•Three isopropylphenyl substituted asymmetric PDI derivatives were reported.•Effects of isopropyl chain position of PDIs on property and PV performance were studied.•These PDIs showed similar LUMO energy levels to PC61BM.•The meta-substituted 3-iPP-PDIs showed the best performance in polymer solar cells.Perylenediimide derivatives (PDI) are among the most promising non-fullerene electron acceptor materials for use in organic solar cells. However, owing to the intensive intermolecular interactions, the non-functionalized PDI molecules showed high tendency of aggregation in solid film, which leads to poor device performance. In this paper molecular geometry of PDI derivatives was finely tuned by introducing a bulky isopropyl group on the bay-phenyl unit, and influences of such a bulky alkyl group on the optical and electrochemical properties were systematically studied. Results indicated that the bulky isopropyl group on the para- and meta-position of the bay-phenyl group has negligible influence on the twist angle between the PDI core and the bay-phenyl unit, and these two compounds (4-iPP-PDI and 3-iPP-PDI) have similar molecular properties. However, large steric hindrance of the ortho-isopropyl group causes a large twist between the PDI core and the bay-phenyl unit, which leads to conjugation break, and consequently to a blue-shifted absorption spectrum and an increased optical band gap for the final PDI compound (2-iPP-PDI). Polymer solar cells using these bay-phenyl functionalized PDIs as the electron acceptor were fabricated and tested. And the meta-substituted PDI compound 3-iPP-PDI show the better device performance than the para- and ortho-substituted compounds (4-iPP-PDI and 2-iPP-PDI), which was ascribed to the proper nano-scale phase separation and high electron mobility of the blended film. The current results proved that molecular geometry of PDI derivatives can be finely regulated through introducing bulky alkyl side chain on the bay-substitution group to achieve a balanced property of crystallinity and electron mobility.

•Large-scale, R2R micro-gravure printing was developed to process ZnO as the electron transport layer (ETL).•The R2R printed ZnO ETL was used to fabricate inverted organic solar cells (OSCs) on flexible substrate.•The inverted OSCs showed comparable performance parameters for using R2R printed and spin-coated ZnO ETLs.•The commercial large-scale, R2R micro-gravure printing process could be potentially used to produce efficient OSCs.Large-scale, roll-to-roll (R2R) micro-gravure printing process was developed to deposit the electron transport layer (ETL) using low-temperature, solution-processable zinc oxide (ZnO) nanoparticle ink on flexible substrate for fabricating inverted organic solar cells (OSCs). The properties of micro-gravure R2R printed ZnO thin film was optimized via using web tension, substrate pre-treatment and printing speed, leading to high-quality and thickness controllable ZnO thin films. The inverted OSCs using R2R micro-gravure printed ZnO thin film as the ETL showed performance parameters comparable to those of spin-coated ZnO thin film ETL on the flexible substrate in both P3HT:PCBM and PTB7-Th:PC71BM based devices. The research demonstrated that the potentially commercial large-scale, R2R micro-gravure printing process could be used to produce high-quality ZnO thin film with controllable thickness for efficient inverted OSCs, which would accelerate the development of fully R2R micro-gravure printing OSCs and their commercialization.Download high-res image (314KB)Download full-size image

Two small molecules named PI-DPP and NI-DPP with a DPP core as the central strong acceptor unit and phthalimide/naphthalimide as the terminal weak acceptor were designed and synthesized. The effects of terminal phthalimide/naphthalimide units on the thermal behavior, optical and electrochemical properties, as well as the photovoltaic performance of these two materials were systematically studied. Cyclic voltammetry revealed that the lowest unoccupied molecular orbitals (LUMO) (~ -3.6 eV) of both molecules were intermediate to common electron donor (P3HT) and acceptor (PCBM). This indicated that PI-DPP and NI-DPP may uniquely serve as electron donor when blended with PCBM, and as electron acceptor when blended with P3HT, where sufficient driving forces between DPPs and PCBM, as well as between P3HT and DPPs should be created for exciton dissociation. Using as electron donor materials, PI-DPP and NI-DPP devices exhibited low power conversion efficiencies (PCEs) of 0.90% and 0.76% by blending with PCBM, respectively. And a preliminary evaluation of the potential of the NI-DPP as electron acceptor material was carried out using P3HT as a donor material, and P3HT:NI-DPP device showed a PCE of 0.6%, with an open circuit voltage (VOC) of 0.7 V, a short circuit current density (JSC) of 1.91 mA•cm-2, and a fill factor (FF) of 45%.

