Jeongho Kim

Enhancement of photovoltaic properties of CH3NH3PbBr3 heterojunction solar cells by modifying mesoporous TiO2 surfaces with carboxyl groups

In a new heterojunction solar cell employing CH3NH3PbBr3 (MAPbBr3) as the light absorber, we found that the introduction of a carboxylate monolayer on the mesoporous TiO2 surfaces significantly enhances JSC as well as VOC. In particular, the presence of a bromoacetate monolayer at the interface of TiO2/MAPbBr3 remarkably increases the photovoltaic conversion efficiency (PCE) from 2.65% to 5.57% with JSC of 5.411 mA cm−2, VOC of 1.372 V, and FF of 0.75. Time-resolved photoluminescence measurements indicate that the presence of the interfacial carboxylate groups expedites electron injection from MAPbBr3 to TiO2. Furthermore, according to pulsed light-induced transient measurements (PLITM) of photocurrent analysis, the lifetime of photoinjected electrons (τe) in the TiO2 conduction band (CB) significantly increases with the passivation of the interface, implying the suppression of charge recombination as well.

Significant light absorption enhancement by a single heterocyclic unit change in the π-bridge moiety from thieno[3,2-b]benzothiophene to thieno[3,2-b]indole for high performance dye-sensitized and tandem solar cells

The molecular design of organic sensitizers is one of the fundamental factors for high-efficiency dye-sensitized solar cells (DSSCs). In this study, we first utilize the alkylated thieno[3,2-b]indole (TI) moiety as the π-bridge unit to enhance the π-bridge capability of the thieno[3,2-b]benzothiophene (TBT) used in organic sensitizers. To improve the spectral response of the SGT-130 reference dye, we strategically designed and synthesized two novel TI-based organic sensitizers, SGT-136 and SGT-137, through a simple change of the π-bridge unit. By replacing the TBT with the alkylated TI moiety, SGT-136 and SGT-137 could have a red-shifted absorption band and upshifted highest occupied molecular orbital (HOMO) energy level. As a result, the SGT-137-based DSSC exhibits a higher PCE (12.45%) than that based on SGT-130 (9.83%) owing to the improvement of current density and retardation of charge recombination by the hexyl substituted TI unit. These results indicate that the TI moiety is a good candidate for remarkable π-electronic mediators in D–π–A organic sensitizers with the characteristic of facile synthesis compared to other complicated π-bridges. Furthermore, the parallel-connected tandem device with SGT-137 and SGT-021 porphyrin-based DSSCs shows a significantly improved current density (22.06 mA cm−2) and PCE (14.64%), which is the highest value reported for organic-based tandem solar cells to date.

Simple synthesis and molecular engineering of low-cost and star-shaped carbazole-based hole transporting materials for highly efficient perovskite solar cells

Perovskite solar cells (PrSCs) have emerged as a very promising technology in the field of photovoltaics by demonstrating power conversion efficiencies (PCEs) soaring from 3.9% to above 22% within the past eight years. To date, perovskite solar cells mainly depend on spiro-OMeTAD to perform a key role as a hole transporting material (HTM). However, the complicated multi-step synthetic procedures and high-cost purification process for spiro-OMeTAD limited its potential for commercial application. Herein, three new carbazole-based HTMs with a starburst structure, coded as SGT-405(3,6), SGT-410(3,6) and SGT-411(3,6) via tuning the substitution position from the (2,7) to the (3,6) position of the carbazole moiety, have been successfully synthesized via three-step synthesis from commercially available reagents and investigated for highly efficient perovskite solar cells. By adopting this strategy, among them, molecularly engineered carbazole derivative SGT-405(3,6) exhibits significantly increased Tg (192.7 °C), improved film forming ability, reduced hole reorganization energy and enhanced hole mobility compared to its parent molecule SGT-405(2,7) and spiro-OMeTAD. Owing to the promising properties of SGT-405(3,6), meso-porous type PrSCs employing SGT-405(3,6) showed a remarkable PCE of 18.87%, which is better than that of the photovoltaic device employing spiro-OMeTAD (17.71%). To the best of our knowledge, the achieved PCE (18.87%) is the highest value reported for devices with the structure of FTO/compact TiO2/meso-porous TiO2/CH3NH3PbI3−xClx/HTM/Au employing small-molecular HTMs. Meanwhile, owing to the simple synthesis of SGT-405(3,6), compared with SGT-405(2,7) previously developed by our group, synthesis cost was much lowered, resulting in low cost compared to the spiro-OMeTAD and SGT-405(3,6), by approximately three times. Furthermore, the long-term device stability of PrSCs was enhanced for SGT-405(3,6) to some extent compared to those of other HTMs studied here due to the good uniform capping layer of SGT-405(3,6) on top of the perovskite layer and the prevention of moisture penetration into the perovskite layer. Therefore, SGT-405(3,6) is a promising low-cost and efficient non-spiro type HTM with potential to replace expensive spiro-OMeTAD for PrSCs.

