Search

The deuterium isotope effect plays a profound role in revealing the kinetics and mechanisms of organic reactions, as well as the hole/electron transport behaviors of semiconductors, yet the application of the deuterium isotope effect in organic solar cells has not been fully explored due to the challenging nature of synthesis. Here, we aim at addressing this conundrum by designing and synthesizing two asymmetric non-fullerene acceptors, BTP-POE-H and BTP-POE-D. Particularly, the effect of deuterium isotope effects on their optoelectronic properties and photovoltaic properties was investigated. Attractively, a higher power conversion efficiency (PCE) of 16.5% and an impressive fill factor of 78% were achieved for the deuterated acceptor BTP-POE-D compared to the control BTP-POE-H under the same conditions. The high fill factor of BTP-POE-D based devices can be attributed to their higher and more balanced charge transport, weaker bimolecular recombination, more ordered molecular stacking and suitable length scale of phase separation, as revealed by grazing incidence X-ray diffraction and transmission electron microscopy measurements. These results not only advance our understanding of the structure–performance relationships of deuterium-substituted acceptors, but also provide a new avenue to design high-performance nonfullerene acceptors. [ABSTRACT FROM AUTHOR]

Asymmetric nonfullerene acceptors (NFAs) are an attractive candidate for high-performance organic solar cells (OSCs), whereas effective asymmetric strategies are highly needed to further advance their device performance. Herein, by alternating the substituted position of trifluorobenzene (Ar3F) in the terminal end group, two asymmetric NFA isomers, namely BTP-γ-Ar3F and BTP-β-Ar3F, are designed, and their photophysical and photovoltaic properties are fully compared. With the γ-substituted fashion, it is shown that BTP-γ-Ar3F features a better planarity, contributing to stronger crystallization and favorable phase separation with the PM6 donor polymer. As a result, a higher power conversion efficiency of 17.42% along with a high open circuit voltage of 0.90 V is realized in the PM6:BTP-γ-Ar3F-based OSCs. The established structure–property relationship provides a rational design principle for asymmetric NFAs toward high photovoltaic performance. [ABSTRACT FROM AUTHOR]

High-performance non-fullerene acceptors (NFAs) are valuable for promoting the power conversion efficiency of organic solar cells (OSCs). Particularly, owing to their adjustable surface tension and molecular packing, asymmetric NFAs are promising for the development of novel organic photovoltaic materials. Herein, three novel asymmetric NFAs, namely Y6-SCz1, Y6-SCz2, and Y6-SCz3, have been designed and synthesized by rationally tuning the branching position of the carbazole-terminal side-chains on the pyrrole ring along with varying halogen substitutions on the indanone end-groups. Consequently, in the PM6:Y6-SCz2 combination, the systematically optimized absorption, energy level, Flory–Huggins interaction parameters, and charge transport process generate a champion PCE of 15.60% with a high open-circuit voltage (VOC) of 0.87 V in OSCs. Our findings deliver new insights into the molecular design of asymmetric NFAs, which are of great importance for further practical implementation of OSCs. [ABSTRACT FROM AUTHOR]

In-depth understanding of the structure–property relationship in organic solar cells (OSCs) is of great importance in the development of the OSC field. In this study, we designed and synthesized two new chlorinated small molecule donor materials, named SM1-α-Cl (with α-chlorinated thiophene as a conjugated side chain) and SM1-β-Cl (with β-chlorinated thiophene as a conjugated side chain), by a side chain isomerization strategy. The morphology results indicate that the two small molecules present different aggregation characteristics, and the SM1-β-Cl:Y6 blend film displays enhanced molecular stacking properties, benefiting the charge transport. Besides, the ultrafast spectroscopic study indicates a higher charge transfer (CT) state yield in the SM1-β-Cl based blend. Therefore, the devices based on SM1-β-Cl:Y6 demonstrate efficient exciton dissociation, balanced electron/hole mobility and a higher power conversion efficiency (PCE) of 12.14%, while SM1-α-Cl:Y6 based devices give an inferior PCE of 8.93%. These results imply that side-chain isomerization of small molecule donors could be an effective method to fine-tune the morphological features and improve the photovoltaic performance. [ABSTRACT FROM AUTHOR]

