High-efficiency Organic Photovoltaics Research Articles

High-Efficiency Organic Photovoltaic Cells With an Antimony Quantum Sheet Modified Hole Extraction Layer

2D materials with unique opto-electronic properties have shown immense potential for application in organic photovoltaics (OPVs). In this article, we reported a facile and efficient strategy to enhance the device performance of OPVs by doping 2D material of antimonene quantum sheets (AMQSs) into PEDOT:PSS emulsion. The interfacial properties between the hole extraction layer (HEL) and PBDB-T:ITIC-based active layer were properly meliorated, achieving boosted power conversion efficiency ( PCE ). Results indicate that the introduced AMQSs could help suppress the recombination loss and improve charge extraction efficiency. Over 8.09% enhancement of PCE is observed compared to that of the control cell with pure PEDOT:PSS. This work provides a simple and valid strategy of HEL modification to realize high efficiency OPVs.

High-Efficiency Indoor Organic Photovoltaics with a Band-Aligned Interlayer

Summary The emergence of indoor electronic devices for internet of things (IoT) has motivated the scientific community to develop photovoltaic devices that can efficiently convert indoor light into electricity. In this paper, we report high-efficiency non-fullerene organic photovoltaic (OPV) cells with over 30% power conversion efficiency (PCE) under indoor conditions. Our results show that the choice of electron-transporting layer (ETL) is critically important to enable such performance. The use of an ETL (named PDI-NO) with a deep highest occupied molecular orbital (HOMO) level can effectively suppress leakage current and reduce trap-assisted recombination of the devices. Thus, using this ETL, we achieve record PCE of 31% by utilizing a low-band-gap acceptor in the bulk-heterojunction (BHJ) blend. Whereas, in another case, by employing a large-band-gap acceptor, a PCE of 26.7% with over 1V is achieved. Our study paves the way toward high-performance indoor OPV devices for powering IoT electronics.

Reducing the Singlet-Triplet Energy Gap by End-Group π-π Stacking Toward High-Efficiency Organic Photovoltaics.

To improve the power conversion efficiencies for organic solar cells, it is necessary to enhance light absorption and reduce energy loss simultaneously. Both the lowest singlet (S1) and triplet (T1) excited states need to energertically approach the charge-transfer state to reduce the energy loss in exciton dissociation and by triplet recombination. Meanwhile, the S1 energy needs to be decreased to broaden light absorption. Therefore, it is imperative to reduce the singlet-triplet energy gap (ΔEST ), particularly for the narrow-bandgap materials that determine the device T1 energy. Although maximizing intramolecular push-pull effect can drastically decrease ΔEST , it inevitably results in weak oscillator strength and light absorption. Herein, large oscillator strength (≈3) and a moderate ΔEST (0.4-0.5 eV) are found for state-of-the-art A-D-A small-molecule acceptors (ITIC, IT-4F, and Y6) owing to modest push-pull effect. Importantly, end-group π-π stacking commonly in the films can substantially decrease the S1 energy by nearly 0.1 eV, but the T1 energy is hardly changed. The obtained reduction of ΔEST is crucial to effectively suppress triplet recombination and acquire small exciton dissociation driving force. Thus, end-group π-π stacking is an effective way to achieve both small energy loss and efficient light absorption for high-efficiency organic photovoltaics.

Bandgap Tailored Nonfullerene Acceptors for Low-Energy-Loss Near-Infrared Organic Photovoltaics

A series of A−π–D−π–A-type nonfullerene acceptors (NFAs) was designed and synthesized with the goal of optimizing light absorption and energy losses in near-infrared (NIR) organic solar cells (OSCs) principally through the use of side-chain engineering. Specific molecules include p-IO1, o-IO1, p-IO2, and o-IO2 with optical bandgaps of 1.34, 1.28, 1.24, and 1.20 eV, respectively. Manipulating the optoelectronic properties and intermolecular organization by substituting bulky phenylhexyl (p-) for linear octyl chains (o-) and replacing bisalkoxy (-O2) with alkyl-alkoxy combination (-O1) allows one to target energy bandgaps and achieve a favorable bulk heterojunction morphology when in the presence of the donor polymer PTB7-Th. Solar cells based on o-IO1 and PTB7-Th exhibit an optimal power conversion efficiency of 13.1%. The excellent photovoltaic performance obtained with the o-IO1 acceptor can be attributed to a short-circuit current of 26.3 mA cm–2 and energy losses on the order of 0.54 eV. These results further highlight how side-chain engineering is a straightforward strategy to tune the molecular design of n-type molecular semiconductors, particularly in the context of NIR high-efficiency organic photovoltaics.

An energy and charge transfer synergetic donor–acceptor heterostructure 2D-COF in photovoltaics

The marriage of fluorescence resonance energy transfer and a charge transfer synergetic D–A heterostructure and a wafer-scale 2D-COF film offers unparalleled advantages in high-efficiency organic photovoltaics (OPV).

