Taehyo Kim

High-Performance Solution-Processed Non-Fullerene Organic Solar Cells Based on Selenophene-Containing Perylene Bisimide Acceptor

Non-fullerene acceptors have recently attracted tremendous interest because of their potential as alternatives to fullerene derivatives in bulk heterojunction organic solar cells. However, the power conversion efficiencies (PCEs) have lagged far behind those of the polymer/fullerene system, mainly because of the low fill factor (FF) and photocurrent. Here we report a novel perylene bisimide (PBI) acceptor, SdiPBI-Se, in which selenium atoms were introduced into the perylene core. With a well-established wide-band-gap polymer (PDBT-T1) as the donor, a high efficiency of 8.4% with an unprecedented high FF of 70.2% is achieved for solution-processed non-fullerene organic solar cells. Efficient photon absorption, high and balanced charge carrier mobility, and ultrafast charge generation processes in PDBT-T1:SdiPBI-Se films account for the high photovoltaic performance. Our results suggest that non-fullerene acceptors have enormous potential to rival or even surpass the performance of their fullerene counterparts.

Small‐Bandgap Polymer Solar Cells with Unprecedented Short‐Circuit Current Density and High Fill Factor

Small-bandgap polymer solar cells (PSCs) with a thick bulk heterojunction film of 340 nm exhibit high power conversion efficiencies of 9.40% resulting from high short-circuit current density (JSC ) of 20.07 mA cm(-2) and fill factor of 0.70. This remarkable efficiency is attributed to maximized light absorption by the thick active layer and minimized recombination by the optimized lateral and vertical morphology through the processing additive.

Capillary Printing of Highly Aligned Silver Nanowire Transparent Electrodes for High-Performance Optoelectronic Devices

Percolation networks of silver nanowires (AgNWs) are commonly used as transparent conductive electrodes (TCEs) for a variety of optoelectronic applications, but there have been no attempts to precisely control the percolation networks of AgNWs that critically affect the performances of TCEs. Here, we introduce a capillary printing technique to precisely control the NW alignment and the percolation behavior of AgNW networks. Notably, partially aligned AgNW networks exhibit a greatly lower percolation threshold, which leads to the substantial improvement of optical transmittance (96.7%) at a similar sheet resistance (19.5 Ω sq(-1)) as compared to random AgNW networks (92.9%, 20 Ω sq(-1)). Polymer light-emitting diodes (PLEDs) using aligned AgNW electrodes show a 30% enhanced maximum luminance (33068 cd m(-2)) compared to that with random AgNWs and a high luminance efficiency (14.25 cd A(-1)), which is the highest value reported so far using indium-free transparent electrodes for fluorescent PLEDs. In addition, polymer solar cells (PSCs) using aligned AgNW electrodes exhibit a power conversion efficiency (PCE) of 8.57%, the highest value ever reported to date for PSCs using AgNW electrodes.

Highly efficient plasmonic organic optoelectronic devices based on a conducting polymer electrode incorporated with silver nanoparticles

Highly efficient ITO-free polymeric electronic devices were successfully demonstrated by replacement of the ITO electrode with a solution-processed PEDOT:PSS electrode containing Ag nanoparticles (NPs). Polymer solar cells (PSCs) and light emitting diodes (PLEDs) were fabricated based on poly(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) (PTBT):PC61BM and Super Yellow as a photoactive layer, respectively. The surface plasmon resonance (SPR) effect and improved electrical conductivity by the Ag NPs clearly contributed to increments in light absorption/emission in the active layer as well as the conductivity of the PEDOT:PSS electrode in PSCs and PLEDs. The ITO-free bulk heterojunction PSCs showed a 1% absolute enhancement in the power conversion efficiency (3.27 to 4.31%), and the power efficiency of the PLEDs was improved by 124% (3.75 to 8.4 lm W−1) compared to the reference devices without Ag NPs. The solution-processable conducting polymer, PEDOT:PSS with Ag NPs, can be a promising electrode for large area and flexible optoelectronic devices with a low-cost fabrication process.

