Article pubs.acs.org/accounts
Organohalide Perovskites for Solar Energy Conversion Published as part of the Accounts of Chemical Research special issue “Lead Halide Perovskites for Solar Energy Conversion”. Qianqian Lin, Ardalan Armin, Paul L. Burn, and Paul Meredith* Centre for Organic Photonics & Electronics, School of Chemistry and Molecular Biosciences, and School of Mathematics and Physics, The University of Queensland, Brisbane, Queensland, Australia 4072
CONSPECTUS: Lead-based organohalide perovskites have recently emerged as arguably the most promising of all next generation thin film solar cell technologies. Power conversion efficiencies have reached 20% in less than 5 years, and their application to other optoelectronic device platforms such as photodetectors and light emitting diodes is being increasingly reported. Organohalide perovskites can be solution processed or evaporated at low temperatures to form simple thin film photojunctions, thus delivering the potential for the holy grail of high efficiency, low embedded energy, and low cost photovoltaics. The initial device-driven “perovskite fever” has more recently given way to efforts to better understand how these materials work in solar cells, and deeper elucidation of their structure−property relationships. In this Account, we focus on this element of organohalide perovskite chemistry and physics in particular examining critical electro-optical, morphological, and architectural phenomena. We first examine basic crystal and chemical structure, and how this impacts important solar-cell related properties such as the optical gap. We then turn to deeper electronic phenomena such as carrier mobilities, trap densities, and recombination dynamics, as well as examining ionic and dielectric properties and how these two types of physics impact each other. The issue of whether organohalide perovskites are predominantly nonexcitonic at room temperature is currently a matter of some debate, and we summarize the evidence for what appears to be the emerging field consensus: an exciton binding energy of order 10 meV. Having discussed the important basic chemistry and physics we turn to more device-related considerations including processing, morphology, architecture, thin film electro-optics and interfacial energetics. These phenomena directly impact solar cell performance parameters such as open circuit voltage, short circuit current density, internal and external quantum efficiency, fill factor, and ultimately the all-important power conversion efficiency. Finally, we address the key challenges pertinent to actually delivering a new and viable solar cell technology. These include long-term cell stability, scaling to the module level, and the toxicity associated with lead. Organohalide perovskites not only offer exciting possibilities for next generation optoelectronics and photovoltaics, but are an intriguing class of material crossing the boundaries of molecular solids and banded inorganic semiconductors. This is a potential area of rich new chemistry, materials science, and physics.
1. INTRODUCTION
with both yet to reach the efficiencies of the organohalide perovskites. The “business case” for the organohalide perovskites is compelling: they can be solution processed from simple earth abundant precursors, or evaporated using relatively low temperatures; they have optical gaps of order 1.6 eV, which are tunable with halide ratio; and they can form efficient thin single photojunctions using simple planar or mesoporous
Organohalide perovskites have recently and spectacularly emerged as the leading, next generation materials option for low cost thin film solar cells.1,2 Power conversion efficiencies exceeding 20% have been demonstrated in laboratory-scale devices.3 This progress has been achieved in less than 5 years since the first viable solar cells were reported by the Miyasaka and Snaith groups,4 an unprecedented rate of development. In comparison, similar technologies such as dye sensitized solar cells and organic solar cells matured over more than a decade, © 2016 American Chemical Society
Received: October 29, 2015 Published: February 10, 2016 545
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Figure 1. (a) Crystal structure of an organohalide lead perovskite (CH3NH3PbI3). (b) Images of organohalide perovskite single crystals. Reproduced with permission from ref 18. Copyright 2015 Nature Publishing Group. (c) Tunable optical gap of organohalide perovskites with different Br/I ratio as shown by the optical absorbance. Reproduced with permission from ref 20. Copyright 2014 Royal Society of Chemistry. (d) Real part of the dielectric constant for CH3NH3PbI3 as a function of frequency from static to the optical regime with associated dominant screening mechanism. (e) Carrier mobility (μ) and trap density (ntraps) measured with the space-charge limited current (SCLC) technique for a CH3NH3PbI3 single crystal. (f) Ionic behavior of an organohalide perovskite diode with electric field poling. Reproduced with permission from refs 16 and 34. Copyright 2015 Nature Publishing Group.
