Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

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Dye-sensitized solar cells strike back.

To develop photosensitizers with high open-circuit photovoltage (Voc) is a crucial strategy to enhance the power conversion efficiency (PCE) of co-sensitized solar cells. Here, we show a judiciously tailored organic photosensitizer, coded MS5, featuring the bulky donor N-(2’,4’-bis(dodecyloxy)-[1,1’-biphenyl]-4-yl)-2’,4’-bis(dodecyloxy)-N-phenyl-[1,1’-biphenyl]-4-amine and the electron acceptor 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid. Employing MS5 with a copper (II/I) electrolyte enables a dye-sensitized solar cell (DSC) to achieve a strikingly high Voc of 1.24 V, with the Voc deficit as low as 130 mV and an ideality factor of merely 1.08. The co-sensitization of MS5 with the wider spectral-response dye XY1b produces a highly efficient and stable DSC with the PCE of 13.5% under standard AM1.5 G, 100 mW cm−2 solar radiation. Remarkably, the co-sensitized solar cell (active area of 2.8 cm2) presents a record PCE of 34.5% under ambient light, rendering it very attractive as an ambient light harvesting energy source for low power electronics. Designing photo-sensitisers with high open-circuit voltage (Voc) is desirable to enhance the power conversion efficiency (PCE) of co-sensitized solar cells. Here, the authors employ a judiciously tailored organic sensitiser MS5 with copper electrolyte to achieve a Voc of 1.24 V, and recorded PCE of 34.5% under ambient light.

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A molecular photosensitizer achieves a V oc of 1.24 V enabling highly efficient and stable dye-sensitized solar cells with copper(II/I)-based electrolyte

Dye-sensitized solar cells (DSSCs), as a cost effective and eco-friendly photovoltaic technology for utilizing solar energy, are promising in meeting the increasing demand of clean and renewable energy resources. Among various sensitizers, porphyrins are crucial candidates with the advantages of strong absorption in a wide spectral range, tunable photophysical and electrochemical properties, and long-lived excited states facilitating electron injection. After decades of development, the power conversion efficiencies of porphyrin-based DSSCs have exceeded 13%, showing the great potential of porphyrins in fabricating highly efficient DSSCs. This review summarizes effective molecular engineering strategies for optimizing porphyrin sensitizers as well as intermolecular engineering of coadsorption and cosensitization systems, with the aim to provide further insight into the molecular structure–photovoltaic performance correlations and an outlook on possible exploration directions in the future for achieving DSSCs with high efficiencies, long-term stability and low cost feasible for practical applications. In addition, the recent advances of porphyrin-based organic solar cells (OSCs) are briefly introduced considering similar design strategies employed for developing porphyrin dyes for DSSCs and active materials for OSCs.

Molecular engineering strategies for fabricating efficient porphyrin-based dye-sensitized solar cells

With the purpose to achieve panchromatic absorption for constructing efficient dye-sensitized solar cells (DSSCs), the cosensitization approach of using two dyes with complementary absorption has been developed with great success. However, this approach usually requires time-consuming optimization of a number of parameters for controlling the ratio and distribution of the two coadsorbed dyes on TiO2 film, which limits the potentials of this strategy. We herein report an alternative approach for developing efficient DSSCs by designing a class of "concerted companion dyes" with two complementary dye components linked covalently. Thus, a newly synthesized organic dye Z2 was linked to a recently reported doubly strapped porphyrin dye XW51 through flexible chains with various lengths to afford XW60-XW63. These dyes exhibit excellent absorption and efficiencies in the range of 8.8%-11.7%. Notably, upon coadsorption with chenodeoxycholic acid, XW61 affords an impressive efficiency of 12.4%, a record for iodine electrolyte-based DSSCs, to the best of our knowledge. In addition, these dyes also exhibit the advantages of easy cell fabrication, simple optimization, as well as excellent photostability.

Efficient Solar Cells Based on Concerted Companion Dyes Containing Two Complementary Components: An Alternative Approach for Cosensitization.

