Semiconducting copolymers based on benzo[1,2-b:4,5-b′]dithiophene and thieno[3,4-c]pyrole-4,6-dione containing oligo(thienylenevinylene) side chains with different lengths were synthesized to examine the effect of the π-conjugated side chains on the performance of polymer solar cells (PSCs). Using the copolymers that had π-conjugated side chains as the PSC electron donor resulted in a higher short circuit current and fill factor compared with the reference copolymers, which had no side chain or an analogous side chain with no π-conjugation, resulting in an increase of power conversion efficiency of 10–22%. Measurements of hole mobility by space-charge-limited current and internal quantum efficiency indicated that introducing the π-conjugated side-chain units can facilitate both charge transport and charge separation in the polymer:PC71BM blend films.

On the basis of naphthalene diimide (NDI) units connected to thiophene (T), thienothiophene (TT), or dithienothiophene (DTT) units via a thiophene π-bridge, three new copolymers—PDTNDI-T, PDTNDI-TT, and PDTNDI-DTT, respectively—were synthesized and used in the fabrication of all-polymer solar cells (all-PSCs). The relationships between the structures of the polymers and their optoelectronic properties and photovoltaic performances as electron acceptors in all-PSCs were investigated in detail. As the number of copolymerized heteroaromatic rings in the DTNDI-based polymers increased, the power conversion efficiencies of the resulting all-PSCs were found to decrease. This decreasing trend in the photovoltaic performance is opposite to the results reported previously for NDI-based polymers lacking the thiophene π-bridge and naphthodithiophene diimide-based polymers. In addition, the three polymers were found to exhibit distinct molecular orientations: a face-on orientation for PDTNDI-T and edge-on orientations for PDTNDI-TT and PDTNDI-DTT. Our results indicate that large fused aromatic rings are not necessarily advantageous in the design of NDI-based polymers containing π-conjugated bridges.Keywords: all-polymer solar cell (all-PSC); dithienothiophene (DTT); naphthalene diimide (NDI); nonfullerene acceptor; thienothiophene (TT);

Heteroblock copolymers consisting of poly(3-hexylthiophene) and fullerene-attached poly(3-alkylselenophene) (T-b-Se-PCBP) were synthesized for organic photovoltaic applications by quasi-living catalyst transfer polycondensation and subsequent conversion reactions. Characterization of the polymers confirmed the formation of well-defined diblock structures with high loading of the fullerene at the side chain (∼40 wt %). Heteroblock copolymer cast as a thin film showed a clear microphase-separated nanostructure approximately 30 nm in repeating unit after thermal annealing, which is identical to the microphase-separated nanostructure of diblock copolymer consisting of poly(3-hexylthiophene) and fullerene-attached poly(3-alkylthiophene) (T-b-T-PCBP). These heteroblock copolymers provide an ideal platform for investigating the effects of nanostructures and interfacial energetics on the performance of organic photovoltaic devices.Keywords: block copolymers; energy cascade; microphase separation; organic solar cells; self-organization; semiconducting polymers; thin films;

Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

Light wave manipulation by using nanostructures is a promising strategy for enhancing the light absorption of thin photoactive layers in organic photovoltaics (OPVs). Here, we propose a method for nanopatterning the multiple interfaces in bulk heterojunction (BHJ) OPVs by using soft imprint lithography at room temperature. The interfaces in the OPVs were separately modified in the front ZnO layers and the back metal electrodes with a grating pattern. Each nanopattern increased the light absorption and the power conversion efficiency of the OPVs by up to 32.5% depending on the materials. Moreover, the nanopatterning at both the front and the back cumulatively increased the light absorption, resulting in the highest efficiency increase of 38.5%. The increases were observed in various BHJ systems with different properties containing the polymers PTB7, PCE10, P3HT, or PNTz4T. A certified performance of 10.31% was achieved for the PNTz4T:PC71BM system in the presence of the nanopatterns. Detailed analysis by using the absorption spectra and optical simulations indicated that the origins of the optical gains from the nanopatterns on the front and the back are different. The front pattern increases the transmittance and the back pattern increases the scattering and excites the surface plasmon polaritons.

