Bifunctional Moth‐Eye Nanopatterned Dye‐Sensitized Solar Cells: Light‐Harvesting and Self‐Cleaning Effects

A nanopatterning technique using nanostamps that provides a facile process to create a nature‐inspired moth‐eye structure achieving high transmittance in the visible range as well as a self‐cleaning effect is reported. Commercially available perfluoropolyether (PFPE) and NOA63 as the mold resin and second replica mold material, respectively, play an important role in fabricating the structure. The structure is found to increase transmittance up to 82% at 540 nm and contact angle up to 150°, representing superhydrophobicity even without the aid of a fluorinated self‐assembled monolayer (SAM) coating. The resulting solid‐state dye‐sensitized solar cells (ssDSSCs) with moth‐eye structures show enhancement of efficiency to 7.3% at 100 mW cm^?2, which is among the highest values reported to date for N719 dye‐based ssDSSCs. This nature‐inspired nanopatterning process could be used for improving light harvesting in any type of photovoltaic cell, and it produces superhydrophobic surfaces, which in turn lead to self‐cleaning for long‐term stability.     

All‐Oxide MoOx/SnOx Charge Recombination Interconnects for Inverted Organic Tandem Solar Cells

Multijunction solar cells are designed to improve the overlap with the solar spectrum and to minimize losses due to thermalization. Aside from the optimum choice of photoactive materials for the respective sub‐cells, a proper interconnect is essential. This study demonstrates a novel all‐oxide interconnect based on the interface of the high‐work‐function (WF) metal oxide MoO_x and low‐WF tin oxide (SnO_x). In contrast to typical p‐/n‐type tunnel junctions, both the oxides are n‐type semiconductors with a WF of 5.2 and 4.2 eV, respectively. It is demonstrated that the electronic line‐up at the interface of MoO_x and SnO_x comprises a large intrinsic interface dipole (≈0.8 eV), which is key to afford ideal alignment of the conduction band of MoO_x and SnO_x, without the requirement of an additional metal or organic dipole layer. The presented MoO_x/SnO_x interconnect allows for the ideal (loss‐free) addition of the open circuit voltages of the two sub‐cells.     

Enhanced Light Harvesting in Organic Solar Cells Featuring a Biomimetic Active Layer and a Self‐Cleaning Antireflective Coating

Advanced light manipulation is extremely attractive for applications in organic optoelectronics to enhance light harvesting efficiency. A novel method of fabricating high‐efficiency organic solar cells (OSCs) is proposed using biomimetic moth eye nanostructures in a quasi‐periodic gradient shape active layer and an antireflective coating. A 24.3% increase in photocurrent is realized without sacrificing dark electrical properties, yielding a 22.2% enhancement in power conversion efficiency to a record of 7.86% for OSCs with a poly(3‐hexylthiophene‐2,5‐diyl):indene‐C60 bis‐adduct (P3HT:ICBA) active layer. The experimental and theoretical characterizations verify that the substantial improvement of OSCs is mainly ascribed to the self‐enhanced absorption resulting from the broadband polarization‐insensitive light trapping in biomimetic nanostructured active layer, the reduction in reflectance by the antireflective coating, and surface plasmonic effect excited by corrugated metallic electrode. It is noteworthy that the pathway described here is promising for opening up opportunities to realize high‐performance OSCs towards the future photovoltaic applications.     

Lateral Organic Solar Cells with Self‐Assembled Semiconductor Nanowires

Solution‐processable organic semiconductor nanowires (NWs) offer a potentially powerful strategy for producing large‐area printed flexible devices. Here, the fabrication of lateral organic solar cells (LOSC) using solution‐processed organic NW blends on a flexible substrate to produce a power source for use in flexible integrated microelectronics is reported. A high photocarrier generation and an efficient charge sweep out are achieved by incorporating 1D self‐assembled poly(3‐hexylthiophene) NWs into the active layer, and an MoO₃ interfacial layer with high work function is introduced to increase the built‐in potential. These structures significantly increase the carrier diffusion/drift length and overall generated photocurrent in the channel. The utility of the LOSCs for high power source applications is demonstrated by using interdigitated electrode patterns that consist of multiple devices connected in parallel or in series. High photovoltage‐producing LOSC modules on plastic substrates for use in flexible optoelectronic devices are successfully fabricated. The LOSCs described here offer a new device architecture for use in highly flexible photoresponsive energy devices.     

