Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 60876006, 60376007), the Natural Science Foundation of Beijing, China (Grant No. 4192016), and the Funding for the Development Project of Beijing Municipal Education Commission of Science and Technology, China (Grant No. KZ201410005008).
Received Date:07 January 2019
Accepted Date:28 June 2019
Available Online:01 September 2019
Published Online:05 September 2019
Abstract:In this paper, the mixed films with different rubrene-to-MoO3 ratios are deposited on the substrates of Si, indium tin oxide and quartz glass by using the thermal evaporation technique. First, these films are characterized by atomic force microscopy and X-ray diffraction in order to identify their surface morphology and their structure properties. The results show that all the films are amorphous and the film has the best flatness when the rubrene-to-MoO3 ratio is 2∶1. Second, the optical properties of the mixed films are investigated by both photoluminescence (PL) spectra and absorption spectra. The optical band gap of rubrene and MoO3 are 2.2 eV and 3.49 eV respectively and there is almost no absorption about rubrene and MoO3 in the near-infrared (NIR) region. However the PL spectrum shows a peak in NIR region and it indicates that the interface between rubrene and MoO3 possesses an abrupt discontinuity at the vacuum level, resulting in electron wave functions overlapping and charge-transfer complex (CTC) forming. The intermediate state within the original band gap of rubrene with energy of 1.25 eV is induced by the CTC, which suggests the possibility of charge transfer exciton generated upon NIR excitation. The absorption spectra of the mixed films show that there is an obvious absorption. All the films have the same absorption peak except the film with a rubrene-to-MoO3 ratio of 4∶1 and it indicates that the concentration of MoO3 has almost no influence on the absorption of the mixed films. The optical band gaps of the mixed thin films are calculated in a spectral range of 345-1035 nm according to the Tauc equation, and the results show that the optical band gap of the film with a rubrene-to-MoO3 ratio of 2∶1 is narrowest (~2.23 eV).In order to study the electrical characteristics of the mixed films, an Al/rubrene:MoO3/ITO device is fabricated. The current density-voltage (J-V) characteristic is also investigated. The analysis of the J-V measurement for the device indicates that the current conduction in the Al/rubrene:MoO3/ITO device is Ohmic type when the rubrene-to-MoO3 ratios are 4∶1 and 2∶1, and it is Schottky type when the ratio is other value. The current for rubrene-to-MoO3 ratio of 1∶1 is larger than that for 1∶2, which indicates that the contact is better when the surface is more smooth. These properties of the mixed films can result in the applications in the near-infrared region. Keywords:mixed film of rubrene∶MoO3/ atomic force microscope/ X-ray diffraction/ properties of optical and electrical
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3.1.混合薄膜结构表征
图1是通过原子力显微镜 (atomic force microscope, AFM)得到的rubrene和MoO3在不同混合比例条件下的表面形貌图, 薄膜衬底为硅衬底, 扫描范围为2.0 μm × 2.0 μm. 从图1可以看出, 不同浓度的MoO3确实对薄膜表面的平整度产生了影响. 在本实验所测量的几个比例中, 当rubrene与MoO3的掺杂比例为2∶1时, 薄膜表面的平整度最佳. 图2给出了不同混合比例条件下的薄膜表面粗糙度RMS (rough mean square), 和薄膜的AFM图对比具有很好的一致性. 随着混合比例的不同, 薄膜表面粗糙度RMS从0.907 nm逐渐下降到0.602 nm. 从图2可知: 五种混合比的薄膜表面粗糙度RMS均在1 nm以下, 说明rubrene和MoO3两种材料的分布较均匀; 但由于两种材料的混合比例不同, 薄膜表面粗糙度RMS又出现差别, 混合比为2∶1时最小, 1∶2时最大, 当混合薄膜中MoO3浓度超过50%时, 薄膜表面粗糙度迅速增加, 这是由于当MoO3浓度过高时, 混合膜不能较好地进行分子排列的优化, 导致薄膜局域化优先成核生长[14]. 另外, 在电场作用下, 薄膜表面粗糙度也会对载流子迁移率造成影响, 而且较大的粗糙度起伏会对界面态密度产生影响, 进而对其电学性质造成不良影响[15], 关于表面粗糙度对电学性质的影响后面将有讨论. 图 1 不同MoO3掺杂比的rubrene∶MoO3薄膜的AFM图像(图像扫描区域2.0 μm × 2.0 μm) (a) 4∶1; (b) 2∶1; (c) 1∶1; (d) 1∶2; (e) 1∶4 Figure1. The AFM images of rubrene∶MoO3 films under different proportion (scan areas are 2.0 μm × 2.0 μm): (a) 4∶1; (b) 2∶1; (c) 1∶1; (d) 1∶2; (e) 1∶4.
