Formation of Organic Alloys in Ternary-Blend Solar Cells with Two

Subscriber access provided by PEPPERDINE UNIV

Letter

The Formation of Organic Alloys in Ternary-Blend Solar Cells with Two Acceptors Having Energy-Level Offsets Exceeding 0.4 eV Petr P. Khlyabich, Melda Sezen-Edmonds, Jenna B. Howard, Barry C. Thompson, and Yueh-Lin Loo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00620 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Energy Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

The Formation of Organic Alloys in Ternary-Blend Solar Cells with Two Acceptors Having EnergyLevel Offsets Exceeding 0.4 eV Petr P. Khlyabich,a Melda Sezen-Edmonds,a Jenna B. Howard,b Barry C. Thompsonb and YuehLin Looa,c* a

Department of Chemical and Biological Engineering, Princeton University,

Princeton, New Jersey 08544, United States. b

Department of Chemistry and Loker Hydrocarbon Research Institute, University of Southern

California, Los Angeles, California 90089-1661, United States. c

Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey

08544, United States. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

1

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

ABSTRACT: Recent studies demonstrated that with proper selection of chemically compatible constituents, the open-circuit voltage (Voc) of ternary-blend solar cells can be tuned across the composition window of the active layer. In this study, we probed the limit of the offset between the lowest unoccupied molecular orbital (LUMO) energy levels of the two acceptors in ternary blends containing one donor and two acceptors. We demonstrate, for the first time, that ternaryblend active layers with two acceptors having energy-level difference between their LUMO levels exceeding 0.4 eV can still result in solar cells exhibiting composition-dependent opencircuit voltage (Voc). Our results suggest strong electronic interactions between the acceptors, with the electron wavefunction delocalized over multiple molecules. These findings have broadened the library of possible candidates for active layers of ternary-blend solar cells with tunable Voc and established guidelines for the design of next-generation of materials for efficient performance of such devices.

TOC GRAPHICS


2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Binary bulk-heterojunction organic solar cells are quickly approaching the practical efficiency limit of 11%.1–8 Further increases in efficiency are possible through the introduction of tandem solar cells in which subcells are connected either in series or in parallel.9,10 Despite the increase in power-conversion efficiencies, such improvements are achieved at the expense of fabrication complexity.11–14 Recently, the use of ternary-blend photoactive layers containing either two donors and one acceptor (D1/D2/A) or one donor and two acceptors (D/A1/A2) has become a comparatively simple approach to realizing single-junction organic solar cells with greater efficiencies.15–20 This approach preserves the simplicity of solar-cell fabrication while paving the way for accessing efficiencies beyond the limit set by single-junction solar cells comprising binary blends of donor and acceptor pairs.21–26 Ternary-blend solar cells benefit from an increase in short-circuit current density (Jsc) upon the introduction of a third photoabsorbing component in the active layer. The Jsc enhancement stems from an increased number of captured photons through the introduction of a constituent having an absorption profile that complements that of the parent donor-acceptor pair.16,27,28 Until recently, the open-circuit voltage (Voc) of ternary-blend solar cells was thought to be pinned by the difference in the energy levels of the higher of the two highest occupied molecular orbitals (HOMO) of the donors and the lowest unoccupied molecular orbital (LUMO) of the acceptor in active layers comprising two donors and an acceptor.29,30 However, recent studies demonstrated that with proper selection of the constituents, the Voc can be continuously tuned between the largest and smallest differences of the HOMO energy levels of the individual donors and the LUMO energy level of the acceptor for D1/D2/A systems, or between the largest and smallest differences of the LUMO energy levels of the individual acceptors and the HOMO energy level of the donor for D/A1/A2 systems without negatively impacting the photocurrent or fill factor


3

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

(FF).15–17,19,31–35 This tunability in Voc in ternary-blend solar cells is described by the organic alloy model17,31 in which individual constituents preserve their excitonic character due to the localized nature of the exciton, yet dissociated holes and electrons are delocalized over much larger distances.32,36 This composition-dependent Voc in ternary-blend solar cells is correlated with the chemical compatibility between the active-layer constituents.36,37 When the two polymer donors in D1/D2/A systems are miscible or can cocrystallize, the corresponding ternary-blend solar cells exhibit composition-dependent Voc. On the other hand, phase separation between the two donor constituents limits the Voc of such ternary-blend solar cells to the energy difference between the LUMO energy level of the acceptor and the higher-lying HOMO energy level of the two donor polymers. Less explored is how the energy-level offsets between the HOMOs of the two donors or the LUMOs of the two acceptors for D1/D2/A or D/A1/A2 systems, respectively, affect Voc. It has been proposed that this tunability in Voc and simultaneous Jsc enhancement in ternary-blend solar cells can only occur when these energy-level offsets are below 0.3 eV38 and experiments to-date have not been inconsistent with this assertion.15,16,35,39,40 While a recent study demonstrated Jsc enhancement in D1/D2/A solar cells when the energy-level offset between the HOMOs of the two donors is 0.37 eV,28 the Voc of these solar cells appear to be pinned by the difference in the energy levels of the LUMO of the acceptor and the higher of the two HOMOs of the donors.28 With a series of D/A1/A2 ternary blends comprising fullerene derivatives, we systematically evaluated how the LUMO-LUMO energy offset of the fullerenes affects the tunability of the Voc in the resulting solar cells. This elucidation should shed light on the connection between the constituent HOMO or LUMO energy-level offsets and the ability to maintain efficient