Synthesis of fluorinated analogues of a practical polymer TQ for improved open-circuit voltages in polymer solar cells

In an attempt to further lower the HOMO of a cost-effective polymer poly(2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-dyl-alt-thiophene-2,5-diyl) (TQ) by adding F atoms onto the existing quinoxaline acceptor within the polymer backbone, we have synthesized two structurally identical fluorinated analogues of TQ (TQ-F (single F) and TQ-FF (double F)), except for the number of F atoms. The effects of inclusion of F atoms on the optical properties, nature of charge transport, and molecular organization are thoroughly investigated. The resulting two fluorinated polymers show a decrease in both the HOMO and the LUMO energy levels relative to non-fluorinated TQ. Moreover, the fluorination of the polymer backbone has lowered the HOMOs more than the LUMOs, slightly widening the energy bandgaps as the number of F atoms increases. Thus, use of these polymers in bulk-heterojunction (BHJ) solar cells, in all cases, leads to large VOC values. The power conversion efficiency (PCE) of the optimized PSCs based on TQ-F reaches 4.41%. In addition, it is interesting to note that, despite TQ-FF having the PCE that is lower than that of TQ-F, an unprecedentedly high VOC of 1.00 V is achieved, which is nearly equal to the highest VOC values ever reported for polymers.

Thienoisoindigo (TIIG)-based small molecules for the understanding of structure–property–device performance correlations

In this contribution, a series of small molecule semiconductors based on the recently conceived thienoisoindigo (TIIG) and three different end-capping moieties (benzene (Bz), naphthalene (Np), and benzofuran (Bf)) with varied electron-donating strength and conformations has been synthesized by Suzuki coupling and utilized for organic photovoltaics (OPVs). Incorporation of different end-capping blocks onto the TIIG core facilitated the tuning of optical properties and the electronic structure (HOMO/LUMO energy levels), solid-state morphology and performance in OPVs. It is apparent that the bandgaps within this series (TIIG-Bz, TIIG-Np, and TIIG-Bf) were progressively red-shifted and the absorption coefficients were enhanced by increasing the conjugation length and/or the donor ability of the end-capping units. In addition, HOMO and LUMO levels were shown to simultaneously follow changes made to the end-capping moieties. The best performing OPVs using TIIG-Np:PC71BM exhibited a power conversion efficiency (PCE) of 1.81% with Jsc = 7.15 mA cm−2, FF = 0.39, and Voc = 0.66 V. With the aim of exploring underlying structure–property relationships for this new class of molecular systems, we have quantitatively investigated various morphological structures in both the pristine small molecule films and small molecule/PC71BM blend films using a combination of grazing incidence wide angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM). In this study, a correlation between the molecular structure, thin film morphology, and photovoltaic properties of these conjugated small molecules was established that provides guidance for the molecular design of new photovoltaic semiconductors based on TIIG units.

Replacing the metal oxide layer with a polymer surface modifier for high-performance inverted polymer solar cells

Replacing ZnO with PEIE, both as a surface modifier for the low work function electrode and as an electron selective layer, enhances the performance and air stability of inverted polymer solar cells by improving electron transport, wettability between the active layer and the cathode, and maximizing light absorption within the active layer without light interference.

Spectroscopically tracking charge separation in polymer : fullerene blends with a three-phase morphology

The coexistence of intermixed amorphous polymer : fullerene phases alongside pure semicrystalline polymer and fullerene phases provides a plausible explanation for effective charge separation in organic photovoltaic blends by providing a cascaded energy landscape. We sought to test this proposal by spectroscopically tracking charge dynamics in 3-phase blends compared with binary counterparts and linking these dynamics to free charge yields. Our study applies broadband transient absorption spectroscopy to a series of closely related alternating thiophene–benzothiadiazole copolymers in which the tuned curvature of the polymer backbone controls the nature and degree of polymer–fullerene intermixing. Free charge generation is most efficient in the 3-phase morphology that features intimately mixed polymer : PCBM regions amongst neat polymer and PCBM phases. TA spectral dynamics and polarization anisotropy measurements reveal the sub-nanosecond migration of holes from intermixed to pure polymer regions of such blends. In contrast, 2-phase blends lack the spectral dynamics of this charge migration process and suffer from severe geminate recombination losses. These results provide valuable spectroscopic evidence for an efficient charge separation pathway that relies on the 3-phase morphology.

