Near-Infrared Ternary Tandem 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.

Ternary Organic Solar Cells with Power Conversion Efficiency Approaching 15% by Fine-Selecting the Third Component

Nonfullerene acceptors with mediate bandgap play a crucial role in ternary devices as the third component, further boosting the performance of organic solar cells (OSCs). Herein, three F-series acceptors (F-H, F-Cl, and F-2Cl) with mediate bandgap are selected and introduced into the PM6:BDT-Br binary system as third component to find the detailed influence of end groups with chlorine (Cl) atom substitution on the performance of ternary organic solar cells. Due to the increased substitution of Cl atoms on the end groups, F-Cl and F-2Cl as guest acceptors reveal a superior ability to regulate the morphology of blend films, contributing to the ordered packing properties and high crystallinity. As a result, F-Cl and F-2Cl based ternary OSCs achieve significantly improved PCEs of 13.89% and 14.67%, respectively, compared with the binary devices (12.70%). On the contrary, F-H without Cl atom displays a poor compatibility with the host system, resulting in an inferior ternary device with a low PCE of 10.79%. This work indicates that F-series acceptors with mediate bandgap are a promising class of third component for high-performance ternary OSCs. And introducing more Cl atoms substitution on the end groups, especially F-2Cl, will own a broad applicability for other binary devices.

Prospects of glove-box versus air-processed organic solar cells

In the search for alternate green energy sources to offset dependence on fossil fuels, solar energy can certainly meet two needs with one deed: fulfil growing global energy demands due to its non-depletable nature and lower greenhouse gas emissions. As such, third generation thin film photovoltaic technology based organic solar cells (OSCs) can certainly play their role in providing electricity at a competing or lower cost than 1st and 2nd generation solar technologies. As OSCs are still at an early stage of research and development, much focus has been placed on improving power conversion efficiencies (PCEs) inside a controlled environment i.e. a glove-box (GB) filled with an inert gas such as N₂. This was necessary until now, to control and study the local nanomorphology of the spin-coated blend films. For OSCs to compete with other solar energy technologies, OSCs should produce similar or even better morphologies in an open environment i.e. air, such that air-processed OSCs can result in similar PCEs in comparison to their GB-processed counterparts. In this review, we have compared GB- vs. air-processed OSCs from morphological and device physics aspects and underline the key features of efficient OSCs, processed in either GB or air.

Interactions between nonfullerene acceptors lead to unstable ternary organic photovoltaic cells

For organic photovoltaic (OPV) devices to achieve consistent performance and long operational lifetimes, organic semiconductors must be processed with precise control over their purity, composition, and structure. This is particularly important for high volume solar cell manufacturing where control of materials quality has a direct impact on yield and cost. Ternary-blend OPVs containing two acceptor-donor-acceptor (A-D-A)-type nonfullerene acceptors (NFAs) and a donor have proven to be an effective strategy to improve solar spectral coverage and reduce energy losses beyond that of binary-blend OPVs. Here, we show that the purity of such a ternary is compromised during blending to form a homogeneously mixed bulk heterojunction thin film. We find that the impurities originate from end-capping C=C/C=C exchange reactions of A-D-A-type NFAs, and that their presence influences both device reproducibility and long-term reliability. The end-capping exchange results in generation of up to four impurity constituents with strong dipolar character that interfere with the photoinduced charge transfer process, leading to reduced charge generation efficiency, morphological instabilities, and an increased vulnerability to photodegradation. As a consequence, the OPV efficiency falls to less than 65% of its initial value within 265 h when exposed to up to 10 suns intensity illumination. We propose potential molecular design strategies critical to enhancing the reproducibility as well as reliability of ternary OPVs by avoiding end-capping reactions.

Efficient and stable organic solar cells enabled by multicomponent photoactive layer based on one-pot polymerization

Degradation of the kinetically trapped bulk heterojunction film morphology in organic solar cells (OSCs) remains a grand challenge for their practical application. Herein, we demonstrate highly thermally stable OSCs using multicomponent photoactive layer synthesized via a facile one-pot polymerization, which show the advantages of low synthetic cost and simplified device fabrication. The OSCs based on multicomponent photoactive layer deliver a high power conversion efficiency of 11.8% and exhibit excellent device stability for over 1000 h (>80% of their initial efficiency retention), realizing a balance between device efficiency and operational lifetime for OSCs. In-depth opto-electrical and morphological properties characterizations revealed that the dominant PM6-b-L15 block polymers with backbone entanglement and the small fraction of PM6 and L15 polymers synergistically contribute to the frozen fine-tuned film morphology and maintain well-balanced charge transport under long-time operation. These findings pave the way towards the development of low-cost and long-term stable OSCs.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Novel Thienyl DPP derivatives Functionalized with Terminal Electron-Acceptor Groups: Synthesis, Optical Properties and OFET Performance

