Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity

Ratcliff, Erin L.; García, Alberto Luis García; Paniagua, Sergio A.; Cowan, Sarah R.; Giordano, Anthony J.; Ginley, David S.; Marder, Seth R.; Berry, Joseph J.; Olson, Dana C.

doi:10.1002/aenm.201200669

Cited by 128 publications

(190 citation statements)

References 82 publications

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“…Indeed, it has been demonstrated that the cell performance is correlated with the substrate Φ SUB , and tuning can induce the V oc variation and thereby the PCE in PSCs. [ 36 ] …”

Section: Substrate-dependent Energeticsmentioning

confidence: 97%

“…Considering the contacts between polymer BHJs and the underlying substrates in PSCs, four commonly used materials on indium-tin-oxide (ITO) glass are used, including poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and molybdenum trioxide (MoO 3 ) as anode contact in regular-structure PSCs for hole extraction, and zinc oxide (ZnO) and cesium fl uoride (CsF) as cathode contact in inverted structure PSCs for electron extraction, respectively. [35][36][37][38][39] Figure 2 displays the UPS spectra of PCDTBT:PC 70 BM BHJs of various blending ratios on four conductive substrates (i.e., CsF, ZnO, PEDOT:PSS, and MoO 3 ). The thicknesses of four substrates are 20, 40, 40, and 20 nm for CsF, ZnO, PEDOT:PSS, and MoO 3 , respectively.…”

Section: Introductionmentioning

confidence: 99%

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Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction

Zhou

Xie

et al. 2015

Adv Materials Inter

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still some obstacles for holding back the further development.The state-of-the-art device structure of PSCs is based on the bulk heterojunction (BHJ) formed by conjugated polymers (electron donor) and fullerene derivatives (electron acceptor). It is commonly recognized that the interfacial properties (e.g., energetics and morphology) between the BHJ and electrodes are decisive for charge transport and extraction process, which determine the device performance and long-term stability. [12][13][14][15] For instance, the energy offset between the lowest unoccupied molecular orbital level (LUMO) of the acceptor and the highest occupied molecular orbital level (HOMO) of the donor sets an upper limit for the open-circuit voltage ( V oc ) in PSCs. [16][17][18][19][20][21][22][23] However, for various organic/substrate contacts and organic donor/acceptor heterostructures with weak interfacial interaction and electronic coupling, energy level alignments (ELAs) at the interfaces are highly dependent on the substrates, revealing a clear transition from the traditionally used vacuum level alignment (VLA) mode to the Fermi level pinning (FLP) mode. [ 12,15,[22][23][24][25][26][27] When the substrate work function (Φ SUB ) lies between the ionization potential (IP) and electron affi nity (EA) values of the organic overlayer, VLA holds for the interface energetics and the work function of the organic overlayer (Φ ORG/SUB ) equals to the Φ SUB . If Φ SUB is larger (or lower) than IP (or EA) of the organic overlayer, spontaneous charge transfer occurs from the organic layer to the substrate (or from the substrate to the organic layer), creating an interface dipole equivalent to Φ SUB -Φ ORG/SUB . The resultant ELAs are determined by the FLP to the HOMO edge (or LUMO edge) of the organic layer. Recently, several models have been proposed to interpret the energetics transition between VLA and FLP. For example, Fahlman and coworkers proposed the integer charge transfer (ICT) model to describe organic/organic interfaces that are characterized by a negligible hybridization of π-electronic molecular orbitals and substrate wave functions. [ 12,28 ] Koch and co-workers used the numerical electrostatic model to reproduce the alignment between the electrode Fermi energy and the transport states in organic semiconductors. [ 26 ] Similarly, the universal ELAs of organic molecules on metal oxides have been experimentally summarized, [ 29,30 ] and theoretically calculated based on Fermi statistics. [ 31 ] In addition, surface properties of the substrates have a substantial infl uence on the fi lm morphology of

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“…Indeed, it has been demonstrated that the cell performance is correlated with the substrate Φ SUB , and tuning can induce the V oc variation and thereby the PCE in PSCs. [ 36 ] …”

Section: Substrate-dependent Energeticsmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction

Zhou

Xie

et al. 2015

Adv Materials Inter

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show abstract

“…The effects of selective layers on OPV diode characteristics in the dark and under illumination are distinct from changes due to shifts in the contact layer work function ( φ ). [ 17 ] Changes in φ tend to only manifest themselves in the open-circuit voltage ( V oc ), whereas changes in selectivity from thermodynamic barriers (with constant φ ) are evident in the reverse saturation current and shunt resistance, in addition to V oc . Additionally, a study comparing injection and extraction effi ciency in OPV devices with and without selective contacts showed that the dominant mechanism controlling V oc is injection, with charge selective contacts having a much lower rate of charge injection into the active layer.…”

Section: Doi: 101002/admi201500346mentioning

confidence: 98%

Doped Interlayers for Improved Selectivity in Bulk Heterojunction Organic Photovoltaic Devices

Mauger

Glasser

Villers

et al. 2015

Adv Materials Inter

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can be inserted between the active layer and the contact layer to create a new interface with more favorable properties, which is the method explored here. These thin layers are often called interlayers and many types of materials have been investigated, such as low work-function metals, [ 3,4 ] metal salts, [ 5 ] silane-based selfassembled monolayers (SAMs), [ 6 ] phosphonic-acid SAMs, [ 7,8 ] metal oxides, [9][10][11] polymers, [12][13][14][15] and polyelectrolytes. [ 16 ] Other examples are also available in the literature, with a thorough review provided by Ma et al. [ 1 ] The majority of these interlayers improve device performance by reducing electronic barriers for extraction of the desired charge carrier and/or increasing charge selectivity by blocking the undesired charge carrier. Creating interfaces or layers that preferentially extract only one polarity of charge carrier are key to optimizing BHJ devices since the intermixed donor and acceptor do not create a diode.Charge selective layers create a thermodynamic barrier for collection of the undesired carrier (electron or hole). The effects of selective layers on OPV diode characteristics in the dark and under illumination are distinct from changes due to shifts in the contact layer work function ( φ ).[ 17 ] Changes in φ tend to only manifest themselves in the open-circuit voltage ( V oc ), whereas changes in selectivity from thermodynamic barriers (with constant φ ) are evident in the reverse saturation current and shunt resistance, in addition to V oc . Additionally, a study comparing injection and extraction effi ciency in OPV devices with and without selective contacts showed that the dominant mechanism controlling V oc is injection, with charge selective contacts having a much lower rate of charge injection into the active layer. [ 18 ] One of the most commonly used charge selective contact layers in OPV devices is the polymer blend poly(3,4-ethylenedi oxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The nanostructure of PEDOT:PSS layers is complex due to the phase-separated colloidal structure of the PEDOT:PSS ink. In the bulk of the layer, elongated ellipsoids of PEDOT are surrounded by thin shells of PSS. [ 19,20 ] This morphology gives rise to its anisotropic conductivity and refractive index. [ 21,22 ] However, the surface of the PEDOT:PSS has a 3-4 nm thick PSS-rich surface layer. [23][24][25] This PSS-rich layer forms due to surface energy considerations. In blend fi lms, the component with the lowest surface energy

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“…S11). Not surprisingly, FETs with high In contents (a-IGO, Ga < 90%; a-GZTO, Ga < 45%) have substantial electron mobilities (μ e ∼ 10 −3 to 8 cm S/m), note that thin, low-conductivity oxide IFLs (≤20 nm) can still achieve efficient, selective charge extraction (53)(54)(55). From these results, the Ga atomic content ≤ 45% for a-GZTO and between 20 and 80 wt % for a-IGO provides acceptable electron transport and will be evaluated with different OPV donor/acceptor systems (vide infra).…”

Section: Significancementioning

confidence: 99%

Amorphous oxide alloys as interfacial layers with broadly tunable electronic structures for organic photovoltaic cells

Zhou

Kim

Loser

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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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...

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Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity

Cited by 128 publications

References 82 publications

Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction

Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction

Doped Interlayers for Improved Selectivity in Bulk Heterojunction Organic Photovoltaic Devices

Amorphous oxide alloys as interfacial layers with broadly tunable electronic structures for organic photovoltaic cells

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Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity

Cited by 128 publications

References 82 publications

Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC70BM Bulk Heterojunction

Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC70BM Bulk Heterojunction

Doped Interlayers for Improved Selectivity in Bulk Heterojunction Organic Photovoltaic Devices

Amorphous oxide alloys as interfacial layers with broadly tunable electronic structures for organic photovoltaic cells

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Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction

Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction