Browsing by Author "Poskela, Aapo"

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Critical analysis on the quality of stability studies of perovskite and dye solar cells
(2018) Tiihonen, Armi; Miettunen, Kati; Halme, Janne; Lepikko, Sakari; Poskela, Aapo; Lund, Peter
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The success of perovskite and dye-sensitized solar cells will depend on their stability over the whole life-time. Aging tests are of utmost importance to identify deficiencies and to suggest cell improvements. Here we analyzed the quality of 261 recent aging tests and found serious shortcomings in current practices. For example, in about 50% of the studies only one sample was considered, meaning that the sample size was too small for statistical significance. We propose a new procedure for aging tests based on careful planning and scientific reporting. This includes estimating the required sample size for an aging test and avoiding so-called nuisance factors, i.e. unintended variations always present in real world testing. The improved procedure can provide more reliable information on stability and lifetime, which could contribute to better understanding of degradation mechanisms important for improving these photovoltaic technologies.
Extreme sensitivity of dye solar cells to UV-induced degradation
(2021-01) Poskela, Aapo; Miettunen, Kati; Tiihonen, Armi; Lund, Peter D.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Present practice to avoid harmful effects of UV light on dye solar cells (DSC) is to use a UV filter. However, we show here that a standard 400 nm UV cutoff filter offers inadequate protection from UV-induced degradation. DSCs that were exposed to only visible light by LED lamps maintained 100% of their initial efficiency after 3000 hours of exposure, whereas the efficiency of DSCs subjected to full light spectrum (Xenon arc lamp) with an efficient UV filter dropped down to 10% of their initial performance already after 1500 hours. Optical analysis of the UV filter confirmed that the amount of light transmitted below 400 nm was negligible. These observations indicate that (a) DSCs can be very sensitive to even minor amount of UV and (b) eliminating the effects of UV light on DSC stability cannot easily be avoided by a UV filter on top of the cell. A detailed analysis of the degradation mechanisms revealed that the culprit to loss of performance was accelerated loss of charge carriers in the electrolyte of the DSCs-a typical symptom of UV exposure. These results suggest that commonly used stability tests under LED illumination are insufficient in predicting the lifetime of DSCs in outdoor conditions. Instead, for such purpose, we recommend solar cell stability to be tested with a full light spectrum and with a suitable UV filter.
From identification of electrolyte degradation rates to lifetime estimations in dye solar cells with iodine and cobalt redox couples
(2017) Miettunen, Kati; Poskela, Aapo; Tiihonen, Armi; Rendon, Sabine; Axenov, Kirill; Kronberg, Leif; Leino, Reko; Lund, Peter
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Degradation of dye solar cells is a major obstacle in their commercialization. Here we look into how much information on the degradation routes and rates one can extract from accessible measurements. Specifically we focus on tracking the color of the cell since all the main components of a dye solar cell have a specific color, and their color changes with degradation. Furthermore we look into extracting the degradation coefficients based on the specific color changes. One of the most vulnerable components of a dye solar cell is the electrolyte. Here we investigate the effect of two most interesting electrolyte compositions: 1) conventional iodine based electrolyte, which to date dominates the stability records of dye solar cells, and 2) cobalt complex electrolyte, which enables record high efficiencies. UV light is known to be highly detrimental as it destroys charge carriers – typically, a UV filter is recommended, but is it enough to prevent the loss of charge carriers? Here expectedly applying a UV filter improved the performance as the cells without a filter had only 4 ± 1 % of the initial efficiency left after a 1,000 hour light soaking test, whereas those with a filter maintained 90 ± 20 % of their initial efficiency. Applying a UV filter only hindered the loss of the charge carriers, but did not eliminate their degradation. From the color changes of the electrolyte, we could identify the degradation coefficient for these electrolytes. This analysis resulted in a highly relevant discovery: the loss rate of the charge carriers in iodine electrolyte was approximately double compared to cobalt electrolyte. Furthermore we could provide indicative estimates of future lifetimes of cells, which could be highly important in improving the lifetime of dye solar cells.