titania forming an electron accepting (n-type) scaffold and contact. They were initially considered in the same way as a dye sensitizer,12 and then as a p-type absorber.13 In both scenarios, the photoexcitation was considered as excitonic at room temperature with a binding energy several times kT.10 Only recently has it been shown that this is not the case; efficient “homojunction” solar cells can be made with no need for an exciton splitting heterojunction interface.8,14 Furthermore, several studies have now confirmed that the organohalide perovskites have a binding energy of 10 meV or less at room temperature.8,14 In addition, light emission has been observed,2 free carrier diffusion lengths of order 100s of micrometers have been measured,15 and trap densities of order 1010 cm−3 determined in high quality thin films and single crystals.16 This collection of properties is more akin to traditional crystalline inorganic semiconductors, not the disordered molecular solids of the a priori assumption. These facts have
scaffold-based architectures. These attributes could deliver the holy grail of efficient solar cells (up to 25%) produced via low cost, low embedded energy manufacturing on flexible substrates. On the down side, the most successful organohalide perovskites to date contain lead, toxic and problematic especially in Europe. In addition, these materials have mixed ionic-electronic character, which may be a problem,5 suffer forward−reverse voltage hysteresis if not appropriately processed,1,6 and achieving the optimum junction morphology is somewhat of an art form.7 Finally, organohalide perovskites are particularly susceptible to moisture and, as with organic solar cells, appropriate encapsulation is essential.8,9 These device-related considerations aside, there is another possibly more intriguing aspect to the organohalide perovskite story: the underlying solid state physics, chemistry, and photophysics.8,10,11 Historically, organohalide perovskite solar cells arose out of dye sensitized solar cells with mesoporous546
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Accounts of Chemical Research solid-state physicists and chemists excited as to the possibilities of this new solution processable silicon. In this Account, we focus on the key features and challenges of organohalide perovskites as solar energy harvesting materials. We try to isolate the facts (the aspects that are generally agreed) from the somewhat more speculative considerations of the field. We start with a basic summary of structural, physical, and optical properties, before moving to chemical and processing related issues which impact morphology. We then turn to device related considerations, architectures and working principles, before concluding with a brief summary of the manufacturing pertinent challenges of stability, scaling, and toxicity.
Accurate determination of the electron and hole mobilities in organohalide perovskites has proven challenging−many of the methods developed for organic semiconductors such as charge extraction by linearly increasing voltage (CELIV) are ineffective. More traditional semiconductor methodologies have been applied with some success; for example Stoumpos et al. measured the mobility of CH3NH3PbI3 via Hall Effect to be ∼66 cm2V−1s−1.22 In a related spectroscopic study, Stranks et al. reported the electron−hole diffusion length in thin films to be 1 μm.23 In a similar THz spectroscopy experiment, Wehrenfennig et al. found the mobilities of CH3NH3PbI3−xClx and CH 3 NH 3 PbI 3 to be 11.6 and 8.0 cm2 V −1 s −1 , respectively.24 These results also suggested that the bimolecular recombination rates were abnormally low and that photogenerated carriers possessed relatively long lifetimes and diffusion lengths.24 More recently measurements on organohalide single crystals have begun to emerge with similar outcomes, for example: Dong et al. reported free carrier diffusion lengths in CH3NH3PbI3 single crystals to be >175 μm with electron and hole mobilities larger than 20 and 100 cm2 V−1 s−1, respectively;15 and Saidaminov et al. estimated the mobility of CH3NH3PbI3 single crystals via space charge limited current (SCLC) to be ∼67 cm2 V−1 s−1 with the low trap