Charge and Triplet Exciton Generation in Neat PC70BM Films and Hybrid CuSCN:PC70BM Solar Cells

Organic solar cells that use only fullerenes as the photoactive material exhibit poor exciton‐to‐charge conversion efficiencies, resulting in low internal quantum efficiencies (IQE). However, the IQE can be greatly improved, when copper(I) thiocyanate (CuSCN) is used as a carrier‐selective interlayer between the phenyl‐C70‐butyric acid methyl ester (PC₇₀BM) layer and the anode. Efficiencies of ≈5.4% have recently been reported for optimized CuSCN:PC₇₀BM (1:3)‐mesostructured heterojunctions, yet the reasons causing the efficiency boost remain unclear. Here, transient absorption (TA) spectroscopy is used to demonstrate that CuSCN does not only act as a carrier‐selective electrode layer, but also facilitates fullerene exciton dissociation and hole transfer at the interface with PC₇₀BM. While intrinsic charge generation in neat PC₇₀BM films proceeds with low yield, hybrid films exhibit much improved exciton dissociation due to the presence of abundant interfaces. Triplet generation with a rate proportional to the product of singlet and charge concentrations is observed in neat PC₇₀BM films, implying a charge–singlet spin exchange mechanism, while in hybrid films, this mechanism is absent and triplet formation is a consequence of nongeminate recombination of free charges. At low carrier concentrations, the fraction of charges outweighs the population of triplets, leading to respectable device efficiencies under one sun illumination.     

Enhancing the Photovoltaic Performance via Vertical Phase Distribution Optimization in Small Molecule:PC71BM Blends

Bulk heterojunction (BHJ) morphologies are vital to the device performance of organic solar cells (OSCs), including phase separation in lateral and vertical directions. However, the morphology developed from the blend solution is not easily predicted and controlled, especially in the vertical direction, because the BHJ morphology is kinetically frozen during the rapid solvent evaporation process. Here, a simple approach to control BHJ morphologies with optimized phase distribution for small molecule:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁ BM) blends by enhancing the substrate temperature during the spin‐coating process. Three molecules with various fluorine atoms in the end acceptor units are selected. The relationship among molecular structures, substrate temperature effects on the morphology, and device performances are symmetrically investigated. Low temperature induces a multiple‐sublayer‐like architecture with significantly varied distributions of composition, morphology, and localized state energy, while high processing temperature induces more uniform film. The short‐circuit current, open‐circuit voltage, and fill factor of the devices are tuned with synergic improvement of efficiency toward over 10% and 11% for conventional and inverted devices. This work reveals the origination of vertical phase segregation, and provides a facile strategy to optimize the hierarchical phase separation for enhancing the performance of OSCs.     

The Effect of Fluorination in Manipulating the Nanomorphology in PTB7:PC71BM Bulk Heterojunction Systems

The best performing low bandgap copolymers PTB series to date which is based on thieno[3,4‐b]thiophene‐alt‐benzodithiophene units blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM), have been the focus of polymer‐based solar cells. Here, novel fluorinated polymers PTB7‐Fx (fluorine units coupled with submonomer thieno[3,4‐b]thiophene) with varied degree of fluorination are used as electron donor materials. The PTB7‐Fx:PC₇₁BM bulk heterojunction (BHJ) films spin‐coated from the host solvent chlorobenzene without and with solvent additive 1,8‐diiodooctane (DIO) and the corresponding solar cell devices are systematically investigated to address the morphology‐efficiency relationship. Self‐assembled BHJ morphology is already observed for as‐spun blend films. After adding the solvent additive DIO, the pronounced ordered structures are suppressed and better intermixed films with much smaller domain sizes result. Full fluorination of the third C‐atom of thienothiophene gives rise to the highest power conversion efficiency. As the absorption properties, film morphology and crystallinity remain similar for different degrees of fluorination, the main influence of the photovoltaic performance is ascribed to the different lowest unoccupied molecular orbital (LUMO) of each polymer instead of the film morphology. Thus the device performance can be efficiently improved by tuning the energy level of the polymer without necessarily changing either the film nanomorphology or crystallinity dramatically.     

On the Effect of Prevalent Carbazole Homocoupling Defects on the Photovoltaic Performance of PCDTBT:PC71BM Solar Cells

The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT (poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt ‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time‐dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents J_SC. With increasing Cbz hc content, J_SC decreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in J_SC alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling‐free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.     

Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71BM Solar Cells

The importance of morphology to organic solar cell performance is well known, but to date, the lack of quantitative, nanoscale and statistical morphological information has hindered obtaining direct links to device function. Here resonant X‐ray scattering and microscopy are combined to quantitatively measure the nanoscale domain size, distribution and composition in high efficiency solar cells based on PTB7 and PC₇₁BM. The results show that the solvent additive diiodooctane dramatically shrinks the domain size of pure fullerene agglomerates that are embedded in a polymer‐rich 70/30 wt.% molecularly mixed matrix, while preserving the domain composition relative to additive‐free devices. The fundamental miscibility between the species – measured to be equal to the device's matrix composition – is likely the dominant factor behind the overall morphology with the additive affecting the dispersion of excess fullerene. As even the molecular ordering measured by X‐ray diffraction is unchanged between the two processing routes the change in the distribution of domain size and therefore increased domain interface is primarily responsible for the dramatic increase in device performance. While fullerene exciton harvesting is clearly one significant cause of the increase owing to smaller domains, a measured increase in harvesting from the polymer species indicates that the molecular mixing is not the reason for the high efficiency in this system. Rather, excitations in the polymer likely require proximity to a pure fullerene phase for efficient charge separation and transport. Furthermore, in contrast to previous measurements on a PTB7‐based system, a hierarchical morphology was not observed, indicating that it is not necessary for high performance.     

Bioscience Methods(BM)

<正>Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by Bio Publisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience,the range of topics including(but are not     

Bioscience Methods(BM)

<正>Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by Bio Publisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience,the range of topics including(but are not limited to)technology review,technique know-how,lab tool,statistical software and known technology modification.Case studies on technologies for gene discovery and function     

Bioscience Methods(BM)

<正>Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by Bio Publisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience,the range of topics including(but are not limited to)technology review,technique know-how,lab tool,statistical software and known technology modification.Case studies on technologies for gene discovery and function validation as well as genetic transformation.The standard article processing charges(APC)collected by Bio Publisher is 1 200 CAD per article.Authors from low-income countries and     

Bioscience Methods(BM)

<正>Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by Bio Publisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience,the range of topics including(but are not limited to)technology review,technique know-how,lab tool,statistical software and known technology modification.Case studies on technologies for gene discovery and function     

Bioscience Methods(BM)

<正>Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by Bio Publisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience,the range of topics including(but are not limited to)technology review,technique know-how,lab tool,statistical software and known technology modification.Case studies on technologies for gene discovery and function validation as well as genetic transformation.The standard article processing charges(APC)     

Bioscience Methods(BM)

<正>Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by Bio Publisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience,the range of topics including(but are not limited to)technology review,technique know-how,lab tool,statistical software and known technology modification.Case studies on technologies for gene discovery and function     

Bioscience Methods(BM)

<正>Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by BioP ublisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience,the range of topics including(but are not limited to)technology review,technique know-how,lab tool,statistical software and known technology modification.Case studies on technologies for gene discovery and function     

Bioscience Methods(BM)

正Bioscience Methods(ISSN 1925-1920)is an open access,peer reviewed journal published online by Bio Publisher.The journal publishes all the latest and outstanding research articles,letters and reviews in all areas of bioscience.The range of topics including(but are not limited to)technology review,technique know-how,lab tool,statistical software and known