•Solvent treatment improves the conductivity of PEDOT:PSS film by removing PSS.•Perovskite film grown on solvent-treated PEDOT:PSS surface shows poor performance.•PSS-rich surface is necessary for high performance inverted perovskite solar cell.•Increased EQE over 600–750 nm was measured for the cells based on modified HTL.Since perovskite precursor solution is typically prepared from high boiling point solvents, understanding the effect of high boiling point solvent treatment of the PEDOT:PSS layer on the performance of perovskite solar cells is important for device processing optimization. In this paper, influence of the surface treatment of the PEDOT:PSS layer with high boiling point solvent, including N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and ethylene glycol (EG), on the device performance of the perovskite solar cells was investigated. Increased conductivity was measured for the PEDOT:PSS film after solvent treatments, which was ascribed to the partial removal of PSS component from the PEDOT:PSS layer, as evidenced by the UV–vis absorption spectroscopy and XPS spectroscopy. In comparison with the reference cell, poorer device performance was obtained for the perovskite solar cells directly deposited on the solvent washed PEDOT:PSS film, which was ascribed to the increased pin hole density of the perovskite films. However, insertion of a thin PSSNa layer between the PEDOT:PSS layer and the perovskite layer greatly improved device performance, demonstrating that PSS-rich surface is favorite for the crystal growth of the perovskite film. Increased external quantum efficiency over 600–750 nm was measured for the cells based on solvent treated PEDOT:PSS layer, leading to a short circuit current and the consequent performance enhancement.Download high-res image (208KB)Download full-size image

•Two branched oligothiophene cored PDI derivatives were synthesis.•These PDIs showed similar LUMO energy levels to PC61BM.•The bigger molecule 6T-PDI4 showed better device performance over 3T-PDI2.•Higher charge carrier mobility and less SRH recombination was found for 6T-PDI4.Two perylenediimide derivatives (3T-PDI2 and 6T-PDI4) based on dendritic terthiophene (3T) and sexithiophene (6T) cores were designed and synthesized. The rigid and dendritic oligothiophene core ensures a non-planar structure of these two compounds, which was confirmed by theoretical molecular structure simulation. The optical and electrochemical investigation results showed that both compounds have similar LUMO energy level of −4.1 eV, demonstrating that the LUMO orbital is mainly localized on the PDI moiety. Polymer solar cells using 3T-PDI2 and 6T-PDI4 as the electron acceptor, and PTB7-Th as the donor were fabricated and tested. A power conversion efficiency (PCE) of 4.12% was obtained for the 6T-PDI4 based device, which is higher than that of 3T-PDI2 based device (3.41%). Higher charge carrier mobility and less Shockley-Read-Hall recombination were also measured for the 6T-PDI4 based device. The current work demonstrates that dendritic PDI derivatives could serve as non-fullerene acceptor for use in polymer solar cells.Download high-res image (195KB)Download full-size image

Three-dimensional (3D) π-conjugated dendrimers are a new class of structurally defined macromolecules for use in organic electronics. Herein, a new family of dendritic oligothiophenes (DOT-c-BTs) up to the 2nd generation with benzothiadiazole (BT) groups at the core have been synthesized by a precise convergent approach. The well-defined chemical structures and the monodispersed nature of these DOT-c-BTs were fully confirmed by NMR spectroscopy, MALDI-TOF mass spectrometry (MALDI-TOF MS), high-resolution mass spectrometry (HR MS), and gel-permeation chromatography (GPC) measurements. The optical and electrochemical properties were investigated by UV-vis absorption, and cyclic voltammetry. The insertion of electron-deficient benzothiadiazole (BT) groups into the core of the conjugated dendritic oligothiophenes resulted in a large redshift compared to all-thiophene dendrimers. Cyclic voltammetry measurements showed one reversible reduction process and multiple oxidation waves for these functionalized dendritic oligothiophenes, due to the reduction of the BT core and the oxidation of different π-conjugated chains, respectively. Applications of DOT-c-BTs in organic solar cells as the electron donor were presented. However, unfavorable nanophase separation in the blended film led to poor device performance.

