Charge Recombination Dynamics in Organic Photovoltaic Systems with Enhanced Dielectric Constant

Increasing the dielectric constant of organic photovoltaic materials to reduce recombination rates has long been pursued, however, material modification often results in the modification of multiple device characteristics, making system comparison difficult. In this study, a fullerene derivative with an increased blend dielectric constant is examined by the addition of a triethylene glycol appendage to the fullerene (TEG‐PCBM). Density functional theory calculations show a small change to the permanent dipole moment between TEG‐PCBM and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₀BM) resulting in similar solubility, morphology, and device performance. TEG‐PCBM is blended with donors P3HT and PTB7‐Th and a comparable performance to PC₆₀BM is found. This model system shows the rarely reported characteristic of an increase in the dielectric constant while leaving its other properties unaltered. Looking at light intensity effects on open‐circuit voltage (V_oc), short‐circuit current (J_sc), and fill factor (FF) along with exciton dissociation efficiency, it is observed that when switching to the TEG‐ modified fullerene derivative, geminate recombination is not reduced, and Shockley–Read–Hall recombination is increased. While triethlyene glycol appendages may prove to be ineffective in improving recombination through increased dielectric constant, an approach for studying recombination in future high dielectric systems is provided.     

Cyclobutadiene–C60 Adducts: N‐Type Materials for Organic Photovoltaic Cells with High VOC

New tetraalkylcyclobutadiene–C₆₀ adducts are developed via Diels–Alder cycloaddition of C₆₀ with in situ generated cyclobutadienes. The cofacial π‐orbital interactions between the fullerene orbitals and the cyclobutene are shown to decrease the electron affinity and thereby increase the lowest unoccupied molecular orbital (LUMO) energy level of C₆₀ significantly (ca. 100 and 300 meV for mono‐ and bisadducts, respectively). These variations in LUMO levels of fullerene can be used to generate higher open‐circuit voltages (V_OC) in bulk heterojunction polymer solar cells. The tetramethylcyclobutadiene–C₆₀ monoadduct displays an open‐circuit voltage (0.61 V) and a power conversion efficiency (2.49%) comparable to the widely used P3HT/PCBM (poly(3‐hexylthiophene/([6,6]‐phenyl‐C61‐butyric acid methyl ester) composite (0.58 V and 2.57%, respectively). The role of the cofacial π‐orbital interactions between C₆₀ and the attached cyclobutene group was probed chemically by epoxidation of the cyclobutene moiety and theoretically through density functional theory calculations. The electrochemical, photophysical, and thermal properties of the newly synthesized fullerene derivatives support the proposed effect of functionalization on electron affinities and photovoltaic performance.     

Degradation Effects Related to the Hole Transport Layer in Organic Solar Cells

The influence of the hole transport layer on device stability in polymer:fullerene bulk‐heterojunction solar cells is reported. Three different hole transport layers varying in composition, dispersion solvent, electrical conductivity, and work function were used in these studies. Two water‐based hole transport layers, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) and polyaniline:poly(styrene sulfonate), and one isopropyl alcohol‐based polyaniline:poly(styrene sulfonate) transport layer were investigated. Solar cells with the different hole transport layers were fabricated and degraded under illumination. Current–voltage, capacitance–voltage, and capacitance–frequency data were collected at light intensities of 16, 30, 48, 80, and 100 mW cm^?2 over a period of 7 h. Device performance and stability were compared between nonencapsulated and encapsulated samples to gain understanding about degradation effects related to oxygen and water as well as degradation mechanisms related to the intrinsic instability of the solar cell materials and interfaces. It is demonstrated that the properties of the hole transport layer can have a significant impact on the stability of organic solar cells.     

Impedance Measurements as a Simple Tool to Control the Quality of Conjugated Polymers Designed for Photovoltaic Applications

The problem of batch‐to‐batch variation of electronic properties and purity of conjugated polymers used as electron donor and photon harvesting materials in organic solar cells is addressed. A simple method is developed for rapid analysis of electronic quality of polymer‐based materials. It is shown that appearance of impurities capable of charge trapping changes electrophysical properties of conjugated polymers. In particular, a clear correlation between the effective relaxation time τeff and relative photovoltaic performance (η/η_max) is revealed for samples of poly(3‐hexylthiophene) intentionally polluted with a palladium catalyst. This dependence is also valid for all other investigated samples of conjugated polymers. Therefore, fast impedance measurements at three different frequencies allow one to draw conclusions about the purity of the analyzed polymer sample and even estimate its photovoltaic performance. The developed method might find extensive applications as a simple tool for product quality control in the laboratory and industrial‐scale production of conjugated polymers for electronic applications.     

