Renzhi Li

High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts

Low-cost excitonic solar cells based on organic optoelectronic materials are receiving an ever-increasing amount of attention as potential alternatives to traditional inorganic photovoltaic devices. In this rapidly developing field, the dye-sensitized solar cell (DSC) has achieved so far the highest validated efficiency of 11.1% (ref. 2) and remarkable stability. However, the cells with the best performance use volatile solvents in their electrolytes, which may be prohibitive for outdoor solar panels in view of the need for robust encapsulation. Solvent-free room-temperature ionic liquids have been pursued as an attractive solution to this dilemma, and device efficiencies of over 7% were achieved by using some low-viscosity formulations containing 1-ethyl-3-methylimidazolium thiocyanate, selenocyanate, tricyanomethide or tetracyanoborate. Unfortunately, apart from tetracyanoborate, all of these low-viscosity melts proved to be unstable under prolonged thermal stress and light soaking. Here, we introduce the concept of using eutectic melts to produce solvent-free liquid redox electrolytes. Using a ternary melt in conjunction with a nanocrystalline titania film and the amphiphilic heteroleptic ruthenium complex Z907Na (ref. 10) as a sensitizer, we reach excellent stability and an unprecedented efficiency of 8.2% under air-mass 1.5 global illumination. Our results are of importance to realize large-scale outdoor applications of mesoscopic DSCs.

Donor/Acceptor Indenoperylene Dye for Highly Efficient Organic Dye-Sensitized Solar Cells

An N-annulated indenoperylene electron-donor decorated with photochemically inactive segments is synthesized and further conjugated via triple bond with electron-acceptor benzothiadiazolylbenzoic acid for a metal-free donor/acceptor dye. Without use of any coadsorbate, the judiciously tailored indenoperylene dye achieves a high-power conversion efficiency of 12.5% under irradiance of 100 mW cm(-2) AM1.5G sunlight.

Design of high-efficiency organic dyes for titania solar cells based on the chromophoric core of cyclopentadithiophene-benzothiadiazole

Based upon the chromophoric core of cyclopentadithiophene-benzothiadiazole, we have finely tailored a metal-free organic dye which displays power conversion efficiencies at various irradiances of the simulated air mass (AM) 1.5 sunlight of 11.5–12.8%, setting a new benchmark for organic dye-sensitized solar cells.

Employ a bisthienothiophene linker to construct an organic chromophore for efficient and stable dye-sensitized solar cells

We employed the bisthienothiophene conjugated linker along with a hydrophobic triphenylamine electron-donor and a hydrophilic cyanoacrylic acid electron-acceptor to construct a high molar extinction coefficient organic photosensitizer, exhibiting a power conversion efficiency of 8.0% measured under irradiation of air mass 1.5 global (AM 1.5G) full sunlight.

A Metal‐Free N‐Annulated Thienocyclopentaperylene Dye: Power Conversion Efficiency of 12 % for Dye‐Sensitized Solar Cells

Reported are two highly efficient metal-free perylene dyes featuring N-annulated thienobenzoperylene (NTBP) and N-annulated thienocyclopentaperylene (NTCP), which are coplanar polycyclic aromatic hydrocarbons. Without the use of any coadsorbate, the metal-free organic dye derived from the NTCP segment was used for a dye-sensitized solar cell which attained a power conversion efficiency of 12% under an irradiance of 100 mW cm(-2), simulated air mass global (AM1.5G) sunlight.

Joint electrical, photophysical and computational studies on D-π-A dye sensitized solar cells: the impacts of dithiophene rigidification

