Letter pubs.acs.org/NanoLett
Exciton Management in Organic Photovoltaic Multidonor Energy Cascades Olga L. Griffith† and Stephen R. Forrest*,†,‡ †
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States Departments of Physics, and Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
‡
S Supporting Information *
ABSTRACT: Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called “energy cascades” drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. KEYWORDS: energy transfer, exciton, multilayer, blocking
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or D2)/tetraphenyldibenzoperiflanthene (DBP, or D3)/C60/ C60 + 4,7-diphenyl-1,10-phenanthroline (BPhen)/BPhen/Ag, we obtain a power conversion efficiency (PCE) of 7.1 ± 0.4%, which is significantly higher than that of bilayer OPVs made with a C60 acceptor and only a single donor from this same materials set. Insight into the exciton transfer and blocking properties of this multidonor system is provided by photoluminescence (PL) measurements and optical modeling of the external quantum efficiency (EQE) to determine the relative photocurrent contributions of the various layers employed. The donor materials in a cascade are chosen based on the following criteria: (i) To ensure an energetic driving force for excitons, the optical energy gaps (Eopt) monotonically decrease from anode to cathode, viz.: Eopt(D1) > Eopt(D2) > Eopt(D3). (ii) The energy offsets between the lowest unoccupied molecular orbitals (LUMOs) of the donors are sufficiently small (to within 0.4 eV from D1 to D3) to minimize exciton dissociation and hence photocurrent generation at intermediate donor interfaces. (iii) The donor, D3, adjacent to the acceptor layer is chosen to ensure a large offset between its highest occupied molecular orbital (HOMO) and the LUMO energy of acceptor, thereby maximizing Voc. These conditions are met by the materials used in the cascade studied here, as shown in Figure 1b and c.
rganic photovoltaic (OPV) cells have the potential for low cost and efficient harvesting of solar energy due to their lightweight, flexibility when deposited onto thin substrates, and low-energy (i.e., “green”) fabrication processes.1−3 One particularly promising architecture for OPVs is the donor cascade that drives energy and/or charge transfer across a stack of multiple layers whose exciton energies monotonically decrease from the donor nearest to the anode, to that nearest to the donor−acceptor heterojunction (DA-HJ).4 In such a cascade, excitons are transferred (primarily via Förster processes)5 through successively lower energy states to the DAHJ for dissociation into electrons and holes. Furthermore, excitons can dissociate at each intermediate donor interface, thus providing additional sites for energy harvesting beyond that of a single DA heterointerface. Both processes have been observed.4,6−10 One advantage of this architecture is that the use of multiple donors allows for flexibility in design: the donor nearest the anode can be optimized for exciton blocking, the next for optical absorption, and the one nearest the DA-HJ can ensure a high open circuit voltage (Voc) due to minimization of the polaron pair recombination rate.11 To realize the full potential of the cascade architecture, losses within and between each donor must also be minimized. In this work we demonstrate a three-stage energy cascade OPV that overcomes most of limitations observed in earlier demonstrations of this architecture. Using the structure in Figure 1a consisting of glass/indium−tin−oxide (ITO)/poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS)/diphenyltetracene (DPT, or D1)/rubrene (RUB, © 2014 American Chemical Society
Received: December 22, 2013 Revised: March 25, 2014 Published: April 4, 2014 2353
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The Förster transfer rate depends on the integrated overlap between the PL spectrum of the ED molecule (PLED) and the absorption of the EA molecule (εEA), given by5,12,14,17 K=
∫ PL ED(λ) εEA(λ) λ 4 dλ
(2)
The Förster radius is then given by ⎛ (cos ϑ − 3 cos φ cos φ )2 Φ ED ⎞1/6 PL ED EA RF = C ⎜⎜ K ⎟⎟ 4 n ⎝ ⎠ r
where C is a constant, ϑ is the angle between the ED and EA dipoles, φED and φEA are the angles between the respective dipoles and ϑ,5,18 nr is the refractive index of the medium at the wavelength of maximum spectral overlap, and ΦED PL is the fluorescence quantum yield of the energy donor. Indices “ED” and “EA” correspond to the energy donor and energy acceptor molecules, respectively. Here we use C = 0.02,17 and the spatially averaged dipole orientation factor is (cos ϑ − 3 cos φED cos φEA)2 = 0.6719 (see Supporting Information, SI for more details). The values for EDs are taken from the literature20 and provided below for energy transfer between a given ED-to-EA pair. The spectral overlap integrals K are calculated using eq 2 employing measured PLED and εEA for the corresponding ED−EA pairs (see SI). The simulated EQE contributions of each active layer to the cascade efficiency are calculated using the exciton current densities of these layers obtained from numerical solution of eq 1, using the exciton diffusion lengths as fitting parameters. Then, the contributions of each active layer to the total Jsc are calculated by integrating the corresponding EQE contributions over the