Power Conversion Efficiency Of Device Research Articles

Layer‐by‐Layer‐Processed Organic Solar Cells with 18.02% Efficiency Enabled by Regulating the Aggregation of Bottom Polymers

The fabrication of organic solar cells (OSCs) by a layer‐by‐layer (LBL) method has attracted growing attention in recent years. As already known, the pre‐aggregates of conjugated polymers in solution have a profound impact on their microstructure morphology in films. Herein, by simply controlling the solution temperature and annealing processes, the pre‐aggregation behavior of D18 polymer in solution can be fine‐tuned and the microstructure of D18 bottom layer is well manipulated. The optimized D18 bottom layer can effectively regulate L8‐BO upper‐layer‐forming suitable networks for efficient charge transportation. In addition, a vertical phase separation with a special D/D:A/A structure (P‐i‐N‐type component distribution) is also formed. As a result, compared to the 16.43% power conversion efficiency (PCE) of the bulk heterojunction devices, such control enables bilayer OSC devices based on the polymer D18 and L8‐BO to deliver an enhanced PCE of 18.02% with simultaneously improved short‐circuit current density, open‐circuit voltage, and fill factor. It is also demonstrated in these results that the LBL deposition process utilizing the pre‐aggregation of polymer and its fiber‐network‐forming ability is a very promising approach to improve charge dynamics, suppress carrier recombination, and fabricate highly efficient OSCs.

Theoretical investigation of silicon thin film solar cell for improving short and long wavelength response

Theoretical investigation or numerical simulation of performance of a solar cell can provide useful information to maximize power conversion efficiency. In that respect we carried out a set of numerical simulation using AFORS-HET simulation program. Separately the optical absorption of the individual layers of thin film solar cell was analyzed. Current-voltage characteristic curve of a reference cell (Cell-A) was used as the starting device. The power conversion efficiency (PCE) of the reference device was 8.85% with short circuit current density (Jsc) of 15.43 mA/cm2 and fill factor (FF) of 68.3%. After suitable optimization the PCE of this device (Cell-B2) improves to 11.59% (Jsc and FF became 13.0 mA/cm2 and 87% respectively). The results show that the effective optical absorption in the active layer can be improved significantly by optimizing the device structure. The short wavelength response can be improved by reducing the parasitic optical absorption of the doped window layer, while the long wavelength response improves by raising effective absorption length of the active layer. Furthermore, optimum thickness of the active layer, for the highest possible PCE, is found to be dependent upon its defect density.

Thermoelectric device performance beyond average ZT: Holistic consideration of materials and design

In the current era where many renewable energy technologies are commercially mature and optimized, thermoelectrics remain relatively under-utilized. This can be partly attributed to their limited device efficiencies despite their high materials performances. On the materials level, the performance of thermoelectrics can be defined by zT or average zT, which is directly correlated to device power conversion efficiency. Due to this direct correlation, it is a longstanding assumption that maximizing average zT will result in higher efficiency. Although this practice holds true for many cases, it does not provide a complete picture due to the complex interplay between electronic and thermal transport properties. In this work, we surveyed the literature to find that high average zT does not always translate to high efficiency. Instead, it was found that compatibility factor, which dictates the optimal ratio of electrical and heat current passing through the material, is equally important in determining the device efficiency. Other factors such as electrical and thermal impedance matching, as well as contact resistance are also discussed. This review concludes with food for thought on future thermoelectric research beyond average zT to realize the untapped potential in this special class of materials.

Enhancement of photovoltaic performance of solvent-free dye-sensitized solar cells with doped poly(3-hexylthiophene)

A hole transporter based on a mixture of an organic semiconductor, poly(3-hexylthiphene) (P3HT), and ionic liquid, 1-methyl-3-propylimidazolium iodide (MPII), was doped with metal bis(trifluoromethanesulfonyl)imides (TFSIs), including LiTFSI, CuTFSI2, and MgTFSI2, and then applied to solvent-free dye-sensitized solar cells (DSCs) together with an additive. The incorporation of metal TFSIs into the P3HT/MPII-based hole transporter led to a significant increase in the short-circuit current (Jsc) values of solvent-free DSCs compared to that of the reference cell with an undoped hole transporter. It was revealed that the internal resistances of the cells sharply decreased with the doping of metal TFSIs, thus inducing a considerable enhancement of the Jsc values. As a result, compared to power conversion efficiency of the reference device, a higher—more than 200%—efficiency was achieved in a solvent-free DSC with a CuTFSI2-doped hole transporter.

