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.     

Improved Performance and Reliability of p‐i‐n Perovskite Solar Cells via Doped Metal Oxides

Perovskite photovoltaics (PVs) have attracted attention because of their excellent power conversion efficiency (PCE). Critical issues related to large‐area PV performance, reliability, and lifetime need to be addressed. Here, it is shown that doped metal oxides can provide ideal electron selectivity, improved reliability, and stability for perovskite PVs. This study reports p‐i‐n perovskite PVs with device areas ranging from 0.09 cm² to 0.5 cm² incorporating a thick aluminum‐doped zinc oxide (AZO) electron selective contact with hysteresis‐free PCE of over 13% and high fill factor values in the range of 80%. AZO provides suitable energy levels for carrier selectivity, neutralizes the presence of pinholes, and provides intimate interfaces. Devices using AZO exhibit an average PCE increase of over 20% compared with the devices without AZO and maintain the high PCE for the larger area devices reported. Furthermore, the device stability of p‐i‐n perovskite solar cells under the ISOS‐D‐1 is enhanced when AZO is used, and maintains 100% of the initial PCE for over 1000 h of exposure when AZO/Au is used as the top electrode. The results indicate the importance of doped metal oxides as carrier selective contacts to achieve reliable and high‐performance long‐lived large‐area perovskite solar cells.     

Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment

Research on the luminescent solar concentrator (LSC) over the past thirty‐odd years is reviewed. The LSC is a simple device at its heart, employing a polymeric or glass waveguide and luminescent molecules to generate electricity from sunlight when attached to a photovoltaic cell. The LSC has the potential to find extended use in an area traditionally difficult for effective use of regular photovoltaic panels: the built environment. The LSC is a device very flexible in its design, with a variety of possible shapes and colors. The primary challenge faced by the devices is increasing their photon‐to‐electron conversion efficiencies. A number of laboratories are working to improve the efficiency and lifetime of the LSC device, with the ultimate goal of commercializing the devices within a few years. The topics covered here relate to the efforts for reducing losses in these devices. These include studies of novel luminophores, including organic fluorescent dyes, inorganic phosphors, and quantum dots. Ways to limit the surface and internal losses are also discussed, including using organic and inorganic‐based selective mirrors which allow sunlight in but reflect luminophore‐emitted light, plasmonic structures to enhance emissions, novel photovoltaics, alignment of the luminophores to manipulate the path of the emitted light, and patterning of the dye layer to improve emission efficiency. Finally, some possible ‘glimpses of the future’ are offered, with additional research paths that could result in a device that makes solar energy a ubiquitous part of the urban setting, finding use as sound barriers, bus‐stop roofs, awnings, windows, paving, or siding tiles.     

Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

Nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C₆₀ is investigated. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. A combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density is applied. In organic solar cells, charge photogeneration and recombination primarily occur at the donor–acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two‐dimensional heterointerface. In order to study nongeminate recombination dynamics in PHJ devices the charge accumulation at this interface is most relavent. As only the spatially averaged carrier concentration can be determined from extraction techniques, the charge carrier density at the interface n_int is derived from the open circuit voltage. Comparing the experimental results with macroscopic device simulation, the differences of recombination and charge carrier densities in CuPc:C₆₀ PHJ and BHJ devices are discussed with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.     

How Molecules with Dipole Moments Enhance the Selectivity of Electrodes in Organic Solar Cells – A Combined Experimental and Theoretical Approach

Simple organic molecules with permanent dipole moments – amino acids and heterocycles – have been successfully employed in bulk‐heterojunction organic solar cells as interlayer between photoactive material and electron contact. A large increase of open‐circuit voltage and fill factor can be observed for four different polymers as donor material in the photoactive layer. A combination of current–voltage curves, scanning Kelvin‐probe atomic force microscopy, ultraviolet photoelectron spectroscopy, and electroluminescence measurements as well as numerical simulations are carried out to clarify in detail the underlying mechanisms. All results fully confirm the hypothesis that the main effect is an accumulation of electrons and a depletion of holes in the photoactive layer in the vicinity of the electron contact induced by a decrease of its effective work function. Further, density functional theory calculations and literature reports of the energy levels of the dipole molecules strongly suggest that the charge carriers tunnel through the thin dipole layer which does however not limit the current. This represents a versatile, simple, and cheap method to realize highly selective contacts which may also be beneficial for other types of solar cells and devices where contact selectivity is crucial.     

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.     

Carrier‐Selectivity‐Dependent Charge Recombination Dynamics in Organic Photovoltaic Cells with a Ferroelectric Blend Interlayer

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the “carrier selectivity” at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution‐processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non‐Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier‐selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier‐selectivity‐controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions.     