simulated solar spectrum from λ = 400 to 700 nm. Finally, the simulated total EQE spectrum is the sum of contributions from all active layers obtained from the best fit to the experimental EQE data. See SI for individual device data and fitting details. The current density vs voltage (J−V) characteristics and EQE of the DPT/RUB/DBP/C60 cascade, and DPT/C60, RUB/C60, DBP/C60 bilayer control OPVs are shown in Figure 2. Here, the optical gaps of DPT (D1), RUB (D2), and DBP (D3) are 2.5, 2.2, and 2.0 eV, respectively (c.f. Figure 1b).4,21 The HOMO of each donor is at 5.4 ± 0.1 eV referenced to vacuum.4,21,22 The DPT/RUB/DBP/C60 cascade has PCE = 7.1 ± 0.4%, or nearly double that of the single-donor DBP/C60 device. The improvement is primarily due to the increase in short-circuit current density from Jsc = 6.7 ± 0.3 mA/cm2 to 10.6 ± 0.5 mA/cm2, with minor increases in Voc and fill factor (FF). All performance parameters for the devices studied are listed in Table 1. The increase in Jsc is also reflected in the increase in EQE from 37 ± 2% to 68 ± 3% at a wavelength of λ = 615 nm, which lies only within the DBP absorption range when 10 nm thick DPT and RUB films are added (see Figure 2b). Further, the EQE due to C60 absorption at λ = 400−550 nm is higher in the DBP/C60 device compared to that in the cascade. The decrease in C60 absorption in the cascade is due to the spectral overlap in absorption of C60 with RUB and DPT, as apparent from their extinction coefficients, k (i.e., the imaginary part of complex refractive index) given in Figure 3. The illumination of cascades lacking a C60 acceptor with the structures: ITO/PEDOT:PSS/DPT/RUB/BPhen/Ag and ITO/PEDOT:PSS/RUB/DBP/BPhen/Ag devices using simulated AM1.5G spectrum with intensity up to 1 sun does not generate photocurrent (see SI). From this we infer that the
Figure 1. (a) Schematic diagram of the three-donor energy cascade organic photovoltaic cell. (b) Transport energy levels of the active layers in a cascade. The highest occupied molecular orbital (HOMO) energy levels of donors, D1, D2, and D3, as measured by ultraviolet photoelectron spectroscopy relative to the vacuum energy level are from refs 4, 21, and 22. The lowest unoccupied MO (LUMO) energies are estimated from the optical gaps of donors relative to the HOMO positions. The energy level offsets of the C60 acceptor (A) are from ultraviolet and inverse photoelectron spectroscopy measurements.32 Energy is transferred from DPT or D1, to RUB or D2, and on to DBP or D3. Both interfaces between D1 and D2 and D2 and D3 are not photoactive. (c) Molecular structural formulas for the three donors used in the cascade.
To understand the role of each donor layer and the relative effects of exciton blocking, quenching, and transfer in the cascade, we use the one-dimensional steady-state exciton diffusion equation that includes Förster energy transfer from the energy donating (ED) to the energy accepting (EA) molecules: L D2
∂ 2n − (1 + τkF)n + τG(x) = 0 ∂x 2
(3)
(1)
Here, LD is the diffusion length, n is the density, G(x) is the generation rate at position x in the stack, τ is the natural lifetime of excitons, and kF = πρRF6/6τd3 is the Förster energy transfer rate from the ED to the EA in the various cascade layers.5,12−16 Also, ρ is the molecular density of the energy acceptor,14 RF is the Förster radius, and d is the distance between ED and EA. 2354
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Figure 3. Extinction coefficients of organic active layers used in the cascade.
Table 2. Parameters Used in Modeling EQE ED−EA DPT−RUB RUB−DBP
Table 1. Electrical Parameters of Three-, Two-, and OneDonor Solar Cells DPT/RUB/DBP RUB/DBP DPT/RUB DBP RUB DPT
Voc, V 0.94 0.94 0.90 0.92 0.89 0.80
± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.03 0.01
Jsc,a mA/cm2 10.6 9.3 3.9 6.7 3.8 2.7
± ± ± ± ± ±
0.5 0.5 0.2 0.3 0.2 0.1
FF, % 71 68 52 69 47 39
± ± ± ± ± ±
1 1 1 2 5 1
PCE, % 7.1 6.0 1.9 4.3 1.6 0.8
± ± ± ± ± ±
20
0.9 0.9
ρ, nm−3
RF, nm
1.45 0.94
2.5 4.7
process. To quantify the role of DPT-to-RUB exciton transfer in the DPT/RUB/C60 cascade, we show simulated relative EQE contributions of DPT, RUB, and C60 to the total cascade EQE represented by the solid lines in Figure 4a, which is compared to experimental EQE data shown by the dotted line. From the fits, we obtain diffusion lengths (LD) of 30 ± 2 nm, 35 ± 2 nm, and 28 ± 4 nm for DPT, RUB, and C60, respectively. The reported LD values for organic molecules vary significantly depending on the purity and morphology of measured thin films. For example, LD of C60 is reported between ∼7 and 40 nm,1,23,24 and that of amorphous RUB varies from ∼6 nm to above 20 nm.25,26 As noted above, it is assumed that the DPT/ RUB interface is not photoactive, as confirmed by experiment. In the DPT/RUB/C60 cascade, the photocurrent contribution of DPT due solely to energy transfer to RUB is 4% (0.15 ± 0.01 mA/cm2), that of absorption in RUB followed by dissociation at C60 is 5% (0.20 ± 0.01 mA/cm2), and absorption in C60 contributes 91% (3.6 ± 0.3 mA/cm2) to the total Jsc (c.f. Table 3). Similarly, in a RUB/DBP/C60 cascade, RUB acts as the ED and DBP as the EA. To quantitatively estimate the effects of exciton blocking and transfer from RUB to DBP, we simulate the EQE spectrum of the RUB/DBP/C60 cascade along with contributions