Heejoo Kim

Electroluminescence in polymer-fullerene photovoltaic cells

We report electroluminescence (EL) in photovoltaic (PV) cells based on semiconducting polymer-fullerene composites. By applying a forward bias to the PV cells, the devices exhibited a clear EL action with a peak around 1.5 eV. We ascribe this peak to an “electric field-assisted exciplex” formed between the electrons in the fullerenes and the holes in the polymers, thereby resulting in radiative recombination in the composites. This finding is totally unexpected because of a strong photoluminescence quenching in the same materials. Since the same devices also showed typical photovoltaic effects under illumination, our results demonstrate a dual functionality in one device; polymer photovoltaic cells and polymer light-emitting diodes.

Organic photovoltaic cells based on conjugated polymer/fullerene composites

Abstract Recently, upon mixing buckminsterfullerene, C60, into semiconducting conjugated polymers, an interesting photophysical phenomenon has been discovered. The photoinduced electron transfer from semiconducting polymers onto C60 is reversible, ultrafast with a quantum efficiency approaching unity, and metastable. This photoinduced electron transfer leads to a number of potentially interesting applications, including photodetectors and photovoltaic cells. In this work, we briefly present the photophysics and associated device applications of conjugated polymer–fullerene composites.

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.

Soluble Transition Metal Oxide/Polymeric Acid Composites for Efficient Hole-Transport Layers in Polymer Solar Cells

We report a new method for developing a low-temperature solution processed vanadium oxide (s-VOx) and poly(4-styrene sulfonic acid) (PSS) composite to act as an efficient hole-transport layer (HTL) in polymer solar cells (PSCs). By compositing the s-VOx and PSS (s-VOx:PSS), the work function values of the s-VOx:PSS changed from 5.0 to 5.3 eV. Therefore, the energy level barrier between the HTL and organic active layer decreased, facilitating charge injection/extraction at the interfaces. In addition, the s-VOx:PSS films were denser and had more pin-hole-free surfaces than pristine s-VOx films, resulting in enhanced PSC performance due to significantly decreased leakage currents and excellent device stability in ambient condition. Because our approach of combining soluble transition metal oxide (TMO) and polymeric acid shows dramatically better performance than pristine TMO, we expect that it can provide useful guidelines for the synthesis and application of TMOs for organic electronics in the future.

Template-mediated nano-crystallite networks in semiconducting polymers

Unlike typical inorganic semiconductors with a crystal structure, the charge dynamics of π-conjugated polymers (π-CPs) are severely limited by the presence of amorphous portions between the ordered crystalline regions. Thus, the formation of interconnected pathways along crystallites of π-CPs is desired to ensure highly efficient charge transport in printable electronics. Here we report the formation of nano-crystallite networks in π-CP films by employing novel template-mediated crystallization (TMC) via polaron formation and electrostatic interaction. The lateral and vertical charge transport of TMC-treated films increased by two orders of magnitude compared with pristine π-CPs. In particular, because of the unprecedented room temperature and solution-processing advantages of our TMC method, we achieve a field-effect mobility of 0.25u2009cm(2)u2009V(-1)u2009s(-1) using a plastic substrate, which corresponds to the highest value reported thus far. Because our findings can be applied to various π-conjugated semiconductors, our approach is universal and is expected to yield high-performance printable electronics.

Origin of the open circuit voltage in conjugated polymer-fullerene photovoltaic cells

We fabricated polymer-fullerene photovoltaic (PV) devices using various conjugated polymers, poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly{2-[4-(3,7-dimethyloctyloxy)-phenoxy]-1,4-phenylenevinyl-ene} (p-DMOP-PPV), and their copolymer system, p-DMOP-co-MEH-PPV. By comparing PV characteristics of such devices with a systematic change of the energy levels (ionization potential Ip and electron affinity Ea), we investigated the origin of open circuit voltage (Voc) of the devices. Our results indicate that the magnitude of Voc is governed by the difference between Ea of the fullerene and Ip of the polymer, which is not consistent with previous consideration of the work function difference between electrodes as in conventional metal-insulator-metal type devices.

Effect of a symmetry breaking layer on the open circuit voltage of conventional bulk-heterojunction solar cells

Solution processable titanium suboxide (TiOx) was introduced as an artificial symmetry breaking layer in bulk-heterojunction (BHJ) solar cells comprising a low band gap conjugated polymer, poly[(4,4′-bis(2-ethylhexyl)dithiene[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT), and a soluble fullerene, [6,6]-phenyl-C71-butyric methyl ester (PC71BM). The inserted TiOx layer obviously extracted the same level of open circuit voltage (Voc) regardless of metal work function. Ultraviolet photoelectron spectroscopy (UPS) results indicated that the formation of the interface dipole between the TiOx symmetry breaking layer and metal electrode successfully modifies the effective work function of the cathode electrode, thereby leading to symmetry breaking in BHJ solar cells.

Homogeneous bulk heterojunction networks via surface energy matching at polymer/fullerene interfaces

This work presents homogeneous bulk heterojunction (BHJ) networks between a push-pull π-conjugated polymer donor, poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene)-alt-5,5-(4,7-bis(2,2-bithien-5-yl)-2,2-dimethyl-2H-benzimidazole)) (PININE-BBTMBI) and a fullerene derivative acceptor, [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). By introducing a bi-thiophene unit between the electron-rich and electron-deficient moieties in a repeating unit, the surface energy of PININE-BBTMBI increased to that of PC71BM, enhancing its miscibility in PININE-BBTMBI/PC71BM blends. Direct evidence obtained via atomic force microscopy and the performance of BHJ solar cells indicate that the introduction of the bi-thiophene unit leads to homogeneous BHJ networks and high-performance BHJ solar cells by matching the surface energies at the polymer/fullerene interfaces.