Excitonic and Band-to-Band Transitions in Temperature-Dependent Optical Absorption Spectra of Cu2SnS3 Thin Films
Excitonic and Band-to-Band Transitions in Temperature-Dependent Optical Absorption Spectra of Cu2SnS3 Thin Films
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Cu2SnS3 薄膜随温度变化的光学吸收光谱中的激子跃迁和带间跃迁
DOI：
10.1002/pssb.201700304
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Kunihiko Tanaka
中科院分区：
文献类型：
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作者：
Naoya Aihara;Hideaki Araki;Kunihiko Tanaka
研究了作为太阳能电池材料的Cu2SnS3(CTS)薄膜的透射率和反射率随温度变化的光吸收光谱。采用热共蒸发和硫化的方法在玻璃衬底上制备了贫铜、近化学计量比和富铜的CTS薄膜样品。在低温下，所有样品都观察到了三个允许在三个上价带和一个下导带之间发生的带-带(BB)转变。绘制了吸收系数与光子能量乘积的平方图，估算了贫铜样品三个能带的带隙能量。另一方面，对于富铜和近化学计量比的样品，观察到了对应于三个带的激子(EX)跃迁。用一个简化的以洛伦兹函数作为EX跃迁的拟合方程解析了具有三个BB和EX跃迁的随温度变化的光吸收光谱。在6 K下，测得富铜样品最低能带的禁带宽度为0.945 eV，EX跃迁为0.936 eV，激子结合能为8.9 meV。在低温区，所有样品的三个带的估算带隙能随温度的升高而出现反常蓝移。
Temperature‐dependent optical absorption spectra from transmittance and reflectance measurements of Cu2SnS3(CTS) thin films which are promising material for solar cells were investigated. Thin film CTS samples with Cu‐poor, near‐stoichiometric, and Cu‐rich compositions were prepared on glass substrates by thermal co‐evaporation and sulfurization. At low temperature, three band‐to‐band (BB) transitions that are allowed between triple upper valence bands and a single lower conduction band were observed for all samples. The square of the product of the absorption coefficient and the photon energy was plotted to estimate the band gap energy for three bands of the Cu‐poor sample. On the other hand, excitonic (EX) transitions that corresponded to three bands were observed for the Cu‐rich and near‐stoichiometric samples. Temperature‐dependent optical absorption spectra with the triple BB and EX transitions were resolved using a simplified fitting equation with Lorentzian functions as the EX transitions. The band gap, EX transition, and exciton binding energies for the lowest energy band of the Cu‐rich sample were determined to be 0.945 eV, 0.936 eV, and 8.9 meV at 6 K, respectively. In the low‐temperature region, anomalous blue‐shifts of the estimated band gap energy with increasing temperature were obtained for three bands of all samples.