Delicate Morphology Control Triggers 14.7% Efficiency All‐Small‐Molecule Organic Solar Cells

Morphology is a critical factor to determine the photovoltaic performance of organic solar cells (OSCs). However, delicately fine‐tuning the morphology involving only small molecules is an extremely challenging task. Herein, a simple, generic, and effective concentration‐induced morphology manipulation approach is demonstrated to prompt both the state‐of‐the‐art thin‐film BTR‐Cl:Y6 and thick‐film BTR:PC₇₁BM all‐small‐molecule (ASM) OSCs to a record level. The morphology is delicately controlled by subtly altering the prepared solution concentration but maintaining the identical active layer thickness. The remarkable performance enhancement achieved by this approach mainly results from the enhanced absorption, reduced trap‐assistant recombination, increased crystallinity, and optimized phase‐separated network. These findings demonstrate that a concentration‐induced morphology manipulation strategy can further propel the reported best‐performing ASM OSCs to a brand‐new level, and provide a promising way to delicately control the morphology towards high‐performance ASM OSCs.     

8.9% Single‐Stack Inverted Polymer Solar Cells with Electron‐Rich Polymer Nanolayer‐Modified Inorganic Electron‐Collecting Buffer Layers

Enhanced power conversion efficiency (PCE) is reported in inverted polymer solar cells when an electron‐rich polymer nanolayer (poly(ethyleneimine) (PEI)) is placed on the surface of an electron‐collecting buffer layer (ZnO). The active layer is made with bulk heterojunction films of poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7) and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM). The thickness of the PEI nanolayer is controlled to be 2 nm to minimize its insulating effect, which is confirmed by X‐ray photoelectron spectroscopy and optical absorption measurements. The Kelvin probe and ultraviolet photoelectron spectroscopy measurements demonstrate that the enhanced PCE by introducing the PEI nanolayer is attributed to the lowered conduction band energy of the ZnO layer via the formation of an interfacial dipole layer at the interfaces between the ZnO layer and the PEI nanolayer. The PEI nanolayer also improves the surface roughness of the ZnO layer so that the device series resistance can be noticeably decreased. As a result, all solar cell parameters including short circuit current density, open circuit voltage, fill factor, and shunt resistance are improved, leading to the PCE increase up to ≈8.9%, which is close to the best PCE reported using conjugated polymer electrolyte films.     

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.     

Origin of the S‐Shaped JV Curve and the Light‐Soaking Issue in Inverted Organic Solar Cells

Inverted organic solar cells generally exhibit a strong s‐shaped kink in the current–voltage characteristics (JV curve) that may be removed by exposure to UV light (light‐soaking) leading to a drastically improved performance. Using in‐device characterization methods the origin of the light‐soaking issue in inverted solar cells employing titanium dioxide (TiO₂) as an electron selective layer is clarified. An injected hole reservoir accumulated at the TiO₂/organic interface of the pristine device is observed from extraction current transients; the hole reservoir increases the recombination and results in an s‐shape in the JV curve of pristine devices. The hole reservoir and the s‐shape is a result of the energetics at the selective contact in the pristine device; the effect of UV exposure is to decrease the work function of the indium tin oxide/TiO₂‐contact, increasing the built‐in potential. This hinders the build‐up of the hole reservoir and the s‐shape is removed. The proposed model is in excellent agreement with drift‐diffusion simulations.     

Overcoming Space‐Charge Effect for Efficient Thick‐Film Non‐Fullerene Organic Solar Cells

Organic solar cells (OSCs) containing non‐fullerene acceptors have realized high power conversion efficiency (PCE) up to 14%. However, most of these high‐performance non‐fullerene OSCs have been reported with optimal active layer thickness of about 100 nm, mainly due to the low electron mobility (≈10^?4–10^?5 cm² V^?1 s^?1) of non‐fullerene acceptors, which are not suitable for roll‐to‐roll large‐scale processing. In this work, an efficient non‐fullerene OSC based on poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′′′‐diyl)] (PffBT4T‐2OD):EH‐IDTBR (consists of electron‐rich indaceno[1,2‐b:5,6‐b′]dithiophene as the central unit and an electron‐deficient 5,6‐benzo[c][1,2,5]thiadiazole unit flanked with rhodanine as the peripheral group) with thickness‐independent PCE (maintaining a PCE of 9.1% with an active layer thickness of 300 nm) is presented by optimizing device architectures to overcome the space‐charge effects. Optical modeling reveals that most of the incident light is absorbed near the transparent electrode side in thick‐film devices. The transport distance of electrons with lower mobility will therefore be shortened when using inverted device architecture, in which most of the excitons are generated close to the cathode side and therefore substantially reduces the accumulation of electrons in the device. As a result, an efficient thick‐film non‐fullerene OSC is realized. These results provide important guidelines for the development of more efficient thick‐film non‐fullerene OSCs.     

