Photophysics of nanomaterials for opto-electronic applications
Kahmann, Simon
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5 Working Mechanism of Ternary
Organic Solar Cells
In this chapter, a ternary organic blend comprising the electron acceptor PC70B M , blended
toget-her with the two donor polymers PDCBT and PTB7-th, is investigated. This mixture allows for the fabrication of well-performing solar cells with a power conversion efficiency exceeding 10%. This is due to an increased coverage of the solar spectrum, as well as a high fill factor and trap-free charge carrier transport. The position of the energy levels of the neat materials suggests a cascade transport, wherein formed holes ultimately end up on PDCBT, before being extracted at the cathode. Spectroscopic studies, in contrast, reveal that holes reside within the PTB7-th phase. The two polymers are furthermore shown to intricately mix and interact through a highly efficient energy transfer from the wider band gap PDCBT towards PTB7-th. The highly crystalline PDCBT therefore acts as an antenna for the PTB7-th:PC70BM sub-cell, in which the charge carrier
genera-tion and conducgenera-tion predominantly take place.
5.1 Introduction
Increasing the power conversion efficiency of organic solar cells is of paramount importance for their future commercialisation. While absorption layers in these devices typically comprise two materials, an electron acceptor (A) and a -donor (D), adding a third component (either A or D) can improve the performance of such a ternary device whilst maintaining the simple
architec-ture of a single junction solar cell.[1]Most commonly, the limited coverage of the solar spectrum
is improved by adding a material with complementary (often near infrared) absorption, to
in-crease the extractable current.[2] Although binary single junction devices have been shown to
reach a high power conversion efficiency exceeding 10%, these devices often involve complex materials which are increasingly difficult and expensive to synthesise or additional laborious
treatments.[3–6]A major advantage of the ternary concept is thus to allow simpler and cheaper
materials to be employed instead and thereby reduce the final cost of fabricated devices.
N. Gasparini, S. Kahmann, M. Salvador, J. Dario Pereira, N. Li, S. Rechberger, E. Spiecker, G. Portale, M. A. Loi, C. J. Brabec, T. Ameri, in preparation.
Solar cells were fabricated and characterised by Nicola Gasparini.
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proved fill factor due to a better charge carrier transport.
Crucially, though, ternary blends rarely outcompete their binary sub-cells, since especially the more complex morphology often hampers the efficient transport of charge carriers or, in other cases, benefits in light harvesting are countered by a reduction in cell voltage.
Depending on the relative position of the HOMO/LUMO levels of the involved materials and their relative amounts, the working mechanisms of ternary devices are conceptually described through a variety of models. For the most common case of two donor and one acceptor material, the ternary device can work as two intermixed binary solar cells (parallel-like), wherein electrons are transported through the acceptor phase and holes through either of the donor phases – de-pending on the material that absorbed the respective photon. In cascade-like devices, on the other hand, the final hole transport always occurs via the donor with the higher lying HOMO le-vel.[1,9]Holes formed on the second donor material will transfer towards the higher HOMO prior to their extraction. If one of the donor materials efficiently harvests incident photons but does not participate in the actual transport of charge carriers, it can be considered as an antenna,
sensitising a well-working binary cell.[10]
For conjugated polymers of different band gap, the absorption and emission spectra are likely to overlap. As discussed in section 2.2.4, this can enable an efficient Förster transfer of the
exci-tation energy between them.[11,12]Although likely, this process is often overlooked in literature
when the energy levels and transport models of ternary devices are discussed.
While ternary organic solar cells have led to impressively high performance in selected cases,[13–15]
the overall yield for an improved efficiency is rather poor. It is thus of utmost importance to pro-perly understand the working mechanism of those few examples that do display a better perfor-mance than their binary counterparts, in order to establish reproducible guidelines for material engineering. Optical spectroscopy is an especially powerful tool to elucidate the intricate inte-raction of the involved materials and to investigate transfer processes between them.
