Phthalimide Polymer Donor Guests Enable over 17% Efficient Organic Solar Cells via Parallel‐Like Ternary and Quaternary Strategies

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.     

On the seasonal distribution of sunspot frequency

Abstract

Using the Zürich sunspot data, the seasonal distribution of all sunspot‐groups and of all new‐formed groups from 1938 to the last solar minimum in 1976 was investigated.

It is shown that there exist different distributions for the northern and southern solar hemisphere with maxima in the third and second quarter of the years, respectively, and seasonal differences in the north‐south asymmetry.

These results confirm the presumption of an influence of interstellar matter on solar activity.     

Fully Solution‐Processed TCO‐Free Semitransparent Perovskite Solar Cells for Tandem and Flexible Applications

Semitransparent perovskite solar cells (st‐PSCs) have received remarkable interest in recent years because of their great potential in applications for solar window, tandem solar cells, and flexible photovoltaics. However, all reported st‐PSCs require expensive transparent conducting oxides (TCOs) or metal‐based thin films made by vacuum deposition, which is not cost effective for large‐scale fabrication: the cost of TCOs is estimated to occupy ≈75% of the manufacturing cost of PSCs. To address this critical challenge, this study reports a low‐temperature and vacuum‐free strategy for the fabrication of highly efficient TCO‐free st‐PSCs. The TCO‐free st‐PSC on glass exhibits 13.9% power conversion efficiency (PCE), and the four‐terminal tandem cell made with the st‐PSC top cell and c‐Si bottom cell shows an overall PCE of 19.2%. Due to the low processing temperature, the fabrication of flexible st‐PSCs is demonstrated on polyethylene terephthalate and polyimide, which show excellent stability under repeated bending or even crumbing.     

Light Trapping with Dielectric Scatterers in Single‐ and Tandem‐Junction Organic Solar Cells

Efficient dielectric scatterers based on a mixture of TiO₂ nanoparticles and polydimethylsiloxane are demonstrated for light trapping in semitransparent organic solar cells. An improvement of 80% in the photocurrent of an optimized semitransparent solar cell is achieved with the dielectric scatterer with ≈100% diffuse reflectance for wavelengths larger than 400 nm. For a parallel tandem solar cell, the dielectric scatterer generates 20% more photocurrent compared with a silver mirror beneath the cell; for a series tandem solar cell, the dielectric scatterer can be used as a photocurrent balancer between the subcells with different photoabsorbing materials. The power conversion efficiency of the tandem cell in series configuration with balanced photocurrent in the subcells exceeds that of an optimized standard solar cell with a reflective electrode. The characteristics of polydimethylsiloxane, such as flexibility and the ability to stick conformably to surfaces, also remain in the dielectric scatterers, which makes the demonstrated light trapping configuration highly suitable for large scale module manufacturing of roll‐to‐roll printed organic single‐ or tandem‐junction solar cells.     

Progress in Tandem Solar Cells Based on Hybrid Organic–Inorganic Perovskites

Owing to their high efficiency, low‐cost solution‐processability, and tunable bandgap, perovskite solar cells (PSCs) made of hybrid organic‐inorganic perovskite (HOIP) thin films are promising top‐cell candidates for integration with bottom‐cells based on Si or other low‐bandgap solar‐cell materials to boost the power conversion efficiency (PCE) beyond the Shockley‐Quiesser (S‐Q) limit. In this review, recent progress in such tandem solar cells based on the emerging PSCs is summarized and reviewed critically. Notable achievements for different tandem solar cell configurations including mechanically‐stacked, optical coupling, and monolithically‐integrated with PSCs as top‐cells are described in detail. Highly‐efficient semitransparent PSC top‐cells with high transmittance in near‐infrared (NIR) region are critical for tandem solar cells. Different types of transparent electrodes with high transmittance and low sheet‐resistance for PSCs are reviewed, which presents a grand challenge for PSCs. The strategies to obtain wide‐bandgap PSCs with good photo‐stability are discussed. The PCE reduction due to reflection loss, parasitic absorption, electrical loss, and current mismatch are analyzed to provide better understanding of the performance of PSC‐based tandem solar cells.     

