The Rise of Highly Efficient and Stable Perovskite Solar Cells

Mar 21, 2017 - Special Issue. Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”...
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The Rise of Highly Efficient and Stable Perovskite Solar Cells Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Michael Graẗ zel* Laboratory of Photonics and Interfaces, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland ABSTRACT: Recently, metal halide perovskite solar cells (PSCs) of the general formular ABX3 where A is a monovalent cation, that is, methylammonium (MA) CH3NH3+•, formamidinium CH2(NH2)2+, Cs+, or Rb+, B stands for Pb(II) or Sn(II), and X for iodide or bromide have achieved solar to electric power conversion efficiencies (PCEs) above 22%, exceeding the efficiency of the present market leader polycrystalline silicon while using 1000 times less light harvesting material and simple solution processing for their fabrication. The top performing devices all employ formulations containing a mixture of up to four A cations and iodide as well as a small fraction of bromide as anion, whose emergence will be described in this Commentary. Apart from leading the current PV efficiency race, these new perovskite materials exhibit intense electroluminescence and an extraordinarily high stability under heat and light stress.

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Originating from liquid electrolyte-based dye sensitized solar cells2,3 solid-state versions have subsequently been introduced4−6 triggering the steep rise of PSCs to become a leader of next generation photovoltaics, an unprecedented phenomenon in the history of the field. Today’s state of the art devices all use multi-A-cation perovskite formulations introduced first in year 2014.7 The multi-A-cation approach has created a new generation of perovskite photovoltaics showing exceptional stability under full solar light exposure at 85 °C and very high open circuit voltages close to the thermodynamic limit, associated with intense electroluminescence, paving the way toward photovoltaic applications on the industrial scale.

ue to their outstanding optoelectronic properties, metal halide perovskites of the generic structure ABX3 where A is a monovalent cation, that is, methylammonium (MA) CH3NH3+•, formamidinium (FA) CH2(NH2)2+, Cs+, or Rb+, while B stands for Pb(II) or Sn(II) and X for iodide as well as bromide have recently become one of the most intensively investigated chemical compounds.1 Within a few years of their inception, perovskite solar cells (PSCs) have achieved solar to electric power conversion efficiencies (PCEs) above 22%, exceeding the efficiency of the present PV market leader polycrystalline silicon, while using 1000 times less light harvesting material and simple low cost solution processing for their fabrication without the need for expensive high temperature or vacuum steps. Covering the thousands of publications that have appeared over the last 3−5 years in this field would go beyond the present Commentary, which focuses on mixed cation/mixed anion containing perovskite formulations that have taken the lead in the race for realizing highly efficient and stable PSCs that are suited for large-scale practical deployment. The crystal structure of the cubic perovskite phase is shown in Figure 1 along with its electronic orbital structure and a cross sectional SEM picture of a typical state of the art working device. A transparent conducting oxide (FTO) deposited on glass covered by a compact TiO2 blocking layer forms the electron-selective contact. This supports the mesoporous scaffold of TiO2 (mp-TiO2) infiltrated by the perovskite light harvester. The latter protrudes from the pores forming a compact capping layer on top of the mesoporous scaffold. This is in turn covered by layer of the organic hole conductor spiroMeOTAD and a thin gold film as back contact. © 2017 American Chemical Society

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THE ADVENT OF MIXED-ORGANIC CATION METAL HALIDE PEROVSKITES The first few years of intense PSC research employed exclusively MAPbI3 as light harvester with MA being used alone as monovalent A cation. However, it became evident that FAPbI3 was a more attractive choice than MAPbI3 due to its smaller band gap, enabling higher solar photocurrents to be generated without sacrificing photovoltage. Moreover, FAPbI3 is more resistant to heat stress than MAPbI38 although both show degradation in ambient air at a relative humidity approaching 100% and their photovoltaic power output may be diminuished by ingression of other device components, such as chemical ingredients of the hole conductor and Au back contact.9 A major caveat of FAPbI3 is that its perovskite phase is only stable at temperatures above 150 °C reverting to a Received: September 30, 2016 Published: March 21, 2017 487