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

photocurrent generation and charge transport in ternary-blend solar cells. Finally, establishing the energetic requirements for tunable Voc in ternary-blend solar cells will also allow rapid screening of potential constituents for active layers, and will simultaneously promote understanding of the operational limits and device physics of ternary-blend solar cells. In our current study, we explored ternary-blend solar cells of the type D/A1/A2 that exhibit efficient charge generation and transport. We demonstrate, for the first time, that the Voc of these solar cells remains tunable across the composition window of the active layer despite an energylevel difference between the LUMOs of the two acceptors that exceeds 0.4 eV, and that is as high as 0.51 eV. Voc tunability requires strong electronic interactions between A1 and A2 so our findings suggest an electron wavefunction that is delocalized over large distances. For this study, we investigated five ternary-blend systems comprising a semicrystalline polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14),41 as the polymer donor (D) and phenyl-C61-butyric acid methyl ester (PC61BM) as the parent acceptor (A1); a second fullerene derivative served as A2. The chemical structures of each of the constituents are shown in Figure 1. We focused on ternary blends containing one donor and two acceptors, D/A1/A2, to elucidate how the energy-level offset between the LUMOs of acceptors A1 and A2 impact device performance. All acceptors have similar surface energies and thus satisfy the morphological criterion of chemical compatibility in order for us to realize ternary-blend solar cells with Vocs that are tunable across the composition window of the active layer.36


5

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

Figure 1. Chemical structures and corresponding energy levels of PBTTT-C14,41 C60, PC61BM, ICBA, bis-PC61BM, tris-PC61BM and ICTA. The LUMO energy levels of the electron acceptors used in the study were measured by solution CV and have been internally referenced to the LUMO energy level of PC61BM for this study. Table 1 contains the ternary-blend compositions, whether the acceptors intercalate between the side chains of PBTTT-C14, and the energy-level offsets between the LUMOs of PC61BM and A2. Ternary blend A comprises bis-PC61BM as A2, providing an energy difference between the LUMOs of the acceptors of 0.12 eV. In ternary blend B, indene-C60 bisadduct, ICBA, is A2. It


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

has a LUMO energy level that is 0.17 eV higher than PC61BM. In ternary blend C, tris-PC61BM is A2; the energy-level difference between the LUMOs of the acceptors is 0.20 eV. We also examined ternary blend D, where indene-C60 trisadduct, ICTA, with a LUMO energy level 0.42 eV higher than that of PC61BM, is A2. Finally, to probe how the LUMO-LUMO offset in A1 and A2 pairs limits Voc tunability, we studied ternary blend E, in which C60 is A1 and ICTA is A2 to give an energy-level difference between the LUMOs of the acceptors of 0.51 eV. We kept the overall polymer:fullerene weight ratio at 1:4 and only changed the weight ratio between A1 and A2. The LUMO energy levels for all of the fullerene derivatives were measured using solution cyclic voltammetry (CV; see Figure S1); the energy-level differences between the LUMOs of PC61BM and A2 for all the blends correlate well with previously reported values.42–44 Table 1. Characterization of ternary blends and the open-circuit voltage of solar cells comprising them. ∆LUMOA1-A2 (eV)

Blend

A1

A2

Morphology

Voc (V)

A

PC61BM

bis-PC61BM

A2 does not intercalate 0.12

0.48 – 0.62

B

PC61BM

ICBA

A1 and A2 intercalate

0.17

0.48 – 0.63

C

PC61BM

tris-PC61BM


0.48 – 0.58

D

PC61BM

ICTA


0.48 – 0.72

E

C60

ICTA


Tunable1

1

The Voc for ternary-blend solar cells containing ternary blend E at 20% C60 loading of the total acceptor content is 0.6 V and is an intermediate value between what one would expect of PBTTT-C14:C60 (below 0.48 V) and PBTTT-C14:ICTA (0.72 V) binary-blend solar cells.


7

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

We fabricated ternary-blend solar cells whose active-layer compositions are specified above. Binary-blend solar cells containing PC61BM demonstrate optimal device performance at 1:4 donor to acceptor weight ratio. While it is known that binary-blend solar cells containing PBTTT-C14 and either bis-PC61BM, tris-PC61BM or ICTA show optimized performance when the active layer comprises a 1:1 donor to acceptor weight ratio, we kept the overall polymer:fullerene ratio at 1:4 since PC61BM is present in all the active layers and PC61BM readily intercalates between the alkyl side chains of PBTTT-C14.45 Keeping the donor:acceptor ratio constant throughout allowed us to minimize the parameters that can affect solar-cell output characteristics. All the devices have low efficiencies because the photocurrents are limited by the low absorption of the acceptors in the active layers. But the preservation of FFs above 0.42 – and in many cases above 0.5 – in all our devices suggests that localized trap states are not limiting device operation. Rather, an imbalance in charge-carrier transport in the active layers results in space-charge limitation.46 This limitation, however, does not impact the conclusions drawn based on the Voc trends we observe herein. Figure 2 demonstrates the Voc behavior of ternary-blend solar cells upon the introduction of different A2 as the third component in these blends. As expected, the introduction of bis-PC61BM (ternary blend A), ICBA (ternary blend B) and tris-PC61BM (ternary blend C) leads to tunable Voc in ternary-blend solar cells as the acceptor pairs are mutually compatible and their LUMO energy level differences are less than 0.2 eV. Solar cells with ternary blend D (ICTA as A2) display similarly tunable Voc across its blend composition window, even when the energy difference between the LUMOs of the two acceptors exceeds 0.4 eV (Figure 2d).


