Multijunction Photovoltaic Devices Research Articles

Promising Proposal for Crystalline Silicon Bottom-Based Multijunction Photovoltaic Devices: Apertures in the Top-Cell Surface

Promising Proposal for Crystalline Silicon Bottom-Based Multijunction Photovoltaic Devices: Apertures in the Top-Cell Surface

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

Multiple tunnel diode peaks in I–V curves of a multijunction laser power converter

How to characterize tunnel diodes (TDs) in multijunction photovoltaic cells is a challenge. Here, we investigate a locally illuminated six-junction InGaAs laser power converter (LPC) by measuring the wide-voltage-range I–V curves. Multiple TD peaks are observed in the reverse breakdown area of the dark-state I–V curve, while with increased laser power, the peaks break into more subpeaks successively, and the subcell photocurrent steps are found to shrink due to the moving subpeaks at the right side of the steps. A model of a multijunction LPC under localized illumination is proposed, and the shrinkage of a step originates from the larger local overproduced subcell photocurrent than the local peak current of the very TD below this subcell. Therefore, the correspondences between TD peaks in the I–V curves and their physical locations in the structure of the LPC are determined. This work gives in-depth insight into the current limiting mechanism of TDs in multijunction photovoltaic devices under non-uniform illumination and expands the knowledge of multijunction photovoltaic devices.

Copper-Arsenic-Sulfide Thin-Films from Local Raw Materials Deposited via RF Co-Sputtering for Photovoltaics.

The inexorable increase of energy demand and the efficiency bottleneck of monocrystalline silicon solar cell technology is promoting the research and development of alternative photovoltaic materials. Copper-arsenic-sulfide (CAS) compounds are still rather unexplored in the literature, yet they have been regarded as promising candidates for use as p-type absorber in solar cells, owing to their broad raw material availability, suitable bandgap and high absorption coefficient. Here, a comprehensive study is presented on the structural and optoelectronic properties of CAS thin-films deposited via radio-frequency magnetron co-sputtering, using a commercial Cu target together with a Cu-As-S target with material obtained from local resources, specifically from mines in the Portuguese region of the Iberian Pyrite Belt. Raman and X-ray diffraction analysis confirm that the use of two targets results in films with pronounced stoichiometry gradients, suggesting a transition from amorphous CAS compounds to crystalline djurleite (Cu31S16), with the increasing proximity to the Cu target. Resistivity values from 4.7 mΩ·cm to 17.4 Ω·cm are obtained, being the lowest resistive films, those with pronounced sub-bandgap free-carrier absorption. The bandgap values range from 2.20 to 2.65 eV, indicating promising application as wide-bandgap semiconductors in third-generation (e.g., multi-junction) photovoltaic devices.

Application of metal, metal‐oxide, and silicon‐oxide based intermediate reflective layers for current matching in autonomous high‐voltage multijunction photovoltaic devices

A logical next step for achieving a cost price reduction per Watt peak of photovoltaics (PV) is multijunction PV devices. In two-terminal multijunction PV devices, the photo-current generated in each subcell should be matched. Intermediate reflective layers (IRLs) are widely employed in multijunction devices to increase reflection at the interface between subcells to enhance current generation in the subcell(s) positioned before the IRL, in reference to the incident light. In this work, the results of over 65 multijunction devices are presented, in order to explore the effect of different current matching approaches. The influence of variations in absorber thickness as well as thickness variations of different IRLs based on silicon-oxide, various transparent conductive oxides (TCO), and metallic layers on all-silicon multijunction PV devices is studied. Specifically, hybrid, 2-terminal, monolithically integrated silicon heterojunction (SHJ) and thin film nanocrystalline silicon (nc-Si:H) and amorphous silicon (a-Si:H) tandem and triple junction devices are processed. Based on these experiments, certain design rules for optimal current matching operation in multijunction devices are formulated. Finally, taking these design rules into account, record all-silicon multijunction devices are processed. Conversion efficiencies close 15% and V oc ≈ 2 $$ {V}_{\mathrm{oc}}\approx 2 $$ V are demonstrated for triple junction SHJ/nc-Si:H/a-Si:H devices. Such conversion efficiencies for a wireless, high-voltage wafer-based all-silicon 2-terminal multijunction PV device opens the way for efficient autonomous solar-to-fuel synthesis systems as well as other wireless innovative approaches in which the multijunction solar cell is used not only as a photovoltaic current-voltage generator, but also as an ion-exchange membrane, electrochemical catalysts, and/or optical transmittance filter.

