With the goal of investigating and enhancing anode performance in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells, the glass/tin-doped indium oxide (ITO) anodes are modified with a series of robust silane-tethered bis(fluoroaryl)amines to form self-assembled interfacial layers (IFLs). The modified ITO anodes are characterized by contact angle measurements, X-ray reflectivity, ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, and cyclic voltammetry. These techniques reveal the presence of hydrophobic amorphous monolayers of 6.68 to 9.76 Å thickness, and modified anode work functions ranging from 4.66 to 5.27 eV. Two series of glass/ITO/IFL/active layer/LiF/Al BHJ OPVs are fabricated with the active layer = poly(3-hexylthiophene):phenyl-C71-butyric acid methyl ester (P3HT:PC71BM) or poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]thi-eno[3,4-b]thiophenediyl]]:phenyl-C71-butyric acid methyl ester (PTB7:PC71BM). OPV analysis under AM 1.5G conditions reveals significant performance enhancement versus unmodified glass/ITO anodes. Strong positive correlations between the electrochemically derived heterogeneous electron transport rate constants (ks) and the device open circuit voltage (Voc), short circuit current (Jsc), hence OPV power conversion efficiency (PCE), are observed for these modified anodes. Furthermore, the strong functional dependence of the device response on ks increases as greater densities of charge carriers are generated in the BHJ OPV active layer, and is attributable to enhanced anode carrier extraction in the case of high-ks IFLs.
The ionization potentials (IPs) and electron affinities (EAs) of widely used conjugated polymers are evaluated by cyclic voltammetry (CV) in conventional electrochemical and lithium-ion battery media, and also by ultraviolet photoelectron spectroscopy (UPS) in vacuo. By comparing the data obtained in the different systems, it is found that the IPs of the conjugated polymer films determined by conventional CV (IPC) can be correlated with UPS-measured HOMO energy levels (EH,UPS) by the relationship EH,UPS = (1.14 ± 0.23) × qIPC + (4.62 ± 0.10) eV, where q is the electron charge. It is also found that the EAs of the conjugated polymer films measured via CV in conventional (EAC) and Li(+) battery (EAB) media can be linearly correlated by the relationship EAB = (1.07 ± 0.13) × EAC + (2.84 ± 0.22) V. The slopes and intercepts of these equations can be correlated with the dielectric constants of the polymer film environments and the redox potentials of the reference electrodes, as modified by the surrounding electrolyte, respectively.
For example, by systematically varying the thickness of the photoactive region (a tedious process), optical cavity modes that serve to enhance absorption can be tuned in frequency. [ 16 ] Furthermore, as we show here, optical transfer matrix simulations can be used to expeditiously optimize photocurrent generation in the photoactive region by shaping its absorption spectrum. In this contribution, transfer matrix calculations are shown to effectively guide OPV performance enhancement by spectral tuning in inverted polymer photovoltaic architectures.Since both donor and acceptor materials in the active layer contact both electrodes in BHJ cells, interfacial layers (IFLs) are typically introduced to minimize leakage currents. [ 17 ] In conventional OPV device architectures, where holes are collected at the transparent indium tin oxide (ITO) anode and electrons at the refl ective metal cathode, the archetypical IFL deposited on the ITO is the hole transport layer poly(3,4-ethylenedioxyle nethiophene):poly(styrenesulphonic acid) (PEDOT:PSS). However, this layer limits device lifetime since it is corrosive, [ 18 ] hygroscopic, [ 19 ] and thermally unstable, [ 20 ] motivating alternative IFL materials strategies. Thus, an inverted device architecture (Figure 1 ), where ITO collects electrons and a high work function metal electrode collects holes, has proven very effective in enhancing both OPV performance and durability. [ 21,22 ] In the present work, an electron transport layer (ETL) coating is deposited on the ITO cathode. Solution deposited zinc oxide (ZnO) is a particularly effective ETL in inverted OPVs due to its large bandgap, [ 23 ] high electron mobility, [ 24 ] solar transparency, [ 25 ] and well-positioned conduction band energy for use with electron acceptors, such as fullerene derivatives. [ 26 ] While recent literature has demonstrated higher PCEs using a polymeric ETL in the inverted OPV architecture, [ 27 ] sol-gel ZnO is inexpensive, environmentally friendly, [ 28 ] and a common ETL in inverted OPVs, motivating this study on its impact in optical cavity strategies. [ 29 ] In addition to its favorable ETL properties, ZnO has also been used as an optical spacer [ 30 ] when adjacent to the refl ective metal electrode, improving the distribution of optical intensity in conventional OPVs. In contrast, this work describes the very signifi cant consequences for the optical intensity distribution of placing a ZnO layer adjacent to the transparent electrode in inverted architecture OPVs.The inverted device architecture in this work utilizes a ZnO ETL and a BHJ active layer composed of the donor poly [[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fl uoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7) and the acceptor [6,6]-phenyl C 71 butyric acid methyl-ester (PC 71 BM; Figure 1 ). The PTB7:PC 71 BM active layer has been previously shown to yield large internal quantum effi ciencies and exhibit absorption across much of the visible spectrum. [ 31,32 ] Furthermore, this a...