8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Figure 2. Open-circuit voltage of optimized ternary-blend solar cells containing (a) PBTTT-C14, PC61BM and bis-PC61BM; (b) PBTTT-C14, PC61BM and ICBA; (c) PBTTT-C14, PC61BM and tris-PC61BM and (d) PBTTT-C14, PC61BM and ICTA as a function of the fraction of PC61BM of the two acceptors; the donor to acceptor weight ratio was kept constant at 1:4. Inspired by the ability to tune the Voc in ternary-blend solar cells with an energy difference between the LUMOs of the two acceptors exceeding 0.4 eV (ternary blend D), we further fabricated ternary-blend solar cells containing PBTTT-C14, C60 and ICTA (ternary blend E) to test the energetic limits on tuning Voc. Substituting PC61BM with C60 resulted in an energy-level difference between the LUMOs of the two acceptors of 0.51 eV. C60, however, has poor solubility in organic solvents, which limited its loading to 20% of the total acceptor content in


9

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

solar cells comprising ternary blend E. At 20% C60 loading, the Voc of these solar cells is at 0.60 V, an intermediate value between what one would expect of PBTTT-C14:C60 (below 0.48 V) and PBTTT-C14:ICTA binary-blend solar cells. This result demonstrates that we can access Voc that is not pinned by those determined by the energy-level difference of PBTTT-C14:C60 and PBTTT-C14:ICTA binary-blend solar cells, even when the LUMO energy-level difference of the two acceptors in our ternary-blend solar cell exceed 0.5 eV. If we invoke the organic alloy model,17,31 the composition-dependent Voc observed in all the ternary-blend solar cells in this study implies strong electronic interactions between A1 and A2 so the electron wavefunction is substantially delocalized across many molecules, even when the LUMO energy levels between A1 and A2 differ by more than 0.5 eV.15,17,31 It follows that the minority fullerene in D/A1/A2 blends is not creating localized traps. Rather, its addition favors delocalization of electronic states over large distances across the composition window, and according to one study, this delocalization can span as many as thirty fullerene molecules.32 Figure S2 contains representative J-V plots for binary- and ternary-blend solar cells in this study. Reference solar cells containing PBTTT-C14:PC61BM, exhibit an average Jsc of 6.4 mA/cm2, which is comparable to previous reports.47 Incorporation of A2 in PBTTT-C14:PC61BM blends result in solar cells with progressively decreasing Jsc. Since the fraction of total acceptor relative to donor is invariant, the reduction in Jsc in all cases stems from the lower absorptivity of bisPC61BM, ICBA, tris-PC61BM and ICTA, relative to that of PC61BM.15,48 Despite a decrease in Jsc, the FF’s of these devices remain at or around 0.50 when PC61BM comprises the major fraction of the acceptor in the blend. We interpret this observation to imply that charge generation and transport in these ternary-blend solar cells are efficient, even when the energylevel difference between the LUMOs of the two acceptors is above 0.4 eV, and as high as 0.5 eV.


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Nonetheless, the decrease in Jsc accompanying the increase in A2 fraction in ternary-blend solar cells results in a gradual decrease in power-conversion efficiency, which stems from reduced absorption of A2 compared to that of PC61BM. More specifically, the reduction in absorption of the active layer when A2 is introduced accounts for more than two-thirds of the reduction in Jsc between ternary-blend and PBTTT-C14:PC61BM solar cells. This analysis assumes that only single-pass photoabsorption contributes to the Jsc when photoabsorption on subsequent passes can account for up to 25% of the Jsc.49 We thus believe that the reduction of absorption of the active layer to be the first order reason for the observed decrease in Jsc in ternary-blend solar cells compared to PBTTT-C14:PC61BM solar cells. It follows that we should be able to increase the power-conversion efficiency by optimizing the thickness of the ternary-blend active layers, but we have opted to maintain identical processing conditions for all active layers to provide a common basis with which we can compare across the solar cells under study. Despite low current densities and power-conversion efficiencies, our model study demonstrates that charge generation and transport can be effectively maintained despite an energy-level difference between the LUMOs of the two acceptors that exceeds 0.4 eV. In binary-blend solar cells with bis-PC61BM, ICBA, tris-PC61BM and ICTA, we observe FF’s of 0.42. The lower FF in these devices compared to binary-blend solar cells of PBTTT-C14:PC61BM solar cells originates from an imbalance in charge-carrier transport,46,50 as the electron mobilities of bis-PC61BM, ICBA, tris-PC61BM and ICTA are low relative to the hole mobility of PBTTT-C14.48 The performance of ternary-blend solar cells in this study supports the notion of an organic alloy, with strong electronic interactions between A1 and A2 such that the electron wavefunction is delocalized across many molecules. In the absence of such delocalization between A1 and A2, we would expect to see evidence of localized trap states or defect states, such as those previously


