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Small organic molecules have emerged as promising component materials for organic photovoltaic devices. Compared to most conjugated polymers for the same application, small molecules have unique morphologies that promote electronic processes relevant to organic solar cell (OSC) function that can be significantly different from those of conjugated polymeric materials. Here we investigate constituent loading-dependent effects on OSC morphology and function using the classical fullerene acceptor, PC61BM, in a heterojunction blend with the small molecule donor, NDT (thiophene-capped diketopyrrolopyrrole naphthodithiophene). The evolution of active layer morphology as well as exciton speciation and dynamics as a function of the PC61BM content are examined in combined structural studies using GIWAXS and spectroscopic studies using transient absorption spectroscopy. We observe evidence for three types of coexisting excitons and details of the interplay between them determines the yield of charge separated states that afford sustained device efficiency.

Processing additives are commonly used to optimize phase separation morphology and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) organic photovoltaics (OPVs), however their exact effects are not well understood. A bulk-heterojunction OPV system containing the model bithiophene imide-benzodithiophene copolymer (PBTIBDT):phenyl-C71-butyric-acid-methyl ester (PC71BM) exhibits a maximum PCE of 5.4% with addition of 3.0 vol% 1,8-diiodooctane (DIO). Here the effects of increasing DIO concentrations (0 → 5 vol%) on the PBTIBDT:PC71BM solutions and resulting thin films are studied by X-ray scattering methodologies and transient optical absorption spectroscopy with 0.1 ps time resolution. As the DIO concentration increases, the radius of gyration of the PC71BM aggregates falls from 17 A to 9 A in solution, and TEM indicates the formation of increasing PC71BM charge percolation pathways in the thin BHJ films. Increased PBTIBDT + PC71BM intermixing not only affects BHJ film charge transport, but also enhances the initial exciton splitting yield in the PBTIBDT cation population. In contrast, the hole carrier population (as represented by the polymer cation) detected several nanoseconds after the photoexcitation is greatest with 3.0 vol% DIO, agreeing well with the corresponding BHJ composition for maximum OPV short circuit current density (Jsc) and fill factor (FF). The increase in initial polymer cation yield with DIO concentration is attributed to enhanced donor–acceptor interfacial area while the increase in long-lived cation population is attributed to formation of a bicontinuous donor polymer–PC71BM acceptor network that promotes large spatial separation of free charges in the device active layer. These results demonstrate the importance of OPV function for the correct balance, as tuned by processing additives, between a high initial donor cation formation yield and high carrier transport efficiency with minimized charge recombination rate.

Improved understanding of fundamental structure–property relationships, particularly the effects of molecular shape and intermolecular packing on film morphology and active layer charge transport characteristics, enables more rational synthesis of new p-type small molecules. Here we investigate a series of small molecules consisting of an acene-based electron-rich core flanked by one or two electron-deficient diketopyrrolopyrrole (DPP) moieties. Through minor changes in the molecule structures, measurable variations in the crystal structure and sizable differences in macroscopic properties are achieved. The molecular symmetry as well as the conformation of the side chains affects the unit cell packing density and strength of the intermolecular electronic coupling in single crystals of all molecules in this series. The addition of a second DPP unit to the benzodithiophene (BDT) core increases molecular planarity leading to decreased reorganization energy, strong cofacial coupling, and moderate hole mobility (2.7 × 10−4 cm2 V−1 s−1). Increasing the length of the acene core from benzodithiophene to naphthodithiophene (NDT) results in a further reduction in reorganization energy and formation of smaller crystalline domains (∼11 nm) when mixed with PCBM. Decreasing the aspect ratio of the core using a “zig-zag” naphthodithiophene (zNDT) isomer results in the highest hole mobility of 1.3 × 10−3 cm2 V−1 s−1 due in part to tight lamellar (d = 13.5 A) and π–π stacking (d = 3.9 A). The hole mobility is directly correlated with the short-circuit current (11.7 mA cm−2) and solar cell efficiency (4.4%) of the highest performing zNDT:PCBM device. For each of these small molecules the calculated π-coupling constant is correlated with the hole mobility as a function of crystal structure and orientation indicating the importance of designing molecules that create extended crystalline networks with maximal π-orbital overlap.