Compatibility of PTB7 and [70]PCBM as a Key Factor for the Stability of PTB7:[70]PCBM Solar Cells

The rapid degradation of organic photovoltaic (OPV) devices compared to conventional inorganic solar cells is one of the critical issues that have to be solved in order to make OPV a competitive commercial technology. The understanding of the fundamental mechanisms that reduce the power conversion efficiency (PCE) over time is beneficial for the design of new materials with enhanced stability. This paper focuses on bulk heterojunction organic solar cells based on thieno [3,4‐b] thiophene‐alt‐benzodithiophene (PTB7) mixed with [6,6]‐phenyl‐C71‐butyric acid methyl esther ([70]PCBM). In spite of being promising in terms of PCE, devices based on this blend are unstable and have a short lifetime. When exposed to light in inert atmosphere, the PCE drops by 15% in less than 1 h and by 35% in 8 h; this degradation is induced by the ultraviolet (UV) part of the spectrum. This paper analyzes the effect induced by UV light on the transport of charges in PTB7:[70]PCBM. Contrary to expectations, the electron transport shows evidence of trapping, while the transport of holes appears unaffected. Furthermore, it is proven that the loss of PCE is due to a reaction between PTB7 and [70]PCBM, while the intrinsic instability of the polymer plays a marginal role.     

Identifying the Real Minority Carrier Lifetime in Nonideal Semiconductors: A Case Study of Kesterite Materials

Time‐resolved photoluminescence (TRPL) is a powerful characterization technique to study carrier dynamics and quantify absorber quality in semiconductors. The minority carrier lifetime, which is critically important for high‐performance solar cells, is often derived from TRPL analysis. However, here it is shown that various nonideal absorber properties can dominate the TRPL signal making reliable extraction of the minority carrier lifetime not possible. Through high‐resolution intensity‐, temperature‐, voltage‐dependent, and spectrally resolved TRPL measurements on absorbers and devices it is shown that photoluminescence (PL) decay times for kesterite materials are dominated by minority carrier detrapping. Therefore, PL decay times do not correspond to the minority carrier lifetime for these materials. The lifetimes measured here are on the order of hundreds of picoseconds in contrast to the nanosecond lifetimes suggested by the decay curves. These results are supported with additional measurements, device simulation, and comparison with recombination limited PL decays measured on Cu(In,Ga)Se₂. The kesterite material system is used as a case study to demonstrate the general analysis of TRPL data in the limit of various measurement conditions and nonideal absorber properties. The data indicate that the current bottleneck for kesterite solar cells is the minority carrier lifetime.     

A Comparison of Five Experimental Techniques to Measure Charge Carrier Lifetime in Polymer/Fullerene Solar Cells

It is important to accurately measure the charge carrier lifetime, a crucial parameter that influences the collection efficiency in organic solar cells. Five transient and small perturbation experimental techniques that measure charge carrier lifetime are applied to a device composed of the polymer PDTSiTTz blended with the fullerene PCBM: time‐resolved charge extraction (TRCE), transient absorption spectroscopy (TAS), photoinduced charge extraction by linearly increasing voltage (photo‐CELIV), transient photovoltage, and electrochemical impedance spectroscopy. The motivation is to perform a comprehensive comparison of several different lifetime measurement techniques on the same device in order to assess their relative accuracy, applicability to operational devices, and utility in data analysis. The techniques all produce similar charge carrier lifetimes at high charge densities, despite previous suggestions that transient methods are less accurate than small perturbation ones. At lower charge densities an increase in the apparent reaction order is observed. This may be related to surface recombination at the contacts beginning to dominate, or an inhomogeneous charge distribution. A combination of TAS and TRCE appears suitable. TAS enables the investigation of recombination mechanisms at early times since it is not limited by RC (resistance‐capacitance product) or charge extraction losses. Conversely, TRCE is useful particularly at low densities when other mechanisms, such as surface recombination, may occur.     

The Emergence of the Mixed Perovskites and Their Applications as Solar Cells

The halide perovskite (PVSK) materials (with ABX₃ formulation) have emerged as “dream materials” for photovoltaic (PV) applications due to their remarkable physical properties such as high optical absorption coefficient, carrier mobility, long carrier diffusion lengths, etc. These properties have enabled the PV devices to reach higher than 20% power conversion efficiencies (PCE) in record time. The further pursuit of higher PCE and improved stability brings forth increasing interests in so‐called “mixed composition” PVSK materials, consisting of partial substitution of the A, B, and/or X‐sites with alternative elements/molecules of similar size. Herein, we highlight the recent advances in developing mixed PVSK for PVs and their relevant optoelectronic properties. We mainly focus on mixed PVSK materials in the form of polycrystalline thin films, but also discuss nanostructured and two‐dimensional (2D) PVSK materials due to the increasing interest of broad readership. Efforts are exerted to elucidate the design principles of mixed PVSK and fabrication techniques for high performance optoelectronic devices, which help deepen our fundamental understanding of mixed PVSK systems. We hope this review will shed light onto the design and synthesis of mixed PVSK materials to further the progress of PVSK photovoltaics towards higher efficiencies and longer lifetimes.     

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

Three 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.     

