High-efficiency Single-junction Research Articles

Surface-functionalized hole-selective monolayer for high efficiency single-junction wide-bandgap and monolithic tandem perovskite solar cells

Carbazole moiety-based 2PACz ([2-(9H-carbazol-9-yl)ethyl]phosphonic acid) self-assembled monolayers (SAMs) are excellent hole-selective contact (HSC) materials with abilities to excel the charge-transfer-dynamics of perovskite solar cells (PSCs). Herein, we report a facile but powerful method to functionalize the surface of 2PACz-SAM, by which reproducible, highly stable, high-efficiency wide-bandgap PSCs can be obtained. The 2PACz surface treatment with various donor number solvents improves assembly of 2PACz-SAM and leave residual surface-bound solvent molecules on 2PACz-SAM, which increases perovskite grain size, retards halide segregation, and accelerates hole extraction. The surface functionalization achieves a high power conversion efficiency (PCE) of 17.62% for a single-junction wide-bandgap (∼1.77 eV) PSC. We also demonstrate a monolithic all-perovskite tandem solar cell using surface-engineered HSC, showing high PCE of 24.66% with large open-circuit voltage of 2.008 V and high fill-factor of 81.45%. Our results suggest this simple approach can further improve the tandem device, when coupled with a high-performance narrow-bandgap sub-cell.

Machine Learning-Aided Band Gap Engineering of BaZrS3 Chalcogenide Perovskite

The non-toxic and stable chalcogenide perovskite BaZrS3 fulfills many key optoelectronic properties for a high-efficiency photovoltaic material. It has been shown to possess a direct band gap with a large absorption coefficient and good carrier mobility values. With a reported band gap of 1.7-1.8 eV, BaZrS3 is a good candidate for tandem solar cell materials; however, its band gap is significantly larger than the optimal value for a high-efficiency single-junction solar cell (∼1.3 eV, Shockley-Queisser limit)─thus doping is required to lower the band gap. By combining first-principles calculations and machine learning algorithms, we are able to identify and predict the best dopants for the BaZrS3 perovskites for potential future photovoltaic devices with a band gap within the Shockley-Queisser limit. It is found that the Ca dopant at the Ba site or Ti dopant at the Zr site is the best candidate dopant. Based on this information, we report for the first time partial doping at the Ba site in BaZrS3 with Ca (i.e., Ba1-xCaxZrS3) and compare its photoluminescence with Ti-doped perovskites [i.e., Ba(Zr1-xTix)S3]. Synthesized (Ba,Ca)ZrS3 perovskites show a reduction in the band gap from ∼1.75 to ∼1.26 eV with <2 atom % Ca doping. Our results indicate that for the purpose of band gap tuning for photovoltaic applications, Ca-doping at the Ba-site is superior to Ti-doping at the Zr-site reported previously.

Mixed Solvents Assisted Post-Treatment Enables High-Efficiency Single-Junction Perovskite and 4T Perovskite/CIGS Tandem Solar Cells.

The interface between the perovskite layer and the hole transport layer (HTL) plays a vital role in hole extraction and electron blocking in perovskite solar cells (PSCs), and it is particularly susceptible to harmful defects. Surface passivation is an effective strategy for addressing the above concerns. However, because of its strong polarity, isopropyl alcohol (IPA) is used as a solvent in all of the surface treatment materials reported thus far, and it frequently damages the surface of perovskite. In this paper, a method is proposed for dissolving the passivation materials, for example, guanidine bromide (GABr), in mixed solvents (1:1) of IPA and toluene (TL), which can efficiently passivate interface and grain boundary defects by minimizing the IPA solubility of the perovskite surface. As a result, all the performance parameters Voc, Jsc, and FF are improved, and the power conversion efficiency (PCE) increased from 20.1 to 22.7%. Moreover, combining the PSCs with GABr post‐treatment in mixed solvents with copper indium gallium selenide (CIGS) solar cells, a 4‐terminal (4T) perovskite/CIGS tandem device is realized and a PCE of 25.5% is achieved. The mixed solvent passivation strategy demonstrated here, hopefully, will open new avenues for improving PSCs’ efficiency and stability.

High-efficiency single-junction organic solar cells enabled by double-fibril network morphology

High-efficiency single-junction organic solar cells enabled by double-fibril network morphology

A general design guideline for strain-balanced quantum-wells toward high-efficiency photovoltaics

Due to the tunable optical absorption threshold and several intriguing physical properties, multiple-quantum-well (MQW) structures have been playing a crucial role in achieving ultra-high efficiency in tandem photovoltaic cells and various optoelectronic technologies. However, devices incorporating such nano-structures suffer from poor perpendicular carrier transport, which hinders device performance. On the other hand, designing MQWs is challenging. Simultaneously optimizing the structural parameters and constituent materials suggests a high design complexity with a large parameter space. In this report, on the basis of the quantitative analysis on the degradation of carrier transport, we propose a general design guideline for MQWs by which the structures and constituent materials can both be optimized targeting at a particular absorption threshold. We firstly demonstrate our design flow by optimizing an InGaP/InGaP strain-balanced (SB) MQW at 1.91 eV to replace In0.49Ga0.51P bulk serving as the top cell of dual-MQW triple-junction solar cells. Secondly, we apply the design concept to a conventional InGaAs/GaAsP SBMQW targeting at 1.23 eV, which is significant for not only high-efficiency single-junction photovoltaics but also current-matched tandem cells. The results illustrate the possibility of further improving current devices in terms of their constituent materials for QWs. Our design optimization method is considered to be applicable to any device based on QW structures.