N-Annulated Perylene Diimide Non-Fullerene Acceptors for Organic Photovoltaics

This work covers the development of non-fullerene acceptors for use in organic photovoltaics built using the N-annulated perylene diimide dye. The classic perylene diimide dye has been extensively used to construct non-fullerene acceptors, leading to device power conversion efficiencies of over 10%. Strong visible light absorption and deep frontier molecular energy levels have made such materials (both molecular and polymeric) near ideal for pairing with narrow-gap conjugated polymers in bulk-heterojunction active layers. The N-annulation of the dye provides an extra site for side-chain engineering and alters the electronic structure of the polycyclic aromatic core. In addition, N-annulation allows for selective bromination of the perylene core, leading to building blocks that are useful for the construction of large molecular frameworks using the atom-economical direct heteroarylation cross-coupling method. Herein, we detail a series of molecules developed by our team that are based on the N-annulated perylene diimide in the form of dimers with different cores (both electron-rich and electron-deficient); dimers with varied side chains; tetramers with varying geometries; and large, asymmetric molecules with internal energy cascades. The use of these molecules as non-fullerene acceptors in organic photovoltaic devices (binary and ternary blends, outdoor and indoor light applications, and spin-coated vs. slot-die-coated photoactive layers) is presented.

Modulation of Dielectric Constant and Photovoltaic Properties of 2,1,3‐benzothiadiazole‐based Alternating Copolymers by Adding Fluorine Atoms to the Backbone of Polymers

AbstractIn organic solar cells (OSCs), fluorine (F) substitution of conjugated polymers has a variety of effects on molecular properties and device performance. In this work, three conjugated polymers, namely PBDT‐4F‐BTs, PBDT‐8F‐FBTs, and PBDT‐10F‐FBTs, with 4, 8 and 10 fluorine atoms in the backbone were prepared and characterized. The conjugated polymers PBDT‐4F‐BTs, PBDT‐8F‐FBTs and PBDT‐10F‐FBTs showed the same highest occupied molecular orbital (HOMO) energy levels of −5.58 eV, which did not decrease with the increasing number of F atoms in each repeat unit of polymers. As the number of fluorine atoms in each repeat unit of polymers in the backbone increases from 4 to 10, the dielectric constant (ϵr) increases from 4.5 to 6.2 at 1 kHz. The bimolecular compounding was suppressed by introducing F into the polymer backbone and influenced the device compounding kinetics of OSCs from the blend films of polymers paired with the phenyl‐C71‐butyric‐acid‐methyl ester (PC71BM). Thereby the fill factor (FF) and power conversion efficiency (PCE) of the OSCs devices have been improved, the fluorine atoms in each repeat unit of polymers backbone increased from 4 to 10, FF increased from 48.7 % to 70.0 %, and the corresponding PCE increased from 3.43 % to 6.46 %.

Slot-Die-Coated Active Layer for Printed Flexible Back-Contact Perovskite Solar Cells

Perovskites have already shown potential as active layers in photovoltaic applications. Furthermore, a low-cost and simple solution processing technology allows perovskites to be used in flexible and printed electronics. Perovskite solar cells (PSC) with a back-contact (BC) structure, in which the electrode system is based on a quasi-interdigitated back-contact (QIBC) design, promise to increase the power conversion efficiency (PCE) of devices beyond those that can be obtained using PSCs with a traditional sandwich structure. While the spin-coating technique is used to deposit the perovskite layer of lab-scale BC PSCs, the application of large-area printing techniques to deposit the perovskite layer of such devices is yet to be explored. Therefore, this work demonstrates an application of the slot-die coating technique to print the perovskite active layer of BC PSCs with QIBC electrodes on flexible polymer substrates. The morphology of the obtained perovskite films on QIBC electrodes are investigated and the primary photoelectric parameters of the resulting BC PSCs are measured. The charge carrier recombination processes in the fabricated BC PSCs are investigated and the dominant mechanism for carrier loss in the devices is determined. The findings of the work give an insight into the properties of the slot-die-coated perovskite active layer of BC PSCs and points to exciting new research opportunities in this direction.

