-
本文中采用渡越时间(time-of-flight, TOF)方法对Alq3中载流子迁移率进行测量。该方法的电路原理如图 2所示。虚线框A中为蒸镀好的三明治式薄膜样品,其中厚度为L的Alq3薄膜两侧为电极,薄膜左侧为ITO电极,与电源负极相连接,右侧为Al电极,与电源正极相连接。R为取样电阻,包括外加电容C(积分电容)以及其它杂散电容在内的所有电容用虚线框B中的符号表示。外加直流电压为U,样品厚度为d。样品中有稳定的电场,其大小为:
$ {E_0} = \frac{U}{d} $
(1) 由于电源对样品的充电,ITO电极和Al电极上分别储存有一定的电荷,分别用-Q和+Q表示。当一个脉冲宽度小于载流子渡越时间的外激光脉冲照射到样品上时,光脉冲产生的载流子将在外电场作用下做宏观定向移动。t时间后距坐标原点的距离为x(取负极为坐标原点,水平向右为正方向)。由于静电感应现象的存在,t时刻,ITO电极上的总电荷为-Q+q1,Al电极上的总电荷为Q+q2(其中q1, q2为感应电荷)。由于感应电荷的出现,样品中的电场将以载流子层为分界层左右发生变化。假设载流子层左侧电场为$\vec E$l,右侧电场为$\vec E$r,由于电源电压不发生变化,因此:
$ {E_{\rm{l}}} \times x + {E_{\rm{r}}} \times \left( {d - x} \right) = {E_{\rm{0}}} \times d $
(2) 载流子生成之前,对Al电极应用高斯定理有:
$ S \cdot {\varepsilon _0}{\varepsilon _{\rm{r}}}{E_0} = Q $
(3) 式中,S为载流子层的面积;ε0和εr分别为真空电容率和样品的相对电容率。
载流子生成之后,对ITO电极和Al电极分别应用高斯定理有:
$ - S \cdot {\varepsilon _0}{\varepsilon _{\rm{r}}}{E_{\rm{l}}} = - Q + {q_1} $
(4) $ S \cdot {\varepsilon _0}{\varepsilon _{\rm{r}}}{E_{\rm{r}}} = Q + {q_2} $
(5) 联立方程(2)式~(5)式解得:
$ \left\{ \begin{array}{l} {q_1} = Ne\left( {1 - \frac{x}{d}} \right)\\ {q_2} = Ne\frac{x}{d} \end{array} \right. $
(6) $ \left\{ \begin{array}{l} {E_{\rm{l}}} = {E_0} - \frac{{Ne}}{{{\varepsilon _0}{\varepsilon _{\rm{r}}}S}}\left( {1 - \frac{x}{d}} \right)\\ {E_{\rm{r}}} = {E_0} + \frac{{Ne}}{{{\varepsilon _0}{\varepsilon _{\rm{r}}}S}} \cdot \frac{x}{d} \end{array} \right. $
(7) 式中,N为激励的总载流子数, e为电子电量。由于载流子在运动过程中受到深陷阱和浅陷阱的影响而不断损失,因此,N将不再保持不变。
ITO以及Al电极上的电荷会由于载流子层的运动而重新分布,当载流子层运动到位置x时,Al电极上的电荷变化量为:
$ \Delta Q = {\varepsilon _0}{\varepsilon _{\rm{r}}}S\Delta {E_{\rm{r}}} = \frac{{Nex}}{d} $
(8) 取样电阻两端电压相应发生变化:
$ \Delta U\left( t \right) = \frac{{Ne}}{{Cd}}x = \frac{{Ne}}{{Cd}}vt $
(9) 式中,v为载流子运动速率,t为时间, C为电容。
当tRCtTOF(其中,tRC为外电路的RC响应时间,tTOF为渡越时间)时,取样电阻上将会产生电流脉冲:
$ i = \frac{{Ne}}{d}v $
(10) 由电流脉冲测量出载流子运行时间,利用下式可计算出载流子的迁移率μ:
$ \mu = \frac{{{L^2}}}{{U{t_{{\rm{TOF}}}}}} $
(11) 然后讨论不同的实验条件对载流子迁移率的影响,给出测量载流子迁移率的最佳实验条件。
-
Scher-Montroll模型就是利用统计力学的概率性原理研究弥散输运中载流子的迁移率问题[15]。假设材料内部存在大量的规则晶胞,但这些晶胞的分布是随机的[16]。电荷的输运就是从一个晶胞到另一个晶胞的过程。由于晶胞的随机分布,载流子的移动速率将随机变化,以至于TOF信号会出现一个长长的尾巴[17]。SCHER和MONTROLL用跳跃时间的分布函数Ψ(t)≈cons(t-(1+α))来描述这种随机分布的影响, 其中α∈(0, 1)。由分布函数可见:α值越小,弥散输运的程度越高[18]。载流子在连续两个晶胞之间移动的平均距离用〈l〉表示,〈l〉∝tα,载流子移动形成的电流用下式表示:
