*3.1. ZnO Electron Collection Layer*

To find the optimal annealing temperature for ZnO films, electron-only devices were fabricated to evaluate the conductivity of ZnO films under different thermal annealing conditions [25]. Figure 2 shows the I-V characteristics of Al/ZnO/Al electron-only device, with 400 nm thick ZnO. Table 1 summarizes the conductivity extracted from the I-V characteristics. The film made with the ramping fabrication method shows the highest conductivity among all films, but the difference is weak, and the conductivity values are in the same order of magnitude. A factor of 3 was observed between the lowest temperature annealing temperature (125 ◦C) and the ramping method (350 ◦C). It is obvious that the conductance of the ZnO thin film becomes higher when the film was annealed at higher temperature. Generally, a high annealing temperature provides a better crystallinity of the films, which directly impacts on the conductivity [26,27]. The reaction phenomena of ZnO sol-gel is reported in a previous report [28]. The ZnO precursor (zinc acetate dehydrate) film is highly resistive, because of the acetate functional group. Therefore, an annealing process is required to induce the reaction between the ZnO precursor with 2-methoxyethanol and the oxidation of a ZnO precursor in air. However, thermal annealing treatments of higher than 200 ◦C were not considered in this study, since the high annealing temperature of CRL could cause damage on the underlying layers in a tandem structure. It is then shown that the annealing temperature could be lowered to 125 ◦C.

**Figure 2.** I-V characteristics of an Al (100 nm)/ZnO (400 nm)/Al (100 nm) electron-only diode device with a ZnO thickness of 400 nm.

**Table 1.** Conductivity value of the ZnO films under different annealing temperatures.