Multi‐Bandgap Quantum Dots Ensemble for Near‐Infrared Photovoltaics

Narrow bandgap quantum dots (QDs) are an important class of materials for near‐infrared (NIR) optoelectronic devices owing to their size‐tunable bandgap and chemical root processing. In photovoltaic applications, NIR QDs could be particularly useful to complement the sub‐bandgap transmission loss of NIR solar radiation from perovskite and c‐Si solar cells. However, insufficient carrier extraction thickness associated with the narrow NIR excitonic bandwidth of QDs limits the conversion efficacy of the broad NIR solar spectrum. Here, we utilize a multi‐bandgap QD ensemble which widens the NIR absorption bandwidth to mimic the broad solar spectrum. A solution‐phase ligand passivation strategy is used to control doping properties and energy level alignment of multi‐bandgap QDs. We successfully developed bulk‐heterojunction solar cells using the multi‐bandgap QD ensemble, which yields higher carrier extraction thickness and broader NIR absorption. The gain from NIR absorption and carrier transport resulted in higher short‐circuit current generation and power conversion efficiency (PCE) in solar cell devices. The champion device shows 8.73% PCE under 1.5 AM solar illumination and 7.44% and 5.05% PCE for the NIR photons transmitted from perovskite and c‐Si layers.

Enhanced Fill Factor and Power Conversion Efficiency of Single Oxide Ferroelectric Photovoltaic Devices with Designed Nanostructures

AbstractThe ferroelectric photovoltaic effect has promising potential for the next generation of solar cells. However, due to disadvantages such as wide bandgap and low fill factor (FF), the power conversion efficiency (PCE) values reported in ferroelectric photovoltaic devices remain considerably below expectations. Herein, enhanced photovoltaic effect in the films with the nanostructure of ferroelectric nanocrystalline particles embedded in the amorphous or poor crystalline matrix is investigated. The nanostructures are realized by controlled crystallization and doping in Zn0.92‐xCux(Fe0.04Li0.04)O (ZCFLO) films. Benefiting from the improved carrier dynamic regulation in ferroelectric/boundary nanostructures and narrowed bandgap, the designed ZCFLO photoferroelectrics films exhibit high efficiency photovoltaic effect under AM 1.5G light, manifesting above‐bandgap photovoltage, markedly improved FF (83.4%), switchable photoresponse (50.3 mA W−1), and high PCE (14.4%). Meanwhile, the simple method presented in this work is fully compatible with large‐scale manufacturing processes and may find applications in cost‐efficiency optoelectronic devices.

Bifunctional modified biopolymer for highly efficient and stable perovskite solar cells and modules

Perovskite solar cells are promising and competitive candidates for future photovoltaic technologies. However, the fabrication of high-quality, efficient, large-area perovskite solar cell modules (PSCMs) is still one of the most significant challenges for future commercialization. In this study, we introduce a sulfonic acid group into poly-l-lysine (PLL) via a one-step ring-opening reaction, and the modified PLL with a sulfonic acid group (PLLS) is expected to synergistically modify the interface between the TiO2 and perovskite layers. The sulfonic acid group in PLLS can modify the electron transport layer by inactivating the under-coordinated titanium (Ti) ion, which reduces the oxygen vacancies and defects on the surface of the TiO2 film and increases the conductivity. In addition, amine groups can passivate deep-level defects on the surface of perovskite to reduce non-radiative recombination. The power conversion efficiency (PCE) of a unit device (0.14 cm2) is increased from 20.60 % to 22.98 % after PLLS modification. The unencapsulated device retained ∼ 80 % of its initial efficiency after 960 h in ambient air with humidity of approximately 10 %. Moreover, the PSCMs achieved a PCE of 20.07 % (with a notable fill factor of 76.75 %) over an active area of 33 cm2 and showed excellent operating stability.

