In-Depth Spectroscopy and New Heights for Organic Solar Cells

In-Depth Spectroscopy and New Heights for Organic Solar Cells

Preview In-Depth Spectroscopy and New Heights for Organic Solar Cells translates into near-room-temperature processing techniques such as standard v...

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In-Depth Spectroscopy and New Heights for Organic Solar Cells

translates into near-room-temperature processing techniques such as standard vacuum deposition or standard coating and printing techniques used to process these semi-crystalline materials into thin layers at low costs using energy-efficient methods.

Bernard Kippelen1,*

For the past decades, the quest for efficient organic solar cells has been characterized by some as an academic exercise despite steady progress and commercialization of the technology. Furthermore, organic photovoltaics were somewhat overshadowed by the stunning progress made in another class of materials that can be processed into thin films easily, namely perovskites. However, during the past 2 years, the increase in power conversion efficiency has accelerated again, with several reports of single-junction cells with efficiencies above 15%2 and a report of an efficiency of 17.29% in a tandem cell3 as shown in Figure 1. This inflection point in the curve after 2016 can in part be attributed to the development of new acceptor molecules4 that are no longer derivatives of fullerenes. These new acceptor molecules can be designed to have superior light absorption properties, high charge mobility, and frontier orbitals with energies that yield high photovoltaic performance. These new heights in performance place organic solar cells in the spotlight again.

In this issue of Joule, Lami et al. describe a method that enables UV photoemission spectroscopy (UPS) in the transverse dimension of polymeric semiconductor layers with nanometer-scale resolution. The approach is based on the use of Argon gas cluster ion beam (GCIB) etching instead of monoatomic ion bean bombardment. The use of GCIB reduces surface damage, enabling in depth UPS. The method is applied to the study of critical electronic levels and photovoltage in organic solar cells.

Minimizing the impacts of climate change and reducing greenhouse gas emissions, while responding to an ever-increasing demand for energy, is the battle of our time. Fortunately, light from the sun delivers one billion watts (1 GW) of optical power per square kilometer—and at no cost. It can produce electricity through the photovoltaic effect with a power conversion efficiency near 20% for current commercial silicon solar modules. During operation, solar cells are the cleanest source of energy as they convert sunlight directly into electricity without releasing any byproducts. However, like with any technology, there are challenges that can impact the scaling and massive deployment of solar energy production. One primary challenge is the problem of storage, a common issue for all photovoltaic technologies, that can be addressed through new battery technology and other energy storage approaches. While silicon appears to be the superior material platform today, the fast deployment of future large-scale solar farms could be slowed down by the energy-intensive manufacturing of the raw high-purity silicon and fabrication of the solar cells. This leads to high energy payback times (EPBT), defined as the time it takes

(typically 1–1.5 years1) for a solar system to generate as much energy as is consumed during its production and lifetime operation. Hence, there is a need to explore and advance alternative photovoltaic technologies that do not just move the needle but have the potential to truly push the envelope. One such intriguing emerging technology is organic photovoltaics. The high absorption coefficient of conjugated molecules and polymers is the hallmark of organic photovoltaics. They absorb light very efficiently within a 200-nm-thick layer to be compared with the thickness of monocrystalline silicon wafers of 100–200 mm. This distinctive feature has important consequences: (1) low quantities of raw materials are needed since only 200 kg of organic active material are required to cover a square kilometer, and (2) reduced thickness means a short path for the carriers that are generated by the light in the absorbing layer in the transverse direction and low resistance for the resulting current that is flowing. The latter means that the requirements for the material in terms of electric charge mobility are quite relaxed and that a fairly high level of disorder can be tolerated in these materials. This

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The design guidelines for molecules and polymers rely heavily on the level of understanding of the photovoltaic effect in devices comprising these materials as light absorbing layers. In polymeric solar cells, this absorber generally consists of blends of donor-like

1Joseph

M. Pettit Professor of Electrical Engineering, Georgia Institute of Technology, Atlanta, GA, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.09.013

Figure 1. Evolution of the Power Conversion Efficiency of Solid-State Organic Photovoltaic Devices Adapted from NREL charts. 5 Circles represent certified efficiencies measured by NREL in single-junction cells. Triangle corresponds to an efficient measured in a tandem structure 3 and is certified by the Chinese CPVT organization (http://www.cpvt.org.cn). Credit for the graph: Felipe A. Larrain.

