The Migration of Constituent Protons in Hybrid Organic-Inorganic

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The Migration of Constituent Protons in Hybrid Organic-Inorganic Perovskite Triggers Intrinsic Doping Carlos Cardenas-Daw, Thomas Simon, Jacek K. Stolarczyk, and Jochen Feldmann J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09319 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Journal of the American Chemical Society

The Migration of Constituent Protons in Hybrid Organic-Inorganic Perovskite Triggers Intrinsic Doping Carlos Cardenas-Daw*, Thomas Simon, Jacek K. Stolarczyk, Jochen Feldmann Photonics and Optoelectronics Group, Department of Physics and Center for Nanoscience (CeNS), Ludwig-MaximiliansUniversität München, Amalienstraße 54, 80799 Munich, Germany. Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany.

Supporting Information Placeholder ABSTRACT: The crucial separation of photocarriers in solar cells can be efficiently driven by contacting semiconductor phases with differing doping levels. Here we show that intrinsic doping surges in methylammonium lead iodide (MAPbI3) crystals as a response to environmental basicity. MAPbI3 crystals were passivated with polybases to induce the deprotonation of its methylammonium ions (MA+). Stable crystals showed marked increases in photoluminescence and radiative decay, attributed to the presence of unbalanced charges acting as doped carriers. This emulates in a controlled manner the proton-withdrawing conditions of polycrystalline films, where excess basic precursors are found between grains. Raman spectroscopy showed the collective alignment of MA+ cations within the intrinsically-doped lattices, thus revealing the build-up of electric fields. On this basis, we propose a mechanism for the formation of doping-gradients towards grain boundaries, potentially explaining the extended photocarrier lifetimes and diffusion lengths observed in perovskite solar cells.

Hybrid organic-inorganic perovskites are extraordinary photovoltaic materials to construct solar cells. Their solution-based processing stands as a sustainable alternative to the energyintensive fabrication of inorganic semiconductor devices which require superior crystallinity.1-4 Remarkably, photocarrier extraction proceeds efficiently in MAPbI3 photovoltaic devices despite relatively high defect concentrations and moderate carrier mobilities.5 Solar cell functionality derives from effective photo-generated charge separation and collection prior to recombination, for which asymmetric electric potentials are crucial. These are built up by contacting materials with different work functions, like phases of a semiconductor with different doped-carrier levels (p-n junctions). In the absence of dopant additives, intrinsic doping can emerge from stoichiometric imbalances, with cation/anion vacancies giving rise to n-type/p-type doping, respectively.6 MAPbI3 shows a distinctive giant dielectric response which is largely attributed to ion migration.7-10 The displacement of its constituent ions has been theoretically investigated,11-14 and experimentally corroborated for MA+ and I- under the driving influence of external electric fields.8,15 Perovskite structures -and specifically MA+ deficient MAPbI3 lattices- have been theoretically16 and experimentally17 identified as capable of holding considerable levels of charge-unbalanced vacancies.

