Solution-Processed Zinc Oxide/Polyethylenimine Nanocomposites as Tunable Electron Transport Layers for Highly Efficient Bulk Heterojunction Polymer Solar Cells

In this study, we employed polyethylenimine-doped sol–gel-processed zinc oxide composites (ZnO:PEI) as efficient electron transport layers (ETL) for facilitating electron extraction in inverted polymer solar cells. Using ultraviolet photoelectron spectroscopy, synchrotron grazing-incidence small-angle X-ray scattering and transmission electron microscopy, we observed that ZnO:PEI composite films’ energy bands could be tuned considerably by varying the content of PEI up to 7 wt %the conduction band ranged from 4.32 to 4.0 eVand the structural order of ZnO in the ZnO:PEI thin films would be enhanced to align perpendicular to the ITO electrode, particularly at 7 wt % PEI, facilitating electron transport vertically. We then prepared two types of bulk heterojunction systemsbased on poly­(3-hexylthiophene) (P3HT):phenyl-C₆₁-butryric acid methyl ester (PC₆₁BM) and benzo­[1,2-b:4,5-<i>b</i>́]­dithiophene-thiophene-2,1,3-benzooxadiazole (PBDTTBO):phenyl-C₇₁-butryric acid methyl ester (PC₇₁BM)that incorporated the ZnO:PEI composite layers. When using a composite of ZnO:PEI (93:7, w/w) as the ETL, the power conversion efficiency (PCE) of the P3HT:PC₆₁BM (1:1, w/w) device improved to 4.6% from a value of 3.7% for the corresponding device that incorporated pristine ZnO as the ETLa relative increase of 24%. For the PBDTTBO:PC₇₁BM (1:2, w/w) device featuring the same amount of PEI blended in the ETL, the PCE improved to 8.7% from a value of 7.3% for the corresponding device that featured pure ZnO as its ETLa relative increase of 20%. Accordingly, ZnO:PEI composites can be effective ETLs within organic photovoltaics

Location and Number of Selenium Atoms in Two-Dimensional Conjugated Polymers Affect Their Band-Gap Energies and Photovoltaic Performance

We synthesized and characterized a series of novel two-dimensional Se-atom-substituted donor (D)−π-acceptor (A) conjugated polymersPBDTTTBO, PBDTTTBS, PBDTTSBO, PBDTSTBO, PBDTTSBS, PBDTSTBS, PBDTSSBO, and PBDTSSBSfeaturing benzodithiophene (BDT) as the donor, thiophene (T) as the π-bridge, and 2,1,3-benzooxadiazole (BO) as the acceptor with different number of Se atoms at different π-conjugated locations, including the π-bridge, side chain, and electron-withdrawing units. We then systematically investigated the effect of different locations and the number of Se atoms in these two-dimensional conjugated polymers on the structural, optical, and electronics such as band-gap energies of the resulting polymers, as determined through quantum-chemical calculations, UV–vis absorption spectra, and grazing-incidence X-ray diffraction. We found that through the rational structural modification of the 2-D conjugated Se-substituted polymers the resulting PCEs could vary over 3-fold (from 2.4 to 7.6%), highlighting the importance of careful selection of appropriate chemical structures such as the location of Se atoms when designing efficient D−π-A polymers for use in solar cells. Among these tested BO-containing polymers, PBDTSTBO that has moderate band gaps and good open-circuit voltages (up to 0.86 V) when mixed with PC₇₁BM (1:2, w/w) provided the highest power conversion efficiency (7.6%) in a single-junction polymer solar cell, suggesting that these polymers have potential applicability as donor materials in the bulk heterojunction polymer solar cells

Distribution of Crystalline Polymer and Fullerene Clusters in Both Horizontal and Vertical Directions of High-Efficiency Bulk Heterojunction Solar Cells

