Thermal Stability Study of Dye-Sensitized Solar Cells with Cobalt Bipyridyl–based Electrolytes
Introduction
Dye-sensitized solar cells (DSSCs) are of significant interest for both academic research and industrial application, because of its low cost, ease of fabrication and also modifiable aesthetic features in color and transparency [1]. One of the key components of DSSCs is the electrolyte and its redox couple system. The iodide/triiodide (I−/I3−) redox couple has dominated in the past, because of its good performance in DSSCs, despite of some well-known drawbacks, like its corrosive nature and high absorption extinction coefficient in the visible light range [2]. In 2010, Feldt et al. revolutionized the field with a new strategy by use of Co(bpy)32+/3+ as the redox couple in combination with an organic sensitizer, D35, achieving an impressive DSSCs efficiency of 6.7% in acetonitrile [3]. This new concept utilized the blocking effects of D35 and bulky cobalt complex, which significantly reduces the fast recombination process normally observed between the injected electrons and the one-electron redox couples. With this development of Co(bpy)32+/3+ as redox mediator in combination with new efficient light harvesting absorbers and sensitization strategy, the record sunlight-to-electrical energy conversion efficiency now exceeds 14% [4], a photovoltaic performance that gives promise for real application. One of the main challenges for commercial application of the cobalt-based DSSCs is to achieve long-term stability in these devices. However, in contrast with the large efforts in improving the device performance, much fewer studies have focused on the stability of these devices. Main efforts among the few studies have been devoted to overcome possible ligand dissociation in Co(bpy)32+/3+, by developing more chemically stable cobalt mediators by using multi-dentate ligands [5]. Recent studies have, however, clearly shown that the presence of cations in the electrolytes and other electrolyte species can also play important role on the stability of these devices, where DSSCs fabricated from Li+-free cobalt electrolytes displayed much higher stability compared to Li+-containing electrolytes [6], [7]. Although it has been stated in the literatures [5], [8] that Co(bpy)32+/3+ is potentially unstable, good stability has been reported for DSSCs with Co(bpy)32+/3+, maintaining 91% of its initial efficiency over 2000 hours under one sun illumination [8]. These results indicate that stability of cobalt-electrolyte based DSSCs is affected by more than the stability of the redox mediator itself. Further insights about the degradation process are therefore important for both fundamental understanding of degradation processes and may determine the direction of research to overcome the main obstacles towards high stable solar cells.
In the present study, we investigate the thermal degradation of DSSCs with cobalt electrolyte in a non-volatile solvent 3-methoxypropionitrile (MPN) in detail, with the aim of understanding the degradation mechanisms. We chose to study the thermal degradation in order to avoid the possible effects of the light soaking, considering the complex photochemistry and electrochemistry mechanisms under light illumination and thermal stress [9], [10]. A well-known organic dye, D35 was selected, since derivatives of D35 have shown great stability in DSSCs [11], while commonly used ruthenium dye may subject to ligand replacement reaction [12], [13], which could interfere with the goal of this study. Since ligand dissociation and exchange could be responsible for degradation in cobalt-complexes based DSSCs, we have investigated the stability of electrolytes with different compositions using Lewis base additives like TBP and MBI that may coordinate the cobalt metal center [14], [15]. The device performance of various electrolyte compositions has been followed up to 50 days. Time resolved transient modulation techniques have been applied to follow the detailed electronic changes of these devices during the process, including charge extraction, electron lifetime and transport time. Also, electrochemical impedance spectroscopy and differential pulse voltammetry have been used to investigate the stability of the electrolytes.
These results have shown further insights about the change of the electronic properties and electrolyte composition changes for the state-of-art cobalt-based DSSCs, together with the current-voltage parameters changes during the degradation process. It is suggested that the stability of DSSCs based on cobalt electrolytes are not limited by the electrolytes, but considerably from a decreased conduction band and reduced electron lifetime, indicating semiconductor/dye/electrolyte interface probably more crucial for a long-term stability of DSSCs under thermal stress.
Section snippets
Experimental Section
All chemicals were purchased from Sigma Aldrich unless stated otherwise. D35 was purchased from Dyenamo. All the chemicals are used as received without further purification.
Current-Voltage characteristics of DSSCs with four electrolytes during thermal degradation process
The initial current-voltage (IV) characteristic parameters of DSSCs with the four electrolytes were similar and are shown in Table S1. Replacement of TBP with MBI as Lewis base additive yields slightly better device performance. Despite more efficient systems have been reported in the literatures, we have chosen the present system in order to focus on the stability of cobalt electrolyte itself and exclude complicated stability contribution from factors like dye degradation and solvent
Discussion
In the present study, we have shown that the degradation of the tested solar cells based on cobalt electrolytes was mainly resulting from the large Voc decrease regardless of the Lewis base additives in the electrolyte. Previous studies have shown rather similar decrease of Voc (> 20%) for the cobalt-electrolyte based DSSCs with ACN as the solvent tested at a temperature of 60 °C in the dark [6], while the light illumination only deteriorated the Voc without changing theI–V evolution trend
Conclusion
Thermal stability of DSSCs fabricated with cobalt bipyridyl–based was studied here motivated by the fast progress and promising commercial industrial application. We have shown that DSSCs fabricated with both MBI and BPY as additives in the standard electrolytes demonstrate highest stability with only 4% relative drop in efficiency at 70 °C in the dark within 50 days, while commonly used TBP electrolytes shown worse stability with a 22% efficiency drop. The Voc decrease of DSSCs is characterized
Acknowledgement
Dr. Tomas Edvisson and Jiajia Gao are acknowledged for experimental helps and discussion. Dr Marina Freitag and Dr. Dongqin Bi are acknowledged for the correction of the manuscript. The authors acknowledge the financial support by the Swedish Energy Agency, the Swedish Research Council (VR), and the STandUP for Energy program. W.Y. gratefully acknowledges the China Scholarship Council (CSC) for a PhD study fellowship.
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Current address: Department of Engineering Sciences and Mathematics, Luleå University of Technology, Sweden.