Zonghao Liu

Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells

Hybrid perovskites have shown astonishing power conversion efficiencies owed to their remarkable absorber characteristics including long carrier lifetimes, and a relatively substantial defect tolerance for solution-processed polycrystalline films. However, nonradiative charge carrier recombination at grain boundaries limits open circuit voltages and consequent performance improvements of perovskite solar cells. Here we address such recombination pathways and demonstrate a passivation effect through guanidinium-based additives to achieve extraordinarily enhanced carrier lifetimes and higher obtainable open circuit voltages. Time-resolved photoluminescence measurements yield carrier lifetimes in guanidinium-based films an order of magnitude greater than pure-methylammonium counterparts, giving rise to higher device open circuit voltages and power conversion efficiencies exceeding 17%. A reduction in defect activation energy of over 30% calculated via admittance spectroscopy and confocal fluorescence intensity mapping indicates successful passivation of recombination/trap centers at grain boundaries. We speculate that guanidinium ions serve to suppress formation of iodide vacancies and passivate under-coordinated iodine species at grain boundaries and within the bulk through their hydrogen bonding capability. These results present a simple method for suppressing nonradiative carrier loss in hybrid perovskites to further improve performances toward highly efficient solar cells.

Low-Temperature TiOx Compact Layer for Planar Heterojunction Perovskite Solar Cells

Here, we demonstrate an effective low-temperature approach to fabricate a uniform and pinhole-free compact TiO2 layer for enhancing photovoltaic performance of perovskite solar cells. TiCl4 was used to modify TiO2 for efficient charge generation and significantly reduced recombination loss. We found that a TiO2 layer with an appropriate TiCl4 treatment possesses a smooth surface with full coverage of the conductive electrode. Further studies on charge carrier dynamics confirmed that the TiCl4 treatment improves the contact of the TiO2/perovskite interface, facilitating charge extraction and suppressing charge recombination. On the basis of the treatment, we improved the open circuit voltage from 1.01 V of the reference cell to 1.08 V, and achieved a power conversion efficiency of 16.4%.

Fine tuning of fluorene-based dye structures for high-efficiency p-type dye-sensitized solar cells.

We report on an experimental study of three organic push-pull dyes (coded as zzx-op1, zzx-op1-2, and zzx-op1-3) featuring one, two, and three fluorene units as spacers between donors and acceptors for p-type dye-sensitized solar cells (p-DSSC). The results show increasing the number of spacer units leads to obvious increases of the absorption intensity between 300 nm and 420 nm, a subtle increase in hole driving force, and almost the same hole injection rate from dyes to NiO nanoparticles. Under optimized conditions, the zzx-op1-2 dye with two fluorene spacer units outperforms other two dyes in p-DSSC. It exhibits an unprecedented photocurrent density of 7.57 mA cm(-2) under full sun illumination (simulated AM 1.5G light illumination, 100 mW cm(-2)) when the I(-)/I3(-) redox couple and commercial NiO nanoparticles were used as an electrolyte and a semiconductor, respectively. The cells exhibited excellent long-term stability. Theoretical calculations, impedance spectroscopy, and transient photovoltage decay measurements reveal that the zzx-op1-2 exhibits lower photocurrent losses, longer hole lifetime, and higher photogenerated hole density than zzx-op1 and zzx-op1-3. A dye packing model was proposed to reveal the impact of dye aggregation on the overall photovoltaic performance. Our results suggest that the structural engineering of organic dyes is important to enhance the photovoltaic performance of p-DSSC.

NiO nanosheets as efficient top hole transporters for carbon counter electrode based perovskite solar cells

Herein, we present fully printable carbon electrode based perovskite solar cells using highly crystalline NiO nanosheets as top hole transport layers, mesoporous TiO2 nanoparticles as a bottom electron transport layer and ZrO2 as an intermediate spacer layer, respectively. Time-resolved photoluminescence decay measurements, electron impedance spectroscopy and transient photovoltage decay measurements have revealed that the NiO nanosheets as top hole transporters exhibit superior charge collection efficiency and a prolonged charge lifetime. As a result, an impressive power conversion efficiency of 14.2% is achieved under standard testing conditions.

Modulated Charge Injection in p-Type Dye-Sensitized Solar Cells Using Fluorene-Based Light Absorbers

In this study, new pull-push arylamine-fluorene based organic dyes zzx-op1, zzx-op2, and zzx-op3 have been designed and synthesized for p-type dye-sensitized solar cells (p-DSCs). In zzx-op1, a di(p-carboxyphenyl)amine (DCPA) was used as an electron donor, a perylenemonoimide (PMID) as an electron acceptor, and a fluorene (FLU) unit with two aliphatic hexyl chains as a π-conjugated linker. In zzx-op2 and zzx-op3, a 3,4-ethylenedioxythiophene (EDOT) and a thiophene were inserted consecutively between PMID and FLU to tune the energy levels of the frontier molecular orbitals of the dyes. The structural modification broadened the spectral coverage from an onset of 700 nm for zzx-op1 to 750 nm for zzx-op3. The electron-rich EDOT and thiophene lifted up the HOMO (highest occupied molecular orbital) levels of zzx-op2 and zzx-op3, making their potential more negative than zzx-op1. When three dyes were employed in p-type DSCs with I(-)/I3(-) as a redox couple and NiO nanoparticles as hole materials, zzx-op1 exhibited impressive energy conversion efficiency of 0.184% with the open-circuit voltage (VOC) of 112 mV and the short-circuit current density (JSC) of 4.36 mA cm(-2) under AM 1.5G condition. Density functional theory calculations, transient photovoltage decay measurements, and electrochemical impedance spectroscopic studies revealed that zzx-op1 sensitized solar cell exhibited much higher charge injection efficiency (90.3%) than zzx-op2 (53.9%) and zzx-op3 (39.0%), indicating a trade-off between spectral broadening and electron injection driving force in p-type DSCs.