DOI: 10.1021/acs.accounts.6b00492 Acc. Chem. Res. 2017, 50, 487−491

Commentary

Accounts of Chemical Research

inserted in the same lattice frame. The fact that the PL maximum of the films shifted gradually to the red upon increasing the FA content confirmed the contention that a solid solution of MA and FA in the perovskite was formed, the ratio of inserted cations being similar to that used in the precursor solution. Choi et al.10 extended the realm of investigations to perovskites containing a mixture of cesium and methylammonium. Similarly to FAPbI3, CsPbI3 also forms a stable perovskite phase only at high temperatures, which reverts to a photoinactive δ phase under ambient conditions. Their study showed that a stable mixed MA/Cs perovskite phase was formed at room temperature. However, the material showed poor stability and the PCE achieved with an inverted planar cell architecture was only about 7%, nearly a factor of 2 below that of MA/FA lead iodide. Remarkably, the (FA)x(MA)1−xPbI3 formulations exhibit superior PV performance to the single A cation perovskites. Figure 2 compares the absorbed photon to electric current

Figure 1. (a) Crystal structure of cubic metal halide perovskites with the generic chemical formula ABX3. The organic or inorganic monovalent cations methylammonium (MA) CH3NH3+, formamidinium (FA) CH2(NH2)2+, Cs+, or Rb+ occupy the A position (green), whereas metal cations and halides are located in the B (gray) and X (purple) positions. (b) Molecular orbitals for MAPbI3. Note that the valence band maximum (VBM) is formed by antibonding orbitals originating from Pb(6s) and I(5p) atomic states, while antibonding Pb(6p) and I(5s) orbitals form the conduction band minimum (CBM). The antibonding nature of the valence band orbitals is the reason for the low concentration of defects in the bandgap acting as charge carrier recombination centers. (c) Cross sectional scanning electron microscopy picture of a typical PSC based on FAxMA1−xPbI3 as light harvesting material. The electron selective contact is formed by the transparent conducting oxide (FTO) deposited on glass covered by a compact TiO2 blocking layer. This serves as a support for the mesoporous scaffold of TiO2 (mp-TiO2) infiltrated by the perovskite light harvester The latter protrudes from the pores forming a compact capping layer on top of the mesoporous scaffold. It is covered by a layer of the organic hole conductor, spiro-MeOTAD, and the thin gold film acting as back contact. The power conversion efficiency of the device is over 21%.

Figure 2. Absorbed photon to electric current conversion efficiency (APCE) for single cation (FAPbI3, MAPbI3) and mixed cation PSCs as a function of the wavelength of incident light. Note the 20 nm redshifted APCE onset from 790 to 810 nm for the mixed-cation perovskite as well as the pure formamidinium lead iodide compared to 790 nm for MAPbI3 indicating a band gap narrowing. Note also the steaper rise of the APCE for the mixed cation pervoskite compared to the single cation formulations. This is keeping with the theoretically predicted increase in the density of occupied levels at the top of the valence band upon mixing the MA and FA cations23 enhancing the cross section for the optical transition near the band gap. This figure is reprinted with permission from reference 7.

conversion efficiency (APCE), or internal quantum efficiency (IQE), of PSCs using the pure phase of FAPbI3 and MAPbI3 or the mixed cation phase (FA)x(MA)1−xPbI3 as a function of the wavelength of incident light. The 20 nm red-shifted APCE onset for (FA)x(MA)1−xPbI3 with respect to MAPbI3 confirms the desired band gap narrowing upon introduction of the FA cations, improving the solar light harvesting. In addition, the APCE spectrum for (FA)x(MA)1−xPbI3 rises more steeply at wavelengths near the absorption threshold compared to that of MAPbI3, indicating that the cross section for the optical transition near the band gap is enhanced by mixing FA and MA cations improving the solar light harvesting by the perovskite. Also, the collection of charge carriers generated by red photons, which penetrate deeper into the perovskite films than light of shorter wavelength is more efficient for (FA)x(MA)1−xPbI3 compared to MAPbI3. This is in keeping with the longer charge carrier lifetimes observed for the former compared to the latter material.7 The seminal results of this first study revealed the large potential for gains in PV performance from using mixed

photoinactive yellow (δ) phase under normal solar operating conditions. A crucial experiment performed by Pellet et al. in 20147 showed that replacing only 20% of the F-cations in FAPbI3 by MA stabilized the black perovskite phase of FAPbI3 down to below room temperature. Detailed analysis of the perovskite films by powder X-ray diffraction, electrical conductivity, absorption and emission spectroscopy, and time-resolved photoluminescence (PL) showed that a mixed 3-dimensional (3D) phase of perovskite with the composition (FA)x(MA)1−xPbI3 was formed, where the two cations are both 488