Design and Demonstration of High-Efficiency Quantum Well Solar Cells Employing Thin Strained Superlattices

Nanostructured quantum well and quantum dot III–V solar cells provide a pathway to implement advanced single-junction photovoltaic device designs that can capture energy typically lost in traditional solar cells. To realize such high-efficiency single-junction devices, nanostructured device designs must be developed that maximize the open circuit voltage by minimizing both non-radiative and radiative components of the diode dark current. In this work, a study of the impact of barrier thickness in strained multiple quantum well solar cell structures suggests that apparent radiative efficiency is suppressed, and the collection efficiency is enhanced, at a quantum well barrier thickness of 4 nm or less. The observed changes in measured infrared external quantum efficiency and relative luminescence intensity in these thin barrier structures is attributed to increased wavefunction coupling and enhanced carrier transport across the quantum well region typically associated with the formation of a superlattice under a built-in field. In describing these effects, a high efficiency (>26% AM1.5) single-junction quantum well solar cell is demonstrated in a device structure employing both a strained superlattice and a heterojunction emitter.

Energy yield analysis of textured perovskite silicon tandem solar cells and modules.

Perovskite silicon tandem solar cells combine potentially low production costs with the ability to surpass the efficiency limit of silicon single junction solar cells. Optical modeling and optimization are crucial to achieve this ambitious goal in the near future. The optimization should seek to maximize the energy yield based on realistic environmental conditions. This work analyzes the energy yield of perovskite silicon tandem solar cells and modules based on realistic experimental data, with a special focus on the investigation of surface textures at the front and rear side of the solar cell and its implication for reflection as well as parasitic absorption properties. The investigation reveals a 7.3%rel higher energy yield for an encapsulated tandem cell with a textured front side compared with an encapsulated high efficiency single junction solar cell with 24.3% harvesting efficiency for irradiance data of the year 2014 in Freiburg/Germany.

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.

Sensitivity Analysis of Optical Metrics for Spectral Splitting Photovoltaic Systems: A Case Study

Spectral splitting of sunlight to increase photovoltaic (PV) efficiency beyond the Shockley–Queisser limit has gained interest in recent years. Sensitivity analysis can be a useful tool for system designers to determine how much deviation from ideal conditions can be tolerated for different optical parameters. Understanding the origin of these sensitivities can offer insight into materials and device design. We employ 2-D TCAD simulations to analyze the sensitivity of system performance to two optical parameters: spectral fidelity (the fraction of photons directed to the intended material) and the spatial uniformity of illumination intensity. We analyze a system using crystalline silicon (Si) and cuprous oxide (Cu2O) as absorbers. We find that the spectral fidelity of the light directed to the Si cell has to be greater than 90% for the system to outperform a high-efficiency single-junction Si device. Varying the fidelity of the light directed to the Cu2O cell from 55% to 90% changes system efficiency by less than 10% relative. In some cases, increasing the fidelity of this light reduces system efficiency. We find no significant impact of spatial variation on length scales from 600 μm to 4.8 mm in devices with emitter sheet resistance less than 500 Ω/□.

High Efficiency Millimeter-Scale Crystalline Perovskite Solar Cells

Current state-of-the-art photovoltaics utilize high purity single-crystal semiconductors grown by sophisticated, high temperature crystal-growth processes. Alternative emerging solar cell technologies based on solution-processed materials promise to deliver power at lower cost, but so far have had limited success due to the presence of defects in the bulk and at the grain boundaries. Improving the crystal size and quality of solution-processed films could lead to unprecedented improvements in photovoltaic performance. However, large-scale crystal growth is challenging in solution-based deposition where molecular kinetics are limited by the low processing temperature. Here, for the first time we demonstrate organometallic perovskites thin-films composed of extra-ordinarily large crystalline grains (1-2 mm), fabricated using a new processing technique. The increase in the grain-size directly correlates to a dramatic increase in the efficiency, as a direct consequence of increased charge carrier mobility and reduction in defect densities. Our measured efficiency values approach ~18%, which is among the highest reported. These devices are extremely robust, exhibiting no degradation with voltage sweep direction or the rate at which the voltage was scanned. We anticipate that this technique for growing high quality large-area crystals at low temperature with low defect concentration will be universally applicable to a wide variety of other material systems thus providing a solution to a long-standing scientific challenge of overcoming polydispersity, defects and grain boundary recombination in solution-processed thin-films. From the perspective of the global photovoltaics community, these results are expected to lead the field towards the synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency single-junction and hybrid (semiconductor and perovskite) tandem planar cells. In addition, I will also discuss design principles that govern the photostability in these systems.