Mapping the Density of States Distribution of Organic Semiconductors by Employing Energy Resolved–Electrochemical Impedance Spectroscopy

Although the density of states (DOS) distribution of charge transporting states in an organic semiconductor is vital for device operation, its experimental assessment is not at all straightforward. In this work, the technique of energy resolved–electrochemical impedance spectroscopy (ER-EIS) is employed to determine the DOS distributions of valence (highest occupied molecular orbital (HOMO)) as well as electron (lowest unoccupied molecular orbital (LUMO)) states in several organic semiconductors in the form of neat and blended films. In all cases, the core of the inferred DOS distributions are Gaussians that sometimes carry low energy tails. A comparison of the HOMO and LUMO DOS of P3HT inferred from ER-EIS and photoemission (PE) or inverse PE (IPE) spectroscopy indicates that the PE/IPE spectra are by a factor of 2–3 broader than the ER-EIS spectra, implying that they overestimate the width of the distributions. A comparison of neat films of MeLPPP and SF-PDI₂ or PC(61)BM with corresponding blends reveals an increased width of the DOS in the blends. The results demonstrate that this technique does not only allow mapping the DOS distributions over five orders of magnitude and over a wide energy window of 7 eV, but can also delineate changes that occur upon blending.     

Efficient Organic Photovoltaic Cells Based on Nanocrystalline Mixtures of Boron Subphthalocyanine Chloride and C60

The electrical and structural behavior of uniformly mixed films of boron subphthalocyanine chloride (SubPc) and C₆₀ and their performance in organic photovoltaic cells is explored. Device performance shows a strong dependence on active‐layer donor–acceptor composition, and peak efficiency is realized at 80 wt.% C₆₀. The origin of this C₆₀‐rich optimum composition is elucidated in terms of morphological changes in the active layer upon diluting SubPc with C₆₀. While neat SubPc is found to be amorphous, mixed films containing 80 wt.% C₆₀ show clear nanocrystalline domains of SubPc. Supporting electrical characterization indicates that this change in morphology coincides with an increase in the hole mobility of the SubPc:C₆₀ mixture, with peak mobility observed at a composition of 80 wt.% C₆₀. Organic photovoltaic cells constructed using this optimum SubPc:C₆₀ ratio realize a power conversion efficiency of (3.7 ± 0.1)% under 100 mW cm^?2 simulated AM1.5G solar illumination.     

有机薄膜太阳电池的研究进展

有机薄膜太阳电池作为一种新型光伏电池,近年来得到了迅猛发展。其制备工艺简单、价格低廉、柔性、质轻,为人类解决能源问题提供了一种崭新的途径。文章综述了近年来有机薄膜太阳电池的发展状况,结合有机薄膜太阳电池的发展历史,分析了单异质结、体异质结和叠层三种典型结构器件的工作原理和研究成果,探讨了各种器件结构的优缺点,并对有机薄膜太阳电池的发展趋势作了展望。     

利用通用迁移率模型计算有机二极管的交流阻抗谱

推导出了有机二极管交流阻抗谱的公式,当迁移率为常数时的计算结果与文献中的解析公式相符合,表明推出的公式和编写的程序是正确的。进一步根据文献中与温度、载流子浓度和电场强度有关的通用迁移率模型,计算了NRS-PPV有机二极管的交流阻抗谱。结果表明阻抗的实部总是取正值,虚部总是取负值。阻抗的实部和虚部的绝对值在低频极限下的数值都是随温度和直流偏压增大而减小。实部总是频率的单调减函数,且减小的速率随温度和直流偏压增大而变慢;虚部绝对值在低温下是频率的单调减函数,在高温下将出现极大值,极值的峰高随温度和直流偏压增大而减小,极值位置相应地出现蓝移。     

Impedance Spectroscopy of π‐Conjugated Organic Materials

AC electrical properties of organic light‐emitting diodes with poly(2‐methoxy‐5‐(2'‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV), poly[2,5‐bia(dimethyloctylsilyl)‐1,4‐phenylene‐vinylene] (BDMOS‐PPV), and tris‐(8‐hydroxyquinolate)‐aluminum (AlQ₃) as light‐emitting materials are studied. The frequency‐dependent real and imaginary parts of impedance were fitted using an equivalent circuit. We found that the conduction mechanism is a space‐charge limited current with exponential trap distribution.     