In this study, we used (i) synchrotron grazing-incidence small-/wide-angle X-ray scattering to elucidate the crystallinity of the polymer PBTC₁₂TPD and the sizes of the clusters of the fullerenes PC₆₁BM and ThC₆₁BM and (ii) transmission electron microscopy/electron energy loss spectroscopy to decipher both horizontal and vertical distributions of fullerenes in PBTC₁₂TPD/fullerene films processed with chloroform, chlorobenzene and dichlorobezene. We found that the crystallinity of the polymer and the sizes along with the distributions of the fullerene clusters were critically dependent on the solubility of the polymer in the processing solvent when the solubility of fullerenes is much higher than that of the polymer in the solvent. In particular, with chloroform (CF) as the processing solvent, the polymer and fullerene units in the PBTC₁₂TPD/ThC₆₁BM layer not only give rise to higher crystallinity and a more uniform and finer fullerene cluster dispersion but also formed nanometer scale interpenetrating network structures and presented a gradient in the distribution of the fullerene clusters and polymer, with a higher polymer density near the anode and a higher fullerene density near the cathode. As a result of combined contributions from the enhanced polymer crystallinity, finer and more uniform fullerene dispersion and gradient distributions, both the short current density and the fill factor for the device incorporating the CF-processed active layer increase substantially over that of the device incorporating a dichlorobenzene-processed active layer; the resulting power conversion efficiency of the device incorporating the CF-processed active layer was enhanced by 46% relative to that of the device incorporating a dichlorobenzene-processed active layer

Fluorene Conjugated Polymer/Nickel Oxide Nanocomposite Hole Transport Layer Enhances the Efficiency of Organic Photovoltaic Devices

A nanocomposite layer comprising the conjugated polymer poly­[(9,9-bis­(3′-(<i>N</i>,<i>N</i>-dimethylamino)­propyl)-2,7-fluorene)-<i>alt</i>-2,7-(9,9-dioctyl)­fluorene] (PFN) and nickel oxide (NiO_<i>x</i>) has been employed as the hole transport layer (HTL) in organic photovoltaics (OPVs) featuring PBDTTBO-C₈ and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the active layer. The optimal device incorporating the PFN:NiO_<i>x</i> nanocomposite as the HTLs displayed a power conversion efficiency (PCE) to 6.2%, up from 4.5% for the corresponding device incorporating pristine NiO_<i>x</i> as the HTL layer: a nearly 40% improvement in PCE. X-ray photoelectron spectroscopy (XPS) was used to determine the types of chemical bonding, ultraviolet photoelectron spectroscopy (UPS) to measure the change in work function, and atomic force microscopy (AFM) to examine the morphology of the composite layers. The growth of nickel trioxide, Ni₂O₃, in the PFN:NiO_<i>x</i> layer played a key role in producing the p-doping effect and in tuning the work function, thereby improving the overall device performance

Symmetry and Coplanarity of Organic Molecules Affect their Packing and Photovoltaic Properties in Solution-Processed Solar Cells

In this study we synthesized three acceptor–donor–acceptor (A–D–A) organic molecules, <b>TB3t-BT</b>, <b>TB3t-BTT</b>, and <b>TB3t-BDT</b>, comprising 2,2′-bithiophene (BT), benzo­[1,2-b:3,4-b′:5,6-d″]­trithiophene (BTT), and benzo­[1,2-b;4,5-b′]­dithiophene (BDT) units, respectively, as central cores (donors), terthiophene (3t) as π-conjugated spacers, and thiobarbituric acid (TB) units as acceptors. These molecules display different degrees of coplanarity as evidenced by the differences in dihedral angles calculated from density functional theory. By using differential scanning calorimetry and X-ray diffractions for probing their crystallization characteristics and molecular packing in active layers, we found that the symmetry and coplanarity of molecules would significantly affect the melting/crystallization behavior and the formation of crystalline domains in the blend film with fullerene, PC₆₁BM. <b>TB3t-BT</b> and <b>TB3t-BDT</b>, which each possess an inversion center and display high crystallinity in their pristine state, but they have different driving forces in crystallization, presumably because of different degrees of coplanarity. On the other hand, the asymmetrical <b>TB3t-BTT</b> behaved as an amorphous material even though it possesses a coplanar structure. Among our tested systems, the device comprising as-spun <b>TB3t-BDT</b>/PC₆₁BM (6:4, w/w) active layer featured crystalline domains and displayed the highest power conversion efficiency (PCE) of 4.1%. In contrast, the as-spun <b>TB3t-BT</b>/PC₆₁BM (6:4, w/w) active layer showed well-mixed morphology and with a device PCE of 0.2%; it increased to 3.9% after annealing the active layer at 150 °C for 15 min. As for <b>TB3t-BTT</b>, it required a higher content of fullerene in the <b>TB3t-BTT</b>/PC₆₁BM (4:6, w/w) active layer to optimize its device PCE to 1.6%