As effective building blocks for high-mobility transistor polymers, oligothiophenes are receiving attention for polymer solar cells (PSCs) because the resulting polymers can effectively suppress charge recombination. Here we investigate two series of in-chain donor-acceptor copolymers, PTPDnT and PBTInT, based on thieno[3,4-c]pyrrole-4,6-dione (TPD) or bithiopheneimide (BTI) as electron acceptor units, respectively, and oligothiophenes (nTs) as donor counits, for high-performance PSCs. Intramolecular S···O interaction leads to more planar TPD polymer backbones, however backbone torsion yields greater open-circuit voltages for BTI polymers. Thiophene addition progressively raises polymer HOMOs but marginally affects their band gaps. FT-Raman spectra indicate that PTPDnT and PBTInT conjugation lengths scale with nT catenation up to n = 3 and then saturate for longer oligomer. Furthermore, the effects of oligothiophene alkylation position are explored, revealing that the alkylation pattern greatly affects film morphology and PSC performance. The 3T with "outward" alkylation in PTPD3T and PBTI3T affords optimal π-conjugation, close stacking, long-range order, and high hole mobilities (0.1 cm(2)/(V s)). These characteristics contribute to the exceptional ∼80% fill factors for PTPD3T-based PSCs with PCE = 7.7%. The results demonstrate that 3T is the optimal donor unit among nTs (n = 1-4) for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering, transmission electron microscopy, and time-resolved microwave conductivity measurements reveal that the terthiophene-based PTPD3T blend maintains high crystallinity with appreciable local mobility and long charge carrier lifetime. These results provide fundamental materials structure-device performance correlations and suggest guidelines for designing oligothiophene-based polymers with optimal thiophene catenation and appropriate alkylation pattern to maximize PSC performance.

Zinc oxide is a promising candidate as an interfacial layer (IFL) in inverted organic photovoltaic (OPV) cells due to the n-type semiconducting properties as well as chemical and environmental stability. Such ZnO layers collect electrons at the transparent electrode, typically indium tin oxide (ITO). However, the significant resistivity of ZnO IFLs and an energetic mismatch between the ZnO and the ITO layers hinder optimum charge collection. Here we report that inserting nanoscopic copper hexadecafluorophthalocyanine (F(16)CuPc) layers, as thin films or nanowires, between the ITO anode and the ZnO IFL increases OPV performance by enhancing interfacial electron transport. In inverted P3HT:PC(61)BM cells, insertion of F(16)CuPc nanowires increases the short circuit current density (J(sc)) versus cells with only ZnO layers, yielding an enhanced power conversion efficiency (PCE) of ∼3.6% vs ∼3.0% for a control without the nanowire layer. Similar effects are observed for inverted PTB7:PC(71)BM cells where the PCE is increased from 8.1% to 8.6%. X-ray scattering, optical, and electrical measurements indicate that the performance enhancement is ascribable to both favorable alignment of the nanowire π-π stacking axes parallel to the photocurrent flow and to the increased interfacial layer-active layer contact area. These findings identify a promising strategy to enhance inverted OPV performance by inserting anisotropic nanostructures with π-π stacking aligned in the photocurrent flow direction.

Rational creation of polymeric semiconductors from novel building blocks is critical to polymer solar cell (PSC) development. We report a new series of bithiopheneimide-based donor-acceptor copolymers for bulk-heterojunction (BHJ) PSCs. The bithiopheneimide electron-deficiency compresses polymer bandgaps and lowers the HOMOs--essential to maximize power conversion efficiency (PCE). While the dithiophene bridge progression R(2)Si→R(2)Ge minimally impacts bandgaps, it substantially alters the HOMO energies. Furthermore, imide N-substituent variation has negligible impact on polymer opto-electrical properties, but greatly affects solubility and microstructure. Grazing incidence wide-angle X-ray scattering (GIWAXS) indicates that branched N-alkyl substituents increased polymer π-π spacings vs linear N-alkyl substituents, and the dithienosilole-based PBTISi series exhibits more ordered packing than the dithienogermole-based PBTIGe analogues. Further insights into structure-property-device performance correlations are provided by a thieno[3,4-c]pyrrole-4,6-dione (TPD)-dithienosilole copolymer PTPDSi. DFT computation and optical spectroscopy show that the TPD-based polymers achieve greater subunit-subunit coplanarity via intramolecular (thienyl)S···O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar ordering, but closer π-π spacing than does the TPD-based analogue. Inverted BHJ solar cells using bithiopheneimide-based polymer as donor and PC(71)BM as acceptor exhibit promising device performance with PCEs up to 6.41% and V(oc)0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher V(oc) with an enhanced PCE of 6.83%, mainly attributable to the lower-lying HOMO induced by the higher imide group density. These results demonstrate the potential of BTI-based polymers for high-performance solar cells, and provide generalizable insights into structure-property relationships in TPD, BTI, and related polymer semiconductors.

Processing additives are used in organic photovoltaic systems to optimize the active layer film morphology. However, the actual mechanism is not well understood. Using X-ray scattering techniques, we analyze the effects of an additive diiodooctane (DIO) on the aggregation of a high-efficiency donor polymer PTB7 and an acceptor molecule PC(71)BM under solar cell processing conditions. We conclude that DIO selectively dissolves PC(71)BM aggregates, allowing their intercalation into PTB7 domains, thereby optimizing both the domain size and the PTB7-PC(71)BM interface.

The influence of solubilizing substituents on the photovoltaic performance and thin-film blend morphology of new benzo[1,2-b:6,5-b']dithiophene (bBDT) based small molecule donor semiconductors is investigated. Solar cells based on bBDT(TDPP)2-PC71BM with two different types of side chains exhibit high power conversion efficiencies, up to 5.53%.