Towards reliable charge-mobility benchmark measurements for organic semiconductors

Blakesley, James C.; Castro, Fernando A.; Kylberg, William; Dibb, George F. A.; Arantes, C.; Valaski, R.; Cremona, M.; Kim, Jong Soo; Kim, Ji-Seon

doi:10.1016/j.orgel.2014.02.008

Cited by 251 publications

(235 citation statements)

References 44 publications

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“…JV characteristics indicate the PFI-modifi ed HILs do not consistently nor signifi cantly improve the hole current density (Figure 6 b). The simulated ohmic JV characteristics are shown as a gray line, for a bare potential of 2.0 V, and µ SCLC = 2 × 10 −4 cm 2 V −1 s −1 taken from the work of Blakesley et al [ 50 ] The measured JV characteristics clearly fall well below the ohmic characteristic at turn-on (i.e., very high contact resistance there), approaching it only at high drive voltages.…”

Section: Resultsmentioning

confidence: 94%

Perfluorinated Ionomer‐Modified Hole‐Injection Layers: Ultrahigh‐Workfunction but Nonohmic Contacts

Belaineh

Tan

Png

et al. 2015

Adv Funct Materials

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Recently it has been reported that Nafion oligomers, i.e., 2‐(2‐sulfonatotetrafluoroethoxy)‐2‐trifluoromethyltrifluoroethoxyfunctionalized oligotetrafluoroethylenes, also called perfluorinated ionomers (PFIs), can be blended into poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDT:PSSH) films to increase their workfunctions beyond 5.2 eV. These PFI‐modified films are useful for energy‐level alignment studies, and have been proposed as hole‐injection layers (HILs). It is shown here however that these HILs do not provide sufficiently fast hole transfer into adjacent polymer semiconductor layers with ionization potentials deeper than ≈5.2 eV. X‐ray and ultraviolet photoemission spectroscopies reveal that these HILs exhibit a molecularly‐thin PFI overlayer that sets up a surface dipole that provides the ultrahigh workfunction. This dipolar layer persists even when the subsequent organic semiconductor layer is deposited, as evidenced by measurements of the diode built‐in potentials. As a consequence, the PFI‐modified HILs produce a higher contact resistance, and a lower equilibrium density of holes at the semiconductor contact than might have been expected from simple thermodynamic considerations of the reduction in hole‐injection barrier. Thus the use of insulating dipolar surface layers at the charge‐injection contact to tune its workfunction to match the relevant transport level of the semiconductor is of limited utility to achieve ohmic contact in these devices.

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Section: Resultsmentioning

confidence: 94%

Perfluorinated Ionomer‐Modified Hole‐Injection Layers: Ultrahigh‐Workfunction but Nonohmic Contacts

Belaineh

Tan

Png

et al. 2015

Adv Funct Materials

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“…In order to correct for the built-in voltage, an effective applied voltage V eff ¼ V-V bi , where the V bi is used as a fitting parameter in the MG law (or other analytical equations of variant types, such as the MH equation or the Murgatroyd equation [30]), is commonly used [31,32]. This indirect approach is, however, rather uncertain, since the band diagram at V ¼ 0 V for a symmetric device and V ¼ V bi for an asymmetric device are not necessarily similar.…”

Section: Built-in Voltagesmentioning

confidence: 99%

Charge Transport in Spiro-OMeTAD Investigated through Space-Charge-Limited Current Measurements

et al. 2018

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Extracting charge-carrier mobilities for organic semiconductors from space-charge-limited conduction measurements is complicated in practice by nonideal factors such as trapping in defects and injection barriers. Here, we show that by allowing the bandlike charge-carrier mobility, trap characteristics, injection barrier heights, and the shunt resistance to vary in a multiple-trapping drift-diffusion model, a numerical fit can be obtained to the entire current density-voltage curve from experimental space-charge-limited current measurements on both symmetric and asymmetric 2; 2 0 ; 7; 7 0 -tetrakis(N; N-di-4-methoxyphenylamine)-9; 9 0 -spirobifluorene (spiro-OMeTAD) single-carrier devices. This approach yields a bandlike mobility that is more than an order of magnitude higher than the effective mobility obtained using analytical approximations, such as the Mott-Gurney law and the moving-electrode equation. It is also shown that where these analytical approximations require a temperature-dependent effective mobility to achieve fits, the numerical model can yield a temperature-, electric-field-, and charge-carrier-density-independent mobility. Finally, we present an analytical model describing trap-limited current flow through a semiconductor in a symmetric single-carrier device. We compare the obtained charge-carrier mobility and trap characteristics from this analytical model to the results from the numerical model, showing excellent agreement. This work shows the importance of accounting for traps and injection barriers explicitly when analyzing current density-voltage curves from space-charge-limited current measurements.