Three novel diketopyrrolopyrrole (DPP) based small molecules have been synthesized and characterized in terms of their chemical-physical, electrochemical and electrical properties. All the molecules consist of a central DPP electron acceptor core symmetrically functionalized with donor bi-thienyl moieties and flanked in the terminal positions by three different auxiliary electron-acceptor groups. This kind of molecular structure, characterized by an alternation of electron acceptor and donor groups, was purposely designed to provide a significant absorption at the longer wavelengths of the visible spectrum: when analysed as thin films, in fact, the dyes absorb well over 800 nm and exhibit a narrow optical bandgap down to 1.28 eV. A detailed DFT analysis provides useful information on the electronic structure of the dyes and on the features of the main optical transitions. Organic field-effect transistors (OFETs) have been fabricated by depositing the DPP dyes as active layers from solution: the different end-functionalization of the dyes had an effect on the charge-transport properties with two of the dyes acting as n-type semiconductors (electron mobility up to 4.4 ⋅ 10-2  cm2 /V ⋅ s) and the third one as a p-type semiconductor (hole mobility up to 2.3 ⋅ 10-3  cm2 /V ⋅ s). Interestingly, well-balanced ambipolar transistors were achieved by blending the most performant n-type and p-type dyes with hole and electron mobility in the order of 10-3  cm2 /V ⋅ s.

Characterizing and Improving the Thermal Stability of Organic Photovoltaics Based on Halogen-Rich Non-Fullerene Acceptors

The thermal stability of inverted, halogen-rich non-fullerene acceptor (NFA)-based organic photovoltaics with MoO_x as the hole transporting layer is studied at temperatures up to 80 °C. Over time, the power conversion efficiency shows a "check-mark" shaped thermal aging pattern, featuring an early decrease, followed by a long-term recovery. A high Cl concentration at the bulk heterojunction (BHJ)/MoO_x interface in the thermally aged device is found using energy dispersive X-ray spectroscopy. X-ray photoelectron spectroscopy shows that the MoO_x is chlorinated after thermal aging. With bulk quantum efficiency analysis, we propose an explanation to the check-mark shaped pattern. Inserting a thin C₇₀ layer between the BHJ and MoO_x suppresses the thermal degradation mechanisms, resulting in three orders of magnitude increase in device lifetime at 80 °C.

Effects of Halogenation on the Benzotriazole Unit of Non-Fullerene Acceptors in Organic Solar Cells with High Voltages

Non-fullerene acceptors (NFAs) can be simply divided into three categories: A-D-A, A-DA'D-A, and A₂-A₁-D-A₁-A₂ according to their chemical structures. Benefiting from the easily modified 1,1-dicyanomethylene-3-indanone end groups, the halogenation on the first two types of materials has been proved to be very effective to modulate their optoelectronic properties and improve their photovoltaic performance. Hence, in this work, we systematically investigate the effect of halogenation on the classic NFA molecule of BTA3, which has the linear A₂-A₁-D-A₁-A₂-type backbone. After fluorination and chlorination, F-BTA3 and Cl-BTA3 have similar optical band gaps but lower energy levels than BTA3. When blending with a linear copolymer PE25 composed of benzodifuran and chlorinated benzotriazole (BTA) according to "Same-A-Strategy", the corresponding V_OC of the halogenated NFAs gradually decreases (1.13 V for F-BTA3 and 1.09 V for Cl-BTA3), compared with that of the BTA3-based device (V_OC = 1.22 V). This tendency mainly comes from the lower lowest unoccupied molecular orbital energy levels due to the strong electron-withdrawing ability of halogen atoms and the larger nonradiative energy loss. However, the power conversion efficiencies of the halogenated materials are slightly improved, from 9.08% for PE25: BTA3 to 10.45% for PE25: F-BTA3 and 10.75% for PE25: Cl-BTA, with the nonhalogenated solvent tetrahydrofuran as the processing solvent. The improved photovoltaic performance of F-BTA3 and Cl-BTA3 should come from the higher carrier mobility, weaker bimolecular recombination, and higher fluorescence quenching rate. This study illustrates that halogenation on the A₁ unit is a promising strategy for developing novel and effective A₂-A₁-D-A₁-A₂-type NFAs.

