Interlayer for modified cathode in highly efficient inverted ITO-free organic solar cells

Interface Engineering for Highly Efficient Organic Solar Cells

Organic solar cells (OSCs) have made dramatic advancements during the past decades owing to the innovative material design and device structure optimization, with power conversion efficiencies surpassing 19% and 20% for single-junction and tandem devices, respectively. Interface engineering, by modifying interface properties between different layers for OSCs, has become a vital part to promote the device efficiency. It is essential to elucidate the intrinsic working mechanism of interface layers, as well as the related physical and chemical processes that manipulate device performance and long-term stability. In this article, the advances in interface engineering aimed to pursue high-performance OSCs are reviewed. The specific functions and corresponding design principles of interface layers are summarized first. Then, the anode interface layer, cathode interface layer in single-junction OSCs, and interconnecting layer of tandem devices are discussed in separate categories, and the interface engineering-related improvements on device efficiency and stability are analyzed. Finally, the challenges and prospects associated with application of interface engineering are discussed with the emphasis on large-area, high-performance, and low-cost device manufacturing.

Non-Covalent Interaction Enhancement on Active/Interfacial Layers via Two-Dimensional Vermiculite Doping for Efficient Organic Solar Cells

Interface modification plays an important role in improving the power conversion efficiency (PCE) of organic solar cells (OSCs). However, the low non-covalent interaction between the cathode interface layer (CIL) and nonfullerene acceptor (NFA) directly affects the charge collection of OSCs. Here, the non-covalent interaction between the CIL and NFA is enhanced by introducing the 2D vermiculite (VML) in the poly(9,9-bis(3'-(N,N-dimethyl)-Nethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) dibromide (PFN-Br) interface layer to form an efficient electron transport channel. As a result, the electron extraction efficiency from the active layer to the CIL is increased, and the PCE of OSCs based on PBDB-T:ITIC is boosted from 10.87% to 12.89%. In addition, the strategy of CIL doping VML is proven to be universal in different CIL materials, for which the PCE is boosted from 10.21% to 11.57% for OSCs based on PDINN and from 9.82% to 11.27% for OSCs based on PNDIT-F3N. The results provide a viable option for designing efficient CIL for high-performance non-fullerene OSCs, which may promote the commercialization of OSCs.

A Transparent Electrode Based on Solution-Processed ZnO for Organic Optoelectronic Devices

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm⁻¹) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices.

A highly conductive and transparent electrode is essential to achieving a high efficiency in indium tin oxide-free optoelectronic devices. Here, the authors strategically prepare sol-gel-grown zinc oxide films based on photoinduced doping effect and demonstrate enhanced performance of devices.

Li-Doped ZnO Electron Transport Layer for Improved Performance and Photostability of Organic Solar Cells

Organic solar cells (OSCs) based on an inverted architecture generally have better stability compared to those based on a standard architecture. However, the photoactive area of the inverted solar cells increases under ultraviolet (UV) or solar illuminatiom because of the too-high conductivity of the UV-illuminated zinc oxide (ZnO) interlayer. This limits the potential of the inverted solar cells for industrial applications. Herein, lithium-doped ZnO (Li-ZnO) films are employed as the cathode interlayer to construct inverted OSCs. The incorporation of Li ions is found to reduce the lateral conductivity of the UV-treated ZnO films because of the presence of Li ions, preventing the high-quality-growth of ZnO nanocrystals. This addresses the problem of having too-high conductivity in the UV-treated ZnO layer, causing the increased photoactive area of inverted solar cells. The overall performance of the solar cell is shown to be higher after the incorporation of Li ions in the ZnO layer, mainly due to the increased fill factor (FF), originating from the reduced trap-assisted recombination losses. Finally, the inverted solar cells based on the Li-ZnO interlayer are demonstrated to have a much better long-term stability, as compared to those based on ZnO. This allows the ZnO-based interlayers to be used for the mass production of organic solar cell modules.

