Summary - Organic Bistable Devices With Interfacial Oxide Layers

Chapter 3 Organic Bistable Devices With Interfacial Oxide Layers

3.3 Summary

The I-V characteristics of the OBD, n-type Si/Alq₃/Al, are investigated. The bistability results from the defects of the Al-O compound layer at the Alq3/Al interface. The electrical characteristics of the OBD can be optimized and tuned according to our needs for different situations based on the trends obtained in these experiments. Of course some tradeoffs must be made. Owing to its simple structure, the OBD can be embedded into the conventional silicon-based fabrication processes. Furthermore, the OBD has great potential for high-density data storage, low-cost memory applications in future nanoelectronics.

Chapter 4 Organic Bistable Devices Using Metal Oxide Nanocluster Layers

4.1 Organic Bistable Devices Using Nanostructured Materials

In organic memory, many published results have focused on ORM with nanostructured materials. [7], [26], [62]-[64], [67], [74], [172]-[178] Because of the distinct properties of the nanostructured materials, ORM with high density, large ON/OFF ratios, and other superior performances will be obtained in the near future. The nanostructured materials can consist of a nanocluster/organic-metal composite layer fabricated by a thermal evaporation method [26], [62], [63], [70] (e.g., Al cores covered with Al oxide shells, [26] and Ag islands in TPD, [70]).

The nanostructured materials can also be composed of a nanocomposite of polymer-Au NPs fabricated by chemical synthesis. [7], [64]-[67] (e.g., Au NPs capped with 2-naphthalenthiol, [67] Au NPs capped with dodecanethiol [64]-[67]). In addition, metal NPs, [62], [175] oxide NPs, [74], [172], [117], [178] C60 NPs, [173], [174] and core/shell type CdSe/ZnS NPs [176]

have been introduced to be the nanostructured materials.

MoOx has been extensively applied in organic electronics (e.g., as a doping layer to raise conductivity, [179] as a buffer layer to increase carrier injection [180] or carrier collection, [181] as an intermediate layer to protect underlayers, [182] and as a capping layer to enhance light coupling [181]). Such wide applications attribute to the characteristics of the MoOx thin films: high work function, high conductivity for holes, and high transparency.

In this chapter, we use a nanostructured MoOx layer as the charge storage layer of an OBD.

The OBD consists of a p⁺-Si/Alq3/nanostructured MoOx/Alq3/Al structure, as shown in Fig.

2.4. The OBD exhibits a high ON/OFF ratio up to 10⁵, long retention time over 4000 s, and a rewritable/reerasable feature. The formation of the bistable switching of the OBD is ascribed to the charge trapping effect of the nanostructured MoOx layer. Moreover, the I-V characteristics of the OBD are quite different from those of an OBD using a MoO_x NPs layer.

[74] No NDR is observed in the I-V curves of our OBD. This phenomenon probably results from the dissimilar surface morphology of the MoO_x layer deposited on the Alq₃ thin film.

Because of both the simple structure of our OBD and the merits of Alq3 (e.g., low cost, easiness of preparation, and high stability), the complexity of fabrication and the production cost have potential to greatly reduce. Furthermore, our OBD can be easily embedded into the well developed semiconductor fabrication processes.

4.2 Results And Discussions

The I-V curves of the OBD with the p⁺-SiAlq3/nanostructured MoO_xAlq3Al structure are shown in Fig. 4.1(a). At first sweeping bias (the olive open square curve), a sweeping bias from 0 to 10 V is applied to the OBD with 100 mA current compliance. Initially, the OBD is in a low conductance state which has a current level of 10⁻¹⁰-10⁻⁵ A. An abrupt increase in the current level is observed when the applied voltage is about 3.5 V. Then, the OBD holds at a high conductance state when the applied bias sweeps to higher voltage. It is clear that the OBD undergoes an electrical transition from an OFF state to an ON state. At next sweeping bias, the OBD still maintains at the high conductance state (the red open circle curve).