Large-Scale Bio-Inspired Flexible Antireflective Film with Scale-Insensitivity Arrays

Natural creatures can always provide perfect strategies for excellent antireflection (AR), which is valuable for photovoltaic industry, optical devices, and flexible displays. However, limited by precision, it is still difficult to guarantee the consistency between the artificial structures and the original biological structures. Here, a novel large-scale flexible AR film is inspired by the cicada wings and successfully fabricated with a recycled template. On the one hand, the adjustable structures on porous templates make it possible to optimize the design of AR structure parameters toward the practical demand. On the other hand, it breaks the limitation of the biological organism size, accomplishing the replication of AR nanostructure units in a large scale. Interestingly, even if the film is covered by enlarged dome cone arrays, it still maintains almost perfect AR property, achieving excellent scale-insensitivity AR performance. This work numerically and experimentally investigates its scale-insensitivity AR performance in detail. Compared with subwavelength nanocones, enlarged cones change the original optical behaviors, and the proportion of transmitted light is reduced while scattering and absorption increase. Based on this, these bio-inspired scale-insensitivity AR arrays could be used in flexible displays, photothermic conversion, solar cells, and so on.

Understanding of the Nearly Linear Tunable Open-Circuit Voltages in Ternary Organic Solar Cells Based on Two Non-fullerene Acceptors

Although the power conversion efficiencies (PCEs) of the state-of-the-art organic solar cells (OSCs) have exceeded 17%, the organic photovoltaic devices still suffer from considerable voltage losses compared with the inorganic or perovskite solar cells. Therefore, the optimization of open-circuit voltage (V_OC) is of great significance for the improvement of the photovoltaic performance of OSCs. The origins of V_OC have been well-established in the binary system; however, the understanding of V_OC in non-fullerene acceptor (NFA)-based ternary OSCs is still lacking. Herein, we have developed a series of ternary organic photovoltaic devices, exhibiting nearly linear increased V_OC as the increase of ITIC third content. We found that both the effective charge-transfer (CT) states and the nonradiative recombination losses of the bulk-heterojunction (BHJ) are altered in the ternary blends, and they collectively contribute to the tunable V_OC. Our results provide a perspective for understanding the origin of V_OC in NFA-based ternary OSCs.

High performance tandem organic solar cells via a strongly infrared-absorbing narrow bandgap acceptor

Tandem organic solar cells are based on the device structure monolithically connecting two solar cells to broaden overall absorption spectrum and utilize the photon energy more efficiently. Herein, we demonstrate a simple strategy of inserting a double bond between the central core and end groups of the small molecule acceptor Y6 to extend its conjugation length and absorption range. As a result, a new narrow bandgap acceptor BTPV-4F was synthesized with an optical bandgap of 1.21 eV. The single-junction devices based on BTPV-4F as acceptor achieved a power conversion efficiency of over 13.4% with a high short-circuit current density of 28.9 mA cm⁻². With adopting BTPV-4F as the rear cell acceptor material, the resulting tandem devices reached a high power conversion efficiency of over 16.4% with good photostability. The results indicate that BTPV-4F is an efficient infrared-absorbing narrow bandgap acceptor and has great potential to be applied into tandem organic solar cells.

Development of tandem organic solar cells has been limited by the choice of near-infrared absorbing materials for the rear cell. Here, the authors report a simple strategy to extend the conjugation length of acceptor Y6 and broaden its absorption range to near-infrared region. A tandem organic solar cell with efficiency of 16.4% was achieved.

Enabling High-Performance Tandem Organic Photovoltaic Cells by Balancing the Front and Rear Subcells

In tandem organic photovoltaics, the front subcell is based on large-bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow-bandgap acceptor for infrared absorption but a large-bandgap donor to realize a high open-circuit voltage. Unfortunately, most of the ultraviolet-visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large-bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole-transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared-absorbing polymer donor as a third component is introduced, which provides a second hole-generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short-circuit current density of the two-terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7 mA cm^-2 (while retaining the open-circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.

n-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications

The use of photovoltaic technologies has been regarded as a promising approach for converting solar energy to electricity and mitigating the energy crisis, and among these, organic photovoltaics (OPVs) have attracted broad interest because of their solution processability, flexibility, light weight, and potential for large-area processing. The development of OPV materials, especially electron acceptors, has been one of the focuses in recent years. Compared with fullerene derivates, n-type non-fullerene molecules have some unique merits, such as synthetic simplicity, high tunability of the absorption and energy levels, and small energy loss. In the last 5 years, organic solar cells based on n-type non-fullerene molecules have achieved a significant breakthrough in the power conversion efficiency from approximately 4% to over 17%, which is superior to those of fullerene-based solar cells; meanwhile, n-type non-fullerene molecules have created brand new opportunities for the application of OPVs in some special situations. This Perspective analyzes the key design strategies of high-performance n-type molecular photovoltaic materials and highlights instructive examples of their various applications, including in ternary and tandem solar cells, high-efficiency semitransparent solar cells for power-generating building facades and windows, and indoor photovoltaics for driving low-power-consumption devices. Moreover, to accelerate the pace toward commercialization of OPVs, the existing challenges and future directions are also reviewed from the perspectives of efficiency, stability, and large-area fabrication.

Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review

The pursuit of low-cost, flexible, and lightweight renewable power resources has led to outstanding advancements in organic solar cells (OSCs). Among the successful design principles developed for synthesizing efficient conjugated electron donor (ED) or acceptor (EA) units for OSCs, chlorination has recently emerged as a reliable approach, despite being neglected over the years. In fact, several recent studies have indicated that chlorination is more potent for large-scale production than the highly studied fluorination in several aspects, such as easy and low-cost synthesis of materials, lowering energy levels, easy tuning of molecular orientation, and morphology, thus realizing impressive power conversion efficiencies in OSCs up to 17%. Herein, an up-to-date summary of the current progress in photovoltaic results realized by incorporating a chlorinated ED or EA into OSCs is presented to recognize the benefits and drawbacks of this interesting substituent in photoactive materials. Furthermore, other aspects of chlorinated materials for application in all-small-molecule, semitransparent, tandem, ternary, single-component, and indoor OSCs are also presented. Consequently, a concise outlook is provided for future design and development of chlorinated ED or EA units, which will facilitate utilization of this approach to achieve the goal of low-cost and large-area OSCs.

Biodegradable π-Conjugated Oligomer Nanoparticles with High Photothermal Conversion Efficiency for Cancer Theranostics

We developed a biodegradable photothermal therapeutic (PTT) agent, π-conjugated oligomer nanoparticles (F8-PEG NPs), for highly efficient cancer theranostics. By exploiting an oligomer with excellent near-infrared (NIR) absorption, the nanoparticles show a high photothermal conversion efficiency (PCE) up to 82%, surpassing those of reported inorganic and organic PTT agents. In addition, the oligomer nanoparticles show excellent photostability and good biodegradability. The F8-PEG NPs are also demonstrated to have excellent biosafety and PTT efficacy both in vitro and in vivo. This contribution not only proposes a promising oligomer-based PTT agent but also provides insight into developing highly efficient nanomaterials for cancer theranostics.

Enhanced Light Utilization in Semitransparent Organic Photovoltaics Using an Optical Outcoupling Architecture

Building-integrated photovoltaics employing transparent photovoltaic cells on window panes provide an opportunity to convert solar energy to electricity rather than generating waste heat. Semitransparent organic photovoltaic cells (ST-OPVs) that utilize a nonfullerene acceptor-based near-infrared (NIR) absorbing ternary cell combined with a thin, semitransparent, high conductivity Cu-Ag alloy electrode are demonstrated. A combination of optical outcoupling and antireflection coatings leads to enhanced visible transmission, while reflecting the NIR back into the cell where it is absorbed. This combination of coatings results in doubling of the light utilization efficiency (LUE), which is equal to the product of the power conversion efficiency (PCE) and the average photopic transparency, compared with a conventional semitransparent cell lacking these coatings. A maximum LUE = 3.56 ± 0.11% is achieved for an ST-OPV with a PCE = 8.0 ± 0.2% at 1 sun, reference AM1.5G spectrum. Moreover, neutral colored ST-OPVs are also demonstrated, with LUE = 2.56 ± 0.2%, along with Commission Internationale d'Eclairage chromaticity coordinates of CIE = (0.337, 0.349) and a color rendering index of CRI = 87.

Two Well-Compatible Acceptors with Efficient Energy Transfer Enable Ternary Organic Photovoltaics Exhibiting a 13.36% Efficiency

Organic photovoltaics (OPVs) are fabricated with PM6 as donor and T6Me, IT-2F, or their mixture as acceptor. A 13.36% power conversion efficiency (PCE) is achieved from the optimized ternary OPVs with 50 wt% IT-2F in acceptors, which is attributed to the enhanced photon harvesting of ternary active layers and improved exciton utilization efficiency through energy transfer from IT-2F to T6Me. The efficient energy transfer from IT-2F to T6Me can be confirmed from the photoluminescence spectra of neat and blend films, which may provide additional channels to enhance exciton utilization efficiency for achieving short-circuit current density (J_SC ) improvement of ternary OPVs. It should be highlighted that the fill factor (FF) of ternary OPVs can be monotonously increased along with the incorporation of IT-2F, indicating the gradually optimized phase separation degree of ternary active layers. The third component IT-2F plays a key role in optimizing phase separation as a morphology regulator. Over 8% PCE improvement is achieved in the optimized ternary OPVs compared with the over 12% PCEs of the corresponding binary OPVs, respectively. This work indicates that the performance of ternary OPVs can be well optimized by carefully picking materials with good compatibility and complementary absorption spectra, as well as the appropriate energy levels.