5.2 Results and Discussion
In this study, a ternary blend involving two conjugated polymers as donor materials and one
fullerene derivative as acceptor were investigated. The former include PTB7-th,[16]a D-A
po-lymer with comparatively narrow band gap, and PDCBT, a polythiophene derivative, similar to
P3HT,[17] but with a different side-chain (consider Figure 5.1 (a) for their molecular structure
and energy levels). The simple structure and thereby cheap synthesis makes the latter an espe-cially attractive candidate for large scale manufacturing, as it will reduce the final price of the devices.The energy levels of the materials, as shown in Figure 5.1 (a), form a cascade, in which
P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
extracted from the J-V characterisation under 1.5 sun illumination
Device VOC/ V JSC/ mA cm−2 F F / % PCE
PDCBT:PC70BM 0.85 11.1 66 6.23
PTB7-th:PC70BM 0.78 15.9 69 8.54
Ternary 0.78 17.5 74 10.12
electrons from PDCBT can either be directly transferred towards PC70BM or with an
interme-diate step via PTB7-th. Similarly, holes from the fullerene can be transferred onto PTB7-th and successively onto PDCBT or directly from the fullerene to the polythiophene.
In blends with the fullerene derivative PC70BM, binary solar cells involving either PTB7-th or
PD-CBT exhibit a PCE of up to 8.5% and 6.2% respectively. Representative J-V-curves are depicted in
Figure 5.1 (b), showing an increased short circuit current JSCfor the ternary device (by 37% and
9% with respect to the PDCBT- and PTB7-th binary cell), giving rise to an improved PCE of up to 10.1%. This increased current density is explained when considering the EQE spectra depicted in (c). The ternary device exhibits a significant contribution at low energy (similar to the PTB7-th binary, but below the onset of the PDCBT-based device), whilst also showing an increased quan-tum efficiency for energy larger than 2 eV.
Interestingly, the ternary device maintains the open circuit voltage (VOC) of 0.78 V,
correspon-ding to the value of the PTB7-th binary cell, instead of an intermediate value.[18] This high
voltage and an improved fill factor (FF) of 74% (as opposed to 66 and 69% of the PDCBT and PTB7-th binary device) furthermore allow for the good performance of the ternary device.
In-vestigating the light intensity dependence of the VOC furthermore reveals the ternary (and the
PTB7-th binary) to exhibit ideal second-order recombination, whereas the PDCBT:PC70BM cell
shows a significant contribution of trap mediated recombination (Figure A1 (a)). Additionally, the charge carrier mobility extracted via the charge extraction by linearly increasing the voltage (CELIV) technique yields the highest value for the ternary device (Figure A1 (b)).
The absorption spectra of relevant films are depicted in Figure 5.1 (d). The two polymers display distinct and complementary absorption regions between 1.6 and 2.0 eV for the narrow band gap
PTB7-th and around 2.4 eV for PDCBT. In neat films and when blended with PC70BM, the
poly-mers also exhibit vibrational sidebands. Notably, the position and shape of these bands differ significantly for the polymer blend and the ternary film. While, for example, neat PTB7-th exhi-bits two peaks at 1.9 and 1.77 eV, the polymer blend only exhiexhi-bits a red-shifted peak at 1.72 eV. Moreover, a broad absorption band from 2 to 2.3 eV characterises its shape, which corresponds to a region in which neat PDCBT merely exhibits a shoulder. In contrast, no contribution from the main peak of neat PDCBT at 2.4 eV can be found. This suggests a profound impact on the
polymer microstructure when mixing them together.[19]
In order to characterise the excited state interaction, steady-state and ultrafast time resolved