density of ∼1010 cm−3 as shown in Figure 1e.16 Alongside the electronic properties detailed above, the dielectric relaxation is a key physical mechanism defining the charge generation physics of optoelectronic devices such as solar cells.8,25 The dielectric properties of organohalide perovskites have been much debated recently in an effort to understand the relative branching fractions of excitonic and nonexcitonic photoexcitations in solar cells at room temperature.1,8 It is recognized in semiconductors that both the real (ε′) and imaginary components (ε″) of the dielectric constant are dispersive as a function of frequency. This appears even more pronounced in the organohalide perovskites. Several recent measurements have shown that in CH3NH3PbI3, for example, ε′ is low (∼6.5) at optical frequencies where the polarization of valence band electrons is the relevant physical screening mechanism.26 This increases to ∼30 in the midfrequency regime where dipolar polarization dominates,27 and can be as high as 70 at frequencies 100 at very low and sub-Hertz frequencies associated with the migration of ions.28 These measurements match predictions from simulations,29 and a typical set of dielectric constants are shown in Figure 1d. The magnitude and dispersion in the dielectric parameters are important since ε′, in particular, has a strong influence on the exciton binding energy (EB). Conventionally, a large ε′ indicates nonexcitonic behavior because EB is small relative to kT (∼25 meV) and the electron−hole Coulomb interaction is effectively screened. This is the case in Si, GaAs, and CdTe8 and leads to efficient charge generation. In contrast, the dielectric constant of organic semiconductors is generally 200 meV), and the added driving force of a heterojunction is required to generate free carriers.30 The Wannier−Mott equation can be used to infer EB if ε′ and the electron−hole pair effective mass are known. Lin et al. used this approach by evaluating the effective mass from their measured ε′ (∼70) and previously reported values of the diamagnetic energy shift (c0).31 They estimated the room temperature binding energy of CH3NH3PbI3 to be of order 2 meV.8 Hirasawa et al. calculated
Fundamental Properties
The perovskite general crystal unit cell is defined by the formula ABX3, where B is a cation and X is an anion, forming an octahedron [BX6]4−. The octahedra are stabilized by a second cation A and a simple and archetypal example of this ABX 3 structure is CaTiO 3 . To determine whether a combination of three elements can form a stable perovskite crystal structure, the tolerance factor t and octahedral factor μ were introduced. Here, t is defined as the ratio of the separation of A and X to the separation of B and X, and μ is the ratio of the B and X ionic radii.1 In organohalide perovskites, the cation A is replaced by small organic molecules, e.g., methylammonium (CH3NH3+)4 or formamidinium (H2NCHNH2+)3. Figure 1a shows a typical lead organohalide perovskite (CH3NH3PbI3) crystal representation where the methylammonium cation sits in the vacancy of eight [PbI6]4− octahedra. The lead cation can be replaced by tin, resulting in the lead-free material CH3NH3SnI3,17 and the halide anion can be bromide or chloride.18 As we shall see next, such changes can be used to tune the optoelectronic properties. This general family of organohalide perovskites are stable to structural variance with a tolerance factor 0.81 < t < 1.11 and octahedral factor 0.44 < μ < 0.90.1 We note the term organohalide perovskite is used throughout the field to reflect the hybrid nature of these materials, and we retain this nomenclature. However, more generic terms such as perovskite or hybrid halide perovskites are also commonplace. Organohalide perovskites are direct gap materials with high absorption coefficients.1 The optical gap can be tuned over several hundred nanometers by changing the chemical composition, including the organic cation A,3 metallic cation B17 and the ratio of the constituent halides.18 Figure 1b shows how the color of organohalide perovskite crystals changes with the halide ratio, indicative of a shift in optical gap.18 The same phenomenon is observed in polycrystalline thin