Enhanced Light Trapping in Multiple Quantum Wells by Thin-Film Structure and Backside Grooves With Dielectric Interface

Insertion of multiple quantum wells (MQWs) into the i-region of GaAs p-i-n solar cells can shift the effective bandgap to the optimal value for single-junction solar cells under high sunlight concentration. The quantum efficiency corresponding to the absorption by MQWs must be sufficiently high for achieving high-efficiency single-junction MQW solar cells. Here, we report light-trapping thin-film MQW solar cells for increasing photoabsorption in MQWs. In order to suppress the free-carrier absorption loss and enhance the light trapping, only the active layers including MQWs were processed to a cell by flip-chip bonding and dissolution of a conductive GaAs substrate. The periodic grooves formed on the back side of the cell scattered photons in the subbandgap range and trapped light inside the cell. For absorption loss reduction in a back contact metal, a dielectric interlayer was introduced between the metal and GaAs. The light-trapping structure resulted in a fivefold increase in effective optical path length compared with the physical thickness of MQWs. External quantum efficiency at wavelengths longer than the GaAs edge exceeded 50% with only 20-period MQWs. As a result, we achieved thin-film light-trapping MQW solar cells with 20% conversion efficiency.

Fabrication and optimization of single-junction nc-Si:H n–i–p solar cells using Si:H phase diagram concepts developed by real time spectroscopic ellipsometry

In this study, we have developed and applied deposition phase diagrams in the plane of the bulk layer thickness db and the H2-dilution ratio R=[H2]/[Si2H6] for Si:H materials deposited by 70MHz VHF PECVD from [H2]+[Si2H6] mixed gases on c-Si/(native-oxide)/n-layer substrates. To establish the phase diagrams, series of Si:H depositions having different R values over the range of 60–150 were measured in real time using a rotating-compensator multichannel ellipsometer. Using phase diagram concepts for guidance, we have fabricated high efficiency single-junction nc-Si:H n–i–p solar cells with ∼3Å/s intrinsic layers using the VHF PECVD process. We have found that the nc-Si:H solar cells with the best performance are obtained by incorporating i-layers deposited in the single-phase nanocrystalline silicon regime near the transition boundary to mixed-phase (a+nc)-Si:H. Applying insights from real time spectroscopic ellipsometry moreover, we have investigated in detail the effects of the phase of the underlying n-layer on the phase evolution of the overdeposited Si:H i-layer and on the overall device performance. With the strategy developed here, a stabilized efficiency of η=9.46% (Voc=0.516V, Jsc=24.65mA/cm2, FF=0.744) has been achieved for nc-Si:H solar cells (0.25cm2 in active area) fabricated with an i-layer deposition rate of ∼2.2Å/s.

Present status and future prospects for advanced space solar cells

Abstract This paper presents an overview of the present status of advanced space solar cells and of our research activities. The emphasis is on high-efficiency single-junction GaAs cells, radiation-resistant InP solar cell, thin-film GaAs-on-Si cells and super high efficency III–V compound-based tandem solar cells. The future prospects for advanced space cells will also be discussed. For widespread applications of advanced cells, the research and development of thin-film technologies and tandem cells for super-high efficiency and low-cost cells are necessary.

Iron Sulfide for Photovoltaics

In recent years, iron sulfide (FeS2), otherwise known as pyrite, has received some attention as a possible candidate for thin-film photovoltaic applications. This interest stems in part from the fact that it has a relatively large absorption coefficient of α &gt; 105 cm−1 for photon energies larger than about 1.3 eV (see Figure 1), such that a thin layer of less than 1,000 Å should effectively absorb the visible part of the solar spectrum. The bandgap of pyrite of 0.95 eV is probably on the borderline of what might still be considered acceptable for a high-efficiency single-junction device, but can be considered ideal in combination with a 1.8 eV material for tandem structures. Another important advantage of this compound semiconductor is that it is composed of cheap, abundant, and non-toxic elements, the latter being of particular importance in times of growing environmental consciousness.Pyrite is a naturally occurring mineral (also known as “fool's gold” because of its golden color) which shows both n- and p-type conduction. This could indicate possible electronic viability of this material. The study of synthetic crystals has been severely impeded by the fact that pyrite undergoes a peritectic decomposition into the more stable FeS and liquid sulfur at 743°C and that therefore the melt growth of high-purity large single-crystalline material is not possible. Nevertheless, it has been shown that polycrystalline pyrite layers could be prepared with different thin-film preparation techniques (see Table I), such that in principle a cost-effective large-scale application could be envisaged.