Patent Application
20110109853
This invention relates to liquid crystal displays and, in particular, to methods and devices for an energy efficient liquid crystal display with embedded photovoltaic cells for each pixel.
For portable display devices, low power consumption, good sun light readability, thin profile, wide viewing angle, and cost competitiveness are highly desirable. Among these requirements, low power consumption is significantly important for the device performance. For example, with low power consumption, the device can be operated for a longer time between battery charges. Previously, in order to save power consumption, a reflective typed LCD which embeds a reflector into LC cell and uses external ambient light to display images was widely employed. Besides, a reflective LCD also can have good image readability under strong sunlight. However, typical reflective LCDs usually have low contrast ratio (resulting from the surface reflection) and insufficient color saturation, which cannot meet the increasing demand of users. As an alternative, transmissive typed LCDs grow very quickly and take a big share in the mobile displays.
A transmissive LCD sandwiches the LC cell between two linear polarizers and uses a backlight, like a cold cathode fluorescent lamp (CCFL) or light emitting diode to display the image, thus its color is vivid and contrast is high. However, the following drawbacks of a pure transmissive LCD are evident: 1) its power consumption is high, resulting from the dependence of a backlight, and 2) its image is easily washed out by strong outdoor sunlight, as its output light cannot compete with the ambient light there. Therefore, these issues require better solutions to improve the performance of LCDs in portable displays.
As a solution to solve the sunlight readability, a transflective LCD that combines both transmissive and reflective functions in one display is introduced. In a typical transflective LCD, each pixel of the display is usually divided into a separate transmissive region and a separate reflective region as described in Zhu et al, “Transflective liquid crystal displays,” J. Display Technology, 1, pp. 15-29, 2005. Thus the display can exhibit high image quality from its transmissive mode, and good sunlight readability from its reflective mode, in a simultaneous or separate means. Besides, by adjusting the ratio of its reflective region and transmissive region, power consumption can also be reduced, e.g., incorporating a large reflective region (while the backlight is turned off when operating in a reflective mode) can efficiently save the display power consumption. However, the power consumption is reduced in a way that sacrifices the transmissive region that has a high image quality.
As another solution for enhancing the energy efficiency of the display, photovoltaic (PV) cells are incorporated into the mobile LCDs. A first LCD embedding PV cells into a display is shown in
In another approach described in U.S. Pat. No. 7,206,044 issued Apr. 17, 2007, a band pass reflector is introduced to a reflective LCD with a PV cell. The device structure is shown in
As an improvement, in another design disclosed in U.S. Pat. No. 7,339,636 issued Mar. 4, 2008 is shown in
As compared to the design using a band pass reflector described in the '044 patent, this method can achieve a true tri-color display in both transmissive and reflective modes, with the help of the color filter 38. However, several problems exist. First, the transflector is far away from the LC cell (separated by a substrate 32a, linear polarizer 31a and other films), thus because the LC cell is usually several microns thick, such a long distance (several hundred micron thickness from the substrate and polarizer) makes the incident light 42a and exit light 42b travel through different pixels, which is called parallax and results in deteriorated images. Besides, because a transflector has both transmission and reflection simultaneously, even working in a transmissive mode with backlight 36 turned on, the small amount of reflection from transflector 35 that has a parallax will affect the image quality as well.
Second, the majority of the charging is accomplished when the transflector is working under a transmissive mode. The efficiency is low, since the overall light transmission after a path from the surface of the top linear polarizer 31b to the exterior surface of the bottom linear polarizer 31a is usually less than 10% in a typical mobile LCD, then further loss after the following stacks finally makes the light to the PV cell less than 4%.
From the analysis of these prior arts, it seems the inclusion of the PV cell is mainly introduced to reflective LCD, and image quality including the contrast ratio of such a display is usually sacrificed. Besides, the light to charge the PV cell is from the ambient lights, while the input efficiency (the amount of light coming to the PV cell surface) is usually low. Better solutions are needed for advanced displays with PV cells.
To overcome the problems with prior art, the present invention provides a device structure that combines the liquid crystal cell and a photovoltaic cell together for high energy efficiency. The present invention is different from prior arts in that, in some embodiments of this display device, the PV cell is formed in a transflective LCD, and the PV cell is below the black matrix region and regions with reflectors of the display. The light for charging the PV cell primarily comes from the backlight, and only a small amount is coming from the external ambient light. In the present invention design, light efficiency is improved, and the display can have high image quality by using a transmissive mode, and good sunlight readability using the reflective mode. Additionally, the display can have reflective functions with high contrast that is parallax free. In another embodiment, the PV cell is formed in a pure transmissive LCD, which is designed to substantially cover the regions that backlight cannot transmit in each LC aperture.