films of relevance to this review and operational solar cells.19 Figure 1c quantifies these changes in the optical gap for a family of formamidinium lead perovskites (H2NCHNH2PbI3) as a function of the Br−/I− ratio. It is worth noting that the gap can be extended to ∼830 nm by replacing the more traditional methylammonium (CH3NH3+) cation with formamidinium (H2NCHNH2+).20 Furthermore, in organohalide tin perovskites (for example, CH3NH3SnI3), the optical gap is even further red-shifted to ∼1000 nm.17 This tunability offers a convenient approach to optimizing the light absorption in solar cells. Furthermore, it also provides control over the open-circuit voltage (Voc) according to the Shockley−Queisser detailed balance limit.21 547
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Figure 2a: lead iodide is first spun-cast onto the substrates to form dense and uniform PbI2 layers; then a methylammonium
EB to be 38 meV based on the optical frequency value of ε′ = 6.5.31 D’Innocenzo et al. estimated the EB of a mixed organohalide perovskite (CH3NH3PbI3−xClx) to have an upper bound of 50 meV from low temperature optical absorption measurements.10 Clearly these various binding energy values deliver different excitonic/nonexcitonic branching fractions. The discrepancy arises from the choice of optical frequency or low frequency dielectric constant−not an issue in conventional inorganic semiconductors where the low and optical frequency values are often similar. Lin et al.8 argued the lower frequency dielectric constant to be relevant for the organohalide perovskites since the exciton radius is large compared to the lattice constant. As such, polarization of the lattice can effectively respond on a time scale relevant to the electron−hole movement.32 Irrespective of the detail of the Wannier−Mott physics, the debate has been somewhat resolved recently by Miyata et al.14 who made direct magneto-optical measurements on CH3NH3PbI3 and found the exciton binding energy to be ∼16 meV at low temperatures. From this, they inferred the room temperature value to be of order a few meV in agreement with Lin et al.8 The exact value of EB is likely to be dependent upon multiple factors such as purity, composition, and morphology.33 However, the community is now coalescing on the view that organohalide perovskites relevant to thin film solar cells have binding energies of order 10 meV at room temperature and are thus predominantly nonexcitonic. This broad statement is in agreement with multiple observations that internal quantum efficiencies approaching 100% can be obtained in simple, single component organohalide photojunctions.8 An intriguing feature of organohalide perovskite solar cells is an unusual hysteresis in the current−voltage response.6 This behavior appears to be dependent upon voltage-scan-rate and electrical history including “poling”. Although the underlying mechanism is still a little uncertain, ionic transport has been proposed to be a significant factor in this phenomenon.5 Eames et al. implicated ionic migration via defects/vacancy-mediated diffusion with an activation energy of ∼0.6 eV.5 Further, Xiao et al. reported a giant switchable photovoltaic effect (Figure 1f), and claimed it was due to electrical poling and ionic behavior.34 Furthermore, Deng et al. reported that light can induce the poling of organohalide perovskite based devices.35 Finally, Jaurez-Perez et al. observed a photoinduced giant dielectric constant which suggests the ionic behavior is enhanced under illumination.25 The evidence that organohalide perovskites are ionic-electronic materials is now compelling and this opens exciting possibilities in hybrid electrical devices. Several groups have now reported stable, hysteresis free high efficiency solar cells and so this ionic nature is not necessarily an impediment for photovoltaics. However, the exact mechanisms underlying this physics are not yet clear.
Figure 2. Organohalide perovskite thin film processing techniques: (a) Two step solution processing approach; (b) mixed halide precursor method; (c) nonsolvent treatment solution processing method; (d) physical vapor deposition coevaporation; and (e) methylammonium iodide vapor treatment.
