Session S20: Hybrid Perovskites -- Photovoltaics

Fundamental Light-Conversion Mechanisms in Metal Halide Perovskites for Photovoltaics

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 22%. We discuss the fundamental processes that have enabled these materials to be such efficient light-harvesters and charge collectors.

We demonstrate that at the intrinsic limit, the mobility of charge-carriers is predominantly governed by interaction of carriers with optical vibrations of the lead halide lattice (Fröhlich interaction)^[1]. In the absence of trap-mediated charge recombination, band-to-band recombination will dominate losses near the Shockley-Queisser limit. We show that in methylammonium lead triiodide perovskite, such processes can be fully explained as the inverse of absorption,^[2] and exhibit a dynamic that is heavily influences by photon reabsorption inside the material.^[3] Finally, we demonstrate that in the absence of degradation, perovskite solar cells can exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they exceed the Shockley-Queisser limit and exhibit elevated open-circuit voltages.^[4,5]

[1] A.D. Wright, C. Verdi, R.L. Milot, G. E. Eperon, M. A. Pérez-Osorio, H. J. Snaith, F. Giustino, M. B. Johnston, L. M. Herz, Nature Communications 2016, 7, 11755
[2] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, L. M. Herz, Nature Communications 2018, 9, 293
[3] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, L. M. Herz, Nano Letters 2017, 17, 5782
[4] Q. Lin, Z. Wang, H. J. Snaith, M. B. Johnston, L. M. Herz, Advanced Science 2018, 5 1700792
[5] Z. Wang, Q. Lin, B. Wenger, M. G. Christoforo, Y. H. Lin, M. T. Klug, M. B. Johnston, L. M. Herz, H. J. Snaith, Nature Energy 2018, 3, 855   

Unraveling the role of low-frequency phonon modes in carrier relaxation and regeneration in lead-halide perovskites

Carrier-phonon scattering, a ubiquitous effect in all condensed matter systems, has been discussed in the halide perovskites in the context of placing an upper limit to the carrier mobility. Using time-resolved terahertz spectroscopy on a formamidinium-based perovskite system, we will show that phonons play a much larger role than thought previously. The combination of phonon emission during carrier relaxation, phonon-assisted exciton dissociation, and free-carrier screening, ensure a large and long-lived carrier population in the material. Our work reveals the delicate interplay among carriers, phonons and excitons, that affect the photo-physical properties of the halide perovskites, that ultimately determines the photovoltaic performance of halide perovskite-based solar cells.   

Electron and Hole Mobilities in Organic-Inorganic Hybrid Perovskites

One of the fundamental semiconducting properties, charge-carrier mobilities have been widely studied in the organic-inorganic hybrid perovskites.However, the reported values of mobilities ranges widely from 0.4 to 600cm²(V s)⁻¹ and most of the experimental methods cannot distinguish the electron and hole mobilities. Other than photoluminescence quenching (PLQ) methods that requires the presence of an extractor layer, here we report an optical method to determine the electron and hole mobilities individually in organic-inorganic hybrid perovskite films and single crystals.   

Long Exciton Diffusion Length in Single Crystal Halide Perovskites

Hybrid inorganic-organic halide perovskites have garnered an incredible amount of attention over the last few years, thanks to power conversion efficiencies climbing from 3% to now 22.7% in less than a decade. The majority of published work thus far has focused on creating higher quality thin films, while the underlying carrier transport processes are not well understood. By studying single crystal methylammonium lead iodide (MAPbI₃) samples, we avoid the issues caused by thin film grain boundaries and shed light on the inherent carrier transport mechanisms of this revolutionary material. We determined the carrier diffusion lengths via scanning photocurrent microscopy (SPCM). We observed excitonic behavior through electric field independent photocurrent distributions as well as a measured exciton diffusion length that increased abruptly below the phase transition temperature from the room temperature tetragonal phase to the low temperature orthorhombic phase. The low temperature phase diffusion reached up to 100 μm at 80 K. We attributed the long exciton diffusion length to the reduced carrier scattering because of the suppressed ion rotation in the orthorhombic phase.   

