Active-layer morphology critically affects the performance of organic photovoltaic cells, and thus its optimization is a key toward the achievement of high-efficiency devices. However, the optimization of active-layer morphology is sometimes challenging because of the intrinsic properties of materials such as strong self-aggregating nature or low miscibility. This study postulates that the “photoprecursor approach” can serve as an effective means to prepare well-performing bulk-heterojunction (BHJ) layers containing highly aggregating molecular semiconductors. In the photoprecursor approach, a photoreactive precursor compound is solution-deposited and then converted in situ to a semiconducting material. This study employs 2,6-di(2-thienyl)anthracene (DTA) and [6,6]-phenyl-C71-butyric acid methyl ester as p- and n-type materials, respectively, in which DTA is generated by the photoprecursor approach from the corresponding α-diketone-type derivative DTADK. When only chloroform is used as a cast solvent, the photovoltaic performance of the resulting BHJ films is severely limited because of unfavorable film morphology. The addition of a high-boiling-point cosolvent, o-dichlorobenzene (o-DCB), to the cast solution leads to significant improvement such that the resulting active layers afford up to approximately 5 times higher power conversion efficiencies. The film structure is investigated by two-dimensional grazing-incident wide-angle X-ray diffraction, atomic force microscopy, and fluorescence microspectroscopy to demonstrate that the use of o-DCB leads to improvement in film crystallinity and increase in charge-carrier generation efficiency. The change in film structure is assumed to originate from dynamic molecular motion enabled by the existence of solvent during the in situ photoreaction. The unique features of the photoprecursor approach will be beneficial in extending the material and processing scopes for the development of organic thin-film devices.Keywords: morphology control; organic solar cells; photoprecursor approach; solution process; α-diketones

Tandem intramolecular electrophilic arene borylation was developed to facilitate access to B-doped polycyclic aromatic hydrocarbons (PAHs). DFT calculations revealed that electrophilic arene borylation occurred via a four-membered ring transition state, in which C–B and H–Br bonds formed in a concerted manner. An organic light-emitting diode employing the B-doped PAH as an emitter and a B-doped PAH-based field-effect transistor were successfully fabricated, demonstrating the potential of B-doped PAHs in materials science.

•Synthesis and reactivity of α-diketone-type acene precursors are reviewed.•Preparation of large or functionalized acenes via α-diketones is overviewed.•The mechanism of photoinduced decarbonylation of α-diketones is discussed.•Fabrication of acene-based devices by the photoprecursor method is introduced.Acenes are highly promising p-type organic semiconductors, and have been the subject of intense studies. However, acenes are often low in solubility and stability, which poses major obstacles in the synthesis and processing of this class of compounds. In order to overcome the problem, a series of α-diketone-type acene precursors have been developed. These precursors are generally more soluble and stable than the corresponding acene compounds, and their quantitative conversion can be achieved simply by photoirradiation both in solution and in the solid state. Further, the irreversible photoinduced removal of the α-diketone unit can be used to alter the optoelectronic properties of fluorophores. This review overviews the synthesis and photochemical properties of α-diketone-type acene precursors, as well as their use as intermediates in preparation of large acenes or highly functionalized acene derivatives. Computational studies on the mechanism of α-diketone photolysis and the use of α-diketone derivatives in fabrication of organic devices are also summarized in this review.

We present herein a linear expanded π-conjugation system comprising azulene units: 2,6′:2′,6″-terazulene. This simple hydrocarbon exhibits excellent n-type transistor performance with an electron mobility of up to 0.29 cm2 V–1 s–1. The lowest unoccupied molecular orbital (LUMO) is well distributed over the entire molecule, whereas the highest occupied molecular orbital (HOMO) is localized at one end. These findings indicate a disadvantage of hole carrier transport and an advantage of n-type-specific transport behavior. This system presents an unconventional concept: polarity control of OFET by molecular orbital distribution control.

Two regioisomeric indolo[3,2-b]carbazoles end-substituted with diketopyrrolopyrrole dyes (3,9-TDPPIC and 2,8-TDPPIC) were synthesized, and their solid-state properties and performance as donor materials for bulk heterojunction solar cells using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor material were evaluated. Depending on the regiochemistry of backbone linkages (3,9- or 2,8-linkage), these two compounds showed quite different solid-state aggregation behaviors (crystalline versus amorphous), leading to a significant difference in the degree of phase separation upon blending with PCBM. As a result, a distinct effect of thermal annealing on the device parameters of solar cells (Voc and Jsc) was observed for these compounds although both the devices showed almost the same power conversion efficiency (PCE) of ca. 0.7% with thermal annealing at 130 °C. For 3,9-TDPPIC, the highest PCE of 1.8% was achieved by further optimization of the device.

6,13-Dihydro-6,13-ethanopentacene-15,16-dione is a soluble precursor of pentacene, which can be converted into pentacene by irradiation in the solid-state. Its photoconversion process in spin-coated films was monitored by UV-visible absorption and IR spectroscopy. A small amount of high-boiling-point additives in the chloroform spin-coating solution promoted photoconversion to obtain high quality films suitable for FETs. The FET mobilities showed a correlation with the additives' boiling points and dissolution abilities, indicating that the retainment of a semidry state during photoconversion is essential to the complete photoconversion. Photoirradiation conditions (irradiation intensity, duration and substrate temperature) were optimized to achieve a field-effect mobility of 0.86 cm2 V−1 s−1, comparable to the performance of vacuum-deposited pentacene films. The prepared films have a partially crystalline morphology different from that of vacuum-deposited films. The high FET mobility of the photoconverted film is attributed to continuously connected grain boundaries arising from partial crystallinity.