Crystallization of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in thin films and in blend films with various polymers was investigated by X-ray diffraction. Thermal annealing induced the crystallization of PCBM in the blend films only through direct contact with a crystallized pure PCBM layer beneath, suggesting that an epitaxial crystallite growth occurred from the bottom interface. The morphology of the crystals depended strongly on the mixing ratio and the crystal structure of the bottom layer, and nanorod-like PCBM crystallites with widths in the range of 100–150 nm and lengths in the range of 150–500 nm were observed. A bulk-heterojunction (BHJ) organic solar cell utilizing the PCBM crystallite as the acceptor showed the highest VOC of 0.83 V for a PTB7:PCBM device to date. These findings offer the ways to use the crystallized PCBM with the controlled nanostructures as the electron conducting materials in organic and hybrid perovskite photovoltaics.

New π-conjugated copolymers based on naphtho[2,3-b:6,7-b′]dithiophene-4,5,9,10-diimide (NDTI) combined with thiophene, thienothiophene, or dithienothiophene units are synthesized and used in field-effect transistors (FETs) and organic solar cells (OSCs). The low-lying lowest unoccupied molecular orbital (LUMO) and high-lying highest occupied molecular orbital (HOMO) levels of the polymers contribute to reducing injection barriers for both electrons and holes, resulting in ambipolar operation of FET devices. The charge mobilities were strongly affected by the molecular orientation of the copolymers, and the highest electron mobility of 0.26 cm2/(V s) was observed for the copolymer with thienothiophene unit with edge-on orientation. On the other hand, OSCs with PTB7 as the electron donor polymer and the copolymers as the acceptor showed a broad photoresponse extending to the near-IR region, and the highest power conversion efficiency of over 3.5% was obtained for the copolymer with dithienothiophene unit that showed the favorable face-on orientation in the neat thin film, though the effect of the molecular orientations in OSCs was not as clear as in OFETs owing to the lower crystallinity of the mixed films.

In conventional organic photovoltaic cells, the active layer consists of a polymeric donor and a molecular acceptor (PD/MA). An unconventional material combination based on molecular donor/polymeric acceptor (MD/PA) emerged in 2014 but attracted limited attention. To broaden photovoltaic material systems and understand the crucial factors related to the photovoltaic performance, in this report, we adopted a molecular donor (p-DTS(FBTTh2)2) and three polymeric acceptors based on perylenediimide (PDI). We find that the high contents (70–80%) of p-DTS(FBTTh2)2 and the better crystallinity and larger grains in the blend films induced by the addition of 1,8-diiodooctane (DIO) play an important role in constructing the continuous and effective donor phase for charge transfer and hole transport in the active layers. The highest PCE of photovoltaic cells reached 3.01% with a VOC of 0.68 V, JSC of 7.59 mA cm−2, and FF of 0.58 for the p-DTS(FBTTh2)2:PSe-PDI active layer, although the hole and the electron mobilities are still unbalanced. Further optimization of the film morphology and improvement of the electron mobility by material design and device engineering are expected to boost the efficiency of MD/PA type fullerene-free solar cells.

The effect of thermal annealing on the energy levels of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films was investigated using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and low-energy inverse photoemission spectroscopy. We observed that thermal annealing at 150 °C induces reductions of both the ionization potential (IP) and the electron affinity (EA) with a narrowing of the band gap by 0.1 eV. These changes are associated with crystallization and a 2.54% reduction in the film thickness. Precise measurements of both the IP and EA enabled an evaluation of the effects of the electronic polarization energy in a model based on the charge localized in a single PCBM molecule.

