2D or not 2D: Eliminating interfacial losses in perovskite solar cells

Perovskite photovoltaics offer advantages for low-cost, efficient-energy production. However, interfacial losses currently limit device performance and stability. In this issue of Chem, Sutanto et al. visualize the energetic landscape of novel 2D/3D perovskite heterojunctions and thereby demonstrate the potential of optimized 2D perovskite layers as transport layers for high-performance, stable perovskite photovoltaics. Perovskite photovoltaics offer advantages for low-cost, efficient-energy production. However, interfacial losses currently limit device performance and stability. In this issue of Chem, Sutanto et al. visualize the energetic landscape of novel 2D/3D perovskite heterojunctions and thereby demonstrate the potential of optimized 2D perovskite layers as transport layers for high-performance, stable perovskite photovoltaics. Photovoltaics based on metal-halide perovskite absorbers have witnessed an unprecedented increase in power conversion efficiencies since the first report in 2009.1Kojima A. Teshima K. Shirai Y. Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Crossref PubMed Scopus (14003) Google Scholar This rapid development has been driven by the convergence of researchers from different backgrounds, resulting in the incredible focus of diverse expertise in device physics, chemistry, material science, and engineering into a single field. Innovations over the last decade in both the device architecture2Stranks S.D. Eperon G.E. Grancini G. Menelaou C. Alcocer M.J.P. Leijtens T. Herz L.M. Petrozza A. Snaith H.J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber.Science. 2013; 342: 341-344Crossref PubMed Scopus (7351) Google Scholar and perovskite composition3Saliba M. Matsui T. Seo J.-Y. Domanski K. Correa-Baena J.-P. Nazeeruddin M.K. Zakeeruddin S.M. Tress W. Abate A. Hagfeldt A. Grätzel M. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar have led to record power conversion efficiencies exceeding 25%.4Jeong J. Kim M. Seo J. Lu H. Ahlawat P. Mishra A. Yang Y. Hope M.A. Eickemeyer F.T. Kim M. et al.Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells.Nature. 2021; 592: 381-385Crossref PubMed Scopus (764) Google Scholar Perovskite solar cells are fabricated in either n-i-p (standard) or p-i-n (inverted) architectures, although ionic defects have been correlated with doping of the perovskite absorber5Fassl P. Lami V. Bausch A. Wang Z. Klug M.T. Snaith H.J. Vaynzof Y. Fractional deviations in precursor stoichiometry dictate the properties, performance and stability of perovskite photovoltaic devices.Energy Environ. Sci. 2018; 11: 3380-3391Crossref PubMed Google Scholar so that the perovskite absorber is not necessarily intrinsic. However, the charge-compensating nature of the perovskite lattice has, to date, prevented the development of reliable protocols6Euvrard J. Yan Y. Mitzi D.B. Electrical doping in halide perovskites.Nat. Rev. Mater. 2021; 6: 531-549Crossref Scopus (53) Google Scholar for creating highly doped perovskite transport layers. As a result, the p and n transport layers are fabricated with different p-type and n-type semiconductors at the anode and cathode, respectively. The complex physico-chemical nature of these heterojunctions between the perovskite absorber and the n-type and p-type transport layers has been identified as one of the critical bottlenecks for both the performance and the stability of perovskite solar cells.7Lira-Cantú M. Perovskite solar cells: stability lies at interfaces.Nat. Energy. 2017; 2: 17115Crossref Scopus (88) Google Scholar Further, the energetic band structure at these interfaces is essentially unknown. As a result, there is a fast-growing interest in dedicated interface engineering, as well as characterization protocols that enable detailed analysis of these buried interfaces. Interfacial losses in solar cells primarily affect the open circuit voltage (VOC) and the fill factor (FF). The VOC is fundamentally limited by the band gap of the absorber layer, which in turn limits the chemical potential or the quasi-Fermi-level splitting (QFLS) of the photogenerated electrons and holes. Losses in carrier density and carrier energy reduce the QFLS and hence the VOC. Therefore, to maximize VOC, the band bending at device interfaces must induce preferential band alignment to promote efficient majority carrier transport while preventing non-radiative recombination losses. At VOC, the resistance at the contact is infinite, and therefore the conductivity of the transport layer is irrelevant, and non-radiative recombination losses that reduce VOC can be identified via optical techniques, such as photoluminescence (PL) spectroscopy. The FF, on the other hand, is limited by the efficiency of carrier extraction from the device close to the maximum power point (MPP). This means that the transport layers must have sufficiently high