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To increase the range of light absorption in polymer solar cells, two absorber donor polymers (blue and red, respectively) are utilized. In article 1604603, H. Ade and co-workers mix these polymers separately with a fullerene acceptor (grey) and process them using a new deposition strategy to create sequentially cast ternary (SeCaT) polymer solar cells that exhibit a bilayer structure without an interlayer.

The morphology, photophysics, and device performance of solar cells based on the low bandgap polymer poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl (PBDTTT-EFT) (also known as PTB7-Th) blended with different fullerene acceptors: Phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), phenyl-C₇₁ -butyric acid methyl ester (PC₇₁BM), or indene-C₆₀ bisadduct (ICBA) are correlated. Compared to PC₇₁ BM-based cells – which achieve a power conversion efficiency (PCE) of 9.4% – cells using ICBA achieve a higher open-circuit voltage (V_OC) of 1.0 V albeit with a lower PCE of 7.1%. To understand the origin of this lower PCE, the morphology and photophysics have been thoroughly characterized. Hard and soft X-ray scattering measurements reveal that the PBDTTT-EFT:ICBA blend has a lower crystallinity, lower domain purity, and smaller domain size compared to the PBDTTT-EFT:PC₇₁BM blend. Incomplete photoluminescence quenching is also found in the ICBA blend with transient absorption measurements showing faster recombination dynamics at short timescales. Transient photovoltage measurements highlight further differences in recombination at longer timeframes due to the more intermixed morphology of the ICBA blend. Interestingly, a mild thermal treatment improves the performance of PBDTTT-EFT:ICBA cells which is exploited in the fabrication of a homo PBDTTT-EFT:ICBA tandem solar cell with PCE of 9.0% and V_OC of 1.93 V.

The mixing behavior of a high-efficiency polymer with various fullerene derivatives is investigated. Compared to solar cells using either PC₆₁BM or PC₇₁ BM as acceptor, cells using ICBA have a lower efficiency due to a more intermixed morphology. ICBA blends, however, show a higher thermal stability which is exploited in the fabrication of homo-tandem cells with 9.0% power conversion efficiency.

Nonfullerene polymer solar cells (PSCs) are fabricated with a perylene monoimide-based n-type wide-bandgap organic semiconductor PMI-F-PMI as an acceptor and a bithienyl-benzodithiophene-based wide-bandgap copolymer PTZ1 as a donor. The PSCs based on PTZ1:PMI-F-PMI (2:1, w/w) with the treatment of a mixed solvent additive of 0.5% N-methyl pyrrolidone and 0.5% diphenyl ether demonstrate a very high open-circuit voltage (V_oc) of 1.3 V with a higher power conversion efficiency (PCE) of 6%. The high V_oc of the PSCs is a result of the high-lying lowest unoccupied molecular orbital (LUMO) of −3.42 eV of the PMI-F-PMI acceptor and the low-lying highest occupied molecular orbital (HOMO) of −5.31 eV of the polymer donor. Very interestingly, the exciton dissociation efficiency in the active layer is quite high, even though the LUMO and HOMO energy differences between the donor and acceptor materials are as small as ≈0.08 and 0.19 eV, respectively. The PCE of 6% is the highest for the PSCs with a V_oc as high as 1.3 V. The results indicate that the active layer based on PTZ1/PMI-F-PMI can be used as the front layer in tandem PSCs for achieving high V_oc over 2 V.

Nonfullerene polymer solar cells with PTZ1 as a donor and PMI-F-PMI as an acceptor demonstrate a very high open-circuit voltage (V_oc) of 1.3 V with a higher power conversion efficiency of 6%. The high V_oc is a result of the high-lying lowest unoccupied molecular orbital (LUMO) of −3.42 eV of the PMI-F-PMI acceptor and the low-lying highest occupied molecular orbital (HOMO) of −5.31 eV of the polymer donor.

A new, easy, and efficient approach is reported to enhance the driving force for charge transfer, break tradeoff between open-circuit voltage and short-circuit current, and simultaneously achieve very small energy loss (0.55 eV), very high open-circuit voltage (>1 V), and very high efficiency (>10%) in fullerene-free organic solar cells via an energy driver.