Comparative photovoltaics studies of prominent PV materials, organometal halide perovskite, CdTe, and GaAs with direct comparison of carrier recombination through power dependent photoluminescence

Comparative Photovoltaics refers to the study of material properties of different PV materials by making direct and meaningful comparison to reveal their relative strength and weakness. Although the hybrid perovskite based solar cell has reached up to 23.3% energy conversion efficiency, it remains unclear what the enabling factors are for the impressive performance, in particular how it differs from the better-known PV materials CdTe and GaAs. Power dependent PL measurements show that perovskite samples tend to have stronger PL than the inorganic counterparts in the very low excitation density region (e.g., 0.1 – 1 sun), indicating that Shockley-Read-Hall (SRH) recombination is relatively less effective in the perovskite. PL lineshape analysis suggests that carriers in the perovskite are not in thermal equilibrium due to structural disordering, which in turn suppresses the nonradiative recombination. Both relatively low effective point defect density and moderate crystalline disordering in the perovskite contribute to the low nonradiative recombination loss, thus, high cell efficiency.   

Substrate Dependent Initial Stage of Coevaporated Perovskite Film Formation

The energy level alignment that occurs at the interfaces in planar-hetero structured perovskite photovoltaic devices strongly influences the charge transport across the interface and thus plays a role in overall device performance. To directly observe the energy level alignment requires pristine homogeneous surfaces that are free of contamination. Coevaporation has been employed to grow perovskite films. The method involves thermally evaporating the perovskite precursors such as PbI₂ and CH₃NH₃I, and some early reports have shown that the perovskite film formation and stoichiometry are problematic at ultralow coverages. Using photoemission spectroscopy, we investigated the perovskite precursors PbI₂ and MAI on gold and highly oriented pyrolytic graphite (HOPG) surfaces. Results show that the nature of the surface and the deposition conditions can strongly influence the film formation. Excessive iodine observed in the initial evaporation stages appears to be substrate dependent, and this may influence the overall energy level alignment.   

Ab-initio Calculations of Surface Defect Migration in Halide Perovskites

Recently, interest in halide perovskites has been on the rise due to their high-power conversion efficiencies exceeding 20%. Nonetheless, a significant current-voltage hysteresis and poor stability are issues that are not fully understood yet. Ionic migration has been suggested as the cause for the current-voltage hysteresis [1]. An open question in this context is how grain boundaries and crystal surfaces affect ion migration.

The present work focuses on halide-related defects, the main contributors to ionic conductivity [2]. We use first principles density functional theory calculations to simulate the migration of halide vacancies in CsPbBr3 perovskite. The well-established nudged elastic band method [3] is used to compute minimum-energy pathways and energy barriers of halide-related defects. We systematically study the change of migration activation energies upon moving from the surface into the bulk. Our results show that ions migrate more easily in soft lattices showing a pronounced dependence on surface proximity, depending on the availability and size of interstitial sites.

[1] Adv. Mater. 2016, 28, 2446-2454
[2] Energy Environ. 2015, 8, 2118-2127
[3] J. Chem. Phys. 2000, 113, 9901-9904   

Raman spectroscopy as a probe of local strain in perovskite solar cells

Hybrid organometallic perovskites are a promising material for thin-film solar cells. Strain can develop due to substrate thermal expansion mismatch during processing, presence of grain boundaries, and even absorption of light [J. Zhao et al., Sci. Adv. 3, eaao5616 (2017)]. These strains may be inhomogeneous and can affect carrier mobility, non-radiative recombination etc. and may be related to degradation. Vibrational frequencies are shifted due to strain in a material and we want to enable use of this phenomenon to map local stress-strain behavior within a perovskite material via Raman spectroscopy, as is done in crystalline silicon and other materials [D. A. Strubbe et al., Phys. Rev. B 92, 241202(R) (2015)]. We study vibrational properties of different phases of methylammonium lead iodide (CH₃NH₃PbI₃) structures and calculate the slope of the shift of each mode for applied strains along different crystal directions to provide a calibration curve for measuring local strain. Our study also gives insight into the interaction between strain, structural changes and vibrational modes which may help to understand the degradation that plagues these materials.   

Direct Optical Characterization of Subgrain Boundaries and Carrier Diffusion Rates in Perovskites

Perovskite solar cells (PSCs) have attracted considerable attention in recent years due to their rapidly increasing power conversion efficiency (PCE), which currently exceeds 23%. An important feature in organic−inorganic halide perovskites (OIHPs) that limits PCE is their defect densities. Here we report an optical spectroscopy method for identifying subgrain special boundaries that introduce defects and limit carrier diffusion. These subgrain boundaries do not exhibit clear surface morphology features that can be recognized by scanning electron microscopy (SEM) or atomic force microscopy (AFM). Such special boundaries may explain why PSC performance has not scaled with apparent grain size. Furthermore, in order to quantitatively study the role of defects, we establish a method to estimate defect density by directly studying the spatial and temporal distribution of carriers in OIHP thin films.