Obviously, the OBD clearly exhibits two different conductance states with an ON/OFF current ratio up to 10⁵. That is to say, the OBD possesses the nature of bistability. In addition, by applying a reversed sweeping bias form 0 to -10 V (the blue open triangle curve), there is a striking decrease in the current level when the applied voltage is about -5 V. In other words, the OBD is switched from the high conductance state to the low conductance state. The OBD can be switched to the high conductance state again when a following sweeping bias from 0 to

Fig. 4.1 (a) The I-V curves of the OBD with the p⁺-Si/Alq3/nanostructured MoOx /Alq3/Al structure. The olive open squares, the red open circles, and the blue open triangles represent the writing, reading, and erasing sweeping biases, respectively. Inset: fitting of the I-V curve of the high conductance state in a log-log scale. (b) The I-V curves of an OBD with a p⁺-Si/Alq3/nanostructured MoOx/Alq3/Ag structure. The olive solid squares, the red solid circles, and the blue solid triangles represent the writing, reading, and erasing sweeping biases, respectively. Inset: The I-V curves of an OBD with a p⁺-Si/Alq3/Ag structure. The green half solid squares and magenta half solid circles are the first and second bias scans, respectively.

10 V is applied. These results suggests that a sweeping bias form 0 to 10 V and a reversed polarity sweeping bias form 0 to -10 V serve as “writing” and “erasing” processes, respectively. Thus, the OBD holds the essences of a memory: bistability and rewritability.

The bistable switching of the OBD is referred to the nanostructured MoO_x layer sandwiched between the Alq3 thin films. But some published results suggested that an Al-O compound near an Al electrode(s) is responsible for the resistance switching of OBDs with an Al electrode(s). [35], [165], [183] As can be seen in Fig. 4.2, an Al-O compound is generated at the interface between the Al and Alq₃ thin films. Namely, the bistable switching of the OBD could be relative to the Al-O compound. To verify this, we use Ag to replace Al as the top electrode. The bistability is again recognized from the I-V curves of an OBD with a p⁺-Si/Alq3/nanostructured MoOx/Alq3/Ag structure, as shown in Fig. 4.1(b). For comparison, the I-V curves of an OBD composed of a p⁺-Si/100 nm Alq₃/Ag structure are shown in the inset of Fig. 4.1(b). No hysteresis as obtained in the I-V curves of the OBDs with the nanostructured MoO_x layer is observed. Therefore, we can conclude that the nanostructured MoOx layer dominates the bistability of the OBD with the p⁺-Si/Alq3/nanostructured

Fig. 4.2 The Al (2p) XPS curve of the Alq₃/Al interface of the OBD with the p⁺-Si/Alq3/nanostructured MoOx/Alq3/Al structure

MoOx/Alq3/Al structure.

Figure 4.3(a) and (b) show the I-V curves of the p⁺-Si/Alq₃/nanostructured MoO_x/Alq₃/Al structures with 3-nm-thick and 8-nm-thick MoOx, respectively. Both of them exhibit resistive switching behavior as the OBD with 5 nm-thick MoO_x. Nevertheless, the device with 3-nm-thick MoOx displays a large current in low conductance state because of less trap sites.

For the device with 8-nm-thick MoO_x, both the currents of the ON state and OFF state are

Fig. 4.3 The I-V curves of the p⁺-Si/Alq₃/nanostructured MoO_x/Alq₃/Al structure. (a) With 3-nm-thick MoO_x (b) With 8-nm-thick MoO_x.

suppressed as a result of much more trap sites. Note that both OBDs cannot reveal rewritibility as the device with 5 nm-thick MoO_x. Even though the actual cause is still unknown, we can deduce that the thickness of the nanostructured MoOx layer influences the electrical identities of the p⁺-Si /Alq₃/nanostructured MoO_x/Alq₃/Al structures, and that the optimized thickness is 5 nm in our case.