The charge transport and microstructural properties of five different molecular weight (MW) batches of the naphthalenediimide-thiophene copolymer P(NDI2OD-T2) are investigated. In particular, the field-effect transistor (FET) performance and thin-film microstructure of samples with MW varying from M_n = 10 to 41 kDa are studied. Unlike conventional semiconducting polymers such as poly(3-hexylthiophene) where FET mobility dramatically drops with decreasing molecular weight, the FET mobility of P(NDI2OD-T2)-based transistors processed from 1,2-dichlorobenzene is found to increase with decreasing MW. Using a combination of grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, atomic force microscopy, and resonant soft X-ray scattering, the increase in FET mobility with decreasing MW is attributed to the pronounced increase in the orientational correlation length (OCL) with decreasing MW. In particular, the OCL is observed to systematically increase from <100 nm for the highest MW samples to ≈1 µm for the lowest MW samples. The improvement in OCL and hence mobility for low MW samples is attributed to the lack of aggregation of low MW chains in solution promoting backbone ordering, with the pre-aggregation of chains in 1,2-dichlorobenzene found to suppress longer-range liquid crystalline order.

The saturation mobility of transistors based on thin films of the polymer P(NDI2OD-T2) is found to systematically increase as molecular weight is decreased from 41 to 10 kDa. This unusual observation is explained by shorter chains being able to better form extended orientationally correlated domains, with the aggregation of longer chains in the solvent 1,2-dichlorobenzene suppressing correlated orienting.

In organic solar cells (OSCs), the energy of the charge-transfer (CT) complexes at the donor–acceptor interface, E _CT, determines the maximum open-circuit voltage (V _OC). The coexistence of phases with different degrees of order in the donor or the acceptor, as in blends of semi-crystalline donors and fullerenes in bulk heterojunction layers, influences the distribution of CT states and the V _OC enormously. Yet, the question of how structural heterogeneities alter CT states and the V _OC is seldom addressed systematically. In this work, we combine experimental measurements of vacuum-deposited rubrene/C₆₀ bilayer OSCs, with varying microstructure and texture, with density functional theory calculations to determine how relative molecular orientations and extents of structural order influence E _CT and V _OC. We find that varying the microstructure of rubrene gives rise to CT bands with varying energies. The CT band that originates from crystalline rubrene lies up to ≈0.4 eV lower in energy compared to the one that arises from amorphous rubrene. These low-lying CT states contribute strongly to V _OC losses and result mainly from hole delocalization in aggregated rubrene. This work points to the importance of realizing interfacial structural control that prevents the formation of low E _CT configurations and maximizes V _OC.

In organic solar cells, the coexistence of donor phases with different degrees of order influences the distribution of charge-transfer (CT) states. Abrupt interfaces between aggregated donor and acceptor phases cause voltage loss as high as ≈300 mV. Interfacial mixing resulting in local disorder between aggregated phases should eliminate the low energy CT states and improve the open-circuit voltage.

A variety of charge extraction (CE) techniques have been developed to measure charge density and recombination coefficients in bulk heterojunction solar cells. Charge recombination during charge extraction as a major limitation of this method has not been systematically quantified. This study reports CE measurements using a newly designed fast switch, which enables the application of a reverse bias to the solar cells facilitating charge extraction. With applied reverse bias, more than 40% increase in the extracted charge is obtained in solar cells with thicker active layers or with fast recombination. The measured charge carrier lifetime increases by up to a factor of three at sufficiently high applied biases (up to 8 V), suggesting significant errors in CE measurements without applied bias. The increased extracted charges with increasing applied bias are attributed to a combination of three cases: (i) slightly faster charge extraction due to the larger electric field; (ii) increased charge extraction rate at high light intensities when the transients are space charge disturbed; (iii) increased charge separated lifetime during charge extraction attributed to the spatial separation of the electron and hole density due to the applied electric field.

Reverse bias during charge extraction is applied through a fast switch to bulk heterojunction solar cells. More than 40% increase in the extracted charge is seen and the recombination lifetime obtained from the relation between the extracted charge and switching delay time is increased up to three times, showing the importance of reverse bias to measure lifetime.

Over the last 5 years, research on the synthesis, device engineering, and device physics of solution-processed small molecule solar cells (SMSCs) has rapidly expanded. Improvements in molecular design and emergent device processing techniques have helped solution-processed SMSCs overcome earlier difficulties in controlling active layer morphology, such that many systems are now at—or approaching—10% power conversion efficiency. In this review, details of the highest performing blend systems are presented in order to identify key trends and provide perspective on current progress in the field. Among the best systems, a planarized molecular structure is prevalent, which can be achieved using large fused-ring moieties, intermolecular non-bonding interactions, and side chain engineering. To obtain efficient devices, the highest performing systems have been optimized through the careful combination of thermal and solvent annealing procedures. Even without additional processing, some systems have been able to obtain interconnected morphologies and efficient charge generation and charge transport. Ultimately, the design of more efficient materials also requires additional understanding of the device physics and loss mechanisms. After highlighting what is known to date on processes limiting device efficiency, an outlook on the most important challenges remaining to the field is provided.

