Effects of Contact-Induced Doping on the Behaviors of Organic

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Effects of Contact-induced Doping on the Behaviors of Organic Photovoltaic Devices Jian Wang, Liang Xu, Yun-Ju Lee, Manuel De Anda Villa, Anton V. Malko, and Julia W.P. Hsu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b03473 • Publication Date (Web): 09 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015

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Effects of Contact-induced Doping on the Behaviors of Organic Photovoltaic Devices

Jian Wang,1 Liang Xu,1 Yun-Ju Lee,1 Manuel De Anda Villa,2 Anton V. Malko,2 and Julia W. P. Hsu1*

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Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, TX, 75080, USA

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Department of Physics, The University of Texas at Dallas, Richardson, TX, 75080, USA

ACS Paragon Plus Environment

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ABSTRACT

Substrates can significantly affect the electronic properties of organic semiconductors. In this paper, we report the effects of contact-induced doping, arising from charge transfer between a high work function hole extraction layer (HEL) and the organic active layer, on organic photovoltaic (OPV) device performance. Employing a high work function HEL is found to increase doping in the active layer and decrease photocurrent. Combined experimental and modeling investigations reveal that higher doping increases polaron-exciton quenching and carrier recombination within the field-free region. Consequently, there exists an optimal HEL work function that enables a large built-in field while keeping the active layer doping low. This value is found to be ~ 0.4 eV larger than the pinning level of the active layer material. These understandings establish a criterion for optimal design of the HEL when adapting a new active layer system, and can shed light on optimizing performance in other organic electronic devices.

KEYWORDS: Fermi level pinning, charge transfer, capacitance-voltage, polaron-exciton quenching, recombination


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It is documented that substrates can significantly affect the electronic properties of a variety of materials that are deposited on top, e.g. organic semiconductors1,2 and 2D materials.3-5 For organic semiconductors, charge transfer to/from the substrate can lead to the formation of an interfacial doped region with different electronic properties from the bulk.1,2 While material property changes have been documented,1,2,6,7 little is known about the impact of substrateinduced doping on device behavior. In this paper, we show that a high work function MoOx contact in organic photovoltaic (OPV) devices induces p-doping across the entire bulk heterojunction (BHJ) active layer (> 200 nm) and produces lower photocurrent. This is the first study that clearly demonstrates that contact-induced doping can have a detrimental effect on OPV device performance. Since charge transfer is a fundamental mechanism that governs organic-inorganic interfaces, understanding its influence on device performance will facilitate better design and optimization of organic electronics beyond OPV. Interfacial contact layers have been shown to dramatically improve the performance and stability of OPV devices.8-11 For hole extraction layers (HELs), most current research activities focus on developing high work function materials, which ensure a high built-in field across the BHJ active layer to enhance charge collection.12-17 However, recent publications show that high work function HELs do not necessarily produce better OPV device performance, yet the cause is unclear.18,19 Therefore, a complete understanding of the effects of HEL work function on OPV device performance is needed to provide a constructive guideline for future HEL designs. In this paper, we study the effects of MoOx HEL work function on OPV device parameters. We choose MoOx for the study because MoOx is commonly used as a HEL in OPVs,8,20-22 as a hole injection layer in organic light-emitting diodes,23 and is inserted into organic field effect transistors to tune the threshold voltage.24 It has been documented that the MoOx work function


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can vary over a large range depending on its cation valence and surface adsorbates.20,25 Utilizing this knowledge, we design a protocol to control the atmosphere and solvent exposure of thermally evaporated MoOx films and vary the work function from 4.90 eV to 5.80 eV (Table S1, Supporting Information). OPV devices are fabricated on these MoOx HELs with a structure of ITO (140 nm) / MoOx (20 nm) / BHJ / Ca (7 nm) / Al (100 nm) with a BHJ active layer of ~ 210 nm poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM), ~ 100 nm thieno[3,4-b]thiophene-alt- benzodithiophene:[6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM), or ~ 70 nm poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-enzothiadiazole)] (PCDTBT):PC71BM. Figure 1 summarizes device open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) for the three BHJ systems made on MoOx HELs of varying work functions (see Fig. S1 in Supporting Information for J-V curves). Clearly, Fig. 1d shows that there exists a maximum PCE value for each BHJ system. The corresponding MoOx work function is marked on x-axis and noted as Φoptimal. The decreases in Voc and FF at low MoOx work function are caused by the decreased built-in field across the device, consistent with previous reports that vary the metal electrode or contact layer work function.13,26 However, the decrease in Jsc at high MoOx work function (Fig. 1b) is surprising and has not been reported previously. Also, the Φoptimal value differs for each donor:acceptor system, which will be discussed later. The goals of this paper are to understand the mechanism(s) leading to Jsc reduction when the HEL work function is higher than the donor specific Φoptimal.