•An n-type low band gap conjugated polymer, PDTS-NDI, was synthesized.•The polymer has two broad absorption bands in the range of 300–500 nm and 500–950 nm.•The PCE of TTV2:PDTS-NDI blend solar cell reached 1.19% with DIO as solvent additive.Naphthalene diimide (NDI) and dithienosilole (DTS) have been well known building blocks for photovoltaic materials. Herein a solution-processable low band gap conjugated polymer, PDTS-NDI, consisting of NDI moiety and DTS unit was synthesized as a new n-type material. The polymer has two broad absorption bands in the range of 300–500 nm and 500–950 nm. Power conversion efficiencies (PCEs) of the polymer blend solar cells in the range of 0.13–0.70% were obtained under simulated AM 1.5, 100 mW/cm2 irradiation, where two polymers with conjugated side chains, PT1 and TTV2, were used as p-type material. For the combination of TTV2 and PDTS-NDI, it was further optimized to give a PCE of 1.19% by adding 1,8-diiodooctane (DIO) as solvent additive, suggesting a potential of PDTS-NDI polymer for the photovoltaic application.

•An n-type amorphous conjugated polymer, PSe-PDI, was synthesized.•The miscibility could be improved by introducing TTV side chain to p-type polymer.•DIO could increase the electron mobility of amorphous PDI-based polymer.•The PCE of TTV7: PSe-PDI blend solar cell reached 2.55% with DIO as solvent additive.An amorphous n-type polymer (PSe-PDI) consisting of perylenediimide and selenophene units was synthesized and applied in bulk heterojunction organic solar cells as an electron acceptor. The all-polymer solar cells based on PTB7 as a donor exhibited power conversion efficiency (PCE) of 1.14%. By using a PTB7 analog with conjugated tris(thienylenevinylene) side chains (TTV7) a superior photovoltaic performance with PCE of 1.81% was achieved due to the improvement of miscibility of the two polymers. Furthermore, by using diiodooctane (DIO) as additive to increase and balance the charge carrier mobility in the TTV7:PSe-PDI blend films, the PCE was increased to 2.55% with a much higher short-circuit current density of 8.34 mA/cm2. This study provides a simple and effective approach to improve the miscibility and charge carrier mobility of amorphous PDI-based all-polymer solar cells.

End-functionalized poly(3-butylthiophene) with a thiol group (P3BT-S) was synthesized and used to form a self-assembled monolayer (SAM). It can induce the end-on orientation in the thin film which has the potential to further enhance hole mobility up to 1.1 × 10−2 cm2 V−1 s−1 in the vertical direction.

Polymer-blend solar cells (all-PSCs) based on a copolymer of naphthodithiophene diimide and bithiophene (PNDTI-BT-DT) as a near-infrared absorber as well as an electron acceptor were fabricated in combination with PTB7 as an electron donor. Notably, the external quantum efficiency spectra of the all-PSCs demonstrated photoresponse up to 900 nm with the efficiency of 25% at 800 nm, which is much higher than that for the previously reported all-PSCs. Power conversion efficiency as high as 2.59% was achieved under the irradiation of simulated solar light (AM1.5, 100 mW/cm2). Both PNDTI-BT-DT and PTB7 formed a crystalline structure in the blend films similar to in the pristine films, leading to the efficient charge generation contributed from both polymers.

Fullerene-based surfactants with semifluoroalkyl chains bearing one of five different functional groups at the end were synthesized and used for the facile surface modification of organic semiconductor films. Surface analysis showed that the modifiers were segregated and the functional groups were exposed at the surface.

•A donor–acceptor dyad molecule was synthesized with naphthalene diimide as acceptor.•Donor and acceptor moieties were crystallized separately in spin-coated films.•Planar shape of the acceptor could be important for the separated crystallization.A dyad molecule was synthesized, with oligo(p-phenylenevinylene) (OPV) as the electron donor and naphthalene diimide (NDI) as the acceptor. The optical, thermal, and X-ray diffraction measurements of the film structure showed that the OPV and NDI moieties of the dyad crystallized separately in the spin-coated films. This is in striking contrast to the previously reported OPV and C70 fullerene dyad, in which the introduction of C70 severely disrupted the OPV packing. The separated crystallization in the OPV–NDI dyad was attributed to the planar shape of the acceptor and the differences in the crystalline structures of planar NDI and rod-like OPV.