Semi‐Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency

Semi‐transparent (ST) organic solar cells with potential application as power generating windows are studied. The main challenge is to find proper transparent electrodes with desired electrical and optical properties. In this work, this is addressed by employing an amphiphilic conjugated polymer PFPA‐1 modified ITO coated glass substrate as the ohmic electron‐collecting cathode and PEDOT:PSS PH1000 as the hole‐collecting anode. For active layers based on different donor polymers, considerably lower reflection and parasitic absorption are found in the ST solar cells as compared to solar cells in the standard geometry with an ITO/PEDOT:PSS anode and a LiF/Al cathode. The ST solar cells have remarkably high internal quantum efficiency at short circuit condition (～90%) and high transmittance (～50%). Hence, efficient ST tandem solar cells with enhanced power conversion efficiency (PCE) compared to a single ST solar cell can be constructed by connecting the stacked two ST sub‐cells in parallel. The total loss of photons by reflection, parasitic absorption and transmission in the ST tandem solar cell can be smaller than the loss in a standard solar cell based on the same active materials. We demonstrate this by stacking five separately prepared ST cells on top of each other, to obtain a higher photocurrent than in an optimized standard solar cell.     

Four‐Terminal Perovskite/Silicon Multijunction Solar Modules

Multijunction solar cells employing perovskite and crystalline‐silicon (c‐Si) light absorbers bear the exciting potential to surpass the efficiency limit of market‐leading single‐junction c‐Si solar cells. However, scaling up this technology and maintaining high efficiency over large areas are challenging as evidenced by the small‐area perovskite/c‐Si multijunction solar cells reported so far. In this work, a scalable four‐terminal multijunction solar module design employing a 4 cm² semitransparent methylammonium lead triiodide perovskite solar module stacked on top of an interdigitated back contact c‐Si solar cell of identical area is demonstrated. With a combination of optimized transparent electrodes and efficient module design, the perovskite/c‐Si multijunction solar modules exhibit power conversion efficiencies of 22.6% on 0.13 cm² and 20.2% on 4 cm² aperture area. Furthermore, a detailed optoelectronic loss analysis along with strategies to enhance the performance is discussed.     

Three‐Dimensional Nanostructured Indium‐Tin‐Oxide Electrodes for Enhanced Performance of Bulk Heterojunction Organic Solar Cells

A three‐dimensional indium tin oxide (ITO) nanohelix (NH) array is presented as a multifunctional electrode for bulk heterojunction organic solar cells for simultaneously improving light absorption and charge transport from the active region to the anode. It is shown that the ITO NH array, which is easily fabricated using an oblique‐angle‐deposition technique, acts as an effective antireflection coating as well as a light‐scattering layer, resulting in much enhanced light harvesting. Furthermore, the larger interfacial area between the electrode and the active layer, together with the enhanced carrier mobility through highly conductive ITO NH facilitate transport and collection of charge carriers. The optical and electrical improvements enabled by the ITO NH electrode result in a 10% increase in short‐circuit current density and power‐conversion efficiency of the solar cells.     

Liquid Norbornadiene Photoswitches for Solar Energy Storage

Due to high global energy demands, there is a great need for development of technologies for exploiting and storing solar energy. Closed cycle systems for storage of solar energy have been suggested, based on absorption of photons in photoresponsive molecules, followed by on‐demand release of thermal energy. These materials are called solar thermal fuels (STFs) or molecular solar thermal (MOST) energy storage systems. To achieve high energy densities, ideal MOST systems are required either in solid or liquid forms. In the case of the latter, neat high performing liquid materials have not been demonstrated to date. Here is presented a set of neat liquid norbornadiene derivatives for MOST applications and their characterization in toluene solutions and neat samples. Their synthesis is in most cases based on solvent‐free Diels‐Alder reactions, which easily and efficiently afford a range of compounds. The shear viscosity of the obtained molecules is close to that of colza oil, and they can absorb up to 10% of the solar spectrum with a measured energy storage density of up to 577 kJ/kg corresponding to 152 kJ mol^–1 (calculated 100 kJ mol^–1). These findings pave the way towards implementation of liquid norbornadienes in closed cycle energy storage technologies.     

Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment

Research on the luminescent solar concentrator (LSC) over the past thirty‐odd years is reviewed. The LSC is a simple device at its heart, employing a polymeric or glass waveguide and luminescent molecules to generate electricity from sunlight when attached to a photovoltaic cell. The LSC has the potential to find extended use in an area traditionally difficult for effective use of regular photovoltaic panels: the built environment. The LSC is a device very flexible in its design, with a variety of possible shapes and colors. The primary challenge faced by the devices is increasing their photon‐to‐electron conversion efficiencies. A number of laboratories are working to improve the efficiency and lifetime of the LSC device, with the ultimate goal of commercializing the devices within a few years. The topics covered here relate to the efforts for reducing losses in these devices. These include studies of novel luminophores, including organic fluorescent dyes, inorganic phosphors, and quantum dots. Ways to limit the surface and internal losses are also discussed, including using organic and inorganic‐based selective mirrors which allow sunlight in but reflect luminophore‐emitted light, plasmonic structures to enhance emissions, novel photovoltaics, alignment of the luminophores to manipulate the path of the emitted light, and patterning of the dye layer to improve emission efficiency. Finally, some possible ‘glimpses of the future’ are offered, with additional research paths that could result in a device that makes solar energy a ubiquitous part of the urban setting, finding use as sound barriers, bus‐stop roofs, awnings, windows, paving, or siding tiles.     

The Role of Antireflective Coating CYTOP,Immersion Oil,and Sensitizer Selection in Fabricating a 2.3 V, 10% Power Conversion Efficiency SSM‐DSC Device

Sequential series multijunction dye‐sensitized solar cells (SSM‐DSCs) can power solar‐to‐fuel processes with a single illuminated area device. Dye selection and strategies limiting photon losses are critical in SSM‐DSC devices for higher performance systems. Herein, an efficient and readily applicable spin coating protocol on glass surfaces with an antireflective fluoropolymer (CYTOP) is applied to an SSM‐DSC architecture. Combining CYTOP with the use of an immersion oil between glass spacers in a three subcell SSM‐DSC with judiciously selected TiO₂ photoanode sensitizers and thicknesses, an overall power conversion efficiency (PCE) of 10.1% is obtained with an output of 2.3 V. Without external bias, this SSM‐DSC configuration shows an impressive overall solar‐to‐fuel conversion efficiency of 6% when powering IrO₂ and Au₂O₃ electrocatalysts for CO₂ and H₂O to CO and H₂ conversion in aqueous solution. The role of CYTOP, immersion oil, sensitizer selection, and film thickness on SSM‐DSC devices is discussed along with the stability of this system.     

Improving the Performance and Stability of Inverted Planar Flexible Perovskite Solar Cells Employing a Novel NDI‐Based Polymer as the Electron Transport Layer

A new naphthalene diimide (NDI)‐based polymer with strong electron withdrawing dicyanothiophene (P(NDI2DT‐TTCN)) is developed as the electron transport layer (ETL) in place of the fullerene‐based ETL in inverted perovskite solar cells (Pero‐SCs). A combination of characterization techniques, including atomic force microscopy, scanning electron microscopy, grazing‐incidence wide‐angle X‐ray scattering, near‐edge X‐ray absorption fine‐structure spectroscopy, space‐charge‐limited current, electrochemical impedance spectroscopy, photoluminescence (PL), and time‐resolved PL decay, is used to demonstrate the interface phenomena between perovskite and P(NDI2DT‐TTCN) or [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM). It is found that P(NDI2DT‐TTCN) not only improves the electron extraction ability but also prevents ambient condition interference by forming a hydrophobic ETL surface. In addition, P(NDI2DT‐TTCN) has excellent mechanical stability compared to PCBM in flexible Pero‐SCs. With these improved functionalities, the performance of devices based on P(NDI2DT‐TTCN) significantly outperform those based on PCBM from 14.3 to 17.0%, which is the highest photovoltaic performance with negligible hysteresis in the field of polymeric ETLs.     

Highly Efficient Polymer Tandem Cells and Semitransparent Cells for Solar Energy

Highly efficient tandem and semitransparent (ST) polymer solar cells utilizing the same donor polymer blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as active layers are demonstrated. A high power conversion efficiency (PCE) of 8.5% and a record high open‐circuit voltage of 1.71 V are achieved for a tandem cell based on a medium bandgap polymer poly(indacenodithiophene‐co‐phananthrene‐quinoxaline) (PIDT‐phanQ). In addition, this approach can also be applied to a low bandgap polymer poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothia‐diazole)] (PCPDTFBT), and PCEs up to 7.9% are achieved. Due to the very thin total active layer thickness, a highly efficient ST tandem cell based on PIDT‐phanQ exhibits a high PCE of 7.4%, which is the highest value reported to date for a ST solar cell. The ST device also possesses a desirable average visible transmittance (≈40%) and an excellent color rendering index (≈100), permitting its use in power‐generating window applications.