With the increasing concern for discovering a new processing additive, the effects of polymer additives (polystyrene, poly(styrene-b-pentafluorostyrene) and poly(pentafluorostyrene)) on the PBDB-TT5:ITIC blend system were thoroughly investigated to obtain a direct comparison with the widely used volatile solvent 1,8-diiodooctane additive. Although different additive types (solid vs. solvent) act via different mechanisms within the active layer, the use of each optimal amount enables improved power conversion efficiencies (PCEs) to various extents. Among them, viscous PPFS was found to be the most effective, resulting in a remarkable improvement in the PCE from 7.90% to 9.29%. Interestingly, the PPFS has a high dielectric constant that can not only induce the enhanced dielectric property of the active layer but also serves as a morphological locking-in agent, conferring long-term and thermal device stabilities. Moreover, we successfully improved the PCEs for other host systems (J71:ITIC, PTB7-Th:PC71BM, and PTB7-Th:PNDI-T10) through PPFS addition. For J71:ITIC, the PCE reached a maximum of up to 10.7%. Our study highlights that the use of PPFS is a simple and practical technique to improve the PCE and stability of devices, with the expectation that further molecular design involving high dielectric constant polymer additives will likely obtain more interesting results in various optoelectronics. [ABSTRACT FROM AUTHOR]

Indacenodithiophene (IDT) is a promising building block for designing organic semiconductors. In this work, a new decacyclic building block (m-IDTIDT) with a double IDT unit and meta-alkyl-phenyl substitutions was synthesized and used for designing a new non-fullerene electron acceptor (m-IDTIDT-FIC). Compared to p-IDTIDT-IC, m-IDTIDT-FIC has a smaller π–π distance, larger crystalline coherence length, and higher electron mobility. Thus, polymer solar cells (PSCs) based on PCE10:m-IDTIDT-FIC can achieve a higher PCE (8.27%) than PCE10:p-IDTIDT-IC-based PSCs (6.48%). In addition, PSCs based on J71:m-IDTIDT-FIC show better photovoltaic performances than PSCs based on PCE10:m-IDTIDT-FIC. This is because the J71:m-IDTIDT-FIC blend film has more balanced charge transport ability, a smooth surface, and higher relative domain purity than the PCE10:m-IDTIDT-FIC blend film. PSCs based on J71:m-IDTIDT-FIC exhibited the best photovoltaic performances with a PCE of 11.32%, an open-circuit voltage (Voc) of 0.92 V, a short-circuit current (Jsc) of 18.01 mA cm−2, and a fill factor (FF) of 68.30%. These results indicate that m-IDTIDT with meta-alkyl-phenyl substitutions is a promising IDT-based building block for high-performance non-fullerene electron acceptors. [ABSTRACT FROM AUTHOR]

Nowadays, constructing ternary polymer solar cells (PSCs) and developing non-fullerene acceptors (NFAs) have emerged as two powerful and efficient means to propel the device efficiency further forward. However, the incorporation of NFAs into a fullerene-based binary PSC to form high-performance donor : fullerene : NFA type ternary PSCs with a thick active layer and large area is still a challenge due to their inferior charge transport properties, complicated blend morphology and unclear aggregation behavior. In this contribution, a new low bandgap NFA (DTCFOIC) based on a fused heptacyclic core (dithienocyclopentafluorene) is developed and blended with a medium bandgap polymer donor PBDB-T to form a binary PSC achieving a power conversion efficiency (PCE) of 6.92%. When a small amount of DTCFOIC was employed as the additional acceptor combined with fullerene-based PC71BM to construct the PBDB-T : PC71BM : DTCFOIC ternary PSCs, an enhanced PCE of 9.40% was obtained after carefully optimizing the blend composition and the amount of the additive 1,8-diiodooctane (DIO) with a thin active layer (100 nm) and normal area (0.04 cm2). This could be mainly attributed to the extended absorption range, improved charge transfer, dissociation and collection properties, and suppressed charge recombination as well as optimized blend morphology in the ternary blend after the addition of NFA and DIO. Moreover, the photovoltaic performance of the ternary PSCs could be further optimized to achieve a higher PCE of 10.13% with a thicker active layer (160 nm) and eventually the highest PCE of 10.41% can be achieved with a thick active layer of 190 nm and a large area of 0.1 cm2, which is among the highest efficiencies of both thick layer and large area ternary PSCs. Besides, the ternary PSCs preserved a high PCE over 9% even with a larger active area of 1 cm2 using a thick active layer (172 nm). This observation demonstrates that incorporating a NFA to construct donor : fullerene : NFA type ternary PSCs is a feasible and effective approach to significantly enhance the performance of the resulting ternary system. Meanwhile, our results related to the thick active layer and large area suggest that this ternary system has a promising application prospect for mass manufacturing high-performance PSCs with a roll-to-roll process. [ABSTRACT FROM AUTHOR]

