Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite

Abstract

Solar cells based on organometal halide perovskites have seen rapidly increasing efficiencies, now exceeding 15%. Despite this progress, there is still limited knowledge on the fundamental photophysics. Here we use microwave photoconductance and photoluminescence measurements to investigate the temperature dependence of the carrier generation, mobility, and recombination in (CH3NH3)PbI3. At temperatures maintaining the tetragonal crystal phase of the perovskite, we find an exciton binding energy of about 32 meV, leading to a temperature-dependent yield of highly mobile (6.2 cm(2)/(V s) at 300 K) charge carriers. At higher laser intensities, second-order recombination with a rate constant of γ = 13 × 10(-10) cm(3) s(-1) becomes apparent. Reducing the temperature results in increasing charge carrier mobilities following a T(-1.6) dependence, which we attribute to a reduction in phonon scattering (Σμ = 16 cm(2)/(V s) at 165 K). Despite the fact that Σμ increases, γ diminishes with a factor six, implying that charge recombination in (CH3NH3)PbI3 is temperature activated. The results underline the importance of the perovskite crystal structure, the exciton binding energy, and the activation energy for recombination as key factors in optimizing new perovskite materials.

Breathing bands due to molecular order in CH3NH3PbI3

“…Frost et al estimated the binding energy of excitons in this system to be about 0.7 meV -too small to affect significantly the photovoltaic performance [16]. This is in contradiction with experimental evidences that indicate a value in the range 6-55 meV [5,37,[67][68][69][70][71][72]. Solutions of the Bethe-Salpeter equation for a single molecular orientation revealed huge exciton binding energies of 153 meV for the cubic phase [39] and 40 meV for the tetragonal phase [32].…”

Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J–V Hysteresis

“…The hole and electron mobilities in MAPI are reported to be in the 2-20 cm 2 /Vs range. 16,26,47 At the 1 sun V oc (~0.85V) there will remain a built in field of perhaps 0.3V. Under this field, the transit time of electrons and holes across the film is only a few ns.…”

Device engineering of perovskite solar cells to achieve near ideal efficiency

“…There has also been efforts towards better understanding of material properties [14][15][16] , including the recombination strengths 4,[17][18][19] and mobility. 4,19,20 Despite the above mentioned exciting achievements in experimental research on perovskite solar cells, the corresponding theoretical understanding is lacking in many aspects. For example, while the detailed balance limits of efficiency based on radiative recombination was already reported 21 , the effect of Auger recombination is not clearly elucidated.…”