$ I\left( t \right) \propto \left\{ \begin{array}{l} {t^{ - \left( {1 - \alpha } \right)}},\left( {\left\langle l \right\rangle < L} \right)\\ {t^{ - \left( {1 + \alpha } \right)}},\left( {\left\langle l \right\rangle \ge L} \right) \end{array} \right. $
(12) 式中,L表示样品薄膜的厚度,此即幂指数定律[19]。根据此式,采用对数坐标变换电流和时间坐标轴,就可以得到载流子的渡越时间。
-
对于Alq3来说,取样电阻值设置为1.5kΩ,光脉冲能量为1.5μJ,外加电压为5V。图 3是Alq3样品中空穴的迁移率随温度的变化关系。
利用同样的方法测量了Alq3中电子的迁移率随温度的变化关系,见图 4。
从图 3和图 4可以看出:对于Alq3样品来说,其内部电子的迁移率在308K~338K的温度范围内与温度的关系满足线性关系,而其内部空穴的迁移率在同样的温度范围内波动较大,因此认为Alq3样品是良好的电子给体, 这与参考文献[20]中报道的基本一致。
-
TOF方法测量载流子迁移率的实验电路,相当于一个RC回路,其中取样电阻是电阻箱,而样品相当于一个平行板电容器,这样,实验的电路部分就构成了一个RC电路。TOF方法的一个基本实验条件即必须是小取样电阻,满足外电路的RC响应时间tRC小于载流子的寿命Tr,这样才能够保证测量的精度。图 5是Alq3中空穴的渡越时间与取样电阻的关系。用同样的方法得到了Alq3中电子的渡越时间与取样电阻的关系, 如图 6所示。
从图 5和图 6可以看出:对于Alq3样品,当取样电阻小于15kΩ时,样品内载流子的渡越时间基本处于恒定状态,在这个电阻范围内,载流子渡越时间基本不受取样电阻的影响。
-
TOF方法的另一个重要条件就是弱光注入,这样才不至于导致体激发。由于实验条件以及光脉冲能量的限制,实验过程中测量了光能量从1.5μJ~3.5μJ等不同光强时的渡越时间,得到了Alq3中电子载流子的渡越时间随光脉冲能量的变化关系,如图 7所示。
从图 7可以看出,当脉冲光的能量不大于3.5μJ时,Alq3中的光生载流子接近薄层分布,反映到渡越时间上就是渡越时间的值基本保持恒定,从而保证了测量的精度。
8-羟基喹啉铝本征薄膜的制备与性质研究
Preparation and study on characteristics of 8-hydroxy-quinoline aluminum film
-
摘要: 8-羟基喹啉铝属于有机半导体材料,在太阳能电池应用领域有较为广阔的应用前景。为了研究8-羟基喹啉铝载流子输运动力学信息,在恒温条件下制备了8-羟基喹啉铝薄膜,采用X射线衍射分析方法对薄膜的性质进行了分析,采用渡越时间方法对影响其载流子迁移率的实验条件进行了理论分析和实验验证。结果表明,在308K~338K温度范围内,8-羟基喹啉铝的载流子输运规律符合浅陷阱模型;取样电阻小于15kΩ及光脉冲能量低于3.5μJ时,载流子渡越时间保持恒定,测试结果可靠。这一结果对有机太阳能电池的制备是有帮助的。Abstract: 8-hydroxy quinoline aluminum is an organic semiconductor material and has broad application prospect in the field of solar cell application. In order to study the transport dynamic information of carriers, 8-hydroxyl quinoline aluminum thin film was prepared under the condition of constant temperature. The method of X-ray diffraction was used to analyze the properties of the film and the method of time-of-flight(TOF) was used to study the experimental conditions affecting the carrier mobility by theoretical analysis and experimental verification. The results show that this method is feasible. The carrier transport law of 8-hydroxy quinoline aluminum in the temperature range of 308K~338K is in accordance with shallow trap model. When the sampling resistance is less than 15kΩ and pulse energy is less than 3.5μJ, the carrier TOF remains constant. The test results are reliable. The result is helpful for the preparation of organic solar cells.