•Poly(4-hexyloxythiazole) was synthesized for the first time.•The polymer films had crystalline nature similar to poly(3-hexylthiophene).•Charge mobility in field effect transistor reached 0.02 cm2/V s.•Bulk heterojunction type solar cells showed a photoresponse up to about 900 nm.A novel semiconducting polymer poly(4-hexyloxythiazole) with a low optical band gap of 1.4 eV was synthesized and used in organic electronic devices. X-ray diffraction (XRD) measurements reveal a high degree of crystallinity and lamellar packing of poly(4-hexyloxythiazole) in the film similar to poly(3-hexylthiophene). Field effect transistor charge mobility of poly(4-hexyloxythiazole) arrived 0.02 cm2/V s. Bulk heterojunction type solar cells based on poly(4-hexyloxythiazole) shows a photoresponse up to about 900 nm.

Controlling the orientation of highly anisotropic structures of polymers is important because the majority of their mechanical, electronic, and optical properties depend on the orientation of the polymer backbone. In thin films, the polymer chains tend to adopt an orientation parallel to the substrate; therefore, forcing the chains to stand perpendicular to the substrate is challenging. We have developed a simple way to achieve this end-on orientation. We functionalized one end of a poly(3-butylthiophene) (P3BT) chain with a 1H,1H,2H,2H,3H,3H-perfluoroundecyl group, which caused spontaneous self-segregation of the polymer (P3BT-F17) to the surface of the polymer film. In P3BT-F17/polystyrene (PS) blend films, a highly ordered end-on orientation of the conjugated backbone was observed in the surface-segregated layer of the crystalline P3BT-F17. Furthermore, when the film was spin-coated from a mixture of P3BT-F17 and P3BT, the chain orientation of P3BT-F17 at the surface forced the P3BT in the bulk of the film to adopt the end-on orientation because of the high crystallinity of P3BT. The electronic conductivity measured perpendicular to the film surface also reflected the end-on orientation in the bulk, resulting in a more than 30-fold enhancement of the hole mobility.

A regioregular poly(3-alkylthiophene) with a statistical sequence of alkyl/semifluoroalkyl side chains (stat-P3DDFT) was synthesized through the copolymerization of mixed monomers. The surface segregation behavior was compared with the corresponding alternating copolymer (alt-P3DDFT) that forms highly ordered dipole layers by surface segregation. X-ray and ultraviolet photoelectron spectroscopies of the polymer films revealed that alt-P3DDFT formed a much more ordered surface segregated monolayer than did stat-P3DDFT, indicating the importance of the alternating sequence of the side chains. When stat-P3DDFT was inserted into the donor/acceptor interfaces in bilayer organic photovoltaic devices, the short circuit current and the open circuit voltages of the devices were related to the disordered interface structures of stat-P3DDFT.

A novel alternating copolymer with a low band gap (Eg = 1.55 eV), PBDT–DTBSe, based on benzodithiophene (BDT) and benzoselenadiazole (BSe) units with thiophene as a π-conjugated bridge, was synthesized and characterized. When 1,8-diiodooctane was used as a solvent additive to optimize the mixing morphology, the maximum power conversion efficiency reached by a polymer solar cell based on PBDT–DTBSe/PC70BM was 5.18%, which was slightly higher than that of the benzothiadiazole (BT)-based analogue (5.01%). These results demonstrated the promising effectiveness of benzoselenadiazole as an electron-deficient unit for the design of the donor–acceptor photovoltaic polymers.

To pursue high power conversion efficiency (PCE) of polymer solar cells (PSCs), many new semiconducting polymers with low band gaps have been developed in the past several years. In this perspective paper, we focused on super low band gap photovoltaic polymers with photocurrent response extending over 1000 nm. This kind of micrometer-response polymers (μmR-polymer) could increase the short circuit current (JSC) due to better match of absorption spectra of the polymers with the solar irradiation and show tremendous potential for application in tandem solar cells and transparent solar cells. The necessary conditions for the design of this kind of μmR-polymers are discussed. Furthermore, the remaining problems and challenges, and the key research direction in near future are discussed.Figure optionsDownload full-size imageDownload as PowerPoint slide

End-functionalized poly(3-butylthiophene) with a thiol group (P3BT-S) was synthesized and used to form a self-assembled monolayer (SAM). It can induce the end-on orientation in the thin film which has the potential to further enhance hole mobility up to 1.1 × 10−2 cm2 V−1 s−1 in the vertical direction.