Multi-channel exciton dissociation in D18/Y6 complexes for high-efficiency organic photovoltaics

Excitons in D18/Y16 solar cells can be efficiently dissociated under small hole-transfer driving forces via multiple channels.

Synthesis of Alkoxyacene‐Based Random Copolymers and Binary Solvent Additive for High Efficiency Organic Photovoltaics

AbstractThe synthesis of three random copolymers—BD2FON‐OB30, BD2FON‐OB50, and BD2FON‐OB70—comprised of different molar ratios of alkoxyphenylene and alkoxynaphthalene is reported by adjusting the ratio of these units for optimized energy levels. When organic photovoltaics are fabricated using these polymers as the electron donor, introduction of the binary solvent additive dramatically enhances power conversion efficiency (PCE) up to 7%, compared to those of the devices without additive (≈2% of PCEmax) and with single additive (≈3% of PCEmax). (without additive: 1–2% and with single additive: 0–3%). These improvements result from minimized bimolecular recombination and balanced electron/hole mobility ratio by optimized bicontinuous D:A morphology in the active layer. Thus, the strategy shows the efficient enhancement of device efficiency by control the morphology using binary solvent additives in the bulk heterojunction film consisting of polymer and fullerene.

Building Blocks for High-Efficiency Organic Photovoltaics: Interplay of Molecular, Crystal, and Electronic Properties in Post-Fullerene ITIC Ensembles.

Accurate single-crystal X-ray diffraction data offer a unique opportunity to compare and contrast the atomistic details of bulk heterojunction photovoltaic small-molecule acceptor structure and packing, as well as provide an essential starting point for computational electronic structure and charge transport analysis. Herein, we report diffraction-derived crystal structures and computational analyses on the n-type semiconductors which enable some of the highest efficiency organic solar cells produced to date, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC) and seven derivatives (including three new crystal structures: 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-propylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC-C3), 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (m-ITIC-C6), and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-butylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC-C4-4F). IDTT acceptors typically pack in a face-to-face fashion with π-π distances ranging from 3.28-3.95 Å. Additionally, edge-to-face packing is observed with S⋯π interactions as short as 3.21-3.24 Å. Moreover, ITIC end group identities and side chain substituents influence the nature and strength of noncovalent interactions (e. g. H-bonding, π-π) and thus correlate with the observed packing motif, electronic structure, and charge transport properties of the crystals. Density functional theory (DFT) calculations reveal relatively large nearest-neighbor intermolecular π-π electronic couplings (5.85-56.8 meV) and correlate the nature of the band structure with the dispersion interactions in the single crystals and core-end group polarization effects. Overall, this combined experimental and theoretical work reveals key insights into crystal engineering strategies for indacenodithienothiophene (IDTT) acceptors, as well as general design rules for high-efficiency post-fullerene small molecule acceptors.

17% Efficient Organic Solar Cells Based on Liquid Exfoliated WS2 as a Replacement for PEDOT:PSS.

The application of liquid-exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene-based organic solar cells is reported. It is shown that solution processing of few-layer WS2 or MoS2 suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS2 are found to exhibit higher uniformity on ITO than those of MoS2 and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short-circuit current (JSC ), and lower series resistance than devices based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoS2 . Cells based on the ternary bulk-heterojunction PBDB-T-2F:Y6:PC71 BM with WS2 as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open-circuit voltage of 0.84 V, and a JSC of 26 mA cm-2 . Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS2 -based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high-efficiency organic photovoltaics.

Fluctuations in the Emission Polarization and Spectrum in Single Chains of a Common Conjugated Polymer for Organic Photovoltaics

Measuring the nanoscale organization of conjugated polymer chains used in organic photovoltaic (OPV) blends is vital if one wants to understand the materials. This is made very difficult with high efficiency OPV polymers such as PTB7 that form aggregates, as a lack of periodicity and a high degree of disorder make understanding of the nanoscale organization challenging. Here, single molecule spectroscopy is used to observe single chains and aggregates of PTB7. Using four detectors the photoluminescence intensity, wavelength, polarization, and lifetime are simultaneously monitored. Fast (milliseconds) and slow (seconds) fluctuations are observed over a time window of 30 s in all of these observables from single aggregates and chains as individual chromophores activate and deactivate, leading to dynamical changes in the emission spectrum and dipole orientation. This information can be used to help reconstruct the spatial and spectral organization of disordered aggregates of PTB7, thereby adding valuable new information on how the chains are arranged in space.