iodide (MAI) solution is deposited on the PbI2 films to create CH3NH3PbI3.36 Subsequently, several modified one-step methods were developed and these are depicted in Figure 2b and c. As an example, in the Figure 2b process, chloride is introduced into the precursor to inhibit rapid crystallization thus resulting in a mixed halide perovskite CH3NH3PbI3−xClx.4 Figure 2c shows an alternative solvent treatment method in which a non solvent (a solvent in which the materials are poorly soluble) is “dripped” onto the wet film during the spin coating of the CH3NH3PbI3 precursor.37 This seeds crystal formation and inhibits large crystal growth.37 Thermal coevaporation is in principle a relatively straightforward route to the production of high quality organohalide perovskite films of various types on a range of surfaces (shown in Figure 2d).13 However, control of component evaporation rates is less than straightforward. Finally, a combined method using spin-coated PbI2 films treated with MAI vapor has also been reported, but is less widely used.38 From our experience, the nonsolvent treatment method is relatively reproducible for small area devices and the coevaporation is more promising for fabricating large area high quality films. Examples of the morphologies that are produced by the various processing methodologies are shown in Figure 3. As with all thin film optoelectronic devices, the correct morphology is crucial for obtaining high quality devices, and in some cases (for example, Figure 3a) the crystal size can be micrometers, orders of magnitude thicker than the junction. In Figure 3b, a film prepared by an improved two step method is shown−pinhole free with uniform crystals. Likewise Figure 3c presents a nonsolvent method film with uniform small crystals of order ∼50−200 nm which can be further increased via annealing39 (Figure 3d).
2. FILM PROCESSING Organohalide perovskite thin films can be fabricated by various relatively simple processing techniques, including spin-coating, spray-coating, low temperature evaporation, and combinations thereof.2 However, it is not trivial with any of these techniques to produce defect free films of the correct stoichiometry and crystal size.7 For example, during solution processing the precursor compounds tend to spontaneously form large crystallites as the solvents evaporate resulting in pin holes and shorting defects. Initially, the most successful processing routes involved two steps to control film quality as shown in 548
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Figure 3. Morphology control of organohalide perovskite thin films. Scanning electron microscopy (SEM) images of CH3NH3PbI3 films obtained from (a) one step spin-coating from DMF solution; (b) the two-steps solution processing method; (c) toluene treatment one step spin-coating method; (d) thermally annealed film of (c) heated at 110 °C for 40 min; and (e) cross-sectional SEM image of an efficient organohalide perovskite solar cell.
∼10% with dramatically improved stabilities were achieved.4,12 The typical mesoporous scaffold structure of these devices is shown in Figure 4a. PCEs have been improved to >20% in the last 3 years.3 (iii) Planar architecture organohalide perovskite solar cells (Figure 4b) based on thin inorganic blocking/transport layers, for example, TiO239 and ZnO41 have been developed in parallel starting with PCEs of ∼15%13 but now reaching ∼19%.39 These cells do not contain a mesoporous scaffold within the junction. (iv) Finally, organic semiconductors such as n-type fullerenes and p-type polymers have been introduced as anode and cathode interlayers. This type of metal-oxide free structure with an organohalide perovskite single layer sandwiched between two electrodes is arguably the simplest of all geometries (Figure 4c). The electrode interlayers can be made extremely thin (1.05 V, confirming the trend. There is still considerable scope for improving the Voc by optimizing the energetics of the contacts and transport layers. As previously mentioned, the optical gap of organohalide perovskites can be tuned by replacing either the organic cations, halide anions, or metal cations. By integrating the standard
AM1.5 illumination spectrum, the maximum short circuit current density (Jsc) can be determined for the perfect case of 100% external quantum efficiency (EQE). In reality, this value must be discounted for reflection and parasitic absorption losses, or imperfect extraction.46 Figure 6b shows such a calculation for CH3NH3PbI3, which is capable of delivering a Jsc ∼25 mA/cm2 for an optical gap of ∼760 nm (∼1.6 eV), and CH3NH3SnI3 with a theoretical Jsc limit of ∼35 mA/cm2 (∼950 nm, ∼1.3 eV). Umari et al. tested this relationship and reported similar results from both a computed and experimentally measured optical gap.47 Formamidinium (FA)-based organohalide perovskites have a narrower gap ∼1.48 eV3 and this leads to a higher Jsc limit of ∼30 mA/cm2. Finally, we turn to quantum efficiency and FF. Organohalide perovskite solar cells, much like organic solar cells, are low finesse optical cavities. The distribution of optical field in this cavity is defined by the optical constants and thicknesses of the constituent layers, and can be simulated using standard transfer matrix analysis. Lin et al.8 recently showed that simple planar CH3NH3PbI3-based devices with junctions ∼100−500 nm thick have two spectral regions in the EQE (Figure 6c): for λ < 500 nm, there is minimal influence of the junction thickness (Beer−Lambert region); but for λ > 500 nm, the EQE shape is strongly dependent on the junction thickness (cavity interference region).8 In addition, Lin et al.8 experimentally determined the IQE of optimized devices to be ∼100%, confirming low recombination losses. This has been further validated by observations of a large linear dynamic range in organohalide perovskite photodiodes and solar cells. (shown in Figure 6d).8 This suggests minimal bimolecular recombination losses48 and is one of the primary reasons for the high FFs approaching 80%.
