Article

Donor Polymer Fluorination Doubles the Efficiency in Non-Fullerene Organic Photovoltaics

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Abstract

Donor polymer fluorination has proven to be an effective method to improve the power conversion efficiency of fullerene-based polymer solar cells (PSCs). However, this fluorine effect has not been well-studied in systems containing new, non-fullerene acceptors (NFAs). Here, we investigate the impact of donor polymer fluorination in NFA-based solar cells by fabricating devices with either a fluorinated conjugated polymer (FTAZ) or its non-fluorinated counterpart (HTAZ) as the donor polymer and a small molecule NFA (ITIC) as the acceptor. We found that, similar to fullerene-based devices, fluorination leads to an increased open circuit voltage (Voc) from the lowered HOMO level and improved fill factor (FF) from the higher charge carrier mobility. More importantly, donor polymer fluorination in this NFA-based system also led to a large increase in short circuit current (Jsc), which stems from the improved charge transport and extraction in the fluorinated device. This study demonstrates that fluorination is also advantageous in NFA-based PSCs and may improve performance to a higher extent than in fullerene-based PSCs. In the context of other recent reports on demonstrating higher photovoltaic device efficiencies with fluorinated materials, fluorination appears to be a valuable strategy in the design and synthesis of future donors and acceptors for PSCs.

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... Given the electron-withdrawing nature of fluorine, it is not surprising that the most common fluorination location is the electrondeficient acceptor moiety. We have previously demonstrated with our FTAZ polymer that the fluorine substituent on the benzotriazole acceptor moiety increased all three major device characteristics: open circuit voltage (V oc ), short circuit current density (J sc ), and fill factor (FF). 10,11 The increase in V oc was due to the deepening of the highest occupied molecular orbital (HOMO) level from the electron-withdrawing strength of the fluorine substituent, and a higher hole mobility allowed for improvements in both J sc and FF. This same trend has been demonstrated by many other polymer systems as well. ...
... 80 We have previously published the morphology of HTAZ with both fullerene and non-fullerene acceptors, and generally amorphous packing is observed with both. 11,73,81 Both the (100) and (010) peaks are broad peaks, thus shorter coherence lengths and a higher degree of disorder compared to dFT-HTAZ. Furthermore, the long period for non-fullerene blends, such as HTAZ:ITIC, shows larger domain spacing values of ∼60 nm. ...
... Furthermore, the long period for non-fullerene blends, such as HTAZ:ITIC, shows larger domain spacing values of ∼60 nm. 11 Additionally, HTAZ blends have generally shown lower relative domain purity compared to the fluorinated alternatives, which can help explain the lower FF values which are seen when comparing HTAZ and dFT-HTAZ blends. ...
Article
While there are numerous approaches to functionalize conjugated polymers for organic solar cells (OSCs), one widely adopted approach is fluorination. Of the many different locations for fluorination, one of the least studied is the conjugated linker which connects the donor and acceptor moieties; further, all existing reports primarily explore monofluorinated thiophene units. Herein, we synthesize and compare two conjugated polymers, HTAZ and dFT-HTAZ, which have different thiophene linkers. In HTAZ, a bare thiophene unit connects the donor and acceptor moieties, while dFT-HTAZ utilizes difluorinated thiophene (dFT) linkers. These polymers serve as the model system to explore the impact of dFT units in OSCs; additionally, this is the first publication to investigate polymers containing dFT units paired with non-fullerene acceptors. Compared to HTAZ, the incorporation of the dFT units maintained the optical properties while lowering the energy levels by ∼0.4 eV, which allowed for a much improved Voc value of ∼1 V. Importantly, when compared with the appropriate non-fullerene acceptor, dFT-HTAZ:ITIC-Th1 blends reached an efficiency of ∼10%, which is nearly 3× that of the nonfluorinated HTAZ. As most OSC polymers have thiophene linkers, using dFT units could serve as a proficient method to increase OSC performance in many polymer systems, especially those that do not have locations for functionalization on the acceptor moiety.
... The inner electrode in Fig. 1, Right was the organic film of the polymer donor, BnDT-FTAZ (31), and the nonfullerene acceptor ITIC (32) as the components of the internal organic solar cell (Fig. 2B). The organic solar cell exploits the highly complementary light absorption (33), favorable morphology (33), and high mobility for the BnDT-FTAZ pair (on the order of 10 −3 ·cm 2 ·V −1 ·s −1 ) (34) with its capability for largearea device fabrication (35). More importantly, as shown in Fig. 2C, the overall light absorption by the Ru(II)polypyridyl dye and the organic cell are highly complementary, enabling a high utilization of the solar input. ...
... The inner electrode in Fig. 1, Right was the organic film of the polymer donor, BnDT-FTAZ (31), and the nonfullerene acceptor ITIC (32) as the components of the internal organic solar cell (Fig. 2B). The organic solar cell exploits the highly complementary light absorption (33), favorable morphology (33), and high mobility for the BnDT-FTAZ pair (on the order of 10 −3 ·cm 2 ·V −1 ·s −1 ) (34) with its capability for largearea device fabrication (35). More importantly, as shown in Fig. 2C, the overall light absorption by the Ru(II)polypyridyl dye and the organic cell are highly complementary, enabling a high utilization of the solar input. ...