图 2 Rubrene与MoO3在不同掺杂比例下的RMS Figure2. The RMS roughness of rubrene∶MoO3 films under different proportion.
本文利用单晶Si(400)作为衬底, 对不同掺杂比例的混合薄膜做了XRD分析, 如图3所示. 从图3可以看出, 在69.8°方向上出现一个特征峰, 据文献[16]报道, 这个特征峰为Si衬底的特征峰. 44°附近出现的是Si表面SiO2对应的衍射峰. 当混合比例为4:1和2:1 (MoO3浓度较小)时, 33°非晶峰中有明显的衍射峰, 而当混合比例为1∶1, 1∶2和1∶4 (MoO3浓度较大)时, 33°非晶峰中的衍射峰消失, 说明当混合薄膜中MoO3含量较少时, 薄膜表面有Rubrene晶粒出现, 图1(a)和图1(b)中的凸起正说明此问题, 33°非晶峰中的衍射峰即为Rubrene衍射峰. 从图3可以看出, 由于薄膜比较薄, 并且衬底温度是室温, MoO3并未晶化, 因此MoO3的掺杂浓度对混合薄膜的晶体结构不产生影响, 任何比例的混合薄膜都表现为非晶态特征. 图 3 不同MoO3掺杂比的rubrene∶MoO3薄膜的XRD图像 Figure3. The XRD image of rubrene∶MoO3 films under different proportion
23.2.混合薄膜的光学和电学性质 -->
3.2.混合薄膜的光学和电学性质
不同掺杂比例的混合薄膜的光致发光(PL)特征如图4所示. 通过对比不同掺杂浓度的混合薄膜发现, 在所研究的五种比例中, 当rubrene∶MoO3等于4∶1和2∶1时, 混合薄膜的PL谱峰波长均为567 nm附近, 两种比例的能隙值基本相同, 后面的讨论进一步说明这个问题. 随着混合薄膜中MoO3浓度的继续增加, 薄膜的光致发光峰发生蓝移. 从该图可以看出: 混合薄膜在近红外区域(~760 nm)还有一个明显的吸收峰, 而rubrene和MoO3的光学带隙分别为2.2 eV(~565 nm)和3.49 eV(~356 nm)[5,17,18], 且rubrene的最高占据分子轨道(HOMO)和MoO3的导带(CB)并未发生重叠, 其能级结构示意图如图5(a)所示, 对于rubrene和MoO3, 在近红外区域均不会有吸收. 然而, 当rubrene和MoO3混合时, rubrene和MoO3的真空能级突然中断, 如图5(b)所示, rubrene和MoO3之间形成电荷转移络合物[5], 在近红外区域出现吸收, 说明rubrene和MoO3混合薄膜对近红外光比较敏感, 在近红外区域显示出潜在的应用前景. 该图还说明, MoO3的掺杂能使混合薄膜的PL谱峰强度发生明显的改变. 图 4 不同MoO3掺杂比的rubrene∶MoO3薄膜的PL谱 Figure4. The PL spectrum of rubrene∶MoO3 films under different proportion.