Multiple Roles of a Non-fullerene Acceptor Contribute Synergistically for High-Efficiency Ternary Organic Photovoltaics

Summary Ternary structure is an important design strategy to obtain high-efficiency non-fullerene organic photovoltaics (OPVs). However, the role of the third component to the standard binary system is still unclear. Here, a wide-bandgap small-molecule acceptor, denoted IDT-T, is synthesized and used together with a wide-bandgap donor polymer, PBDB-T, and a low-bandgap acceptor, ITIC, for fullerene-free ternary solar cells. The ternary cell features an enhanced power conversion efficiency (PCE) up to 12.2%, together with improved photocurrent density, open-circuit voltage (VOC), and fill factor. Studies of the thin films indicate that IDT-T functions as an energy-level mediator, a fluorescence resonance energy-transfer donor, an electron acceptor, and a crystallization modulator in the blend, which contribute synergistically in the ternary blend to deliver a higher VOC, more efficient exciton generation, suppressed bimolecular charge recombination and enhanced charge transport, and an overall high photovoltaic performance.

Donor–Acceptor–Acceptor's Molecules for Vacuum‐Deposited Organic Photovoltaics with Efficiency Exceeding 9%

AbstractThree vacuum‐deposited donor–acceptor–acceptor (d–a–a') small molecule donors are studied with different side chains attached to an asymmetric heterotetracene donor block for use in high efficiency organic photovoltaics (OPVs). The donor with an isobutyl side chain yields the highest crystal packing density compared to molecules with 2‐ethylhexyl or n‐butyl chains, leading to the largest absorption coefficient and short circuit current in an OPV. It also exhibits a higher fill factor, consistent with its preferred out‐of‐plane molecular π–π stacking arrangement that facilitates charge transport in the direction perpendicular to the substrate. A power conversion efficiency of 9.3 ± 0.5% is achieved under 1 sun intensity, AM 1.5 G simulated solar illumination, which is significantly higher than 7.5 ± 0.4% of the other two molecules. These results indicate that side chain modification of d–a–a' small molecules offers an effective approach to control the crystal packing configuration, thereby improving the device performance.

Amplitude-Mode Spectroscopy of Charge Excitations in PTB7 π -Conjugated Donor-Acceptor Copolymer for Photovoltaic Applications

The authors show that the photogenerated polarons in the PTB7/PCBM copolymer blend, used in organic photovoltaic (OPV) solar cells, couple strongly to six vibrational modes that can be described by the amplitude-mode model. From the relative strengths of the doping-induced absorption of the polaron vibrations and electronic transitions, the polaron's effective mass is obtained, and its influence on carrier mobility unraveled. This understanding of the photophysics of such blends is the next step in engineering lightweight, low-cost, high-efficiency OPV solar panels.

Role of Crystallization in the Morphology of Polymer:Non-fullerene Acceptor Bulk Heterojunctions.

Many high efficiency organic photovoltaics use fullerene-based acceptors despite their high production cost, weak optical absorption in the visible range, and limited synthetic variability of electronic and optical properties. To circumvent this deficiency, non-fullerene small-molecule acceptors have been developed that have good synthetic flexibility, allowing for precise tuning of optoelectronic properties, leading to enhanced absorption of the solar spectrum and increased open-circuit voltages (VOC). We examined the detailed morphology of bulk heterojunctions of poly(3-hexylthiophene) and the small-molecule acceptor HPI-BT to reveal structural changes that lead to improvements in the fill factor of solar cells upon thermal annealing. The kinetics of the phase transformation process of HPI-BT during thermal annealing were investigated through in situ grazing incidence wide-angle X-ray scattering studies, atomic force microscopy, and transmission electron microscopy. The HPI-BT acceptor crystallizes during film formation to form micron-sized domains embedded within the film center and a donor rich capping layer at the cathode interface reducing efficient charge extraction. Thermal annealing changes the surface composition and improves charge extraction. This study reveals the need for complementary methods to investigate the morphology of BHJs.

High-Efficiency Organic Photovoltaics with Two-Dimensional Conjugated Benzodithiophene-Based Regioregular Polymers

We synthesized and characterized two kinds of regioregular polymers that were based on thieno[3,4-b]thiophene as an electron-accepting unit and benzo[1,2-b:4,5-b′]dithiophene as the electron-donating unit with different side chain, alkylthio and alkyl thiophenes, named rr-PTBS and rr-PTB7-Th, respectively. Because of the partial introduction of the alkylthio thiophene side chain, rr-PTBS showed red-shifted absorption and a deeper HOMO level compared to those of rr-PTB7-Th. In addition, both rr-PTBS:PC71BM and rr-PTB7-Th:PC71BM blended films showed face-on orientations stronger than those of regiorandom PTB7-Th. However, the rr-PTB7-Th:PC71BM blended film showed a peak in the out-of-plane direction much weaker than those of rr-PTBS:PC71BM and PTB7-Th:PC71BM blended films. Moreover, the rr-PTBS:PC71BM blended film exhibited charge carrier mobility (μe/μh ∼ 1.01) much more balanced than that of the rr-PTB7-Th:PC71BM blended film (μe/μh ∼ 1.23). The bulk-heterojunction organic photovoltaic (OPV) device based ...