from RUB, DBP, and C60 as shown in Figure 4b (solid lines) along with experimental EQE data (dotted line). From the fits, we obtain LD = 20 ± 1 nm for DBP that is on the higher end of reported values. For example, the reported LD of DBP as measured by PL quenching method21 is 9 ± 3 nm, and the modeled LD of DBP from various optical simulations27,28 ranges from 5 to 20 nm. The EQE spectrum is dominated by contributions from DBP (yielding an integrated current density of 3.6 ± 0.3 mA/cm2, or 51% of the total Jsc) and C60 (3.4 ± 0.2 mA/cm2 or 47% of the total Jsc), whereas the contribution from RUB due to energy transfer to DBP represents only 2% of the total. As above, the photocurrent contribution of RUB is due only to exciton transfer to DBP, since the RUB/DBP interface also does not generate photocurrent. These results are summarized in Table 3. The photocurrent of the RUB/DBP/
Figure 2. (a) Current density−voltage (J−V, at 1 sun light intensity, AM1.5G spectrum) and (b) external quantum efficiency (EQE) characteristics of a DPT/RUB/DBP/C60 cascade solar cell and the corresponding control bilayer solar cells.
donor
ref
0.4 0.3 0.1 0.2 0.1 0.1
a
Jsc obtained from data using 1 sun AM 1.5G simulated illumination were spectrally corrected. Values calculated from the integrated EQE spectra and spectral mismatch factors are provided in the Supporting Information.
DPT/RUB and RUB/DBP interfaces are not photoactive and hence do not lead to the dissociation of excitons into free charges in the three-donor cascade device. To understand the role played by each donor layer in the DPT/RUB/DBP/C60 cascade, we quantitatively analyze exciton blocking, quenching, and transfer at individual DPT/ RUB and RUB/DBP interfaces used in dual donor DPT/RUB/ C60 and RUB/DBP/C60 cascades, respectively. The molecular densities of energy acceptors and the Förster transfer radii for DPT-to-RUB and RUB-to-DBP exciton transfer calculated using eqs 2 and 3 are given in Table 2. In the DPT/RUB/C60 cascade, DPT is the exciton donor (ED), and RUB is the exciton acceptor (EA) in the transfer 2355
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exciton blocking at the anode by RUB are primarily responsible for the increased efficiency of the RUB/DBP/C60 cascade over that of the individual single donor devices. Note that the EQE contribution of DBP at λ = 615 nm in the dual donor cascade (64 ± 3%) is almost the same as in the DPT/RUB/DBP/C60 cascade (68 ± 3%). To confirm the effects of exciton blocking and transfer at the RUB/DBP interface, we measured the PL spectra of DBP (60 nm thick) deposited on quartz and capped with 8 nm thick films of either RUB or BPhen (the latter used as a reference exciton blocker). From these measurements (see Supporting Information, Figure S6, and the accompanying discussion), we infer that 85% of the increase in DBP PL capped with RUB is due to exciton blocking, and 15% is due to RUB-to-DBP exciton transfer. These results are in reasonable agreement with the EQE data. The simulated EQE of the DPT/RUB/DBP/C60 cascade is shown in Figure 4c. Here, DPT transfers excitons to RUB leading to the photocurrent contribution of 0.13 ± 0.01 mA/ cm2, while blocking RUB excitons from reaching the anode. Further, RUB transfers excitons to DBP, contributing approximately the same photocurrent, leading to a total contribution of 0.24 ± 0.02 mA/cm2 due to exciton transfer. The DBP and C60 layers contribute the most to the cascade photocurrent: 3.9 ± 0.3 mA/cm2 and 3.5 ± 0.2 mA/cm2, respectively (see Table 3). The overall increase in Jsc using the DPT/RUB/DBP/C60 cascade is 1.7 ± 0.1 mA/cm2 compared to the bilayer DBP/C60 cell, with 14% of this increase due to DPT-to-RUB-to-DBP exciton transfer. The photocurrent contribution from DBP is almost double in the cascade compared with a DBP/C60 bilayer due to efficient exciton blocking by DPT/RUB. From these analyses, we conclude that the EQE increase observed for the DPT/RUB/DBP/C60 cascade compared to the DBP/C60 bilayer cell is primarily due to blocking of DBP excitons by the DPT/RUB layer. Importantly, exciton blocking by RUB leads to an additional photocurrent of 1.6 ± 0.1 mA/ cm2 (from 2.0 ± 0.1 to 3.6 ± 0.3 mA/cm2) generated in DBP along with 0.12 mA/cm2 due to RUB-to-DBP energy transfer; and when both DPT and RUB are included in the three-donor cascade, the DBP response increases by 1.9 ± 0.1 mA/cm2 (to 3.9 ± 0.3 mA/cm2) due to improved exciton blocking by DPT/ RUB, followed by an additional 0.24 ± 0.01 mA/cm2 due to DPT-to-RUB-to-DBP energy transfer. Note that the differences in EQE between these several devices are also affected by the corresponding changes in the light intensity distribution across the active layers, although these effects are relatively small and are accounted for in simulations of the optical fields in our analysis. We investigated exciton blocking, quenching, and transfer in multidonor energy cascade OPVs using a combination of EQE
Figure 4. Simulated EQE of two- and three-donor cascade OPVs obtained by summing (solid black line) the relative contributions from the individual donor layers (indicated). Experimental EQE data are also shown (dotted line). (a) DPT/RUB/C60 cascade; (b) RUB/ DBP/C60 cascade; (c) DPT/RUB/DBP/C60 cascade. DPT (a, c) and RUB (b, c) contributions are due to Förster energy transfer.