Steric Engineering Enables Efficient and Photostable Wide-Bandgap Perovskites for All-Perovskite Tandem Solar Cells.

Wide-bandgap (WBG, ≈1.8eV) perovskite is a crucial component to pair with narrow-bandgap perovskite in low-cost monolithic all-perovskite tandem solar cells. However, the stability and efficiency of WBG perovskite solar cells (PSCs) are constrained by the light-induced halide segregation and by the large photovoltage deficit. Here, a steric engineering to obtain high-quality and photostable WBG perovskites (≈1.8eV) suitable for all-perovskite tandems is reported. By alloying dimethylammonium and chloride into the mixed-cation mixed-halide perovskites, wide bandgaps are obtained with much lower bromide contents while the lattice strain and trap densities are simultaneously minimized. The WBG PSCs exhibit considerably improved performance and photostability, retaining >90%of their initial efficiencies after 1000 h of operation at maximum power point. With the triple-cation/triple-halide WBG perovskites enabled by steric engineering, a stabilized power conversion efficiency of 26.0% in all-perovskite tandem solar cells is further obtained. The strategy provides an avenue to fabricate efficient and stable WBG subcells for multijunction photovoltaic devices.

The fundamental operation mechanisms of nc-SiOX≥0:H based tunnel recombination junctions revealed

Two terminal multi-junction (MJ) photovoltaic (PV) devices are well established concepts to increase the solar-to-electrical power conversion in reference to single PV junctions. In multi-junction PV devices two consecutive sub-cells are interconnected using a tunnel recombination junction (TRJ) in which the light excited holes of one sub-cell recombine with the light excited electrons of the other sub cell. An ideal TRJ is an ohmic contact with non-rectifying behaviour. TRJ’s based on p- and n-doped silicon-oxides have been successfully applied in a variety of hybrid multi-junction PV devices in which tunnelling and trap-assisted tunnelling over width of 5–20 nm rules the TRJ’s recombination kinetics. In this contribution the qualitative fundamental working principles of tunnel recombination junctions based on p- and n-doped silicon and silicon-oxide alloys are revealed using both electrical modelling and experiments based on a unique set of tandem lab cells (four types based on four different PV materials) combined with structural variations in TRJ architectures. The study results in design rules for the integration of silicon-oxide based TRJ’s and provides fundamental insights into the sensitivity of the electrical performance of the TRJ’s to doping concentrations, to alignment of the conduction and valence bands of consecutive sub-cells, to the nature of interface defects, to the growth of amorphous and crystalline phases and its dependence on substrate or seed layers and to the nanoscale thicknesses of the TRJ layers. • Experimental results of over 100+ different tandem device architectures are presented. • Four types of tandem PV devices are presented based on four different silicon alloys. • p-layer design is structurally varied across different multijunction PV architectures. • Energy band diagrams provide fundamental insight into the working principles of TRJ’s. • The study results in design rules for the integration of silicon-oxide based TRJ’s.

Optoelectronic Properties of Tin-Lead Halide Perovskites.

Mixed tin–lead halide perovskites have recently emerged as highly promising materials for efficient single- and multi-junction photovoltaic devices. This Focus Review discusses the optoelectronic properties that underpin this performance, clearly differentiating between intrinsic and defect-mediated mechanisms. We show that from a fundamental perspective, increasing tin fraction may cause increases in attainable charge-carrier mobilities, decreases in exciton binding energies, and potentially a slowing of charge-carrier cooling, all beneficial for photovoltaic applications. We discuss the mechanisms leading to significant bandgap bowing along the tin–lead series, which enables attractive near-infrared bandgaps at intermediate tin content. However, tin-rich stoichiometries still suffer from tin oxidation and vacancy formation which often obscures the fundamentally achievable performance, causing high background hole densities, accelerating charge-carrier recombination, lowering charge-carrier mobilities, and blue-shifting absorption onsets through the Burstein–Moss effect. We evaluate impacts on photovoltaic device performance, and conclude with an outlook on remaining challenges and promising future directions in this area.