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Figure 1. Device parameters (a) Voc, (b) Jsc, (c) FF, and (d) PCE of P3HT:PC61BM (black circles), PTB7:PC71BM (red squares) and PCDTBT:PC71BM (blue diamonds) active layers versus MoOx work function. Inset of (b) shows device structure. Arrows in (d) mark Φoptimal values with colors corresponding to the 3 active layers. Note: data on ITO substrates for P3HT:PC61BM (cross symbols) are included for completeness. Error bars represent standard deviations of device parameters, averaged over ~ 12 diodes per device condition.

A close look at the dark J-V curves of P3HT:PCBM devices (Fig. S2, Supporting Information) reveals that higher work function MoOx devices exhibit higher injection current at positive bias. In other words, the device Jsc and injection current are inversely correlated. This result clearly illustrates that a good hole injection layer, i.e. high work function MoOx, does not necessarily make a good HEL for OPV devices, although the two terms are used interchangeably in OPV


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literature.22 The inverse correlation between Jsc and injection current (Fig. S2c, Supporting Information) provides a clue to understand the low Jsc found in devices made on high work function MoOx HELs. Since P3HT:PCBM active layers have been previously determined to be p-type,27 we first examine the doping density (NA) in the device using capacitance-voltage (C-V) measurements. The 1/C2 vs. V plot (Fig. 2a) shows that NA of active layer increases by an order of magnitude, from 2.6 × 1016 cm-3 to 2.6 × 1017 cm-3, when MoOx work function varies from 4.90 to 5.80 eV. Here NA is obtained using the following equation: = 2⁄ ∗ Δ ⁄Δ1⁄

,28 where q is elemental charge, ε is the dielectric constant of the active layer, A is device area, C is measured capacitance, and V is applied bias. We use 3.8 ε0 for ε, where ε0 is the vacuum permittivity.28 The independence of the capacitance value on frequency up to ~ 105 Hz (Supporting Information Fig. S3) implies that carriers probed in these C-V measurements are mobile holes, not traps. The observation of increasing NA is consistent with the increase of dark injection current as MoOx work function increases. On the other hand, J-V characteristics of hole only devices (Fig. S4, Supporting Information) show that an injection barrier does not exist at the MoOx/P3HT:PCBM interface, and that carrier mobility decreases as MoOx work function increases, which should lead to lower injection current if it were the dominant mechanism. Therefore, the increased NA with high work function MoOx is responsible for the increased injection current, which is correlated with decreased Jsc. Figure 2b shows that Jsc linearly decreases with increasing NA, and more than 50% of Jsc (~5 mA/cm2) is lost when NA increases from 2.6 × 1016 cm-3 to 2.6 × 1017 cm-3 for P3HT:PCBM devices.


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Figure 2. (a) 1/C2 vs. V plots for P3HT:PCBM devices on different work function MoOx substrates. Orange lines represent the fitting according to the equation shown in the main text. (b) Jsc vs NA for P3HT:PCBM devices. (c) Schematic illustration of contact-induced doping mechanism of BHJ from MoOx HEL based on ICT model. (d) NA as a function of active layer position , for P3HT:PCBM devices on different work function MoOx substrates. Here = − ⁄ , and d is the active layer thickness (210 nm). In (a) and (d), symbols represent different work function MoOx: 4.90eV (black circles), 5.25eV (red squares), 5.80eV (blue diamonds). In (d), position 0 nm represents the MoOx/P3HT:PCBM interface, and 210 nm represents the P3HT:PCBM/Ca interface.


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As nothing in the device is changed except for the MoOx function, we attribute the increased NA to contact-induced doping by MoOx. When the work function of the substrate is higher than the pinning level of positive polarons in the BHJ, electrons flow from the BHJ to MoOx until equilibrium is reached and Fermi level pinning occurs.1,2 Although different models have been proposed to explain the origins of Fermi level pinning in organic materials, they concur that the semiconductor becomes positively charged (p-doped) at equilibrium and there exists a vacuum level offset ∆ at the MoOx/BHJ interface.1,2 For the rest of the paper, we will interpret our results following the integer charge transfer (ICT) model (Fig. 2c) and refer to the pinning level as EICT+.1 It has been documented that MoO3 can p-dope small-molecule semiconductors when they are co-evaporated as a mixed film.29 Another study reported that up to ~ 20 nm P3HT can be pdoped when deposited on top of a flat MoO3 substrate, where the p-doping is attributed to oxidation of sulfur atoms in the P3HT molecule.7 Here we find that high work function MoOx HELs can induce doping across the entire thickness (>200 nm) of the active layer as seen in Fig. 2d, a much larger scale than reported previously. Recently, Deledalle et. al. showed that doping in the active layer negatively impacts OPV performance, but the source of dopants in their study is unspecified.30 In our system, we show unambiguously that a high work function HEL contact is the source of doping that leads to degraded OPV performance. Therefore background carrier density of the active layer can be experimentally controlled through a judicious choice of contact work function.