Hybrid Cathode Interlayer Enables 17.4% Efficiency Binary Organic Solar Cells

With the emergence of fused ring electron acceptors, the power conversion efficiency of organic solar cells reached 19%. In comparison with the electron donor and acceptor materials progress, the development of cathode interlayers lags. As a result, charge extraction barriers, interfacial trap states, and significant transport resistance may be induced due to the unfavorable cathode interlayer, limiting the device performances. Herein, a hybrid cathode interlayer composed of PNDIT-F3N and PDIN is adopted to investigate the interaction between the photoexcited acceptor and cathode interlayer. The state of art acceptor Y6 is chosen and blended with PM6 as the active layer. The device with hybrid interlayer, PNDIT-F3N:PDIN (0.6:0.4, in wt%), attains a power conversion efficiency of 17.4%, outperforming devices with other cathode interlayer such as NDI-M, PDINO, and Phen-DPO. It is resulted from enhanced exciton dissociation, reduced trap-assisted recombination, and smaller transfer resistance. Therefore, the hybrid interlayer strategy is demonstrated as an efficient approach to improve device performance, shedding light on the selection and engineering of cathode interlayers for pairing the increasing number of fused ring electron acceptors.

Performance Enhancement of Organic Solar Cells by Adding a Liquid Crystalline Molecule in Cathode and Anode Interlayers

In this study, an effective and simple approach for optimizing the performance of both cathode and anode interlayers in OSCs is demonstrated using 4-heptyl-4'-cyanobiphenyl (7CB) to dope a classic cathode (ZnO and SnO₂) or an anode interlayer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)]. Because of the enhanced light absorption, improved physical contact between a photoactive layer and an interlayer, and increased carrier recombination, all of the devices based on a 7CB-doped interlayer show increased short-circuit current density (J_sc), fill factor (FF), and power conversion efficiency (PCE) compared to the corresponding undoped interlayer, regardless it is the anode interlayer or the cathode interlayer, which is a rare phenomenon in the interlayer modification field.

The impact of thermal treatment on the performance of benzo[1,2-b:4,5-b′]difuran-based organic solar cells

The new low bandgap benzo[1,2-b:4,5-b′]difuran (BDF)-based organic small molecule, namely B1, was synthesized by Stille coupling polymerization reactions. B1 was found to be soluble in common organic solvents such as chloroform, toluene and chlorobenzene with excellent film forming properties. The structure of B1 was verified by ¹H NMR, GC-MS and elemental analysis. The B1 films exhibit broad absorption bands from 300 to 750 nm. The hole mobility of B1 : PC₆₁BM (1 : 1, w/w) blend film reached up to 7.7 × 10⁻² cm V⁻¹ s⁻¹ after thermal annealing by the space-charge-limited current method. BHJ organic solar cells (OSCs) were fabricated with a device structure of ITO/PEDOT : PSS/B1 : C₆₁BM/LiF/Al. When the active layer was thermally annealed at 120 °C, B1 showed the best photovoltaic performance, with a PCE up to 5.0%. We also studied the connection between the morphologies of the active layers and the photovoltaic performance by AFM, PL, etc. Our observation will guide future design for even better small molecules for highly efficient OSCs.

The new low bandgap benzo[1,2-b:4,5-b′]difuran (BDF)-based organic small molecule, namely B1, was synthesized by Stille coupling polymerization reactions.

Organic Photovoltaics over Three Decades

The development of organic semiconductors for photovoltaic devices, over the last three decades, has led to unexpected performance for an alternative choice of materials to convert sunlight to electricity. New materials and developed concepts have improved the photovoltage in organic photovoltaic devices, where records are now found above 13% power conversion efficiency in sunlight. The author has stayed with the topic of organic materials for energy conversion and energy storage during these three decades, and makes use of the Hall of Fame now built by Advanced Materials, to present his view of the path travelled over this time, including motivations, personalities, and ambitions.

Poly(4-vinylpyridine): A New Interface Layer for Organic Solar Cells

Poly(4-vinylpyridine) (P4VP) was used as a cathode interface layer in inverted organic solar cells (OSCs) fabricated using poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and PC₇₁BM (phenyl C₇₁ butyric acid methyl ester) as the donor and acceptor materials, respectively. We successfully demonstrate that the work function of underlying indium tin oxide (ITO) electrode can be significantly reduced by ∼0.7 eV, after modification of the surface with a thin film of P4VP. Photoconversion efficiency of 4.7% was achieved from OSCs incorporating P4VP interface layer between the ITO and bulk heterojunction (BHJ). Thin P4VP layer, when used to modify ZnO electron transport layer in inverted OSCs, reduced the ZnO work function from 3.7 to 3.4 eV, which resulted in a noteworthy increase in open-circuit voltage from 840 to 890 mV. On simultaneous modification of ZnO with P4VP and optimization of the BHJ morphology by using solvent additive chloronapthalene, photoconversion efficiency of OSCs was significantly increased from 4.6% to 6.3%. The enhanced device parameters are also attributed to an energetically favorable material stratification, as a result of an enrichment of PC₇₁BM toward the P4VP interface.