An expedient semi-empirical modelling approach for optimal bandgap profiling of stoichiometric absorbers: A case study of thin film amorphous silicon germanium for use in multijunction photovoltaic devices

Bandgap energy profiling is applied in a variety of materials for photovoltaic technologies, such as chalcogenides, III–V materials and perovskites. Bandgap profiling of the absorber layer is used to fight the fundamental loss mechanisms imposed by the bandgap energy of the absorber for the maximum voltage and current that a photovoltaic device can generate. The bandgap profile can be affected by a number of profiling strategies, such as the difference between the maximum and minimum bandgap energy, the position of the minimum bandgap energy, the width over which this minimum bandgap energy occurs and the total absorber width. These parameters have a complex effect on output characteristics of a photovoltaic device. Varying multiple parameters at once further increases the complexity, limiting the effectiveness of rigorous physical opto-electrical modelling. In this work we therefore present an expedient semi-empirical approach for the optimal bandgap profiling of stoichiometric absorbers.Using PECVD processed amorphous silicon germanium as a model, we present a unique set of semi-empirical relations that simulate the VOC and JSC of solar cells as a function of the bandgap energy profile. For this model, the influence of deposition conditions such as the relative germane flow rate, the deposition power and substrate temperature on the opto-electrical properties of a-SiGe:H films is first characterized. Opting for the relative germane flow rate to control the bandgap profiling, the experimental results of a large number of solar cells with profiled a-SiGe:H absorber are presented, varying: 1. the absorber thickness, 2. the peak germane flow rate, so minimum bandgap energy, and 3. the introduction of a plateau at the minimum bandgap energy. Using this experimental data and optical simulations, the expedience and effectiveness of the semi-empirical approach is demonstrated.

High current density tunnel diodes for multi-junction photovoltaic devices on InP substrates

InAlGaAs tunnel diodes, lattice-matched to InP and grown by molecular beam epitaxy, are demonstrated with peak tunneling current densities exceeding 1200 A/cm2. This was achieved by a 20 °C reduction in growth temperature for the p-type tunnel diode layers, resulting in up to two orders of magnitude improvement in the peak tunneling current density. Secondary ion mass spectrometry measurements reveal that the lower growth temperature reduces unwanted segregation of p-type Be dopants, improving dopant incorporation within the active tunnel diode layers. The diodes are transparent to wavelengths above 1000 nm and are compatible with the bottom junctions of InP-based multi-junction solar cells and with InP-based photonic power converters operating in the telecommunication O- and C-bands. When incorporated into a dual-junction photonic power converter test structure, measurements under 1319-nm laser illumination demonstrate integrated tunnel diode operation, enabling a halving of the short-circuit current and doubling of the open-circuit voltage as compared to a single junction reference device.

The impact of processing conditions and post-deposition oxidation on the opto-electrical properties of hydrogenated amorphous and nano-crystalline Germanium films

Low-cost multijunction photovoltaic devices are the next step in the solar energy revolution. Adding a bottom junction with a low bandgap energy material through plasma enhanced chemical vapor deposition (PECVD) processing could potentially provide a low-cost boost in conversion efficiency. A logical candidate for this low bandgap material is germanium. In this work we investigate the growth of PECVD processed hydrogenated amorphous/nano-crystalline germanium (a/nc-Ge:H), by characterizing over 100 samples, processed with a wide range of deposition pressures, powers, temperatures and GeH4 dilution in hydrogen, using elemental analysis, vibrational analysis and analysis of the opto-electrical properties. We have identified a small processing window in which nc-Ge:H films are processed reproducibly. We also report on the strong correlation between the refractive index of the films and the presence- and extent of post-deposition oxidation. Notably, the oxidation generally increased the photoresponse of the films, as it results in a decrease of room temperature σd by 1-3 orders of magnitude. However, oxidation results in an increase of the bandgap energy and therefore impedes the development of a low bandgap material. The lowest E04 we report is about 1.1eV, with an ETauc of 0.9eV and an σph/σd of 3.4.

