrid perovskite series MAPb(Br1-x Clx )3 , with x=0, 0.33, 0.5, 0.67, and 1. From difference Fourier maps, the H and C/N positions were located within the PbX6 lattice; the refinement of the crystal structures unveiled the MA conformations. Three different orientations were found to exist as a function of the chlorine content (x) and, therefore, of the cubic unit-cell size. These conformations are stabilized by H-bond interactions with the halide ions, and were found to agree with those reported from theoretical calculations.

Metal-induced gap states in passivating metal/silicon contacts

Passivating metal/silicon contacts combine low carrier recombination with low contact resistivities, enabled by a low gap state density at their interface. Such contacts find applications in high-efficiency solar cells. We perform first-principles calculations based on density functional theory to investigate the surface defect and metal-induced gap state density of silicon in close contact with metals (Al and Ag). We confirm that surface hydrogenation fully removes surface-defect gap states of (111)-oriented silicon surfaces. However, the metal-induced gap state density increases significantly when metals are closer than 0.5 nm to such surfaces. These results highlight the importance of the tunneling-film thickness in achieving effective passivating-contact formation.

Molecular beam epitaxial growth of interdigitated quantum dots for heterojunction solar cells

Interdigitated quantum dots, which are multiple stacks of type-I InAs/GaAs quantum dots and type-II GaSb/GaAs quantum dots, are grown using molecular beam epitaxy. By incorporating the interdigitated quantum dots into a p-i-n AlGaAs/GaAs heterojunction solar cell structure, we demonstrate a photovoltaic effect with a 20.6% improvement in open-circuit voltage, when compared to that of another cell incorporating the same quantum dots but with a p-i-n GaAs homojunction architecture. A transmission electron microscopy is performed to analyze strain-induced defects created in the multi-stack quantum dot structures. The heterojunction solar cell incorporating the interdigitated quantum dots realized in this work would find potential applications in high-efficiency single-junction intermediate band solar cells operating under concentrated sunlight.

Fabrication of high-performance F and Al co-doped ZnO transparent conductive films for use in perovskite solar cells

Fluorine and aluminum co-doped ZnO (FAZO) films were deposited on glass substrates by RF magnetron sputtering. The effects of the substrate temperature (Tsub) on the structural, morphological, electrical, and optical properties of the films were investigated by X-ray diffraction analysis, atomic force microscopy, photoluminescence spectroscopy, Hall effect measurement, and ultraviolet–visible–near infrared spectrophotometry. All the thin films showed typical wurtzite structure with the c axis preferentially oriented perpendicular to the substrate. With increasing Tsub, the grain size shows a parabolic-like enhancement; the film surface became relatively smooth and dense, and the crystallinity was also improved. At Tsub = 320 °C, the films showed the best electrical properties, with a resistivity of 3.53 × 10−4 Ω cm, Hall mobility of 39.33 cm2/V s, and carrier concentration of 4.50 × 1020 cm−3. An optical transmittance of more than 80% in a wide wavelength range from 400 to 1200 nm was also achieved. Perovskite solar cells employing the FAZO film had a higher power conversion efficiency than reference cells using F-doped SnO2 and Sn-doped In2O3, demonstrating its suitability for application in high-efficiency solar cells.

Exceptional silicon surface passivation by an ONO dielectric stack

Immeasurably low surface recombination of crystalline-silicon wafers is demonstrated with an oxide-nitride-oxide (ONO) corona charged dielectric stack. We detail experimental variations to each layer of the dielectric stack to establish a procedure which provides outstanding passivation properties on textured and planar silicon wafers. We demonstrate surface recombination velocities of < 1 cm/s and surface recombination prefactors of < 1 fA/cm2, and we show that passivation remains stable over a 2-year period when stored in ambient conditions. The effective carrier lifetimes of n-type silicon are found to exceed the commonly accepted intrinsic lifetime limit, and in one case, a lifetime of 170 ms is attained. These high lifetimes indicate that ONO passivation is amongst the best dielectric passivation, and as such, might find applications in high-efficiency silicon solar cells.