Article
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Artificial photosynthesis provides a way to store solar energy in chemical bonds. Achieving water splitting without an applied external potential bias provides the key to artificial photosynthetic devices. We describe here a tandem photoelectrochemical cell design that combines a dye-sensitized photoelectrosynthesis cell (DSPEC) and an organic solar cell (OSC) in a photoanode for water oxidation. When combined with a Pt electrode for H 2 evolution, the electrode becomes part of a combined electrochemical cell for water splitting, 2H 2 O → O 2 + 2H 2 , by increasing the voltage of the photoanode sufficiently to drive bias-free reduction of H ⁺ to H 2 . The combined electrode gave a 1.5% solar conversion efficiency for water splitting with no external applied bias, providing a mimic for the tandem cell configuration of PSII in natural photosynthesis. The electrode provided sustained water splitting in the molecular photoelectrode with sustained photocurrent densities of 1.24 mA/cm ² for 1 h under 1-sun illumination with no applied bias.
... 11 Among various electron withdrawing functionalities reported, we have chosen small sized, s-electron withdrawing, fluorine (F and CF 3 ) units as end-capped substitutions. Various experimental studies have revealed that fluorination is one of the unique approaches in several ways: (1) it attains a high oxidative stability by energy level lowering; 12,13 (2) it converts the p-type materials into n-type/ambipolar materials; 14 (3) it shifts the energy levels through various intermolecular interactions, such as C-HÁ Á ÁF, C-FÁ Á ÁF, and C-FÁ Á Áp contacts upon solid state packing or thin film formation; 15 (4) efficient contacts with PEDOT:PSS or the electrode surface, which can reduce the resistance resulting in charge transport facilitation; 16,17 (5) increasing the crystalline nature of the materials, etc. 9,10,18 Much effort has been devoted to the fluorination on the electron donor and acceptor materials for organic solar cell applications. 18 In particular, recently, J. Lee and co-workers have developed the isomorphic fluorobenzo[c] [1,2,5] thiadiazole-based polymers with an ultrahigh mobility of 17.8 cm 2 V À1 s À1 , revealing the benefits of fluorination. ...
... Various experimental studies have revealed that fluorination is one of the unique approaches in several ways: (1) it attains a high oxidative stability by energy level lowering; 12,13 (2) it converts the p-type materials into n-type/ambipolar materials; 14 (3) it shifts the energy levels through various intermolecular interactions, such as C-HÁ Á ÁF, C-FÁ Á ÁF, and C-FÁ Á Áp contacts upon solid state packing or thin film formation; 15 (4) efficient contacts with PEDOT:PSS or the electrode surface, which can reduce the resistance resulting in charge transport facilitation; 16,17 (5) increasing the crystalline nature of the materials, etc. 9,10,18 Much effort has been devoted to the fluorination on the electron donor and acceptor materials for organic solar cell applications. 18 In particular, recently, J. Lee and co-workers have developed the isomorphic fluorobenzo[c] [1,2,5] thiadiazole-based polymers with an ultrahigh mobility of 17.8 cm 2 V À1 s À1 , revealing the benefits of fluorination. 2 Combining these two individual inherent electron mobility building blocks (PzDP and fluorine units at the central/end-capped positions, respectively) into a single p-conjugated skeleton can accelerate the electron mobility to a higher extent. ...
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Three different fluorine (4-CF3, 3, 5-CF3 and 3, 4, 5-F) end-capped small molecular non-fullerene acceptor molecules (1CFPzDP, 2CFPzDP, and 3FPzDP) based on A2-π-A1-π-A2 configuration containing a common core acceptor as...
... 31 In this regard, understanding the relationships between the molecular structures of non-fullerene acceptors and their blend morphology, as well as photovoltaic performances is highly necessary, so as to nd out the guidance for the future molecular design of acceptors with a better morphology and performances. [32][33][34][35] At present, the molecular optimization of nonfullerene acceptors mainly focuses on the tuning of energy levels and absorption, [36][37][38][39] and less attention has been paid to the molecular packing effects on the blend morphology and device performances. 40 Recently, we reported a new type of unfused-core-based non-fullerene acceptor with the structure of acceptor-donor-core-donor-acceptor (A-D-C-D-A). ...
Article
Although the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) based on non-fullerene accceptors have been increasing rapidly within couple of years, little is known about the correlations between molecular structures and blend morphologies. In this work, we design and synthesize three acceptor-donor-core-donor-acceptor (A-D-C-D-A) type non-fullerene acceptors, HF-PCIC, HFO-PCIC and OF-PCIC, which possess the same electron-donating parts (D) and electron-accepting terminals (A), but different benzene-based cores (C). We observe that such minor chemical variations can lead to distinct differences in their photovoltaic properties. The resulting PSCs based on HF-PCIC with 2,5-difluorobenzene core yield a good PCE of 10.97%, which is higher than those of HFO-PCIC and OF-PCIC based PSCs (8.36% and 9.09%). If the processing solvent is changed from chlorobezene to chloroform, a further improved PCE of 11.49% is obtained for HF-PCIC-based PSCs due to the formation of finer phase separation domains. This is the highest value among the PSCs with non-fullerene acceptors possessing unfused-cores. Through a series of characterizations, we disclose that the diverse benzene-based cores influence the molecular geometries of three non-fullerene acceptors, resulting in the varied molecular packing modes and film morphologies. The results suggest that the tuning of non-fullerene acceptors’ geometries is an effective method to optimize the film morphology and thus the photovoltaic properties. The unfused-core in A-D-C-D-A small molecules provides a good building block to contruct high-performance non-fullerene acceptors.