Halide Segregation versus Interfacial Recombination in Bromide-Rich Wide-Gap Perovskite Solar Cells

Perovskites offer exciting opportunities to realize efficient multijunction photovoltaic devices. This requires high-VOC and often Br-rich perovskites, which currently suffer from halide segregatio...

Efficiency increase in multijunction monochromatic photovoltaic devices due to luminescent coupling

We present a multijunction detailed-balance model that includes the effects of luminescent coupling, light trapping, and nonradiative recombination, suitable for the treatment of multijunction solar cells and photonic power converters—photovoltaic devices designed to convert the narrow-band light. The model includes both specular and Lambertian reflections using a ray-optic formalism and treats nonradiative processes using an internal radiative efficiency. Using this model, we calculate and optimize the efficiency of multijunction photonic power converters for a range of material qualities and light-trapping schemes. Multijunction devices allow increased voltage with lower current, decreasing series resistance losses. We show that efficiency increases significantly with an increased number of junctions, even without series resistance, when the device has an absorbing substrate. Such an increase does not occur when the device has a back reflector. We explain this effect using a simplified model, which illustrates the origin of the decreased radiative losses in multijunction devices on substrates.

Spectral response measurements of each subcell in monolithic triple-junction GaAs photovoltaic devices

Multijunction photovoltaic devices have gained attention for both solar cells and wireless power transmission. Herein, we present the spectral response of each subcell in a monolithic triple-junction photovoltaic device composed of only GaAs subcells with different thicknesses, which clarifies current matching among the GaAs subcells under simulated solar illumination conditions. In addition, the spectral response curves of each subcell show that the power-dependent spectral responses under laser illumination with wavelengths of 405, 660, and 785 nm are explained by the top GaAs subcell, which has a slightly lower shunt resistance and experiences the effects of luminescent coupling.

Tailored lead iodide growth for efficient flexible perovskite solar cells and thin-film tandem devices

Flexible perovskite solar cells (PSCs) hold great promise for the low-cost roll-to-roll production of lightweight single- and multijunction photovoltaic devices. Among the different deposition methods used for the perovskite absorber, the two-step hybrid vacuum-solution approach enables precise control over the thickness and morphology of PbI2. However, efficient conversion to perovskite is limited by diffusion of the organic cations in the compact lead halide layer. Herein, a multistage absorber deposition is developed by thermal evaporation of PbI2 and spin coating of CH3NH3I (MAI). The process relies on the different types of growth of vacuum-deposited PbI2 onto amorphous and crystalline surfaces. This approach represents a way to effectively increase the absorber thickness while tackling the limited MAI diffusion in the compact PbI2 film via a two-step deposition method. The efficiency of flexible PSCs is improved from 14.2 to 15.8% with multistage deposition. Furthermore, the use of an amorphous transparent conductive oxide (TCO), InZnO, enhances the mechanical resistance against bending with respect to conventional crystalline TCO-based flexible devices. Near-infrared transparent flexible PSCs are developed with an efficiency of 14.0% and average transmittance of ~74% between 800 and 1000 nm. Flexible perovskite/CIGS thin-film tandem devices are demonstrated with an efficiency of 19.6% measured in the four-terminal configuration.

A differential spectral responsivity measurement system constructed for determining of the spectral responsivity of a single- and triple-junction photovoltaic cells

A differential spectral responsivity (DSR) measurement system has been designed and constructed at National Metrology Institute of Turkey (TUBITAK UME) to determine the spectral responsivity (SR) of a single- or a multi-junction photovoltaic device (solar cell). The DSR setup contains a broad band light bias source composed of a constructed Solar Simulator based on a 1000 W Xe-arc lamp owning a AM-1.5 filter and 250 W quartz-tungsten-halogen lamp, a designed and constructed LED-based Bias Light Sources, a DC voltage bias circuit, and a probe beam optical power tracking and correction circuit controlled with an ADuC847 microcontroller card together with an embedded C based software, designed and constructed in TUBITAK UME under this project. By using the constructed DSR measurement system, the SR calibration of solar cells, the monolitic triple-junction solar cell GaInP/GaInAs/Ge and its corresponding component cells have been performed within the EURAMET Joint Research Project SolCell.