Fogging, reflection, and dust-free transparent conducting glasses based on superhydrophilic nanotextures for organic photovoltaics

Abstract We present the preparation of high-efficiency organic photovoltaics (OPVs) using functional transparent conducting oxide glasses (functional TCOs) based on superhydrophilic nanotextures under fogging conditions. The superhydrophilic nanotextures are easily prepared with spin-coating of aged colloidal silica suspensions on the non-conducting side of indium tin oxide glasses (ITOs). PEDOT:PSS is used as a hole transport layer (HTL) in OPVs with PTB7/PC71BM as a bulk hetero-junction layer (BHJ layer). The OPVs based on functional TCOs were confirmed by atomic force microscopy (AFM), scanning electron microscopy (SEM), water contact angle, UV–vis spectroscopy, and external quantum efficiency (EQE) measurements. Compared with the conventional TCOs, the functional TCOs, which have superhydrophilicity, axial gradient in refractive index, substantially suppress fogging and reduce reflection, leading to significantly improved light harvesting. The efficiency of OPVs based on functional TCOs (4.4%) were higher than those of OPVs with conventional TCOs (2.9%) under fogging conditions. This work has also demonstrated on superhydrophilic nanotextures based functional TCOs with excellent dust-free and self-cleaning performance.

Efficiency Improvement of Organic Photovoltaics Adopting Li- and Cd-Doped ZnO Electron Extraction Layers

High-efficiency organic photovoltaics (OPV) with a PTB7: PC70BM photoabsorption layer were fabricated by adopting Li- and Cd-doped ZnO electron extraction layers. For devices with both Li- and Cd-doped ZnO, the performances were significantly improved, including the short-circuit current, open-circuit voltage, fill factor (FF), and power conversion efficiency. Furthermore, film characterization with techniques including X-ray diffraction, high-resolution transmission electron microscopy, and resistivity analysis was performed. The results suggest that the improved electrical properties of Li- and Cd-doped ZnO electron extraction layers with a doping ratio of approximately 6 at% are highly effective in achieving a power conversion efficiency of up to 8%.

Light Manipulation in Organic Photovoltaics.

Organic photovoltaics (OPVs) hold great promise for next‐generation photovoltaics in renewable energy because of the potential to realize low‐cost mass production via large‐area roll‐to‐roll printing technologies on flexible substrates. To achieve high‐efficiency OPVs, one key issue is to overcome the insufficient photon absorption in organic photoactive layers, since their low carrier mobility limits the film thickness for minimized charge recombination loss. To solve the inherent trade‐off between photon absorption and charge transport in OPVs, the optical manipulation of light with novel micro/nano‐structures has become an increasingly popular strategy to boost the light harvesting efficiency. In this Review, we make an attempt to capture the recent advances in this area. A survey of light trapping schemes implemented to various functional components and interfaces in OPVs is given and discussed from the viewpoint of plasmonic and photonic resonances, addressing the external antireflection coatings, substrate geometry‐induced trapping, the role of electrode design in optical enhancement, as well as optically modifying charge extraction and photoactive layers.

Green-solvent-processable organic solar cells

Solution-processable organic photovoltaics (OPV) has emerged as a promising clean energy-generating technology due to its potential for low-cost manufacturing with a high power/weight ratio. The state-of-the-art OPV devices are processed by hazardous halogenated solvents. Fabricating high-efficiency OPV devices using greener solvents is a necessary step toward their eventual commercialization. In this review, recent research efforts and advances in green-solvent-processable OPVs are summarized, and two basic strategies including material design and solvent selection of light-harvesting layers are discussed. In particular, the most recent green-solvent-processable OPVs with high efficiencies in excess of 9% are highlighted.

Effect of crystal packing on the excitonic properties of rubrene polymorphs

Singlet fission (SF), the conversion of one singlet exciton into two triplet excitons, may lead to the realization of high efficiency organic photovoltaics by generating two carriers from one photon. Recently, SF has been observed in molecular crystals of rubrene. While the orthorhombic form of rubrene is most often observed under ambient conditions, metastable monoclinic and triclinic polymorphs are known. Here, dispersion-inclusive density functional theory (DFT) is used to investigate the relative stability of all three phases. Many-body perturbation theory is then employed to study the effect of crystal structure on the electronic and excitonic properties. Band structures are calculated within the GW approximation and optical properties are calculated by solving the Bethe–Salpeter equation (BSE). We find that crystal packing significantly affects the electronic and excitonic properties of rubrene. Based on our calculations, the triclinic and especially the monoclinic forms of rubrene are expected to exhibit higher SF efficiencies than the orthorhombic form.