Effects of Water/Alcohol Soluble Cationic Polythiophenes as Cathode Interlayers for Eco‐Friendly Solar Cells

AbstractThree new ionic polythiophene derivatives, soluble in polar solvents, are synthesized with good yields using simple, low‐cost, and straightforward procedures. They are investigated as interfacial cationic conjugated polyelectrolyte (CPE) layers for halogen‐free bulk heterojunction polymeric solar cells, based on a water‐soluble electron‐donor polymer (poly[3‐(6‐diethanolaminohexyl)thiophene]) and a water‐soluble electron‐acceptor fullerene derivative (malonodiserinolamide fullerene). The simple insertion of the CPE interlayer between the active layer and the aluminum cathode dramatically increases the power conversion efficiency of the final device up to nearly 5%, resulting from a decrease of the electrode work function, improved electron extraction, and optimization of the morphology of the layers. The obtained results demonstrate that the incorporation of CPE layer is a powerful and convenient methodology for the development of highly efficient and eco‐friendly processable polymeric solar cells.

Humidity-Insensitive, Large-Area-Applicable, Hot-Air-Assisted Ambient Fabrication of 2D Perovskite Solar Cells.

2D layered perovskites (LPs) have shown great potential to deliver high-performance photovoltaic devices with long-term stability. Despite many signs of progress being made in film quality and device performance, LP films are mainly processed in strict conditions and through non-scalable techniques. Here, the hot-air-assisted ambient fabrication technique is introduced to prepare LP films for efficient and stable solar cells. The high-quality LP films with good crystallinity, preferable orientation and desirable morphology are obtained by balancing the crystal nucleation and growth processes. Employing the synchrotron-based in situ grazing-incidence X-ray diffraction technique, hot air induces the solidification of solutes and forms an intermediate at the air-liquid interface, which transforms into 3D-like perovskite, followed by the growth of the 2D species toward the substrate. The optimal LP film delivers a device power conversion efficiency of 16.36%, the best value for the LP-based solar cells prepared by the non-spin-coating techniques. The solar cell performance is insensitive to the film processing humidity and the device size is upscalable, which promises real-world deployment of LP-based optoelectronic devices.

Growth of (Ag,Cu)(In,Ga)Se2 Absorbers under Band Gap Variation and Characterization with a Focus on Optical Spectroscopy

Whilst Cu(In,Ga)Se2 (CIGSe) is an extremely promising material for solar cell fabrication, the widening of the band gap beyond the standard 1.1 eV is highly desirable for semitransparent applications. By replacing Cu with Ag and increasing the Ga content, we fabricate ACIGSe absorbers with band gaps ranging from 1.27–1.55 eV. An Ag/(Ag + Cu) ratio from 0.36–1.00 is chosen, as well as a Ga/(Ga + In) ratio from 0.25–0.59. The larger Ag and Ga contents lead to the expected band gap widening, which is, together with high sub-gap transparency, essential for semitransparent applications. The crystalline properties are confirmed by Raman spectroscopy and X-ray diffraction, which both reveal peak shifts according to the composition variations: a higher Ag content results in lower Raman shifts as well as in lower angles of X-ray diffraction for the main peaks due to the larger mass of Ag compared to Cu and the larger lattice constant of Ag-rich compounds. Increased open circuit voltages and decreased short circuit current densities are confirmed for higher band gaps. An overall trend of increased power conversion efficiency of the related devices is promising for future research of wide band gap Ag-chalcopyrites and their semitransparent application.