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Commentary

Accounts of Chemical Research FA/MA-cation perovskites, which was borne out by the ensuing development of high efficiency PSCs.11−13 These devices all employed FA/MA perovskite formulations in conjunction with mixed anions, their chemical composition being (MAPbBr3)x(FAPbI3)1−x, and reached record PCEs exceeding 22%.14 However, the mixtures remained prone to decomposition upon exposure to heat and humidity, due to the presence of MAI, which is easily removed from the perovskite by moisture and heat forming PbI2. Hence it was very important to develop perovskite structures that are stable in full sunlight over both low and high temperature ranges and withstand degradation by water.

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THE MAGICAL MIX: HEAT- AND MOISTURE-RESISTANT AND LIGHT-FAST CESIUM−FORMAMIDINIUM CATION BASED PEROVSKITES Purely inorganic perovskites of the general formula CsPbX3, such as CsPbI3 and CsPbBr3, have been applied as luminescent materials and in high-energy radiation detectors.15−20 The CsPbBr3 based devices exhibit excellent thermal stability and generate high photovoltages.21 However, they produce low photocurrents in sunlight due to their wide band gap (2.36 eV) resulting in a modest overall power conversion efficiency, which is given by the equation PCE = VocJsc FF

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where Voc is the open circuit voltage, Jsc is the short circuit photocurrent, and FF is the fill factor. These device performance metrics are determined by recording a J−V curve under standard reporting conditions, that is, air mass (AM) 1.5 sunlight with an intensity of 1000 W/m2 and at a temperature of 25 °C. While the cubic perovskite phase of CsPbI3 has a narrower band gap of 1.73 eV, it is only stable above 308 °C, reverting to the yellow (δ) orthorhombic phase upon cooling22 via a firstorder, reversible phase transition.21 This renders CsPbI3 unsuitable for photovoltaic applications, hence the great importance of the finding24−26 that mixing the two yellow delta modifications of CsPbI3 and FAPbI3 produces a black perovskite phase containing both FA and Cs cations of the composition FAxCs1−xPbI3 that is stable over a large temperature range and exhibits excellent photovoltaic performance as well as moisture stability. First principle calculations25 show that the remarkable stabilization of the perovskite phase by mixing the Cs and FA cations stems from entropic gains and the small internal energy input required for the formation of their solid solution. In contrast, the energy of formation of the δ phase containing mixed cations is too large to be compensated by this configurational entropy increase. Thus, for the δ phase, cation mixing increases the free energy preventing the formation of mixed crystals. Figure 3 summarizes the results of our quantitative energetic analysis25 along with a graphical presentation of the structural features for the phases involved. Replacement of FA+ in the δ phase of FAPbI3 by Cs+ phase leads to a significant destabilization with respect to the two pure FA or CsPbI3 δ phases, that is not compensated by the mixing entropy, TΔSδ‑CsxFA1−xPbI3 = kBT[x log x + (1 − x) log(1 − x)]. By contrast, forming the α or β type perovskite phases is thermodynamically favorable as the sum of the enthalpy and mixing entropy contributions to the free energy is negative,

Figure 3. Structural features of crystalline FAPbI3 and CsPbI3 in the α, β, and δ phase. Internal energy and entropy contributions to the thermodynamic driving force, ΔF, of forming mixed crystals in the α, β, and δ phases. Panels a and b represent the structure of the δ phase for pure FA and Cs perovskites, respectively, while panels c and d refer to α and β FAPbI3, respectively. Panel e shows the structure for the cubic phase of the CsPbI3. Panel f shows the variation of internal energy, ΔE (dashed blue line), mixing entropy contribution, −TΔS (dot-red dashed line), and free energy, ΔF = ΔE − TΔS (black continuous line) as a function of Cs content, x. This figure is reprinted with permission from reference 25.

