Conformational Disorder Enhances Solubility and Photovoltaic Performance of a Thiophene–Quinoxaline Copolymer

Wang, Ergang; Bergqvist, Jonas; Vandewal, Koen; Ma, Zaifei; Hou, Lintao; Lundin, Angelica; Himmelberger, Scott; Salleo, Alberto; Müller, Christian; Inganäs, Olle; Zhang, Fengling; Andersson, Mats R.

doi:10.1002/aenm.201201019

Cited by 92 publications

(102 citation statements)

References 37 publications

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“…This is in good agreement with the area per repeat unit (≈36 Å 2 ) considering a π-π distance of about 4 Å and a chain backbone repeat distance of 9 A. [ 17 ] In Figure 2 b, AFM analysis, performed in defective regions of the fi lm, reveals a Langmuir close-packed monolayer with out-of-plane molecular order and a fi lm thickness of about 2 nm. The surface pressure versus Mma for rr-P3HT is reported in Figure 2 c. The limiting molecular area is of about 12 Å 2 , which is comparable with previous values reported for a close packed rr-P3HT monolayer, [ 18 ] assuming a main polymer chain parallel to the air/water interface with thiophene rings standing edge-on.…”

Section: Langmuir-shäfer Filmsupporting

confidence: 82%

Energy Level Bending in Ultrathin Polymer Layers Obtained through Langmuir–Shäfer Deposition

Bao

Fabiano

Andersson

et al. 2015

Adv Funct Materials

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polycrystalline and defects/variations in the local order of OSC fi lm create a distribution of ionization energies. [ 1 ] Every molecule/polymer in a fi lm will thus have its own individual ionization potential (IP) and electron affi nity (EA), with the fi lm IP and EA then represented by the smallest/ largest individual IP/EA. The frontier parts of the resulting occupied and unoccupied state distributions forming the energy gap are often modeled as being either Gaussian or exponential, [ 2 ] and the most easily oxidized/reduced states in an OSC fi lm are typically referred to as tail states or gap states. [ 3 ] Due to processing conditions relevant to printed electronics, the fi lms typically are physisorbed on substrates, forming weakly interacting interfaces. The energy level alignment at interfaces [3][4][5][6][7] and over heterolayer stacks [ 8 ] featuring OSC is an intensively studied topic and the integer charge transfer (ICT) model [ 4,5,9 ] was developed to describe the energy level alignment for the particular case of weakly-interacting metal/OSC and OSC/OSC interfaces obtained from physisorbed fi lms. Figure 1 a shows the typical energy level alignment behavior for OSC weakly interacting interfaces that follow the ICT model, where the resulting work function (WF) is either substrate independent (i, iii) or linearly dependent with a slope of ≈1 (ii). The Fermi level pinning is a result of spontaneous charge transfer across the interface when the substrate work function is larger (smaller) than the energy required to oxidize (gained from reducing) a molecule/polymer at an interface forming an integer charge transfer state (due to interaction with the transferred charge/image charge on the substrate). The most easily oxidized (or reduced) molecules/polymers adjacent to the interface are "used up" in this process until enough charge has been transferred across the interface to create a potential step that equilibrates the Fermi level, with the resulting pinning energies being referred to as the E ICT+,− (Figure 1 a). The integer charge transfer state (ICT+,−) energies depend on the inter and intramolecular order, [ 5 ] so that at a given interface, there hence will be a distribution of ICT+ and ICT− energies, see Figure 1 b. The ICT model enables energy level offsets at heterojunctions to be predicted, but the extent of the resulting energy level bending (ELB) region is fi ercely debated with a variety of results ranging from abrupt space charge regions with no ELB extending beyond the fi rst monolayer at an interface, to virtually no potential drop at the interface and ELB regions extending several tens Energy Level Bending in Ultrathin Polymer Layers Obtained through Langmuir-Shäfer DepositionQinye Bao , Simone Fabiano , Mattias Andersson , Slawomir Braun , Zhengyi Sun , Xavier Crispin , Magnus Berggren , Xianjie Liu , and Mats Fahlman * The semiconductor-electrode interface impacts the function and the performance of (opto)electronic devices. For printed organic electronics the electrode surface...

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Section: Langmuir-shäfer Filmsupporting

confidence: 82%

Energy Level Bending in Ultrathin Polymer Layers Obtained through Langmuir–Shäfer Deposition

Bao

Fabiano

Andersson

et al. 2015

Adv Funct Materials

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“…In order to study the connection between the donor polymer composition and Voc different ternary BHJ OPV were fabricated by mixing the acceptor PC71BM with pairs of the donor polymers TQp6, TQm6 and TQm12 in compositional steps of 10%. The polymers were synthesized as indicated in Reference 13 and their molecular structures and full names can be found in Figure S1 in the Supporting Information (SI). These polymers were chosen for their similar structure leading to a good miscibility.…”

Section: Open Circuit Voltage Of Ternary Blendsmentioning

confidence: 99%

Open circuit voltage and efficiency in ternary organic photovoltaic blends

Felekidis

Wang

Kemerink

2016

Energy Environ. Sci.