C60 cascade is approximately 1.2 mA/cm2 greater than that of the DBP/C60 bilayer cell; 10% of this increase is due to RUBto-DBP exciton transfer, and the remaining 90% is due to exciton blocking by RUB (see Table 3). The relative contribution of RUB-to-DBP energy transfer is relatively minor due to the low k of RUB. Hence, the effects of
Table 3. Short-Circuit Current Density Contributions Calculated for Each Organic Active Layer to the Total in the Cascades
a
donor
Jsc of DPTa
Jsc of RUBa
Jsc of DBPa
DPT/RUB/DBP RUB/DBP DPT/RUB DBP RUB DPT
0.13 (1.8%)
0.11 (1.5%) 0.12 (1.7%) 0.20 (5.0%)
3.88 (51.0%) 3.63 (50.7%)
0.15 (3.8%)
2.01 (33.9%) 0.20 (6.1%) 0.21 (8.4%)
Jsc of C60a 3.48 3.41 3.57 3.92 3.14 2.28
(45.7%) (47.6%) (91.2%) (66.1%) (93.9%) (91.6%)
total Jsca 7.61 7.16 3.92 5.93 3.34 2.49
Relative percentage contributions to Jsc are in parentheses. All values are in mA/cm2. 2356
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are shown in the SI. Given the number of interfaces in the cascade, the EQE was also measured under a white light bias with AM1.5G simulated irradiance at 1 sun intensity. Details are provided in the SI. Experimental errors for Voc and FF arise from variations between devices, and the error in Jsc (∼5%) is primarily due to uncertainties in the intensity of the lamp, which also dominates the error in PCE. Photoluminescence (PL) measurements of active layer, DBP, were performed using the structures: quartz/active layer 60 nm/cap layer 8 nm, where the capping layers for DBP were RUB and BPhen.13 The photoluminescence emission spectra of these samples were measured using a PTI QuantaMaster spectrofluorometer at an incident angle of 30° in a high purity N2 atmosphere to prevent atmospheric degradation of the films. The samples were illuminated through the capping layer. The PL signal was corrected for lamp intensity fluctuations during the measurement, leading to an error in PL intensity of ±5%. All PL data were normalized to the number of absorbed photons in the active layer to account for the differences in the refractive indices of the capping layers using the transfer matrix method.1,31
and PL measurements, along with an analysis of the contributions from each donor layer based on the steadystate exciton diffusion equation assuming Förster energy transfer between donor layers. The multilayer cascade offers the flexibility of using several donor materials in a single device, each optimized for a different purpose. Indeed, we find that a DPT/RUB/DBP/C60 cascade has PCE = 7.1 ± 0.4%, a substantial improvement over past reports of cascade devices.4,6−10 The improvement is primarily due to efficient exciton blocking of the widest energy gap donor layer positioned nearest to the anode where quenching can occur. While exciton transfer between donors is small, it can be increased by the use of materials with comparable oscillator strengths but without significant absorption spectral overlap, thereby eliminating spectral blocking by multiple layers. We note that while this concept has been applied to the donor side of the heterojunction, similar advantages are expected when cascades are implemented on the acceptor side, provided that suitable material combinations can be found. In addition, the model developed to analyze exciton dynamics in cascade OPVs can be expanded to analyze other device architectures and energy transfer across organic interfaces. The advantage of the model is its utility in accurately calculating photocurrent contributions of multiple active layers in devices. This leads to insights regarding the origins and efficiency of energy transfer within multilayer structures, ultimately leading to high performance devices. Experimental Section. ITO-coated (15 Ω/sq) glass substrates (from Bayview) were detergent- and solvent-cleaned and exposed to ultraviolet light and ozone for 10 min prior to depositing PEDOT:PSS films and loading into a vacuum thermal evaporation (VTE) chamber (base pressure of
Exciton Management in Organic Photovoltaic Multidonor Energy Cascades Olga L. Griffith† and Stephen R. Forrest*,†,‡ †
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States Departments of Physics, and Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
‡
S Supporting Information *
ABSTRACT: Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called “energy cascades” drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. KEYWORDS: energy transfer, exciton, multilayer, blocking