... The HOMO levels of P Zn −2FTPA, P Zn −3FTPA, and P Zn −TPA, determined from the first oxidation onset of the CV curves, were −5.37 (E ox : −0.69 eV), −5.24 (E ox : 0.56 eV), and 5.14 eV (E ox : 0.46 eV), respectively ( Figure 1b). The downshift of the HOMO level by fluorination, which has often been reported in organic semiconductors, 28,29 was in the order of P Zn −2FTPA > P Zn −3FTPA > P Zn −TPA. This indicates that it was influenced more effectively by fluorination adjacent to the P Zn core. ...
... eV) match well with ITBC, and its strong absorption from 400 to 600 nm is complementary with the UV−vis spectrum of ITBC. 24,42,46 The film UV−vis absorption of neat FTAZ and ITBC and their blend film (1:1 ratio) are shown in Figure 4a. The blend film exhibited a broad absorption spectrum from 300 nm in the UV region to 800 nm in the near IR region. ...
... Bulk heterojunction (BHJ) organic solar cells (OSCs), with unique advantages of low-cost, flexibility, and capability of roll-to-roll highspeed printing technique, have attracted considerable attention recently [1][2][3][4][5]. With extensive efforts devoted to the innovations in photovoltaic materials and devices, great advances have been made for OSCs, especially in the case of the successful introduction of nonfullerene acceptors over the last few years [6][7][8][9][10][11][12][13][14][15][16], and power conversion efficiencies (PCE) for OSCs over 16% have been reported by several research groups [17][18][19][20][21][22][23]. Over the years, lots of methods have been explored to further improve the performance of OSCs, including terpolymers [24][25][26][27][28][29][30], ternary blends [31][32][33][34][35][36][37], and optimizations of device fabricating process [38][39][40][41]. ...
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In this wok, a D1-A-D2-A random terpolymer donor PFBT4T-BDT10 was synthesized for application in organic solar cells (OSCs). With fullerene derivative PC71BM as the acceptor, the PFBT4T-BDT10:PC71BM binary active layer delivered a very low short-circuit current (Jsc) of 5.87 mA cm⁻² and a low fill factor (FF) of 53.8%, leading to a poor efficiency of 2.40%. With a small fused ring electron acceptor IDIC as the third component, the PFBT4T-BDT10:IDIC:PC71BM ternary active layers displayed a largely elevated efficiency up to 10.17%, corresponding to significantly elevated Jsc of 21.12 mA cm⁻² and FF of 63.9%. The favorable morphology and the broader absorption are the main reasons for the elevation of Jsc. It was found that IDIC could act as a morphology regulator in the ternary blends, not only restricting the aggregation of PC71BM but also maintaining the crystallization of PFBT4T-BDT10. Our work highlights the importance of ternary blend strategy for random terpolymers.
... The sp 3 hybridized nitrogen atoms in benzotriazole units can introduce alkyl side chains to increase solubility without affecting the planarity of the main chain [15][16][17][18]. A series of conjugated polymers with fluoro-substituted benzotriazole and benzotriazole as receptor units have been synthesized for PSC [19][20][21][22][23][24].Based on the above properties, we have designed and synthesized two kinds of D-A conjugated polymers BDT-TTBTA and BDT-TTFBTA based on benzotriazole using BDT as donor unit (Scheme 1). By introducing the F atom in the benzotriazole monomer to explore the effect of F atom on the photovoltaic properties of benzotriazole polymer. ...
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Two kinds of wide bandgap conjugated polymers, named as BDT-TTBTA and BDT-TTFBTA, were designed and synthesized based on alkylthiophene-modified benzo[1,2-b:4,5-b’]dithiophene (BDT) as the donor unit, benzotriazole (BTA)/fluorobenzotriazole (FBTA) as acceptor units and alkylthiophenothiophene as π-bridge. The thermal stability, absorption spectra, electrochemical properties and active layer morphology of these polymers were investigated to explore the impacts of F atoms on the photovoltaic performance of polymer. By introducing F atom, BDT-TTFBTA:ITIC devices obtained higher open circuit voltage (Voc) of 0.75 eV, fill factor (FF) of 48%, and short-circuit current (Jsc) of 12.39 mA·cm⁻² than BDT-TTBTA:ITIC devices, giving rise to a higher power conversion efficiency (PCE) of 4.5% compare to BDT-TTBTA:ITIC devices.
... This fluorine impact continues to be studied today, in both fullerene acceptors and non-fullerene acceptors based OPV systems [10][11][12][13][14][15][16][17][18][19]. For example, Zhang et al. investigated the impact of fluorinating the thiophene linker groups in a copolymer containing benzodithiophene (BnDT) and benzotriazole (TAZ) [9]. ...
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In this work, the photostability of certain organic photovoltaic (OPV) active layers was demonstrated to improve by as much as a factor of five under white light illumination in air with the use of 1,7-bis-trifluoromethylfullerene (C60(CF3)2) as the acceptor in place of PC60BM. However, the results were highly dependent on the structure and functionality within the donor material. Twelve combinations of active layer blends were studied, comprised of six different high-performance donor polymers (two fluorinated and four non-fluorinated donors) and two fullerene acceptors (PC60BM and C60(CF3)2). The relative rates of irreversible photobleaching of the active layer blends were found to correlate well with the electron affinity of the fullerene when the polymer and fullerene were well blended, but a full rationalization of the photobleaching data requires consideration of both the electron affinity of the fullerene as well as the relative miscibility of the polymer–fullerene components in the blend. Miscibility of those components was probed using a combination of time-resolved photoluminescence (TRPL) measurements and scanning tunneling microscopy (STM) imaging. The presence of fluorinated aromatic units in the donor materials tend to promote more intimate mixing with C60(CF3)2 as compared to PC60BM. The full results of these photobleaching studies and measurements of donor–acceptor miscibility, considered alongside additional photoconductance measurements and preliminary device work, provide new molecular optimization insights for improving the long-term stability of OPV active layers.