resulting in a stabilization of the mixed phases. The stabilization energy is of the order of 0.05 eV (∼2kBT at room temperature) and 0.02 eV (∼0.8kBT) per stoichiometric unit for the α and β phase, respectively. Within this simple model, the δ → α or β transition temperature is reduced by ∼200−300 K when going from the pure FAPbI3 to the mixed system, which explains why the perovskite phase is stable at room temperature for the mixed compound. The increase of compound stability upon mixing may also contribute to the observed resistance toward decomposition at higher temperatures of these mixed cation perovskites. We investigated how the A-cation mixing affects the electronic and photovoltaic properties of perovskites.25 The absorption onset of β-Cs0.25(FA)0.75PbI3 is blue-shifted by ∼15 nm due to the distortion of the PbI3 lattice upon mixing. The average Pb−I−Pb angle passes from 167° in the pure β FAPbI3 to 164.5° at ∼25% Cs content. The distortion of the lattice reduces the 5p-I/6s-Pb antibonding overlap of the valence band 489

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Accounts of Chemical Research

Figure 4. Record efficiency, open-circuit voltage, electroluminescence, and high temperature stability. (A) Current-density/voltage (JV) curve, taken at 10 mV s−1 scan rate, of the best performing solar cell at 21.8% efficiency (Voc = 1180 mV, Jsc = 22.8 mA cm−2, and a fill factor of 81%). The forward and reverse scan is shown in Table S2 of ref 15). The inset shows the scan rate independent maximum power point (MPP) tracking for 60 s resulting in a stabilized efficiency of 21.6% at 977 mV and 22.1 mA cm−2 (displayed as the red triangles in the JV and MPP scan, respectively). (B) JV curve of the highest open-circuit voltage device. The inset shows the Voc over 120 s resulting in 1240 mV (displayed as the red triangles in the JV and Voc scan, respectively). (C) EQE electroluminescence (EL) as a function of voltage. The left inset shows the corresponding EL spectrum over wavelength. The image in the inset is a solar cell with 2 active areas. The left area is operated as an LED displaying a clearly visible red emission even under ambient light. At the same time, the right area can be operated as a solar cell or a photodetector underlining the versatility of the perovskite material. (D) Thermal stability test of a perovskite solar cell. The device is aged for 500 h at 85 °C under continuous full sun illumination and maximum power point tracking in a nitrogen atmosphere (red curve, circles). This aging routine exceeds industry norms. During the full sunlight soaking at 85 °C, the device retains 95% (dashed line) of its initial performance. This figure is reprinted with permission from reference 28.

and maximum power point tracking. Figure 4 summarizes these impressive experimental findings that pave the way toward an industrially deployable, new generation of perovskite photovoltaics.

maximum (VBM, Figure 1b), resulting in a reduction of its energy and an increase of the bandgap. Replacement of iodide by bromide in mixed Cs/FA perovskites results in a more pronounced blue shift of the absorption edge. This increase of the bandgap is due again to a decrease of the antibonding overlap between the X-np (X = Br or I) and Pb-6s orbitals with respect to the pure iodide perovskite as the smaller bromide ions distort the PbI3−yBry framework. Remarkably these mixed cation/mixed anion perovskites are stable under photoillumination and do not show the undesirable light induced segregation into the pure phases that has been observed for MAPb(I,Br) based mixed anions, preventing their application in tandem solar cells. The mixed cation perovskites show good stability when exposed to ambient air with no discernible decrease in efficiency of unsealed solar cells following a time lapse of 1000 h under ambient conditions in the dark. However, with a PCE below 18%, their solar conversion efficiency remained significantly below the performance achieved with the FA/MA mixed organic cation formulations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: michael.graetzel@epfl.ch. ORCID

Michael Graẗ zel: 0000-0002-0068-0195 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS I thank my co-workers as well as all colleagues that have contributed to the work described here for their precious help. Special thanks are due to Simone Meloni for providing the orbital diagram shown in Figure 1b. I am also grateful for all private and public institutions that have enabled this work by financially supporting our research.

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GROUND-BREAKING ADVANCE TO TRIPLE AND QUADRUPLE MIXED CATION PEROVSKITES Blending the highly efficient FA/MA mixed cation formulations with Cs and Rb27,28 enabled a breakthrough for realizing PSCs that exhibit both very high efficiency and unprecedented stability. We showed recently that the small Rb+ can be embedded into a “cation cascade” to create perovskite materials with excellent material properties. We achieved stabilized efficiencies of up to 21.6% as well as an external quantum efficiency of electroluminescence of 3.8%. The open-circuit voltage of 1.24 V at a band gap of 1.63 eV leads to loss-inpotential of only 0.39 V versus 0.4 V for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85 °C for 500 h under full solar illumination

REFERENCES

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DOI: 10.1021/acs.accounts.6b00492 Acc. Chem. Res. 2017, 50, 487−491