Surface engineering to achieve organic ternary memory with a high device yield and improved performance

Organic memories fabricated on surface-engineered indium tin oxide show the highest ternary yield (82%) to date and better performance.

Squaraine molecules deposited on indium tin oxide (ITO) substrates modified with phosphonic acids crystalize more orderly than do those on untreated ITO. The as-fabricated electro-resistive memories show the highest ternary device yield observed to date (82%), a narrower switching voltage distribution, and better retention as well as resistance uniformity.

Development of polymer–fullerene solar cells

Global efforts and synergetic interdisciplinary collaborations on solution-processed bulk-heterojunction polymer solar cells (PSCs or OPVs) made power conversion efficiencies over 10% possible. The rapid progress of the field is credited to the synthesis of a large number of novel polymers with specially tunable optoelectronic properties, a better control over the nano-morphology of photoactive blend layers, the introduction of various effective interfacial layers, new device architectures and a deeper understanding of device physics. We will review the pioneering materials for polymer–fullerene solar cells and trace the progress of concepts driving their development. We discuss the evolution of morphology control, interfacial layers and device structures fully exploring the potential of photoactive materials. In order to guide a further increase in power conversion efficiency of OPV, the current understanding of the process of free charge carrier generation and the origin of the photovoltage is summarized followed by a perspective on how to overcome the limitations for industrializing PSCs.

Inverted organic photovoltaic cells

The advance in lifestyle, modern industrialization and future technological revolution are always at high expense of energy consumption. Unfortunately, there exist serious issues such as limited storage, high cost and toxic contamination in conventional fossil fuel energy sources. Instead, solar energy represents a renewable, economic and green alternative in the future energy market. Among the photovoltaic technologies, organic photovoltaics (OPVs) demonstrate a cheap, flexible, clean and easy-processing way to convert solar energy into electricity. However, OPVs with a conventional device structure are still far away from industrialization mainly because of their short lifetime and the energy-intensive deposition of top metal electrode. To address the stability and cost issue simultaneously, an inverted device structure has been introduced into OPVs, bridging laboratory research with practical application. In this review, recent progress in device structures, working mechanisms, functions and advances of each component layer as well their correlations with the efficiency and stability of inverted OPVs are reviewed and illustrated.

Stability of Polymer Interlayer Modified ITO Electrodes for Organic Solar Cells

Indium-tin-oxide (ITO) electrode surfaces were modified using thin polymeric films of ethoxylated polyethylenimine (PEIE) and poly(3,3′-([(9′,9′-dioctyl-9H,9′H-[2,2′-bifluorene]-9,9-diyl)bis(4,1-phenylene)]bis(oxy))bis(N,N-dimethylpropan-1-amine)) (PFPA-1) to investigate the resultant work function and its stability in ambient atmosphere. Both PEIE and PFPA-1 were found to significantly reduce the ITO work function, as a result of a surface dipole at the ITO–polymer interface. After aging for two weeks in ambient air atmosphere, the N-side groups and OH groups in PEIE-modified ITO were found to realign themselves away from the polymer surface, resulting in an orientation more parallel to the surface normal and thus in an increase in work function from 3.5 to 3.8 eV. The work function of PFPA-1-modified ITO was found to increase from 3.65 to 4.1 eV after two weeks of aging in air due to a complete re-orientation of the polar side chains away from the surface, aligning the dipoles more parallel to the surface normal. In both PEIE and PFPA-1 samples, the hydrophobic aliphatic carbon was found to dominate the polymer surface, after aging.

Solution combustion synthesis of metal oxide nanomaterials for energy storage and conversion

The design and synthesis of metal oxide nanomaterials is one of the key steps for achieving highly efficient energy conversion and storage on an industrial scale. Solution combustion synthesis (SCS) is a time- and energy-saving method as compared with other routes, especially for the preparation of complex oxides which can be easily adapted for scale-up applications. This review summarizes the synthesis of various metal oxide nanomaterials and their applications for energy conversion and storage, including lithium-ion batteries, supercapacitors, hydrogen and methane production, fuel cells and solar cells. In particular, some novel concepts such as reverse support combustion, self-combustion of ionic liquids, and creation of oxygen vacancies are presented. SCS has some unique advantages such as its capability for in situ doping of oxides and construction of heterojunctions. The well-developed porosity and large specific surface area caused by gas evolution during the combustion process endow the resulting materials with exceptional properties. The relationship between the structural properties of the metal oxides studied and their performance is discussed. Finally, the conclusions and perspectives are briefly presented.