Bulk heterojunction photovoltaics based on small molecule donors have greatly improved their performance thanks to a growing multidisciplinary effort. This review presents recent advances in molecular design, control and characterization of bulk heterojunction film morphology, and understandings of charge generation and recombination in these devices. The review is concluded with a discussion of questions that need further research to push small molecule photovoltaics higher.

Charge transport in organic photovoltaic (OPV) devices is often characterized by space-charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi-equilibrium energies in the disorder-broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time-resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time-resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum-power point (short-circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.

Detailed comparison of steady-state and ultrafast time-resolved experiments reveals that photogenerated carrier mobility in organic solar cells is significantly higher than that probed by space-charge limited currents (SCLCs). The SCLC method is only valid for materials with a sufficiently low energetic disorder, where photogenerated carrier thermalization can be neglected. The over-simplified use of quasi-equilibrium mobility data in literature requires re-evaluation.

Focusing on the bottleneck of molecularly engineered organic semiconductors, a breakthrough is made to tune the electronic properties of organic semiconductors from p-type to n-type by using fluorinated metal phthalocyanines as examples. The experimentally observed p-type to n-type transition characteristics of single-crystal field-effect devices result from a combination of extrinsic and intrinsic properties of materials with different fluoridation substitution.

A novel wide-gap conjugated polymer PhF2,5 (E_g = 1.9 eV) is designed to contain alternating cyclopentadithiophene and difluorophenylene unit with the goal of favoring unipolar organic field effect transistor characteristics. The higher lowest unoccupied molecular orbital energy of PhF2,5 increases the barrier to electron injection, leading to unipolar transport and higher on/off ratios, without sacrificing desirable high hole mobilities.

In article 1604044, T. Wang and co-workers report compositional and surface modifications to low-temperature-processed TiO₂ films as electron transport layers in inverted polymer solar cells. This approach not only increases the power conversion efficiency of photovoltaic devices to 10.5%, but more importantly, eliminates the light-soaking problem that is commonly observed in polymer solar cells employing metal oxides as the charge-transport layers.

The device efficiency of polymer:fullerene bulk heterojunction solar cells has recently surpassed 11%, as a result of synergistic efforts among chemists, physicists, and engineers. Since polymers are unequivocally the “heart” of this emerging technology, their design and synthesis have consistently played the key role in the device efficiency enhancement. In this article, the first focus is a discussion on molecular engineering (e.g., backbone, side chains, and substituents), then the discussion moves on to polymer engineering (e.g., molecular weight). Examples are primarily selected from the authors contributions; yet other significant discoveries/developments are also included to put the discussion in a broader context. Given that the synthesis, morphology, and device physics are inherently related in explaining the measured device output parameters (J_sc, V_oc and FF), we will attempt to apply an integrated and comprehensive approach (synthesis, morphology, and device physics) to elucidate the fundamental, underlying principles that govern the device characteristics, in particular, in the context of disclosing structure-property correlations. Such correlations are crucial to the design and synthesis of next generation materials to further improve the device efficiency.

Recent progress (2012–2016) in polymer:fullerene bulk-heterojunction solar cells is reviewed. The intrinsic complexity of such solar cells urges the community to apply an integrated and comprehensive approach – including synthesis, morphology, and device physics – to elucidate the fundamental underlying principles that govern the device performance, in particular, in the context of disclosing structure–property correlations.

Efficient ternary polymer solar cells are constructed by incorporating an electron-deficient chromophore (5Z,5′Z)-5,5′-((7,7′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(6-fluorobenzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (IFBR) as an additional component into the bulk-heterojunction film that consists of a wide-bandgap conjugated benzodithiophene-alt-difluorobenzo[1,2,3]triazole based copolymer and a fullerene acceptor. With respect to the binary blend films, the incorporation of a certain amount of IFBR leads to simultaneously enhanced absorption coefficient, obviously extended absorption band, and improved open-circuit voltage. Of particular interest is that devices based on ternary blend film exhibit much higher short-circuit current densities than the binary counterparts, which can be attributed to the extended absorption profiles, enhanced absorption coefficient, favorable film morphology, as well as formation of cascade energy level alignment that is favorable for charge transfer. Further investigation indicates that the ternary blend device exhibits much shorter charge carrier extraction time, obviously reduced trap density and suppressed trap-assisted recombination, which is favorable for achieving high short-circuit current. The combination of these beneficial aspects leads to a significantly improved power conversion efficiency of 8.11% for the ternary device, which is much higher than those obtained from the binary counterparts. These findings demonstrate that IFBR can be a promising electron-accepting material for the construction of ternary blend films toward high-performance polymer solar cells.