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or D2)/tetraphenyldibenzoperiflanthene (DBP, or D3)/C60/ C60 + 4,7-diphenyl-1,10-phenanthroline (BPhen)/BPhen/Ag, we obtain a power conversion efficiency (PCE) of 7.1 ± 0.4%, which is significantly higher than that of bilayer OPVs made with a C60 acceptor and only a single donor from this same materials set. Insight into the exciton transfer and blocking properties of this multidonor system is provided by photoluminescence (PL) measurements and optical modeling of the external quantum efficiency (EQE) to determine the relative photocurrent contributions of the various layers employed. The donor materials in a cascade are chosen based on the following criteria: (i) To ensure an energetic driving force for excitons, the optical energy gaps (Eopt) monotonically decrease from anode to cathode, viz.: Eopt(D1) > Eopt(D2) > Eopt(D3). (ii) The energy offsets between the lowest unoccupied molecular orbitals (LUMOs) of the donors are sufficiently small (to within 0.4 eV from D1 to D3) to minimize exciton dissociation and hence photocurrent generation at intermediate donor interfaces. (iii) The donor, D3, adjacent to the acceptor layer is chosen to ensure a large offset between its highest occupied molecular orbital (HOMO) and the LUMO energy of acceptor, thereby maximizing Voc. These conditions are met by the materials used in the cascade studied here, as shown in Figure 1b and c.
rganic photovoltaic (OPV) cells have the potential for low cost and efficient harvesting of solar energy due to their lightweight, flexibility when deposited onto thin substrates, and low-energy (i.e., “green”) fabrication processes.1−3 One particularly promising architecture for OPVs is the donor cascade that drives energy and/or charge transfer across a stack of multiple layers whose exciton energies monotonically decrease from the donor nearest to the anode, to that nearest to the donor−acceptor heterojunction (DA-HJ).4 In such a cascade, excitons are transferred (primarily via Förster processes)5 through successively lower energy states to the DAHJ for dissociation into electrons and holes. Furthermore, excitons can dissociate at each intermediate donor interface, thus providing additional sites for energy harvesting beyond that of a single DA heterointerface. Both processes have been observed.4,6−10 One advantage of this architecture is that the use of multiple donors allows for flexibility in design: the donor nearest the anode can be optimized for exciton blocking, the next for optical absorption, and the one nearest the DA-HJ can ensure a high open circuit voltage (Voc) due to minimization of the polaron pair recombination rate.11 To realize the full potential of the cascade architecture, losses within and between each donor must also be minimized. In this work we demonstrate a three-stage energy cascade OPV that overcomes most of limitations observed in earlier demonstrations of this architecture. Using the structure in Figure 1a consisting of glass/indium−tin−oxide (ITO)/poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS)/diphenyltetracene (DPT, or D1)/rubrene (RUB, © 2014 American Chemical Society
Received: December 22, 2013 Revised: March 25, 2014 Published: April 4, 2014 2353
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The Förster transfer rate depends on the integrated overlap between the PL spectrum of the ED molecule (PLED) and the absorption of the EA molecule (εEA), given by5,12,14,17 K=
∫ PL ED(λ) εEA(λ) λ 4 dλ
(2)
The Förster radius is then given by ⎛ (cos ϑ − 3 cos φ cos φ )2 Φ ED ⎞1/6 PL ED EA RF = C ⎜⎜ K ⎟⎟ 4 n ⎝ ⎠ r
where C is a constant, ϑ is the angle between the ED and EA dipoles, φED and φEA are the angles between the respective dipoles and ϑ,5,18 nr is the refractive index of the medium at the wavelength of maximum spectral overlap, and ΦED PL is the fluorescence quantum yield of the energy donor. Indices “ED” and “EA” correspond to the energy donor and energy acceptor molecules, respectively. Here we use C = 0.02,17 and the spatially averaged dipole orientation factor is (cos ϑ − 3 cos φED cos φEA)2 = 0.6719 (see Supporting Information, SI for more details). The values for EDs are taken from the literature20 and provided below for energy transfer between a given ED-to-EA pair. The spectral overlap integrals K are calculated using eq 2 employing measured PLED and εEA for the corresponding ED−EA pairs (see SI). The simulated EQE contributions of each active layer to the cascade efficiency are calculated using the exciton current densities of these layers obtained from numerical solution of eq 1, using the exciton diffusion lengths as fitting parameters. Then, the contributions of each active layer to the total Jsc are calculated by integrating the corresponding EQE contributions over the simulated solar spectrum from λ = 400 to 700 nm. Finally, the simulated total EQE spectrum is the sum of contributions from all active layers obtained from the best fit to the experimental EQE data. See SI for individual device data and fitting details. The current density vs voltage (J−V) characteristics and EQE of the DPT/RUB/DBP/C60 cascade, and DPT/C60, RUB/C60, DBP/C60 bilayer control OPVs are shown in Figure 2. Here, the optical gaps of DPT (D1), RUB (D2), and DBP (D3) are 2.5, 2.2, and 2.0 eV, respectively (c.f. Figure 1b).4,21 The HOMO of each donor is at 5.4 ± 0.1 eV referenced to vacuum.4,21,22 The DPT/RUB/DBP/C60 cascade has PCE = 7.1 ± 0.4%, or nearly double that of the