— Photographs showing the operation of the tandem device: (a) low ambient, RLCD-off; (b) low ambient, RLCD-on; (c) high ambient, RLCD-off; (e) high ambient, RLCD-on. OLED is always lit on in all cases. 

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... measurement of the A-CR was accomplished by controlling the optical intensity of a He–Ne laser illumi- nated normally upon the stacked device. Without the pho- toluminescence effect, our green-emission OLED, with peaks at 540 nm for the top and bottom sides, respectively, was transparent to the red emission from the He–Ne laser, with a peak wavelength at 632.8 nm. The reflected signal and light emission from the OLED were determined by a photodetector from the normal direction via a beam splitter. To measure the A-CR, a broadband white-light source with a photometric unit is more suitable. Although the definition of the light source and measuring unit were different from the original cases, the results were still meaningful. 8 Figures 3(a) and 3(b) show the measured A-CR at normal incidence for the RLCD, OLED, and (RLCD + OLED) modes of operation, corresponding to the device structures shown in Figs. 1(a) and 1(b), respectively. Here, a NB VALC was used for the RLCD operation. The dots represent the experimental results while the lines represent the fit- tings with Eq. (1) without any adjustable parameter. Under low ambient, the A-CR of the OLED was extremely high, but decreased sharply as the ambient intensity increased. On the other hand, the A-CR of the RLCD held stable at ~105:1 and was insensitive to the ambient light intensities for both device configurations. When the RLCD and transparent OLED were both turned on, the overall A-CR was even higher than that for the individual devices. Compared with Fig. 3(a), the A-CR in Fig. 3(b) was higher for the T- and (R + T)-modes because the transparent OLED was placed on the polarizer of the RLCD, which decreased the optical loss and increased the transmission of the T-mode. As shown in Table 1, one can also see that there was a tre- mendous improvement (112%) in the maximum transmittance when placing the OLED on top of RLCD. In this configuration, the transmittance was as high as 75.7%, sur- passing even the theoretical limits of a transflective LCD, i.e., 50%. To simplify the fabrication process and further improve the transmittance, a PCGH RLCD was employed. Figure 3(c) shows the A-CR measurement at different ambient intensities for the configuration shown in Fig. 2(d). One can see that the A-CR for the R-mode was only 13.3, comparable to the A-CR of a sheet of newspaper. The maximum transmittance, as shown in Table 1, could go as high as 82.2%. However, the overall A-CR when turning on the OLED and RLCD was lower than that for the VA LC as shown in Figs. 3(a) and 3(b). This resulted from the low A-CR of the PCGH RLCD. A benchmark for the A-CR values of this work, compared with the existing transflective LCD, can be a CR > 100:1 over 85° and CR > 10:1 over 70° for the T- and R- modes, respectively. 18 In our tandem device, for T-mode operation, because there is no light leakage during OLED off-state, the CR ratio is infinite at all viewing angles. Besides, the vertically stacking structure increases the aperture ratio of the T-mode (as well as R-mode) region compared with a transflective LCD. Hence, under the same display luminance and ambient condition, A-CR under T-mode operation in our tandem device is always higher than that of a conventional transflective LCD. In our previous paper, 8 for R-mode operation, our simulation results showed that a CR > 2:1 over a 55° viewing cone (in this case, A-CR is equal to CR) can be achieved by using VA-LC. For the PCGH case which is an absorption-type LC, the CR is typically limited to ~5:1 due to the insufficient dichroic ratio of the guest dye. 6 Figure 4 consists of photos taken under different ambient intensities (Thorlabs OSL1 Fiber illuminator), which illustrate the operation principles of the tandem device. A sheet of white paper was used as a diffused reflector. The OLED with green emission was always lit for all the cases. Figures 4(a) and 4(b) show the cases under low ambient (below 1 nit). There was no discernible difference when the RLCD was turned off (a) or on (b). On the other hand, the transparent OLED was clearly visible which meant the T-mode of this transflective device was functioning properly under low ambient. One can also see some parallax images of the OLED at the right and bottom sides in Fig. 4(b), which originated from the multiple reflections of the glass substrate due to the increased reflection of the RLCD. Figures 4(c) and 4(d) demonstrate the device appearance under high ambient (over 10,000 nits). One would not be able to observe the OLED emission (although it was always on) because it was completely washed out by the high ambient. On the other hand, one could switch the RLCD off and on, as shown in Figs. 4(c) and 4(d), respectively, under high ambient. For the human eyes, the RLCD and OLED were the dominant device at high and low ambient, respectively. A high contrast for this tandem device was obtained with both the RLCD and OLED on [as shown in Figs. 4(b) at low ambient, and 4(d), at high ambient] and off simultaneously. As shown in Figs. 3 (a)–3(c), although A-CR measurements were meaningful, however, (1) the light source was a well-collimated monochrome laser beam in our experiments and (2) unwanted light was filtered out by a spatial filter. These may lead to the wrong impression in real devices, as shown in Figs. 2(c) and 2(d): that the contrast ratios in such mechanical stacking devices are also high (105 and 13.3 for VA and PCGH devices, respectively) in real ambient illumination. By analyzing the gray levels in Figs. 4(c) and 4(d), which are 44 and 87, respectively, A- CR = 1.97 at the “display region” can be achieved. Such a low A-CR resulted from (1) broadband light-source illumination, (2) oblique incident light, (3) scattering from the white paper, and (4) complex interface reflections. By using a laser as the light source, together with a pinhole to filter out the reflection at different interfaces (air/glass and air/metal), a CR (or A-CR in R-mode) = 13.3 was achieved for our PCGH LC at the device normal. This may be the highest value that can be obtained based on the same materials and device configuration. It is also possible to further improve the CR by using a new device structure and fabrication. For example, a CR = ~200:1 was achieved by using a dye-doped gel. 19 In our experiments, as shown in Fig. 1(d), there were four air/glass interfaces. At a rough ...