Polymeric solar cell with 18.06% efficiency based on poly(para-nitroaniline)/TiO2 composites

In this paper, the poly para-nitro aniline [PPNN]/titanium (IV) oxide [TiO2]NPs composite was synthesized and investigated followed by the fabrication of thin film. The sol gel and physical vapor deposition (PVD) techniques have been successfully used to deposit [PPNN/TiO2]MNC thin film with good adhesion and film thickness ≅ 100 ± 3 nm. The structural and morphological properties of [PPNN/TiO2]MNC thin films were studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). The measured optical properties of [PPNN/TiO2]MNC thin films as Reflectance (R) in the UV–vis–NIR spectrum, Absorbance (Abs), and Transmittance (T) have been employed to probe the optical characteristics at room temperatures. As well as the calculations of TD-DFT (time-dependent density functional theory) and the optimization through TD-DFT/Mol3 and Cambridge Serial Total Energy Bundle (TD-FDT/CASTEP) were employed to study the geometrical characteristics. The rectifier ration (RR), indeality factor (n) and barrier potential ΦB (eV) values are 0.74 × 105, 2.98 and 1.05 at 375 K, respectively. The Au/[PPNN/TiO2]MNC/p-Si/Al heterojunction device's power conversion efficiency are 18.06 at 200 W. cm−2. The obtained results candidates [PPNN/TiO2]MNC thin film as a good material for solar cells and optoelectronic devices.

Recent Developments of Polymer Solar Cells with Photovoltaic Performance over 17%

AbstractWith the emergence of ADA'DA‐type (Y‐series) non‐fullerene acceptors (NFAs), the power conversion efficiencies (PCEs) of organic photovoltaic devices have been constantly refreshed and gradually reached 20% in recent years (19% for single junction and 20% for tandem device). The acceptors possess specific design concept, which greatly enrich the NFA types and have excellent compatibility with many donor materials. It is gratifying to note that the previously underperforming donor materials combine with these regulated acceptors to shine again. Nowadays, the concept of modular design is widely used in the research of acceptors and donors, injecting new vitality into the field of organic photovoltaics. Furthermore, these acceptors also promote the research of multicomponent devices, tandem devices, bilayer devices, processing solvent engineering, and additive engineering. Herein, the latest progresses of polymer solar cells with efficiency over 17% are briefly reviewed from the aspects of active material design, interface material development, and device technology. At last, the opportunities and challenges of organic photovoltaic commercialization in the future are discussed.

Phase Stability, Band Gap Tuning, and Rashba Splitting in Selenium-Alloyed Bournonite: CuPbSb(S1–xSex)3

Recently, bournonite (CuPbSbS3) has been identified as a potential ferroelectric photovoltaic (PV) material with a 1.3 eV band gap, which falls in an appropriate range for single-junction PV devices. Progress in applying bournonite has yielded several studies reporting successful thin-film processing, the most recent of which demonstrated a 2.65% power conversion efficiency for a bournonite-based PV device. In an effort to explore bournonite band gap engineering for PV and other prospective applications (e.g., thermoelectric, spintronic), we here consider the solid-state synthesis of selenium-alloyed bournonite, CuPbSb(S1–xSex)3, across the full range of x (0.0 ≤ x ≤ 1.0) and report phase purity for 0.0 ≤ x ≤ 0.5. We characterize the crystal structure and band gap of the samples using X-ray diffraction and diffuse reflectance spectroscopy, determining a band gap decrease for single-phase samples from 1.25 eV at x = 0.0 to 1.06 eV at x = 0.5. Formation energies determined using dispersion-corrected hybrid density functional theory (DFT) support experimental findings and are consistent with the stability (instability) of the x = 0.5 (x = 1.0) structure relative to starting materials. The computed band structures show a decreasing trend in the band gap with increasing x, again consistent with experiment, with the states at the conduction (valence) band minima (maxima) being principally derived from Pb (S/Se) states. Spin texture analysis and detailed comparison of spin splitting parameters show that Se alloying modifies the band gap while maintaining Rashba spin splitting character within the electronic structure. Rashba splitting is most pronounced for the conduction band minimum along the Γ–X path in the Brillouin zone. Energy separation between spin states is maintained at ΔE ∼0.2 eV for the various Se contents, with σz as the spin direction. A smaller spin splitting occurs along Γ–Z (decreasing value with increasing x), with σx as the spin direction.

