Polymer solar cells are one of the promising energy sources because of the easy solution-processable production with large area at a low cost without toxicity. Among the polymer materials, a donor-acceptor conjugated copolymer PTB7 has been extensively studied because of the typical high-performance polymer solar cells. Here, we show operando direct observation of charge accumulation in PTB7:PCBM blend solar cells from a microscopic viewpoint using electron spin resonance spectroscopy. The accumulation of ambipolar charges in the PTB7-based cells is directly observed for the first time, which shows a clear correlation with the performance deterioration during device operation. The sites of the ambipolar charge accumulation are elucidated at the molecular level, whose information would be useful for improving the cell durability in addition to the performance improvement.

Electron spin resonance (ESR) spectroscopy of 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) thin films and perovskite (CH3NH3PbI3)/spiro-OMeTAD layered films are reported. Clear ESR signals (g = 2.0030) were observed by adding a dopant lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) to the spiro-OMeTAD thin films, which directly showed the spin (hole) formation in spiro-OMeTAD by the Li-TFSI doping. The number of spins in the spiro-OMeTAD thin film has increased by more than two orders of magnitude by the Li-TFSI doping under dark conditions, which demonstrates from a microscopic viewpoint that Li-TFSI has high doping effects for the spiro-OMeTAD thin films. Under simulated solar irradiation, the spin density in the spiro-OMeTAD thin films and the perovskite/spiro-OMeTAD layered films largely increased by the Li-TFST doping due to the formation of long-lived holes in spiro-OMeTAD. The transient responses of the number of photogenerated spins, Nspin, of the layered films upon the light irradiation showed the increase and the decrease in the Nspin due to the hole transfer and recombination at the perovskite/spiro-OMeTAD interface. The states of long-lived holes in the spiro-OMeTAD layers were analyzed using the simulation of the ESR spectra, which reveals the mobile photogenerated holes with a lifetime >10 μs.

Using electron spin resonance (ESR), we clarified the origin of the efficiency degradation of polymer solar cells containing a lithium-fluoride (LiF) buffer layer created by a thermal annealing process after the deposition of an Al electrode (post-annealing). The device structure was indium-tin-oxide/ poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/LiF/Al. Three samples consisting of quartz/P3HT:PCBM/LiF/Al, quartz/P3HT:PCBM/Al, and quartz/PCBM/LiF/Al were investigated and compared. A clear ESR signal from radical anions on the PCBM was observed after LiF/Al was deposited onto a P3HT:PCBM layer because of charge transfer at the interface between the PCBM and the LiF/Al, which indicated the formation of PCBM−Li+ complexes. The number of radical anions on the PCBM was enhanced remarkably by the post-annealing process; this enhancement was caused by the surface segregation of PCBM and by the dissociation of LiF at the Al interface by the post-annealing process. The formation of a greater number of anions enhanced the electron scattering, decreased the electron-transport properties of the PCBM molecules, and caused an energy-level shift at the interface. These effects led to degradation in the device performance.

Charge accumulation states in organic solar cells were studied in detail by a microscopic characterization with light-induced electron spin resonance (ESR) spectroscopy during device operation using a typical polymer solar cell of indium-tin oxide (ITO)/poly (3,4-ethylenedioxythiophene):poly (4-styrenesulfonate) (PEDOT:PSS)/regioregular poly(3-hexylthiophene) (P3HT):fullerene [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM)/LiF/Al. Two light-induced ESR signals with a narrow and broad linewidth were observed where the broad component with a slow accumulation rate clearly correlated with the deterioration of the device performance. From the ESR analysis, the charge accumulation state causing the device deterioration is ascribed to holes at P3HT polymer-chain ends with residual bromines in amorphous regions in the active layer. Preventing the charge accumulation seems indispensable to develop highly durable polymer solar cells with high performance.

A full range spectral domain optical coherence tomography (SD-OCT) technique that relies on the linear phase modulation of one of the interferometer arms has been widely utilized. Although this method is useful, the mirror image elimination is not perfect for samples in which regions with high axial motion exist. In this paper, we introduce a new modulation pattern to overcome this mirror image elimination failure. This new modulation is a parabolic phase modulation in the transverse scanning direction, and is applied to the SD-OCT reference beam by an electro-optic modulator. Flow phantom and in vivo experiments demonstrate that for moving structures with large velocities, this parabolic phase modulation technique presents better mirror image elimination than a standard linear phase modulation method. A direct consequence of this enhanced mirror image removal is an improved velocity range obtained with phase-resolved Doppler imaging. Consequently, applying the proposed technique in retinal blood flow measurements may be useful for ophthalmologic diagnosis.