Porphyrin-based compounds and their applications in materials and medicine

Four novel porphyrin dyes XW48–XW51 with a doubly strapped porphyrin framework and judicious optimization of the substituents of the phenothiazine donor have been synthesized and utilized as DSSC dyes. On the basis of XW40, bulky 2,6-bis(hexyloxy)phenyl groups were incorporated into the phenothiazine donor by replacing the hexyl, 4-hexyloxyphenyl and both of them, to afford XW48, XW49 and XW50, respectively. On the basis of XW50, XW51 was synthesized by using 2,4-bis(hexyloxy)phenyl in place of the 2,6-bis(hexyloxy)phenyl units. As a result, the Voc values were enhanced stepwisely from 730 mV (XW40) to ca. 750 mV (XW48/XW49) and then to 761 mV (XW50). Compared with XW50, XW51 contains a bulky donor with more evenly distributed alkoxyl chains, which tend to form a more compact self-assembled monolayer to impede the penetration of I3−. Hence, an extraordinarily high Voc of 781 mV was achieved for XW51. As a result, the efficiency was dramatically improved to 11.1%, which is the highest efficiency for DSSCs based on an individual sensitizer using the iodine electrolyte, to the best of our knowledge. Furthermore, high efficiencies of 10.1% and 9.7% were also obtained for XW50 and XW51, respectively, based on the cobalt electrolyte.

Systematic optimization of the substituents on the phenothiazine donor of doubly strapped porphyrin sensitizers: an efficiency over 11% unassisted by any cosensitizer or coadsorbent

Concerted companion dyes (CC dyes) like XW61 have been demonstrated to be an effective platform for developing efficient DSSCs. However, the moderated phenothiazine-based electron donor in XW61 results in unsatisfactory Jsc. To address this problem, a stronger fluorenyl indoline-based electron donor has been used to construct porphyrin dye XW68 and organic dyes Y1-Y2. The stronger electron-donating character of the fluorenyl indoline unit leads to an enhanced Jsc value (20.48 mA·cm-2) for the individual dye XW68. On this basis, CC dyes XW69-XW70-C8 have been designed and synthesized by combining the frameworks of Y1 and Y2 with XW68. The complementary absorption characters of the porphyrin and the organic dye moieties lead to panchromatic absorption with a strong light-harvesting capability from 350 to 700 nm and the onset wavelength extended to ca. 840 nm in the IPCE curves. As a result, excellent Jsc values have been achieved (>22 mA·cm-2). In addition to the advantages of high Jsc, bulky octyl groups have been introduced into the donor of XW70-C8 to reduce dye aggregation and suppress charge recombination. Finally, a highest PCE of 11.1% with a satisfactory Jsc (22.25 mA·cm-2) and an enhanced Voc (750 mV) has been achieved upon coadsorption of XW70-C8 with CDCA. In addition, the CC dye XW70-C8-based solar cells exhibit excellent long-term photostability. These results provide an effective method for rationally improving the photovoltaic behavior, especially the Jsc of CC dyes, by introducing strong electron donor moieties with suitable substituents.

Fluorenyl Indoline as an Efficient Electron Donor for Concerted Companion Dyes: Enhanced Light-Harvesting and Photocurrent.

Porphyrin dyes have been widely used for the fabrication of efficient dye-sensitized solar cells (DSSCs). However, dye aggregation and charge recombination still exert negative effects on photovolt...

Yingying Chen

Papers

Efficient Solar Cells Based on Concerted Companion Dyes Containing Two Complementary Components: An Alternative Approach for Cosensitization.

Systematic optimization of the substituents on the phenothiazine donor of doubly strapped porphyrin sensitizers: an efficiency over 11% unassisted by any cosensitizer or coadsorbent

Fluorenyl Indoline as an Efficient Electron Donor for Concerted Companion Dyes: Enhanced Light-Harvesting and Photocurrent.

A new type of multibenzyloxy-wrapped porphyrin sensitizers for developing efficient dye-sensitized solar cells