Organic Salts as a Route to Energy Level Control in Low Bandgap,High Open‐Circuit Voltage Organic and Transparent Solar Cells that Approach the Excitonic Voltage Limit

A new series of organic salts with selective near‐infrared (NIR) harvesting to 950 nm is reported, and anion selection and blending is demonstrated to allow for fine tuning of the open‐circuit voltage. Extending photoresponse deeper into the NIR is a significant challenge facing small molecule organic photovoltaics, and recent demonstrations have been limited by open‐circuit voltages much lower than the theoretical and practical limits. This work presents molecular design strategies that enable facile tuning of energy level alignment and open‐circuit voltages in organic salt‐based photovoltaics. Anions are also shown to have a strong influence on exciton diffusion length. These insights provide a clear route toward achieving high efficiency transparent and panchromatic photovoltaics, and open up design opportunities to rapidly tailor molecules for new donor–acceptor systems.     

Lifetime of Organic Photovoltaics: Status and Predictions

The results of a meta‐analysis conducted on organic photovoltaics (OPV) lifetime data reported in the literature is presented through the compilation of an extensive OPV lifetime database based on a large number of articles, followed by analysis of the large body of data. We fully reveal the progress of reported OPV lifetimes. Furthermore, a generic lifetime marker has been defined, which helps with gauging and comparing the performance of different architectures and materials from the perspective of device stability. Based on the analysis, conclusions are drawn on the bottlenecks for stability of device configurations and packaging techniques, as well as the current level of OPV lifetimes reported under different aging conditions. The work is summarized by discussing the development of a tool for OPV lifetime prediction and the development of more stable technologies. An online platform is introduced that will aid the process of generating statistical data on OPV lifetimes and further refinement of the lifetime prediction tool.     

Recent Development of Quinoxaline Based Polymers/Small Molecules for Organic Photovoltaics

Among the various molecular designs developed for the synthesis of conjugated polymers and small molecules for optoelectronic applications, the donor: acceptor (D–A) approach is the most widely explored method over the past decades. Through the covalent linkage of electron‐rich and electron‐deficient units, a plethora of medium‐low band gap materials has been developed and tested in organic photovoltaic devices. In particular, the quinoxaline aromatic structure and its derivatives are among the most studied electron deficient aromatic units used in D–A structures. Quinoxaline based materials are endowed with characteristics that are useful for large scale production in real world applications, such as easy synthetic procedures and excellent stability in air. Moreover, the use of quinoxaline based polymers/small molecules in bulk heterojunction (BHJ) devices led to power conversion efficiencies over 9%. Considering the potential of quinoxaline based materials, this review gathers together quinoxaline based polymers and small molecules reported in the literature during the last 5 years, summarizing and discussing the structure‐properties relationships for this class of organic semiconductors, aiming to serve as a background and to promote efforts for the further development of new quinoxaline derivatives with improved and advanced properties for future applications.     

Modulation of End Groups for Low‐Bandgap Nonfullerene Acceptors Enabling High‐Performance Organic Solar Cells

The field of nonfullerene organic solar cells (OSCs) has seen an impressive progress, largely due to advances in high‐performance small molecule acceptors (SMAs). As a large portion of the solar energy is located in the near‐infrared region, it is important to develop ultralow‐bandgap SMAs that have extended absorption in the spectral range of 800–1000 nm to maximize light absorption and efficiencies. In this work, three low‐bandgap SMAs, namely, IXIC, IXIC‐2Cl, and IXIC‐4Cl, are designed and synthesized with same fused terthieno[3,2‐b]thiophene donor unit and different end groups (EGs). The three SMAs all have low optical bandgap (E_g) of 1.35, 1.30, and 1.25 eV, respectively. The chlorination on EGs can lower the energy level and broaden absorption range of the SMAs. As a result, the V_oc of the devices is reduced but the J_sc is significantly increased. In addition, the addition of chlorine atoms can enhance π–π stacking and crystallinity of the SMAs, which result in high fill factors. Overall, the optimum EGs are monochlorine‐substituted IC and OSCs based on PBDB‐T:IXIC‐2Cl that can achieve remarkable power conversion efficiencies (PCEs) of 12.2%, which is one of the highest PCEs for nonfullerene organic solar cells based on low‐bandgap SMAs.     