single-donor DBP/C60 device. The improvement is primarily due to the increase in short-circuit current density from Jsc = 6.7 ± 0.3 mA/cm2 to 10.6 ± 0.5 mA/cm2, with minor increases in Voc and fill factor (FF). All performance parameters for the devices studied are listed in Table 1. The increase in Jsc is also reflected in the increase in EQE from 37 ± 2% to 68 ± 3% at a wavelength of λ = 615 nm, which lies only within the DBP absorption range when 10 nm thick DPT and RUB films are added (see Figure 2b). Further, the EQE due to C60 absorption at λ = 400−550 nm is higher in the DBP/C60 device compared to that in the cascade. The decrease in C60 absorption in the cascade is due to the spectral overlap in absorption of C60 with RUB and DPT, as apparent from their extinction coefficients, k (i.e., the imaginary part of complex refractive index) given in Figure 3. The illumination of cascades lacking a C60 acceptor with the structures: ITO/PEDOT:PSS/DPT/RUB/BPhen/Ag and ITO/PEDOT:PSS/RUB/DBP/BPhen/Ag devices using simulated AM1.5G spectrum with intensity up to 1 sun does not generate photocurrent (see SI). From this we infer that the
Figure 1. (a) Schematic diagram of the three-donor energy cascade organic photovoltaic cell. (b) Transport energy levels of the active layers in a cascade. The highest occupied molecular orbital (HOMO) energy levels of donors, D1, D2, and D3, as measured by ultraviolet photoelectron spectroscopy relative to the vacuum energy level are from refs 4, 21, and 22. The lowest unoccupied MO (LUMO) energies are estimated from the optical gaps of donors relative to the HOMO positions. The energy level offsets of the C60 acceptor (A) are from ultraviolet and inverse photoelectron spectroscopy measurements.32 Energy is transferred from DPT or D1, to RUB or D2, and on to DBP or D3. Both interfaces between D1 and D2 and D2 and D3 are not photoactive. (c) Molecular structural formulas for the three donors used in the cascade.
To understand the role of each donor layer and the relative effects of exciton blocking, quenching, and transfer in the cascade, we use the one-dimensional steady-state exciton diffusion equation that includes Förster energy transfer from the energy donating (ED) to the energy accepting (EA) molecules: L D2
∂ 2n − (1 + τkF)n + τG(x) = 0 ∂x 2
(3)
(1)
Here, LD is the diffusion length, n is the density, G(x) is the generation rate at position x in the stack, τ is the natural lifetime of excitons, and kF = πρRF6/6τd3 is the Förster energy transfer rate from the ED to the EA in the various cascade layers.5,12−16 Also, ρ is the molecular density of the energy acceptor,14 RF is the Förster radius, and d is the distance between ED and EA. 2354
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Figure 3. Extinction coefficients of organic active layers used in the cascade.
Table 2. Parameters Used in Modeling EQE ED−EA DPT−RUB RUB−DBP
Table 1. Electrical Parameters of Three-, Two-, and OneDonor Solar Cells DPT/RUB/DBP RUB/DBP DPT/RUB DBP RUB DPT
Voc, V 0.94 0.94 0.90 0.92 0.89 0.80
± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.03 0.01
Jsc,a mA/cm2 10.6 9.3 3.9 6.7 3.8 2.7
± ± ± ± ± ±
0.5 0.5 0.2 0.3 0.2 0.1
FF, % 71 68 52 69 47 39
± ± ± ± ± ±
1 1 1 2 5 1
PCE, % 7.1 6.0 1.9 4.3 1.6 0.8
± ± ± ± ± ±
20
0.9 0.9
ρ, nm−3
RF, nm
1.45 0.94
2.5 4.7
process. To quantify the role of DPT-to-RUB exciton transfer in the DPT/RUB/C60 cascade, we show simulated relative EQE contributions of DPT, RUB, and C60 to the total cascade EQE represented by the solid lines in Figure 4a, which is compared to experimental EQE data shown by the dotted line. From the fits, we obtain diffusion lengths (LD) of 30 ± 2 nm, 35 ± 2 nm, and 28 ± 4 nm for DPT, RUB, and C60, respectively. The reported LD values for organic molecules vary significantly depending on the purity and morphology of measured thin films. For example, LD of C60 is reported between ∼7 and 40 nm,1,23,24 and that of amorphous RUB varies from ∼6 nm to above 20 nm.25,26 As noted above, it is assumed that the DPT/ RUB interface is not photoactive, as confirmed by experiment. In the DPT/RUB/C60 cascade, the photocurrent contribution of DPT due solely to energy transfer to RUB is 4% (0.15 ± 0.01 mA/cm2), that of absorption in RUB followed by dissociation at C60 is 5% (0.20 ± 0.01 mA/cm2), and absorption in C60 contributes 91% (3.6 ± 0.3 mA/cm2) to the total Jsc (c.f. Table 3). Similarly, in a RUB/DBP/C60 cascade, RUB acts as the ED and DBP as the EA. To quantitatively estimate the effects of exciton blocking and transfer from RUB to DBP, we simulate the EQE spectrum of the RUB/DBP/C60 cascade along with contributions from RUB, DBP, and C60 as shown in Figure 4b (solid lines) along with experimental EQE data (dotted line). From the fits, we obtain LD = 20 ± 1 nm for DBP that is on the higher end of reported values. For example, the reported LD of DBP as measured by PL quenching method21 is 9 ± 3 nm, and the modeled LD of DBP from various optical simulations27,28 ranges from 5 to 20 nm. The EQE spectrum is dominated by contributions from DBP (yielding an integrated current density of 3.6 ± 0.3 mA/cm2, or 51% of the total Jsc) and C60 (3.4 ± 0.2 mA/cm2 or 47% of the total Jsc), whereas the contribution from RUB due to energy transfer to DBP represents only 2% of the total. As above, the photocurrent contribution of RUB is due only to exciton transfer to DBP, since the RUB/DBP interface also does not generate photocurrent. These results are summarized in Table 3. The photocurrent of the RUB/DBP/
