Behind the Breakthrough of the ∼30% Perovskite Solar Cell

In the December 11, 2020 issue of Science, Al-Ashouri and colleagues reported a certified monolithic perovskite/silicon tandem solar cell with the power conversion efficiency 29.15%. The improvements of the ideality factor and the fill factor are key to achieving efficient perovskite tandem solar cells, which lead a new direction of the perovskite research. In the December 11, 2020 issue of Science, Al-Ashouri and colleagues reported a certified monolithic perovskite/silicon tandem solar cell with the power conversion efficiency 29.15%. The improvements of the ideality factor and the fill factor are key to achieving efficient perovskite tandem solar cells, which lead a new direction of the perovskite research. Recently in Science, Al-Ashouri et al., a group at Helmholtz-Zentrum Berlin (HZB), reported a certified 29.15% perovskite-silicon tandem solar cell by using a self-assembled monolayer (SAM) with methyl group substitutions as the hole-selective layer to improve the hole extraction efficiency and reduce the fill factor (FF) loss.1Al-Ashouri A. Köhnen E. Li B. Magomedov A. Hempel H. Caprioglio P. Márquez J.A. Morales Vilches A.B. Kasparavicius E. Smith J.A. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (418) Google Scholar By using the ideality factor and the FF loss-analyzing techniques, it is proved that their strategy can effectively reduce the non-radiative losses and increase the open-circuit voltage (Voc) of the tandem devices to 1.92 V. This breakthrough will lead to several changes, including the technology developments and the commercialization of the perovskite solar cells (PSCs). The first effect is to facilitate healthy competition in the PSC research community. Previously, Oxford PV held the highest tandem cell efficiency records since 2018 (27.3% in 2018 and then 28% in 2019).2NRELBest Research-Cell Efficiencies.https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdfDate: 2020Google Scholar However, the understanding of the tandem PSCs with efficiency beyond 28% is very limited, especially the device details and the science fundamentals. Two weeks after HZB published their 29.15% details, Oxford PV announced their new efficiency world record of the perovskite tandem solar cells at 29.52%, certified by the National Renewable Energy Laboratory (NREL).2NRELBest Research-Cell Efficiencies.https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdfDate: 2020Google Scholar Both groups state their roadmaps to push the efficiency over 30% in the near future. It shows the healthy competitive relationship between Oxford PV and HZB, which is valuable for the development in the PSC field. From HZB's publication,1Al-Ashouri A. Köhnen E. Li B. Magomedov A. Hempel H. Caprioglio P. Márquez J.A. Morales Vilches A.B. Kasparavicius E. Smith J.A. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (418) Google Scholar one can realize more details to fabricate the >28% efficient perovskite tandem cells. For example, Figure 1 shows the external quantum efficiency (EQE) curves of 28% and 29.15% records of Oxford PV and HZB, respectively.3Green M. Dunlop E. Hohl-Ebinger J. Yoshita M. Kopidakis N. Hao X. Solar Cell Efficiency Tables (Version 57).Prog. Photovolt. Res. Appl. 2020; 1: 1-13Google Scholar Combining this curve and the device details addressed by HZB,1Al-Ashouri A. Köhnen E. Li B. Magomedov A. Hempel H. Caprioglio P. Márquez J.A. Morales Vilches A.B. Kasparavicius E. Smith J.A. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (418) Google Scholar researchers can analyze and understand the device structure differences between the two records according to the observations from spectroscopic evidence. Consequently, the entire PSC field shows rapid growth when more and more technical contents are released. In this way, racing for the crown of the best cell efficiency will then become helpful to the whole PSC society. The second effect is about the research focus. The efficiency-breakthrough focus of PSCs is changed from focusing on the Voc loss to the ideality factor and the FF loss. Figure 2 shows the best research-cell efficiencies from the NREL's efficiency chart2NRELBest Research-Cell Efficiencies.https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdfDate: 2020Google Scholar and their vital parameters calculated from Shockley-Queisser (S-Q) detailed balance limit.5Shockley W. Queisser H.J. Detailed Balance Limit of Efficinecy of p-n Junction Solar Cells.J. Appl. Physiol. 1961; 32: 510-519Crossref Scopus (8866) Google Scholar Among them, Jsc,sq, Voc,sq, and FFrad are the S-Q limitation values of the short-circuit current density, the open-circuit voltage, and the fill-factor, respectively. They represent the highest theoretical values of the Jsc, Voc, and FF under the thermal dynamic equivalent condition. Jsc/Jsc,sq, Voc/Voc,sq, and FF/FFrad are the percentages of the theoretical limit. The performance reaches the theoretical limit, if the value is equal to 100%. The conversion efficiency is increased, almost linearly, from 20.1% to 25.5% (2014∼2020) in Figure 2. However, the value of Jsc/Jsc,sq is close to 92% from 2014 to 2020. Since then, the Jsc is thought to have reached the theoretical limit. Certainly, if the optical reflection loss of a glass substrate (∼8%) is considered, the internal efficiency is really close to 100%. At that time, the improvement of the Voc loss was the key to the high efficiency.4Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface Passivation of Perovskite Film for Efficient Solar Cells.Nat. Photonics. 