TiO 2 Surface Cleaning

One of the most attractive advantages of organic solar cells is the low cost production process, mainly attributed to the ultra-thin films that could allow roll-to-roll processing to be applied and the cheaper materials synthesis matters to reduce the cost of devices production [16]. On the other hand, this is a challenge for the device architecture because thin film design will increase the chance of short circuits induced by the defects in the thin film or on its surface. In this case, a smooth and dense blocking layer is necessary for the inverted blend solar cells. As described

in the fabrication process of spray pyrolysis (see section 2.2.2.1), TiO2 film is obtained by the pyrolysis of organic precursor at 450°C in open air, with the substrates placed in a chemical fume hood. The whole process takes about 2 hours. During the fabrication process explored in air, defects on the TiO2 surface forms due to the incomplete decomposition of the TiO2 precursor, as well as the dust particles in the environment falling down to the film. The existence of defects is confirmed in Chapter 4, which can significantly influence the device performance. The particles attached on the TiO2 film surface can increase the chance of short circuits in the device during characterizations. Therefore a proper cleaning process on the TiO2 film is necessary for a working device.

Mechanical Cleaning

Figure 3.1 TiO2 films magnified image before (a) and after (b) mechanical cleaning

Initially, TiO2 films are cleaned by a mechanical method after fabrication. The process is described as follows: the surface of the TiO2 films is wiped using a cotton Q-tip, keeping the sample in a petri dish filled with ethanol. Photographic images of TiO2 films surface are shown in Figure 3.1. It is obvious that the mechanical cleaning can efficiently remove the particles on the film to obtain a macroscopically clean and smooth surface.

Ultrasonic Cleaning

3.1 Blocking layers improvement

substrates are placed in a beaker filled with ethanol and ultrasonic cleaned for 2 min. Then blend solar cells are produced on TiO2 films. The I-V characterizations of samples cleaned by three different cleaning processes: no cleaning, mechanical cleaning and mechanical cleaning followed by ultrasonic cleaning, are shown in figure 3.2. The I-V curves exhibit that the performance of devices are influenced by the different cleaning steps. A significant increasing of PCE is observed from the I-V curves of TiO2 solar cells after cleaning. After the first mechanical cleaning step, PEC of device enhance from 1.53% to 1.72%, while a further improvement to 1.96% after the second step of ultrasonic cleaning. As a result, a nearly 30% increasing is observed for the device built on the TiO2 been cleaned by both mechanical cleaning and ultrasonic cleaning.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 -7 -6 -5 -4 -3 -2 -1 0

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Figure 3.2: I-V curves of ITO/ TiO2/ P3HT:PCBM/ Ag devices treated by different cleaning methods on the

TiO2 surface. No cleaning (black squares), Mechanical cleaning (red circles) and Mechanical-ultrasonic cleaning

(green triangles). Samples are characterized under illumination with simulated AM 1.5G solar light (100 mW/cm2).

Plasma Cleaning

Plasma cleaning is widely accepted as an efficient method to modify the surface of ITO [129-131], in order to clean the organic impurities formed during production or the other

processing steps, and to improve the hydrophilic property for a better coating of polymer layer. The plasma cleaning is assumed to be favorable for TiO2 surface, with expectation of removing the organic compounds remained from the decomposition of precursor and the organic cleaning solution. Therefore blend solar cells are fabricated after a plasma cleaning process has been applied to the TiO2 blocking layer, and I-V measurements are recorded for the devices performance. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -7 -6 -5 -4 -3 -2 -1 0 -0.3 0.0 0.3 0.6 0.9 1E-5 1E-4 1E-3 0.01 0.1 1 10

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Figure 3.3 I-V curves of device with TiO2 surface cleaned by O2 plasma. Characterization implement on samples

as prepared and after storing in dark for 1 and 4 days Samples are characterized under illumination with simulated AM 1.5G solar light (100 mW/cm2).

As shown in Figure 3.3, an unexpected degradation is observed for the plasma cleaned TiO2 film. With the apparent S shape shown in the I-V curve, the performance of device is significantly reduced attributed to the low ISC, VOC, and FF (below 25%). When stored in dark for 1 day, obviously this sample recovers in terms of devices performance, and this improvement increases with increasing aging time. Four days after being produced, the devices perform remarkably well with an enhanced efficiency by a factor of 3 compared to those freshly prepared.

3.1 Blocking layers improvement

It is interesting to observe this reduced performance caused by plasma cleaning on TiO2 surface, although this is not a helpful optimization for its fabrication process. The reason need to be further investigated fundamentally, however many previous research confirmed that plasma surface treatment could change the oxygen binding state [130, 131]. Therefore it is reasonable assumed that extra oxygen band generated by oxygen plasma can attach to the TiO2 surface. As a direct result, the work function is changed and the charge transport will be influenced, which is represented by the S-shape of I-V curve. Additionally, the O- rich surface is more active under UV light illumination, resulting in photogeneration of electron-hole pairs. Electrons are very efficiently transferred to bound oxygen to generate the superoxide radical anion O-2, which in turn can deacidize many organic compounds including polymers. The photogenerated holes can react with surface hydroxyl groups to produce TiOH+ that again can react and degrade adsorbed organic compounds [49, 132], and reduce the device performance. On the other hand, since the solar cells do recover after a few days, it suggests that the Ti-O binding state induced by plasma is not stable, and the scission of this chemical bond takes place in the low-oxygen environment [133]. More evidences are required to support the assumption given above, such as XPS measurement which is responsible for electrochemical characteristics of the TiO2 film

surface. Limited by the measurement equipment, further research of oxygen plasma working mechanics has not implemented. Since the initial results demonstrate that plasma on TiO2 film

causes negative effect on their performance, therefore this step will not be included in the cleaning process for device fabrication.