11

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

observed when PC61BM is mixed with PC84BM,51 manifested as a precipitous drop in FF because of extensive recombination and a sharp decrease in Jsc even with a minute addition of A2. Instead, our ternary-blend solar cells maintain FFs above 0.42 throughout the composition window and the drop in Jsc only comes when A2 with lower absorptivity is incorporated substantially. We thus surmise that our ternary blends form organic alloys with extensive electron wavefunction delocalization between A1 and A2 in order for us to observe compositiondependent Voc in the resulting solar cells. However, further studies are necessary to estimate the extent of electron wavefunction delocalization.51 The fullerene acceptors explored herein are chemically compatible with each other and can be divided into two categories: those that intercalate between the side chains of the PBTTT-C14 (PC61BM and ICBA) to form bimolecular crystals,45 and those that do not,45 presumably due to sterics (bis-PC61BM, tris-PC61BM and ICTA). This intercalation can be studied by X-ray diffraction, but more easily tracked via absorption measurements. Figure 3a shows the absorption profile of PBTTT-C14. We observe in Figure 3b that the introduction of ICTA does not alter the absorption profile of PBTTT-C14 significantly. ICTA absorbs in the near-UV; we see an increase in the absorption below 400 nm attributable to ICTA. The absorption at longer wavelengths that is associated with PBTTT-C14 remains unchanged. Consistent with literature reports,45 the comparison between Figures 3a and b indicate that ICTA does not intercalate between the side chains of PBTTT-C14. Figure 3c shows the absorption profiles of ternary blends of PBTTT-C14:PC61BM:ICTA across the composition window. While the absorption profiles are substantially different from that of PBTTT-C14, they are marginally different from each other. These absorption profiles are different from that of PBTTT-C14 because PC61BM readily intercalates between the side chains of the polymer donor, resulting in bimolecular


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

crystals whose absorption is distinct from that of PBTTT-C14.52 The observation that adding ICTA and increasing its concentration does not substantially alter the absorption profiles in Figure 3c indicates the preservation of bimolecular crystals at all compositions. The absorption profiles with the addition of A2 are qualitatively similar for ternary blends A and C. Our data thus indicate that in ternary blends containing PC61BM and a non-intercalating fullerene derivative (ternary blends A, C and D), the presence of A2 does not disrupt the formation of bimolecular crystals between PBTTT-C14 and PC61BM. Figure 3d shows the absorption profiles of ternary blends of PBTTT-C14:PC61BM:ICBA across the composition window. Since ICBA also readily intercalates between the side chains of PBTTT-C14, the similarity of absorption profiles across the composition window suggests that the bimolecular crystals are structurally similar, whether they form between PBTTT-C14 and PC61BM, between PBTTT-C14 and ICBA, or at any other intermediate compositions.


13

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

Figure 3. (a) UV-vis absorbance spectrum of a PBTTT-C14 thin film. (b) UV-vis absorbance spectrum of a thin film of PBTTT-C14:ICTA at a 1:4 weight ratio. (c) UV-vis absorbance spectra of ternary-blend films containing PBTTT-C14, PC61BM and ICTA with increasing PC61BM content. (d) UV-vis absorbance spectra of ternary-blend films containing PBTTT-C14, PC61BM and ICBA with increasing PC61BM content. Complementary x-ray diffraction analyses are provided in supporting information. Figure S3 contains the x-ray diffraction images, along with the collapsed one-dimensional x-ray diffraction traces, of thin films containing ternary blends at different A1:A2 ratios. The ternary blends are semi-crystalline, as evidenced by the presence of a strong, out-of-plane reflection along qxy = 0. Figure S4 shows quantification of the positions of the primary reflections extracted from the


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

GIXD patterns of binary and ternary blends. Consistent with the picture that binary blends form bimolecular crystals because one or both of the acceptors is/are capable of intercalating between the side chains of PBTTT-C14, the x-ray diffraction patterns reveal a primary reflection associated with the characteristic spacing of the bimolecular crystals at 0.22 Å-1 and at 0.24 Å-1 for PBTTT-C14:PC61BM and PBTTT-C14:ICBA, respectively. Binary blends comprised of fullerene derivatives that are bulkier reveal a primary reflection between 0.26 Å-1 to 0.28 Å-1. This larger q-spacing corresponds to a smaller characteristic distance that is comparable to that of PBTTT-C14 alone; this observation thus indicates that bis-PC61BM, tris-PC61BM and ICTA do not to intercalate between the side chains of PBTTT-C14.45 The GIXD patterns of all ternaryblend thin films in this study yielded primary reflections between 0.22 Å-1 and 0.24 Å-1. This observation is consistent with our absorption studies and is evidence that bimolecular crystals are formed in all the ternary blends, independent of whether A2 intercalates because PC61BM does so readily. Figure 4 provides a simple illustration of these two scenarios: one in which both the acceptors intercalate between the side chains of PBTTT-C14 (Figure 4a; for ternary blend B, with ICBA as A2), and one in which only PC61BM intercalates between the side chains of PBTTT-C14 (Figure 4b). The latter is the scenario for ternary blends A, C, and D; while A2 in these ternary blends does not readily form bimolecular crystals with PBTTT-C14, they do not preclude the formation of bimolecular crystals between PBTTT-C14 and PC61BM.


