Amaresh Mishra

Metal-free organic dyes for dye-sensitized solar cells: from structure: property relationships to design rules.

Dye-sensitized solar cells (DSSC) have attracted considerable attention in recent years as they offer the possibility of low-cost conversion of photovoltaic energy. This Review focuses on recent advances in molecular design and technological aspects of metal-free organic dyes for applications in dye-sensitized solar cells. Special attention has been paid to the design principles of these dyes and on the effect of various electrolyte systems. Cosensitization, an emerging technique to extend the absorption range, is also discussed as a way to improve the performance of the device. In addition, we report on inverted dyes for photocathodes, which constitutes a relatively new approach for the production of tandem cells. Special consideration has been paid to the correlation between the molecular structure and physical properties to their performance in DSSCs.

Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology

This article is written from an organic chemists point of view and provides an up-to-date review about organic solar cells based on small molecules or oligomers as absorbers and in detail deals with devices that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an acceptor (n-type semiconductor) material. The article pays particular attention to the design and development of molecular materials and their performance in corresponding devices. In recent years, a substantial amount of both, academic and industrial research, has been directed towards organic solar cells, in an effort to develop new materials and to improve their tunability, processability, power conversion efficiency, and stability. On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches to enhancing the efficiency of organic solar cells are analyzed.

Functional Oligothiophenes: Molecular Design for Multidimensional Nanoarchitectures and Their Applications†

3.2. Thienothiophenes 1216 3.2.1. Thieno[3,4-b]thiophene Analogues 1216 3.2.2. Thieno[3,2-b]thiophene Analogues 1217 3.2.3. Thieno[2,3-b]thiophene Analogues 1218 3.3. , ′-Bridged Bithiophenes 1219 3.3.1. Dithienothiophene (DTT) Analogues 1220 3.3.2. Dithieno[3,2-b:2′3′-d]thiophene-4,4-dioxides 1221 3.3.3. Dithienosilole (DTS) Analogues 1221 3.3.4. Cyclopentadithiophene (CPDT) Analogues 1221 3.3.5. Nitrogen and Phosphor Atom Bridged Bithiophenes 1222

Highly efficient photocathodes for dye-sensitized tandem solar cells

Thin-film dye-sensitized solar cells (DSCs) based on mesoporous semiconductor electrodes are low-cost alternatives to conventional silicon devices. High-efficiency DSCs typically operate as photoanodes (n-DSCs), where photocurrents result from dye-sensitized electron injection into n-type semiconductors. Dye-sensitized photocathodes (p-DSCs) operate in an inverse mode, where dye-excitation is followed by rapid electron transfer from a p-type semiconductor to the dye (dye-sensitized hole injection). Such p-DSCs and n-DSCs can be combined to construct tandem solar cells (pn-DSCs) with a theoretical efficiency limitation well beyond that of single-junction DSCs (ref. 4). Nevertheless, the efficiencies of such tandem pn-DSCs have so far been hampered by the poor performance of the available p-DSCs (refs 3, 5-15). Here we show for the first time that p-DSCs can convert absorbed photons to electrons with yields of up to 96%, resulting in a sevenfold increase in energy conversion efficiency compared with previously reported photocathodes. The donor-acceptor dyes, studied as photocathodic sensitizers, comprise a variable-length oligothiophene bridge, which provides control over the spatial separation of the photogenerated charge carriers. As a result, charge recombination is decelerated by several orders of magnitude and tandem pn-DSCs can be constructed that exceed the efficiency of their individual components.

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.

Application of the Tris(acetylacetonato)iron(III)/(II) Redox Couple in p‐Type Dye‐Sensitized Solar Cells

An electrolyte based on the tris(acetylacetonato)iron(III)/(II) redox couple ([Fe(acac)3](0/1-)) was developed for p-type dye-sensitized solar cells (DSSCs). Introduction of a NiO blocking layer on the working electrode and the use of chenodeoxycholic acid in the electrolyte enhanced device performance by improving the photocurrent. Devices containing [Fe(acac)3](0/1-) and a perylene-thiophene-triphenylamine sensitizer (PMI-6T-TPA) have the highest reported short-circuit current (J(SC)=7.65 mA cm(-2)), and energy conversion efficiency (2.51%) for p-type DSSCs coupled with a fill factor of 0.51 and an open-circuit voltage V(OC)=645 mV. Measurement of the kinetics of dye regeneration by the redox mediator revealed that the process is diffusion limited as the dye-regeneration rate constant (1.7×10(8) M(-1) s(-1)) is very close to the maximum theoretical rate constant of 3.3×10(8) M(-1) s(-1). Consequently, a very high dye-regeneration yield (>99%) could be calculated for these devices.