4. CONCLUSION In concluding, we now comment on the challenges facing organohalide perovskite solar cells. From the above commentary, it is clear that the basic working principles and the 551
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Accounts of Chemical Research structure−property relationships defining performance are becoming clearer, with more recent focus on the basic physics and chemistry. The ultimate goal of introducing any new solar cell technology is to progress it to manufacturing. Several key issues need to be addressed in this regard: (i) Long-term stability: organohalide perovskites are hygroscopic and ionic. They are susceptible to moisture ingress which can for example, change the stoichiometry or introduce surface/interfacial traps.49 It is thus clear that effective encapsulation will be an important part of delivering a stable, viable solar module. (ii) Large area cells and modules: defect density scales exponentially with area in thin film solar cells and controlling crystal size, uniformity, and defects such as pinholes are a significant challenge. The transparent conducting electrode has also been recognized as a limit on the maximum collection pathway and thus cell area. (iii) Toxicity and lead-free solar cells: the toxicity of lead cannot be ignored. There is a strong drive to replace lead with less toxic materials. The first lead-free organohalide perovskite solar cells have been reported but have lower efficiencies, and tin-based systems in particular suffer from the chemical instability of Sn2+.17 There is still much to learn about these intriguing materials, and their potential will broaden into other optoelectronic applications if solar cell development progresses toward deployment.
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sustainable advanced materials with a focus on the physics of disordered semiconductors and conductors.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Qianqian Lin is a PhD candidate at The University of Queensland, Centre for Organic Photonics & Electronics (COPE). His research interests include perovskite/organic optoelectronic devices, charge transport measurement, and numerical simulations. Ardalan Armin obtained his PhD in 2015 from the University of Queensland in the physics of disordered semiconductors. He is currently a Postdoctoral Research Associate at the University of Queensland (COPE) as part of the Australian Centre for Advanced Photovoltaics. Paul L. Burn is a Fellow of the Australian Academy of Science and Royal Society of Chemistry. He is a Professor of Chemistry (COPE) at The University of Queensland, Australia. He received his PhD from the University of Sydney before moving to Cambridge University where he was a Dow Research Fellow at Christ’s College. In 1992 he moved to the University of Oxford and then in 2007 to The University of Queensland as an Australian Research Council Federation Fellow. His research focuses on the development of organic optoelectronic materials and their application. Paul Meredith is a Professor of Materials Physics (COPE) at the University of Queensland, Australia and an Australian Research Council Discovery Outstanding Researcher Award Fellow. He received his PhD in Optoelectronics from Heriot-Watt University, Edinburgh in 1993, and after a postdoctoral position at Cambridge University and period of industrial research with Proctor and Gamble, joined the University of Queensland in 2001. His research interests span 552
DOI: 10.1021/acs.accounts.5b00483 Acc. Chem. Res. 2016, 49, 545−553
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DOI: 10.1021/acs.accounts.5b00483 Acc. Chem. Res. 2016, 49, 545−553