Three-dimensional π-conjugated dendrimers are a class of structure defined macromolecules for use in organic electronics. Herein, a new family of dendritic oligothiophenes (DOT-p-DPPs) that are functionalized with the diketopyrrolopyrrole group at the periphery were synthesized by a precise stepwise approach. The chemical structure and the monodisperse nature of these DOT-p-DPPs were confirmed by NMR, MALDI-TOF MS, HR MS, and GPC measurements. UV-vis absorption and fluorescence spectra and cyclic voltammetry data of these compounds were also measured. Small band gaps (∼1.8 eV) and almost identical HOMO/LUMO energy levels (−5.2/−3.5 eV) were measured for these DOT-p-DPPs independent of the molecular size. However, the molecular molar extinction coefficient (ε) of DOT-p-DPPs was found to be linearly correlated with the number of terminal DPP units, and a high ε of 3.6 × 105 cm−1 L mol−1 was measured for the bigger molecules. These results in combination with theoretical calculation results confirm that the frontier molecular orbitals are mostly localized over the peripheral DPP units. The applications of DOT-p-DPPs in organic solar cells as the electron donor are presented. However, unfavorable nanophase separation and lower DOT-p-DPP content in the blended films led to poor device performance. The two photon absorption cross section of these DPP decorated dendrimers was measured, and high cross section values of over 2000 GM were measured for these dendritic molecules, among which the G1 dendrimer 6T-p-DPP with a high TPA cross section value close to 7000 GM was achieved.

Understanding the influence of external loads on the degradation behavior of polymer solar cells (PSCs) is highly important for gaining deep insight into the degradation mechanism of PSCs, as well as for establishing standard stability test protocols for PSCs. In this paper, the degradation behavior of inverted P3HT:PC61BM solar cells operated under different external load conditions were investigated. External load-dependent “burn-in” performance decays were demonstrated, i.e., devices degrade much faster in the open-circuit state than in the short-circuit state, where the short-circuit current (JSC) loss was found to dominate the decay in performance and is the parameter that is most sensitive to the load conditions. The device performance of aged cells maintained 84% of the initial value after thermal annealing, regardless of the load conditions during aging, demonstrating that the external load-dependent performance decay of the P3HT:PC61BM cells is partially reversible. Analyzing the light intensity-dependent open-circuit voltage (VOC) characteristics and dark-current density–voltage (J–V) curves of the aged devices revealed that trap-assisted charge recombination increased in the aged devices. The appearance of PC61BM dimers was confirmed by analyzing the products of the blend film after light illumination, which is ascribed to the main reason for the “burn-in” performance decay. The faster formation of PC61BM dimers is directly correlated to a higher exciton concentration within the photoactive layer when the device is operated under higher load conditions, which explains the external load dependence of the performance decay. In addition to the formation of PC61BM dimers, formation of PC61BM clusters that leads to the nanomorphology changes during aging was ascribed to the second reason for the performance decay, which is external load independent and thermal irreversible. By blending the P3HT:PC61BM photoactive layer with piperazine, a triplet quencher for fullerene molecules, the external load-dependent “burn-in” loss was fully suppressed, which unambiguously confirms that the initial “burn-in” loss in P3HT:PC61BM solar cells is related to the PC61BM excited state. The present work not only clearly demonstrates the behavior and mechanism for the external load-dependent degradation of P3HT:PC61BM solar cells but also provides an effective way to improve the device stability.