Two fullerene/non-fullerene hybrids were designed and synthesized for polymer solar cells as small molecule acceptors. Polymer solar cells based on J71:A2 achieved a power conversion efficiency of 4.52% with the open circuit voltage at nearly 1.0 V. The success of the hybrids may pave a new way for small molecule acceptors. [ABSTRACT FROM AUTHOR]

The effects of fused-ring regiochemistry on the physicochemical and photovoltaic properties of n-type organic semiconductor (n-OS) acceptors are investigated. Two n-OS isomers TPTC and TPTIC were prepared with different oxygen positions in the central fused-ring unit of the acceptor molecules: oxygen is connected with benzene in TPTC and it is connected with two thiophenes in TPTIC. It is found that TPTC tends to cause excessive self-aggregation with several different packing motifs or polymorphs, while TPTIC with compact alkyl chains forms well-defined crystals. The electron mobility of TPTC, which is measured by the space-charge-limited current (SCLC) method, is much lower than that of TPTIC. When blending these acceptors with the polymer PTQ10, excessive self-aggregation of TPTC leads to large phase separation and exhibits little change after thermal annealing treatment, while the intermolecular interaction in TPTIC is appropriate to achieve suitable phase separation in its blend films with PTQ10, and the stacking of both crystallites was obviously improved after thermal annealing. Thus the PSCs with TPTIC as the acceptor show a much higher power conversion efficiency (PCE) of 10.42%, in comparison with that (1.97%) of the device with TPTC as the acceptor. These results indicate that the regiochemistry of the n-OS acceptors greatly influences the aggregation behavior of the molecules, which strongly affects the performance of the PSCs, and the structure–property relationship of the materials with the regiochemistry could guide the development of high performance n-OS acceptors. [ABSTRACT FROM AUTHOR]

In this work, a novel oligomer based on the introduction of alkylthio-thienylenevinylene thiophene (TVT-SR) side groups into benzo[1,2-b:4,5-b′]dithiophene (BDT) as a central donor unit, BDT(TVT-SR)2, is developed and used as the donor material for all-small molecule OSCs (SM-OSCs) with IDIC as the acceptor. BDT(TVT-SR)2 exhibits red-shifted absorption, a down-shifted HOMO energy level and higher hole mobility in comparison with its corresponding molecule H11 with alkylthiothienyl side chains. The power conversion efficiency (PCE) of the SM-OSCs based on BDT(TVT-SR)2:IDIC with thermal annealing (TA) at 110 °C for 10 min reached 11.1%. This TA treatment improves the film morphology, charge transport properties and thus device performance of the SM-OSCs. Besides, these devices show good morphological stability and photo-stability. [ABSTRACT FROM AUTHOR]

Solution-processed organic solar cells (OSCs) have been attracting more and more attention for a series of well-known advantages, and power conversion efficiencies (PCEs) of over 11% have been reported. However, the highest PCE of the OSCs based on small molecule donor/polymer acceptor blends is only 4.82%, which was much lower than those of other types of OSCs due to weak absorption of the polymer acceptor and the unbalanced charge carrier mobility of the small molecule donor and the polymer acceptor. Here, we fabricated small molecule donor/polymer acceptor-based OSCs using the wide bandgap SM1 and DR3TBDTT as the small molecular donor and the low-bandgap n-type conjugated polymer PZ1 as the polymer acceptor. With the treatment of a solvent additive, which can promote the absorption intensity, enhance the carrier mobility and suppress the charge carrier recombination, the SM1-based devices and the DR3TBDTT-based devices show PCEs of 3.97% and 5.86%, respectively. It is worth mentioning that the PCE of 5.86% is the state-of-the-art efficiency for OSCs based on the small molecular donor/polymer acceptor system. [ABSTRACT FROM AUTHOR]