Indium tin oxide-free tandem polymer solar cells on opaque substrates with top illumination

Top-illuminated, indium tin oxide (ITO)-free, tandem polymer solar cells are fabricated on opaque substrates in an inverted device configuration. In the tandem cell, a wide band gap subcell, consisting of poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) blended with [70]PCBM is combined with a small band gap subcell consisting of a mixture of poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}-alt-{[2,2'-(1,4-phenylene)bisthiophene]-5,5'-diyl}] (PDPPTPT) and [60]PCBM. Compared to the more common bottom-illuminated inverted tandem polymer solar cells on transparent ITO substrates, the front and back cells must be reversed when using opaque substrates and a transparent and conductive top contact must be employed to enable top illumination. A high conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer in combination with Ag lines surrounding the active area as current collection electrode is used for this purpose. The tandem polymer solar cell on an opaque glass/metal substrate yields a power conversion efficiency of 6.1% when the thicknesses of the photoactive layers are balanced for optimum performance. This is similar to the equivalent inverted tandem device fabricated on a transparent glass/ITO substrate.

Charge carrier generation and transport in different stoichiometry APFO3:PC61BM solar cells

In this paper we studied carrier drift dynamics in APFO3:PC61BM solar cells of varied stoichiometry (2:1, 1:1, and 1:4 APFO3:PC61BM) over a wide time range, from subpicoseconds to microseconds with a combination of ultrafast optical electric field probing and conventional transient integrated photocurrent techniques. Carrier drift and extraction dynamics are strongly stoichiometry dependent: the speed of electron or hole drift increases with higher concentration of PC61BM or polymer, respectively. The electron extraction from a sample with 80% PC61BM takes place during hundreds of picoseconds, but slows down to sub-microseconds in a sample with 33% PC61BM. The hole extraction is less stoichiometry dependent: it varies form sub-nanoseconds to tens of nanoseconds when the PC61BM concentration changes from 33% to 80%. The electron extraction rate correlates with the conversion efficiency of solar cells, leading to the conclusion that fast electron motion is essential for efficient charge carrier separation preventing their geminate recombination.

25th anniversary article: organic photovoltaic modules and biopolymer supercapacitors for supply of renewable electricity: a perspective from Africa

The role of materials in civilization is well demonstrated over the centuries and millennia, as materials have come to serve as the classifier of stages of civilization. With the advent of materials science, this relation has become even more pronounced. The pivotal role of advanced materials in industrial economies has not yet been matched by the influence of advanced materials during the transition from agricultural to modern societies. The role of advanced materials in poverty eradication can be very large, in particular if new trajectories of social and economic development become possible. This is the topic of this essay, different in format from the traditional scientific review, as we try to encompass not only two infant technologies of solar energy conversion and storage by means of organic materials, but also the social conditions for introduction of the technologies. The development of organic-based photovoltaic energy conversion has been rapid, and promises to deliver new alternatives to well-established silicon photovoltaics. Our recent development of organic biopolymer composite electrodes opens avenues towards the use of renewable materials in the construction of wooden batteries or supercapacitors for charge storage. Combining these new elements may give different conditions for introduction of energy technology in areas now lacking electrical grids, but having sufficient solar energy inputs. These areas are found close to the equator, and include some of the poorest regions on earth.

Recent advances in polymer solar cells: realization of high device performance by incorporating water/alcohol-soluble conjugated polymers as electrode buffer layer

This Progress Report highlights recent advances in polymer solar cells with special attention focused on the recent rapid-growing progress in methods that use a thin layer of alcohol/water-soluble conjugated polymers as key component to obtain optimized device performance, but also discusses novel materials and device architectures made by major prestigious institutions in this field. We anticipate that due to drastic improvements in efficiency and easy utilization, this method opens up new opportunities for PSCs from various material systems to improve towards 10% efficiency, and many novel device structures will emerge as suitable architectures for developing the ideal roll-to-roll type processing of polymer-based solar cells.

Conductive conjugated polyelectrolyte as hole-transporting layer for organic bulk heterojunction solar cells

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been extensively used as the hole-transporting layer (HTL) in bulk heterojunction (BHJ) solar cells, however, its anisotropic electrical conduction and intrinsic acidic nature generally limit the device performance. Here we demonstrate the application of a water/alcohol soluble CPE (CPE-K) as HTLs in BHJ solar cells, achieving a PCE up to 8.2%. The more superior and uniform vertical electrical conductivity found in CPE-K reduces the series resistance and provides efficient hole extraction.