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(c) PDCBT PTB7-th PC70BM a) b) c) d) 0.0 0.2 0.4 0.6 0.8 1.0 -20 -15 -10 -5 0 5 PDCBT:PC70BM 1:1 PDCBT:PTB7-th:PC70BM 0.5:0.5:1.5 PTB7-th:PC 70BM 1:1.5 Curre nt density (mA cm -2) Voltage (V) 400 500 600 700 800 0 10 20 30 40 50 60 70 80 90 PDCBT:PC70BM 1:1 PDCBT:PTB7-th:PC70BM 0.5:0.5:1.5 PTB7-th:PC70BM 1:1.5 EQE (%) Wavelength (nm) 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Norm. Absorbance Wavelength (nm) PDCBT:PC70BM PDCBT:PTB7-th:PC70BM PTB7-th:PC70BM PDCBT PTB7-th PC70BM a) b) c) d) 0.0 0.2 0.4 0.6 0.8 1.0 -20 -15 -10 -5 0 5 PDCBT:PC70BM 1:1 PDCBT:PTB7-th:PC70BM 0.5:0.5:1.5 PTB7-th:PC70BM 1:1.5 Curre nt density (mA cm -2) Voltage (V) 400 500 600 700 800 0 10 20 30 40 50 60 70 80 90 PDCBT:PC70BM 1:1 PDCBT:PTB7-th:PC70BM 0.5:0.5:1.5 PTB7-th:PC70BM 1:1.5 EQE (%) Wavelength (nm) 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Norm. Absorbance Wavelength (nm) PDCBT:PC70BM PDCBT:PTB7-th:PC70BM PTB7-th:PC70BM PDCBT PTB7-th PC70BM a) b) c) d) 0.0 0.2 0.4 0.6 0.8 1.0 -20 -15 -10 -5 0 5 PDCBT:PC70BM 1:1 PDCBT:PTB7-th:PC70BM 0.5:0.5:1.5 PTB7-th:PC70BM 1:1.5 Curre nt density (mA cm -2) Voltage (V) 400 500 600 700 800 0 10 20 30 40 50 60 70 80 90 PDCBT:PC70BM 1:1 PDCBT:PTB7-th:PC70BM 0.5:0.5:1.5 PTB7-th:PC70BM 1:1.5 EQE (%) Wavelength (nm) 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Norm. Absorbance Wavelength (nm) PDCBT:PC70BM PDCBT:PTB7-th:PC70BM PTB7-th:PC70BM Ener gy PDCBT PTB7-th PC70BM (d)
Figure 5.1: Molecular structure and relative position of the energy levels of the involved materials (a).
Ter-nary solar cells exhibit an improved PCE due to a higher JSCand F F compared with the binary cells (b).
The increased current of this device can be tracked back to a broadened and improved EQE as shown in (c). Absorption spectra of the respective films (d) display significant changes in peak position when blending the two polymers together.
photoluminescence measurements were performed on neat, binary and ternary films. The neat materials’s PL, shown in Figure 5.2 (a) (red, orange and grey), displays a strong peak (1.6 for
PTB7-th, 1.77 for PC70BM and 1.85 eV for PDCBT) with a shoulder at lower energy in all three
ca-ses. In the ternary blend (blue), only PC70BM and PTB7-th still emit. The emission from PC70BM
stems from agglomerates formed due to the addition of the selective solvent 1,8-diiodooctane
(consider methods).[20]The PTB7-th emission changes from a sharp peak around 1.6 eV and a
shoulder at 1.5 eV, found in the neat film, to a broader, featureless emission with a maximum at 1.55 eV, again indicating morphological changes upon blending.
In order to understand the mechanisms in the ternary film, the different binary sub-systems
were examined separately. For the PTB7-th:PC70BM blend (green), the PL spectrum displays a
superposition of the two neat films, showing that the PTB7-th morphology is hardly affected by
PC70BM. The PL decay of neat PTB7-th, can be fitted bi-exponentially, offering a lifetime of 79
and 426 ps (consider Table 5.2). Both components are reduced dramatically to 3 and 52 ps, when blended with the electron acceptor, signifying an efficient electron transfer from the polymer
to-P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
Figure 5.2: Steady state (a) and time resolved PL spectra (b) of the neat and blended materials. The steady-state spectra reveal distinct peaks for all three neat materials. When blended together with PTB7-th or in the ternary film, emission from PDCBT cannot be observed. The decay traces displayed in (b) show a strong
reduction of the PL lifetime for both polymers when blended with PC70BM.