Low band gap S,N-heteroacene-based oligothiophenes as hole-transporting and light absorbing materials for efficient perovskite-based solar cells

Novel low band gap oligothiophenes incorporating S,N-heteropentacene central units were developed and used as hole-transport materials (HTMs) in solid-state perovskite-based solar cells. In addition to appropriate electronic energy levels, these materials show high photo-absorptivity in the low energy region, and thus can contribute to the light harvesting of the solar spectrum. Solution-processed CH3NH3PbI3-based devices using these HTMs achieved power conversion efficiencies of 9.5–10.5% in comparison with 7.6% obtained by reference devices without HTMs. Photoinduced absorption spectroscopy gave further insight into the charge transfer behavior between photoexcited perovskites and the HTMs.

A dopant-free spirobi[cyclopenta[2,1-b:3,4-b′]dithiophene] based hole-transport material for efficient perovskite solar cells

We present the design and synthesis of a promising hole transporting material (HTM) using the 4,4′-spirobi[cyclopenta[2,1-b:3,4-b′]dithiophene] derivative (spiro-CPDT) as the core and triarylamines as terminal units. The implementation of the new HTM in CH3NH3PbI3-based perovskite solar cells exhibited an excellent overall power conversion efficiency (PCE) of 13.4% without the use of any dopants and additives which is comparable to 15.0% obtained using p-doped spiro-MeOTAD-based devices. Furthermore, the device based on the new HTM generated a slightly higher open circuit voltage (VOC) of 971 mV compared to a spiro-MeOTAD (VOC = 951 mV) based device. The present results demonstrate that spiro-CPDT could be an excellent building block to prepare dopant-free HTMs for perovskite solar cells and holds promise to replace the p-doped spiro-OMeTAD, which is important for the fabrication of cost-effective devices in the future.

A-D-A-D-A-type oligothiophenes for vacuum-deposited organic solar cells.

Novel A-D-A-D-A-type oligothiophenes incorporating electron-withdrawing benzo[c][1,2,5]thiadiazole (BTDA) as core and trifluoroacetyl (TFA) as terminal acceptor groups have been developed. Vacuum-processed planar heterojunction organic solar cells incorporating these new oligomers as donor and C(60) as acceptor showed very high open circuit voltages up to 1.17 V, resulting in power conversion efficiencies of 1.56% under AM1.5G conditions.

Click-chemistry approach in the design of 1,2,3-triazolyl-pyridine ligands and their Ru(II)-complexes for dye-sensitized solar cells

The synthesis of new 1,2,3-triazolyl-pyridine ligands via “click-chemistry” and their corresponding Ru(L)(2,2′-bipyridyl-4,4′-dicarboxylic acid)(NCS)2 complexes (L = 1,2,3-triazolyl-pyridine) are presented. The complexes have been photophysically and electrochemically characterized and have been used as sensitizers in dye-sensitized solar cells (DSSC). In DSSCs with an acetonitrile-based electrolyte the cells comprising of Ru(2-(1-(4-hexylphenyl)-1H-1,2,3-triazol-4-yl)pyridine)(2,2′-bipyridyl-4,4′-dicarboxylic acid)(NCS)2 TBA salt 1 showed an overall power conversion efficiency of 7.8% under full sunlight intensity, and Ru(2-(4-(4-hexylphenyl)-1H-1,2,3-triazol-1-yl)pyridine)(2,2′-bipyridyl-4,4′-dicarboxylic acid)(NCS)2 TBA salt 2 an efficiency of 4.7%. Transient photovoltage and photocurrent decay measurements showed an enhanced performance for dye 1 due to faster electron transport into the TiO2 film and lower recombination rate in comparison to dye 2 sensitized devices. Additionally, solid-state devices were prepared with 2 μm thick TiO2 films using spiro-OMeTAD as a hole-transport material. The solid-state dye-sensitized solar cells showed power conversion efficiencies of 1.92% and 0.38% for sensitizer 1 and 2, respectively.