15

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Figure 4. Illustration of ternary-blend morphologies where (a) both acceptors, A1 and A2, intercalate (ternary blend B) and (b) only one acceptor, A1, intercalates (ternary blends A, C, D and E) between the side-chains of PBTTT-C14. Dash circles represent the extent of delocalization of free charges (electron). Earlier studies have demonstrated that the morphology of ternary blends determines the Voc behavior in ternary-blend solar cells.36 As expected, the introduction of ICBA to PBTTTC14:PC61BM (ternary blend B) leads to tunable Voc in ternary-blend solar cells as both acceptors can intercalate between side chains of PBTTT-C14 and they are known to be miscible with each other (Figure 2a).53 This observation is consistent with the organic alloy model previously used to describe D1/D2/A and D/A1/A2 systems, in which the electron wavefunction is delocalized over large distances and is determined by the average composition of D1/D2 or A1/A2.15,17,31,54 In ternary blends where A2 does not intercalate between the side chains of PBTTT-C14, the Voc of


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

these solar cells still falls between those corresponding to binary-blend solar cells of PBTTTC14:A1 and PBTTT-C14:A2. Still in the framework of the organic alloy model, this observation implies strong electronic interactions between constituents A1 and A2. The similarity in how the Voc varies across the composition window in solar cells containing ternary blends A and B (Figures 2a and b) further suggests A1 and A2 must maintain strong electronic interactions with each other so the electron wavefunction is delocalized over large distances whether A2 intercalates. We have schematized this distance over which delocalization takes place with circles in Figure 4b. We believe electron delocalization to extend beyond the intercalated acceptors; the Voc of devices comprising such ternary blends is thus determined by the average composition of A1:A2. A recent study demonstrated that the electron wavefunction can extend over 30 fullerene molecules for systems containing one donor and two fullerene acceptors.32 As a result, it is quite possible for changes in the A1:A2 ratio to alter the overall electron wavefunction, leading to the observation of tunable Voc in ternary-blend solar cells.15– 17,32

Figure 2 also shows that the Voc behavior extracted from solar cells comprising ternary blend C is qualitatively different from those comprising the other ternary blends. The Voc of solar cells comprising ternary blends A, B and D can be tuned continuously over 60 to 130 mV across the entire composition window. This behavior is consistent with earlier observations of the Voc behavior in ternary-blend solar cells containing one donor and two fullerene acceptors.15,39,55 Solar cells containing ternary blend C exhibit Voc that can be tuned by only 20 mV as PC61BM content is increased from 20 to 80%. Qualitatively, this behavior is consistent with those more commonly observed in ternary-blend solar cells containing D1/D2/A blends.16,33 We speculate that this subtle difference in the Voc behavior between the ternary-blend solar cells stems from


17

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

small differences in the morphology of the active layer. In the context of the organic alloy model, the introduction of tris-PC61BM beyond 20% does not alter the electronic structure of the delocalized state. We surmise that this invariance in delocalization stems from an invariance of the local composition spanning the distance over which the fullerenes are delocalized, which then leads to an invariance in the composition at the donor: acceptor interface. This hypothesis further suggests that tris-PC61BM is only partially miscible beyond 20% in PC61BM, leading to phase separation of two solid solutions comprising both fullerenes but this phase separation must occur sufficiently far away from the donor:acceptor interface where charge separation takes place to maintain a near-constant Voc that is intermediate of those of the respective binary-blend solar cells. Clearly, further studies are necessary to investigate the origin of this difference.23 The ability to tune the Voc in ternary-blend solar cells when the energy-level offset between the HOMOs of the two donors or LUMOs of the two acceptors exceeds 0.4 eV provides a pathway to achieve power-conversion efficiencies beyond the practical efficiency limit for binary-blend solar cells, which has been estimated to be approximately 11% for those containing fullerenes as acceptors.1–8 To exceed this limit, the donor and acceptor constituents of the ternary-blend active layer must be judiciously selected so (a) their absorption profiles are complementary to maximize Jsc, and (b) they readily form an organic alloy so the Voc is tunable across the composition window. Case in point would be a ternary-blend active layer that contains two donors, a low bandgap constituent with strong absorption characteristics in the near-IR and a large bandgap constituent whose absorption starts in the UV and extends to the visible range of the solar spectrum. Provided that the energy levels are appropriately matched with the acceptor constituent to enable efficient exciton dissociation and there is extensive wavefunction delocalization between the two donor constituents,36,37 we should be able to extend the practical


18

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

efficiency limit of single-junction solar cells. A recent estimate of the efficiency limit for ternary-blend solar cells is 14%;38 this estimate assumes an energy-level offset between the HOMOs of the two donors or LUMOs of the two acceptors of 0.3 eV. That these energy-level offsets can be greater than 0.3 eV affords greater tunability of the Voc, which should extend the efficiency limit beyond what had been predicted. In summary, we have demonstrated ternary-blend solar cells containing one donor and two fullerene acceptors (D/A1/A2) with tunable Voc when the LUMO-LUMO offsets of the two acceptors exceeds 0.4 eV, and is as high as 0.5 eV. Independent of whether the second acceptor intercalates between the side chains of PBTTT-C14, chemical compatibility between the acceptors ensure strong electronic interactions, which allows delocalization of the electron wavefunction over both A1 and A2. Accordingly, the Voc of such ternary-blend solar cells is tunable with its magnitude determined by the overall acceptor composition. Our results have broadened the library of possible candidates for active layers of ternary-blend solar cells with tunable Voc, providing a pathway towards materials design for more efficient single-junction solar cells. ASSOCIATED CONTENT Supporting Information. Materials, materials characterization, synthetic procedures, UV-vis absorption, CV traces, device fabrication and characterization, J-V curves of ternary-blend solar cells, GIXD images and out-of-plane X-ray diffraction traces, the positions of the primary reflections extracted from the GIXD images, tabulated solar cells data. AUTHOR INFORMATION


19

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

Corresponding Author: E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT GIXD experiments were conducted at the Cornell High Energy Synchrotron Source supported by the NSF & NIH/NIGMS via NSF award DMR-1332208. We acknowledge funding from the National Science Foundation (ECCS-1549619 and CMMI-1537011) and Princeton Center for Complex Materials funded under NSF-MRSEC (DMR-1420541) for support of PPK, MSE and YLL. JBH and BCT acknowledge support by the National Science Foundation (CBET Energy for Sustainability) CBET-1436875. REFERENCES (1)

Thompson, B. C.; Khlyabich, P. P.; Burkhart, B.; Aviles, A. E.; Rudenko, A.; Shultz, G.