A nonfullerene acceptor (5Z,5′Z)-5,5′-((7,7′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(6-fluorobenzo[c]-[1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) is introduced as an electron-cascade acceptor material in blends of a wide-bandgap donor poly(benzodithiophene-alt-difluorobenzo[1,2,3]triazole) and a fullerene acceptor [6,6]-phenyl-C₆₁-butyric acid methyl ester to fabricate ternary blend polymer solar cells (PSCs). The ternary device exhibits a power conversion efficiency of 8.11%, which is much higher than those obtained from the binary PSCs.

Zn(II)–porphyrin sensitizers, coded as SGT-020 and SGT-021, are designed and synthesized through donor structural engineering. The photovoltaic (PV) performances of SGT sensitizer-based dye-sensitized solar cells (DSSCs) are systematically evaluated in a thorough SM315 as a reference sensitizer. The effect of the donor ability and the donor bulkiness on photovoltaic performances is investigated for establishing the structure–performance relationship in the platform of porphyrin-triple bond-benzothiadiazole-acceptor sensitizers. By introducing a more bulky fluorene unit to the amine group in the SM315, the power conversion efficiency (PCE) is enhanced with the increased short-circuit current (J_sc) and open-circuit voltage (V_oc), due to the improved light-harvesting ability and the efficient prevention of charge recombination, respectively. As a consequence, a maximum PCE of 12.11% is obtained for SGT-021, whose PCE is much higher than the 11.70% PCE for SM315. To further improve their maximum efficiency, the first parallel tandem DSSCs employing cobalt electrolyte in the top and bottom cells are demonstrated and an extremely high efficiency of 14% is achieved, which is currently the highest reported value for tandem DSSCs. The series tandem DSSCs give a remarkably high V_oc value of >1.83 V. From this DSSC tandem configuration, 7.4% applied bias photon-to-current efficiency is achieved for solar water splitting.

The effect of the donor ability and bulkiness on DSSC photovoltaic performances is investigated to exceed a world champion porphyrin. The first parallel tandem DSSCs employing cobalt electrolyte are demonstrated with an extremely high efficiency of 14%. The series tandem DSSCs give a remarkably high V_oc value of >1.83 V, achieving 7.4% applied bias photon-to-current efficiency for solar water splitting.

The clinical application of cell-based therapy is severely hampered by restricted cell survival and poor understanding of cell fate after transplantation. C. Shi and co-workers describe a novel near-infrared mitochondria-targeting heptamethine dye (NIRCP-61) on page 8397, developed to simultaneously enable cell tracking and cytoprotection in cytotherapy. This small-molecule fluorophore maintains superior fluorescent imaging properties while inducing cytoprotective effects against oxidative stress to augment tissue repair and regeneration.

Conjugated polymer semiconductors P1 and P2 with bithienopyrroledione (bi-TPD) as acceptor unit are synthesized. Their transistor and photovoltaic performances are investigated. Both polymers display high and balanced ambipolar transport behaviors in thin-film transistors. P1-based devices show an electron mobility of 1.02 cm² V⁻¹ s⁻¹ and a hole mobility of 0.33 cm² V⁻¹ s⁻¹, one of the highest performance reported for ambipolar polymer transistors. The electron and hole mobilities of P2 transistors are 0.36 and 0.16 cm² V⁻¹ s⁻¹, respectively. The solar cells with PC₇₁BM as the electron acceptor and P1/P2 as the donor exhibit a high V_oc about 1.0 V, and a power conversion efficiency of 6.46% is observed for P1-based devices without any additives and/or post treatment. The high performance of P1 and P2 is attributed to their crystalline films and short π–π stacking distance (<3.5 Å). These results demonstrate (1) bi-TPD is an excellent versatile electron-deficient unit for polymer semiconductors and (2) bi-TPD-based polymer semiconductors have potential applications in organic transistors and organic solar cells.

Versatile polymer semiconductors containing bithienopyrroledione unit are designed and synthesized. They exhibit balanced ambipolar transport behaviors with an electron mobility of 1.02 cm² V⁻¹ s⁻¹ and a hole mobility of 0.33 cm² V⁻¹ s⁻¹ in thin-film transistors, and photovoltaic properties with a power conversion efficiency of 6.46% and a high V_oc of 1.02 V in polymer solar cells.