Table 5.2: Extracted lifetimes for the mono- or bi-exponential fits to the transient photoluminescence of organic films Sample Emission τ1/ps τ2/ ps PC70BM PC70BM 689 -PDCBT PDCBT 31 1029 PDCBT:PC70BM PDCBT 22 96 PTB7-th PTB7-th 79 426 PTB7-th:PC70BM PTB7-th 3 52 PTB7-th:PDCBT PTB7-th 345 -Ternary PTB7-th 4 229
wards the fullerene. This observation corresponds to the well performing binary solar cell. The extracted lifetime is similarly found for the ternary blend (Table 5.2).
For PDCBT:PC70BM, one finds a varying emission spectrum that strongly depends on the
exci-ted sample area (consider Figure A2), suggesting a generally heterogeneous film. Nevertheless, in accordance with the efficient solar cell, also the PL lifetime of PDCBT is significantly reduced (dark blue in Figure 5.2 (b)) due the efficient electron transfer.
Having established that both polymer:fullerene blends work efficiently, the interplay of the two polymers was examined in the following. The most efficient ternary blend in solar cells requires a polymer ratio of 1:1. Notably, steady-state spectra are homogeneous over the entire sample area, indicating a fine intermixing. Strikingly, and in accordance with the ternary blend, the only detectable emission stems from PTB7-th. This observation could be explained by a highly asymmetric charge transfer, i.e. a rapid electron transfer from PDCBT towards PTB7-th, and
a hampered hole transfer in opposite direction. Since, however, PC70BM generally acts as an
efficient electron acceptor, but does not quench the PDCBT emission nearly as completely as
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transferred to PTB7-th.
The broad overlap between the emission of PDCBT and the absorption of PTB7-th (as shown in Figure 5.3 (a)), allows exactly for such an efficient transfer via the Förster mechanism (consider section 2.2.4). To investigate this behaviour, the PL was investigated for samples with a varied concentration of the supposed acceptor (PTB7-th, from 1% to 20%). The steady-state spectra in Figure 5.3 (b) display that already for 1% PTB7-th in a matrix of PDCBT, there is hardly any emis-sion from the latter left. A small peak around 1.9 eV is still visible in that region, which, upon further increasing the concentration over 2.5 to 5%, becomes weaker and shifts towards hig-her energy. For a PTB7-th concentration exceeding 5%, virtually no emission from PDCBT can be seen. Simultaneously, the emission spectrum associated with PTB7-th changes significantly upon varying its concentration. For 1%, the PL maximum is located at 1.66 eV and is relatively narrow. Increasing the concentration first shifts the peak towards 1.65 eV and a shoulder emer-ges at 1.55 eV. At higher concentration, the latter region dominates the emission profile of PTB7-th, but the band is significantly broader and less structured than in its neat film. In accordance with above discussed aspects for the absorption spectra and the PL of the ternary device, this behaviour points to the strong effect on the morphology of PTB7-th, when blended with PDCBT. Additional morphological characterisation using transmission electron microscopy and X-ray scattering support this proposition (consider original publication). In particular, such distinct emission regions were associated with isolated single chains for high energy PL and aggregates
in case of lower energy emission.[21,22]This underlines the assumption of a good intermixture of
the two polymers for small amounts of PTB7-th. Upon increasing its concentration, the materi-als phase separate and PTB7-th forms agglomerates.
The corresponding time resolved PL of PDCBT and PTB7-th are depicted in (c) and (d). As ex-pected, a strong lifetime reduction for the former is already observable at 1% concentration of PTB7-th. For amounts as high as 10%, the PL of PDCBT decays almost completely within the first
100 ps. Notably, this reduction is significantly more efficient than for the blend with PC70BM.
To quantify the efficiency of the PL quenching, the average lifetimeτav =PiAiτi of the
bi-exponential decay was determined for each case and presented in Table 5.3. The efficiencyη
was calculated asη = 1 − τD A/τD, whereτD Ais the average lifetime for the respective blend and
τD the average lifetime of the neat donor PDCBT.[23]For a PTB7-th concentration of 10%, the
energy transfer already occurs with an efficiency exceeding 98%.