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Poor miscibility between the polymer donor and acceptor in the active layer leads to low fill factors (FF). PCl and PCl-Si are synthesized by polymerization of the accessible and inexpensive IDIC-C16 with BDT-Cl and BDT-Cl-Si, respectively. PCl and PCl-Si involve a BDT skeleton that is definitely used in most highly efficient polymer donors, such as PM6. Guided by the law of similarity and intermiscibility, the similar building block acts as a bridge to improve the interfacial interaction and miscibility between the donor and acceptor, leading to a favorable morphology of the active layer. It is found that the miscibility of the active layer is sensitive to the structural similarity degree of the similar unit of the donor and acceptor. The PCl-Si-based device delivers a power conversion efficiency (PCE) of 9.25% with a moderate FF of 67.86%, whereas the PM6:PCl-based device achieves a PCE of 10.02% with a higher FF of 70.25%, which is the highest FF of the device with an IDIC-C16-based polymer acceptor. In addition, the improved interaction between the donor and acceptor improves the device stability. These results demonstrate that regulating the structural similarity between donor and acceptor is a promising strategy to optimize and stabilize morphology for high-performance all-polymer solar cells.
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The post-fullerene indacenodithiophene acceptor ITIC is a highly effective n-type component of high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs) for reasons that are not well-understood. Here, the impact of the ITIC alkyl sub-stituent architecture on PSC active layer film morphology, charge transport, and photovoltaic (PV) performance is investigated with the donor polymers PBDB-T and PBDB-TF. On progressing from n-propyl to n-hexyl to n-nonyl ITIC substituents, PSC power conversion efficiency (PCE) increases from <0.1 % to 9.31% to 10.24%, respectively. BHJ blend morphology, carrier recombina-tion dynamics, and PV performance with both donor polymers as probed by AFM, XRD, GIWAXS, and light intensity dependence correlate with marked differences in ITIC acceptor crystallinity. The DSC cold crystallization temperatures of the n-hexyl and n-nonyl-functionalized acceptors are found to closely track the anneal-ing temperatures for optimum PSC performance. These results identify a promising strategy for optimizing the performance of post-fullerene acceptor PSCs.
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A series of new non-fullerene small molecule acceptors (NTIC, NTIC-Me, NTIC-OMe and NTIC-F) based on the acceptor-donor-acceptor (A-D-A) architecture, using hexacyclic naphthalene-(cyclopentadithiophene) as the central unit were designed and synthesized. The non-fullerene OSC device based on PBDB-T:NTIC showed a highest PCE of 8.63%. With the relative high-lying LUMO level of NTIC-OMe, the PBDB-T:NTIC-OMe based device obtained a comparatively high Voc of 0.965 V and a PCE of 8.61% simitanousely. The results demonstrate that naphthalene core is a promising building block for constructing high efficicent non-fullerene accepotors and furthur boost the photovaltaic perfomance of the devices.
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In this work, high-efficiency nonfullerene polymer solar cells (PSCs) are developed based on a thiazolothiazole-containing wide bandgap polymer PTZ1 as donor and a planar IDT-based narrow bandgap small molecule with four side chains (IDIC) as acceptor. Through thermal annealing treatment, a power conversion efficiency (PCE) of up to 11.5% with an open circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 16.4 mA cm−2, and a fill factor of 76.2% is achieved. Furthermore, the PSCs based on PTZ1:IDIC still exhibit a relatively high PCE of 9.6% with the active layer thickness of 210 nm and a superior PCE of 10.5% with the device area of up to 0.81 cm2. These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free PSCs and large-scale devices fabrication.
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A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed, with the design and synthesis of a side-chain-conjugated acceptor (ITIC2) based on a 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']di(cyclopenta-dithiophene) electron-donating core and 1,1-dicyanomethylene-3-indanone electron-withdrawing end groups. ITIC2 with the conjugated side chains exhibits an absorption peak at 714 nm, which redshifts 12 nm relative to ITIC1. The absorption extinction coefficient of ITIC2 is 2.7 × 10(5) m(-1) cm(-1) , higher than that of ITIC1 (1.5 × 10(5) m(-1) cm(-1) ). ITIC2 exhibits slightly higher highest occupied molecular orbital (HOMO) (-5.43 eV) and lowest unoccupied molecular orbital (LUMO) (-3.80 eV) energy levels relative to ITIC1 (HOMO: -5.48 eV; LUMO: -3.84 eV), and higher electron mobility (1.3 × 10(-3) cm(2) V(-1) s(-1) ) than that of ITIC1 (9.6 × 10(-4) cm(2) V(-1) s(-1) ). The power conversion efficiency of ITIC2-based organic solar cells is 11.0%, much higher than that of ITIC1-based control devices (8.54%). Our results demonstrate that side-chain conjugation can tune energy levels, enhance absorption, and electron mobility, and finally enhance photovoltaic performance of nonfullerene acceptors.
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Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all-polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all-polymer bulk-heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade-coated films. Resonant soft X-ray scattering is used to elucidate the multiphasic morphology of these printed all-polymeric films and complements with the use of grazing incidence wide-angle X-ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive-free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.