High mobilities have been calculated for guest protons through interstitial position in MAPbI3 lattices.18 Indeed, protons are the smallest ions in nature and as such, optimal candidates for ionic conduction. Intrinsically, the only possible source of protons in MAPbI3 is the MA+ cation, which is formed upon the reversible protonation of neutral methylamine (MA) molecules (CH3NH2 + H+ ⇌ CH3NH3+). Nevertheless, self-dissociation of MA+ within the crystal lattice as a source of free protons is energetically unfavorable.19 Proton transfers in the solid state require the presence of free proton bases of sufficient strength, with a proper supramolecular arrangement to facilitate ion transfer.20 Photoluminescence measurements of MAPbI3 films have shown that at high photocarrier densities (>1016 cm-3), the recombination of band-edge electrons and holes proceeds with bimolecular (second-order) dynamics, which is indicative of free charges.21-23 Nevertheless, as the photocarrier density is decreased towards levels characteristic of regular solar cell operation, recombination switches to monomolecular (first-order) dynamics. As the PL wavelength remains unchanged throughout this transition, the effect has been attributed to the recombination of minority photo-carriers with outnumbering unintentionally-doped carriers.21 This interpretation is consistent with Hall-effect measurements24 and Mott-Schottky analysis25 showing intrinsic doping at comparable concentrations. Intrinsic doping content can be controlled by changing precursor raitos,24 and modified during solar cell operation due to photochemical and electrochemical processes in which H+ is reduced or I- oxidized, leaving behind chargeunbalanced vacancies.17 Here we demonstrate that intrinsic n-type doping markedly increases in MAPbI3 crystals surrounded by bases of sufficient strength to deprotonate MA+. We attribute this effect to protonmigration processes initiated at crystal boundaries, which result in partially deprotonated lattices with unbalanced negative charges. The treatment of MAPbI3 with amines has been reported to deactivate photoelectron surface traps; an effect attributed to the formation of N→Pb2+ dative bonds.26 Here we consider also the function of amines as proton acceptors. We passivated MAPbI3 crystals with polybases consisting of amine repeating units. Due to conformational constrains, only a fraction of those moieties bond to the crystal surface while the rest remains near the surface and free for proton uptake. In effect, passivation with polybases can be understood as wrapping the crystals with supramolecular proton sponges. Sub-micron MAPbI3 crystal suspensions were treated with one of the polybases: polyvinylpyridine (PVP), linear polyethyl-

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enimine (LPEI), or branched polyethylenimine (BPEI). Excess reactants were removed by centrifugation and rinsing. Stable passivated crystals were drop-cast and protected with PMMA films for further analysis. As explained in Fig. 1, this combination of poly-bases provides a basis to differentiate the effect of crystal deprotonation from that of surface-trap deactivation. While all of these polybases readily form N→Pb2+ bonds,27-28 they differ in proton basicity. PVP consists of aromatic amines which are considerably weaker proton bases compared to neutral MA.29 In contrast, LPEI and BPEI consist of aliphatic amines which are comparable in basicity to neutral MA, itself an aliphatic amine. Thus, partial deprotonation is primarily expected with LPEI and BPEIpassivation.

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bimolecular recombination of photocarriers and doped negative carriers (see supporting information; results in Table 1).

Figure 2. (a) Photoluminescence spectra of MAPbI3 perovskite crystals, pristine and polybase-passivated, normalized to quantum yields. Time-resolved photoluminescence of MAPbI3 pristine crystals compared to samples passivated with (b) polyvinylpyridine (PVP), (c) linear polyethylenimine (LPEI), and (d) branched polyethylenimine (BPEI). The solid lines correspond to modeled curves.

Table 1. Fitted values for radiative recombination(kr) and trapping(kT) rates; and for trap(T) and doped carrier(D) concentrations. Figure 1. Passivation of MAPbI3 crystals with polybases. Steady-state PL spectra and quantum yields (PLQY) (λex=480nm) of pristine and polybase passivated MAPbI3 crystals are shown in Fig. 2a. The low PLQY value for pristine MAPbI3 is consistent with reports on freshly-prepared films.30 The generalized increase in PLQY upon polybase passivation is in line with the expected deactivation of surface traps. Nevertheless, LPEI and BPEI -which are strong enough to deprotonate MA+ ions- trigger markedly stronger increases compared to PVP, which is weaker as to deprotonate MA+ ions. Time resolved PL data provided further insight. Decay curves shown in Fig. 2b,c,d correspond to initial photocarrier densities in the order of ∼6·1015 cm-3 (λex=480nm, 36nJ·cm-2·pulse-1) which emulate regular solar cell operation conditions.31 PVP-passivated crystals show slower decay along the entire time range compared to pristine MAPbI3. This implies that the observed increase in PLQY results from a decrease in non-radiative decay rate. Interestingly, despite showing higher PLQY, LPEI and BPEIpassivated crystals show markedly faster initial decays compared to pristine MAPbI3; at least to the point in which over 95% of radiative decay has already taken place. This divergent effect implies that the observed enhancement in PLQY cannot be solely explained by the deactivation of surface traps. Accordingly, in crystals passivated with polybases which are strong enough to deprotonate MA+ ions, the enhancement of PLQY also results from an increase in the radiative decay of electron-hole pairs. As partially deprotonated crystals imply unbalanced negative charges within the lattice, we consider the surge of n-type doped carriers. We modeled decay curves as a combination of trapping and