The ON/OFF state switching of the OBD shown in Fig. 4.1 primarily results from hole trapping/de-trapping provided by the nanostructured MoOx layer, as shown in Fig. 4.4. At the low applied voltage of the first sweeping bias in Fig. 4.1(a), the current is very small. Most of holes injected from the p⁺-Si into the OBD are trapped by charge trapping centers given by the nanostructured MoO_x layer. As a result, the OBD stays at the low conductance state. By applying a voltage above the threshold, numerous holes are injected into the OBD, and the charge trapping centers are filled sufficiently. Then, the nanostructured MoO_x layer is polarized, and the interfacial resistance of the nanostructured MoOx layer is decreased. The OBD is switched to the high conductance state. On the contrary, while an adequate

Fig. 4.4 The iIllustration of the writing and erasing processes of the OBD using a MoOx

nanoclusterlike layer.

reversed bias is applied on the OBD, the trapped holes in the charge trapping centers are released. The OBD is restored to the low conductance state.

By fitting the I-V curve of the high conductance state of the OBD in a log-log scale, linear relation between current and voltage is found, as shown in the inset of Fig. 4.1(a). Such relation indicates that carrier transportation at the high conductance state is influenced by space charges. In addition, NDR observed in the I-V curves of an OBD using a MoO_x nanoparticles layer [74] is not obtained in Fig. 4.1(a). Such dissimilarity probably originates from the surface morphology of the MoO_x layer. As shown in Fig. 4.5(a), the surface morphology of a 5-nm-thick MoOx layer grown on the p⁺-Si/Alq3 is not a nanoparticle feature, but a nanoclusterlike feature. Polarization resulted from the charge trapping of the MoO_x nanoclusterlike layer is more random than that of the MoOx nanoparticles layer. This means that the effective polarization effect on the carrier transportation of the MoO_x nanoclusterlike layer is weaker than that of the MoOx nanoparticles layer, as shown in Fig. 4.5(b). Therefore, after switched from the low conductance state to the high conductance state, our OBD maintained at the high conductance state without NDR. These results also suggest that the

Fig. 4.5 (a) The surface morphology of the 5-nm-thick MoO_x layer deposited on the p⁺-Si/Alq₃. (b) The illustration of the effective fields of ordered and random dipoles.

surface morphology of a nanostructured MoOx layer plays an important role in the electrical characteristics of OBDs using nanostructured MoO_x.

Two important properties of our OBD are shown in Fig. 4.6, retention time and rewritable/re-erasable ability. The retention measurement of our OBD was carried out by

Fig. 4.6 (a) The retention measurement of the OBD with the p⁺-Si/Alq3/nanostructured MoOx/Alq3/Al structure. The red open squares and blue open circles correspond to the high and low conductance states. (b) The reading currents after writing and erasing of the OBD with the p⁺-Si/Alq3/nanostructured MoOx/Alq3/Al structure for the first eight cycles. The red solid squares and blue solid circles correspond to the high and low conductance states.

applying a bias at 1 V, as shown in Fig. 4.6(a). There is no appreciable change in the current of the high conductance state. Oppositely, current fluctuation occurs in the low conductance state. The current fluctuation of the low conductance state is because of the incomplete erasing of trapped charges after an erasing process. Nevertheless, the current of the low conductance state is getting smaller and more stable with time, and the ON/OFF current ratio becomes larger. Overall, a clear conductance difference between ON/OFF states can be recognized, and our device possesses long retention time over 4000 s. Fig. 4.6(b) shows the currents of the OBD at reading voltage after writing (high conductance state) and erasing (low conductance state). The reading voltage, writing sweeping bias, and erasing sweeping bias are 1 V, from 0 to 10 V, and from 0 to -10 V, respectively. The current of the high conductance state of each cycle is quite stable. However, the current fluctuation of the low conductance state of each cycle is again observed because of the incomplete erasing of trapped charges. In spite of the current fluctuation of the low conductance state, the ON/OFF current ratio of each cycle can still be easily distinguished.