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Rapid advances have been recently demonstrated in polymer solar cells (PSCs) with fused-ring electron acceptors (FREAs), which have low bandgap and high electron mobility. Semiconducting polymer donors with medium bandgap to complement the absorption and proper energy level alignments to minimize energy loss are preferred in this system, but there are few studies on them. Here, we explore thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) based polymers for high performance PSCs with FREAs. A new TPD polymer, PMOT16 is developed with 4-methoxyl thiophene as conjugated side chains on benzo[1,2-b:4,5-b']dithiophene unit. PMOT16 exhibits lower energy levels and enhanced interactions compared to the thiophene counterpart, PBDTT-6ttTPD. However, in PSCs with ITIC as acceptor, PMOT16 shows inferior performance to PBDTT-6ttTPD on short circuit current (JSC) and fill factor. When IDIC with lower energy levels is employed as acceptor, PMOT16 PSCs show decent power conversion efficiencies (PCEs) of around 10% with low energy loss, which surpasses that of PBDTT-6ttTPD due to increase of open circuit voltage. It is found that the lower JSC and inferior PCE in PMOT16:ITIC are ascribed to the approaching of energy levels between PMOT16 and ITIC. Our studies highlight the potential of TPD based polymers for high performance FREA-PSCs and the necessity for tuning energy level alignments.
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A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 10(5) m(-1) cm(-1) , a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10(-3) cm(2) V(-1) s(-1) . The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.
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A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
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A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).
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We design and synthesize four fused-ring electron acceptors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0 to 2 fluorine substituents as the electron-deficient units. These four molecules exhibit broad (550-850 nm) and strong absorption with high extinction coefficients of (2.1-2.5) ×10(5) M(-1) cm(-1). Fluorine substitution down shifts LUMO energy level, red shift absorption spectrum, and enhance electron mobility. The polymer solar cells based on the fluorinated electron acceptors exhibit power conversion efficiencies as high as 11.5%, much higher than that of their nonfluorinated counterpart (7.7%). We investigate the effects of the fluorine atom number and position on electronic properties, charge transport, film morphology, and photovoltaic properties.
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To achieve efficient organic solar cells, the design of suitable donor-acceptor couples is crucially important. State-of-the-art donor polymers used in fullerene cells may not perform well when they are combined with non-fullerene acceptors, thus new donor polymers need to be developed. Here we report non-fullerene organic solar cells with efficiencies up to 10.9%, enabled by a novel donor polymer that exhibits strong temperature-dependent aggregation but with intentionally reduced polymer crystallinity due to the introduction of a less symmetric monomer unit. Our comparative study shows that an analogue polymer with a C2 symmetric monomer unit yields highly crystalline polymer films but less efficient non-fullerene cells. Based on a monomer with a mirror symmetry, our best donor polymer exhibits reduced crystallinity, yet such a polymer matches better with small molecular acceptors. This study provides important insights to the design of donor polymers for non-fullerene organic solar cells.
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Fine energy-level modulations of small-molecule acceptors (SMAs) are realized via subtle chemical modifications on strong electron-withdrawing end-groups. The two new SMAs (IT-M and IT-DM) end-capped by methyl-modified dicycanovinylindan-1-one exhibit upshifted lowest unoccupied molecular orbital levels, and hence higher open-circuit voltages can be observed in the corresponding devices. Finally, a top power conversion efficiency of 12.05% is achieved.
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Solution-processed bulk heterojunction solar cells have experienced a remarkable acceleration in performances in the last two decades, reaching power conversion efficiencies above 10%. This impressive progress is the outcome of a simultaneous development of more advanced device architectures and of optimized semiconducting polymers. Several chemical approaches have been developed to fine-tune the optoelectronics and structural polymer parameters required to reach high efficiencies. Fluorination of the conjugated polymer backbone has appeared recently to be an especially promising approach for the development of efficient semiconducting polymers. As a matter of fact, most currently best-performing semiconducting polymers are using fluorine atoms in their conjugated backbone. In this review, we attempt to give an up-to-date overview of the latest results achieved on fluorinated polymers for solar cells and to highlight general polymer properties’ evolution trends related to the fluorination of their conjugated backbone.
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The active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure–property relationships that are emerging.
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Side chains and fluorine (F) substituents on conjugated polymers have shown significant impact on the photovoltaic properties of polymer-based bulk heterojunction (BHJ) solar cells, but their respective impact is largely studied independently. In order to disentangle the effect of side chains and F substituents, we comprehensively investigate a series of conjugated polymers with an identical backbone (PNDT–DTBT) but different combinations of side chains and F substituents. Surprisingly, these seemingly marginal changes to the polymer backbone strongly influence the morphology and structure in BHJ thin films (e.g., domain size/purity and the relative orientation of polymer crystallites), as manifested by resonant soft X-ray scattering (R-SoXS) and grazing-incidence wide-angle X-ray scattering (GI-WAXS), thereby exerting significant impact on the photovoltaic properties of these conjugated polymer-based BHJ cells. Devices based on the polymer with long bulky side chains and F substituents (C8,4-C6,2F) simultaneously exhibit large open circuit voltage (Voc), high short circuit current (Jsc) and good fill factor (FF), with an efficiency as high as 5.6% for this series of PNDT–DTBT polymers.