Two innovative research studies are reported in this paper. One is the sorting of semiconducting carbon nanotubes and ink formulation by a novel semiconductor copolymer and second is the development of CMOS inverters using not the p-type and n-type transistors but a printed p-type transistor and a printed ambipolar transistor. A new semiconducting copolymer (named P-DPPb5T) was designed and synthesized with a special nonlinear structure and more condensed conjugation surfaces, which can separate large diameter semiconducting single-walled carbon nanotubes (sc-SWCNTs) from arc discharge SWCNTs according to their chiralities with high selectivity. With the sorted sc-SWCNTs ink, thin film transistors (TFTs) have been fabricated by aerosol jet printing. The TFTs displayed good uniformity, low operating voltage (±2 V) and subthreshold swing (SS) (122–161 mV dec−1), high effective mobility (up to 17.6–37.7 cm2 V−1 s−1) and high on/off ratio (104–107). With the printed TFTs, a CMOS inverter was constructed, which is based on the p-type TFT and ambipolar TFT instead of the conventional p-type and n-type TFTs. Compared with other recently reported inverters fabricated by printing, the printed CMOS inverters demonstrated a better noise margin (74% 1/2 Vdd) and was hysteresis free. The inverter has a voltage gain of up to 16 at an applied voltage of only 1 V and low static power consumption.

We have demonstrated in this article that both power conversion efficiency (PCE) and performance stability of inverted planar heterojunction perovskite solar cells can be improved by using a ZnO:PFN nanocomposite (PFN: poly[(9,9-bis(3′-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene]) as the cathode buffer layer (CBL). This nanocomposite could form a compact and defect-less CBL film on the perovskite/PC61BM surface (PC61BM: phenyl-C61-butyric acid methyl ester). In addition, the high conductivity of the nanocomposite layer makes it works well at a layer thickness of 150 nm. Both advantages of the composite layer are helpful in reducing interface charge recombination and improving device performance. The power conversion efficiency (PCE) of the best ZnO:PFN CBL based device was measured to be 12.76%, which is higher than that of device without CBL (9.00%), or device with ZnO (7.93%) or PFN (11.30%) as the cathode buffer layer. In addition, the long-term stability is improved by using ZnO:PFN composite cathode buffer layer when compare to that of the reference cells. Almost no degradation of open circuit voltage (VOC) and fill factor (FF) was found for the device having ZnO:PFN, suggesting that ZnO:PFN is able to stabilize the interface property and consequently improve the solar cell performance stability.

•BDT-cored A-π-D-π-A molecules with regioregular oligothiophene π-bridge were reported.•Systematical investigation on the π-conjugation length effect was carried out.•A positive correlation of chain length and performance was reported for the first time.•A high PCE of 5.14% was achieved for the COOP-4HT-BDT:PC71BM based device.A-π-D-π-A type conjugated small molecules (COOP-nHT-BDT (n=1–4)) with a benzo[1,2-b:4,5-b']dithiophene (BDT) as the electron-donating core, 2-cyano-3-octyloxy-3-oxo-1-propenyl (COOP) as the terminal electron-accepting unit, and two regioregular oligo(3-hexylthiophene) (nHT) as π-conjugated bridges were synthesized and characterized. TGA results confirmed that all these compounds are thermally stable with decomposition temperatures higher than 340 °C. Broad absorption bands over 350 to 600 nm with small optical band gaps (Egopt) of 1.9–1.8 eV were measured for these compounds. Cyclic voltammetry measurements indicated that HOMO energy level of these COOP-nHT-BDTs increases slightly with the increase of nHT chain length, whereas all these compounds display similar LUMO energy level of −3.60 eV. These were supported by density function calculation results, that electronic wave functions of the HOMO orbitals are delocalized over the BDT-core with the adjacent 2–3 thiophene units, whereas the electronic wave functions of the LUMO orbital are mostly localized on the terminal COOP unit with 1–2 thiophene units. Gradually increasing of the photovoltaic performance was found for these compounds with the increase of the π-bridge chain length, and the COOP-4HT-BDT/PC71BM (2:1, w/w) based device displayed a best power conversion efficiency of 5.14% with an open circuit voltage of 0.90 V, a short circuit current of 8.27 mA cm−2and a fill factor of 0.69. A positive correlation between long-term stability of PCE and chain length was also obtained. COOP-4HT-BDT based devices showed the highest device stability with only 15% decay after 320 h continuous illumination.