A Tandem Organic Solar Cell with PCE of 14.52% Employing Subcells with the Same Polymer Donor and Two Absorption Complementary Acceptors

The tandem structure is an efficient way to simultaneously tackle absorption and thermalization losses of the single junction solar cells. In this work, a high-performance tandem organic solar cell (OSC) using two subcells with the same donor poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T) and two acceptors, F-M and 2,9-bis(2-methylene-(3(1,1-dicyanomethylene)benz[f ]indanone))7,12-dihydro-(4,4,10,10-tetrakis(4-hexylphenyl)-5,11-diocthylthieno[3',2':4,5]cyclopenta[1,2-b]thieno[2″,3″:3',4']cyclopenta[1',2':4,5]thieno[2,3-f][1]benzothiophene (NNBDT), with complementary absorptions is demonstrated. The two subcells show high V_oc with value of 0.99 V for the front cell and 0.86 V for the rear cell, which is the prerequisite for obtaining high V_oc of their series-connected tandem device. Although there is much absorption overlap for the subcells, a decent J_sc of the tandem cell is still obtained owing to the complementary absorption of the two acceptors in a wide range. With systematic device optimizations, a best power conversion efficiency of 14.52% is achieved for the tandem device, with a high V_oc of 1.82 V, a notable FF of 74.7%, and a decent J_sc of 10.68 mA cm^-2 . This work demonstrates a promising strategy of fabricating high-efficiency tandem OSCs through elaborate selection of the active layer materials in each subcell and tradeoff of the V_oc and J_sc of the tandem cells.

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution-processed bulk-heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA-based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all-small-molecule OSCs, semi-transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA-OSCs for future material design and challenges ahead is provided.

Intrinsic measurements of exciton transport in photovoltaic cells

Organic photovoltaic cells are partiuclarly sensitive to exciton harvesting and are thus, a useful platform for the characterization of exciton diffusion. While device photocurrent spectroscopy can be used to extract the exciton diffusion length, this method is frequently limited by unknown interfacial recombination losses. We resolve this limitation and demonstrate a general, device-based photocurrent-ratio measurement to extract the intrinsic diffusion length. Since interfacial losses are not active layer specific, a ratio of the donor- and acceptor-material internal quantum efficiencies cancels this quantity. We further show that this measurement permits extraction of additional device-relevant information regarding exciton relaxation and charge separation processes. The generality of this method is demonstrated by measuring exciton transport for both luminescent and dark materials, as well as for small molecule and polymer active materials and semiconductor quantum dots. Thus, we demonstrate a broadly applicable device-based methodology to probe the intrinsic active material exciton diffusion length.

Zhang et al. develop a device-based method to probe intrinsic exciton transport in photovoltaic cells. The broad utility of this method is demonstrated by measuring exciton transport for both luminescent and dark organic semiconductors as well as semiconductor quantum dots.

Highly Efficient Semitransparent Organic Solar Cells with Color Rendering Index Approaching 100

Neutral-colored semitransparent organic solar cells (ST-OSCs) have attracted considerable attention owing to their unique application in no-visual-obstacle building-integrated photovoltaics. Toward this promising potential application, a synergistic effect is first proposed by employing a dielectric mirror and ternary photoactive layer with near-infrared absorption to tune the color perception as well as ST-OSC performance precisely. As a result, a neutral-color ST-OSC with high average transmittance of over 21% is successfully constructed, and a remarkable color-rendering index approaching 100 and high power conversion efficiency (PCE) of 9.37% are simultaneously achieved. To the best of our knowledge, this is the highest PCE reported for neutral-color ST-OSCs to date. Importantly, this synergistic effect is demonstrated to be a universal strategy that is not only suitable for various photoactive layer systems, but can also be implanted in flexible substrate. The resulting neutral-color flexible ST-OSCs also show a promising PCE of 8.76%.

Effect of fluorine substitution in organoboron electron acceptors for photovoltaic application

Fluorine substitution at the core unit or the endcapping groups has an interesting effect on the opto-electronic properties and device behaviors of organoboron electron acceptors.