and an acceptor-like organic semiconductors. The resulting nanoscale morphology of this blend, referred to as a bulk heterojunction, is complex and critical to enabling efficient exciton dissociation—converting absorbed photons into mobile holes and electrons. It should also provide efficient pathways for these carriers toward the charge collecting layers that are adjacent to the absorber on its opposite sides, generating the highest possible photocurrent density. Highresolution structural characterization enabled by advanced X-ray light sources and sophisticated techniques such as grazing-incidence wide-angle X-ray scattering (GIWAXS) have played an important role in the last decade to inform the fabrication of devices with optimized layer morphology, yielding increased photocurrents. However, photovoltaic devices produce power; thus, optimizing the photocurrent or even fill factor is not sufficient. The other important photovoltaic performance metric is the photovoltage or the open-circuit voltage (VOC). In this issue of Joule, Professor Yana Vaynzof6 and co-workers demonstrate that UV photoemission spectroscopy

(UPS) can be implemented in organic solar cells by using depth profiling in order to gain information on the energy of the relevant electronic levels of these materials throughout the entire thickness of the device and establish what the authors call ‘‘energetic landscapes.’’ UPS is a well-known technique that is used to gain information about the electronic levels of materials.7 The new twist in the reported research is that the authors take UPS, a surfacesensitive technique, into the third dimension with nanometer-scale resolution by combining it with Argon gas cluster ion beam (GCIB) etching. In contrast to standard monoatomic ion beam bombardment, GCIB is known to reduce damage on the surface of the etched film. Since UPS measures surface-related properties, the quality of the surface of the film after each etching step is of paramount importance. With the improved sensitivity gained by GCIB etching, the authors tackle one of the most critical questions in organic photovoltaics: namely, what limits VOC. While the notion of a current density can be conceptualized in its simplest form by the classical picture of multiple charges (with charge measured in Coulombs) moving per unit time through a surface (A/cm2 with A = C/s), there is no single particle analogy to describe a photovoltage. It is a concept based on ensembles of particles that can be described by statistical thermodynamics. Spatially inhomogeneous distributions of holes and electrons are described in terms of their quasi-Fermi level energies or their electrochemical potentials. The increased complexity associated with the concept of photovoltage has sparked a somewhat less-consensual debate in the community about what design criteria to use to inform the design of molecules and polymers for optimized VOC. While no real consensus has emerged yet in the scientific community, it is generally admitted that the relative en-

ergy of the frontier orbitals of the materials in the bulk heterojunction plays an important role. Furthermore, as was suggested in 2008 by our group8 and later confirmed experimentally,9 the ground-state charge transfer (CT) complex formed by the electronic coupling at the donor and acceptor interface is significant in determining the value of VOC. The photon energy of this CT complex depends on the relative frontier orbitals of the donor- and acceptor-like materials that are blended in the bulk heterojunction—in particular, the energy difference of the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. However, since the CT complex is localized at the interface, it is critical to account for the effects of electronic coupling between the donor and acceptor that lead to modified HOMO and LUMO energies. Note that these values at the interface are not necessarily those measured for these molecules when they are non-interacting. Hence, it becomes clear that in order to inform the design of materials that yield optimized VOC, it is crucial to have advanced and high-resolution spectroscopies that can characterize the frontier orbitals in bulk heterojunction organic solar cells, as shown by Lami et al.6 How well this characterization tool will serve the scientific community to further advance the understanding and optimization of VOC in organic solar cells remains an open question. While being able to measure and predict the CT complex energy is useful, other techniques can yield that information, and other properties such as the mechanisms of charge recombination (band to band versus trap-assisted) are as important in informing the prediction of the value of VOC. These recombination mechanisms strongly depend on the morphology of the film. Even if GCIB can be considered a big leap forward in reducing surface damage, future studies will ultimately decide

