Direct Measurement of the Exciton Binding Energy and Effective Masses for Charge carriers in an Organic-Inorganic Tri-halide Perovskite

Abstract: Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown remarkable progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance due to spontaneous free carrier generation following light absorption. Additionally, we determine the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties.

• The paper presents direct magneto-optical measurements that reveal an exciton binding energy of 16 meV, pivotal for free carrier generation in perovskites.
• The methodology employs high magnetic fields up to 150T to accurately determine the effective mass of charge carriers at 0.104mₑ.
• These insights refine electronic models and advance perovskite solar cell design by elucidating crucial charge carrier dynamics.

Exciton Binding Energy and Effective Mass Measurement in Organic-Inorganic Tri-halide Perovskites

The study of organic-inorganic tri-halide perovskites has delineated significant insights into their photophysical properties critical for photovoltaic applications. This paper focuses on the exciton binding energy and the effective masses of charge carriers within the archetypal material CH$_3$NH$_3$PbI$_3$, presenting new measurements that challenge previously held assumptions in the context of perovskite solar cells.

The authors employ high magnetic fields to perform inter-band magneto-absorption studies, allowing for a direct and accurate measurement of the exciton binding energy. The significant findings include a measured exciton binding energy of only 16 meV at low temperatures, a value substantially lower than prior assumptions, which had ranged from 30 to 50 meV. The implication of this lower binding energy is the efficient generation of free carriers at room temperature, a hypothesis corroborated by binding energy values dropping to just a few meV. Additionally, the determined excitonic reduced effective mass of 0.104m$_e$ is notably in agreement with recent theories but diverges from earlier experimental estimates.

One of the salient outcomes of these measurements is the resolution of an ongoing debate regarding the nature of charge carriers in perovskites. The smaller exciton binding energy supports the observation that these materials, in practical conditions such as room temperature, primarily exhibit free carrier behavior. This directly explains the superior performance of perovskite-based photovoltaic devices, which benefit from efficient charge separation and transport.

The methodology involved magneto-optical techniques with fields up to 150T, aiding in comprehensive spectral analysis, including the resolution of multiple excitonic transitions. The full numerical calculations employed account for the transitions as the material shifts from low to high magnetic fields, reinforcing the findings’ accuracy. The high-quality crystalline films and advanced measurement conditions were pivotal in achieving these precise results, contrasting with earlier studies limited by lower magnetic field capabilities and less refined spectroscopy techniques.

The implications of this research extend to both theoretical understanding and practical advancement in the development of perovskite solar cells. The accurate values of exciton binding energies and effective masses redefine the analytical models needed for these materials. Moreover, they provide vital parameters for optimizing photovoltaic device architectures, potentially leading to enhanced cell efficiencies beyond the current benchmarks.

Future work could explore the dynamic behavior of excitons and free carriers under varied environmental influences, such as different temperatures and magnetic fields. Further investigations could focus on the role of structural phase transitions in modifying electronic properties, possibly enhancing the control over charge recombination and separation processes in device applications.

In summary, by redefining critical parameters within organic-inorganic tri-halide perovskites, this research contributes to a deeper understanding of the fundamental electronic mechanisms that underpin high-performance photovoltaic devices. Such insights might facilitate the development of next-generation solar cells with improved efficiencies and stability, targeting the increasing demand for cost-effective and scalable renewable energy solutions.