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Figure 3. (a) Normalized time-resolved PL decay curves for pure P3HT on glass (green), and on MoOx substrates with different work functions: 4.90eV (black), 5.25eV (red), and 5.80eV (blue), and P3HT:PCBM (orange dashed) on glass. (b) Experimentally measured Jsc (black circles), calculated Jgen from integrating free-carrier generation rate profiles (blue diamonds), simulated Jsc (red squares), and Jrec (green triangles) from SCAPS, as a function of NA in P3HT:PCBM active layer. (c) SCAPS simulated Ec, Ev, (solid lines) and recombination rate profile (symbols) for devices with NA of 2.6 × 1016 cm-3 (black, 4.90eV), 5.8 × 1016 cm-3 (red, 5.25eV), and 2.6 × 1017 cm-3 (blue, 5.80eV). Position 0 nm represents the MoOx/active layer interface, and 210 nm represents the active layer/Ca interface. (d) Normalized EQE spectra of P3HT:PCBM devices with different NA of 2.6 × 1016 cm-3 (black circles), 5.8 × 1016 cm-3 (red squares), and 2.6 × 1017 cm-3 (blue diamonds). Un-normalized EQE spectra are shown in Supporting Information Fig. S8. Inset: integrated Jsc from EQE spectra agrees well with experimental Jsc. Both units are mA/cm2.


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To understand the physics behind Jsc reduction upon increased NA, we examined the carrier generation

and

recombination

within

device

active

layers.

First,

time-resolved

photoluminescence (PL) measurements (Fig. 3a) show that the exciton decay rate (1/lifetime, lifetime summarized in Table S2, in Supporting Information) for pure P3HT films increase as the MoOx work function increases (from black to red to blue solid lines), due to the increased NA in the P3HT film. This phenomenon can be explained by polaron-exciton quenching.31-36 Notably, the exciton decay rate with the highest NA (blue line, Φ=5.80 eV) is approaching the decay rate of a 1:1 P3HT:PCBM blend film on glass (orange dash line). Hence, the polaron-exciton quenching process competes with the exciton dissociation by PCBM and decreases free carrier generation within the device. Additionally, polaron-exciton quenching has been attributed to a higher J-V slope under reverse bias in bilayer OPV systems.35,36 Figure S6 shows that our devices made on higher work function MoOx exhibit such J-V characteristics. On the other hand, the exciton decay rate with the lowest NA (black line, at Φoptimal ~ 4.90 eV) is comparable to the decay rate of a pure P3HT film on glass (green solid line), indicating that the MoOx at Φoptimal introduces little additional exciton quenching. Using these exciton decay rates measured by PL, we estimate the free carrier dissociation percentage (Discussions following Table S2 in Supporting Information), which is then used to multiply the exciton generation rate profile calculated from the transfer matrix model to obtain the free carrier generation rate profile (Fig. S5, Supporting Information).37,38 Figure 3b plots the calculated generation current (blue diamonds), which decreases from 10.0 mA/cm2 to 7.8 mA/cm2 as NA increases from 2.6 × 1016 cm-3 to 2.6 × 1017 cm-3. However, the decrease in the generation current due to exciton-polaron quenching is not sufficient to account for the observed decrease in Jsc (Fig. 3b black circles).