For PTB7-th, the lifetime is largest for small concentrations (Figure 5.3 (d)). More important, though, is the early behaviour of the emission. The maximum intensity for 1% PTB7-th is only reached after approximately 190 ps. This delayed maximum is a clear sign for an energy transfer from PDCBT. For up to 5% of the acceptor, such a rise-time in emission intensity is observed and attributed to diffusion limited transfer of excitation energy. For larger concentration of PTB7-th, the acceptor sites are ubiquitous in the film and the transfer occurs quasi-instantaneously,
wit-P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
(d) (c)
Figure 5.3: Overlap of the emission of PDCBT and absorption of PTB7-th (a), allowing for a Förster transfer. Concentration variation shows that already small amounts of PTB7-th strongly quench the emission of the PDCBT matrix. Time resolved spectra of PDCBT (c) and PTB7-th (d) display a reduced lifetime for PDCBT-and a delayed maximum of the PTB7-th emission at low concentration, indicating an energy transfer.
hout undergoing a pronounced diffusion regime.
The degree of the PL quenching of PDCBT and prolonged emission of PTB7-th can be attributed to both a fine intermixing of the two components, and the long PL lifetime of neat PDCBT, allo-wing excitons in this material to travel long distances before encountering PTB7-th acceptors. The combined PL investigations thus show that both polymers work as donor-acceptor couples,
when blended with PC70BM. Blending the polymers together enables a rapid energy transfer
to-wards the narrow band gap PTB7-th. These discoveries, however, do not reveal whether only one of the polymers – and if so, which – acts as the hole transport phase in ternary solar cells. It could, for example, be argued that subsequent to the energy transfer towards PTB7-th and
ex-citon dissociation at the interface with PC70BM, holes are transferred back towards PDCBT to
become extracted.
In order to determine the location of formed charge carriers, steady-state photoinduced absorp-tion spectroscopy was carried out both in the MIR and in the NIR spectral region. Data for the
former are shown in Figure 5.4 (a). As outlined in Chapter 4, the position of the P1polaron
tran-W OR KING M E CH AN IS M OF T E R N AR Y OR GA N IC
The average lifetime is calculated asτav=PiAiτi Sample A1 τ1/ ps A2 τ2/ ps τav η / % PDCBT 0.10 55 0.90 1026 929 -1% 0.31 53 0.69 256 193 79.2 2.5% 0.63 38 0.37 124 70 92.5 5% 0.83 29 0.17 110 43 95.4 10% 0.87 12 0.13 42 16 98.3 (b) (a)
Figure 5.4: Steady state photoinduced absorption spectra in the MIR (a) and NIR (b) spectral region. In both cases, the ternary blend exhibits distinct markers associated with the PTB7-th polaron. No signatures from PDCBT can be found for the ternary film.
sition and especially the IRAVs can be used as a fingerprint of the charge carrying material.
The-refore, the spectra of the binary blends (polymer:PC70BM) are shown along the ternary film.
PTB7-th:PC70BM displays a broad absorption feature between approximately 0.4 down to 0.1 eV,
which is superimposed with narrow IRAVs and two broader dips around 0.16 eV (green). The spectrum was determined to be independent of the excitation energy (2.75, 2.3, 1.95 eV).
PDCBT reveals a similar picture, when blended with PC70BM (black). Comparing the two spectra
with each other, however, one finds distinct features to distinguish the polymers (as indicated by the shaded regions in Figure 5.4) –namely, the less pronounced peak for PDCBT around 0.22 eV, the significantly different slope at 0.13 eV and, most prominently, the peak at 0.14 eV, which in-cludes three narrow peaks for PDCBT and two (wider separated) peaks for PTB7-th.
The spectrum for the ternary blend (blue) only shows evidence for the PTB7-th polaron. There is no indicator of PDCBT and the similarities to the PTB7-th binary blend are remarkable. Espe-cially for the peak at 0.14 eV and the slope around 0.13 eV the spectra are almost identical. This suggests charge carriers to only reside on the narrow band gap polymer.
To corroborate above claims, quasi-steady-state PIA in the NIR was carried out on the same
sam-ples. This allows for probing the spectral region, in which the ground state bleach and P2polaron
transition become visible.