Achieving Both Enhanced Voltage and Current through Fine‐Tuning Molecular Backbone and Morphology Control in Organic Solar Cells

It is a great challenge to simultaneously improve the two tangled parameters, open circuit voltage (V_oc) and short circuit current density (J_sc) for organic solar cells (OSCs). Herein, such a challenge is addressed by a synergistic approach using fine‐tuning molecular backbone and morphology control simultaneously by a simple yet effective side chain modulation on the backbone of an acceptor–donor–acceptor (A–D–A)‐type acceptor. With this, two terthieno[3,2‐b]thiophene (3TT) based A–D–A‐type acceptors, 3TT‐OCIC with backbone modulation and 3TT‐CIC without such modification, are designed and synthesized. Compared with the controlled molecule 3TT‐CIC, 3TT‐OCIC shows power conversion efficiency (PCE) of 13.13% with improved V_oc of 0.69 V and J_sc of 27.58 mA cm^?2, corresponding to PCE of 12.15% with V_oc of 0.65 V and J_sc of 27.04 mA cm^?2 for 3TT‐CIC–based device. Furthermore, with effective near infrared absorption, 3TT‐OCIC is used as the rear subcell acceptor in a tandem device and gave an excellent PCE of 15.72%.     

Constructing High‐Performance Organic Photovoltaics via Emerging Non‐Fullerene Acceptors and Tandem‐Junction Structure

In consideration of the unique advantages of new non‐fullerene acceptors and the tandem‐junction structure, organic photovoltaics (OPVs) based on them are very promising. Studies related to this emerging area began in 2016 with achieved power conversion efficiencies (PCEs) of 8–10%, which have now been boosted to 17%. In this essay, the construction of high‐performance OPVs is discussed, with a focus on combining the advantages of new non‐fullerene acceptors and the tandem‐junction structure. In order to achieve higher PCEs, methods to enable high short‐circuit current density, open‐circuit voltage, and fill factor are discussed. In addition, the stability and reproducibility of high‐efficiency OPVs are also addressed. Herein, it is forecast that the new non‐fullerene acceptors‐based tandem‐junction OPVs will become the next big wave in the field and achieve high PCEs over 20% in the near future. Some promising research directions on this emerging hot topic are proposed which may further push the field into the 25% high efficiency era and considerably advance the technology beyond laboratory research.     

Perylene Sensitization of Fullerenes for Improved Performance in Organic Photovoltaics

We present the addition of an energy relay dye to fullerenes resulting in increased light harvesting and significantly improved power conversion efficiency for organic photovoltaic (OPV) devices. Although exhibiting excellent properties as electron acceptors, visible light absorption of fullerenes is limited. Strongly light absorbing donor materials are needed for efficient light harvesting in the thin active layer of OPV devices. Therefore, photocurrent generation and thus power conversion efficiency of this type of solar cell is confined by the overlap of the relatively narrow absorption band of commonly used donor molecules with the solar spectrum. Herein the concept of fullerene dye sensitization is presented, which allows increased light harvesting on the electron acceptor side of the heterojunction. The concept is exemplarily shown for an UV absorbing small molecule and a near infrared absorbing polymer, namely hexa‐peri‐hexabenzocoronene (HBC) and Poly[2,1,3‐benzothiadiazole‐4,7‐diyl[4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b']dithiophene‐2,6‐diyl]] (PCPDTBT), respectively. In both systems remarkably higher power conversion efficiency is achieved via perylene sensitization of the fullerene acceptor. Steady state photoluminescence, transient absorption and transient photocurrent decay studies reveal pathways of the additionally generated excited states at the sensitizer molecule. The findings suggest fluorescence resonance energy transfer from the photo‐excited dye to the fullerene enabling decoupling of light absorption and charge transport. The presented sensitization method is proposed as a viable new concept for performance enhancement in organic photovoltaic devices.     

Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high‐temperature preparation of metal oxides as a charge transport layer in most state‐of‐the‐art PSCs has been becoming a big stumbling block for future low‐temperature and large‐scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low‐temperature‐processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all‐solution‐processed PSCs, we propose a novel n‐type sulfur‐containing small molecule hexaazatrinaphtho[2,3‐c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10^?2 cm² V^?1 s^?1 as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via S? I or S? Pb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n‐type organic small molecules for high‐performance PSCs.     

High‐Performance,Transparent, Dye‐Sensitized Solar Cells for See‐Through Photovoltaic Windows

Dye‐sensitized solar cells (DSCs) have attracted great interest as one of the most promising photovoltaic technologies, and transparent DSCs show potential applications as photovoltaic windows. However, the competition between light absorption for photocurrent generation and light transmittance for obtaining high transparency limits the performance of transparent DSCs. Here, transparent DSCs exhibiting a high light transmittance of 60.3% and high energy conversion efficiency (3.66%) are reported. The strategy is to create a cocktail system composed of ultraviolet and near‐infrared dye sensitizers that selectively and efficiently harvest light in the invisible or low‐eye‐sensitivity region while transmitting light in high‐eye‐sensitivity regions. This new design provides a reasonable approach for realizing high efficiency and transparency DSCs that have potential applications as photovoltaic windows.     

Contact Doping with Sub‐Monolayers of Strong Polyelectrolytes for Organic Photovoltaics

Barriers to charge transfer at electrode‐semiconductor contacts are ubiquitous and limit the applicability of organic semiconductors in electronic devices. Molecular or ionic doping near contacts can alleviate charge injection or extraction problems by enabling charge tunneling through contact barriers, but the soft nature of organic materials allows for small molecule dopants to diffuse and migrate, degrading the performance of the device and limiting effective interfacial doping. Here, it is demonstrated that contact doping in organic electronics is possible through ionic polymer dopants, which resist diffusion or migration due to their large size. Sub‐monolayer deposition of non‐conjugated strong polyelectrolytes, e.g., sulfonated poly(sulfone)s, at the anode‐semiconductor interface of organic photovoltaics enables efficient hole extraction at the anode. The performance of contact‐doped organic photo­voltaics nearly matches the performance of devices composed of traditional hole transport layers such as poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The degree of sulfonation of the dopant polymer and the thickness of the ionic dopant layer is shown to be critical for optimizing doping and the efficiency of the device.     

Real‐Time Photoluminescence Studies of Structure Evolution in Organic Solar Cells

A method to study the structural evolution of organic bulk heterojunctions via real‐time, in‐situ, steady‐state photoluminescence (PL) is presented. In‐situ PL, in combination with real‐time transmission and reflection measurements, allows us to quantitatively describe the progression of intimate mixing during blade coating of two OPV systems: the common model system poly(3‐hexylthiophene‐2,5‐diyl)/phenyl‐C61‐butyric‐acid‐methyl ester (P3HT/PCBM), and the higher power conversion efficiency system 7,7′‐(4,4bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(6‐fluoro‐5‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5]thiadiazaole), p‐DTS(FBTTh₂)₂/[70]PCBM. Evaluating the time dependence of the PL intensity during drying using a 3D‐random‐walk diffusion model allows for the quantitative determination of the ratio of characteristic domain size to exciton diffusion length during solidification in the presence of the processing additives 1‐chloronaphtalene (CN), 1,8‐octanedithiol (ODT), and 1,8‐diiodooctane (DIO). In both cases, the obtained results are in good agreement with the typically observed fibril widths and grain sizes, for P3HT and p‐DTS(FBTTh₂)₂, respectively.     

Design of Nonfullerene Acceptors with Near‐Infrared Light Absorption Capabilities

A series of narrow bandgap electron acceptors is designed and synthesized for efficient near‐infrared (NIR) organic solar cells. Extending π‐conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7‐Th, IOTIC‐2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm^?2 with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7‐Th:IOTIC‐2F‐based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC‐2F, into the PTB7‐Th:IOTIC‐2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy‐level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics.     