Influence of Materials and Aging Test Design on Dye Solar Cell Stability
(2021) Poskela, Aapo
School of Science | Doctoral dissertation (article-based)
Dye solar cells are an emerging third-generation photovoltaic technology. Its main advantages are the low cost of materials and manufacturing, and the possibility to adjust the colour of the cell by selecting different dyes. Currently one of the main obstacles for commercialisation of dye solar cells is their relatively short lifetime. Silicon solar panels are often given guarantees of over two decades, while dye solar cells can reliably function only for a few years in outdoor conditions. This work focuses on dye solar cell stability from multiple perspectives: the impact of different materials and structures, effect of operating conditions to the cell lifetime, and analysis of the methods used for studying dye solar cell stability. A practical challenge in the production of robust and large dye solar cells is reliably sealing its liquid electrolyte within the cell. It was discovered that soaking a nanocellulose or nanochitin membrane in an electrolyte solution and then sealing it inside the dye solar cell not only simplifies the assembly process, but also increases the initial energy conversion efficiency of the solar cells. Research for improved dye solar cell lifetime can be supported studying how operating conditions affect it, i.e. what kind of factors and mechanisms are most detrimental to stability and should always be taken into consideration in cell design. Since a dye solar cell can function for some years in standard operation, stability studies generally rely on accelerated tests. One focus of this thesis was to study how well approximations made in accelerated tests correspond to realistic conditions. A common mistake in dye solar cell stability testing is to underestimate the degrading effects of UV light. It was found that even a UV filter is inadequate to protect dye solar cells fully, suggesting that UV degradation of dye solar cells is still an issue requiring further research. Another topic that is discussed is how the circuit state of the cells impacts their degradation rate. When solar cells are operating in practice, they are connected to an electric load that brings them close to their maximum power point. Often this is overlooked in research and dye solar cells are aged in open circuit conditions, which may artificially lengthen the lifetime of dye solar cells. This thesis demonstrates that the difference between the two states is small. The work also includes pioneering aging tests in harsh northern conditions. The stability research practices in the field of emerging photovoltaics was reviewed, providing suggestions on how to improve the corresponding research to obtain more accurate results.
Nanocellulose aerogel membranes for optimal electrolyte filling in dye solar cells
(2014) Miettunen, Kati; Vapaavuori, Jaana; Tiihonen, Armi; Poskela, Aapo; Lahtinen, Panu; Halme, Janne; Lund, Peter
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A new method for depositing electrolyte in dye solar cells (DSCs) is introduced: a nanocellulose hydrogel membrane is screen printed on the counter electrode and further freeze-dried to form a highly porous nanocellulose aerogel, which acts as an absorbing sponge for the liquid electrolyte. When the nanoporous dye-sensitized TiO2 photoelectrode film is pressed against the wetted aerogel, it becomes filled with the electrolyte. The electrolyte flows inside the TiO2 film only about ten micrometers (i.e. the TiO2 film thickness) whereas in the conventional filling method, where the electrolyte is pumped through the cell, it flows about 1000-times longer distance, which is known to cause uneven distribution of the electrolyte components due to a molecular filtering effect. Furthermore, with the new method there is no need for electrolyte filling holes which simplifies significantly the sealing of the cells and eliminates one common pathway for leakage. Photovoltaic analysis showed that addition of the nanocellulose aerogel membrane did not have a statistically significant effect on cell efficiency, diffusion in the electrolyte or charge transfer at the counter electrode. There was, however, a clear difference in the short circuit current density and open circuit voltage between the cells filled with the aerogel method and in the reference cells filled with the conventional method, which appeared to be caused by the differences in the electrolyte filling instead of the nanocellulose itself. Moreover, accelerated aging tests at 1 Sun 40 °C for 1000 h showed that the nanocellulose cells were as stable as the conventional DSCs. The nanocellulose aerogel membranes thus appear inert with respect to both performance and stability of the cells, which is an important criterion for any electrolyte solidifying filler material.
Nanocellulose and Nanochitin Cryogels Improve the Efficiency of Dye Solar Cells
(2019-06-17) Poskela, Aapo; Miettunen, Kati; Borghei, Maryam; Vapaavuori, Jaana; Greca, Luiz G.; Lehtonen, Janika; Solin, Katariina; Ago, Mariko; Lund, Peter D.; Rojas, Orlando J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Biobased cryogel membranes were applied as electrolyte holders in dye solar cells (DSC) while facilitating carrier transport during operation. They also improved device performance and stability. For this purpose, cellulose nanofibers (CNF), TEMPO-oxidized CNF (TOCNF), bacterial cellulose (BC), and chitin nanofibers (ChNF) were investigated. The proposed materials and protocols for incorporating the electrolyte, via simple casting, avoided the typical problems associated with injection of the electrolyte through filling holes, a major difficulty especially in manufacturing large area cells. Owing to the fact that cryogel membranes did not require any orifice for injection, they were effective in minimizing leakage and in retaining liquid electrolyte. The results indicated the reduction of performance losses compared to conventional electrolyte filling, likely due to the better spatial distribution of electrolyte. DSCs based on BC cryogels had an initially higher performance and similar stability compared to those of the reference cells. When compared to reference cells, CNF and ChNF cryogels produced higher initial performance, but they underwent a faster degradation. The difference in stability was attributed to the effect of residual components, including lignin in CNF and proteins in ChNF, as demonstrated in bleaching experiments. TOCNF indicated a relatively poor performance, most likely because of residual aldehydes. Overall, we offer a comprehensive evaluation based on current-voltage (IV) profiles under simulated sunlight, incident photon-to-charge carrier efficiency (IPCE), electrochemical impedance spectroscopy (EIS), and color image processing, together with accelerated DSC stability tests, to unveil the effects of new membrane-based assembly. Our results give guidelines for future developments related in particular to the effects of the tested biomaterials on device stability.