Structural Evolution of Crystalline Conjugated Polymer/Fullerene Domains from Solution to the Solid State in the Presence and Absence of an Additive

The power conversion efficiencies of polymer/fullerene solar cells are critically dependent on the nanometer-scale morphologies of their active layers, which are typically processed from solution. Using synchrotron wide- and small-angle X-ray scattering, we have elucidated the intricate mechanism of the structural transitions from solutions to solid films of the crystalline polymer poly­[bis­(dodecyl)­thiophene-thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione] (PBTTPD) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), including the effect of the solvent additive 1,6-diiodohexane (DIH). We found that the local assembly of rigid-rod PBTTPD segments that formed in solution instantly and then relaxed within several hundred seconds upon cooling to room temperature from 90 °C could re-emerge and develop into seeds for subsequent crystallization of the polymer in the solid films. At room temperature (25 °C), the presence of DIH in chlorobenzene slightly enhanced the formation of local assembly PBTTPD segments in the supersaturated PBTTPD in PBTTPD/PC₇₁BM blend solution. Two cases of films were subsequently developed from these blend solutions with drop-casted and spin-coated methods. For spin-coated thin films (90 nm thick), which evolve quickly, polymer’s crystallinity and the fullerene packing in the solid-state thin films were enhanced in the case of involving DIH. Regarding the effect of DIH for processing the drop-casted thick films (2.5 μm thick), which evolve slowly, DIH has no observable effect on PBTTPD/PC₇₁BM structure. Our results provide some understanding of the mechanism behind the structural development of polymer/fullerene blends upon their transitions from solution to the solid state, as well as the key functions of the additive

Side Chain Structure Affects the Photovoltaic Performance of Two-Dimensional Conjugated Polymers

We used Stille coupling of electron-rich benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT) presenting conjugated alkylthiophene (T), alkylphenyl (P), or alkylfuran (F) side chains with electron-deficient alkoxy-modified 2,1,3-benzooxadiazole (BO) moieties to obtain a series of two-dimensional, conjugated, D−π–A polymers (PBDTTBO, PBDTPBO, and PBDTFBO). The side chains of the BDT units altered the solubility, conformations, and electronic properties of the synthesized conjugated polymers, allowing tuning of their photovoltaic properties when blended with fullerenes. Density functional theory calculations revealed that the presence of these side chain groups on the BDT donor units affected the torsion angles between the side chain groups and the conjugated main chains but resulted in only slightly different energy levels for the highest occupied molecular orbitals for these polymers, consistent with results obtained experimentally using cyclic voltammetry. These polymers displayed excellent thermal stabilities (5 wt % degradation temperatures: >330 °C) and broad spectral absorptions (from 450 to 700 nm). Transmission electron microscopy images revealed that the morphologies of active layers comprising these two-dimensional conjugated polymers and the fullerene derivative PC₇₁BM did, however, vary substantially depending on the structure of the side chains that affects the solubility of the polymers. As a result, the efficiencies of photovoltaic devices incorporating PBDTFBO, PBDTPBO, or PBDTTBO polymers and PC₇₁BM varied greatly, from 3.6 to 5.9%. When using 1-chloronaphthalene (1 vol %) or 1,8-diiodooctane (1 vol %) as an additive for processing the active layer, the power conversion efficiencies (PCEs) of photovoltaic devices incorporating blends of PBDTFBO, PBDTPBO, or PBDTTBO and PC₇₁BM (1:2) improved to 5.4, 6.4, and 7.4%, respectively, due to their optimized morphologies, with the PCE of 7.4% being among the highest values reported for conjugated polymers involving BO moieties. Thus, the photovoltaic properties of these conjugated polymers were highly tunable through slight modifications of their side chain structures