Unlike fullerene derivatives, current nonfullerene n-type organic semiconductor (n-OS) electron acceptors can rarely work efficiently with different band-gap polymer donors in polymer solar cells (PSCs). To address this issue, we designed and synthesized a new n-OS electron acceptor, namely m-MeIC, which is proved to be effective with different band-gap polymer donors including wide band-gap J71, medium band-gap PBDB-T and low band-gap PCE-10. Photovoltaic devices based on J71:m-MeIC and PBDB-T:m-MeIC achieved power conversion efficiencies (PCEs) as high as 12.08% and 10.93%, respectively. The difference in PCEs between the devices based on J71 and PBDB-T is mainly ascribed to the gap of the open circuit voltage. It should be mentioned that the J71-based device exhibited an energy loss of 0.62 eV, which is significantly lower than the most reported energy loss. For the low band-gap polymer donor PCE-10, the device also showed a relatively high PCE of 8.34%. The different device performances are analyzed in depth from charge transport and collection, to recombination loss mechanism and morphology of blend films. This work reveals that the new n-type organic semiconductor is very promising as a universal nonfullerene acceptor, pairing with different band-gap polymer donors. [ABSTRACT FROM AUTHOR]

Semi-transparent polymer solar cells (ST-PSCs) have attracted great attention recently because of their potential for application in smart windows, etc. Here, we fabricated ST-PSCs based on a low band-gap conjugated polymer, PTB7-Th, as the donor and a narrow band-gap n-type organic semiconductor (n-OS), ITVfIC, as the acceptor. The active layer (with a thickness of 100 nm) of the ST-PSC exhibits a high average transmittance (AT) of 73.46% in the wavelength range of 400–600 nm. The as-cast ST-PSC with 15 nm Ag as the cathode without any additive or thermal-annealing treatment shows a higher power conversion efficiency (PCE) of 8.21% with an AT of 33.7%, which is one of the highest values for ST-PSCs without extra treatment. In addition, the ST-PSCs show good thermal stability with 91% of their original PCE value retained after high temperature treatment at 200 °C for 2 hours. The higher PCE and good stability indicate that the ST-PSC has potential for practical application and the narrow bandgap ITVfIC could be a promising n-OS acceptor for the fabrication of ST-PSCs. [ABSTRACT FROM AUTHOR]

Quinoxaline (Qx) has an easily modifiable structure, which allows for fine-tuning its properties through optimizing the length of side chains and the kinds of aromatic rings in conjugated side chains. Qx based polymer PBDTT-ffQx with alkoxy substituted fluorobenzene side chains shows a power conversion efficiency (PCE) of 8.47% in nonfullerene solar cells. In this work, we change the alkoxy substituted fluorobenzene side chains to alkyl substituted fluorothiophene in the Qx unit and employ the benzodithiophene (BDT) unit with different lengths of alkyl thiophene side chains to obtain two new Qx based polymers, named TTFQx-T1 and TTFQx-T2, respectively. The TTFQx-T1:ITIC blend film has clear nanoscale phase separation and a suitable domain size, and the device with the TTFQx-T1:ITIC blend film shows lower geminate recombination and higher charge mobility than those with TTFQx-T2:ITIC. Therefore, the TTFQx-T1:ITIC based device exhibits a higher PCE of 10.52% than that based on the TTFQx-T2:ITIC blend with a moderate PCE of 7.22%. The inverted devices from TTFQx-T1:ITIC blends with a larger active area of 16 mm2 than the conventional device (4.5 mm2) show a good PCE of 9.21%. The results highlight that the alkyl lengths and the kinds of aromatic rings in side chains are significant for the development of high performance photovoltaic polymers. Optimizing the conjugated side chains of Qx based polymers is an efficient way to improve photovoltaic properties, and Qx based polymers are potential candidates for fabricating highly efficient polymer solar cells. [ABSTRACT FROM AUTHOR]

Double bond π-bridges were introduced into an A–D–A structured n-type organic semiconductor (n-OS) IDT-IC and a new n-OS acceptor SJ-IC was synthesized. In comparison with IDT-IC, the SJ-IC film shows significantly red-shifted absorption, improved electron mobility and tuned crystallinity, which make its blend film with a polymer donor easier to form appropriate phase separation. The polymer solar cells with polymer J61 as a donor and SJ-IC as an acceptor demonstrated a higher PCE of 9.27% with a higher Jsc of 16.99 mA cm−2, while the device based on J61/IDT-IC only delivered a PCE of 6.95% with a Jsc of 13.70 mA cm−2, which should be ascribed to the red-shifted and broadened absorption of SJ-IC. Therefore, inserting double bond π-bridges into n-OS acceptor molecules is a simple and effective way to broaden and red-shift their absorption to improve their photovoltaic performance. In addition, the near-infrared absorption of the SJ-IC acceptor should be beneficial to its future application in semitransparent and tandem PSCs. [ABSTRACT FROM AUTHOR]