New Semi-Solid Electrolytes for Mass Production and Increased Lifetime of Dye Solar Cells
(2017-02-14) Poskela, Aapo
Perustieteiden korkeakoulu | Master's thesis
This study focuses on improving either mass production or lifetime of dye solar cells. The first goal was to study how the state of the external circuit affects the degradation of dye solar cells under illumination. This effect is relevant when considering the operation and aging of cells in actual use. Cells were aged in open circuit, short circuit and under a load close to their maximum power point. It was determined that aging in short circuit had clearly detrimental effect on the long-term stability of the cells, while cells in open circuit and under load performed similarly to each other. The reasons for the different behaviours of the cells was discussed further based on the extensive measurements performed on the cells before, during and after the aging. The cause of the rapid decrease in the performance of short circuited cells was identified to be the degradation of charge carriers in the electrolyte. The second goal was to discover a method to increase dye solar cell stability by adding absorber polymers into the electrolyte of the cells. The hypothesis was that the absorbing polymers would absorb water, which is generally harmful to dye solar cells, during the operation of the cells thus allowing the cells to function longer. Unfortunately, the polymers decreased cell performance notably. As the degradation was apparent instantly after the cell assembly, it is likely that the polymers reacted with the electrolyte of the cells. The final goal was to enhance dye solar cell fabrication methods with the help of biomaterial aerogels. The usual method of injecting liquid electrolyte into a cell through small holes in the substrates has few practical issues, especially when filling a cell with a large area. In this work the aerogels were inserted between the electrodes of the cells and then soaked in electrolyte before the cell was sealed. The aerogels successfully held the liquid electrolyte in place, thus also improving the cell resistance against leaking. The measurements also indicated that this assembly method avoided the problems present in injecting the electrolyte. The performance of the cells assembled with the best of the aerogels did not differ from the performance of the reference cells, demonstrating the validity of this assembly technique.
Predictive Modeling of Dye Solar Cell Degradation
(2022-06) Poskela, Aapo; Tiihonen, Armi; Palonen, Heikki; Lund, Peter D.; Miettunen, Kati
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Degradation of dye solar cell performance based on the early changes in electrolyte color is predicted, allowing to estimate the lifetime of the dye solar cells even before their efficiency declines. Previous predictive models commonly rely on regression analysis of the predicted parameter; thus, they are unable to capture degradation before a significant decrease in performance. Degradation tests, even when accelerated, may take thousands of hours. As such, recognizing degradation trends early can lead to rewarding cuts in the duration of solar cell development pipelines. With accurate lifetime predictions, researchers can steer materials research to reach longer lifetimes in shorter cycles. The predictive power of our model relies on color changes in the electrolyte that directly correlate with the concentration of tri-iodide charge carriers within it, the loss of which is the predominant degradation mechanism for most liquid-electrolyte dye solar cells. By linking the physical mechanisms inside the cell, which eventually start to degrade the performance of dye solar cells, an early prediction of the lifetime can be made even when the device performance still appears stable. It is exemplified with dye solar cells that integrating architecture-specific knowledge on degradation mechanisms has potential to improve lifetime predictions for photovoltaics.
Review of Quasi-Solid Electrolytes for Dye-Sensitized Solar Cells and Their Application as Printable Electrolytes
(2013-10-14) Poskela, Aapo
Perustieteiden korkeakoulu | Bachelor's thesis
Testing dye-sensitized solar cells in harsh northern outdoor conditions
(2018-06) Lepikko, Sakari; Miettunen, Kati; Poskela, Aapo; Tiihonen, Armi; Lund, Peter D.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Here, we report on the behavior of dye solar cells in real-life weather conditions from a northern outdoor test covering for the first time cell performance measurements in harsh conditions with varying weather from mildly warm conditions to freezing and snowy. The effect of different weather conditions on the cell performance is quantitatively investigated by using correlations coefficients of weather parameters to cell performance. No degradation was observed during the frosty period, but instead during the warmer, rainy periods with high moisture levels. Nevertheless, after 6 weeks of outdoor testing in varying harsh conditions, the cells maintained on average 88% of their initial efficiency. Tracking the cell performance during the aging showed that the test cells generated roughly as much current at subzero temperatures as at warmer temperatures. Investigations of the degradation reactions revealed that while photoelectrode degradation was the main cause of degradation during this test, the loss of charge carriers, which had only a minor effect on performance during the test, would likely become a major degradation factor during the next 1000 h of testing. Furthermore, the test showed that the cells even doubled their efficiency in low light intensity conditions compared with the standard reporting conditions. Thus, the overall conversion efficiency during the whole experiment reached up to 50% higher values compared with the results in standard testing conditions.