P–n junction solar cells based on anthradithiophene (ADT) as a p-type semiconductor were fabricated by using the photoprecursor method in which an α-diketone-type precursor was spin-coated and then transformed to ADT in situ by photoirradiation. Combination with PC71BM as an n-layer material led to 1.54% photoconversion efficiency.

•Soluble photoprecursor of pentacene is used for solar cells for the first time.•The photoconversion process strongly depends on the spin-coating solvent.•The photo-converted pentacene showed comparable APCE with vacuum-deposited one.A soluble pentacene diketone (PDK) photoprecursor, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was used with C60 to fabricate hetero-layered organic photovoltaic devices. The PDK spin-coated film converts to an insoluble pentacene film upon photoirradiation. The photoconversion process strongly depends on the solvent used for spin-coating. A mixture of chloroform and dichlorobenzene (ratio 1:5) gave the best photovoltaic device performance. The absorbed photon-to-current efficiency in the photo-converted-pentacenefilm device was similar to that of the vacuum-deposited pentacene film.Graphical abstract

Hole mobility was evaluated by top-contact bottom gate field effect transistor and time resolved microwave conductivity measurements in 2,6-dithienylanthracene and hexyl-substituted 2,6-dithienylanthracene films prepared by spin-coating of their α-diketone precursors followed by photoirradiation, revealing enough high potentials for semiconducting films with charge carrier mobilities of 0.8–0.9 cm2 V−1 s−1 in the photo-irradiated films.

5,12-Dicyanonaphthacene and 6,13-dicyanopentacene have been synthesized for the first time. The LUMO and HOMO levels are deepened as predicted and fabricated organic field-effect transistors (OFETs) showed ambipolar responses with carrier mobilities of 10−3 cm2/V·s.

The on/off ratio of a vertical-type metal-base organic transistor was significantly improved by subjecting it to heat treatment in air. The heat treatment of the collector layer and the base electrode reduced the off current that is mainly due to leakage current between the base and the collector, resulting in a considerable decrease in the off current. As a result, a high on/off ratio greater than 105 was achieved, in addition to the low-voltage and high current operation of the vertical-type organic transistor.

•Soluble photoprecursor of pentacene is used for solar cells for the first time.•The photoconversion process strongly depends on the spin-coating solvent.•The photo-converted pentacene showed comparable APCE with vacuum-deposited one.A soluble pentacene diketone (PDK) photoprecursor, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was used with C60 to fabricate hetero-layered organic photovoltaic devices. The PDK spin-coated film converts to an insoluble pentacene film upon photoirradiation. The photoconversion process strongly depends on the solvent used for spin-coating. A mixture of chloroform and dichlorobenzene (ratio 1:5) gave the best photovoltaic device performance. The absorbed photon-to-current efficiency in the photo-converted-pentacenefilm device was similar to that of the vacuum-deposited pentacene film.Graphical abstractDownload full-size image

P–n junction solar cells based on anthradithiophene (ADT) as a p-type semiconductor were fabricated by using the photoprecursor method in which an α-diketone-type precursor was spin-coated and then transformed to ADT in situ by photoirradiation. Combination with PC71BM as an n-layer material led to 1.54% photoconversion efficiency.

Hole mobility was evaluated by top-contact bottom gate field effect transistor and time resolved microwave conductivity measurements in 2,6-dithienylanthracene and hexyl-substituted 2,6-dithienylanthracene films prepared by spin-coating of their α-diketone precursors followed by photoirradiation, revealing enough high potentials for semiconducting films with charge carrier mobilities of 0.8–0.9 cm2 V−1 s−1 in the photo-irradiated films.

Two regioisomeric indolo[3,2-b]carbazoles end-substituted with diketopyrrolopyrrole dyes (3,9-TDPPIC and 2,8-TDPPIC) were synthesized, and their solid-state properties and performance as donor materials for bulk heterojunction solar cells using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor material were evaluated. Depending on the regiochemistry of backbone linkages (3,9- or 2,8-linkage), these two compounds showed quite different solid-state aggregation behaviors (crystalline versus amorphous), leading to a significant difference in the degree of phase separation upon blending with PCBM. As a result, a distinct effect of thermal annealing on the device parameters of solar cells (Voc and Jsc) was observed for these compounds although both the devices showed almost the same power conversion efficiency (PCE) of ca. 0.7% with thermal annealing at 130 °C. For 3,9-TDPPIC, the highest PCE of 1.8% was achieved by further optimization of the device.

6,13-Dihydro-6,13-ethanopentacene-15,16-dione is a soluble precursor of pentacene, which can be converted into pentacene by irradiation in the solid-state. Its photoconversion process in spin-coated films was monitored by UV-visible absorption and IR spectroscopy. A small amount of high-boiling-point additives in the chloroform spin-coating solution promoted photoconversion to obtain high quality films suitable for FETs. The FET mobilities showed a correlation with the additives' boiling points and dissolution abilities, indicating that the retainment of a semidry state during photoconversion is essential to the complete photoconversion. Photoirradiation conditions (irradiation intensity, duration and substrate temperature) were optimized to achieve a field-effect mobility of 0.86 cm2 V−1 s−1, comparable to the performance of vacuum-deposited pentacene films. The prepared films have a partially crystalline morphology different from that of vacuum-deposited films. The high FET mobility of the photoconverted film is attributed to continuously connected grain boundaries arising from partial crystallinity.