A new small molecule, ITTC, bearing an indacenodithieno[3,2-b]thiophene core and a 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile end group, was designed, synthesized and characterized as a non-fullerene electron acceptor. The ITTC possesses strong and broad light absorption, high and balanced charge mobility and a nanoscale interpenetrating morphology when blended with a recently synthesized hexafluoroquinoxaline based polymer donor-HFQx-T. HFQx-T was obtained from a Stille coupling copolymerization of a 2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene electron donating unit and a hexafluoroquinoxaline based electron accepting unit. The device employing HFQx-T as the donor and ITTC as the acceptor delivered a power conversion efficiency (PCE) of 8.19% without any post-treatment. After thermal annealing, an impressive PCE of 10.4% was obtained. This performance is among the highest PCEs reported for fullerene-free polymer solar cells up to date. This study demonstrates the great potential of ITTC as n-type materials for organic electronics. [ABSTRACT FROM AUTHOR]

Ladder-type conjugated structures with rigid and coplanar molecular frameworks feature longer effective conjugation, affirmative optoelectronic properties and strong intermolecular π–π interactions, which are ideal characteristics for organic photovoltaics. Here, a new “zigzag” angular-fused naphthodifuran (zNDF) based on alkoxyphenyl side chains was designed and synthesized. The distannylated zNDF building block was copolymerized with 4,7-di(5-bromothiophen-2-yl)-5,6-dioctyloxybenzo[c][1,2,5]thiadiazole and 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(4-(2-ethylhexyloxy)-3-fluorophenyl)-6,7-difloroquinoxaline (Br-BT and Br-ffQx) acceptor units by Stille cross coupling reaction to form two new medium bandgap donor–acceptor polymers PzNDFP-BT and PzNDFP-ffQx, respectively. The photovoltaic properties of the copolymers blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor were investigated. A 6.9% efficiency was achieved from the single device based on the PzNDFP-BT : PC71BM (1 : 1.5, w/w) blend film with a 0.25% 1,8-diiodooctane (DIO) additive, which is among the highest efficiency for zNDF-based polymer solar cells. [ABSTRACT FROM AUTHOR]

A new electron-acceptor, thiophene-fused benzotriazole (BTAZT), is designed for the first time to construct donor–acceptor copolymers with bithienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) as an electron-donor unit for polymer solar cells (PSCs). Compared with their analogous polymers, which contain a similar conjugated backbone but with the benzotriazole (BTAZ) unit as the acceptor, the BTAZT-based polymers show evident red-shifted absorption with narrower optical band gaps due to the fusion of the thiophene moiety onto the BTAZ unit to further stabilize the quinoid structure of their conjugation backbones. Furthermore, two types of side-chain groups, the alkylcarboxyl group (in PBDTT-BTAZT-1) or alkylcarbonyl group (in PBDTT-BTAZT-2), are introduced to adjust the energy levels and modulate the solubility of the PBDTT-BTAZT based polymers. It is found that the stronger electron-withdrawing ability of the alkylcarboxyl group relative to that of the alkoxycarbonyl group at the α-position of the fused thiophene can further reduce the HOMO energy level to enhance the open voltage (Voc) in the PBDTT-BTAZT-2 based device. PSC devices based on these two polymers as electron-donating materials and a fullerene derivative (PC71BM) as an electron-accepting material are fabricated, and device optimization with additives (DIO or NMP) to modify the morphologies of the active layers and cathode buffer layers (PDIN or PDINO) to modify the cathode interlayers is investigated. The best PCE of approximately 7% is obtained for the PBDTT-BTAZT-1 based device with the DIO additive and PDIN-modified cathode interlayer. Notably, its optimal short circuit current density reaches 14.41 mA cm−2, which is one of the highest values ever reported in BTAZ-based PSCs with a PCBM acceptor. The best PCE of the PBDTT-BTAZT-2 based device also reaches 6.16% with the NMP additive and PDINO-modified cathode interlayer. [ABSTRACT FROM AUTHOR]