A primary objective of the invention is to provide devices and methods for a LCD with photovoltaic cells that can have a high energy efficiency, which could make the display device a longer working time than conventional displays without PV cells.
A secondary objective of the invention is to provide devices and methods for a LCD with photovoltaic cells that can have a transmissive function that can provide high image quality including high contrast, high color saturation, and a functional reflective mode that has a good sunlight readability of displayed images.
A first embodiment provides a transflective liquid crystal display with a photovoltaic cell that includes a lower transparent substrate having a first linear polarizer laminated on an exterior surface, an upper transparent glass substrate having a second linear polarizer laminated on an exterior surface, the upper substrate positioned adjacent to a viewer and a liquid crystal layer sandwiched between the lower and upper substrates. The transflective display also includes a backlight unit adjacent to the exterior of the lower substrate, plural pixel regions between the upper and lower substrates. Each pixel includes a first sub-region having a data line and a gate line connected to a thin-film-transistor to switch the pixel, a storage capacitor and a reflector between the first substrate and the liquid crystal layer, the reflector reflecting ambient light to display images, a photovoltaic cell formed in the first sub-region between the lower substrate and the reflector, the light from the backlight in the first sub-region striking the photovoltaic cell to generate current and voltage for charging a battery of the liquid crystal display, and a second sub-region having transparent electrodes on at least one of the upper and lower transparent substrates for driving the pixel, the second sub-region transmitting light from the backlight unit through the liquid crystal layer for displaying images while part of the ambient light passing the through the second sub-region toward the backlight unit is scattered and redirected to the photovoltaic cell.
The photovoltaic cell can be made of active photovoltaic materials including amorphous silicon materials in a single p-n junction or multiple p-n junction structure, the said transparent electrodes in the transmissive region can be made of conductive materials including indium-tin-oxide and indium-zinc-oxide; the reflector can be made of one of aluminum and silver; liquid crystal cell can be a twist-nematic cell, a homogeneous cell, or a vertical alignment cell; and liquid crystal cell can include at least one compensation film selected from a group consisting of discotic film, uniaxial film, and biaxial film. The ratio between the first sub-region and the sub-region is adjusted from approximately 2:8 to approximately 8:2.
Another embodiment provides a method for fabricating the transflective liquid crystal display with an embedded photovoltaic cell by providing a first and second substrate with a linear polarizer laminated on an exterior surfaces, a color filter on the second transparent substrate and a liquid crystal layer between the substrates as a liquid crystal cell having a plurality of pixels. Each pixel includes a transparent pixel electrode on an interior surface of the first transparent substrate and a transparent common electrode on an interior surface of one of the first and second transparent substrates, and a thin-film transistor, a gate line and a data line on the first substrate, the gate line and the data line each connected to one of the source and the drain of the thin-film-transistor. A photovoltaic cell is formed on a portion of the first transparent substrate in each pixel region to form a reflective sub-region with a reflector between the photovoltaic cell and the liquid crystal layer. A backlight unit is positioned below the first substrate so the light emitted from the backlight unit strikes the photovoltaic cell in the reflective sub-region and transmitting through the liquid crystal cell the remaining transmission sub-region. The method includes depositing an alignment layer on the interior surfaces of the first and second substrates, depositing a color filter on the second transparent substrate, and connecting the data line and gate line to an external driving circuit and connecting the photovoltaic cell to an external charging circuit to charge a battery.
A second embodiment provides a transmissive liquid crystal display with a photovoltaic cell including a lower transparent substrate, a backlight unit positioned adjacent to the exterior surface of the lower substrate, an upper transparent substrate positioned for viewing by a viewer with linear polarizers laminated on an exterior surface of the substrates, and a liquid crystal layer between the lower and upper substrates forming a plurality of pixel regions between the upper and lower substrates. Each pixel region includes a first sub-region having a data line and a gate line each connected to one of a thin-film-transistor drain and source and a storage capacitor, a black matrix between the upper substrate and the liquid crystal layer covering a portion of each pixel to block light from the backlight unit, a photovoltaic on the lower substrate aligned with the black matrix to absorb light from the backlight unit, and a second sub-region having transparent pixel and common electrodes on at least one of the lower and the upper substrate for driving the liquid crystal cell and transmitting light incident from the backlight unit through the liquid crystal cell to display images, the ambient light passing through the second sub-region of the pixel scattering with a portion of the scattered light striking the photovoltaic.