P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
bleaches around 2.05 (PDCBT) and 1.65 eV (PTB7-th) emerge for excitation with 2.3 eV. For the former, one additionally finds a narrow absorption at 1.92 eV and a broader absorption band around 1.3 eV. In contrast, the PTB7-th sample exhibits a pronounced PIA band around 1.0 eV with two separated peaks. Again, as seen for the MIR investigation, the two polymers exhibit features that allow to distinguish their contribution to the spectrum of the ternary sample. The ternary film, eventually, offers an increased photobleach for PTB7-th at 1.65 eV and from 1.9 to 2.0 eV. A bleach for PDCBT cannot be observed, which is in accordance with the ultrafast energy transfer towards PTB7-th, discussed above. Additionally, there are no signs of PIA stem-ming from PDCBT. The broad absorption band around 1 eV, on the contrary, precisely follows
the course of the binary PTB7-th:PC70BM sample. The investigation in the NIR as well as in the
MIR spectral region therefore conclusively show that only the narrow band gap polymer PTB7-th carries PTB7-the hole polaron in PTB7-the ternary blend. AlPTB7-though exhibiting a higher lying HOMO level, PDCBT does not receive significant amounts of charge carriers.
These observations fit the above-mentioned trap free transport of the ternary solar cell, which is
also observed for the PTB7-th binary. Furthermore, the ternary also exhibits the same VOCas the
PTB7-th device, which indicates that the states through which charge carrier transport occurs in these two blends, are the same.
5.3 Conclusion
In conclusion, a highly efficient ternary blend of PDCBT:PTB7-th:PC70BM for solar cell
applica-tions was investigated. Despite the cascade-like alignment of the HOMO/LUMO energy levels, charge carrier transport occurs solely through the fullerene and the narrow band gap PTB7-th, which exhibits a lower lying HOMO than PDCBT. Excitons generated on the latter are rapidly transferred to PTB7-th and the subsequently formed holes do not transfer back.
Using a variety of spectroscopic and morphological investigation tools, it could furthermore be shown that the two polymers intermix well and therefore exhibit a complete energy transfer al-ready at a concentration below 20% of PTB7-th in PDCBT. The favourable microstructure of the polymers is considered the core reason for the improved performance of the ternary device. This investigation shows that not only charge transfer processes need to be taken into account when discussing the mechanisms occurring in ternary blends. The observed rapid energy trans-fer furthermore highlights the possibility of using large quantities of cheap materials as antenna for harvesting sunlight without corrupting the performance of the solar cell.
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Solar cell fabrication: Solar cells were fabricated using an inverted device layout based on in-dium tin oxide (ITO)/ZnO/active layer/MoOx/Ag. In order to achieve the best performance, the amounts of the polymers were varied, while the overall polymer to fullerene ratio was kept con-stant at 1:1.5 by weight. An optimal performance was achieved with a 1:1 polymer ratio. Films
were spin-cast at 2000 rpm in a glove box from solution of 20 mg mL−1concentration. For an
op-timum result, 3 vol-% of 1,8-diiodooctane was added approximately four hours prior to casting the films.
Photoinduced absorption spectroscopy: For MIR investigations, solutions were spin cast on ZnSe substrates and mounted into a cryostat without having been exposed to air. To avoid per-turbations from the pump light, a GaAs filter was placed in front of the detector. Spectra were
acquired with a resolution of 5 cm−1and the measurement cycle was run at least 512 times.
Quasi-steady-state PIA studies in the NIR were performed by exciting the sample with a 532 nm laser, chopped at 141 Hz, and probing with the continuous spectrum of a Xe lamp. The
trans-mitted light is dispersed by a 1200 lines mm−1grating monochromator (iHR320, Horiba) and
detected by a Si detector down to an energy of 1.1 eV and an InGaAs detector for lower values. Additional measurements with a blocked Xe lamp account for the sample PL.
In both cases, measurements occurred at cryogenic temperature.
Photoluminescence Spectroscopy: Films were deposited on quartz substrates, mounted into a nitrogen filled sample holder, and excited at 400 nm using the second harmonic of a mode-locked Ti:sapphire laser at a repetition rate of 76 MHz. Steady-state spectra were recorded with an InGaAs detector from Andor. Time-resolved traces were taken with a Hamamatsu streak ca-mera working in synchroscan mode.
P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
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