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We then performed 1D drift-diffusion modeling using SCAPS software39,40 with free carrier generation profiles described above (Fig. S5, Supporting Information) to examine the effects of increased NA on carrier recombination. Figure 3c shows the simulated energy band diagram and the recombination rate (assuming bimolecular processes only) at short circuit as a function of active layer position. Because of the p-type doping, the active layer is split into a depletion region near the cathode where the electric field is high, and a field-free region near the anode. Movement of photogenerated carriers in the field-free region is mainly diffusion driven, compared with drift-field driven movement in the depletion region.27 When NA increases, not only does the field-free region extend closer to the cathode (Fig. 3b from black to blue), the recombination rate also increases within the field-free region because more holes are available to recombine with photogenerated electrons that are not efficiently swept out. As a result, recombination current (Fig. 3b green triangles) increases from 0.4 mA/cm2 to 3.1 mA/cm2 as NA increase from 2.6 × 1016 cm-3 to 2.6 × 1017 cm-3. Combining both the effects of decreased generation and increased recombination, a good agreement is reached between the simulated Jsc (Fig. 3b red squares) and experimental Jsc (Fig. 3b black circles) as a function of NA, as well the overall J-V curves (Fig. S7, Supporting Information). In addition, by examining the spectral changes in the external quantum efficiency (EQE) spectra, we can verify the spatial locations of photocurrent loss.41 Figure 3d shows that a depression in the wavelength range of 500~600 nm evolves as NA increases. As photons of those wavelengths are predominantly absorbed at the front half of the active layer (Fig. S9, Supporting Information), such a depression indicates a decreased collection efficiency from the front half of the device, consistent with the simulation results from SCAPS (Fig. 3c) that recombination increases at the front half of the device with increased NA. Note that the previous report from Deledalle et. al. attributes the effect of doping


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solely to increased recombination within the field-free region and did not include the polaronexciton quenching effect. Our results clearly show that these two mechanisms have comparable contributions to Jsc reduction (Fig. 3b). The discussions above clearly elucidate the effects of contact-induced doping on carrier generation and recombination in the P3HT:PCBM system. The existence of an optimal HEL work function is the result of competition between minimum doping in the active layer and maintaining a sufficiently high built-in field. Close examination of PTB7 and PCDTBT systems confirms the findings in the P3HT system: the device behaviors as a function of MoOx work function are the same for all three donor materials, with the exception of Φoptimal being specific to the active layer. Figure 4a shows the measurement of the Fermi level pinning position (EICT+) for three BHJ active layers,1 which are 4.51 ± 0.05 eV (P3HT:PC61BM), 4.64 ± 0.05 eV (PTB7:PC71BM), and 4.86 ± 0.06 eV (PCDTBT:PC71BM). We find a constant difference of ~ 0.40 ± 0.02 eV between Φoptimal and EICT+ for all three BHJ systems (Fig. 4b and Table S4 in Supporting Information). This result clearly indicates that the contact-induced doping mechanism governs device behaviors in PTB7:PC71BM and PCDTBT:PC71BM systems as well. The ~ 0.4 eV difference between Φoptimal and EICT+ would give a target of HEL work function in terms of device optimization for future active layer systems, where values lower than Φoptimal would result in lower Voc & FF due to low built-in field, while values higher than Φoptimal would result in lower Jsc due to HEL-induced doping. It should be noted that the Φoptimal only predicts the work function value at which Jsc starts to decrease, but not the amplitude of Jsc reduction. For thin active layer devices, which tend to be more depleted, Jsc decreases less (Fig. 1d, PTB7 and PCDTBT).


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Figure 4. (a) Active layer work function vs. MoOx HEL work function for P3HT:PC61BM (black circles), PTB7:PC71BM (red squares) and PCDTBT:PC71BM (blue diamonds). The orange dashed line represents the regime where Fermi level is not pinned and the active layer work function equals the MoOx work function. Note: cross symbols represent active layers on top of ITO substrates. (b) Summarized band diagram: the optimal work function of the MoOx HEL is found to be 0.4 eV deeper than the EICT+ level of a specific BHJ.

In conclusion, we report the effects of contact-induced doping from high work function MoOx HELs on OPV performance. The increased doping in the active layer is deleterious as it decreases Jsc via increased polaron-exciton quenching and carrier recombination within the field-


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free region. Therefore, a Φoptimal exists for HELs that enables a large built-in field while keeping active layer doping low, and it is empirically found to be ~ 0.4 eV larger than the pinning energy of the BHJ active layer material. This research provides an in-depth understanding to guide future HEL design/selection for OPV devices. Furthermore, since contact-induced doping is a fundamental mechanism that governs organic-inorganic interfaces, the knowledge gained from OPVs will benefit other organic electronics, such as organic light-emitting diodes and organic field effect transistors.

Supporting Information. Experimental details, supporting figures, and tables are provided in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT We thank Prof. X. Gong for providing PTB7 materials and Prof. R. Wallace for the use of spectroscopic ellipsometer. We thank Trey Daunis for reading through the manuscript and correcting the grammatical errors. Device fabrication, characterization, and simulation [J. W, L. X, Y-J. L, and J.W.P.H.] were sponsored by the National Science Foundation (NSF) DMR1305893. Time-resolved PL spectroscopy measurement [M. V. A and A. V. M] was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DESC0010697. J.W.P.H. acknowledges the support from Texas Instruments Distinguished Chair in Nanoelectronics.


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Effects of Contact-Induced Doping on the Behaviors of Organic

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