Minjung Shin

Distorted Asymmetric Cubic Nanostructure of Soluble Fullerene Crystals in Efficient Polymer:Fullerene Solar Cells

We found that 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C(61) (PCBM) molecules make a distorted asymmetric body-centered cubic crystal nanostructure in the bulk heterojunction films of reigoregular poly(3-hexylthiophene) and PCBM. The wider angle of distortion in the PCBM nanocrystals was approximately 96 degrees , which can be assigned to the influence of the attached side group to the fullerene ball of PCBM to bestow solubility. Atom concentration analysis showed that after thermal annealing the PCBM nanocrystals do preferentially distribute above the layer of P3HT nanocrystals inside devices.

Temperature/time-dependent crystallization of polythiophene: fullerene bulk heterojunction films for polymer solar cells.

We report the temperature/time-dependent crystallization of poly(3-hexylthiophene) (P3HT) in blend films of P3HT and [6,6]-phenyl-C(61)-butyric acid methyl ester (PC₆₁BM). The crystallization behaviour of P3HT:PC₆₁BM blend films was measured as a function of annealing time at two different temperatures (150°C and 160°C) by employing a synchrotron-radiation grazing-incidence angle X-ray diffraction (GIXD) technique. The crystallization behaviour was correlated with corresponding solar cells annealed under the same conditions. Results showed that the trend of device performance was almost in accordance with that of the (100) GIXD intensity, indicating that the nanostructure change in blend films does affect the device performance. However, the intermediate zones related to nanomorphology fluctuations, which were observed for lower temperature (140°C) annealing, were significantly suppressed at higher temperature (150°C and 160°C) annealing.

Nanomorphology-driven two-stage hole mobility in blend films of regioregular and regiorandom polythiophenes

We report the nanomorphology-driven two-stage hole mobility in the blend films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) polymers of which regioregularity was 92.2% and 33.0%, respectively. The hole mobility of blend films was measured by employing a top-contact type organic field-effect transistor which has an aromatic polyimide gate insulating layer and silver source/drain electrodes. Results showed that the hole mobility of blend films was suddenly reduced as large as two orders of magnitude as the bulk regioregularity of blend films decreased from 89.8% to 86.3%, even though the hole mobility change was far less than one order of magnitude after and before this boundary condition. The discontinuous two-stage hole mobility trend has been attributed to the destruction of P3HT chain ordering/alignment in the blend films at the boundary blend composition, as evidenced from the huge changes in optical absorption coefficient, surface nanomorphology, and in-plane/out-of-plane nanostructures in the blend films.

Initial performance changes of polymer/fullerene solar cells by short-time exposure to simulated solar light.

Recently, polymer solar cells have attracted keen interest as sustainable energy conversion media owing to their potential for low-cost manufacturing and advanced features, including their semi-transparency and light-weight, ultrathin, and flexible form factors. These characteristics are expected to broaden the application of polymer solar cells towards mobile and/or consumer electronics; no longer limited to conventional solar power modules fixed to land installations and/or buildings. So far, the power conversion efficiency (PCE) of polymer solar cells has been gradually improved by controlling the nanomorphology of the active layer as well as by introducing new tailored polymer materials based on bandgap engineering. Recent reports have forecast that the PCE of polymer solar cells can reach 11–16 % via further bandgap engineering of semiconducting polymers and acceptor molecules. 13] However, in addition to enhancing the efficiency, the stability of polymer solar cells is a critical issue for their commercialization. To date, tens of reports on the stability and/or lifetime of polymer solar cells have appeared. These reports have shown the importance of encapsulation to avoid attack by oxygen, as well-documented in the study of organic light-emitting devices (OLEDs). The studies pay particular attention to the time-dependent decay of short-circuit current density (Jsc) and open-circuit voltage (Voc), in the presence of performance oscillation, in polymer solar cells and/or modules prepared using blend films of poly(3-hexylthiophene) (P3HT) and 1-(3methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). [25] The parameters Jsc and Voc quickly degraded after about 200 h at 65 8C, whereas the fill factor (FF) increased up to 600 h. However, the initial quick drop in Jsc and Voc was not clearly addressed although it might be key to understanding the stability issues of polymer solar cells. Hence, in this work, we attempt to describe the initial performance changes in P3HT:PCBM solar cells during continuous illumination by simulated solar light. To concentrate on the very early stages of illumination, we collected device data every hour, from 0 to 11 h (the longest time here). To understand the performance changes, the blend films were analyzed by optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and phase-contrast atomic force microscopy (AFM) before and after illumination (11 h). Results and Discussion