•Three p-alkylphenyl substituted asymmetric PDI derivatives were synthesized.•These PDIs were used as electron acceptor in solution processed organic solar cells.•Effects of p-alkyl chain length on property and PV performance of PDIs were studied.•PDI molecule with more intensive steric hindrance gave better device performance.In this report, three asymmetrical perylenediimide derivatives (PDI) substituted on the bay-position with para-alkylphenyl groups were synthesized, on which the substituted alkyl side chain was n-propyl (4-PP-PDI), n-hexyl (4-HP-PDI), or n-nonyl (4-NP-PDI) group. The effect of alkyl chain length on the optical and electrochemical properties, thermal behavior, as well as the photovoltaic performance of these materials in solution processed polymer solar cells were systematically studied. Results indicated that the para-alkyl side chain length showed negligible influence on the spectroscopy and redox behaviors of the materials, but significant influence on the photovoltaic performance. The propyl substituted compound 4-PP-PDI showed the best photovoltaic performance of VOC = 0.63 V, JSC = 1.93 mA cm−2, FF = 0.63 and a power conversion efficiency of 0.77%, which was attributed to the balanced intermolecular interaction of PDI molecules and the donor-acceptor phase separation of the blend films.

Inverted planar perovskite (PVSK) solar cells with a device structure of ITO/PEDOT:PSS/PVSK/PC61BM/Al have emerged as a new generation solar cell owing to their advantages of high power conversion efficiency (PCE), low processing temperature and potential low cost. In this paper, the polar solvent treatment effect of the PEDOT:PSS anode interlayer on PVSK solar cell performance was investigated. The conductivity of the PEDOT:PSS film was found to increase by washing with polar solvents, including H2O, ethanol (EtOH), and a mixture solvent of ethanol and H2O (EtOH:H2O = 8:2 v/v), which was attributed to the removal of the PSS component from the PEDOT:PSS film, leading to a PEDOT-rich surface. However, the PCE of perovskite solar cells decreased from 9.39% for the pristine PEDOT:PSS film based device to 4.21%, 8.35%, 7.13% for the H2O-, EtOH- and EtOH:H2O-treated PEDOT:PSS film based devices, respectively, suggesting that the high conductivity of the PEDOT:PSS film does not ensure a high device performance of the inverted PVSK solar cells. UV-Vis absorption spectra, AFM surface morphology and SEM images of the PVSK films deposited on different PEDOT:PSS surfaces were studied, and the results showed that PEDOT-rich surface is not favorable for the crystal growth of PVSK layer, and consequently leads to poor device performances. This conclusion was further supported by the improved device performance of PVSK solar cells based on a PEDOT:PSS/PSSNa anode buffer layer, where an additional poly(sodium p-styrenesulfonate) (PSSNa) layer was deposited on the PEOT:PSS surface.

Solution-processed organic–inorganic hybrids composing of MoO3 nanoparticles and PEDOT:PSS were developed for use in inverted organic solar cells as hole transporting layer (HTL). The hybrid MoO3:PEDOT:PSS inks were prepared by simply mixing PEDOT:PSS aqueous and MoO3 ethanol suspension together. A core–shell structure was proposed in the MoO3:PEDOT:PSS hybrid ink, where PEDOT chains act as the core and MoO3 nanoparticles connected with PSS chains act as the composite shell. The mixing with PEDOT:PSS suppressed the aggregation of MoO3 nanoparticles, which led to a smoother surface. In addition, since the hydrophilic PSS chains were passivated through preferentially connection with MoO3, the stronger adhesion between MoO3 nanoparticles and the photoactive layer improved the film forming ability of the MoO3:PEDOT:PSS hybrid ink. The MoO3:PEDOT:PSS hybrid HTL can therefore be feasibly deposited onto the hydrophobic photoactive polymer layer without any surface treatment. The use of the MoO3:PEDOT:PSS hybrid HTL resulted in the optimized P3HT:PC61BM- and PTB7:PC61BM-based inverted organic solar cells reaching highest power conversion efficiencies of 3.29% and 5.92%, respectively, which were comparable with that of the control devices using thermally evaporated MoO3 HTL (3.05% and 6.01%, respectively). Furthermore, less HTL thickness dependence of device performance was found for the hybrid HTL-based devices, which makes it more compatible with roll-to-roll printing process. In the end, influence of the blend ratio of MoO3 to PEDOT:PSS on photovoltaic performance and device stability was studied carefully, results indicated that the device performance would decrease with the increase of MoO3 blended ratio, whereas the long-term stability was improved.Keywords: hole transporting layer; improved wettability and film forming ability; inverted organic solar cell; MoO3:PEDOT:PSS hybrid; stability; thickness dependence

