Spectral Signatures of Positive and Negative Polarons in Lead-Halide Perovskite Nanocrystals
Spectral Signatures of Positive and Negative Polarons in Lead-Halide Perovskite Nanocrystals
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DOI：
10.1021/acs.jpcc.9b08044
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发表时间：
2020-01-09
影响因子：
3.7
通讯作者：
Kilin, Dmitri
中科院分区：
文献类型：
--
作者：
Forde, Aaron;Inerbaev, Talgat;Kilin, Dmitri
APbX（3）（A = Cs，甲基铵{MA}; X = I，Br，Cl）卤化铅钙钛矿对于发光应用是令人感兴趣的，这是由于其带隙在可见光和近红外光谱（IR）上的可调谐性以及有效的光致发光量子产率（PLQY）。人们普遍推测，光激发的电子和空穴在空间上分离成大的（Frolich）负极化子和正极化子，它们被A阳离子稳定。极化子被认为是光学活性的，最近的红外瞬态吸收实验显示出与极化子光电离成连续带态相一致的光谱特征。对于中间耦合区域中的大极化子，还期望观察到产生极化子激发态的极化子势阱内的跃迁的光谱特征。从极化子激发态，我们预测，大极化子应该能够自发辐射（光致发光）在中红外到远红外制度的基础上的概念，逆职业内的极化子势阱。为了验证这一假设，我们使用密度泛函理论（DFT）为基础的计算，使用CsPbBr 3双原子模型作为主机材料的负（电子）或正（空穴）极化子。我们动态耦合电子和核自由度计算非绝热耦合，使我们能够探索激发极化子态的非辐射弛豫。从自发辐射的爱因斯坦系数中发现了极化子激发态的辐射弛豫。极化子发射效率由非辐射复合率（k（NR））和辐射复合率（k（R））确定为k（R）/（k（R）+ k（NR））。发现正负极化子从弛豫激发态（RES）到极化子基态（PGS）都表现出明亮的吸收特性和光致发光，但这是一个低效率的过程（PLQY类似于10（-4）-10（-7））。讨论了改进极化子发射PLQY计算的方法，如Marcus速率修正和相干性。这项工作为观察IR极化子吸收提供了计算支持，并为将APbX（3）钙钛矿的发射能力扩展到中红外到远红外区域提供了潜在的方向。
APbX(3) (A = Cs, methylammonium {MA}; X = I, Br, Cl) lead halide perovskites are of interest for light-emitting applications due to the tunability of their bandgap across the visible and near-infrared spectrum (IR) coupled with efficient photoluminescence quantum yields (PLQYs). It is widely speculated that photoexcited electrons and holes spatially separate into large (Frolich) negative and positive polarons which are stabilized by the A cations. Polarons are expected to be optically active, with recent IR transient absorption experiments showing spectral features consistent with photoionization of the polaron into the continuum band states. For large polarons in the intermediate coupling regime, it would also be expected to observe spectral signatures of transitions within the polaronic potential well producing polaron excited-states. From the polaron excited-state we predict that large polarons should be capable of spontaneous emission (photoluminescence) in the mid-IR to far-IR regime based on the concept of inverse occupations within the polaron potential well. To test this hypothesis, we use density-functional theory (DFT) based calculations using a CsPbBr3 nanocrystal atomistic model as a host material for either negative (electron) or positive (hole) polarons. We dynamically couple electronic and nuclear degrees of freedom by computing nonadiabatic couplings which allow us to explore nonradiative relaxation of excited polaronic states. Radiative relaxation of excited polaronic states is found from Einstein coefficients for spontaneous emission. Efficiency of polaron emission is determined from rates of nonradiative recombination (k(NR)) and radiative recombination (k(R)) as k(R)/(k(R) + k(NR)). It is found that both the positive and negative polaron show bright absorption features and photoluminescence from the relaxed-excited state (RES) to the polaron ground states (PGS), but it is an inefficient process (PLQY similar to 10(-4)-10(-7)). Methodology considerations for improving the computed PLQY of polaron emission are discussed, such as Marcus rate corrections and coherence. This work provides computational support for observation of IR polaron absorption and a potential direction toward extending the emission capabilities of APbX(3) perovskites into the mid-IR to far-IR regime.