Quinoxaline–thiophene based thick photovoltaic devices with an efficiency of ∼8%

A series of difluoroquinoxaline–thiophene based reduced band gap polymers was designed and synthesized by considering non-covalent coulombic interactions in a polymeric main chain. The insertion of different numbers of thiophene moieties allows for the adjustment of the absorption range, frontier energy levels, crystalline self-organization, film morphology and the resulting photovoltaic properties. A thick blend film of poly(thiophene-alt-(2,3-bis(3,4-bis(octyloxy)phenyl)-6,7-difluoroquinoxaline)) (PDFQx-T):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) showed a rough, inhomogeneous and largely phase-separated surface morphology compared to a typical film with ∼100 nm thickness. A similar trend was observed in the surface morphology of a poly(2,2′-bithiophene-alt-(2,3-bis(3,4-bis(octyloxy)phenyl)-6,7-difluoroquinoxaline)) (PDFQx-2T) blend film, showing deteriorated photovoltaic properties with increasing film thickness. In contrast, poly(2,2′:5′,2′′-terthiophene-alt-2,3-bis(3,4-bis(octyloxy)phenyl)-6,7-difluoroquinoxaline) (PDFQx-3T) had a similar blend film morphology for both thick and thin active layers, showing a homogeneous and smooth morphology with a face-on orientation and tight π–π stacking (d-spacing = 3.6 A). The optimized photovoltaic cell based on PDFQx-3T : PC71BM achieved a power conversion efficiency (PCE) of 8% with an open-circuit voltage of 0.74 V, a short-circuit current of 17.19 mA cm−2 and a fill factor of 0.63 at an active layer thickness of ∼270 nm. It is still a challenge to develop photovoltaic polymers which allow efficient charge transport and extraction at a device thickness of ∼300 nm. Fine-adjustment of intra- and interchain interactions must be considered carefully to achieve high device properties for thick devices without deterioration in the blend morphology and charge recombination. This high PCE at an active layer thickness of ∼300 nm may suggest great potential for the mass production of printed polymer solar cells via industrial solution processes.

Straight chain D–A copolymers based on thienothiophene and benzothiadiazole for efficient polymer field effect transistors and photovoltaic cells

Three types of linear and planar-structured donor (D)–acceptor (A) type alternating copolymers were synthesized by incorporating intrachain noncovalent Coulombic interactions, based on thieno[3,2-b]thiophene and benzothiadiazole (BT) moieties. The chain linearity and fine adjustment of interchain organization by the incorporation of different numbers of electronegative fluorine atoms onto BT, significantly affected the frontier energy levels, film morphology, and the resulting charge transport properties. The semi-crystalline morphology and charge carrier transport properties were studied by grazing incidence wide-angle X-ray scattering and polymer field-effect transistor (PFET) characteristic measurements. A hole mobility as high as 0.1 cm2 V−1 s−1 in PFET was obtained for poly[2,5-bis(decyltetradecyloxy)benzene-alt-4,7-bis(thieno[3,2-b]thiophene)-5,6-difluoro-2,1,3-benzothiadiazole] (PPDTT2FBT), suggesting a strong self-organization due to the linear chain configuration with conformation lock. The difluorinated PPDTT2FBT also showed the highest power conversion efficiency (PCE, 6.4%) by blending with PC71BM, but a poorer photovoltaic performance was obtained compared to the wavy-structured counterpart, poly[2,5-bis(2-hexyldecyloxy)phenylene-alt-5,6-difluoro-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole] (PPDT2FBT), reported previously. The mainly edge-on orientation of PPDTT2FBT (with π–π stacking in both xy and z directions) is attributed to the moderate PCE in the blends. Fine modulation of chain linearity may suggest an effective way to control the desirable interchain ordering and bulk film morphology for specific application in polymer solar cells or field effect transistors.