MAPbI3

kr/cm3⋅s-1

kT/cm3⋅s-1

T/cm-3

D/cm3

2.0⋅10-11

2.9⋅10-8

6.8⋅1015

1.0⋅1016

-11

-8

15

1.1⋅1016

PVP-MAPbI3

2.0⋅10

3.7⋅10

6.5⋅10

LPEI- MAPbI3

2.0⋅10-11

7.4⋅10-8

6.3⋅1015

1.1⋅1017

BPEI- MAPbI3

2.0⋅10-11

7.6⋅10-8

5.8⋅1015

2.8⋅1017

The passivation of MAPbI3 crystals with PVP, which is a weaker base relative to MA, triggers only a minor increase in doped carrier concentration. In contrast, the passivation of MAPbI3 crystals with either LPEI or BPEI, which are strong enough to deprotonate MA+, triggers marked doped-carrier concentration increases. The largest increase is observed with BPEI, which provides a higher density of basic units per passivated surface area (figure 1). A correlation is identified between prospective crystal deprotonation and doped-carrier concentration. This can be explained by the surge of unbalanced electrons in the crystal after MA+ deprotonation. The overall increase in radiative recombination indicates that these unbalanced electrons are energetic enough to reach the conduction band, as outlined in Fig. 1. After positive charges from MA+ are lost, the electrostatic balance is disrupted so electrons from I- p orbitals -the primary contributors to the valence band-32 are shifted towards Pb+2 p orbitals, the main contributors of the conduction band. To confirm the role of MA+ deprotonation in the surge of MAPbI3 radiative decay, we synthesized control samples replacing this organic cation with Cs+. As CsPbI3 is not stable


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beyond the nanoscale, we synthetized both, CsPbBr3 and MAPbBr3 for direct comparison. Increased PLQY values were observed in polybase-passivated MAPbBr3 crystals, which is expected upon MA+ deprotonation. In contrast, polybasepassivation of CsPbBr3 -which has no deprotonable cationsresulted in modest to medium decreases of PLQY (figure S1). This divergent effect confirms that MA+ cations are essential to trigger an increase in the radiative decay of lead halide perovskites upon polybase-passivation. Our findings have important implications in the context of reports on neutral MA molecules trapped at the inter-grain region of annealed polycrystalline MAPbI3 films.33 Neutral MA -the deprotonated form of the MA+ ions composing the MAPbI3 lattice- provides a proton withdrawing environment of sufficient strength to partially deprotonate the crystals. Considering that solar cells are built of such films while this study was carried on solvent-rinsed pristine crystals, it can be argued that LPEI/BPEI passivation mimics the conditions for MAPbI3 crystals in photovoltaic devices. In that sense, we find this deprotonation mechanism potentially explains the positive effect of moderate humidity on MAPbI3 solar cells,34,35 despite the well-known fact that water degrades the lattice.36 Water is both a proton acceptor and a good solvent for neutral MA, and its presence can strongly modify inter-grain basicity. We investigated the impact of polybase-passivation on the chemical structure of MAPbI3 crystals using Raman spectroscopy. The Raman spectrum of pristine and polybase-passivated MAPbI3 crystals is show in figure 3. According to theoretical calculations,37 the weaker bands around 93 cm-1 and below correspond to vibrational modes of the inorganic Pb-I cage, while the two well-defined bands around 110 cm-1 and 160 cm-1 correspond to MA+ libration modes. Those calculations also show that the libration mode around 160 cm-1 shifts to higher frequencies when MA+ cations are collectively aligned head-totail within the inorganic cage. As shown in figure 3, such shifts are observed upon polybase-passivation. A modest shift is observed with PVP, while more pronounced shifts are observed with LEP and BPEI. This reveals a correlation between intrinsic doping and the degree of collective MA+ alignment within MAPbI3 crystal.