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We present the development and characterization of a dedicated resonant soft x-ray scattering facility. Capable of operation over a wide energy range, the beamline and endstation are primarily used for scattering from soft matter systems around the carbon K-edge (∼285 eV). We describe the specialized design of the instrument and characteristics of the beamline. Operational characteristics of immediate interest to users such as polarization control, degree of higher harmonic spectral contamination, and detector noise are delineated. Of special interest is the development of a higher harmonic rejection system that improves the spectral purity of the x-ray beam. Special software and a user-friendly interface have been implemented to allow real-time data processing and preliminary data analysis simultaneous with data acquisition.
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Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first order recombination at short circuit to bimolecular recombination at open circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The "missing 0.3V" inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open circuit voltage at room temperature results from the temperature dependence of the quasi-Fermi-levels in the polymer and fullerene domains - a conclusion based upon the fundamental statistics of Fermions. Comment: Accepted for publication in Physical Review B. http://prb.aps.org/accepted/B/6b07cO3aHe71bd1b149e1425e58bf2868cda2384d?ajax=1&height=500&width=500
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Two new soft X-ray scanning transmission microscopes located at the Advanced Light Source (ALS) have been designed, built and commissioned. Interferometer control implemented in both microscopes allows the precise measurement of the transverse position of the zone plate relative to the sample. Long-term positional stability and compensation for transverse displacement during translations of the zone plate have been achieved. The interferometer also provides low-distortion orthogonal x, y imaging. Two different control systems have been developed: a digital control system using standard VXI components at beamline 7.0, and a custom feedback system based on PC AT boards at beamline 5.3.2. Both microscopes are diffraction limited with the resolution set by the quality of the zone plates. Periodic features with 30 nm half period can be resolved with a zone plate that has a 40 nm outermost zone width. One microscope is operating at an undulator beamline (7.0), while the other is operating at a novel dedicated bending-magnet beamline (5.3.2), which is designed specifically to illuminate the microscope. The undulator beamline provides count rates of the order of tens of MHz at high-energy resolution with photon energies of up to about 1000 eV. Although the brightness of a bending-magnet source is about four orders of magnitude smaller than that of an undulator source, photon statistics limited operation with intensities in excess of 3 MHz has been achieved at high energy resolution and high spatial resolution. The design and performance of these microscopes are described.
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Organic solar cells (OSCs) have been a rising star in the field of renewable energy since the introduction of the bulk heterojunction (BHJ) in 1992. Recent advances have pushed the efficiencies of OSCs to over 13%, an impressive accomplishment via collaborative efforts in rational materials design and synthesis, careful device engineering, and fundamental understanding of device physics. Throughout these endeavors, several design principles for the conjugated donor polymers used in such solar cells have emerged, including optimizing the conjugated backbone with judicious selection of building blocks, side-chain engineering, and substituents. Among all of the substituents, fluorine is probably the most popular one; improved device characteristics with fluorination have frequently been reported for a wide range of conjugated polymers, in particular, donor–acceptor (D–A)-type polymers. Herein we examine the effect of fluorination on the device performance of solar cells as a function of the position of fluorination (on the acceptor unit or on the donor unit), aiming to outline a clear understanding of the benefits of this curious substituent.
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A thieno[3,4-b]thiophene-based donor polymer PTBTz-2 was employed to construct fullerene-free solar cell with the classical acceptor ITIC. Interestingly, due to the high extinction coefficients and wide absorption for these two materials, the active layer can harvest a larger fraction of the coverage solar spectrum even in ultrathin film. Furthermore, the simultaneous advantages of appropriate cascade energy level, well balanced hole/electron mobility (μh/μe=1.16), and low charge accumulation and recombination, make the PTBTz-2/ITIC-based solar cells exhibit an excellent power conversion efficiency of 10.92% with large short circuit current density of 20.34 mA cm⁻². The results indicate that fine-tailored thieno[3,4-b]thiophene-based polymers would be another type of promising donor materials except for widely reported efficient benzo[d][1,2,3]triazole (BTA) or benzo[1,2-c:4,5-c']-dithiophene-4,8-dione (BDD) based polymers, and would enrich the reservoir of high performance light-harvesting conjugated polymers.
Article
Two novel wide bandgap copolymers based on quinoxalino[6,5-f]quinoxaline (NQx) acceptor block, PBDT–NQx and PBDTS–NQx, are successfully synthesized for efficient nonfullerene polymer solar cells (PSCs). The attached conjugated side chains on both benzodithiophene (BDT) and NQx endow the resulting copolymers with low-lying highest occupied molecular orbital (HOMO) levels. The sulfur atom insertion further reduces the HOMO level of PBDTS–NQx to −5.31 eV, contributing to a high open-circuit voltage, Voc, of 0.91 V. Conjugated n-octylthienyl side chains attached on the NQx skeletons also significantly improve the π–π* transitions and optical absorptions of the copolymers in the region of short wavelengths, which induce a good complementary absorption when blending with the low bandgap small molecular acceptor of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene. The wide absorption range makes the active blends absorb more photons, giving rise to a high short-circuit current density, Jsc, value of 15.62 mA cm−2. The sulfur atom insertion also enhances the crystallinity of PBDTS–NQx and presents its blend film with a favorable nanophase separation, resulting in improved Jsc and fill factor (FF) values with a high power conversion efficiency of 11.47%. This work not only provides a new fused ring acceptor block (NQx) for constructing high-performance wide bandgap copolymers but also provides the NQx-based copolymers for achieving highly efficient nonfullerene PSCs.
