•We use DSSC dye as electron donor in organic photovoltaics and receive high photovoltaic performance.•This is the first time that organic dye with carboxyl group was used for evaporated device.•The highest PCE of up to 4.5% was achieved under air mass AM 1.5G solar illumination.We demonstrated the use of an asymmetrical donor–acceptor-type indoline dye—D131, developed for dye-sensitized solar cells, as an electron donor and fullerene C70 as an electron acceptor for thermal co-evaporated bulk-heterojunction organic solar cells (OSCs). In spite of the presence of intermolecular hydrogen bonds among D131 molecules, they can be thermally evaporated in high vacuum at a relatively low temperature of 220 °C. The blend ratio and thickness of the active layer of D131/C70 blend films in OSCs were optimized to achieve a maximum power-conversion efficiency of 4.5% with a short-circuit current of 9.1 mA cm−2, an open-circuit voltage of 0.89 V, and a fill factor of 0.56 under AM 1.5G solar illumination (100 mW cm−2), which is the best value reported so far for OSCs based on indoline-based donor materials.Graphical abstract

•Organic photovoltaics using ambipolar chlorophyll derivatives as either donors or acceptors were fabricated.•The maximum power conversion efficiency of up to 2.6% has been achieved under AM 1.5 (100 mW/cm2) solar illumination.•The electron mobility of organic photovoltaics was determined by that of both donor and acceptor molecules.Two ambipolar chlorophyll derivatives, namely, 32,32-dicyano-pyropheophorbide-a (Chl-1) and methyl 131-deoxo-131-(dicyanomethylene) pyropheophorbide-a (Chl-2), were synthesized for use as either the electron acceptor or the electron donor in organic planar-heterojunction solar cells. Despite the higher electron mobilities of these chlorophyll derivatives compared with their hole mobilities, devices using them as the electron donor with fullerene C70 give much better photovoltaic performance than when they are used as the electron acceptor with copper phthalocyanine. In these Chl-based solar cells, the energy gap between the LUMO levels of the donor and acceptor molecules substantially affects the charge separation and resultant photocurrent and photovoltaic performance. The highest solar energy-to-electricity conversion efficiency of up to 2.3% has been achieved using the Chl-2/C70 solar cell, under AM1.5 solar illumination (100 mW/cm2) after thermal annealing of the device. It was also confirmed that the electron mobility of blend films containing Chls and fullerene derivative PC70BM was determined not only by the electron mobility of PC70BM but also by that of Chls.Graphical abstract

In this work, we demonstrate utilization of natural carotenoids (Cars), namely, fucoxanthin, β-carotene, and lycopene, as electron-donor molecules together with the electron-acceptor fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in organic solar cells (OSCs). Unlike fucoxanthin and β-carotene, which form amorphous films, lycopene readily forms aggregates through a simple spin coating process. A high carrier mobility of up to 2.1 × 10–2 cm2/(V s) was observed for lycopene, which is three orders of magnitude greater than those of fucoxanthin and β-carotene, with values of (8.1 and 1.8) × 10–5 cm2/(V s), respectively. OSCs with different Car:PCBM blend ratios were optimized for these Cars. The highest photovoltaic performance was obtained for lycopene with a blend ratio of 1:1, at which the film morphology and charge transport were optimized. Replacement of the acceptor molecule PCBM with a high-lowest-unoccupied-molecular-orbital fullerene derivative indene-C60 bisadduct improved the overall conversion efficiency of lycopene-based OSCs by enhancing the open-circuit current (Voc). Interestingly, further investigation on charge-separation dynamics revealed that photocurrent is generated only from the S2 (1Bu+) state, and the others underwent ultrafast excitation relaxation through S2 → S1 (2Ag–) → S0 (ground state), leaving much room for further improvement.

In order to realize solution-processed, electrolyte-free organic solar cells with near-infrared absorption purpurins, organic semiconductors fullerene C70 and its derivative PC70BM were used instead of inorganic semiconductors TiO2 and SnO2 used in our previous publication for dye-sensitized solar cells (J. Phys. Chem. C2011, 115, 24394–24402). The LUMO energy levels of both metal-free and zinc purpurin sensitizers, H2P and ZnP, were higher than that of the examined fullerenes to promise efficient charge separation. These purpurin molecules can readily form aggregates through a simple spin-coating process. In C70-based planar-heterojunction (PHJ) solar cells, a thin active layer of 5 nm gave the best solar power conversion efficiencies of up to 1.3% and 1.7% for H2P and ZnP, respectively. To improve the photovoltaic performance, bulk-heterojunction (BHJ) solar cells with the purpurin:PC70BM blends were fabricated. The best blend ratio of 1:4 has been found to afford the highest photovoltaic performance for both of the purpurin sensitizers, due to the most efficiently balanced charge transport toward both electrodes. Zinc oxide (ZnO) as the hole blocking layer improved both the photocurrent and photovoltage for the ZnP-based BHJ solar cells but not for the H2P-based BHJ solar cells. This difference has been attributed to an inefficient charge separation at the H2P/ZnO interface that competes with the normal charge separation at the H2P/PC70BM interface. The photovoltaic performances recorded at AM 1.5 solar illumination (100 mW/cm2) for the BHJ solar cells were improved at high temperature to 1.7% and 2% for H2P and ZnP, respectively, which assures these solar cells to work at ambient environments in real-world applications.

Purpurin sensitizers with and without the central zinc, ZnP and H2P, have been synthesized and used in dye-sensitized solar cells. Both the sensitizers readily formed aggregates on the semiconductor surface. The DFT and TD-DFT calculations suggest that the major difference between the two sensitizers is ascribable to the energy levels of their four molecular orbitals. With biased potential in the solid-state photovoltaic diodes, the photoresponse of ZnP and H2P started from −2 and −2.5 V, respectively, and the observed difference is in agreement of the difference of calculated LUMO energy level for the two sensitizers. ZnP gave much better photovoltaic performance than H2P, when TiO2 electrode and 4-tert-butylpyridine (TBP)-free electrolyte were employed. The decrease of photocurrent of ZnP-based solar cell in TBP-containing electrolyte is attributed to the change in energy level of the electron acceptor, while that of H2P-based solar cell in TBP-containing electrolyte is ascribed to the change of electron donor state. The replacement of TiO2 with SnO2 substantially improved the photocurrent of solar cells because the electron injection from LUMO orbital of the dye sensitizers becomes favorable. A clear observation of photocurrent generation from the dye aggregate suggests that the photon-generating excitons can diffuse over the dye aggregate and finally reach the semiconductor surface. TBP in electrolyte can disturb the dye aggregation, and this will reduce the possibility of exciton annihilation in dye layer, which was supported by the sub-picosecond time-resolved absorption spectra.