We report here the discovery of a unique room-temperature water-vapor annealing (WVA) method to fabricate high performance planar heterojunction perovskite solar cells (pero-SCs) based on CH3NH3PbI3. High crystallinity and void-free perovskite films were obtained by the WVA treatment at room temperature without thermal annealing (TA) and the power conversion efficiency (PCE) of the pero-SCs reached 16.4% with a high open circuit voltage (Voc) of 1 V. A mechanism is proposed that the water vapor molecules catalyzed the dissolution and re-crystallization of the perovskite at the grain edges and void area, resulting in void-free and large sized crystals in the perovskite films and the improved photovoltaic performance. This WVA approach provides a simple, high-efficiency and energy conservation processing method in the fabrication of planar perovskite solar cells towards commercialization. [ABSTRACT FROM AUTHOR]

An alternating copolymer (P(IDT-NDI)) containing indacenodithienothiophene (IDT) and naphthalene diimide (NDI) units was synthesized for application as an acceptor material in all-polymer solar cells (all-PSCs). The polymer possesses a low bandgap of 1.51 eV, a suitable LUMO level of −3.84 eV and a HOMO level of −5.75 eV for use as an acceptor material instead of PCBM. Three conjugated polymers including J50 and J51 with a medium bandgap (ca. 1.9 eV) and PTB7-Th with a low bandgap (1.59 eV) were selected as donor materials for the investigation of the photovoltaic performance of the nonfullerene acceptor P(IDT-NDI). The champion all-PSCs with P(IDT-NDI) as an acceptor demonstrated power conversion efficiencies of 3.63%, 4.12% and 5.33% for the polymer donors PTB7-Th, J50 and J51, respectively. The results indicate that the complementary absorption of the polymer donor with polymer acceptor is very important for high performance all-PSCs and P(IDT-NDI) is a promising polymer acceptor for all-PSCs. [ABSTRACT FROM AUTHOR]

A non-fullerene electron acceptor bearing a fused 10-heterocyclic ring (indacenodithiopheno-indacenodithiophene) with a narrow band gap (∼1.5 eV) was designed and synthesized. It possesses excellent planarity and enhanced effective conjugation length compared to previously reported fused-ring electron acceptors. When this acceptor was paired with PTB7-Th and applied in polymer solar cells, a power conversion efficiency of 6.5% was achieved with a high open circuit voltage of 0.94 V. More significantly, an energy loss as low as 0.59 eV and an external quantum efficiency as high as 63% were obtained simultaneously. [ABSTRACT FROM AUTHOR]

The synthesis of a series of A–π–D–π–A oligomers bearing coplanar electron-donating dithieno[3,2-b:2′,3′-d]silole (DTS) unit linked through bithiophene π-bridges with the electron-withdrawing alkyldicyanovinyl (alkyl-DCV) groups is described. This study demonstrates a systematic investigation of structure–property relationships in this type of oligomer and shows obvious benefits of alkyl-DCV groups as compared to the commonly used DCV ones, in terms of elaboration of high performance organic solar cells (OSCs). Considerable efforts have been made to improve the power conversion efficiency (PCE) of oligomer-based OSCs by diverse strategies including fine-tuning of the oligomer properties via variation of their terminal and central alkyl chains, blend morphology control via solvent vapor annealing (SVA) treatment, and surface modification via interfacial engineering. These efforts allowed achieving PCEs of up to 6.4% for DTS(Oct)2-(2T-DCV-Me)2 blended with PC70BM. Further morphological investigations demonstrated that the usage of SVA treatment indeed effectively results in increased absorption and ordering of the BHJ composite, with the only exception for the most soluble oligomer DTS(Oct)2-(2T-DCV-Hex)2. Besides, a detailed study analyzed the charge transport properties and recombination loss mechanisms for these oligomers. This study not only revealed the importance of integrated alkyl chain engineering on gaining morphological control for high performance OSCs, but also exhibited a clear correlation between molecular ordering and charge carrier mobility respective to carrier dynamics. These results outline a detailed strategy towards a rather complete characterization and optimization methodology for organic photovoltaic devices, thereby paving the way for researchers to easily find the performance parameters adapted for widespread applications. [ABSTRACT FROM AUTHOR]