conductivity to promote majority carrier extraction. A drop in conductivity between the absorber layer and the transport layer manifests itself as a resistance with a corresponding voltage drop between the absorber layer and the contact. Losses in the FF can be monitored only via measurements on fully contacted devices. Therefore, assessing the quality of transport layers and contact materials requires an integrated experimental approach to identify the underlying nature of the loss mechanism and its impact on different operating points of the solar cell.8Hutter E.M. Kirchartz T. Ehrler B. Cahen D. Von Hauff E. Pitfalls and prospects of optical spectroscopy to characterize perovskite-transport layer interfaces.Appl. Phys. Lett. 2020; 116: 100501Crossref Scopus (18) Google Scholar Interfacial engineering in perovskite photovoltaics has been largely hampered to date by the lack of suitable transport materials, as well as the poor understanding of interfacial energetics and carrier dynamics. Generally, transport-layer materials are associated either with poor chemical stability when interfaced with the perovskite absorber or with low conductivity, thereby preventing efficient charge extraction from the device. Recently, 2D perovskites were identified as potentially promising for the formation of stable, chemically compliant contact interfaces with suitable conductivity for carrier extraction for perovskite solar cells.9Grancini G. Nazeeruddin M.K. Dimensional tailoring of hybrid perovskites for photovoltaics.Nat. Rev. Mater. 2019; 4: 4-22Crossref Scopus (425) Google Scholar However, this field is still in its infancy given that the energetic structure and electrical transport properties at the buried 2D/3D perovskite heterojunction remain elusive. In this issue of Chem, Sutanto et al.10Sutanto A.A. Caprioglio P. Drigo N. Hofstetter Y.J. Garcia-Benito I. Queloz V.I.E. Neher D. Nazeeruddin M.K. Stolterfoht M. Vaynzof Y. et al.2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells.Chem. 2021; 7: 1903-1916Abstract Full Text Full Text PDF Scopus (37) Google Scholar visualize the vertical energetic profile of the 2D/3D perovskite heterojunctions for the first time. Their findings reveal that the preferential band bending between the bulk 3D perovskite absorber and the 2D perovskite hole transport layer effectively reduces non-radiative recombination losses at the anode. They correlate these findings with improvements in VOC and demonstrate that optimized 2D/3D perovskite interfaces display no non-radiative losses. They also observed enhanced device stability for solar cells fabricated with the optimized 2D layers. This study’s detailed insight into the electrostatics of the 2D/3D interface represents an important and fundamental step toward mitigating interfacial losses in perovskite solar cells. The authors applied the innovative approach of combining ultraviolet photoelectron spectroscopy (UPS) with argon cluster etching to obtain nanometer resolution of the energetic landscape at the 2D/3D perovskite interface. They applied this technique to study the interface between the 3D bulk perovskite absorber layer ([(FAPbI3)0.87(MAPbBr3)0.13]0.92(CsPbI3)0.08) and 2D perovskite layers formed from novel thiophene-methylammonium salts, (2-thiophenemethylammonium iodide (2-TMAI), 2- thiophene methylammonium bromide (2-TMABr), or 2-thiophenemethylammonium chloride (2-TMACl)) (see schematic in Figure 1). The device architecture employs an FTO/c-TiO2/mp-TiO2/SnO2 cathode and a Spiro-OMeTAD/Ag anode. 2D layers were solution processed on top of the absorber layer before the anode was applied. Reference devices had the same structure except for a 2D layer (Figure 1 of Sutanto et al.10Sutanto A.A. Caprioglio P. Drigo N. Hofstetter Y.J. Garcia-Benito I. Queloz V.I.E. Neher D. Nazeeruddin M.K. Stolterfoht M. Vaynzof Y. et al.2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells.Chem. 2021; 7: 1903-1916Abstract Full Text Full Text PDF Scopus (37) Google Scholar). The comparison of three different building blocks for preparing 2D layers offers insight into the impact of relevant design parameters on device performance, such as the chemical structure of building blocks, as well as the morphology of the resulting layer. The UPS depth profiling revealed the energetic band alignment at the 2D/3D interface located at the solar anode. The perovskite absorber layer was n-type as a result of the excess of PbI2 used for fabrication. 