Inaugural Article: Singlet exciton binding energy in poly(phenylene vinylene)
Inaugural Article: Singlet exciton binding energy in poly(phenylene vinylene)
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首篇文章：聚亚苯基亚乙烯基中的单线态激子结合能
DOI：
10.1073/pnas.241497098
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发表时间：
2001
影响因子：
11.1
通讯作者：
S. Brazovski
中科院分区：
文献类型：
--
作者：
Daniel Moses;Jian Wang;A. Heeger;N. Kirova;S. Brazovski
共轭聚合物领域的一个中心问题是相对于带宽的电子-电子相互作用的强度（1）：成对电子-空穴对的吸引力是否如此之强，以至于光激发是局部的和强关联的Frenkel激子？或者说，载流子是否被充分屏蔽，使得由电子-声子相互作用（极化子形成）和电子-电子相互作用（弱束缚激子）补充的能带图像是合理的？激子结合能（Eb）的测定对于回答这些问题，从而理解半导体聚合物的电子结构至关重要。
由于这个问题尚未解决，关于半导体（共轭）聚合物光学性质的大量文献包含两个相互冲突的最低能量π-π* 吸收分配（1）。
最低能量的π-π* 吸收来自于紧密结合的中性单线态激子的产生，带间跃迁在显著更高的能量处开始，例如，在分子晶体中，如蒽（2）。
最低能量的π-π* 吸收是由直接带间跃迁引起的，例如在直接带隙半导体如GaAs中。
这两个不同的分配意味着非常不同的结果，为光生带电激发。当最低能量π-π* 吸收由直接带-带跃迁产生时，人们期望观察到能量接近吸收开始（EJ 1-π）的电荷载流子的光生阈值;即，在
1a
然而，如果Eb大，则期望观察到在大于Eb的光吸收开始的能量下经由最低带间跃迁的电荷载流子的光生的阈值;即，在
1b
对于聚（亚苯基亚乙烯基）（PPV）及其几种可溶性衍生物，在零外场下，使用红外活性振动模式的超快光诱导吸收测量了带电激发（极化子）光生的量子效率（3-5）。结果表明，电荷载流子光生具有单一阈值，该阈值在能量上接近于吸收的开始，与Eq. 1a（5）。虽然光电导数据已经报道了第二个阈值远高于吸收的开始（6，7），我们已经检测到并表征了来自电子光电发射（8）的瞬态和稳态光电流的贡献。在猝灭光电发射贡献之后，PPV和所有PPV衍生物中的真实体光电导数据显示接近于光吸收开始的阈值，高于该阈值，光电流几乎与激发能量无关（高达6.2eV），再次与Eq. 1a（8）。这些数据表明，PPV（及其可溶性衍生物）的最低能量光学吸收带中的振子强度来自最低能量带间跃迁。
理论模型已经得出PPV中Eb的估计值，范围从0.1 eV到1 eV（6，7，9）。此外，光致发光与光强度（I）而不是I2成比例，指示来自束缚电子-空穴对的发射。因此，发现激子与光谱精度和测量的Eb仍然是重要的目标，半导体聚合物的电子物理学的实验研究。
我们已经测量了Eb在链取向PPV通过高分辨率激发轮廓光谱的稳态光电流（Iphoto）在各种外场（F）和温度（T），并在样品中具有不同的缺陷浓度。在激发曲线中观察到激子的光谱特征，作为在增加外部电场或缺陷密度时出现在带边缘下方的窄峰。因为激子吸收和发射平行于链轴偏振，所以用平行和垂直于PPV链轴偏振的光测量Iphoto的激发分布使得能够通过激子解离从通过π-π* 带间跃迁的载流子产生来识别（和分离）载流子产生。从这些研究中，我们已经确定了带隙能量（Eg）（2.42 eV）和Eb（2.60 meV），我们已经澄清了外部场和缺陷在载流子光生过程中的作用。
A central issue in the field of conjugated polymers is the strength of the electron-electron interaction relative to the bandwidth (1): Is the attraction of a geminate electron-hole pair so strong that the photoexcitations are localized and strongly correlated Frenkel excitons? Or rather, are the charge carriers sufficiently well screened that a band picture supplemented by the electron-phonon interaction (polaron formation) and the electron-electron interaction (weakly bound excitons) is justified? Determination of the exciton binding energy (Eb) is critically important to answering these questions and thereby to understanding the electronic structure of semiconducting polymers.
Because this issue has not been resolved, the extensive literature on the optical properties of semiconducting (conjugated) polymers contains two conflicting assignments for the lowest energy π-π* absorption (1).
The lowest energy π-π* absorption results from the creation of tightly bound neutral singlet excitons with the onset of the interband transition at a significantly higher energy, as for example, in molecular crystals such as anthracene (2).
The lowest energy π-π* absorption results from a direct band-to-band transition, as for example, in direct gap semiconductors such as GaAs.
These two different assignments imply very different results for the photogeneration of charged excitations. When the lowest energy π-π* absorption results from a direct band-to-band transition, one expects to observe a threshold for photogeneration of charge carriers close in energy to the onset of absorption (Eπ-π); i.e., at
1a
If, however, Eb is large, one expects to observe the threshold for photogeneration of charge carriers via the lowest band-to-band transition at energy greater than the onset of optical absorption by Eb; i.e., at
1b
For poly(phenylene vinylene) (PPV) and several of its soluble derivatives, the quantum efficiency for photogeneration of charged excitations (polarons) has been measured, in zero external field, using ultrafast photo-induced absorption by infra-red active vibrational modes (3–5). The results demonstrated charge carrier photogeneration with a single threshold that is close in energy to the onset of absorption, in agreement with Eq. 1a (5). Although photoconductivity data have been reported with a second threshold well above the onset of absorption (6, 7), we have detected and characterized a contribution to the transient and steady-state photocurrent that originates from electron photoemission (8). After quenching the photoemission contribution, the true bulk photoconductivity data in PPV and all of the PPV derivatives show a threshold close to the onset of optical absorption, above which the photocurrent is nearly independent of excitation energy (up to 6.2 eV), again in agreement with Eq. 1a (8). These data demonstrate that the oscillator strength in the lowest energy optical absorption band of PPV (and its soluble derivatives) arises from the lowest energy band-to-band transition.
Theoretical models have yielded estimates for the Eb in PPV that range from values of order 0.1 eV to 1 eV (6, 7, 9). Moreover, the photoluminescnce is proportional to the light intensity (I) rather than I2, indicative of emission from a bound electron-hole pair. Thus, finding the exciton with spectroscopic accuracy and measuring the Eb remain as important goals for experimental studies of the photophysics of semiconducting polymers.
We have measured the Eb in chain-oriented PPV through high-resolution excitation profile spectroscopy of the steady-state photocurrent (Iphoto) at various external fields (F) and temperatures (T), and in samples with different defect concentrations. The spectral signature of the exciton is observed in the excitation profile as a narrow peak that emerges just below the band edge upon increasing the external electric field or the defect density. Because the exciton absorption and emission are polarized parallel to the chain axis, measuring the excitation profile of Iphoto with light polarized parallel and perpendicular to the PPV chain axis enables the identification (and separation) of carrier generation by means of exciton dissociation from carrier generation by means of the π-π* interband transition. From these studies, we have determined band gap energy (Eg) (2.42 eV) and Eb (≈60 meV), and we have clarified the role of the external field and defects in the carrier photogeneration process.