The photovoltaic cell can be made of active photovoltaic materials including amorphous silicon materials in a single p-n junction or multiple p-n junction structure, the said transparent electrodes in the transmissive region can be made of conductive materials including indium-tin-oxide and indium-zinc-oxide; the reflector can be made of one of aluminum and silver; liquid crystal cell can be a twist-nematic cell, a homogeneous cell, or a vertical alignment cell; and liquid crystal cell can include at least one compensation film selected from a group consisting of discotic film, uniaxial film, and biaxial film.
Another embodiment provides a method for fabricating the transflective liquid crystal display with a photovoltaic cell including constructing a liquid crystal cell having a first substrate and a second substrate, laminating a first and second linear polarizer on the exterior surface of the substrates and forming a plurality of pixels between the first and second substrate each pixel having a thin-film-transistor and a data line and a gate line, forming transparent pixel electrodes on the first substrate and a transparent common electrodes on one of the substrates, connecting the data line and the gate line to one of the source and drain of the thin-film-transistor and connecting pixel electrode to the gate of the thin-film-transistor; forming a photovoltaic cell on a portion of the first transparent substrate in each pixel region; depositing black-matrix on the second transparent substrate; interposing a liquid crystal layer between the first substrate and the second substrate; and disposing a backlight unit below the first transparent substrate, the photovoltaic converting part of the backlight emission into electrical energy.
Further objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments which are illustrated schematically in the accompanying drawings.
a shows a top view of the first embodiment of the transflective LCD device with a photovoltaic cell.
b is a schematic diagram showing an equivalent circuit of present invention.
Before explaining the disclosed embodiments of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.
The following is a list of the reference numbers used in the drawings and the detailed specification to identify components:
Each pixel is divided into two regions: region 110a that can transmit the light 112 from the backlight unit 108 to the viewer when the display is working under white mode (LC cell and the two polarizers pass the light), and region 110b that blocks the light 111 from the backlight unit 108, regardless of the working mode of the LC cell 103. In addition, on the bottom substrate 101a, a photovoltaic cell 105 made of semiconductor materials like silicon is formed to substantially cover the backlight blocking region 110b in each pixel.
A reflector 104 made of metals like aluminum and silver is also formed on the bottom substrate 101a to reflect the incident ambient light 113 to the viewer as reflected light 114, which functions for displaying images in the reflective mode. The backlight 108 transmitted into the backlight-blocking region 110b is absorbed by the PV cell 105 for charging the battery using charging circuits. As a benchmark, in a conventional transmissive LCD that does not have a reflector 104, the aperture ratio (the effective region that passes the light to the viewer from the backlight) is usually less than 50%, and the light 111 transmitted to the backlight-blocking region 110b is usually absorbed by the black-matrix (not shown), thus is wasted. Unlike the prior art, the configuration of the present invention, part of light 111 in the blocking region 110b can be used to illuminate the PV cell 105 for charging batteries.
The photovoltaic cell 105 shown is a device using semiconductor materials like silicon in the fundamental form of a p-n junction or in a multiple p-n junction structure, which can directly convert light into electricity. Two electrodes (not shown) are formed on two surfaces of the p-n junction structured photovoltaic cell 105, and the two electrodes are further connected to external circuits for charging the battery. Light impinging on the PV cell produces both a current and a voltage to generate electric power. First, the light is absorbed to raise electrons and stimulate them to a higher energy state, then these electrodes move from the PV cell to an external circuit. The electrons then dissipate their energies into the external circuits (for example, the energy is directed to charge the external batteries) and return to the PV cell again. The process can repeat as long as light is continuously impinging on the surface of the PV cell 105 and the PV cell 105 is connected to the external circuits in a closed loop.