Figure 2. (a) Current density−voltage (J−V, at 1 sun light intensity, AM1.5G spectrum) and (b) external quantum efficiency (EQE) characteristics of a DPT/RUB/DBP/C60 cascade solar cell and the corresponding control bilayer solar cells.
donor
ref
0.4 0.3 0.1 0.2 0.1 0.1
a
Jsc obtained from data using 1 sun AM 1.5G simulated illumination were spectrally corrected. Values calculated from the integrated EQE spectra and spectral mismatch factors are provided in the Supporting Information.
DPT/RUB and RUB/DBP interfaces are not photoactive and hence do not lead to the dissociation of excitons into free charges in the three-donor cascade device. To understand the role played by each donor layer in the DPT/RUB/DBP/C60 cascade, we quantitatively analyze exciton blocking, quenching, and transfer at individual DPT/ RUB and RUB/DBP interfaces used in dual donor DPT/RUB/ C60 and RUB/DBP/C60 cascades, respectively. The molecular densities of energy acceptors and the Förster transfer radii for DPT-to-RUB and RUB-to-DBP exciton transfer calculated using eqs 2 and 3 are given in Table 2. In the DPT/RUB/C60 cascade, DPT is the exciton donor (ED), and RUB is the exciton acceptor (EA) in the transfer 2355
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exciton blocking at the anode by RUB are primarily responsible for the increased efficiency of the RUB/DBP/C60 cascade over that of the individual single donor devices. Note that the EQE contribution of DBP at λ = 615 nm in the dual donor cascade (64 ± 3%) is almost the same as in the DPT/RUB/DBP/C60 cascade (68 ± 3%). To confirm the effects of exciton blocking and transfer at the RUB/DBP interface, we measured the PL spectra of DBP (60 nm thick) deposited on quartz and capped with 8 nm thick films of either RUB or BPhen (the latter used as a reference exciton blocker). From these measurements (see Supporting Information, Figure S6, and the accompanying discussion), we infer that 85% of the increase in DBP PL capped with RUB is due to exciton blocking, and 15% is due to RUB-to-DBP exciton transfer. These results are in reasonable agreement with the EQE data. The simulated EQE of the DPT/RUB/DBP/C60 cascade is shown in Figure 4c. Here, DPT transfers excitons to RUB leading to the photocurrent contribution of 0.13 ± 0.01 mA/ cm2, while blocking RUB excitons from reaching the anode. Further, RUB transfers excitons to DBP, contributing approximately the same photocurrent, leading to a total contribution of 0.24 ± 0.02 mA/cm2 due to exciton transfer. The DBP and C60 layers contribute the most to the cascade photocurrent: 3.9 ± 0.3 mA/cm2 and 3.5 ± 0.2 mA/cm2, respectively (see Table 3). The overall increase in Jsc using the DPT/RUB/DBP/C60 cascade is 1.7 ± 0.1 mA/cm2 compared to the bilayer DBP/C60 cell, with 14% of this increase due to DPT-to-RUB-to-DBP exciton transfer. The photocurrent contribution from DBP is almost double in the cascade compared with a DBP/C60 bilayer due to efficient exciton blocking by DPT/RUB. From these analyses, we conclude that the EQE increase observed for the DPT/RUB/DBP/C60 cascade compared to the DBP/C60 bilayer cell is primarily due to blocking of DBP excitons by the DPT/RUB layer. Importantly, exciton blocking by RUB leads to an additional photocurrent of 1.6 ± 0.1 mA/ cm2 (from 2.0 ± 0.1 to 3.6 ± 0.3 mA/cm2) generated in DBP along with 0.12 mA/cm2 due to RUB-to-DBP energy transfer; and when both DPT and RUB are included in the three-donor cascade, the DBP response increases by 1.9 ± 0.1 mA/cm2 (to 3.9 ± 0.3 mA/cm2) due to improved exciton blocking by DPT/ RUB, followed by an additional 0.24 ± 0.01 mA/cm2 due to DPT-to-RUB-to-DBP energy transfer. Note that the differences in EQE between these several devices are also affected by the corresponding changes in the light intensity distribution across the active layers, although these effects are relatively small and are accounted for in simulations of the optical fields in our analysis. We investigated exciton blocking, quenching, and transfer in multidonor energy cascade OPVs using a combination of EQE
Figure 4. Simulated EQE of two- and three-donor cascade OPVs obtained by summing (solid black line) the relative contributions from the individual donor layers (indicated). Experimental EQE data are also shown (dotted line). (a) DPT/RUB/C60 cascade; (b) RUB/ DBP/C60 cascade; (c) DPT/RUB/DBP/C60 cascade. DPT (a, c) and RUB (b, c) contributions are due to Förster energy transfer.