Indolizine‐Based Donors as Organic Sensitizer Components for Dye‐Sensitized Solar Cells

Strong electron‐donating functionality is desirable for many organic donor‐π‐bridge‐acceptor (D‐π‐A) dyes. Strategies for increasing the electron‐donating strength of common nitrogen‐based donors include planarization of nitrogen substituents and the use of low resonance‐stabilized energy aromatic ring‐substituted nitrogen atoms. Organic donor motifs based on the planar nitrogen containing heterocycle indolizine are synthesized and incorporated into dye‐sensitized solar cell (DSC) sensitizers. Resonance active substitutions at several positions on indolizine in conjugation with the D‐π‐A π‐system are examined computationally and experimentally. The indolizine‐based donors are observed to contribute electron density with strengths greater than triarylamines and diarylamines, as evidenced by UV/Vis, IR absorptions, and oxidation potential measurements. Fluorescence lifetime studies in solution and on TiO₂ yield insights in understanding the performance of indolizine‐based dyes in DSC devices.     

Thermally Stable All‐Polymer Solar Cells with High Tolerance on Blend Ratios

Tuning the blend composition is an essential step to optimize the power conversion efficiency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor:acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high‐performance organic nonfullerene BHJ blends PBDB‐T:ITIC and PBDB‐T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer‐polymer (PBDB‐T:N2200) blend are more tolerant to composition changes, relative to polymer‐molecule (PBDB‐T:ITIC) devices. In both systems, short‐circuit current density (J_sc) is tracked closely with PCE, indicating that exciton dissociation and transport strongly influence PCEs. With dilute acceptor concentrations, polymer‐polymer blends maintain high electron mobility relative to the polymer‐molecule blends, which explains the dramatic difference in PCEs between them as a function of D:A blend ratio. In addition, polymer‐polymer solar cells, especially at high D:A blend ratio, are stable (less than 5% relative loss) over 70 d under continuous heating at 80 °C in a glovebox without encapsulation. This work demonstrates that all‐polymer solar cells show advantage in operational lifetime under thermal stress and blend‐ratio resilience, which indicates their high potential for designing of stable and scalable solar cells.     

Donor–Acceptor Shape Matching Drives Performance in Photovoltaics

While the demonstrated power conversion efficiency of organic photovoltaics (OPVs) now exceeds 10%, new design rules are required to tailor interfaces at the molecular level for optimal exciton dissociation and charge transport in higher efficiency devices. We show that molecular shape‐complementarity between donors and acceptors can drive performance in OPV devices. Using core hole clock (CHC) X‐ray spectroscopy and density functional theory (DFT), we compare the electronic coupling, assembly, and charge transfer rates at the interface between C₆₀ acceptors and flat‐ or contorted‐hexabenzocorone (HBC) donors. The HBC donors have similar optoelectronic properties but differ in molecular contortion and shape matching to the fullerene acceptors. We show that shape‐complementarity drives self‐assembly of an intermixed morphology with a donor/acceptor (D/A) ball‐and‐socket interface, which enables faster electron transfer from HBC to C₆₀. The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved efficiency of organic photovoltaic devices that incorporate these concave/convex D/A materials.     

A Halogenation Strategy for over 12% Efficiency Nonfullerene Organic Solar Cells

Three acceptor–donor–acceptor type nonfullerene acceptors (NFAs), namely, F–F, F–Cl, and F–Br, are designed and synthesized through a halogenation strategy on one successful nonfullerene acceptor FDICTF (F–H). The three molecules show red‐shifted absorptions, increased crystallinities, and higher charge mobilities compared with the F–H. After blending with donor polymer PBDB‐T, the F–F‐, F–Cl‐, and F–Br‐based devices exhibit power conversion efficiencies (PCEs) of 10.85%, 11.47%, and 12.05%, respectively, which are higher than that of F–H with PCE of 9.59%. These results indicate that manipulating the absorption range, crystallinity and mobilities of NFAs by introducing different halogen atoms is an effective way to achieve high photovoltaic performance, which will offer valuable insight for the designing of high‐efficiency organic solar cells.     

Efficient Defect Passivation for Perovskite Solar Cells by Controlling the Electron Density Distribution of Donor‐π‐Acceptor Molecules

Organic–inorganic hybrid perovskite solar cells (PSCs) are a promising photovoltaic technology that has rapidly developed in recent years. Nevertheless, a large number of ionic defects within perovskite absorber can serve as non‐radiative recombination center to limit the performance of PSCs. Here, organic donor‐π‐acceptor (D‐π‐A) molecules with different electron density distributions are employed to efficiently passivate the defects in the perovskite films. The X‐ray photoelectron spectroscopy (XPS) analysis shows that the strong electron donating N,N‐dibutylaminophenyl unit in a molecule causes an increase in the electron density of the passivation site that is a carboxylate group, resulting in better binding with the defects of under‐coordinated Pb²⁺ cations. Carrier lifetime in the perovskite films measured by the time‐resolved photoluminescence spectrum is also prolonged by an increase in donation ability of the D‐π‐A molecules. As a consequence, these benefits contribute to an increase of 80 mV in the open circuit voltage of the devices, enabling a maximum power conversion efficiency (PCE) of 20.43%, in comparison with PCE of 18.52% for the control device. The authors' findings provide a novel strategy for efficient defect passivation in the perovskite solar cells based on controlling the electronic configuration of passivation molecules.     

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.     

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.