V.; Ng, C. F.; Mangubat, L. B. Polymer-Based Solar Cells: State-of-the-Art Principles for the Design of Active Layer Components. Green 2011, 1, 29–54. (2)

Janssen, R. A. J.; Nelson, J. Factors Limiting Device Efficiency in Organic Photovoltaics.

Adv. Mater. 2013, 25, 1847–1858. (3)

Scharber, M. C.; Sariciftci, N. S. Efficiency of Bulk-Heterojunction Organic Solar Cells.

Prog. Polym. Sci. 2013, 38, 1929–1940.


20

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(4)

Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganäs, O.; Gao, F.; Hou, J. Fullerene-Free

Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734–4739. (5)

Liu, C.; Yi, C.; Wang, K.; Yang, Y.; Bhatta, R. S.; Tsige, M.; Xiao, S.; Gong, X. Single-

Junction Polymer Solar Cells with Over 10% Efficiency by a Novel Two-Dimensional Donor– Acceptor Conjugated Copolymer. ACS Appl. Mater. Interfaces 2015, 7, 4928–4935. (6)

Huang, J.; Li, C.-Z.; Chueh, C.-C.; Liu, S.-Q.; Yu, J.-S.; Jen, A. K.-Y. 10.4% Power

Conversion Efficiency of ITO-Free Organic Photovoltaics Through Enhanced Light Trapping Configuration. Adv. Energy Mater. 2015, 5, 1500406. (7)

Zhang, S.; Ye, L.; Hou, J. Breaking the 10% Efficiency Barrier in Organic Photovoltaics:

Morphology and Device Optimization of Well-Known PBDTTT Polymers. Adv. Energy Mater. 2016, 6, 1502529. (8)

Bin, H.; Gao, L.; Zhang, Z.-G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang,

C.; Xiao, M.; et al. 11.4% Efficiency Non-Fullerene Polymer Solar Cells with Trialkylsilyl Substituted 2D-Conjugated Polymer as Donor. Nat. Commun. 2016, 7, 13651. (9)

Ameri, T.; Dennler, G.; Lungenschmied, C.; Brabec, C. J. Organic Tandem Solar Cells:

A Review. Energy Environ. Sci. 2009, 2, 347. (10)

Ameri, T.; Li, N.; Brabec, C. J. Highly Efficient Organic Tandem Solar Cells: A Follow

up Review. Energy Environ. Sci. 2013, 6, 2390.


21

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(11)

Page 22 of 28

Zheng, Z.; Zhang, S.; Zhang, J.; Qin, Y.; Li, W.; Yu, R.; Wei, Z.; Hou, J. Over 11%

Efficiency in Tandem Polymer Solar Cells Featured by a Low-Band-Gap Polymer with FineTuned Properties. Adv. Mater. 2016, 28, 5133–5138. (12)

Zhou, H.; Zhang, Y.; Mai, C.-K.; Collins, S. D.; Bazan, G. C.; Nguyen, T.-Q.; Heeger, A.

J. Polymer Homo-Tandem Solar Cells with Best Efficiency of 11.3%. Adv. Mater. 2015, 27, 1767–1773. (13)

You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-

C.; Gao, J.; Li, G.; et al. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, 1446. (14)

Zhang, K.; Gao, K.; Xia, R.; Wu, Z.; Sun, C.; Cao, J.; Qian, L.; Li, W.; Liu, S.; Huang,

F.; et al. High-Performance Polymer Tandem Solar Cells Employing a New n-Type Conjugated Polymer as an Interconnecting Layer. Adv. Mater. 2016, 28, 4817–4823. (15)

Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Efficient Ternary Blend Bulk

Heterojunction Solar Cells with Tunable Open-Circuit Voltage. J. Am. Chem. Soc. 2011, 133, 14534–14537. (16)

Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Compositional Dependence of the

Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells Based on Two Donor Polymers. J. Am. Chem. Soc. 2012, 134, 9074–9077. (17)

Khlyabich, P. P.; Burkhart, B.; Rudenko, A. E.; Thompson, B. C. Optimization and

Simplification of Polymer–fullerene Solar Cells through Polymer and Active Layer Design. Polymer 2013, 54, 5267–5298.


22

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(18)

Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J. Organic Ternary Solar Cells: A Review.

Adv. Mater. 2013, 25, 4245–4266. (19)

An, Q.; Zhang, F.; Zhang, J.; Tang, W.; Deng, Z.; Hu, B. Versatile Ternary Organic Solar

Cells: A Critical Review. Energy Environ. Sci. 2016, 9, 281–322. (20)

Huang, H.; Yang, L.; Sharma, B. Recent Advances in Organic Ternary Solar Cells. J.