Article
Fluorinated conjugated polymers leading to enhanced photovoltaic device performance has been widely observed in a variety of donor-acceptor copolymers; however, almost all these polymers have fluorine substituents on the acceptor unit. Building upon our previously reported PBnDT-FTAZ, a fluorinated donor-acceptor conjugated polymer with impressive device performance, we set this study to explore the effect of adding the fluorine substituents onto the flanking thiophene units between the donor unit (BnDT) and the acceptor unit (TAZ). We developed new synthetic approaches to control the position of the fluorination (3’ or 4’) on the thiophene unit, and synthesized four additional PBnDT-TAZ polymers incorporating the fluorine-substituted-thiophene (FT) units, 3’-FT-HTAZ, 4’-FT-HTAZ, 3’-FT-FTAZ and 4’-FT-FTAZ. We discover that relocating the fluorine substituents from the acceptor to the flanking thiophene units have negligible impact on the device characteristics (short circuit current, open circuit voltage, and fill factor) when comparing 4’-FT-HTAZ with the original FTAZ. Combining these two fluorination approaches together, 4’-FT-FTAZ shows even higher device performance than FTAZ (7.7% vs. 6.6%) with active layers over 200 nm in thickness. Furthermore, high values of fill factor ~ 70% are all achieved for photovoltaic devices based on 3’-FT-HTAZ, 4’-FT-HTAZ or 4’-FT-FTAZ, ascribed to the observed high hole mobilities (over 1 × 10-3 cm2/Vs) in these devices. Our study offers a new approach to utilize the fluorinated thiophene units in developing new conjugated polymers to further improve the device performance of polymer solar cells.
Article
Morphology can play a critical role in determining function in organic photovoltaic (OPV) systems. Recently molecular acceptors have showed promise to replace fullerene derivatives as acceptor materials in bulk heterojunction solar cells and have achieved >10% efficiencies in single junction devices. The nearly identical mass/electron densities between the donor (polymer) and acceptor (molecule) materials results in poor material contrast compared to fullerene-based OPVs and therefore morphology characterization using techniques that rely on mass/electron density variations poses a challenge. This inhibits a fundamental understanding of the structure–property relationships for non-fullerene acceptor materials. We demonstrate that low angle annular dark field scanning transmission electron microscopy and resonant soft X-ray scattering form a set of complementary tools that can provide quantitative characterization of fullerene as well as non-fullerene based organic photovoltaic systems.
Article
A new acceptor–donor–acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V OC of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.
Article
Non-fullerene acceptors (NFAs) are becoming a serious contender to fullerene-based electron acceptors in organic photovoltaics, due to their structural versatility and easily tunable optical and electronic properties. However, NFA-based solar cells often have a decreased short-circuit current (Jsc) and fill factor (FF) compared to their fullerene-based counterparts. Here, we investigate the fundamental causes of this decrease in the performance of solar cells using a non-fullerene acceptor (SF-PDI2) paired with two polymer donors, FTAZ and PyCNTAZ, compared with their fullerene-based counterparts. Through a number of experimental techniques and morphological studies, we show that the SF-PDI2-based solar cells suffer from insufficient charge generation, transport, and collection when compared with the PCBM-based solar cells. The SF-PDI2-based solar cells show increased bimolecular recombination, which, together with other recombination loss mechanisms in these cells, causes a significant decrease in their Jsc and FF. Notably, the less pure domains, low electron mobility (on the order of 10⁻⁵ cm² V⁻¹ s⁻¹), and imbalanced mobility (in regard to the hole mobility) further explain the low FF. On the other hand, the higher open-circuit voltage (Voc) in the SF-PDI2 devices is mainly due to the increase in the CT state energy. It is worth mentioning that the PyCNTAZ-based devices show an ultralow charge separation energy (ΔECS), close to 0 eV. Our results demonstrate that further increasing the mobility (both of electrons and holes) in these NFA-based solar cells would be a viable approach to further enhance the efficiency of these new types of solar cells, ideally, without losing the high Voc of such cells.
Article
A nonfullerene polymer solar cell with a high efficiency of 9.26% is realized by using benzodithiophene-alt-fluorobenzotriazole copolymer J51 as a medium bandgap polymer donor and the low bandgap organic semiconductor ITIC with high extinction coefficients as acceptor.
Article
This review paper summarizes the recent progress of highly efficient copolymers with the fluorination strategy for photovoltaic applications. We first present a brief introduction of the fundamental principles of polymer solar cells, and then the functions of fluorine atoms on the polymer donor materials. Finally, we review the research progress of the reported copolymers by classification of the fluorinated acceptor units and donor units, respectively. The resulting structure-property correlations of these copolymers are also discussed which shall certainly facilitate widespread utilization of this strategy for constructing high-performance photovoltaic copolymers in the future.
Article
We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = -5.66 eV, LUMO = -3.93 eV; ITIC: HOMO = -5.48 eV, LUMO = -3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-bandgap polymer donors and wide-bandgap polymer donors. ITIC-Th has higher electron mobility (6.1 X 10-4 cm2 V-1 s-1) than ITIC (2.6 X 10-4 cm2 V-1 s-1) due to enhanced intermolecular interaction induced by sulfur-sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-bandgap and wide-bandgap polymer donors. In one case a power conversion efficiency of 9.57% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.