Interfacial Contact Management in Perovskite Solar Cells with Carbon Top Electrode

Perovskite solar cells (PSCs) represent a promising photovoltaic technology that has great potential for mass application. The main advantages of the perovskite solar cells are related to simplicity of their fabrication, tunable optical properties, and low cost of active materials. For further reducing the device costs, graphite‐based contacts are often used to replace an energy‐inefficient deposition of metal contacts, which is a trade‐off between the cost and power conversion efficiency (PCE) of devices. Herein, a new approach to increase the PCE of hole transport layer‐free perovskite cells with carbon‐based electrodes by the sequential treatment of the perovskite absorber layer and perovskite/carbon electrode interface with methylammonium iodide is presented. The proposed passivation technique leads to a dramatic improvement in the device power conversion efficiency from the initial 3.9% to 7.9%. The origin of the enhanced device performance is attributed to the passivation of defects on the perovskite/carbon interface and, most importantly, improving the electrical contact between the top electrode and absorber layer.

Air-processed MAPbI3 perovskite solar cells achieve 20.87% efficiency and excellent bending resistance enabled via a polymer dual-passivation strategy.

In recent years, air-processed MAPbI3 perovskite solar cells (PSCs) have attracted widespread interest from researchers worldwide because of their simple and low-cost fabrication process. Nonetheless, the ambient conditions usually bring about many adverse effects, such as imperfect crystallization and numerous defects in perovskite films, which seriously impact both the photoelectric performance and stability of the device. Therefore, in this work, a polymer dual-passivation strategy was employed by introducing ammonium polyphosphate (APP) as an additive to the green anti-solvent to accurately modify the perovskite layer. APP, which has abundant phosphate and ammonium groups, could simultaneously fill the I/Pb vacancies by Lewis acid-base reactions to restrain defect formation and improve the power conversion efficiency (PCE) of the ultimate device. On the other hand, the long molecular chains of the polymer with a certain flexural ability were easily congregated at the grain boundaries of the perovskite grains, thus enhancing the bending resistance. Consequently, high-quality perovskite films with a dense morphology and large grain size were obtained. Because of the reduced defect density and suppressed non-radiative recombination, the optimal PSC attained a champion PCE of 20.87% with negligible hysteresis. Furthermore, the non-encapsulated APP-modified flexible device also exhibited excellent bending resistance. Only 20% of its normalized PCE was lost after 150 bending cycles at room temperature. This simple, green, low-cost, and reliable strategy for preparing high-efficiency PSCs with good stability can facilitate its commercialization.

Low-dimensional antimony selenosulfide as an emerging material for solar cell applications

Antimony chalcogenides (Sb2X3), including Sb2S3, Sb2Se3, and the alloy-type Sb2(S,Se)3, have been considered as a promising absorber materials for photovoltaic applications. Owing to its unique quasi-one-dimensional crystal structure, it displays distinct defect and carrier transport properties and requires special material synthesis strategy compared with the traditional three-dimensional crystal structure semiconductor materials. Recent studies on this class of materials have generated new understandings in film fabrication, defect characteristics and passivation, interfacial engineering, and efficiency improvement. With these efforts, the power conversion efficiency of the solar cell device has been increased from below 3% to 10.7% over the past 10 years. This efficiency achievement suggests that Sb2X3 possesses great potential for practical applications with further efficiency enhancement. This perspective article presents the critical development in the Sb2X3 materials and solar cells in recent years, including the unique crystal structure for solar cells, the preparation method for obtaining high-quality Sb2X3 films, and the discovery and passivation of unusual and complex defects. Finally, we propose several strategies for future efficiency improvement.

Spontaneously spreading film process to improve the photovoltaic performance of organic solar cells with PHJ structure.

Efficient charge transport and extraction within the active layer plays a major role in the photovoltaic performance of organic solar cells (OSCs). In this work, the spontaneously spreading (SS) process was utilized to achieve sequential deposition of the active layer with a planar heterojunction (PHJ) structure. The SS process avoids the damage of the upper layer solution to the lower layer film by the spin coating process. The film with PHJ structure exhibits notable vertical phase separation compared to the bulk heterojunction (BHJ). Moreover, the power conversion efficiency (PCE) of the PHJ device (12.00%) is significantly higher than that of the BHJ (10.84%) due to the efficient charge transport. This work offers a novel fabrication method and device structure to enhance the photovoltaic performance of OSCs.