Light-Induced Degradation Mechanism in Poly(3-hexylthiophene)/Fullerene Blend Solar Cells

The mechanism of light-induced degradation in organic solar cells based on regioregular poly(3-hexylthiophene) and indene-C60 bisadduct is studied by transient absorption (TA) and electron spin resonance (ESR) measurements. After 45 h light exposure under simulated solar illumination at 100 mW cm-2, the short-circuit current density, open-circuit voltage, and fill factor are all degraded by about 20%-30% relative to the initial photovoltaic parameters. For the assignment of limiting conversion processes in the degraded solar cells, exciton diffusion into a donor/acceptor interface, charge transfer at the interface, charge dissociation into free charge carriers, and charge collection to each electrode are observed before and after the light exposure by the TA measurement. As a result, it is found that the charge collection deteriorates after the light exposure because of light-induced charge trap formation in the bulk of the active layer. The origin of charge traps is further discussed on the basis of ESR measurements and density functional theory calculation

Direct Observation of Radical States and the Correlation with Performance Degradation in Organic Light-Emitting Diodes During Device Operation

Microscopic characterization of radical states in organic light‐emitting diodes (OLEDs) during device operation is useful for elucidating the degradation mechanism because the radical formation has been considered as non‐radiative recombination centers. Electron spin resonance (ESR) spectroscopy is suitable for such characterization because it can directly observe radicals in OLEDs. In this work, the detailed ESR investigation into the radical states in OLEDs during device operation is firstly reported using a typical light‐emitting Alq3‐based OLEDs. The simultaneous measurements of the ESR signal and the luminance of the same OLED are performed to study the direct correlation between the radical states and the performance degradation. These characteristics show that the luminance monotonically decreases and an ESR signal concomitantly increases as the duration of the device operation increases after operating the OLED. Using the analysis of density functional theory (DFT) calculation, the origin of the newly emerged ESR signal is ascribed to the cationic species due to decomposed Alq3 molecules. The elucidation of the radical species formed in OLEDs during device operation has been demonstrated at a molecular level for the first time. This ESR analysis would provide useful knowledge for understanding the degradation mechanism in the OLEDs at the molecular level

Mechanistic investigation into the light soaking effect observed in inverted polymer solar cells containing chemical bath deposited titanium oxide

In the glass-indium tin oxide (ITO)/titanium oxide (TiOx)/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS)/Au cell (TiOx cell), which contains amorphous titanium oxide prepared by chemical bath deposition and dried at 150 °C, a light soaking effect has been observed upon irradiation with white light. In contrast, in ITO/titanium oxide (TiO2)/P3HT:PCBM/PEDOT:PSS/Au cell (TiO2 cell), which contains anatase titanium oxide prepared by heat treatment at 450 °C, the maximum power conversion efficiency was obtained just after irradiation with white light. The number of P3HT+• cation radicals in the quartz-ITO/TiOx and TiO2/P3HT:PCBM substrates was estimated by ESR measurements at room temperature upon irradiation with white light. It increased gradually with an increase in irradiation time for the TiOx substrate but increased only slightly just after light irradiation for the TiO2 substrate. Upon irradiation with UV-cut light, the performance of the TiOx cell was inferior to that of the TiO2 cell. This could be related to the resistances of the P3HT:PCBM layers which were estimated by alternating current impedance spectroscopy. The resistance of the P3HT:PCBM layer in the TiOx cell was much larger than that in the TiO2 cell, though the difference between the two cells was merely heat treatment temperature of titanium oxide films using as electron collection layers. That is, the concentration of photocarriers in the P3HT:PCBM of the TiOx cell was significantly less than that in the P3HT:PCBM of the TiO2 cell. From these experimental results, the light soaking effect could be reasonably explained by assuming the existence of charge recombination centers in the TiOx near the TiOx/P3HT:PCBM interface

これまでのプレイスメントテスト実施を振り返る

2002年度の工学部カリキュラム改正において,「基礎科目」が設置されるとともに,基礎教養科目等で習熟度別クラス編成による授業が行われることとなった。このクラス分けのために,現在,本学新入学学生は4月最初にいわゆるプレイスメントテストを受けている。プレイスメントテストについては毎年度,簡単な報告を行ってきたが,2002年度以来5回の実施を経たいま,これまでの経過報告を含めた全体報告を行おうと考えている。まず,プレイスメントテストに関しての経緯・状況を述べ,プレイスメントテストで得られたデータからいくつかの事実を報告する。次に各実施科目でのプレイスメントテストの状況報告を行う。In 2002, we started a new educational program for the freshmen in the faculty of engineering. We put new introductory courses for several subjects in the new program. Then the freshmen in our university started their study in classes to which they are arranged according to their proficiency level. For this classification, the freshmen have to take the "Placement Test" in the beginning of April. Every year since 2002, we have reported the results of the Placement Test briefly. In this article, we will report the processes and results of these five years

Improvement of open-circuit voltage due to reverse electron transfer in inverted perovskite solar cells

 The data of the paper "Improvement of open-circuit voltage due to reverse electron transfer in inverted perovskite solar cells".  </p