Fullerene Adducts Bearing Cyano Moiety for Both High Dielectric Constant and Good Active Layer Morphology of Organic Photovoltaics

Power conversion efficiency (PCE) of organic photovoltaics (OPVs) lags behind of inorganic photovoltaics due to low dielectric constants (ε _r) of organic semiconductors. Although OPVs with high ε _r are attractive in theory, practical demonstration of efficient OPV devices with high‐ε _r materials is in its infancy. This is largely due to the contradiction between the requirements of high ε _r and good donor:acceptor blend morphology in the bulk heterojunction. Herein, a series of fullerene acceptors is reported bearing a polar cyano moiety for both high ε _r and good donor:acceptor blend morphology. These cyano‐functionalized acceptors (ε _r = 4.9) have higher ε _r than that of the widely used acceptor, [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) (ε _r = 3.9). The high ε _r is realized without decrease of electron mobility and change of the lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy levels. Although the cyano‐functionalized acceptors have increased polarity, they still exhibit good compatibility with the typical donor polymer. Polymer solar cells based on the cyano‐functionalized acceptors exhibit good active layer morphology and show better device performance (PCE = 5.55%) than that of PC₆₁BM (PCE = 4.56%).     

Imaging the Electric Potential within Organic Solar Cells

The charge transport in organic solar cells is investigated by surface potential measurements via scanning Kelvin probe microscopy. Access to the solar cell's cross‐section is gained by milling holes with a focused ion beam which enables the direct scan along the charge transport path. In a study of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)propyl‐1‐phenyl[6,6]C61 (P3HT:PCBM) bulk heterojunction solar cells, the open circuit voltage is built up at the top contact. A comparison of the potential distribution within normal and inverted solar cells under operation exhibits strongly different behaviors, which can be assigned to a difference in interface properties.     

Exciton‐Charge Annihilation in Organic Semiconductor Films

Time‐resolved optical spectroscopy is used to investigate exciton‐charge annihilation reactions in blended films of organic semiconductors. In donor–acceptor blends where charges are photogenerated via excitons, pulsed optical excitation can deliver a sufficient density of temporally overlapping excitons and charges for them to interact. Transient absorption spectroscopy measurements demonstrate clear signatures of exciton‐charge annihilation reactions at excitation densities of ≈10¹⁸ cm^?3. The strength of exciton‐charge annihilation is consistent with a resonant energy transfer mechanism between fluorescent excitons and resonantly absorbing charges, which is shown to generally be strong in organic semiconductors. The extent of exciton‐charge annihilation is very sensitive not only to fluence but also to blend morphology, becoming notably strong in donor–acceptor blends with nanomorphologies optimized for photovoltaic operation. The results highlight both the value of transient optical spectroscopy to interrogate exciton‐charge annihilation reactions and the need to recognize and account for annihilation reactions in other transient optical investigations of organic semiconductors.     

Effect of Trace Solvent on the Morphology of P3HT:PCBM Bulk Heterojunction Solar Cells

The performance of bulk‐heterojunction (BHJ) solar cells is strongly correlated with the nanoscale structure of the active layer. Various processing techniques have been explored to improve the nanoscale morphology of the BHJ layer, e.g., by varying the casting solvent, thermal annealing, solvent annealing, and solvent additives. This paper highlights the role of residual solvent in the “dried” BHJ layer, and the effect of residual solvents on PCBM diffusion and ultimately the stability of the morphology. We show that solvent is retained within the BHJ film despite prolonged heat treatment, leading to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM agglomerates. The addition of a small volume fraction of nitrobenzene to the casting solution inhibits the diffusion of PCBM in the dry film, resulting in smaller PCBM agglomerates, and improves the fill factor of the BHJ device to 0.61 without further tempering. The addition of nitrobenzene also increases the P3HT crystalline content, while increasing the onset temperature for melting of P3HT side chains and backbone. The melting temperature for PCBM is also higher with the nitrobenzene additive present.     