-
Key words:
- optoelectronics /
- transport dynamics /
- time-of-flight method /
- mobility
-
[1] SUNDAR V C, ZAUMSEIL J, PODZOROV V, et al. Elastomeric transistor stamps:reversible probing of charge transport in organic crystals[J]. Science, 2004, 303(5664):1644-1646. doi: 10.1126/science.1094196 [2] LUO Y, BRUN M, RANNOU P, et al. Growth of rubrene thin film, spherulites and nanowires on SiO2[J]. Physica Status Solidi, 2007, 204(6):1851-1855. doi: 10.1002/pssa.v204:6 [3] PARK S W, JEONG S H, CHOI J M, et al. Rubrene polycrystalline transistor channel achieved through in situ vacuum annealing[J]. Applied Physics Letters, 2007, 91(3):26-43. [4] DENG J X, KANG Ch L, YANG B, et al. Deposition and characterization of vacuum evaporated Rubrene films[J]. Chinese Journal of Vacuum Science and Technology, 2012, 32(8):678-681(in Ch-inese). [5] SHI Y M, ZHOU W, LU A Y, et al. Van der waals epitaxy of MoS2 layers using grapheme as growth templates[J]. Nano Letters, 2012, 12(6):2784-2791. doi: 10.1021/nl204562j [6] YU L L, HAN W, LEE Y H, et al. Graphene/MoS2 hybrid technology for large-scale two-dimensional electronics[J]. Nano Letters, 2014, 14(6):3055-3063. doi: 10.1021/nl404795z [7] HUO N J, KANG J, WEI Zh M, et al. Novel and enhanced optoelectronic performances of multilayer MoS2-WS2 heterostructure transistors[J]. Advanced Functional Materials, 2014, 24(44):7025-7031. doi: 10.1002/adfm.201401504 [8] CHEN Y Q, ZHANG Ch T, ZHANG J H. Simulation of temperature field of graphene substrate fabricated by laser chemical vapor deposition[J]. Laser Technology, 2015, 39(5):648-653(in Chinese). [9] HOU Zh J, TANG X, LUO M W, et al. Study on laser-induced chemical liquid deposition Fe film[J]. Laser Technology, 2016, 40(1):136-140(in Chinese). [10] ZHANG N, ZHOU B Q, ZHANG L R, et al. Research of hot wire chemical vapor deposition and micro-structure of a-SiNx: H thin film[J]. Laser Technology, 2016, 40(3):413-416(in Chinese). [11] LEE Y H, ZHANG X Q, ZHANG W J, et al. Synthesis of large-area MoS2 atomic layers with chemical vapor deposition[J]. Advanced Materials, 2012, 24(17):2320-2325. doi: 10.1002/adma.201104798 [12] SHI G, MEI L, GAO J S, et al. DUV LaF3 thin film deposited by IBS, thermal boat and electron beam evaporation[J]. Laser Technology, 2013, 37(5):592-595(in Chinese). [13] JIA F, CAO P J, ZENG Y X. Effect of substrate temperature on the properties of ZnO prepared with pulsed laser deposition method[J]. Laser Technology, 2010, 34(3):357-359(in Chinese). [14] LIU Q H, ZHANG Z H, LIU Zh Ch, et al. Study on measurement method of carrier mobility in weak photoconductive material[J]. Laser Technology, 2014, 38(4):445-448(in Chinese). [15] HUANG Ch H, LI F Y, HUANG Y Y. Optoelectronic functional ultrathin films[M]. Beijing:Peking University Press, 2004:238-298(in Chinese). [16] SCHER H, ELLIOTT W, MONTROL L. Anomalous transit-time dispersion in amorphous solids[J]. Physical Review, 1975, B12(6):2455-2477. [17] WU F, TIAN W J, ZHANG Zh M, et al. Organic electioluminescent device based on balanced carriers injection and transportation[J]. Thin Solid Films, 2000, 363(1):214-217. [18] BASSLER H. Hopping conduction in polymers[J]. International Journal of Modern Physics, 1994, B8(7):847-854. [19] FORERO S, NGUYEN P H, BRUTTING W, et al. Charge carrier transport in poly(p-phenylenevinylene) light-emiting devices[J]. Physical Chemistry Chemical Physics, 1999, 1(8):1769-1776. doi: 10.1039/a808614a [20] LIN P, LIANG Ch J, DENG Zh B, et al. Photovoltaic character of organic el devices MEH-PPV/Alq3[J]. Spectroscopy and Spectral Analysis, 2005, 25(1):23-25(in Chinese).