•ZnO:PFN nanocomposite was developed as cathode buffer layer for use in OPV.•Higher device performance was achieved for ZnO:PFN based solar cells.•Less thickness dependence was found for ZnO:PFN based cathode buffer layer.•Devices having ZnO:PFN buffer layer showed improved long term stability.Cathode buffer layer (CBL) is one of the key issues influencing the performance of organic solar cells. In this article, a nanocomposite of zinc oxide nanoparticles (ZnO) together with a conjugated polymer, poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN) was developed, and used as the CBL for solution processed inverted organic solar cells (OSC). In comparison with the bare ZnO, PFN or ZnO/PFN stacked layer, the ZnO:PFN nanocomposite CBL employing device showed superior device performance, in particular significantly improved fill factor. Optimized power conversion efficiency (PCE) of P3HT:PC61BM and PTB7:PC61BM device using ZnO:PFN composite CBLs reached to 3.56% and 7.17%, respectively. Influences of the mixing ratio and the layer thickness of the ZnO:PFN nanocomposite CBL on solar cell performance were carefully studied, and results indicated that ZnO:PFN CBL showed a wide tolerance of blend ratio and layer thickness. In particular, no obvious thickness dependent-device performance was found, even when the CBL layer thickness was higher than 125 nm, providing a good printing processibility of the nanocomposite. Photoelectron spectroscopy, photoluminescence, as well as electric conductivity of the ZnO:PFN films were studied, and the results were compared with that of the other three reference CBLs. Results demonstrated that the interaction between ZnO and PFN decreases the work function of the blended film, leading to a more favorable energy level alignment for electron injection at the interface. Analysis on the dark J–V curves of the solar cells revealed that device using ZnO:PFN CBL had the best diode characteristics including lowest reverse saturated current density (J0), ideal factor (n), and series resistance (Rs), and highest shunt resistance (Rsh). Such an improvement was ascribed to the defect passivation by the conjugated polymer, which led to an improved charge carrier selectivity of the CBL and consequently enhanced the FF of the solar cells. In addition, long-term stability of organic solar cells was also improved by using ZnO:PFN nanocomposite as the CBL. The current work provides a valuable guideline for the development of high performance cathode buffer layer material for printable organic solar cells.

•Three dicyanovinyl-end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) were synthesized.•All these compounds showed low LUMO energy level.•“All-thiophene” solar cells using these DCV-OTs as the electron acceptor were fabricated.•Molecular structure depended property and performance were investigated.Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C61-butyric acid methyl ester (PC61BM, ELUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, ELUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)2, whereas PTB7:DTSi(THDCV)2 based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10−5 cm2 V−1 s−1 by space charge limited current method, which is much lower than that of PC61BM, and was considered as one of the reason for the low photovoltaic performance.

•Pd-catalyzed direct CH polymerization of 2-bromo-3-hexylthiophene was investigated.•Solvent shows great influence on the terminal groups and molecular weight of P3HT.•β-Coupling was proposed to be the reason for the low regioregularity of P3HT.•Application of the synthesized P3HT in polymer solar cell was investigated.The effect of reaction conditions on Pd-catalyzed direct CH arylation polycondensation of 2-bromo-3-hexylthiophene to regioregular poly(3-hexylthiophene) (rr-P3HT) is exhibited. A wide range of reaction parameters, including palladium catalyst, ligand, reaction temperature and solvent, have been evaluated with a view to optimizing the polymerization conditions. For the first time, the end-groups of the synthesized polymer chains, confirmed by MALDI-TOF-MS measurement, were found to be significantly depended on the solvent used in reaction. Furthermore, homo-coupling and α or β coupling side-reactions were detected in a model oligomer synthesis reaction, which was supposed to be the reason for the low regioregularity of the resulting P3HT. Optical and electrochemical properties, as well as photovoltaic performance of the synthesized P3HT were investigated.A series of influence parameters on Pd-catalyzed CH arylation polycondensation of 2-bromo-3-hexylthiophene are investigated. Solvent shows an important role on the terminal group. The β-coupling side reaction was confirmed by a model oligomer reaction, which was proposed as the reason for the low regioregularity of the polymers.