crystal provides the conditions for the transfer of protons between MA bases of equal strength. Ultimately, an equilibrated distribution of protons is reached as a compromise between the amount of neutral MA present over the crystal surface, and the energetic cost of leaving unbalanced charges in the lattice, as schematically represented in Fig. 4

Figure 4. Schematic representation of the proton migration process in MAPbI3 crystals. Accordingly, a decreasing gradient in proton-vacancies can be expected towards the interior of the crystal, with their corresponding charge-balancing protons being stabilized over the crystal surface. Such arrangement implies the build-up of electric potential within crystals. The feasibility of this mechanisms is supported by theoretical calculations showing low energetic barriers for the deprotonation of MA+ ions inside the MAPbI3 lattice by strong bases adsorbed to the surface,39 and for the migration of protons through lattice interstitial positions.18 These findings provide a model explanation for efficient photocarrier separation and collection in MAPbI3 solar cells. Recombination rates are unexpectedly low and carrier diffusion lengths extraordinarily large considering the relatively moderate carrier mobility of this material (namely, the free-diffusion Langevin recombination model does not apply on MAPbI3).40,41 In this context, the grain-boundary potential proposed in this work promotes the separation of photo-generated charges, as schematically explained in Fig. 5. In other words, we propose that MAPbI3 has the property to self-equilibrate into polycrystalline films with internal p-n junctions, such as those accessible through targeted doping along grain boundaries.42,43 This interpretation is consistent with reports on the heterogeneity of photocurrent within individual grains.44 Also, it conciliates evidence on the beneficial role of MAPbI3 grain boundaries as regions for photocarrier separation and collection45,46 with reports on the detrimental character of these grain boundaries as photocarrier deep-trap sites.23

c Figure 3. Raman spectra of pristine and polybase-passivated MAPbI3 crystals.

Figure 5. Proposed charge separation mechanism in MAPbI3 solar cells facilitated by the surge of electric fields near grain boundaries.

Theoretical calculations38 have revealed that MA+ cations randomly rotate within MAPbI3 crystals at room temperature, thus leading to conclude that the collective alignment of MA+ dipoles is rather to be expected under the effect of electric fields. To explain these observations, we propose that the regular arrangement of MA+ ions at equidistant positions throughout the

In conclusion, self-doping of MAPbI3 crystals can be driven by a dynamic, proton-transfer equilibration process with its surroundings. Stable, partially deprotonated MAPbI3 lattices contain unbalanced negative charges which function as doped carriers. Proton migration directly responds to the chemistry of the crystal surface, and proceeds without the need of external fields. In that



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sense, MAPbI3 crystals can be understood as smart responsive materials in which doping -an intrinsic property- can be controlled by an external stimulus, like basicity. We believe that the uncovering of this mechanism lays the grounds for the potential development of alternative systems replicating the polarizability of photovoltaic crystals by incorporating proton-transport networks as part of charge-balancing hybrid structures.

ASSOCIATED CONTENT Supporting Information Experimental details and photoluminescence decay model.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Bavarian State Ministry of Science, Research, and Arts through the grant “Solar Technologies go Hybrid (SolTech)”. We thank J. Sichert for technical support and Dr. K. Milowska and Dr. A. Urban for scientific discussion.

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The Migration of Constituent Protons in Hybrid Organic-Inorganic

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