“Highly Efficient and Stable” Perovskite Solar Cells: Hype Versus Reality

Abstract

No doubt, in a short span of about 12 years, perovskite solar cells have made significant impacts both on fundamental research and in the development of photovoltaic (PV) technology. (1,2) The certified single-junction solar cell efficiency increased from 14.1% (2013) to 26.7% (2024), (2) although most of the recent champion solar cell efficiency gains were made through decreasing the area of the cell. (3) Si-perovskite tandem solar cells have attained a photoconversion efficiency (PCE) of 34.6%. The enthusiastic determination of the scientific community during the past decade was a key factor in turning academic research into practical reality. Efforts are now underway to produce large panels for solar PV installations in China and elsewhere. Despite this progress, one catchy phrase that we continue to see in the titles of scientific papers is “efficient and stable”, or even “highly efficient and stable”, or some variation thereof. In order to highlight a newly adopted procedure to formulate perovskite solar cells, authors often use such adjectives in the title of their paper. Since most research laboratories can now produce perovskite solar cells with ∼24% PCE, there is no need to claim high efficiency every time one reports a perovskite solar cell performance. Unless it is a record efficiency, any other claim of high efficiency appears to be an unnecessary hype. Looking back into the perovskite solar cell literature, the first reference to “efficient and stable” appeared in 2014. (4) Since then, this phrase has become a mantra for publishing perovskite papers, more noticeably even in “prominent” journals. Interestingly, its use in the title of perovskite solar cell articles is on the rise. Figure 1 shows the increased use of this phrase in journal titles during the last ten years. (Note: ACS Energy Letters has published ∼37 such articles during this period, mostly prior to 2021. We now discourage the use of nonquantifiable phrases in the title. (5)) Figure 1. Number of perovskite solar cells/photovoltaics papers (2014–2024) published with the phrase “efficient and stable” in the title in all journals. The entry for 2024 is partial data. Source: Web of Science, Clarivate Analytics, Dec 23, 2024. Many questions arise when one sees perovskite photovoltaics papers claiming “Efficient” or “Highly Efficient” in the title: Is this a real breakthrough or just another hyped claim? What makes the new “highly efficient” perovskite solar cell article distinctively different from the previous one published by the same group or in the same journal? Is it the chemical treatment that the authors employed or the new solar cell design that represents the scientific advance? Is the previous report, published by the same group six months earlier with a similar claim, now redundant? Unless one compares the record-breaking PV performance of a previously reported work, it is difficult to assess the merit of the new claim. Often, such comparisons to previous advances are not discussed while making new claims. In other words, authors often fail to explain why the current solar cell performance is better than the previously reported claim of a similar nature. Another aspect is the stability of perovskite solar cells presented in the papers. It is common practice to demonstrate the stability of the cells by comparing the normalized PCE of a newly designed perovskite solar cell with that of a poorly performing control cell, monitored at the maximum power point. Figure 2 shows a simulated stability plot illustration (not a real performance curve) to highlight the stability claims typically made in these papers. Often, the control curve shows a rapid decay, while the newly developed solar cell shows stable performance with less than 85% decrease in normalized PCE. The time scale (X-axis) is determined by the evaluation of the stability period of the new cell, which can vary from a few hundred hours to a few thousand hours. Earlier, a consensus report based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols was drafted by several leading researchers to test perovskite solar cell stability. (6) The report pointed out many deficiencies in published stability tests, including testing only a single cell, presenting normalized efficiency instead of measured efficiency, and presenting analyses of light/dark cycles to simulate day/night operation. A similar emphasis on reporting stability parameters of perovskite solar cells was also made in another editorial recently. (7) It is important that the scientific community take these recommendations into account seriously while conducting stability tests. Many questions remain unanswered when we read extraordinary claims about the stability of perovskite solar cells. Does the stability curve refer to a single champion cell or a collection of multiple cells? How does the stability performance differ under maximum power point and open circuit operational conditions? How does the stability performance of the newly reported cell compare to that of another stable cell reported by the same group a few months earlier? Why do authors often compare the stability performance to that of a poorly performing control and not the previously reported stable cell? Figure 2. A simulated plot to illustrate typical stability comparison between newly reported solar cell and a control cell. (The data points are intended to mimic the trend and do not represent real data. Usually X = 10, 100 or 1000, depending on the reported stability time scale.) It has been more than ten years since we started referring to perovskite solar cells as “efficient and stable”. Given the success of perovskite solar cell research, isn’t it time for us to report on perovskite solar cell performance without hyped claims? When it became a common practice to use “Novel” in titles, many leading journals discouraged the use of such nonquantifiable adjectives. (8) The perovskite research community can come together and evaluate research progress based on scientific merit without the emphasis on superlatives. At ACS Energy Letters, we appeal to our authors to refrain from using nonscientific or nonquantifiable adjectives (for example, excellent, unprecedented, exceptional, sustainable, efficient, superior, high, remarkable, extraordinary, promising, innovative, etc.) in the title of submitted papers. In an earlier editorial, we discussed the correct use of the terms “efficiency” or “efficient” in the context of a catalytic process and the importance of using precise and objective language that accurately reflects the data and the conclusions. (5) A plea was also made to avoid terms such as “high efficiency” in the manuscript titles. (5,9) “Sandwich papers” that report minor improvements in efficiency and stability as compared to control cells, achieved through routine chemical treatment or device engineering, fail to impress reviewers or readers. (10) It is important for our authors to focus on the new scientific advances with minimal use of superlatives. Papers that focus on new insights will have a long-term impact and will, therefore, gain the attention of peers. This article references 10 other publications. This article has not yet been cited by other publications.