C60 cascade is approximately 1.2 mA/cm2 greater than that of the DBP/C60 bilayer cell; 10% of this increase is due to RUBto-DBP exciton transfer, and the remaining 90% is due to exciton blocking by RUB (see Table 3). The relative contribution of RUB-to-DBP energy transfer is relatively minor due to the low k of RUB. Hence, the effects of
Table 3. Short-Circuit Current Density Contributions Calculated for Each Organic Active Layer to the Total in the Cascades
a
donor
Jsc of DPTa
Jsc of RUBa
Jsc of DBPa
DPT/RUB/DBP RUB/DBP DPT/RUB DBP RUB DPT
0.13 (1.8%)
0.11 (1.5%) 0.12 (1.7%) 0.20 (5.0%)
3.88 (51.0%) 3.63 (50.7%)
0.15 (3.8%)
2.01 (33.9%) 0.20 (6.1%) 0.21 (8.4%)
Jsc of C60a 3.48 3.41 3.57 3.92 3.14 2.28
(45.7%) (47.6%) (91.2%) (66.1%) (93.9%) (91.6%)
total Jsca 7.61 7.16 3.92 5.93 3.34 2.49
Relative percentage contributions to Jsc are in parentheses. All values are in mA/cm2. 2356
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are shown in the SI. Given the number of interfaces in the cascade, the EQE was also measured under a white light bias with AM1.5G simulated irradiance at 1 sun intensity. Details are provided in the SI. Experimental errors for Voc and FF arise from variations between devices, and the error in Jsc (∼5%) is primarily due to uncertainties in the intensity of the lamp, which also dominates the error in PCE. Photoluminescence (PL) measurements of active layer, DBP, were performed using the structures: quartz/active layer 60 nm/cap layer 8 nm, where the capping layers for DBP were RUB and BPhen.13 The photoluminescence emission spectra of these samples were measured using a PTI QuantaMaster spectrofluorometer at an incident angle of 30° in a high purity N2 atmosphere to prevent atmospheric degradation of the films. The samples were illuminated through the capping layer. The PL signal was corrected for lamp intensity fluctuations during the measurement, leading to an error in PL intensity of ±5%. All PL data were normalized to the number of absorbed photons in the active layer to account for the differences in the refractive indices of the capping layers using the transfer matrix method.1,31
and PL measurements, along with an analysis of the contributions from each donor layer based on the steadystate exciton diffusion equation assuming Förster energy transfer between donor layers. The multilayer cascade offers the flexibility of using several donor materials in a single device, each optimized for a different purpose. Indeed, we find that a DPT/RUB/DBP/C60 cascade has PCE = 7.1 ± 0.4%, a substantial improvement over past reports of cascade devices.4,6−10 The improvement is primarily due to efficient exciton blocking of the widest energy gap donor layer positioned nearest to the anode where quenching can occur. While exciton transfer between donors is small, it can be increased by the use of materials with comparable oscillator strengths but without significant absorption spectral overlap, thereby eliminating spectral blocking by multiple layers. We note that while this concept has been applied to the donor side of the heterojunction, similar advantages are expected when cascades are implemented on the acceptor side, provided that suitable material combinations can be found. In addition, the model developed to analyze exciton dynamics in cascade OPVs can be expanded to analyze other device architectures and energy transfer across organic interfaces. The advantage of the model is its utility in accurately calculating photocurrent contributions of multiple active layers in devices. This leads to insights regarding the origins and efficiency of energy transfer within multilayer structures, ultimately leading to high performance devices. Experimental Section. ITO-coated (15 Ω/sq) glass substrates (from Bayview) were detergent- and solvent-cleaned and exposed to ultraviolet light and ozone for 10 min prior to depositing PEDOT:PSS films and loading into a vacuum thermal evaporation (VTE) chamber (base pressure of