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“…[5] However, fullerene derivatives have intrinsic shortcomings, such as high cost of synthesis, low absorption coefficients in the visible spectral region, limited variability in energy level, and morphological instability in the blended films. [6] The development of new electron acceptors which overcome the drawbacks associated with fullerene-based acceptors is thus vital for further advancing OSCs. [7] Encouragingly, several studies have reported BHJ solar cells with PCEs >8% based on non-fullerene acceptors.…”

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confidence: 99%

The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk‐Heterojunction Solar Cells

et al. 2016

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Four hexachlorosubphthalocyanines SubPcCl6-X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution processed bulk-heterojunction organic solar cells. Subphthalocyanines are aromatic chromophore molecules with cone-shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptors and PTB7-Th as the electron donor exhibit a power conversion efficiency up to 4% and an external quantum efficiency approaching 60% due to significant contributions from both the electron donor and acceptor to the photocurrent, indicating a promising prospect of non-fullerene acceptors based on subphthalocyanines and structurally related systems.Solution processed bulk-heterojunction (BHJ) organic solar cells (OSCs) are a promising renewable energy technology towards future efficient, large-area, flexible photovoltaic modules. [1] The main component of an OSC is its BHJ active layer, consisting of an electron donor and an electron acceptor phase separated into a bi-continuous interpenetrating network morphology.[2] Power conversion efficiencies (PCEs) exceeding 11% have been achieved recently.[3] While numerous electron donors, including semiconducting polymers and small molecules, have been assessed, [4] electron acceptor components are still dominated by fullerene derivatives because of their high electron mobility, ideal frontier orbital energy levels, and isotropic charge transport properties.[5] However, fullerene derivatives have intrinsic shortcomings, such as high cost of synthesis, low absorption coefficients in the visible spectral region, limited variability in energy level, and morphological instability in the blended films.[6] The development of new electron acceptors which overcome the drawbacks associated with fullerene-based acceptors is thus vital for further advancing OSCs. [7] Encouragingly, several studies have reported BHJ solar cells with PCEs >8% based on non-fullerene acceptors.[8]Subphthalocyanines (SubPcs) are aromatic chromophore molecules including a boron atom at their inner cavity, with intense optical absorption in the 460−580 nm spectral region, [9] and relatively high electron mobilities.[10] Traditionally, they have been used as electron donors in vacuum deposited planar heterojunction solar cells.[9a] However, the electronic properties of SubPcs can be easily adjusted by introducing axial and/or peripheral substituents.[9a] Hence, by rational molecular design, i.e.introducing peripheral electron withdrawing groups, SubPcs have been transformed into electron acceptor molecules. [11] In this context, non-fullerene, vacuum evaporated solar cells containing SubPc molecules achieved a PCE of 8.4%.[12] The cone-shaped structure of SubPcs prevents excessive aggregation in solution and solid state, providing good solution processability even without the assistance of electrically i...

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Towards reliable charge-mobility benchmark measurements for organic semiconductors

Cited by 251 publications

References 44 publications

Perfluorinated Ionomer‐Modified Hole‐Injection Layers: Ultrahigh‐Workfunction but Nonohmic Contacts

Perfluorinated Ionomer‐Modified Hole‐Injection Layers: Ultrahigh‐Workfunction but Nonohmic Contacts

Charge Transport in Spiro-OMeTAD Investigated through Space-Charge-Limited Current Measurements

The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk‐Heterojunction Solar Cells

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