Reduced-Dimensional α-CsPbX3 Perovskites for Efficient and Stable Photovoltaics

Summary Inorganic CsPbX3 perovskites have gained great attention owing to their excellent thermal stability and carrier transport properties. However, the power conversion efficiency (PCE) of solution-processed CsPbX3 perovskite solar cells is still far inferior to that of their hybrid analogues. Insufficient film thickness and undesirable phase transition are the two major obstacles limiting their device performance. Here, we show that by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). We further introduced a judicious amount of phenylethylammonium iodide (PEAI) into the system to induce reduced-dimensional perovskite formation. Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed undesirable phase transition and thus reduced the film's trap density. Following this approach, we reported a state-of-the-art PCE to date, 12.4%, for reduced-dimensional α-CsPbI3 perovskite photovoltaics with greatly improved performance longevity.

Perfect metamaterial absorber for applications in sustainable and high-efficiency solar cells

The current state of energy is characterized by complex challenges in production processes and environmental issues. With the world population continuing to multiply faster and the globalization process, additional energy production is needed to meet future demands. Solar energy is one of the best sustainable energy resources, which is expected to play a vital role in this scenario. One of the best techniques to harvest this resource is through solar photovoltaic technology, which produces electricity directly from solar radiation. But, one of the problems still persisting is its low efficiency. To harness this technology, this problem needs to be addressed. Metamaterial (MTM) technology has enabled the creation of advanced devices for various applications. Solar cell technology is one of the fields to benefit from this technology. MTM perfect absorber can be used in solar cells to improve their absorption. Multiple-bands MTM absorber for next generation high-efficiency solar cells is proposed. The design gives a nearly perfect absorption (99.94%) with a bandwidth of 23.4% in visible spectrum. In addition, the geometric flexibility of a proposed design causes its absorption rate to be insensitive of polarization angles and angles of incident electromagnetic radiations.

Well-aligned NiPt alloy counter electrodes for high-efficiency dye-sensitized solar cell applications

Abstract Development of cost-effective and robust counter electrodes (CEs) is a persistent objective for high-efficiency dye-sensitized solar cells (DSSCs). To achieve this goal, we present here the hydrothermal synthesis of well-aligned NiPt alloy CEs, which is templated by ZnO nanowires and nanosheets. The preliminary results demonstrate that NiPt alloy electrodes are featured by increased charge-transfer processes and electrocatalytic activity in comparison with expensive Pt CE, yielding power conversion efficiencies of 8.29% and 7.41% in corresponding DSSCs with NiPt nanowire and nanosheet alloy CEs, respectively. Additionally, the NiPt alloy CEs also display extraordinary dissolution-resistant ability when suffering long-term utilization in liquid-junction DSSCs.

Switchable ferroelectric photovoltaic effects in epitaxial h-RFeO3 thin films.

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects and narrow band-gap properties using hexagonal ferrite (h-RFeO3) thin films, where R denotes rare-earth ions. FPVs with narrow band gaps suggest their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations (4.7-8.5 μC cm-2), which possibly reduces a rapid recombination rate of the photo-generated electron-hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive to the magnitude of polarization. In the case of the h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. The effect of electrical fatigue on FPV responses has been further investigated. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.

Research on controllable preparation and antireflection properties of zigzag SiNWs arrays

ABSTRACTIn this work, an alternative metal-assisted chemical etching method is proposed to fabricate large-area uniform zigzag silicon nanowires. The effects of deposition time, etching time, HF concentration and H2O2 concentration on the nanostructure of nanowires were systematically investigated. The results demonstrate the suitable preparation concentrations in the alternating etching process are HF([SI]/[SII] = 9.2 M/2.3 M) and H2O2{[SI]/[SII] = (0.04 M∼0.26 M)/(0.4 M∼0.65 M)}. Moreover, the anti-reflec-tion properties of the resulting zigzag SiNWs arrays have been studied, the average reflectance is almost lower than 10% at wavelength range of 200∼1200 nm. The superior anti-reflection performance of zigzag SiNWs shows a huge potential application in high-efficiency silicon solar cells.

Routes for realizing high-performing Si solar cells by using periodic structures

Theoretical suggestion indicated the substantial improvement of solar cells by using periodic structures. Yet, the promise has not been practically realized due to the lack of electrical analyses of periodical light-absorbing structure. Here, we report the record high-efficiency of 16.9% for periodically patterned silicon (Si) solar cells. Periodic Si micro-pillars were fabricated for large-scale solar cells. The Si pillars with a height of about 5μm provided significantly enlarged light-active surface, which improved the photo-generated carrier collection efficiency. From optical aspects, the pillar structures reduced light reflection and hence effectively drove more photons into the absorber layer. Additionally, the enhanced pillar-structured Si surface definitely contributed to reduce an electrical resistance of a solar cell. We demonstrated that the surficial enhanced Si design could be a promising approach for high-efficient solar cell applications. We may suggest a route for the optimum electrical designs of periodic structured solar cells.

18.87%-efficient inverted pyramid structured silicon solar cell by one-step Cu-assisted texturization technique

We achieved an inverted pyramid structure, meeting the tradeoff between the light reflection minimization and carrier recombination by adjusting the one-step Cu-assisted texturization of silicon wafer, and silicon solar cells based on this structure were fabricated, which gained a high conversion efficiency of 18.87% without using any complex techniques. These data were compared with the performance of conventional upright pyramid silicon solar cells as manufactured using identical raw wafers, the Cu-etched inverted pyramid silicon cells collected 0.59mA/cm2 more short-circuit current density and 0.47% more efficiency. Importantly, our data demonstrate the better performance and manufacturability of inverted pyramid structured silicon solar cell and as such may open new perspectives for high efficiency solar cell applications.

Sulfur-rich sulfurization and solution stability of Cu2ZnSnS4 solar cells fabricated by 2-Methoxyethanol-based process

Cu2ZnSnS4 (CZTS) solar cells were fabricated by 2-Methoxyethanol (EGME) solution process. The sulfurization of the CZTS precursors were carried out in a sulfur-rich ambient by refilling the sulfur powders every 30 min during the heat treatment process. A reference sample was prepared by puting the sulfur powders nearby the sample just at the beginning of the sulfurization. It was found that the CZTS sulfurized in a sulfur-rich ambient (re-fill the sulfur powders every 30 min) was critical to obtain the large grain, voids-free Mo/CZTS interface, and a CZTS film without second phases. The cell parameters were improved and the sulfurization time was shortened using the sulfur-rich heat treatment. Based on our proposed sulfurization process, the conversion efficiency of the CZTS solar cell was 5.8%, which is the best result among CZTS solar cells fabricated through EGME solution processes. The efficiency of the CZTS solar cell fabricated by the solution that was stored for 60 days was still larger than 5%. Deterioration of the solution and the CZTS film was found when the solution was stored for 120 days. The sulfur-rich sulfurization condition and the highly stable EGME solution were proposed for high-efficiency CZTS solar cell application.

Single-crystal microplates of two-dimensional organic–inorganic lead halide layered perovskites for optoelectronics

Organic–inorganic hybrid perovskites attract considerable attention owing to their applications in high-efficiency solar cells and light emission. Compared with three-dimensional perovskites, two-dimensional (2D) layered hybrid perovskites have a higher exciton binding energy and potentially higher light-emission efficiency. The growth of high-quality crystalline 2D perovskites with a well-defined nanoscale morphology is desirable because they can be suitable building blocks for integrated optoelectronics and (nano)photonics. Herein, we report the facile solution growth of single-crystal microplates of 2D perovskites based on a 2-phenylethylammonium (C6H5CH2CH2NH 3 + , PEA) cation, (PEA)2PbX4 (X = Br, I), with a well-defined rectangular geometry and nanoscale thickness through a dissolution–recrystallization process. The crystal structures of (PEA)2PbX4 are first confirmed using single-crystal X-ray diffraction. A solution-phase transport-growth process is developed to grow microplates with a typical size of tens of micrometers and thickness of hundreds of nanometers on another clean substrate different from the substrate coated with lead-acetate precursor film. Surface-topography analysis suggests that the formation of the 2D microplates is likely driven by the wedding-cake growth mechanism. Through halide alloying, the photoluminescence emission of (PEA)2Pb(Br,I)4 perovskites with a narrow peak bandwidth is readily tuned from violet (~410 nm) to green (~530 nm).