4.3 Summary

The electrical properties of an OBD with a p⁺-Si/Alq3/nanostructured MoOx/Alq3/Al structure are studied. We show that the bistability of the OBD mainly attribute to the charge trapping effect of the MoOx nanoclusterlike layer interposed between the Alq3 thin films. The resistance switching without NDR is observed in the I-V curves of the OBD, which differs from those of an OBD with a MoOx nanoparticles layer. According to the results in this chapter, we can figure out that the electrical characteristics of OBDs using nanostructured MoOx layers are largely affected by the surface morphology of the nanostructured MoOx

layers.

Chapter 5 High Open Circuit Voltage Metal-Insulator-Semiconductor Solar Cells

5.1 A Stacking Metal-Insulator-Semiconductor Solar Cell

Extending photovoltaic applications plays a prominent role in the following evolution of photovoltaics. Using solar cells to convert solar energy for hydrogen production (solar hydrogen) [184]-[186] can be considered a promising application (a perfect renewable energy source), as shown in Fig. 5.1(a). Generated hydrogen can convert into not only thermal energy but also electricity and mechanical energy, and return to water finally (a clean, renewable energy cycle), as shown in Fig. 5.2(b).

One of essential factors in solar hydrogen is that the Voc‟s of solar cells have to be larger than the dissociating voltage of water (1.23V). [184]-[186] However, the Voc‟s of conventional single junction solar cells are not compatible with the dissociating voltage. For matching the requirement, we can probably adopt following four strategies.

Fig. 5.1 (a) The illustration of converting solar energy for hydrogen production using photovoltaics. (b) The clean, renewable energy cycle of solar hydrogen.

(a) Increasing the n of solar cells

According to Eq. (1.2), increasing n can make V_oc larger, but it degrades the FF as well. [159]

(b) Reducing the surface defects of solar cells

Surface defects enlarge the surface recombination rates of solar cells, and thus lower V_oc. For reducing surface recombination rates, many surface passivation methods (such as SiNx passivation [155], and AlOx passivation [187]) are demonstrated to decrease surface defects, and therefore, enhance V_oc. Unfortunately, the augment is limited. Take crystalline silicon solar cells with excellent surface passivation for example. The measured V_oc‟s are unable to be larger than the built-in voltages of pn junctions (~0.7 V).

For single junction solar cells, the upper limits of Voc‟s are the bandgaps of active layers. It suggests that we can use active materials with larger bandgaps to get higher Voc‟s. Nevertheless, using the active materials will make less incident photons be absorbed by solar cells. [188]

(d) Integrating solar cells with different bandgaps (tandem solar cells)

It is an effective way to raise V_oc, and incident photons can be efficiently absorbed by solar cells. [103]

Based on the concept of tandem solar cells, we propose a stacking MIS (crystalline silicon) solar cell, which integrates an n-type MIS (crystalline silicon) solar cell with a p-type MIS one, to effectively increase Voc, as shown in Fig. 2.7. There are two advantages using MIS (crystalline silicon) solar cells. First, the fabrication of MIS (crystalline silicon) solar cells is low-cost, low-temperature, simple processes. Second, the V_oc‟s of MIS (crystalline silicon) solar cells are comparable with those of pn junction (crystalline silicon) solar cells.

This is because the ultrathin oxide layers can block the majority carriers from transporting to

the metal electrodes, and are easy for minority carriers to tunnel through, so the effect of carrier recombination at the MS junctions is vastly decreased. [151], [152], [159], [190], [191]

Our experimental results show that by comparing with other MIS solar cells with the same output power, this stacking structure has potential to give high Voc and low Jsc, which can reduce the electrical loss in MIS solar cell applications. Furthermore, this stacking structure shows an effective way to enlarge V_oc and has potential to achieve a V_oc larger than 1.23 V, although the Voc‟s of conventional MIS solar cells are not high enough to match the requirement for dissociating water (1.23 V) [184]–[186]. Based on the stacking structure, we believe that MIS solar cells are very promising for photoelectrochemical (PEC) water splitting and that monolithic MIS photovoltaic-PEC devices can be realized in the future for both solar photovoltaics and solar hydrogen.