2019; 13: 460-466Crossref Scopus (2317) Google Scholar Indeed, the efficiency breakthroughs were accompanied by the improvement of the Voc loss from 2014 to 2020 (Figure 2). It is worth noting that, in 2020, Jeong et al. reported 25.5% efficiency with the low Voc loss, which pushed the Voc/Voc,sq to ∼96%.7Jeong M. Choi I.W. Go E.M. Cho Y. Kim M. Lee B. Jeong S. Jo Y. Choi H.W. Lee J. et al.Stable perovskite solar cells with efficiency exceeding 24.8% and 0.3-V voltage loss.Science. 2020; 369: 1615-1620Crossref PubMed Scopus (621) Google Scholar Nevertheless, the FF/FFrad results still stay at the same level, around 92% of the FF S-Q limit, compared with that of the previous work (in Figure 2).8Green M. Dunlop E. Levi D.H. Hohl-Ebinger J. Yoshita M. Ho-Bailie A.W.Y. Solar Cell Efficiency Tables (Version 54).Prog. Photovolt. Res. Appl. 2019; 27: 565-575Crossref Scopus (573) Google Scholar It means that the main limiting factor is not the Voc loss. Therefore, HZB proposes that the ideality factor and the FF loss are the key factors to advance the efficiency of PSCs.1Al-Ashouri A. Köhnen E. Li B. Magomedov A. Hempel H. Caprioglio P. Márquez J.A. Morales Vilches A.B. Kasparavicius E. Smith J.A. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (418) Google Scholar By adding the proper SAM in single-junction PSCs, the ideality factor is improved to 1.26 and the FF is increased to 84%. This strategy is proven to be successful, and the tandem efficiency can reach 29.15%. On the other hand, the 84% FF of HZB's results, the FF/FFrad only reaches 93% of the S-Q limit. Compared with 96% of Voc/Voc,sq, the FF has more room for improvement. Consequencely, the ideality factor and the FF are the key factors to lead efficiency breakthrough of the PSCs, rather than Voc. The third effect is that the main inspection techniques will be focused on the efficiency potential characterization of the neat-perovskite-films instead of complete-device efficiency inspections. There are 15 characterization tools in the publication,1Al-Ashouri A. Köhnen E. Li B. Magomedov A. Hempel H. Caprioglio P. Márquez J.A. Morales Vilches A.B. Kasparavicius E. Smith J.A. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (418) Google Scholar including 12 optical methods. The reason they utilize so many tools is to understand the deep nature of the physics, defects, and recombination mechanisms to further overcome the difficulties and challenges of PSCs. Traditionally, the efficiency or transporting-mechanism-related measurements of the PSCs can only be performed after the metal contact process.9Stolterfoht M. Grischek M. Caprioglio P. Wolff C.M. Gutierrez-Partida E. Peña-Camargo F. Rothhardt D. Zhang S. Raoufi M. Wolansky J. et al.How To Quantify the Efficiency Potential of Neat Perovskite Films: Perovskite Semiconductors with an Implied Efficiency Exceeding 28.Adv. Mater. 2020; 32: e2000080Crossref PubMed Scopus (65) Google Scholar For instance, the FF loss caused by the non-radiative recombination in the interfaces and electrodes of the PSCs can be analyzed quantitatively by the intensity-dependent Voc measurements only in a complete-device form after the metallization process. Before the metallization procedure, researchers do not have proper tools to quantitatively study the potential efficiency of PSCs. For neat perovskite films, HZB proposes and promotes the non-destructive optical methods, such as the internal quasi-Fermi level splitting (QFLS) via photoluminescence (PL) measurements, to quantify the efficiency potential of neat perovskite compositions and identify the non-radiative loss in neat materials, perovskite/hole-transporting-layer (HTL) interfaces, and perovskite/HTL/metal interfaces. In the work, the computed QFLS values (or implied Voc (iVoc)) can be reconstructed to "pseudo-J-V" curves from intensity-dependent absolute PL spectra. The pseudo-FF values of bare perovskite films grown on different HTLs can be extracted as well. Subsequently, the potential efficiencies of different neat perovskite layers are predicted from the pseudo-J-V curves. This neat-film-inspection method will become normal and popular for two reasons. One is that it can characterize and quantify the efficiency potential of any perovskite films on glasses in the absence of carrier transport limitations. The other reason is that it can significantly save time for trial-and-error efforts and device performance tuning before finishing the metallization. As a result, it will become mainstream in future PSC research. As the authors mentioned, perovskite/silicon tandem cells have several technological advances from the matured silicon solar cell industry. This work not only takes the breakthrough in efficiency, but also describes new perspectives for developing PSCs. It brings the healthy competitions in the PSC society and gives the boost to the commercialization of the PSC technology as well. The authors acknowledge the support of research and development projects from Southern Taiwan Science Park ( STSP ) and Department of Indusrtrial Technology ( DOIT) .