Defect chemistry plays a critical role in determining the optoelectronic properties of perovskite semiconductors. These materials, particularly hybrid organic-inorganic perovskites like methylammonium lead iodide (MAPbI₃), exhibit a complex landscape of point defects that influence charge carrier dynamics, recombination, and stability. Understanding the nature of these defects—vacancies, interstitials, and anti-site defects—is essential for optimizing material performance.

Vacancies are among the most common defects in perovskite semiconductors. In MAPbI₃, iodine vacancies (V_I) and lead vacancies (V_Pb) are frequently observed due to their low formation energies. Iodine vacancies, for instance, have formation energies as low as 0.1–0.3 eV under iodine-poor conditions, making them highly probable. These vacancies act as shallow donors or acceptors depending on the charge state. V_I typically introduces shallow donor states near the conduction band, while V_Pb creates deep acceptor states near the valence band. The presence of these defects can lead to unintentional doping, altering carrier concentrations and Fermi level positions.

Interstitial defects, such as interstitial iodine (I_i) or methylammonium (MA_i), also contribute to defect-mediated recombination. Iodine interstitials tend to form deep traps, particularly when in a neutral or positively charged state. Their formation energy is higher than vacancies but can still occur under non-stoichiometric growth conditions. Interstitial defects often act as non-radiative recombination centers, reducing photoluminescence quantum yield and increasing charge carrier losses.

Anti-site defects, where atoms occupy incorrect lattice sites, are less common but can have significant impacts. In MAPbI₃, Pb_I (lead on an iodine site) and I_Pb (iodine on a lead site) are possible anti-site defects. Pb_I introduces deep trap states within the bandgap, while I_Pb can act as a shallow acceptor. The formation energy of anti-site defects is generally higher than vacancies, but under certain synthesis conditions, such as rapid crystallization or extreme stoichiometric imbalances, their concentration can become non-negligible.

The charge state of defects is highly dependent on the Fermi level and synthesis environment. For example, V_I can exist in neutral (V_I⁰), positive (V_I⁺), or doubly positive (V_I²⁺) states. Under iodine-rich conditions, the formation energy of V_I increases, suppressing its concentration. Conversely, in iodine-deficient environments, V_I becomes more prevalent. Similar trends apply to other defects, where synthesis conditions dictate defect populations and charge states.

Non-radiative recombination is a major consequence of defect activity in perovskites. Deep-level defects, such as V_Pb or I_i, create mid-gap states that facilitate Shockley-Read-Hall (SRH) recombination. This process competes with radiative recombination, reducing the internal quantum efficiency of light-emitting devices and solar cells. The capture cross-section of these defects determines their recombination strength, with some defects exhibiting cross-sections as high as 10⁻¹⁴ cm², making them highly effective non-radiative centers.

Defect passivation is a key strategy for mitigating non-radiative losses. Halogen-rich synthesis, where excess iodine or bromine is introduced during growth, helps suppress vacancy formation by maintaining stoichiometric balance. Additive engineering, such as incorporating potassium iodide (KI) or formamidinium bromide (FABr), has been shown to passivate grain boundaries and surface defects. These additives interact with undercoordinated lead atoms, neutralizing deep traps and improving carrier lifetimes.

Another effective passivation approach involves post-synthesis treatments. Exposure to iodine vapor can heal iodine vacancies, while Lewis base molecules like thiophene or pyridine can coordinate with Pb²⁺, reducing defect density. Surface passivation with insulating polymers or 2D perovskite layers has also proven effective in suppressing interfacial recombination.

Characterization techniques are crucial for identifying and quantifying defects in perovskite semiconductors. Deep-level transient spectroscopy (DLTS) is a powerful tool for probing trap states, providing information on defect energy levels and concentrations. For example, DLTS measurements on MAPbI₃ have revealed trap densities ranging from 10¹⁴ to 10¹⁶ cm⁻³, depending on synthesis conditions.

Photoluminescence (PL) spectroscopy is another essential technique for assessing defect activity. The PL decay lifetime is directly influenced by non-radiative recombination, with shorter lifetimes indicating higher defect densities. Time-resolved PL can distinguish between bulk and surface recombination, helping to pinpoint defect localization. Low-temperature PL measurements further reveal defect-related emission peaks, such as those associated with bound excitons or trap-assisted transitions.

Secondary ion mass spectrometry (SIMS) provides quantitative data on elemental distribution and impurity concentrations, aiding in the identification of defect origins. X-ray diffraction (XRD) can detect strain or lattice distortions caused by point defects, while scanning transmission electron microscopy (STEM) offers atomic-scale visualization of defect structures.

In summary, defect chemistry in perovskite semiconductors is a multifaceted field with significant implications for material performance. Vacancies, interstitials, and anti-site defects each contribute to charge trapping and non-radiative recombination, with their populations dictated by formation energies and synthesis conditions. Effective passivation strategies, including halogen-rich growth and additive engineering, can mitigate these defects, while advanced characterization techniques enable precise defect identification and analysis. A thorough understanding of these mechanisms is essential for advancing perovskite-based optoelectronic applications.