Four benzodithiophene (BDT)–thiophene (T) copolymers with conjugated side chains containing electron-withdrawing ethylrhodanine acceptor units, PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, were designed and synthesized for investigating the effect of side chains on the physicochemical properties and photovoltaic performance of the conjugated polymers. All the four copolymers possess an identical conjugated backbone of alternative benzodithiophene–thiophene, but different side chains on BDT and thiophene units, respectively. Polymer solar cells (PSCs) with these polymers as donors and PC70BM as acceptors exhibit an initial power conversion efficiency (PCE) of 0.61% for PHDBDT-T-R, 2.32% for PEHBDT-T-R, 1.46% for PHDBDT-T-TR, and 2.36% for PEHBDT-T-TR. After the treatment with 3 vol% DIO additive and with methanol, the highest PCE was increased up to 1.01%, 4.04%, 3.47%, and 4.25% for PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, respectively, with significantly increased Jsc and FF. The effects of methanol treatment on the photovoltaic performance of the PSCs can be ascribed to the increased carrier transport, improved exciton dissociation and optimized phase separation of the active layer. This work indicates that side-chain engineering plays a key role in molecular structures and optoelectronic properties. [ABSTRACT FROM AUTHOR]

Four new D–A type copolymers, namely, PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT, were designed and synthesized to investigate the influence of conjugated side chain pattern on photovoltaic properties of conjugated polymers. All the four copolymers have an identical conjugated backbone comprising benzo[1,2-b:4,5-b′]dithiophene (BDT) donor unit and quinoxaline (Qx) acceptor unit, but with varying conjugated side chains, p-alkoxyphenyl or 2-alkylthienyl, attached to the donor and acceptor units, respectively. As evidenced by UV/Vis absorption spectra, electrochemical cyclic voltammetry, density functional theory (DFT), grazing incidence X-ray scattering (GIXS), transmission electron microscope (TEM) and photovoltaic measurements, the difference in conjugated side chain modulation led to totally different physicochemical properties. Among the four copolymers, PBDT-DFQX-TT exhibits the broadest absorption spectrum, the most close-packed structure as well as a finest fibril structure when blended with PC71BM. After systematic device optimization, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ) photovoltaic devices based on the blends of PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT with PC71BM achieved 3.96%, 6.08%, 6.54% and 7.68%, respectively. By systematic varying the side chains of the copolymers from all phenyl groups to all thienyl ones, PCEs was increased by 250% from 3.96% to 7.68%. To date, PBDT-DFQX-TT is one of a few Qx-based PSCs that exhibits PCE exceeding 7.5%, and the results suggest that simultaneously modulating the conjugated side chains on both donor and acceptor units of copolymers could be an effective strategy for constructing high performance photovoltaic copolymers. [ABSTRACT FROM AUTHOR]

An easy-accessible amine group functionalized fullerene derivative, DMAPA-C60, is explored as a cathode buffer layer (CBL) in polymer solar cells (PSCs) for our presently tested three different material systems, namely P3HT:PCBM, PBDTTT-C:PC70BM and PBDTTT-C-T:PC70BM. The power conversion efficiencies of the three systems with DMAPA-C60 as the CBL reach 3.88%, 6.29% and 7.42%, respectively, which are much higher than those of the corresponding PSCs with the Al-only cathode and even slightly higher than those of the corresponding Ca/Al devices of these systems. The DMAPA-C60 CBL also allows high work function metals (Ag, Cu, and Au) as cathodes. [ABSTRACT FROM AUTHOR]

A new copolymer of dithienosilole and dithienylthiazolo[5,4-d]thiazole with a hexyl side chain on the 4-position of thiophene units was designed and synthesized. Polymer solar cells based on the polymer as a donor and PC70BM as an acceptor demonstrated a power conversion efficiency up to 5.88% with a high fill factor of 71.6%. [ABSTRACT FROM AUTHOR]

A star-shaped oligothiophene based on triphenylamine as a core and 2-ethylhexyl cyanoacetate as end groups (S(TPA-3T-CA)) was synthesized. S(TPA-3T-CA) exhibited strong absorption and high hole mobility. Solution-processed solar cells based on S(TPA-3T-CA):PC(71)BM showed a power conversion efficiency of 3.60% and a fill factor of 0.56.