Multilayer organic solar cells with wet-processed polymeric bulk heterojunction film and dry-processed small molecule films

Here, we report multilayer organic solar cells fabricated using a mix of solution (wet) and thermal evaporation (dry) techniques, which consist of indium-tin oxide (ITO), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), poly(3-hexylthiophene) (P3HT):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), C60, bathocuproine (BCP), and aluminum layers. Results show that the short circuit current density (JSC) of a ITO∕PEDOT:PSS∕C60/BCP/Al device was greatly improved by inserting a pristine P3HT light-absorbing layer between the PEDOT:PSS and C60 layers. Addition of PCBM to the P3HT layer lowered the JSC in the devices compared to the pristine P3HT layer and, in general, the JSC continued to decline with increasing PCBM content.

Influence of thermal annealing on the deformation of a lithium fluoride nanolayer in polymer : fullerene solar cells

In this letter we attempted to examine the possible deformation of a lithium fluoride (LiF) nanolayer inserted in between a light-absorbing polymeric layer (a mix of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)) and a metal (Al) electrode in polymer : fullerene solar cells. The direct annealing of devices with the LiF nanolayer resulted in a gradual decay in device performances with annealing time at an optimum temperature, after exhibiting a maximum efficiency by very short-time annealing (3 min). In contrast, the study of film annealing and post-deposition of the LiF nanolayer showed that a longer-time (120 min) annealing led to a maximum efficiency. These results indicate that the LiF nanolayer in devices is vulnerable to the deformation upon thermal annealing.

Effect of Long Time Annealing and Incident Light Intensity on the Performance of Polymer: Fullerene Solar Cells

In this paper, we report the effect of long annealing time device and the influence of incident light intensity on the performance of polymer solar cells prepared using a Ca/Al top electrode and a blend film of regioregular poly(3-hexylthiophene) and soluble fullerene as a photoactive layer. Results showed that the device performance exhibited a four-stage trend with annealing time, which was attributed to the continuous change of nanomorphology in blend films. In particular, the longest time annealing (300 min) did not degrade the device performance, thus indicating durability of the thin Ca electrode. Interestingly, an extremely high fill factor (76%-82%) was obtained at a low light intensity.

Influence of hole-transporting material addition on the performance of polymer solar cells

We report the effect of hole-transporting material (HTM) addition on the performance of polymer solar cells based on blends of poly(3-hexylthiophene) (P3HT) and soluble fullerene. N,N′-Diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-diphenyl]-4,4′-diamine (TPD) was chosen as a HTM because it is one of the well-established HTMs despite its drawback of wide band gap (3.1 eV) for solar cell applications. Two specialized measurement systems, synchrotron radiation grazing incidence angle X-ray diffraction (GIXD) and phase-mode atomic force microscopy (AFM), were employed to understand the correlation between device performance and nanostructures of blend films. Results showed that the addition of 3–7 wt% TPD improved the short circuit current density of unannealed devices due to the improved P3HT crystallization induced by the presence of TPD molecules. Although the short circuit current density of the binary blend device was recovered to the highest value after thermal annealing, the improved fill factor of TPD-added ternary blend devices at the 3–7 wt% TPD content led to the slightly enhanced power conversion efficiency at 3 wt% TPD in spite of reduced optical absorption in the ternary blend film.

Wide band gap triarylamine derivative doped with organosulfonic acid and its application for organic light-emitting devices

We report a bis-type wide band gap triarylamine derivative, N, N′-diphenyl-N, N-bis(3-methylphenyl)-[1,1′-diphenyl]-4,4′-diamine (TPD), that was chemically doped with camphorsulfonic acid (CSA) in chloroform. Optical measurements showed that new optical absorption peak, leading to a broadband (480–620 nm) photoluminescence, was created in the TPD–CSA after doping, while the redox peak of TPD was shifted by the CSA doping. A tuneable (from green to red) electroluminescence (EL) was measured from organic light-emitting devices with the TPD–CSA-dispersed polymer layer and a green emission layer (tris(8-hydroxyquinolinato)aluminium – Alq3). The red EL, which is actually impossible from the Alq3 layer that intrinsically emits typical green light, has been ascribed to the contribution from both newly generated gap state in the TPD–CSA molecules and shifted charge recombination zone owing to hole mobility changes in the TPD–CSA-dispersed polymer layers.