5.2 Results And Discussions

For obtaining the optimized process conditions of the top and bottom cells of the stacking MIS solar cell, we fabricated n-type and p-type MIS solar cells under different process conditions, and analyzed their characteristics.

The red square curves of Fig. 5.2(a) show the J-V curves of the n-type and p-type MIS solar cells with as-deposited ultrathin SiO₂ layers deposited at 20 mTorr under light illumination, and the measured parameters are listed in Table 5.1. The MIS solar cells exhibit poor photovoltaic characteristics probably because of not good interface properties between the ultrathin SiO2 layers and Si. To analyze the interface properties of the n-type and p-type MIS solar cells, high-frequency C-V curves were recorded, as shown by the red square curves in Fig. 5.2(b). Large capacitance leakage is observed in both C-V curves. It indicates that there are a great number of trap states at the interfaces, such as silicon dangling bonds and silanol groups. [192], [193] Charge carriers can flow via the trap states to cause current leakage, and the capacitance is reduced. To passivate these trap states, H₂ annealing was

introduced. The blue circle curves in Figs. 5.2(a) and (b) show the J-V and C-V curves of MIS solar cells with additional H₂ annealing, respectively. The MIS solar cells with additional H₂ annealing exhibit better photovoltaic properties, and their C-V curves display smaller drops at the accumulation regions. These attribute to the formation of Si-H bonds at the interfaces, and then the trap states are diminished and the leakage currents are decreased. Therefore, H2

Fig. 5.2 (a) The J-V curves of MIS solar cells with and without H2 annealing under light illumination. (b) Corresponding C-V curves. All capacitance values are normalized by the largest value of each curve.

annealing can improve the performance of MIS solar cells with ultrathin SiO2 layers deposited by RF sputtering because of the passivation of trap states.

The working pressure of depositing the ultrathin SiO2 layers plays an important role in the V_oc of the MIS solar cells with ultrathin SiO₂ layers. The J-V curves of n-type and p-type MIS solar cells with fixed ultrathin sputtering SiO2 layer thicknesses (n-type with about 2 nm and p-type with about 1 nm) deposited under different working pressures with H₂ annealing are shown in Fig. 5.3 and measured parameters are listed in Table 5.2. For the p-type MIS solar cells, 475 mV, 386 mV, and 350 mV open-circuit voltages, corresponding to working pressures at 20 mTorr, 30 mTorr, and 40 mTorr, respectively, are obtained. The Voc‟sof the n-type MIS solar cells are 422 mV, 339 mV, and 313 mV corresponding to working pressures at 20 mTorr, 30 mTorr, and 40 mTorr, respectively. The relationships between the Voc‟s and the working pressures during deposition are shown in Fig. 5.4(a). For both MIS solar cells, lower working pressures results in larger Voc‟s.

The corresponding C-V measurements are shown in Fig. 5.4(b). The C-V curves of the n-type/p-type MIS solar cells shift toward positive/negative bias with decreasing working pressure. At a lower working pressure, Ar⁺ ions get more energy to sputter a SiO₂ target due to a larger mean free path. As a result, more surface charges are introduced in the ultrathin sputtering SiO₂ layers on the Si substrates because of oxygen vacancies. Moreover, the Table 5.1 The measured parameters of MIS solar cells with and without H₂ annealing.

deposition rates of the ultrathin sputtering SiO2 layers increase with decreasing working pressure in our case. A higher deposition rate reduces the ion bombardment duration at the interfaces. As a result, less trap states are produced, and consequently, Voc is enlarged.

According to these two factors, the C-V curves of the p-type MIS solar cells shift toward a Table 5.2 The measured parameters of MIS solar cells with the fixed thicknesses of ultrathin sputtering SiO2 layers (n-type ~ 2 nm, p-type ~ 1 nm) deposited under different working

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