Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals

Abstract

Solution-processed hybrid organolead trihalide (MAPbX 3 ) perovskite solar cells (PSCs) have now achieved 20.1% certified power conversion efficiencies (1), following a rapid surge of development since perovskite based devices were first reported in 2009 (2). A key to the success of PSCs is the long diffusion length of charge carriers in the absorber perovskite layer (3). This parameter is expected to depend strongly on film crystallinity and morphology. Thermally evaporated MAPbI 3 films fabricated using a Cl --based metal salt precursor were reported to exhibit carrier diffusion lengths three times those of the best solution-processed materials, yet no measurable Cl -was incorporated in the final films, hinting at amajor but unclear mechanism in the control of crystallinity and morphology (4, 5). These observations suggest that there may be room to improve upon already remarkable PSC efficiencies via the optimization of three key parameters: charge carrier lifetime, mobility, and diffusion length.The quest for further improvements in these three figures of merit motivated our exploration of experimental strategies for the synthesis of large single-crystal MAPbX 3 perovskites that would exhibit phase purity and macroscopic (millimeter) dimensions. Unfortunately, previously published methods failed to produce single crystals with macroscopic dimensions large enough to enable electrode deposition and practical characterization of electrical properties (6). Past efforts based on cooling-induced crystallizationwere hindered by (i) the limited extent to which solubility could be influenced by controlling temperature, (ii) the complications arising from temperature-dependent phase transitions inMAPbX3, and(iii) the impact of convective currents (arising from thermal gradients in the growth solution) that disturb the ordered growth of the crystals.We hypothesized that a strategy using antisolvent vapor-assisted crystallization (AVC), in which an appropriate antisolvent is slowly diffused into a solution containing the crystal precursors, could lead to the growth of sizableMAPbX3 crystals of high quality (with crack-free, smooth surfaces,well-shaped borders, and clear bulk transparency). Prior attempts to grow hybrid perovskite crystals with AVC have fallen short of these qualities-a fact we tentatively attributed to the use of alcohols as antisolvents (7). Alcohols act as good solvents for the organic salt MAX (8) due to solventsolute hydrogen bond interactions; as a result, they can solvate MA+ during the ionic assembly of the crystal, potentially disrupting long-range lattice order.We instead implemented AVC (Fig. 1A) using a solvent with high solubility and moderate coordination for MAX and PbX 2 [N,Ndimethylformamide (DMF) or g-butyrolactone (GBA)] and an antisolvent in which both perovskite precursors are completely insoluble [dichloromethane (DCM)]. We reasoned that DCM, unlike alcohols, is an extremely poor solvent for both MAX and PbX 2 and lacks the ability to form hydrogen bonds, thus minimizing asymmetric i...

Lead‐Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?

“…The valence band maximum (VBM) of the MA 3 Bi 2 I 9 was measured to be 5.63 eV by photoelectron spectroscopy in the air (PESA)89 and 5.9–6.0 eV by ultraviolet photoelectron spectroscopy (UPS)87, 89 in vacuum, respectively, which are all well aligned with the conduction band of TiO 2 . The electron mobility of MA 3 Bi 2 I 9 single crystal is 29.7 cm 2 V −1 s −1 as estimated by the space charge limited conduction (SCLC),89 which is comparable to that of MAPbI 3 (38 cm 2 V −1 s −1 ) 186. The same carrier mobility was estimated to be 1 cm 2 V −1 s −1 by Hall Effect 87.…”

Recent Progress in Single‐Crystalline Perovskite Research Including Crystal Preparation, Property Evaluation, and Applications

“…Furthermore, additional optoelectronic properties have been discovered, including long balanced hole–electron diffusion length,19, 20, 21, 22, 23 high carrier mobility,21, 24 high absorption coefficient in visible region,25, 26 and long carrier lifetime 21, 22, 25, 27, 28, 29. These optoelectronic properties make hybrid perovskites ideal candidates for solar cell applications,22, 25, 30, 31, 32, 33, 34, 35, 36, 37, 38 LEDs,39, 40, 41 lasers,42, 43 etc.…”

“…Over the last 2 years, an increasing number of researchers have begun studying single‐crystalline perovskites, including crystal preparations,21, 27, 28, 43, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65 property studies,21, 27, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80 and applications 56, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91. For example, an extremely long carrier lifetime (up to 20 µs) and carrier diffusion length (175 µm) have been found,21 which has received a great deal of attention.…”

“…To date, all reported perovskite crystals have been prepared with the liquid growth method 21, 27, 28, 55, 61. Generally, crystals will form by slowly reducing the solubility of the target samples in a precursor solution.…”

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode

“…MAPbI 3 is an unusual solar absorber material, in which efficient carrier transport5, 6, 7, 8 coexists with a high density of defects 9, 10. This is related to the soft lattice and the large static dielectric constant (60–70)11, 12, 13 of MAPbI 3 , which, on one hand, promote the defect formation and, on the other hand, suppress carrier trapping at charged defects and impurities 14, 15, 16.…”