2-MAI and 2-TMACl were found to result in the formation of n-type 2D layers and therefore have negligible impact on band bending at the anode interface. Consistent with this result, the solar cell parameters of these devices were comparable to those of the reference device. In contrast, 2-TMABr was found to result in the formation of a p-type transport layer and, correspondingly, the formation of a p-n junction with the n-type perovskite absorber layer (Figure 3 of Sutanto et al.10Sutanto A.A. Caprioglio P. Drigo N. Hofstetter Y.J. Garcia-Benito I. Queloz V.I.E. Neher D. Nazeeruddin M.K. Stolterfoht M. Vaynzof Y. et al.2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells.Chem. 2021; 7: 1903-1916Abstract Full Text Full Text PDF Scopus (37) Google Scholar). This was correlated with an increase in the solar cell VOC from 1.141 V (control device) to 1.193 V (2-TMABr). PL spectroscopy revealed that the non-radiative recombination losses in the 2-TMABr devices were lower than in the reference device. From the PL, the QFLS was determined and found to be higher in devices prepared with 2-TMABr, consistent with the observed increase in VOC (Figure 4 of Sutanto et al.10Sutanto A.A. Caprioglio P. Drigo N. Hofstetter Y.J. Garcia-Benito I. Queloz V.I.E. Neher D. Nazeeruddin M.K. Stolterfoht M. Vaynzof Y. et al.2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells.Chem. 2021; 7: 1903-1916Abstract Full Text Full Text PDF Scopus (37) Google Scholar). Notably, the authors show that QFLS values extracted from PL measurements on uncontacted 3D perovskite absorber layers and from perovskite absorbers interfaced with 2D layers (2-TMABr) were equivalent. In other words, these optimized 2D/3D heterojunctions have no associated non-radiative losses. Given that the measured VOC (1.193 V) in these devices was still considerably lower than expected (the band gap of the perovskite absorber was 1.61 eV), the losses in VOC can be attributed to non-radiative losses in the perovskite absorber and at the cathode interface and to energetic misalignment between the 2D perovskite and the Spiro-OMeTAD. Engineering functional interfaces in perovskite solar cells with no radiative losses is an important step toward reaching the theoretical efficiency limit. These results demonstrate the potential of 2D/3D heterojunctions to maximize VOC while shedding light on the importance of localizing sources for non-radiative losses in perovskite solar cells. Further optimization of the 2D layers is required, as indicated by the other solar cell parameters (Figure 2 of Sutanto et al.10Sutanto A.A. Caprioglio P. Drigo N. Hofstetter Y.J. Garcia-Benito I. Queloz V.I.E. Neher D. Nazeeruddin M.K. Stolterfoht M. Vaynzof Y. et al.2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cells.Chem. 2021; 7: 1903-1916Abstract Full Text Full Text PDF Scopus (37) Google Scholar). The FF of the solar cells fabricated with 2D layers was found to be slightly lower (between 0.74 and 0.76) than in reference devices (0.79), which was attributed to reduced conductivity in the 2D layers arising from the bulky, insulating organic groups, resulting in less efficient charge extraction. The short-circuit current density (JSC) was also found to be slightly lower in solar cells prepared with 2-TMABr (23.2 mA/cm2) and 2-TMACl (22.8 mA/cm2) than in the reference device and the device prepared with 2-TMAI (24.2 mA/cm2 for both devices), and this result is consistent with the slightly lower absorption observed in these devices. The promising results presented here will certainly motivate future studies into optimizing the fabrication of the 2D transport layers (including variations in the chemical structure of organic groups on the building blocks), as well as optimizing the thickness and morphology of the layers, to minimize further optical and electrical losses. Dedicated engineering of 2D/3D interfaces represents a promising route to tackling non-radiative losses in perovskite solar cells. The enhanced stability in the devices prepared with optimized 2D transport layers further underlines the interesting potential of 2D/3D perovskite heterostructures for efficient and stable perovskite solar cells. By applying a comprehensive experimental approach that correlates the energetic landscape at the anode interface with QFLS in the absorber and with the resulting solar cell parameters, the authors were able to demonstrate a promising route toward eliminating non-radiative recombination losses at the anode interface. The approach and insight presented here will serve as a valuable guide for the dedicated design of high-performance perovskite solar cells. 2D/3D perovskite engineering eliminates interfacial recombination losses in hybrid perovskite solar cellsSutanto et al.ChemMay 3, 2021In BriefUnderstanding interface energetics and physics is essential for gaining fundamental knowledge of perovskite solar cells’ operation and optimization. 2D/3D perovskite interfaces incorporating thiophene-based bulky organic cations are investigated. By fine tuning the halide counterion of the thiophene-based cation, a p-n junction is formed at the interface. The formation of a p-n junction eliminates the interfacial recombination losses by reducing the electron density, resulting in devices with high VOC, which approach the potential internal quasi-Fermi level splitting of the perovskite absorber. Full-Text PDF Open Archive