Jongjin Lee

Electrostatically Self‐Assembled Nonconjugated Polyelectrolytes as an Ideal Interfacial Layer for Inverted Polymer Solar Cells

Polymer solar cells (PSCs) based on the bulk heterojunction (BHJ) blend of a conjugated polymer donor and a fullerene acceptor have gained considerable attention as a cost-effi cient, fl exible, and portable energy source. [ 1–3 ] Considering the practical aspects of its commercialization, the inverted device structure of PSCs (I-PSCs), consisting of a BHJ photoactive layer between indium tin oxide (ITO) as a bottom cathode and a high work function (WF) metal (Ag or Au) as a top anode, is an advantageous approach due to its superior long-term stability and printability. [ 4–6 ] In this type of inverted device structure, an additional interfacial layer between the active layer and ITO electrode must be introduced to establish the device concept. The high WF of the bottom ITO cathode (approximately 4.8 eV) hampers the formation of an ohmic contact with the lowest unoccupied molecular orbital (LUMO) level of fullerene and makes the creation of a high built-in fi eld used to break the electrical symmetry of the device diffi cult. Thus, many researchers explored a variety of interfacial materials to shift and modify the energy level of the ITO cathode. [ 4–16 ]

Light-soaking issue in polymer solar cells: Photoinduced energy level alignment at the sol-gel processed metal oxide and indium tin oxide interface

We report the origin of the strong UV-irradiation dependence, generally known as a “light-soaking” process, in inverted polymer solar cells (I-PSCs) using the interface of an sol-gel processed titanium sub-oxide (TiOx) and indium tin oxide (ITO) cathode. When I-PSCs incorporating TiOx as an electron-selecting layer were fabricated, the as-prepared devices exhibited an anomalous J-V curve with a kink shape, resulting in an extremely low efficiency. However, the kink shape disappeared after white light irradiation for considerable duration, after which the device parameters recovered the normal values expected for this class of devices. By using electrochemical impedance spectroscopy and by measuring the contact potential difference and transient photoconductivity of the TiOx layer, we found that the light-soaking process in I-PSCs originates from the photoinduced “rearrangement of the Fermi levels” at the sol-gel processed TiOx and ITO cathode interface together with trap sites existing in the TiOx layer. ...

Direct observation of internal potential distributions in a bulk heterojunction solar cell

Cross-sectional potential distributions of a polymer bulk heterojunction (BHJ) solar cell, consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester blends, were directly observed by using Kelvin probe force microscopy. It was found that the bulk electric field of BHJ is nearly field-free compared with that of classical metal-insulator-metal model, whereas the electric field of BHJ device is confined to the cathode interface. The cathode-interfaced rectification junction is studied to originate from the formation of bilayered p-n junction in the BHJ.

Biased internal potential distributions in a bulk-heterojunction organic solar cell incorporated with a TiOx interlayer

External-biased potential distributions of a polymer bulk-heterojunction (BHJ) solar cell, incorporated with electron/hole transporting layers, were directly obseved through a cross-sectional Kelvin probe force microscopy. The bulk electric field of BHJ was found to be nearly field-free even under reverse biases, and the field-free region was probed to expand with the incorporation of TiOx electron transporting layer; as a result, inducing a decrease of quasi-Fermi level splitting region in obtaining a high fill factor in the TiOx-interlayered junction photodiodes.

Seamless polymer solar cell module architecture built upon self-aligned alternating interfacial layers

An efficient module cell architecture of a polymer solar cell built upon self-aligned alternating interfacial layers is presented. Alternating conventional and inverted subcells are serially connected on a single compound electrode with self-aligned interfacial layers. A high relative power conversion efficiency of 82% of the large-area module cell (4.24%) to the small-sized laboratory cells (5.19%) could be obtainable.

Magnetization states of canted antiferromagnetic YFeO3 investigated by terahertz time-domain spectroscopy

We investigate magnetization states of a canted antiferromagnetic YFeO3 by using terahertz (THz) time-domain spectroscopy under a variation of an external magnetic field. As a quasi-ferromagnetic mode near 0.3 THz is excited by the magnetic field of the THz pulse, a precession of the magnetic moment and its induction decay radiate the THz wave. Since oppositely aligned magnetic domains emit the circularly/elliptically polarized THz waves with opposite helicities, a detection of specific polarization of the THz wave shows a clear hysteresis behavior in good agreement with a magnetization curve obtained as a function of the applied magnetic field. Furthermore, time-domain measurement of excited magnetic moment can provide direct information about the magnetization decay dynamics.