Conventional polymer solar cells with power conversion efficiencies increased to >9% by a combination of methanol treatment and an anionic conjugated polyelectrolyte interface layer

The power conversion efficiency of a PTB7:PC₇₁BM polymer solar cell was improved up to 9.1% by a combination of methanol treatment followed by conjugation of a water- or alcohol-soluble polyelectrolyte thin layer.

A hyperbranched conjugated polymer as the cathode interlayer for high-performance polymer solar cells

An alcohol-soluble hyperbranched conjugated polymer HBPFN with a dimethylamino moiety is synthesized and used as a cathode interlayer. A PCE of 7.7% is obtained for PBDTTT-C-T/PC71 BM based solar cells. No obvious interfacial dipole is found at the interface between the active layer and HBPFN however, an interfacial dipole with the cathode could be one of the reasons for the enhanced performance.

N-acyldithieno[3,2-b:2',3'-d]pyrrole-based low-band-gap conjugated polymer solar cells with amine-modified [6,6]-phenyl-C61-butyric acid ester cathode interlayers

Efficient low-band-gap polymers are one key component for constructing tandem solar cells with other higher-band-gap materials to harvest wide absorption of the solar spectrum. The N-acyldithieno[3,2-b:2',3'-d]pyrrole (DTP) building block is used for making low-band-gap polymers. It is attractive because of its strong donating ability and relatively low highest-occupied-molecular-orbital level in comparison with the N-alkyl DTP building block. However, additional solubilizing groups on the accepting units are needed for soluble donor-acceptor polymers based on the N-alkanoyl DTP building block. Combining N-benzoyl DTP with a 4,7-dithieno-2,1,3-benzothiadiazole building block, a polymer with a low band gap of 1.44 eV, delivers a high short-circuit current of 17.1 mA/cm(2) and a power conversion efficiency of 3.95%, which are the highest for the devices with DTP-containing materials. Herein, an alcohol-soluble diamine-modified fullerene cathode interfacial layer improved the device efficiency significantly more than the mono-amine analogue.

Enhancing the performance of polymer photovoltaic cells by using an alcohol soluble fullerene derivative as the interfacial layer

Alcohol soluble fullerene derivative (FN-C60) has been synthesized and used as a cathode interfacial layer for high-efficiency polymer solar cells (PSCs). To examine the function of the FN-C60 interfacial layer, polymer solar cells were fabricated with blends of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM as the active layer. In comparison to the bare Al electrode, power conversion efficiencies (PCEs) of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM based PSCs were increased from 3.50 to 4.64%, 4.69 to 5.25%, 2.70 to 4.60%, and 1.52 to 2.29%, respectively, when FN-C60/Al was used as the electrode. Moreover, the overall photovoltaic performances of PSCs with the FN-C60/Al electrode were better than those of cells with LiF/Al electrode, indicating that FN-C60 is a potential interfacial layer material to replace LiF.

Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements

Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photogenerated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 × 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.

High-efficiency polymer solar cells enhanced by solvent treatment

A significant enhancement of efficiency in thieno[3,4-b]-thiophene/benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester (PTB7:PC70 BM) solar cells can be achieved by methanol treatment. The effects of methanol treatment are shown in an improvement of built-in voltage, a decrease in series resistance, an enhanced charge-transport property, an accelerated and enlarged charge extraction, and a reduced charge recombination, which induce a simultaneous enhancement in open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) in the devices.

Optical design of transparent thin metal electrodes to enhance in-coupling and trapping of light in flexible polymer solar cells

ITO-free polymer solar cells with efficiencies as high as 6.6% and 5.8% are fabricated on glass and polyethylene naphthalate (PEN) by using TeO(2) to enhance the in-coupling of light in an Ag-Ag microcavity. These cells exhibit higher performance, selective microcavity resonance as a function of the thickness of TeO(2) , and better bending stability than flexible devices made with ITO.

Device engineering for highly efficient top-illuminated organic solar cells with microcavity structures

Small-molecule organic solar cells with microcavity structures utilizing very thin solar-absorbing active layers are simulated and fabricated. By carefully fine-tuning the in-cell spacer layer and out-of-cell capping layer, highly efficient top-illuminated indium tin oxide-free solar cells are created on glass and flexible polyethylene terephthalate substrates with efficiencies of up to 5.5% and 5%, respectively.