104

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Organic bulk heterojunction solar cells based on ternary blends of two donor absorbers and one acceptor are investigated by experiments and modeling. The commonly observed continuous tunability of the open circuit voltage with donor1:donor2 ratio can quantitatively be explained as quasi-Fermi level splitting due to photocreated charges filling a joint density of states that is broadened by Gaussian disorder. On this basis, a predictive model for the power conversion efficiency that accounts for the composition-dependent absorption and the shape of the current-voltage characteristic is developed. When all other parameters, most notably the fill factor, are constant, we find that for state-of-the-art absorbers, having a broad and strong absorption spectrum, ternary blends offer no advantage over binary ones. For absorbers with a more narrow absorption spectrum ternary blends of donors with complementary absorption spectra, offer modest improvements over binary ones. In contrast, when, upon blending, transport and/or recombination kinetics are improved, leading to an increased fill factor, ternaries may offer significant advantages over binaries. Broader contextSolar cells based on blends of organic semiconductors almost routinely reach power conversion efficiencies (PCE) in the 10% range. One of the strategies that has been proposed to push this limit further upward is the use of so-called ternary blends in which two instead of one absorbing donor material is blended with the electron acceptor. Systematic improvement of PCE by ternary blending has been reported. Despite the experimental success, the current understanding of both the PCE enhancement and the underlying puzzling tunability of the open circuit voltage Voc is limited and essentially qualitative. In this contribution we develop and validate a quantitative and predictive framework for both the Voc and PCE of ternary organic solar cells, which allows us to propose design rules. In contrast to current wisdom, we demonstrate that PCE enhancement in ternary blends hardly results from optimizing the overlap with the solar spectrum but predominantly from synergetic improvement of the charge transport and recombination kinetics. The model can be easily extended to include more detail and will allow other researchers to upfront evaluate the photovoltaic potential of 'any' ternary material combination. 10 On the same basis Savoie et al. proposed an empirical expression for Voc as a current-weighted average of the two sub-systems and used this to estimate a maximal theoretical PCE (of 13%) for varying donor bandgaps. 11 Apart from lacking a formal basis, there are two problems with the parallel junction picture. First, it relies on a specific morphology in the active layer that allows different regions to act as independent binary cells that are electrically decoupled. Even if a fortunate morphology would lead to the presence of two (hole-) percolating networks that are electrically isolated from each other, the presence of metal contacts does unavoidably enforce...

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“…This planar geometry could be the reason why the absorption onset in APFO3-F8BT is red shifted than APFO15-F8BT. On the other hand, in APFO3-F8BT a favorable π-π intermolecular stacking of the polymer chains can form ordered arrangements and allows for a well-defined low energy absorption band to be observed as compared to the less structured low energy band that is observed in APFO15-F8BT [25]. Besides to the geometry of the backbone of the polymers, the electron accepting capacity of the quinoxaline unit in APFO15-F8BT is lower due to the electron donating alkoxy phenyl rings that are attached as compared to the benzothiadiazole unit in APFO3-F8BT.…”

Section: Resultsmentioning

confidence: 99%

Random polyfluorene <i>co</i>-polymers designed for a better optical absorption coverage of the visible region of the electromagnetic spectrum

Gedefaw¹,

Ma²,

Henriksson³

et al. 2014

Bull. Chem. Soc. Eth.

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ABSTRACT. Two alternating polyfluorenes (APFO15-F8BT and APFO3-F8BT) with full absorption of the visible region of the electromagnetic radiation were designed and synthesized for bulk-heterojunction solar cell devices. The optical and electrochemical properties of the two polymers were studied. The two polymers exhibited strong absorption in the visible region with no significant valley over the visible region extending up to 650 nm. Deep HOMO and ideally situated LUMO energy levels were the characteristics of the two polymers as revealed from the square wave voltammogram study: desired properties for extracting high open circuit voltage and for a facile charge transfer to the acceptor component in devices to take place, respectively. Photovoltaic devices were fabricated by blending the two polymers with PCBM[70] and up to ~2% power conversion efficiency were obtained.

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Conformational Disorder Enhances Solubility and Photovoltaic Performance of a Thiophene–Quinoxaline Copolymer

Cited by 92 publications

References 37 publications

Energy Level Bending in Ultrathin Polymer Layers Obtained through Langmuir–Shäfer Deposition

Energy Level Bending in Ultrathin Polymer Layers Obtained through Langmuir–Shäfer Deposition

Open circuit voltage and efficiency in ternary organic photovoltaic blends

Random polyfluorene <i>co</i>-polymers designed for a better optical absorption coverage of the visible region of the electromagnetic spectrum

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