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the full potential of this approach. For now, it is a welcome and promising new tool to help advance organic photovoltaics and other device platforms based on organic semiconductors. 1. Fraunhofer Institute (2015). Photovoltaics Report. https://www.ise.fraunhofer.de. 2. Yuan, J., Zhang, Y., Zhou, L., Zhang, G., Yip, H.-L., Lau, T.-K., Lu, X., Zhu, C., Peng, H., Johnson, P.A., et al. (2019). Singlejunction organic solar cell with over 15%

efficiency using fused-ring acceptor with electron-deficient core. Joule 3, 1140–1151. 3. Meng, L., Zhang, Y., Wan, X., Li, C., Zhang, X., Wang, Y., Ke, X., Xiao, Z., Ding, L., Xia, R., et al. (2018). Organic and solution-processed tandem solar cells with 17.3% efficiency. Science 361, 1094–1098. 4. Lin, Y., Wang, J., Zhang, Z.G., Bai, H., Li, Y., Zhu, D., and Zhan, X. (2015). An electron acceptor challenging fullerenes for efficient polymer solar cells. Adv. Mater. 27, 1170–1174. 5. NREL (2019). Best Research-Cell Efficiencies. NREL. https://www.nrel.gov/.

6. Lami, V., Weu, A., Zhang, J., Chen, Y., Fei, Z., Heeney, M., Friend, R.H., and Vaynzof, Y. (2019). Visualizing the vertical energetic landscape in organic photovoltaics. Joule 3, this issue, 2513–2534. 7. Kahn, A. (2016). Fermi level, work function and vacuum level. Mater. Horiz. 3, 7–10. 8. Postcavage, W.J., Yoo, S., and Kippelen, B. (2008). Origin of the open-circuit voltage in multilayer heterojunction organic solar cells. Appl. Phys. Lett. 93, 193308. 9. Vandewal, K., Tvingstedt, K., Gadisa, A., Ingana¨s, O., and Manca, J.V. (2009). On the origin of the open-circuit voltage of polymer-fullerene solar cells. Nat. Mater. 8, 904–909.

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Relaxor Ferroelectric Capacitors Embrace Polymorphic Nanodomains Xiaodong Jian,1 Xin Chen,1 and Q.M. Zhang1,* Among the energy storage devices, the dielectric capacitors possess the fastest charge/discharge speed and highest power density. However, their energy density is low. Recently in Science, Pan et al. demonstrated a relaxor ferroelectric ceramic thin film with polymorphic nanodomains that achieves an energy density of 112 J/cm3 with a high discharge/change efficiency. Dielectric capacitors store and regulate charges/electric energy and are widely used in electronic and electrical systems.1 In general, the polarization of the dielectric materials utilized in capacitors can be changed rapidly by external applied electric fields. In fact, among all the energy storage devices such as batteries, fuel cells, and supercapacitors, the dielectric capacitor is the one that can be charged and discharged rapidly, with a rate that can reach much less than microseconds.1 It is the fast discharge of the dielectric capacitor in releasing the stored electric energy that quickly turns on the bright flashlight of cameras and delivers electrical pulses to restore a normal heart rhythm in implantable

cardioverter-defibrillators (ICDs). The increased functionality and miniaturization of modern devices demands higher energy density and better charge/ discharge efficiency of dielectric capacitors than the state of the art. For example, a smaller dielectric capacitor that stores and delivers more electric energy efficiently can help shrink the size of smart phones and ICDs. Therefore, there is a great demand to increase the energy density of dielectric materials. Recently in Science, Pan et al. report the development of a dielectric ceramic thin film with a thinness of ca. 500 nm that possesses a high polarization level, approaching 1 C/m2 with a high dielec-

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tric breakdown strength (~5 MV/cm) and low leakage current (~10 6 A/cm2 under 1.5 MV/cm).2 As a result, the ceramic thin films deliver an energy density of 112 J/cm3, approaching that of supercapacitors, with 80% of discharge/charge efficiency. In general, the energy density Ue stored in a dielectric can be deduced from the polarization P change with external electric field E; e.g., Z Ue =

Pm

E dp;

(Equation 1)

Pr

where Pr and Pm are the initial (E = 0) and final (at Em) polarizations, respectively (see illustration in Figure 1A). Hence, in order to achieve a high Ue, a dielectric should possess a high Pm and high Em at which the dielectric reaches Pm. Among various dielectrics, the ferroelectric ceramics are the ones that possess the highest Pm.1,3 In their investigation, Pan et al. selected BiFeO3 (BFO) as a main component, which generates a large Pm, ~100 mC/cm2, which is among the highest in known leadfree ferroelectrics.3 However, in normal ferroelectrics, the polarization forms

1Department

of Electrical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA, 16802, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.09.008