provide good solubility in organic solvents and processability, we attached branched alkyl substituents at the DTP-nitrogen (ethylhexyl (EH) 1 , 4 , octylnonyl (ON) 2 , 5 , hexyldecyl (HD) 3 , 6 ) and hexyl side chains to each thiophene unit in a regioregular fashion as a mimic of the well-known and frequently used regioregular poly(3-hexylthiophene). [ 26 ] Thereby, the hexyl chains were attached either at the outer ( 1 -3 ) or inner ( 4 -6 ) positions thus yielding three isomeric pairs, which only differ in the positioning of the side chains. As a result, excellent solubilities of ≥120 mg mL −1 in chloroform for molecules 5 and 6 have been obtained. First solar cell results of oligomer pair 1 and 4 have recently been published and we discovered a striking difference between the photovoltaic behavior of the two isomers. [ 16 ] Whereas the so far highest PCE of 4.8% for any DTP-based oligomer has been obtained for oligothiophene 1 and PC 61 BM in BHJSCs, isomer 4 showed a lower performance (0.8%), which we attributed to the different blend morphologies and phase separation probed by atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. [ 16 ] The detailed photovoltaic behavior, further optimization, and characterization of the photoactive layers of the whole series of soluble DTP-oligothiophenes 1 -6 is now described. Signifi cant improvement of the oligomer BHJSC performance was obtained by SVA, which was investigated by absorption spectroscopy, grazing incidence XRD (GI-XRD), and AFM. After extensive optimization, power conversion effi ciencies up to 6.1% and fi ll factors greater than 70% were achieved.

IntroductionSmall molecule organic solar cells (SMOSCs) have attracted extensive attention due to their well-defi ned molecular structures, high reproducibility, easy purifi cation as well as low Morphological modifi cation using solvent vapor annealing (SVA) provides a simple and widely used fabrication option for improving the power conversion effi ciencies of solution-processed bulk heterojunction (BHJ) small molecule solar cells. Previous reports on SVA have shown that this strategy infl uences the degree of donor/acceptor phase separation and also improves molecular donor ordering. A blend composed of a dithienopyrrole containing oligothiophene as donor (named UU07) and [6,6]-phenyl-C61-butyric acid methyl ester as acceptor is investigated with respect to SVA treatment to explore the dynamics of the BHJ evolution as a function of annealing time. A systematic study of the time dependence of morphology evolution clarifi es the fundamental mechanisms behind SVA and builds the structureproperty relation to the related device performance. The following two-stage mechanism is identifi ed: Initially, as SVA time increases, donor crystallinity is improved, along with enhanced domain purity resulting in improved charge transport properties and reduced recombination losses. However, further extending SVA time results in domains that are too large and a few large donor crystallites, depleting donor component in the mixed domain. Moreover, the larger domain microstructure suffers from enhanced recombination and overall lower bulk mobility. This not only reveals the importance of precisely controlling SVA time on gaining morphological control, but also provides a path toward rational optimization of device performance.

Semitransparent solar cells (SSCs) can open photovoltaic applications in many commercial areas, such as power‐generating windows and building integrated photovoltaics. This study successfully demonstrates solution‐processed small molecule SSCs with a conventional configuration for the presently tested material systems, namely BDTT‐S‐TR:PC70BM, N(Ph‐2T‐DCN‐Et)3:PC70BM, SMPV1:PC70BM, and UU07:PC60BM. The top transparent cathode coated through solution processes employs a highly transparent silver nanowire as electrode together with a combination interface bilayer of zinc oxide nanoparticles (ZnO) and a perylene diimide derivative (PDINO). This ZnO/PDINO bilayer not only serves as an effective cathode buffer layer but also acts as a protective film on top of the active layer. With this integrated contribution, this study achieves a power conversion efficiency (PCE) of 3.62% for fully solution‐processed SSCs based on BDTT‐S‐TR system. Furthermore, the other three systems with various colors exhibited the PCEs close to 3% as expected from simulations, demonstrate the practicality and versatility of this printed semitransparent device architecture for small mole­cule systems. This work amplifies the potential of small molecule solar cells for window integration.

Solvent vapor annealing (SVA) can strongly influence the morphology of oligomer:fullerene based organic solar cells.

Solvent vapor annealing transforms the morphology of blends by a three-stage mechanism.