Varying nitrogen background pressure; an efficient approach to improve electrical properties of MBE-grown GaAs1−xNx thin films with less atomic disorder

Epitaxial GaAs1−xNx thin films were grown on GaAs (001) substrates through solid-source molecular beam epitaxy using an RF plasma source for incorporating nitrogen. We examined the thin films grown under different nitrogen background pressures (NBPs) using various ex-situ techniques. GaAs0.969N0.031 thin films grown under a high NBP of 5.5 × 10−6 Torr exhibited thickness variation-induced Pendellösung fringes, a smooth surface with few Ga droplets, and a high electron mobility of 4020 cm2/(Vs), all of which evidence the high crystal quality. This is attributed to the reduction in surface recombination centers at elevated nitrogen concentrations. A high intensity of the GaAs LO1 and low intensity of the GaN-like local vibrational mode of the relaxed films were observed, attributed to less atomic disorder in the alloys. GaAs0.969N0.031 thin films exhibited near-band emission at 1.14 eV, close to the ideal band gap of (In)GaAsN (∼1 eV). Moreover, the photoluminescence intensity of films annealed at 750 °C exhibited a ∼200-fold improvement over films annealed at 650 °C. Hence, superior-quality epitaxial GaAs0.0969N0.031 thin films grown through SS-MBE are an ideal substitute for (In)GaAsN in high-efficiency multijunction solar cell applications.

Direct-indirect character of the bandgap in methylammonium lead iodide perovskite.

Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data.

Improvement of parameters in a-Si(p)/c-Si(n)/a-Si(n) solar cells

We analyzed and discussed the influence of thickness and doping concentration of the different layers in a-Si(p)/c-Si(n)/a-Si(n) photovoltaic (PV) cells with the aim of increasing its efficiency while decreasing its global cost. Compared to the efficiency of a standard marketed PV cell, elaborated with a ZnO transparent conductive oxide (TCO) layer but without Back Surface Field (BSF) layer, an optimization of the thickness and dopant concentration of both the emitter a-Si(p) and absorber c-Si(n) layers will gain about 3% in the global efficiency of the cell. The results also reveal that with introduction of the third layer, i.e. the BSF layer, the efficiency always achieves values above 20% and all other parameters of the cell, such as the open-circuit voltage, the short-circuit current and the fill-factor, are strongly affected by the thickness and dopant concentration of the layers. The values of all parameters are given and discussed in the paper. Thereby, the simulation results give for an optimized a-Si(p)/c-Si(n)/a-Si(n) PV cells the possibility to decrease the thickness of the absorber layer down to 50 μm which is lower than in the state-of-the-art. This structure of the cell achieves suitable properties for high efficiency, cost-effectiveness and reliable heterojunction (HJ) solar cell applications.

Heteroepitaxy of Ge on Cube-Textured Ni(001) Foils Through CaF2 Buffer Layer

ABSTRACTEpitaxial Ge films are useful as a substrate for high-efficiency solar cell applications. It is possible to grow epitaxial Ge films on low cost, cube textured Ni(001) sheets using CaF2(001) as a buffer layer. Transmission electron microscopy (TEM) analysis indicates that the CaF2(001) lattice has a 45o in-plane rotation relative to the Ni(001) lattice. The in-plane epitaxy relationships are CaF2[110]//Ni[100] and CaF2[$\bar 1$10]//Ni[010]. Energy dispersive spectroscopy (EDS) shows a sharp interface between Ge/CaF2 as well as between CaF2/Ni. Electron backscatter diffraction (EBSD) shows that the Ge(001) film has a large grain size (∼50 μm) with small angle grain boundaries (&lt; 8o). The epitaxial Ge thin film has the potential to be used as a substrate to grow high quality III-V and II-VI semiconductors for optoelectronic applications.

Photoresponse properties of BaSi2 film grown on Si (100) by vacuum evaporation

We have succeeded in the observation of high photoresponsivity of orthorhombic BaSi2 film grown on crystalline Si by a vacuum evaporation method, raising the prospect of its promising application in high-efficiency thin-film solar cells. Photocurrent was observed at photon energies larger than 1.28 eV, which corresponds to the band gap of evaporated BaSi2 film, indicating that the photoresponsivity originates from the BaSi2 film. The effect of the substrate temperature on the film’s properties was also investigated. The films grown at a substrate temperature larger than 500 °C are single-phase polycrystalline BaSi2 films, while those grown at a substrate temperature of 400 °C is a mixture of phases. We confirmed that undoped evaporated BaSi2 films are an n-type material with high carrier concentration. High carrier lifetime of 4.8 and 2.7 μs can be found for the films grown at 500 °C and 400 °C, respectively. BaSi2 film grown at a substrate temperature of 500 °C, which is crack-free and single-phase, shows the best photoresponsivity. The maximum value of photocurrent was obtained at photon energy of 1.9 eV, corresponding to an external quantum efficiency of 22% under reverse applied voltage of 2 V.