Article
Solar cells, a renewable, clean energy technology that efficiently converts sunlight into electricity, are a promising long-term solution for energy and environmental problems caused by a mass of production and the use of fossil fuels. Solution-processed organic solar cells (OSCs) have attracted much attention in the past few years because of several advantages, including easy fabrication, low cost, lightweight, and flexibility. Now, OSCs exhibit power conversion efficiencies (PCEs) of over 10%. In the early stage of OSCs, vapor-deposited organic dye materials were first used in bilayer heterojunction devices in the 1980s, and then, solution-processed polymers were introduced in bulk heterojunction (BHJ) devices. Relative to polymers, vapor-deposited small molecules offer potential advantages, such as a defined molecular structure, definite molecular weight, easy purification, mass-scale production, and good batch-to-batch reproducibility. However, the limited solubility and high crystallinity of vapor-deposited small molecules are unfavorable for use in solution-processed BHJ OSCs. Conversely, polymers have good solution-processing and film-forming properties and are easily processed into flexible devices, whereas their polydispersity of molecular weights and difficulty in purification results in batch to batch variation, which may hamper performance reproducibility and commercialization. Oligomer molecules (OMs) are monodisperse big molecules with intermediate molecular weights (generally in the thousands), and their sizes are between those of small molecules (generally with molecular weights <1000) and polymers (generally with molecular weights >10000). OMs not only overcome shortcomings of both vapor-deposited small molecules and solution-processed polymers, but also combine their advantages, such as defined molecular structure, definite molecular weight, easy purification, mass-scale production, good batch-to-batch reproducibility, good solution processability, and film-forming properties. Therefore, OMs are a good choice for solution-processed reproducible OSCs toward scalable commercialized applications. Considerable efforts have been dedicated to developing new OM electron donors and electron acceptors for OSCs. So far, the highest PCEs of solution-processed OSCs based on OM donors and acceptors are 9-10% and 6-7%, respectively. OM materials have become promising alternatives to polymer and/or fullerene materials for efficient and stable OSCs. In this Account, we present a brief survey of the recent developments in solution-processable OM electron donors and acceptors and their application in OSCs. Rational design of OMs with star- and linear-shaped structures based on triphenylamine, benzodithiophene, and indacenodithiophene units and their impacts on device performance are discussed. Structure-property relationships are also proposed. Furthermore, the remaining challenges and the key research directions in the near future are also addressed. In the next years, an interdisciplinary approach involving novel OM materials, especially electron acceptor materials, accurate morphology optimization, and advanced device technologies will probably bring high-efficiency and stable OSCs to final commercialization.
Article
In the past two years, non-fullerene acceptors including polymers and small molecules have become the focus of many research efforts. Fullerene-free organic solar cells (OSCs) have shown efficiencies of up to 6.8% for solution-processed devices, and even up to 8.4% for vacuum-deposited devices, which have been significantly improved relative to those disclosed 2 years ago (generally <4%). Non-fullerene acceptor materials are a new focus in the OSC field. Tailoring extended fused-rings with electron-deficient groups is an effective strategy for design of acceptors. Here, very recent developments in several systems of fused ring-based electron acceptors, such as halogenated (sub or subna)phthalocyanine, imide-functionalized rylene, and linear fused-rings end capped with electron deficient blocks, are reviewed.
Article
A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings, for example, weak absorption in the visible spectral region and limited energy level variability, of fullerene acceptors is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion efficiencies of up to 6.8%, a record for fullerene-free PSCs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Article
Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.
Article
Analysis of measured charge carrier mobilities and fill factors in solution processable small molecule bulk heterojunction solar cells reveals that in order to achieve high FF, hole and electron mobilities must be >10(-4) cm(2) /Vs. Neat film mobility measurements are also found to be a useful predictor of the maximum blend film mobility and FF obtained in blend film solar cells.
Article
Nika is an Igor Pro -based package for correction, calibration and reduction of two-dimensional area-detector data into one-dimensional data (`lineouts'). It is free (although the user needs a paid license for Igor Pro ), open source and highly flexible. While typically used for small-angle X-ray scattering (SAXS) data, it can also be used for grazing-incidence SAXS data, wide-angle diffraction data and even small-angle neutron scattering data. It has been widely available to the user community since about 2005, and it is currently used at the SAXS instruments of selected large-scale facilities as their main data reduction package. It is, however, also suitable for desktop instruments when the manufacturer's software is not available or appropriate. Since it is distributed as source code, it can be scrutinized, verified and modified by users to suit their needs.
Article
The molecular weight (MW) of PBnDT-FTAZ can be precisely controlled by adjusting the stoichiometric ratio of two monomers, following the Carothers equation. Study of a set of PBnDT-FTAZ with different MW reveals that the MW significantly influences the morphology and structural order of PBnDT-FTAZ in its bulk heterojunction solar cells, with the highest efficiency (over 7%) achieved with the use of a MW of 40 kg/mol.
Article
Solution-processed small molecule p-DTS(FBTTh2)2:PC71BM bulk-heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a Calcium (Ca) layer between the BHJ layer and the Al cathode; for 20nm Ca thickness, the FF ~ 73 %, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100 % in the spectral range 600-650 nm and well over 80 % across the entire spectral range from 400 - 700 nm. Analysis of the current-voltage (J-V) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the J-V curves are dominated by first-order recombination from the short circuit condition to the maximum power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open circuit condition in the optimized device with Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley-Read-Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) leads to the high fill factor.
Article
Plastic solar cells bear the potential for large-scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5%. This article reviews the processes and limitations that govern device operation of polymer:fullerene BHJ solar cells, with respect to the charge-carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build-up of the space–charge that will significantly reduce the power conversion efficiency. Dissociation of electron–hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open-circuit voltage (VOC) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in VOC of polymer:fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short-circuit current increase simultaneously.
Article
Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.