Jungwoo Heo

A universal processing additive for high-performance polymer solar cells

To optimize the performance of polymer solar cells, various techniques have been developed and reported from various research fields. The introduction of processing additives in the polymer : PCBM bulk-heterojunction solution is one of the efficient strategies used to improve the cell performance. Although numerous solvents have been presented as processing additives, an appropriate processing additive is always different for each polymer solar cell. In this manuscript, we demonstrate that diphenyl ether (DPE) works as a widely beneficial processing additive, which provides high-performance polymer solar cells from all types of photovoltaic polymers. DPE acts like a theta solvent to photovoltaic polymers, helps to form ideal bulk-heterojunction film morphologies and suppresses bimolecular charge recombination. This study suggests an efficient way to optimize the performance of polymer solar cells using DPE regardless of the photovoltaic polymers used.

Solution-processed CdS transistors with high electron mobility

Solution-processed CdS field effect transistors (FETs) and solar cells are demonstrated via spin-coating and thermal annealing of soluble cadmium thiolate compounds. The synthesis is carried out in one simple step using cadmium oxide and tertiary alkane thiols. The cadmium thiolates are soluble in organic solvents such as chloroform and may be spin-coated, like organic semiconductors, to form thin films. The cadmium thiolate films decompose rapidly at 300 °C to yield semiconducting cadmium sulfide films. FETs are easily fabricated using these films and exhibit electron mobilities of up to 61 cm2 V−1 s−1, which compare favourably to FETs prepared from other solution-processed materials such as organic semiconductors, inorganic nanoparticles or chalcogenide films. Initial attempts to prepare hybrid bilayer solar cells were successfully realized by spin-coating a p-type semiconducting polymer layer on top of the n-type CdS film. These devices show significant photocurrent response from both the CdS and polymer layers, indicating that the CdS films are able to participate in photo-induced electron transfer from the polymer to the CdS layer as well as photo-induced hole transfer from CdS to the polymer layer.

Highly efficient polymer solar cells with a thienopyrroledione and benzodithiophene containing planar random copolymer

We synthesized and characterized a new low band-gap copolymer, PBTTFB, incorporating N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) as the acceptor and benzodithiophene (BDT) and (2,5-difluorophenylene)dithiophene as the donor units with S⋯F and S⋯O non-covalent intramolecular interactions. The PBTTFB polymer replaced bis(dodecyloxy)benzo[c][1,2,5]thiadiazole (BT) in P1, a previously reported polymer, with 5-dodecyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione and exhibited improved macromolecular planarity and molecular ordering of the molecular structure. UV-vis absorption, electrochemical properties, bulk-heterojuction (BHJ) film morphology, and molecular ordering as well as photovoltaic charaterization derived from PBTTFB were studied and analyzed to explore the effect of the thienopyrroledione unit instead of the benzodithiophene unit in the molecular backbone of the polymer. From photovoltaic charaterization, we obtained an enhanced Jsc value of 14.51 mA cm−2 from the PBTTFB polymer compared to the Jsc value of 10.54 mA cm−2 from P1 due to improved macromolecular planarity. Furthermore, PBTTFB exhibited the highest PCE of 8.25% by adding DPE as a processing additive due to better interpenetration networks for improving charge transport and collection.

Clean thermal decomposition of tertiary-alkyl metal thiolates to metal sulfides: environmentally-benign, non-polar inks for solution-processed chalcopyrite solar cells

We report the preparation of Cu2S, In2S3, CuInS2 and Cu(In,Ga)S2 semiconducting films via the spin coating and annealing of soluble tertiary-alkyl thiolate complexes. The thiolate compounds are readily prepared via the reaction of metal bases and tertiary-alkyl thiols. The thiolate complexes are soluble in common organic solvents and can be solution processed by spin coating to yield thin films. Upon thermal annealing in the range of 200–400 °C, the tertiary-alkyl thiolates decompose cleanly to yield volatile dialkyl sulfides and metal sulfide films which are free of organic residue. Analysis of the reaction byproducts strongly suggests that the decomposition proceeds via an SN1 mechanism. The composition of the films can be controlled by adjusting the amount of each metal thiolate used in the precursor solution yielding bandgaps in the range of 1.2 to 3.3 eV. The films form functioning p-n junctions when deposited in contact with CdS films prepared by the same method. Functioning solar cells are observed when such p-n junctions are prepared on transparent conducting substrates and finished by depositing electrodes with appropriate work functions. This method enables the fabrication of metal chalcogenide films on a large scale via a simple and chemically clear process.

Peroptronic devices: perovskite-based light-emitting solar cells

Solution processed perovskite semiconductors have developed rapidly over the past decade to yield excellent performance in both solar cell and light emitting diode devices. Both of these device types are prepared using similar materials and architectures, raising the possibility of perovskite based light emitting solar cells. Recent reports have indicated that some low band gap perovskite solar cells are able to emit infrared light efficiently, however, intermediate band gap perovskite solar cells which emit visible light have not, to the best of our knowledge been deliberately designed or extensively characterized. In this work, we have investigated the use of different electron transport layers in order to minimize energetic barriers to electron injection and extraction in methylammonium lead bromide (MAPbBr3) films. We demonstrate that through appropriate band structure engineering, MAPbBr3 can be used to make such “peroptronic” light-emitting solar cells, which simultaneously exhibit efficient solar cell power conversion efficiencies over 1% and 0.43 lm W−1 green light emission.

Formamidinium-based planar heterojunction perovskite solar cells with alkali carbonate-doped zinc oxide layer

We herein demonstrate n-i-p-type planar heterojunction perovskite solar cells employing spin-coated ZnO nanoparticles modified with various alkali metal carbonates including Li2CO3, Na2CO3, K2CO3 and Cs2CO3, which can tune the energy band structure of ZnO ETLs. Since these metal carbonates doped on ZnO ETLs lead to deeper conduction bands in the ZnO ETLs, electrons are easily transported from the perovskite active layer to the cathode electrode. The power conversion efficiency of about 27% is improved due to the incorporation of alkali carbonates in ETLs. As alternatives to TiO2 and n-type metal oxides, electron transport materials consisting of doped ZnO nanoparticles are viable ETLs for efficient n-i-p planar heterojunction solar cells, and they can be used on flexible substrates via roll-to-roll processing.

Nanoparticle‐Enhanced Silver‐Nanowire Plasmonic Electrodes for High‐Performance Organic Optoelectronic Devices

Improved performance in plasmonic organic solar cells (OSCs) and organic light-emitting diodes (OLEDs) via strong plasmon-coupling effects generated by aligned silver nanowire (AgNW) transparent electrodes decorated with core-shell silver-silica nanoparticles (Ag@SiO2 NPs) is demonstrated. NP-enhanced plasmonic AgNW (Ag@SiO2 NP-AgNW) electrodes enable substantially enhanced radiative emission and light absorption efficiency due to strong hybridized plasmon coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) modes, which leads to improved device performance in organic optoelectronic devices (OODs). The discrete dipole approximation (DDA) calculation of the electric field verifies a strongly enhanced plasmon-coupling effect caused by decorating core-shell Ag@SiO2 NPs onto the AgNWs. Notably, an electroluminescence efficiency of 25.33 cd A-1 (at 3.2 V) and a power efficiency of 25.14 lm W-1 (3.0 V) in OLEDs, as well as a power conversion efficiency (PCE) value of 9.19% in OSCs are achieved using hybrid Ag@SiO2 NP-AgNW films. These are the highest values reported to date for optoelectronic devices based on AgNW electrodes. This work provides a new design platform to fabricate high-performance OODs, which can be further explored in various plasmonic and optoelectronic devices.