Understanding the External Quantum Efficiency of Organic Homo‐Tandem Solar Cells Utilizing a Three‐Terminal Device Architecture

A fundamental analysis of the external quantum efficiency (EQE) of organic tandem solar cells with equal absorbers in both subcells (homo‐tandem solar cells) is presented. Providing direct access to both subcells by introducing a conductive intermediate polymer electrode into the recombination zone, without changing the optical and electric device properties, the three‐terminal device becomes a proxy to the two‐terminal tandem solar cell properties. From the spectrally resolved EQE of the subcells in three‐terminal configuration wavelength and intensity of suitable bias light as well as bias voltage are determined that in turn allow for accurate EQE measurements of the common two‐terminal tandem solar cells. Theoretic considerations allow the prediction of the tandem solar cell's EQE from its subcells' EQEs as well as the prediction of the tandem cell EQE under monochromatic bias light illumination being in excellent agreement with experimental results. All findings discussed herein can be applied to more common hetero‐tandem solar cell architectures likewise.     

Additive‐Free Organic Solar Cells with Power Conversion Efficiency over 10%

Nowadays, solvent additives are widely used in organic solar cells (OSCs) to tune the nano‐morphology of the active blend film and enhance the device performance. With their help, power conversion efficiencies (PCEs) of OSCs have recently stepped over 10%. However, residual additive in the device can induce undesirable morphological change and also accelerate photo‐oxidation degradation of the active blend film. Thereby, their involvements are actually unfavorable for practical applications. Here, a donor material PThBDTP is employed, and PThBDTP:PC71BM based OSCs are fabricated. A PCE of over 10% is achieved without using any additives and film post‐treatments. The device displays a high open‐circuit voltage of 0.977 V, a large short‐circuit current density of 13.49 mA cm^‐2, and a high fill factor of 76.3%. These results represent an important step towards developing high‐efficiency additive‐free OSCs.     

Enhanced Device Efficiency of Bilayered Inverted Organic Solar Cells Based on Photocurable P3HTs with a Light‐Harvesting ZnO Nanorod Array

Periodically patterned zinc oxide nanorod (P‐ZnO NR) layers are directly prepared from a pre‐patterned ZnO seed layer using a polydimethylsiloxane (PDMS) elastomeric stamp and then applied in inverted organic photovoltaic devices (IOPVs). The IOPV is assembled with a hydrothermally grown zinc oxide nanorod patterns with a (100) preferential crystal orientation as an electron transport buffer layer (ETBL) and photoactive bilayer consisting of methacylate end‐functionalized poly(3‐hexylthiophene) (P3HT‐MA), phenyl‐C₆₀‐butyric acid methyl ester (PC₆₀BM) and indene‐C₆₀ bis‐adduct (IC₆₀BA). In te IOPVs, the P‐ZnO NR is found to induce efficient light harvesting and the photocrosslinkable P3HTs afford solution‐processed bilayer architecture in IOPVs to show improved device stability and performance (PCE_max= 5.95%), as the bilayered structure allowed direct exciton splitting, thus reducing the charge recombination.     

Nonfullerene n‐Type Organic Semiconductors for Perovskite Solar Cells

Compared to inorganic semiconductors and/or fullerene derivatives, nonfullerene n‐type organic semiconductors present some advantages, such as low‐temperature processing, flexibility, and molecule structure diversity, and have been widely used in perovskite solar cells (PSCs). In this research news article, the recent advances in nonfullerene n‐type organic semiconductors which function as electron‐transporting, interface‐modifying, additive, and light‐harvesting materials in PSCs are summarized. The remaining challenges and promising future directions of nonfullerene‐based PSCs are also discussed.