New thieno[3,2-b][1]benzothiophene-based organic sensitizers containing π-extended thiophene spacers for efficient dye-sensitized solar cells

Three new thieno[3,2-b][1]benzothiophene (TBT)-based D–π–A organic sensitizers containing the thiophene π-spacer (SGT-121, 123 and 125) have been synthesized for the application of dye-sensitized solar cell (DSSC), where TBT was employed as a new fused π-bridge unit using the advantages of good co-planarity with the linkage between the thiophene unit and the phenyl unit of the triphenylamine group. Specifically, the combination of a dihexyloxyphenyl-substituted biphenylamine donor and the TBT π-bridge plays multifunctional roles, e.g., the enhanced ability of the π-bridge and donor, slow charge recombination and prevention of dye aggregation in the D–π–A sensitizer. The photophysical, electrochemical and photovoltaic properties of the SGT sensitizers were systematically investigated. As a strategy for the improvement of absorption abilities, the various thiophene derivatives, e.g., those with thiophene (T, SGT-121), bithiophene (BT, SGT-123) and thienothio[3,2-b]thiophene (TT, SGT-125) moieties, were incorporated as π-spacers between the TBT π-bridge and the acceptor unit. The introduction of thiophene π-spacers significantly improved the photovoltaic performance (in particular, in terms of the photocurrent Jsc and open-circuit voltage Voc) compared to SGT-127 without the thiophene unit. The SGT sensitizers were systematically evaluated for DSSCs based on the Co(bpy)32+/3+ (bpy = 2,2′-bipyridine) redox couple. Among the four SGT sensitizers, SGT-123-based DSSC including the BT moiety exhibited the highest power conversion efficiency of 9.69%, Jsc of 16.16 mA cm−2, Voc of 830 mV and fill factor of 0.72. These results present the impact of thiophene π-spacers for enhancing the photovoltaic performances of a D–π–A organic sensitizer.

Enhanced Polarization Ratio of Electrospun Nanofibers with Increased Intrachain Order by Postsolvent Treatments

Polarized emission that is beneficial to lighting and display applications can be demonstrated by aligning emissive chromophores, which can be achieved using an electrospinning technique. We investigate the photophysical properties of nanofibers based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/poly(ethylene oxide) blends both with and without postsolvent treatments. Two different solvents were sequentially used in an attempt to extract the insulating electrospinnable polymer and increase the polarization ratio of the nanofiber meshes by molecular reorganization. The polarization ratio of emission from the nanofiber meshes treated with N,N-dimethylformamide (DMF) following treatment with acetonitrile solvents was found to be increased. An increase in the 0-0 emission vibronic intensity relative to that of the 0-1 peak and a reduction in the photoluminescence (PL) bandwidth were found. In addition, the PL decays faster and the parallel component along the nanofiber axis increases after the DMF treatment, indicating that the radiative recombination process becomes faster. Our results consistently show that postsolvent treatment promotes stronger J-aggregate character, with longer coherence lengths of the exciton along the long axis of the nanofibers, due to enhanced intrachain order.

Silver bismuth iodides in various compositions as potential Pb-free light absorbers for hybrid solar cells

Polycrystalline silver bismuth iodide (SBI) powders of various compositions (Ag : Bi = 2 : 1–1 : 1 in atomic ratio) were synthesized via a solid-state reaction in an evacuated Pyrex tube. Regardless of the composition of Ag and Bi, Ag2BiI5 in the hexagonal phase was preferentially formed and BiI3 impurity in the rhombohedral phase was formed with the increase of the Bi component. The synthesized SBI powders of various compositions were applied as light absorbers for hybrid solar cells (HSCs), which employ mesoporous TiO2 as the electron transporting layer and pristine spiro-OMeTAD as the hole transporting layer. To deposit light absorber layers, the coating solutions were prepared by dissolving the synthesized SBI powders in DMSO/DMF/HI and spin-coated over the mesoporous TiO2 layer. For the solar cell employing pure Ag2BiI5, a photovoltaic conversion efficiency (PCE) of 1.74% was achieved, whereas the inclusion of the BiI3 impurities in the Ag2BiI5 phase significantly increased the device performance. The highest PCE of 2.31% was achieved with the SBI of Ag : Bi = 55 : 45. Furthermore, the SBI solar cells show no hysteresis in the J–V curve measurements and are highly stable under ambient conditions, exhibiting excellent long-term stability at a relative humidity of 50%.

Silicotungstate, a Potential Electron Transporting Layer for Low-Temperature Perovskite Solar Cells

Thin films of a heteropolytungstate, lithium silicotungstate (Li4SiW12O40, termed Li-ST), prepared by a solution process at low temperature, were successfully applied as electron transporting layer (ETL) of planar-type perovskite solar cells (PSCs). Dense and uniform Li-ST films were prepared on FTO glass by depositing a thin Li-ST buffer layer, followed by coating of a main Li-ST layer. The film thickness was controlled by varying the number of coating cycles, consisting of spin-coating and thermal treatment at 150 °C. In particular, by employing 60 nm-thick Li-ST layer obtained by two cycles of coating, the fabricated CH3NH3PbI3 PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with JSC of 22.16 mA cm-2, VOC of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO2 layer processed at the same temperature (PCE = 12.27%). Spectroscopic analyses by time-resolved photoluminescence and pulsed light-induced transient measurement of photocurrent indicate that the Li-ST layer collects electrons from CH3NH3PbI3 more efficiently and also exhibits longer electron lifetime than the TiO2 layer thermally treated at 150 °C. Thus, Li-ST is considered to be a promising ETL material that can be applied for the fabrication of flexible PSC devices.