The rigidification of π-conjugated segments represents a feasible tactic towards energy-level engineering of organic D-π-A dyes in mesoscopic titania solar cells. In this work, comparions of four dyes with the di(3-hexylthiophene), dihexyldithienosilole, dihexylcyclopentadithiophene and N-hexyldithienopyrrole linkers have revealed some general influences of π-linker rigidification on the optoelectronic features of titania solar cells employing a cobalt(II/III) redox electrolyte, in terms of energetic and kinetic viewpoints. Compared to a dye with the di(3-hexylthiophene) linker, its three counterparts with rigidified dithiophene blocks present bathochromic and hyperchromic absorptions of solar photons. Transient absorption measurements have shown that the incorporation of Si-, C- and N-bridged dithiophene segments decelerates the dye regeneration kinetics. The rigidification of π-conjugated dithiophene linkers brings forth a general open-circuit photovoltage diminishment, in the range from 60 to 190 mV. Further insightful impedance analyses have disclosed that the open-circuit photovoltage reduction, due to the π-linker alternation from di(3-hexylthiophene) to N-hexyldithienopyrrole, is predominantly caused by an adverse downward displacement of the titania conduction band edge, despite a positive contribution from attenuated charge recombination at the titania/electrolyte interface.

Organic dye-sensitized solar cells with a cobalt redox couple: influences of π-linker rigidification and dye–bath solvent selection

The rigidification of π-conjugated linkers represents a viable strategy towards the energy-level engineering of organic push–pull photosensitizers in dye-sensitized solar cells. In this paper we deploy 4-hexylphenyl substituted cyclopenta[1,2-b:5,4-b′]dithiophene[2′,1′:4,5]thieno[2,3-d]thiophene as the π-linker of a D-π-A dye, which displays an improved molar absorption coefficient and a red-shifted absorption peak in contrast to its model dye with the 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene segment. The energy-gap reduction is concomitant with negative and positive shifts of ground-state and excited-state redox potentials, which however do not exert an adverse impact on the net charge separation yield at the titania/dye/electrolyte interface, probably due to the formation of a favored microstructured dye assembly. Furthermore, the dye load amount can be tuned by changing the bath solvent and has a significant influence on some key photovoltaic features such as the photocurrent and photovoltage, the latter of which is dissected via the joint charge extraction and photovoltage decay experiments. The charge recombination lifetime could be roughly rationalized by analyzing the damping of signals on emitted electrons from titania in the X-ray photoelectron spectroscopy measurements.

Modulating the assembly of organic dye molecules on titania nanocrystals via alkyl chain elongation for efficient mesoscopic cobalt solar cells

Through elongating the end or side alkyl chains of dye molecules, we decorate anatase nanocrystals with a thicker organic assembly featuring a smaller tilt angle of the D-π-A backbone with respect to the surface normal, which retards the interfacial charge recombination and confers a higher photovoltage output on mesoscopic cobalt solar cells displaying an over 10% power conversion efficiency at the AM1.5G conditions.

Oligothiophene dye-sensitized solar cells

We scrutinize the energetic and kinetic interplays in oligothiophene dye-sensitized solar cells via analyzing electrical impedance as well as transient emission and absorption spectroscopies.

Electrical and photophysical analyses on the impacts of arylamine electron donors in cyclopentadithiophene dye-sensitized solar cells

The electron donor of a D-π-A dye is known for its capability to tune both the electronic trait and packing mode of dye molecules chemisorbed on titania nanocrystals of dye-sensitized solar cells (DSCs), bringing on the opportunity to impact cell performance by modulating the physicochemical characteristics at the titania/dye/electrolyte interface. In this paper, we scrutinize the influences of arylamine electron donors on the optoelectronic features of thin-film DSCs employing a tris(1,10-phenanthroline)cobalt(II/III) redox electrolyte, by use of four cyclopentadithiophene dyes (C218, C244, C245 and C246) with the respective dihexyloxy-, diphenothiazinyl- or di-tert-butylphenyl-substituted triphenylamine and N-hexyl-carbazole electron donors. Amongst these electron donors, dihexyloxy-substituted triphenylamine is found to present the strongest electron-donating capacity, endowing the corresponding C218 dye with evidently red-shifted light absorption in comparison with the other three congeners. Transient absorption measurements show that all DSCs exhibit expeditious dye regeneration, guaranteeing efficient long-distance charge separation at the titania/dye/redox couple interface. Furthermore, it is worthwhile noting that the C218 dye prompts the highest open-circuit photovoltage amongst these chromophores, which is primarily attributed to the positive effect of slow cobalt(III) interception of titania electrons, highlighting the superiority of applying dihexyloxy-substituted triphenylamine as the electron donor for a cyclopentadithiophene dye.