Mater. Chem. A 2017, 5, 11501–11517. (21)

Park, K. H.; An, Y.; Jung, S.; Park, H.; Yang, C. The Use of an N-Type Macromolecular

Additive as a Simple yet Effective Tool for Improving and Stabilizing the Performance of Organic Solar Cells. Energy Environ. Sci. 2016, 9, 3464–3471. (22)

Zhao, W.; Li, S.; Zhang, S.; Liu, X.; Hou, J. Ternary Polymer Solar Cells Based on Two

Acceptors and One Donor for Achieving 12.2% Efficiency. Adv. Mater. 2017, 29, 1604059. (23)

Baran, D.; Ashraf, R. S.; Hanifi, D. A.; Abdelsamie, M.; Gasparini, N.; Röhr, J. A.;

Holliday, S.; Wadsworth, A.; Lockett, S.; Neophytou, M.; et al. Reducing the Efficiency– stability–cost Gap of Organic Photovoltaics with Highly Efficient and Stable Small Molecule Acceptor Ternary Solar Cells. Nat. Mater. 2016, 16, 363–369. (24)

Kumari, T.; Lee, S. M.; Kang, S.-H.; Chen, S.; Yang, C. Ternary Solar Cells with a

Mixed Face-on and Edge-on Orientation Enable an Unprecedented Efficiency of 12.1%. Energy Environ. Sci. 2017, 10, 258–265. (25)

Zhang, G.; Zhang, K.; Yin, Q.; Jiang, X.-F.; Wang, Z.; Xin, J.; Ma, W.; Yan, H.; Huang,

F.; Cao, Y. High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer


23

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Containing a Liquid Crystalline Small Molecule Donor. J. Am. Chem. Soc. 2017, 139, 2387– 2395. (26)

Gasparini, N.; Lucera, L.; Salvador, M.; Prosa, M.; Spyropoulos, G. D.; Kubis, P.;

Egelhaaf, H.-J.; Brabec, C. J.; Ameri, T. High-Performance Ternary Organic Solar Cells with Thick Active Layer Exceeding 11% Efficiency. Energy Environ. Sci. 2017, 10, 885–892. (27)

Cho, Y. J.; Lee, J. Y.; Chin, B. D.; Forrest, S. R. Polymer Bulk Heterojunction

Photovoltaics Employing a Squaraine Donor Additive. Org. Electron. 2013, 14, 1081–1085. (28)

Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L. Ternary Blend Polymer Solar Cells with

Enhanced Power Conversion Efficiency. Nat. Photonics 2014, 8, 716–722. (29)

Koppe, M.; Egelhaaf, H.-J.; Dennler, G.; Scharber, M. C.; Brabec, C. J.; Schilinsky, P.;

Hoth, C. N. Near IR Sensitization of Organic Bulk Heterojunction Solar Cells: Towards Optimization of the Spectral Response of Organic Solar Cells. Adv. Funct. Mater. 2010, 20, 338– 346. (30)

Ameri, T.; Min, J.; Li, N.; Machui, F.; Baran, D.; Forster, M.; Schottler, K. J.; Dolfen, D.;

Scherf, U.; Brabec, C. J. Performance Enhancement of the P3HT/PCBM Solar Cells through NIR Sensitization Using a Small-Bandgap Polymer. Adv. Energy Mater. 2012, 2, 1198–1202. (31)

Street, R. A.; Davies, D.; Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Origin of the

Tunable Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Organic Solar Cells. J. Am. Chem. Soc. 2013, 135, 986–989.


24

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(32)

Street, R. A.; Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C. Electronic States in

Dilute Ternary Blend Organic Bulk Heterojunction Solar Cells. J. Phys. Chem. C 2014, 118, 26569–26576. (33)

Khlyabich, P. P.; Rudenko, A. E.; Burkhart, B.; Thompson, B. C. Contrasting

Performance of Donor–Acceptor Copolymer Pairs in Ternary Blend Solar Cells and TwoAcceptor Copolymers in Binary Blend Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 2322– 2330. (34)

Lu, H.; Zhang, J.; Chen, J.; Liu, Q.; Gong, X.; Feng, S.; Xu, X.; Ma, W.; Bo, Z. Ternary-

Blend Polymer Solar Cells Combining Fullerene and Nonfullerene Acceptors to Synergistically Boost the Photovoltaic Performance. Adv. Mater. 2016, 28, 9559–9566. (35)

Lee, T. H.; Uddin, M. A.; Zhong, C.; Ko, S.-J.; Walker, B.; Kim, T.; Yoon, Y. J.; Park, S.

Y.; Heeger, A. J.; Woo, H. Y.; et al. Investigation of Charge Carrier Behavior in High Performance Ternary Blend Polymer Solar Cells. Adv. Energy Mater. 2016, 6, 1600637. (36)

Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C.; Loo, Y.-L. Structural Origins for

Tunable Open-Circuit Voltage in Ternary-Blend Organic Solar Cells. Adv. Funct. Mater. 2015, 25, 5557–5563. (37)

Gobalasingham, N. S.; Noh, S.; Howard, J. B.; Thompson, B. C. Influence of Surface

Energy on Organic Alloy Formation in Ternary Blend Solar Cells Based on Two Donor Polymers. ACS Appl. Mater. Interfaces 2016, 8, 27931–27941. (38)

Savoie, B. M.; Dunaisky, S.; Marks, T. J.; Ratner, M. A. The Scope and Limitations of

Ternary Blend Organic Photovoltaics. Adv. Energy Mater. 2015, 5, 1400891.