图 5 MoO3 和Rubrene的能级结构示意图 (a) MoO3与Rubrene各自能级图; (b) MoO3与Rubrene相互作用后的能级图 Figure5. A schematic diagram of energy level alignments about MoO3 and Rubrene: (a) Before interaction; (b) after interaction
为了利用带间跃迁理论分析光学吸收[19—21], 更好地研究混合薄膜的光学性质, 给出了混合薄膜的吸收光谱, 如图6所示, 其中纵坐标表示吸收率A. 混合薄膜在496 nm和533 nm处出现了多峰结构, 这种多峰的吸收光谱可以归为红荧烯的S0→S0跃迁的达维多夫(Davydov)分裂, 这种分裂现象的产生与红荧烯的结构性质有关[22—24]. 混合薄膜除了出现多峰结构, 在300 nm附近还有明显的吸收峰出现, 这个吸收峰来自于红荧烯, 由红荧烯薄膜中最高占据分子轨道到最低未占据分子轨道之间的吸收系数决定[25]. 混合薄膜的吸收强度明显增强, 且随着掺杂浓度的不同, 吸收还是略有区别, 在所研究的五种成分当中, 除rubrene与MoO3的比例为4∶1时对应的峰值波长较短, 其余四种比例的峰值波长基本相同, 说明在浅紫外区域, MoO3的掺杂对混合薄膜的吸收影响不大, 但随着MoO3掺杂浓度的增大, 混合薄膜在近红外区域出现明显的吸收, 说明rubrene和MoO3相互作用产生中间能级(如图5(b)), 形成电荷转移络合物. 但除混合比例为2∶1的混合薄膜之外, 其他4种比例对应的薄膜表面平整度不好, 表面比较粗糙, 通过对薄膜电学性质的讨论发现, 薄膜表面平整度对薄膜的电学性质产生较大影响. 图 6 不同MoO3掺杂比的Rubrene∶MoO3薄膜的吸收光谱 Figure6. The absorption spectra of Rubrene∶MoO3 films under different proportions
图7给出了不同掺杂浓度的混合薄膜的${(\alpha hv)^2}$和$hv$的关系曲线. 由方程(3)可知, 曲线的斜率与横坐标的交点即为光学带隙. 因此可以得到五种混合比例薄膜的光学带隙宽度如表1所列. 图 7 不同MoO3掺杂浓度的rubrene∶MoO3混合薄膜的${(\alpha hv)^2}$与$hv$的关系曲线 Figure7.${(\alpha hv)^2}$versus $hv$ plot of rubrene∶MoO3 films under different proportion
Rubrene∶MoO3
4∶1
2∶1
1∶1
1∶2
1∶4
能隙Eg/eV
2.24
2.23
2.25
2.25
2.25
表1利用图7关系所得能隙值 Table1.The value of energy gap received from Fig. 7.
分析表格中的能隙值发现, rubrene∶MoO3混合比为2∶1时的能隙值最小, 而在偏离它的两个方向上, 能隙均较大. 因此, 当混合比例为2∶1时, 薄膜中的电子更容易激发, 而且从图1可知, 此种比例下的薄膜表面平整度也是最好的. 本文研究了4∶1, 2∶1, 1∶1, 1∶2, 1∶4五种比例的混合薄膜的电学性质, 图8是测试混合薄膜电学性质的器件结构示意图. 图9是五种比例下混合薄膜的J-V特性. 图 8 Al/rubrene∶MoO3/ITO器件结构示意图(0.8 cm × 0.8 cm) Figure8. Schematic illustration of the Al/rubrene∶MoO3/ITO (0.8 cm × 0.8 cm)
图 9 室温下不同MoO3掺杂浓度的rubrene∶MoO3混合薄膜的J-V特性曲线 Figure9. Current density-voltage characteristics of rubrene∶MoO3 films under different proportion at room temperature