Squaraine (SQ) dye-based organic semiconductor hybrids 6T-SQ and 18T-SQ functionalized with oligothiophene dendrons were synthesized via Suzuki–Miyaura coupling. The electronic coupling between the oligothiophene dendrons and the squaraine core was rather weak, as suggested from UV–vis spectra, cyclic voltammetry measurements and molecular modeling. Thin films of pure SQ were characterized by a pronounced solvent- and heat-induced crystallization tendency. The dendrons substantially hindered the squaraine core crystallization, and 18T-SQ films remained amorphous after annealing or storage for several weeks. PCBM disrupted dye crystallization in blends, and smooth and stable films could be coated. Heat treatment of blended films induced dewetting for SQ:PCBM and 6T-SQ:PCBM, but 18T-SQ:PCBM remained again stable. These morphological film features could consistently explain the performance of dye-fullerene solar cells. The best performance (η ∼ 1.5%) was obtained for simple bilayer 6T-SQ:C60 or 18T-SQ:C60 cells without annealing. Our results demonstrate that the attachment of decorating moieties to a central light-absorbing core unit in molecular push–pull systems can be used to adjust the optoelectronic and morphological film properties of small molecular semiconductors with a strong tendency towards crystallization.Graphical abstractHighlights► Oligothiophene dendron–squaraine hybrids were used as donors in organic solar cells. ► The electronic coupling between the dendrons and the squaraine core was rather weak. ► The dendrons substantially hindered the pronounced squaraine dye crystallization.

Understanding the influence of external loads on the degradation behavior of polymer solar cells (PSCs) is highly important for gaining deep insight into the degradation mechanism of PSCs, as well as for establishing standard stability test protocols for PSCs. In this paper, the degradation behavior of inverted P3HT:PC61BM solar cells operated under different external load conditions were investigated. External load-dependent “burn-in” performance decays were demonstrated, i.e., devices degrade much faster in the open-circuit state than in the short-circuit state, where the short-circuit current (JSC) loss was found to dominate the decay in performance and is the parameter that is most sensitive to the load conditions. The device performance of aged cells maintained 84% of the initial value after thermal annealing, regardless of the load conditions during aging, demonstrating that the external load-dependent performance decay of the P3HT:PC61BM cells is partially reversible. Analyzing the light intensity-dependent open-circuit voltage (VOC) characteristics and dark-current density–voltage (J–V) curves of the aged devices revealed that trap-assisted charge recombination increased in the aged devices. The appearance of PC61BM dimers was confirmed by analyzing the products of the blend film after light illumination, which is ascribed to the main reason for the “burn-in” performance decay. The faster formation of PC61BM dimers is directly correlated to a higher exciton concentration within the photoactive layer when the device is operated under higher load conditions, which explains the external load dependence of the performance decay. In addition to the formation of PC61BM dimers, formation of PC61BM clusters that leads to the nanomorphology changes during aging was ascribed to the second reason for the performance decay, which is external load independent and thermal irreversible. By blending the P3HT:PC61BM photoactive layer with piperazine, a triplet quencher for fullerene molecules, the external load-dependent “burn-in” loss was fully suppressed, which unambiguously confirms that the initial “burn-in” loss in P3HT:PC61BM solar cells is related to the PC61BM excited state. The present work not only clearly demonstrates the behavior and mechanism for the external load-dependent degradation of P3HT:PC61BM solar cells but also provides an effective way to improve the device stability.