Baselines for Lifetime of Organic Solar Cells

The process of accurately gauging lifetime improvements in organic photovoltaics (OPVs) or other similar emerging technologies, such as perovskites solar cells is still a major challenge. The presented work is part of a larger effort of developing a worldwide database of lifetimes that can help establishing reference baselines of stability performance for OPVs and other emerging PV technologies, which can then be utilized for pass‐fail testing standards and predicting tools. The study constitutes scanning of literature articles related to stability data of OPVs, reported until mid‐2015 and collecting the reported data into a database. A generic lifetime marker is utilized for rating the stability of various reported devices. The collected data is combined with an earlier developed and reported database, which was based on articles reported until mid‐2013. The extended database is utilized for establishing the baselines of lifetime for OPVs tested under different conditions. The work also provides the recent progress in stability of unencapsulated OPVs with different architectures, as well as presents the updated diagram of the reported record lifetimes of OPVs. The presented work is another step forward towards the development of pass‐fail testing standards and lifetime prediction tools for emerging PV technologies.     

Light Harvesting at Oblique Incidence Decoupled from Transmission in Organic Solar Cells Exhibiting 9.8% Efficiency and 50% Visible Light Transparency

For many years, it has been recognized that potential organic photovoltaic cells must be integrated into elements requiring high transparency. In most of such elements, sunlight is likely to be incident at large angles. Here it is demonstrated that light transmission can be largely decoupled from harvesting by optically tailoring an infrared shifted nonfullerene acceptor based organic cell architecture. A 9.67% power conversion efficiency at 50° incidence is achieved together with an average visual transmission above 50% at normal incidence. The deconstruction of a 1D nanophotonic structure is implemented to conclude that just two λ/4 thick layers are essential to reach, for a wide incidence angle range, a higher than 50% efficiency increase relative to the standard configuration reference. In an outdoor measurement of vertically positioned 50% visible transparent cells, it is demonstrated that 9.80% of sunlight energy can be converted into electricity during the course of 1 day.     

Single Material Solar Cells: the Next Frontier for Organic Photovoltaics?

An overview of various approaches for the realization of single‐material organic solar cells (SMOCs) is presented. Fullerene‐conjugated systems dyads, di‐block copolymers, and self‐organized donor‐acceptor molecules all represent different possible approaches towards SMOCs. Although each of them presents specific advantages and poses specific problems of design and synthesis, these different routes have witnessed significant progress in the past few years and SMOCs with efficiencies in the range of 1.50% have been realized. These performances are already higher than those of bi‐component bulk heterojunction solar cells some ten years ago, demonstrating that SMOCs can represent a credible approach towards efficient and simple organic solar cells. Possible directions for future research are discussed with the aim of stimulating further research on this exciting topic.     

Bandgap Narrowing in Non‐Fullerene Acceptors: Single Atom Substitution Leads to High Optoelectronic Response Beyond 1000 nm

Two narrow bandgap non‐fullerene acceptors (NBG‐NFAs), namely, COTIC‐4F and SiOTIC‐4F, are designed and synthesized for the fabrication of efficient near‐infrared organic solar cells (OSCs). The chemical structures of the NBG‐NFAs contain a D′‐D‐D′ electron‐rich internal core based on a cyclopentadithiophene (or dithienosilole) (D) and alkoxythienyl (D′) core, end‐capped with the highly electron‐deficient unit 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile (A), ultimately providing a A‐D′‐D‐D′‐A molecular configuration that enhances the intramolecular charge transfer characteristics of the excited states. One can thereby reduce the optical bandgap (E_g^opt) to as low as ≈1.10 eV, one of the smallest values for NFAs reported to date. In bulk‐heterojunction (BHJ) OSCs, NBG‐NFA blends with the polymer donor PTB7‐Th yield power conversion efficiencies (PCE) of up to 9.0%, which is particularly high when compared against a range of NBG BHJ blends. Most significantly, it is found that, despite the small energy loss (E_g^opt ? eV_OC) of 0.52 eV, the PTB7‐Th/NBG‐NFA bulk heterojunction blends can yield short‐circuit current densities of up to 22.8 mA cm^?2, suggesting that the design and application of NBG‐NFA materials have substantial potential to further improve the PCE of OSCs.