Molecular Packing and Dielectric Property Optimization through Peripheral Halogen Swapping Enables Binary Organic Solar Cells with an Efficiency of 18.77%

Peripheral halogen regulations can endow non-fullerene acceptors (NFAs) with enhanced features as organic semi-conductors and further boost efficient organic solar cells (OSCs). Herein, based on a remarkable molecular platform of CH14 with more than six halogenation positions, a preferred NFA of CH23 is constructed by synergetic halogen swapping on both central and end units, rendering the overall enlarged molecular dipole moment, packing density and thus relative dielectric constant. Consequently, the CH23-based binary OSC reaches an excellent efficiency of 18.77% due to its improved charge transfer/transport dynamics, much better than that of 17.81% for the control OSC of CH14. This work demonstrates the great potential for further achieving state-of-the-art OSCs by delicately regulating the halogen formula on these newly explored CH-series NFAs.     

Photovoltaic Function and Exciton/Charge Transfer Dynamics in a Highly Efficient Semiconducting Copolymer

Exciton dissociation is a key step for the light energy conversion to electricity in organic photovoltaic (OPV) devices. Here, excitonic dissociation pathways in the high‐performance, low bandgap “in‐chain donor–acceptor” polymer PTB7 by transient optical absorption (TA) spectroscopy in solutions, neat films, and bulk heterojunction (BHJ) PTB7:PC₇₁BM (phenyl‐C₇₁‐butyric acid methyl ester) films are investigated. The dynamics and energetics of the exciton and intra‐/intermolecular charge separated states are characterized. A distinct, dynamic, spectral red‐shift of the polymer cation is observed in the BHJ films in TA spectra following electron transfer from the polymer to PC₇₁BM, which can be attributed to the time evolution of the hole–electron spatial separation after exciton splitting. Effects of film morphology are also investigated and compared to those of conjugated homopolymers. The enhanced charge separation along the PTB7 alternating donor–acceptor backbone is understood by intramolecular charge separation through polarized, delocalized excitons that lower the exciton binding energy. Consequently, ultrafast charge separation and transport along these polymer backbones reduce carrier recombination in these largely amorphous films. This charge separation mechanism explains why higher degrees of PCBM intercalation within BHJ matrices enhances exciton splitting and charge transport, and thus increase OPV performance. This study proposes new guidelines for OPV materials development.     

涂层圆柱爬行波传播常数和模式阻抗的分析与计算

本文分析了介质涂层圆柱爬行波的电磁特性。找到了一种求解爬行波传播常数和模式阻抗的有效方法－微扰－迭代法，这种方法简单、快速，便于工程应用。最后给出公式并计算了一些实例。     

Identifying a Threshold Impurity Level for Organic Solar Cells: Enhanced First‐Order Recombination Via Well‐Defined PC84BM Traps in Organic Bulk Heterojunction Solar Cells

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap‐assisted recombination rate when [6,6]‐phenyl C₈₄ butyric acid methyl ester (PC₈₄BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]‐phenyl C₆₁ butyric acid methyl ester (PC₆₀BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley–Read–Hall recombination; the latter is dramatically enhanced by the addition of the PC₈₄BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap‐induced recombination loss.     

Analysis of Characteristic Impedance and Effective Dielectric Constant of Asymmetrical Coplanar Waveguide with Finite Dimensions

Coplanar waveguide transmission lines have become quite attractive due to its applica-tion to microwave or millimeter-wave integrated circuits and devices.Calculations of char-acteristic impedance and effective dielectric constant of...     

Analysis of Characteristic Impedance and Effective Dielectric Constant of Asymmetrical Coplanar Waveguide with Finite Dimensions

Conformal mapping techniques are used to obtain the closed-form expressions of the characteristic impedance and effective dielectric constant of the asymmetrical coplanar waveguide with finite line dimensions and substrate thickness. These closed-form expressions are generally applicable to design various particular types of coplanar waveguide transmission lines.