In diverse classes of organic optoelectronic devices, controlling charge injection, extraction, and blocking across organic semiconductor-inorganic electrode interfaces is crucial for enhancing quantum efficiency and output voltage. To this end, the strategy of inserting engineered interfacial layers (IFLs) between electrical contacts and organic semiconductors has significantly advanced organic light-emitting diode and organic thin film transistor performance. For organic photovoltaic (OPV) devices, an electronically flexible IFL design strategy to incrementally tune energy level matching between the inorganic electrode system and the organic photoactive components without varying the surface chemistry would permit OPV cells to adapt to ever-changing generations of photoactive materials. Here we report the implementation of chemically/environmentally robust, low-temperature solution-processed amorphous transparent semiconducting oxide alloys, In-Ga-O and Ga-Zn-Sn-O, as IFLs for inverted OPVs. Continuous variation of the IFL compositions tunes the conduction band minima over a broad range, affording optimized OPV power conversion efficiencies for multiple classes of organic active layer materials and establishing clear correlations between IFL/photoactive layer energetics and device performance.interface | amorphous oxide | photovoltaic | interfacial layers S olar to electrical energy conversion technologies have received great attention as abundant and sustainable resources (1-5). The diffuse nature of solar energy requires low-cost, largearea devices while maintaining high power conversion efficiency (PCE) (3). As a universal design strategy, many of the emerging thin film photovoltaic (PV) technologies such as bulk heterojunction (BHJ) organic, perovskite, quantum dot (QD), and CIGS (Cu-InGa-Se) solar cells are fabricated using a trilayer architecture, where light absorbers are sandwiched between two electrodes coated with various interfacial layers (IFLs) (6-9). Stringent requirements govern ideal IFL materials design. Energetically, their respective band positions should match those of the photoinduced built-in potentials to provide energetically continuous carrier transport pathways and to accommodate the maximum allowed output voltage. In recent reports, PV performance enhancement via IFL energetic tuning has been demonstrated for very specific BHJ organic, QD, and perovskite cell compositions (6,8,10,11). However, true IFL energetic tunability has not been achieved and offers a challenging opportunity to optimize device performance.Fabricable from energetically diverse organic active layers, organic photovoltaics (OPVs) provide an excellent test bed for tuning IFL energetics and are the subject of this study. The basic BHJ OPV architecture contains a mesoscopically heterogeneous and isotropic, phase-separated donor-acceptor blend-a strategy to overcome the relatively short exciton diffusion lengths (∼10 nm) (2, 12, 13), sandwiched between hole-transporting (HT) and electron-transporting (ET) IFLs (2). Th...
To achieve densely packed charge-selective organosilane-based interfacial layers (IFLs) on the tin-doped indium oxide (ITO) anodes of organic photovoltaic (OPV) cells, a series of Ar2N-(CH2)n-SiCl3 precursors with Ar = 3,4-difluorophenyl, n = 3, 6, 10, and 18, was synthesized, characterized, and chemisorbed on OPV anodes to serve as IFLs. To minimize lateral nonbonded -NAr2···Ar2N- repulsions which likely limit IFL packing densities in the resulting self-assembled monolayers (SAMs), precursor mixtures having both small and large n values are simultaneously deposited. These "heterogeneous" SAMs are characterized by a battery of techniques: contact angle measurements, X-ray reflectivity, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry, and DFT computation. It is found that the headgroup densities of these "supersaturated" heterogeneous SAMs (SHSAMs) are enhanced by as much as 17% versus their homogeneous counterparts. Supersaturation significantly modifies the IFL properties including the work function (as much as 16%) and areal dipole moment (as much as 49%). Bulk-heterojunction OPV devices are fabricated with these SHSAMs: ITO/IFL/poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][2-[[(2-ethylhexyl)oxy]carbonyl]-3-fluorothieno[3,4-b]thiophenediyl]]:phenyl-C71-butyric acid methyl ester (PTB7:PC71BM)/LiF/Al. OPVs having SHSAM IFLs exhibit significantly enhanced performance (PCE by 54%; Voc by 35%) due to enhanced charge selectivity and collection, with the PCE rivaling or exceeding that of PEDOT:PSS IFL devices -7.62%. The mechanism underlying the enhanced performance involves modified hole collection and selectivity efficiency inferred from the UPS data. The ITO/SAM/SHSAM surface potential imposed by the dipolar SAMs causes band bending and favorably alters the Schottky barrier height. Thus, interfacial charge selectivity and collection are enhanced as evident in the greater OPV Voc.
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