Crystallization of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in thin films and in blend films with various polymers was investigated by X-ray diffraction. Thermal annealing induced the crystallization of PCBM in the blend films only through direct contact with a crystallized pure PCBM layer beneath, suggesting that an epitaxial crystallite growth occurred from the bottom interface. The morphology of the crystals depended strongly on the mixing ratio and the crystal structure of the bottom layer, and nanorod-like PCBM crystallites with widths in the range of 100–150 nm and lengths in the range of 150–500 nm were observed. A bulk-heterojunction (BHJ) organic solar cell utilizing the PCBM crystallite as the acceptor showed the highest VOC of 0.83 V for a PTB7:PCBM device to date. These findings offer the ways to use the crystallized PCBM with the controlled nanostructures as the electron conducting materials in organic and hybrid perovskite photovoltaics.

Fullerene-based surfactants with semifluoroalkyl chains bearing one of five different functional groups at the end were synthesized and used for the facile surface modification of organic semiconductor films. Surface analysis showed that the modifiers were segregated and the functional groups were exposed at the surface.

A regioregular poly(3-alkylthiophene) with a statistical sequence of alkyl/semifluoroalkyl side chains (stat-P3DDFT) was synthesized through the copolymerization of mixed monomers. The surface segregation behavior was compared with the corresponding alternating copolymer (alt-P3DDFT) that forms highly ordered dipole layers by surface segregation. X-ray and ultraviolet photoelectron spectroscopies of the polymer films revealed that alt-P3DDFT formed a much more ordered surface segregated monolayer than did stat-P3DDFT, indicating the importance of the alternating sequence of the side chains. When stat-P3DDFT was inserted into the donor/acceptor interfaces in bilayer organic photovoltaic devices, the short circuit current and the open circuit voltages of the devices were related to the disordered interface structures of stat-P3DDFT.

In conventional organic photovoltaic cells, the active layer consists of a polymeric donor and a molecular acceptor (PD/MA). An unconventional material combination based on molecular donor/polymeric acceptor (MD/PA) emerged in 2014 but attracted limited attention. To broaden photovoltaic material systems and understand the crucial factors related to the photovoltaic performance, in this report, we adopted a molecular donor (p-DTS(FBTTh2)2) and three polymeric acceptors based on perylenediimide (PDI). We find that the high contents (70–80%) of p-DTS(FBTTh2)2 and the better crystallinity and larger grains in the blend films induced by the addition of 1,8-diiodooctane (DIO) play an important role in constructing the continuous and effective donor phase for charge transfer and hole transport in the active layers. The highest PCE of photovoltaic cells reached 3.01% with a VOC of 0.68 V, JSC of 7.59 mA cm−2, and FF of 0.58 for the p-DTS(FBTTh2)2:PSe-PDI active layer, although the hole and the electron mobilities are still unbalanced. Further optimization of the film morphology and improvement of the electron mobility by material design and device engineering are expected to boost the efficiency of MD/PA type fullerene-free solar cells.

Light wave manipulation by using nanostructures is a promising strategy for enhancing the light absorption of thin photoactive layers in organic photovoltaics (OPVs). Here, we propose a method for nanopatterning the multiple interfaces in bulk heterojunction (BHJ) OPVs by using soft imprint lithography at room temperature. The interfaces in the OPVs were separately modified in the front ZnO layers and the back metal electrodes with a grating pattern. Each nanopattern increased the light absorption and the power conversion efficiency of the OPVs by up to 32.5% depending on the materials. Moreover, the nanopatterning at both the front and the back cumulatively increased the light absorption, resulting in the highest efficiency increase of 38.5%. The increases were observed in various BHJ systems with different properties containing the polymers PTB7, PCE10, P3HT, or PNTz4T. A certified performance of 10.31% was achieved for the PNTz4T:PC71BM system in the presence of the nanopatterns. Detailed analysis by using the absorption spectra and optical simulations indicated that the origins of the optical gains from the nanopatterns on the front and the back are different. The front pattern increases the transmittance and the back pattern increases the scattering and excites the surface plasmon polaritons.