In the configuration of the present invention, the backlight-blocking region 110b shown in
With color filters (not shown) laminated on one of the interior surfaces of the LC cell, a specific color can be displayed. At the same time, the driving voltage from the data line 132 is also applied onto the storage capacitor 135 to store the electric charges which can help the LC cell 103 hold the voltage for a longer length of time. An illustration of the equivalent circuit of the display is shown in
Referring to
Therefore, in the total pixel region 137, the total effective region with light for displaying images is the transmissive region 130 which is transparent to the backlight and ambient light and the reflective region 140 which is transparent to ambient light but not to backlight. Nevertheless, as shown in
For the LC cell 103 shown in
The ratio of the reflective region 140 to the transmissive region 130 (R/T) in
According to the performance of the configuration shown in this embodiment, the light incident from the backlight unit 108 is unpolarized before it reaches the surface of the bottom linear polarizer 101a. As a result, only about 50% of the light can pass through the bottom linear polarizer 101a. However, if backlight recycling is employed by methods such as arranging a reflective polarizer like a wire grid polarizer located below the bottom linear polarizer, a factor of approximately 1.6× improvement can be achieved for the light to pass the bottom linear polarizer.
Usually as discussed above, according to the opaque regions from the black-matrix covered TFT area, metal data and gate lines, storage capacitor, the effective region for light transmission and reflection (the sum of the reflective region 140 and the transmissive region 130) usually takes less than 60% of the total pixel area 137. For example, if in the effective region, the reflective region 140 and the transmissive region 130 take an area ratio of 1:1, then backlight-blocking region 110b in the pixel would be larger than approximately 70% which includes 40% from the non-effective regions including the black matrix covered areas and other opaque area, and 30% from the effective regions which is the reflective region. As a result, approximately 56% (in this example 56%=70%×50%×1.6) of the incident light from the bottom backlight unit can be used to illuminate the PV cell, which is relatively high in practical applications.
As a comparison, in prior arts, the efficiency of ambient light after it passes the two sheet linear polarizers and further to the PV cell at the backplane of the backlight unit is less than 4%. In practical applications, the ambient light can also contribute to the light source for the PV cell 105. As shown in
In the preferred embodiment shown in
Similarly, in each pixel, the pixel region is divided into two regions: region 210a that transmit the light 212 from the backlight unit 208 to the viewer when the LCD is working under a white mode, and region 210b that blocks the light 211 from the backlight unit 208 by the opaque regions of the pixel like black matrix, regardless of the working mode of the LC cell 203. On the bottom substrate 201a, a photovoltaic cell 205 made of semiconductor materials like silicon is formed to substantially cover the backlight blocking region 210b in each pixel. The backlight 211 emission into the backlight-blocking region 210b is absorbed by the PV cell 205 for charging the battery by certain charging circuits (not shown). The PV cell 205 in this example is a device using semiconductor materials like silicon in the fundamental form of a p-n junction, which can directly convert light into electricity.
A detailed top view of each repetitive pixel region 237 is shown in
Similarly, as shown in
For the LC cell 203 in
According to the performance of the configuration shown in this embodiment, about 32% (in this example 32%=40%×50%×1.6) of the total backlight incident from the bottom backlight unit 208 can be used to illuminate the PV cell 205, which is relatively high in practical applications. The number is derived from the following accumulation: first, the linear polarizer 201a causes a loss of approximately 50% for an unpolarized light incident from bottom, but when a reflective polarizer like a wire grid polarizer or polymer films is used, the light from the backlight passing the bottom linear polarizer 201a could be enhanced by about 1.6× to approximately 50%×1.6=80%. Second, according to the 60% aperture ratio of the display, the PV cell covers about 40% of the backlight blocking region. Therefore, the light to the PV cell in total can be about 32%. As a comparison, in prior arts, the efficiency of ambient light after it passes the two sheet linear polarizers and further to the PV cell at the backplane of the backlight unit is less than 4%.
For the PV cell, the light from backlight unit at the backplane of the LC cell is major light source. However, the light from the incoming ambient can also be used to charge the PV cell. As shown in
In summary, the LCDs of the present invention with embedded photovoltaic cell can convert light which was previously lost due to the low aperture ratio of the display as the light source of the PV cell, which in turn charges the batteries via external circuits. Additionally, part of the ambient light can also be directed to the PV cell after it passes the backlight system and is reflected. As a result, the energy efficiency of the display is greatly enhanced. In one embodiment, the display works in a transflective LCD configuration that has both transmissive and reflective functions. In another embodiment, the display works in a pure transmissive LCD configuration that relies on the backlight unit to display images.
As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalences of such metes and bounds are therefore intended to be embraced by the appended claims.
This application claims the benefit of priority to U.S. Provisional Application No. 61/258,785 filed on Nov. 6, 2009.
| Number | Date | Country | |
|---|---|---|---|
| 61258785 | Nov 2009 | US |