Semitransparent Fully Air Processed Perovskite Solar Cells

UNLABELLEDnSemitransparent solar cells are highly attractive for application as power-generating windows. In this work, we present semitransparent perovskite solar cells that employ conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (nnnPEDOTnPSS) film as the transparent counter electrode. ThennnPEDOTnPSS electrode is prepared by transfer lamination technique using plastic wrap as the transfer medium. The use of the transfer lamination technique avoids the damage of the CH3NH3PbI3 perovskite film by direct contact ofnnnPEDOTnPSS aqueous solution. The semitransparent perovskite solar cells yield a power conversion efficiency of 10.1% at an area of about 0.06 cm(2) and 2.9% at an area of 1 cm(2). The device structure and the fabrication technique provide a facile way to produce semitransparent perovskite solar cells.

Fluorene functionalized porphyrins as broadband absorbers for TiO2 nanocrystalline solar cells

Three 9,9-dihexyl-9H-fluorene (DHF) functionalized zinc porphyrin dyes (coded as ZZX-N3, ZZX-N4, and ZZX-N5) were designed and synthesized for dye-sensitized solar cells. Then, DHF and benzoic acid were conjugated to the porphyrin ring through triple bonds to act as a spacer to elongate the π-conjugation and as an acceptor for an efficient electron injection, respectively. A bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BFA) and a bis(4-hexylphenyl)amine (BPA) were further linked to DHF to act as electron donors in ZZX-N3 and ZZX-N4, respectively. ZZX-N5 did not have any electron donor and served as a reference. Moreover, ZZX-N3- and ZZX-N4-sensitized cells exhibited broader sunlight absorption than ZZX-N5, and as a result, higher photon-to-electricity efficiency (PCE) (ZZX-N3, 3.83%; ZZX-N4, 4.2%; ZZX-N5, 3.70%) was observed. The results are consistent with well-separated HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) in ZZX-N3 and ZZX-N4 than in ZZX-N5. However, the overall conversion efficiency of ZZX-N3- and ZZX-N4-sensitized cells was low, which is due to significant dye aggregation induced by the extra long alkyl-chains on the donor groups. This was evidenced by blue and red shifts of the absorption spectra of dye-coated TiO2 films. In addition, the extra long-chains also did not offer better shielding to prevent electron recombination of injected electrons with I3− in electrolyte as revealed by electrochemical impedance spectroscopy. When a co-sensitizer (coded as PBS) was used, a new peak corresponding to the absorption of PBS at 560 nm was observed on the incident photon to charge carrier efficiency (IPCE) spectra; however, the overall photovoltaic performance was not improved due to the significant decrease of dye-loading density of porphyrin dyes, indicating a need to break off the trade-off between dye-loading and light-harvesting.

Near-infrared absorbing porphyrin dyes with perpendicularly extended π-conjugation for dye-sensitized solar cells

Two new porphyrin dyes, coded as ZZX-N6 and ZZX-N6C12, with photon-to-electron response up to 800 nm in the solar spectrum were synthesized for dye-sensitized solar cells (DSCs). Four alkynyl groups were conjugated perpendicularly to the donor–π–acceptor axis of the porphyrins through triple bonds, resulting in 40–56 nm red-shift of absorption onset compared to porphyrin dye YD2-o-C8, leading to higher short-circuit current density. Post treatment of dye-coated TiO2 films by chenodeoxycholic acid (CDCA) with a concurrent increase of iodide concentration in an electrolyte improved the cell performance. Both impedance spectroscopic and photovoltage decay measurements showed that charge resistance at the TiO2–electrolyte interface increased after the CDCA treatment. Increasing iodide concentration in the electrolyte enhanced the reduction of I3− at the Pt counter electrode, leading to a higher short-circuit current density (JSC) and fill factor (FF). Under the optimized conditions, the ZZX-N6 based device exhibited an energy conversion efficiency of 7.21%, which is comparable to the YD2-o-C8 based device. The results demonstrate an alternative to broaden the light absorption capability of porphyrin dyes for high efficiency solar cells.

Solvent effects on adsorption kinetics, dye monolayer, and cell performance of porphyrin-sensitized solar cells

The effect of three dye loading solvents (THF, ethanol, and n-butanol) on the adsorption kinetics, the binding mode of the dye molecules on the TiO2 nanoparticles, and photovoltaic performance in dye-sensitized solar cells was reported. The kinetic study indicated that solvents greatly affected the adsorption amount and adsorption rate on the surface of TiO2 nanoparticles. The adsorption data in THF, ethanol and n-butanol were analyzed by applying an intraparticle diffusion model, pseudo-first order model, and pseudo-second order model, respectively. It was found that adsorption rate was controlled by mass transfer processes. UV-vis spectra and ATR-FTIR spectra of porphyrin-coated electrodes were measured to study the intermolecular interaction and binding mode on TiO2 films, respectively. Finally, the cell performance was tested under different solvents and the best efficiency of 8.18% was obtained when n-butanol was used as the solvent for dye-loading.