All Electrodeposited Nanowire Array Based Multi-Junction Photovoltaic Devices

Large-scale solar fuel production using semiconductor based photoelectrochemical system requires inexpensive, efficient and stable devices that can provide photovoltages sufficient to overcome both the thermodynamic and overpotential losses of the redox reaction. Here we report all electrodeposited multi-junction photovoltaic (PV) devices for solar fuel production using serially connected multiple Cd/CdSe/Au Schottky barrier junctions with photovoltages exceeding 1.0 V. These PV devices are synthesized inside nanoporous alumina membranes with each nanoscale solar conversion units physically and electronically isolated from each other, making the entire device fault tolerant. The alumina membranes also serves to protect the semiconductor from chemical and photoelectrochemical corrosion. Ensemble and individual nanowire measurements were performed to understand charge carrier energetics and transport pathways across multiple interfaces.

The numerical computation of lumped parameter values using the multi-dimensional Newton-Raphson method for the characterisation of a multi-junction CPV module using the five-parameter approach

The mathematical modelling of photovoltaic devices is heavily relied upon due to their highly variable yield while operating under varying environmental conditions. A number of methods currently exist to characterise the non-linear electrical behaviour of these devices, with the single-diode, five-parameter model extensively used. In this study, we examine the use of this model for a triple-junction III–V type solar cell operating under concentrated sunlight. In order to use this method, however, five characterisation parameters must be calculated for the specific device under study. Their calculation is difficult due to the intrinsic non-linear operational characteristics of these devices, as such a number of methods currently exist for this purpose. A new method is presented here based on the examination of the power-voltage data from which a system of five residual equations are derived and solved via the multi-variable Newton-Raphson method.The new approach was validated experimentally by means of both qualitative and quantitative assessment. Results revealed that the proposed method was able to accurately model this device type and when compared to several alternative methods, yield the lowest average root mean square error (RMSE) with a 7–55% improvement offered over the alternative methods investigated. The proposed method will be useful to engineers and scientists who wish to characterise the performance of multi-junction photovoltaic devices using the single-diode, five-parameter model and require a method to extract the values of their characterisation parameters. A limitation with this approach is the computational effort required to solve the system of non-linear residual equations and the need for good initial estimates of the lumped parameters to initiate the search.

Imaging Atomic-Scale Clustering in III-V Semiconductor Alloys.

Quaternary alloys are essential for the development of high-performance optoelectronic devices. However, immiscibility of the constituent elements can make these materials vulnerable to phase segregation, which degrades the optical and electrical properties of the solid. High-efficiency III-V photovoltaic cells are particularly sensitive to this degradation. InAlAsSb lattice matched to InP is a promising candidate material for high-bandgap subcells of a multijunction photovoltaic device. However, previous studies of this material have identified characteristic signatures of compositional variation, including anomalous low-energy photoluminescence. In this work, atomic-scale clustering is observed in InAlAsSb via quantitative scanning transmission electron microscopy. Image quantification of atomic column intensity ratios enables the comparison with simulated images, confirming the presence of nonrandom compositional variation in this multispecies alloy.

Quantitative Determination of Luminescent Coupling in Multijunction Solar Cells from Spectral Photovoltage Measurements

We present a simple method to quantify the magnitude of luminescent coupling (LC) between stacked subcells in multijunction photovoltaic devices. The effect of luminescence produced at high-band-gap subcells on underlying low-gap units within the same device can be directly accessed as a measurable open-circuit voltage difference by comparing two photovoltage spectra. Additionally, our study unambiguously identifies LC as the modulation mechanism across multijunction solar cells generating a response from buried subcells in photoreflectance measurements.