25

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(39)

Page 26 of 28

Cheng, P.; Li, Y.; Zhan, X. Efficient Ternary Blend Polymer Solar Cells with Indene-C60

Bisadduct as an Electron-Cascade Acceptor. Energy Environ. Sci. 2014, 7, 2005–2011. (40)

Li, H.; Zhang, Z.-G.; Li, Y.; Wang, J. Tunable Open-Circuit Voltage in Ternary Organic

Solar Cells. Appl. Phys. Lett. 2012, 101, 163302. (41)

McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.;

Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; et al. Liquid-Crystalline Semiconducting Polymers with High Charge-Carrier Mobility. Nat. Mater. 2006, 5, 328–333. (42)

Faist, M. A.; Keivanidis, P. E.; Foster, S.; Wöbkenberg, P. H.; Anthopoulos, T. D.;

Bradley, D. D. C.; Durrant, J. R.; Nelson, J. Effect of Multiple Adduct Fullerenes on Charge Generation and Transport in Photovoltaic Blends with Poly(3-Hexylthiophene-2,5-Diyl). J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 45–51. (43)

Faist, M. A.; Kirchartz, T.; Gong, W.; Ashraf, R. S.; McCulloch, I.; de Mello, J. C.;

Ekins-Daukes, N. J.; Bradley, D. D. C.; Nelson, J. Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells. J. Am. Chem. Soc. 2012, 134, 685–692. (44)

Nardes, A. M.; Ferguson, A. J.; Whitaker, J. B.; Larson, B. W.; Larsen, R. E.; Maturová,

K.; Graf, P. A.; Boltalina, O. V.; Strauss, S. H.; Kopidakis, N. Beyond PCBM: Understanding the Photovoltaic Performance of Blends of Indene-C 60 Multiadducts with Poly(3Hexylthiophene). Adv. Funct. Mater. 2012, 22, 4115–4127. (45)

Miller, N. C.; Cho, E.; Gysel, R.; Risko, C.; Coropceanu, V.; Miller, C. E.; Sweetnam, S.;

Sellinger, A.; Heeney, M.; McCulloch, I.; et al. Factors Governing Intercalation of Fullerenes


26

Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

and Other Small Molecules Between the Side Chains of Semiconducting Polymers Used in Solar Cells. Adv. Energy Mater. 2012, 2, 1208–1217. (46)

Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M. Space-Charge Limited Photocurrent.

Phys. Rev. Lett. 2005, 94, 126602. (47)

Miller, N. C.; Sweetnam, S.; Hoke, E. T.; Gysel, R.; Miller, C. E.; Bartelt, J. A.; Xie, X.;

Toney, M. F.; McGehee, M. D. Molecular Packing and Solar Cell Performance in Blends of Polymers with a Bisadduct Fullerene. Nano Lett. 2012, 12, 1566–1570. (48)

Kang, H.; Cho, C.-H.; Cho, H.-H.; Kang, T. E.; Kim, H. J.; Kim, K.-H.; Yoon, S. C.;

Kim, B. J. Controlling Number of Indene Solubilizing Groups in Multiadduct Fullerenes for Tuning Optoelectronic Properties and Open-Circuit Voltage in Organic Solar Cells. ACS Appl. Mater. Interfaces 2012, 4, 110–116. (49)

Song, Y.; Chang, S.; Gradecak, S.; Kong, J. Visibly-Transparent Organic Solar Cells on

Flexible Substrates with All-Graphene Electrodes. Adv. Energy Mater. 2016, 6, 1600847. (50)

Faist, M. A.; Shoaee, S.; Tuladhar, S.; Dibb, G. F. A.; Foster, S.; Gong, W.; Kirchartz, T.;

Bradley, D. D. C.; Durrant, J. R.; Nelson, J. Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors. Adv. Energy Mater. 2013, 3, 744– 752. (51)

Street, R. A.; Krakaris, A.; Cowan, S. R. Recombination Through Different Types of

Localized States in Organic Solar Cells. Adv. Funct. Mater. 2012, 22, 4608–4619.


27

ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(52)

Page 28 of 28

Parmer, J. E.; Mayer, A. C.; Hardin, B. E.; Scully, S. R.; McGehee, M. D.; Heeney, M.;

McCulloch, I. Organic Bulk Heterojunction Solar Cells Using Poly(2,5-Bis(3Tetradecyllthiophen-2-Yl)Thieno[3,2,-b]Thiophene). Appl. Phys. Lett. 2008, 92, 113309. (53)

Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. Influence of Polymer

Compatibility on the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 9913–9919. (54)

Mollinger, S. A.; Vandewal, K.; Salleo, A. Microstructural and Electronic Origins of

Open-Circuit Voltage Tuning in Organic Solar Cells Based on Ternary Blends. Adv. Energy Mater. 2015, 5, 1501335. (55)

Kang, H.; Kim, K.-H.; Kang, T. E.; Cho, C.-H.; Park, S.; Yoon, S. C.; Kim, B. J. Effect

of Fullerene Tris-Adducts on the Photovoltaic Performance of P3HT:Fullerene Ternary Blends. ACS Appl. Mater. Interfaces 2013, 5, 4401–4408.


28

Formation of Organic Alloys in Ternary-Blend Solar Cells with Two

Recommend Documents