Patent Publication Number: US-2009229667-A1

Title: Translucent solar cell

Description:
BACKGROUND 
     1. Field 
     This disclosure relates, in general, to solar cells. 
     2. General Background 
     Solar cells made from organic materials and polymers are considered as promising alternatives to their inorganic counterparts. Ever since their first report, polymer/fullerene bulk-heterojunction (BHJ) solar cells, more commonly known as plastic solar cells, have attracted a lot of positive attention. 
     SUMMARY 
     A translucent solar cell has a transparent substrate and a first translucent electrode that is the anode. A transparent active layer, that is a substantially organic material layer, is formed on top of the anode. On top of the active layer, a second translucent electrode is formed. The second translucent electrode is the cathode. In a variation, the first translucent electrode is the cathode and the second translucent electrode is the anode. The flexibility in choosing the order of the anode and cathode relative to the transparent substrate allows for an increase in processing techniques and, thus, the amount of utilizable materials to increase the power conversion efficiency of translucent solar cells. 
     Translucent solar cells have a low cost for their raw material and their manufacturing. From a raw material point of view, a polymer is derived from organic elements having great abundance and availability. From a manufacturing point of view, the solar cells utilize solution processing, thus yielding an easier fabrication process that requires less energy input than their silicon or other inorganic counterparts. 
    
    
     
       DRAWINGS 
       The above-mentioned features and objects of the present disclosure will become more apparent with reference to the following description taken in conjunction with the accompanying drawings wherein like reference numerals denote like elements and in which: 
         FIG. 1  is an exemplary embodiment of a translucent solar cell. 
         FIG. 2  is an exemplary embodiment of a translucent solar cell. 
         FIG. 3  is a process flow diagram for a method of making a translucent solar cell in accordance with the present disclosure. 
         FIG. 4  is a process flow diagram for a method of making a translucent solar cell in accordance with the present disclosure. 
         FIG. 5  is a process flow diagram for a method of making a translucent solar cell in accordance with the present disclosure. 
         FIG. 6  is a process flow diagram for a method of making a translucent solar cell in accordance with the present disclosure. 
         FIG. 7  is a process flow diagram for a method of making a translucent solar cell in accordance with the present disclosure. 
         FIG. 8  is a process flow diagram for a method of making a translucent solar cell in accordance with the present disclosure. 
         FIG. 9  is a table showing various solar cell properties after annealing at different temperatures, in accordance with the present disclosure. 
         FIG. 10  is a curve that shows improved performance of polymer solar cells upon thermal annealing, in accordance with the present disclosure. 
         FIG. 11  is a schematic of multiple device tandem structure solar cells, in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Polymer active layers used in plastic solar cells are usually about 50-200 nm thick. This small thickness results in inefficient absorption because the maximum absorption wavelength for a polymer active layer is usually about 650 nm. For example, maximum absorption in a 80 nm thick poly(3-hexylthiophene): [6,6]-phenyl C 61 -butyric acid methyl ester (P3HT:PCBM) film, the most commonly used active layer, has been shown to be less than 40% at the peak absorption wavelength. At other wavelengths in the absorption range an even higher percentage of light is transmitted without being absorbed. 
     The active layer of the plastic solar cells is semi-transparent or translucent in the visible light range. This semi-transparent or translucent property of the active layer can be used to the advantage of fabricating translucent plastic solar cells. In order to make the plastic solar cells translucent, the bottom and top contacts have to be made semi-transparent. The photons that are unabsorbed in the active layer should be transmitted through the cell, without any significant reduction in intensity. 
     The present disclosure makes use of the following processes described herein: 
     Thermal annealing: Thermal annealing is a process in which the substrates, which have various layers deposited on top, are provided thermal energy (heat) by placing the substrates on a hot plate, which is maintained at a certain temperature for a certain period of time. The temperature is referred to as the annealing temperature and the time as annealing time. The thermal annealing may also be done by providing the thermal energy in non-contact mode where the substrate does not come in contact with the hot plate (or heat source), such as placing the substrates in an oven under controlled temperature for a certain period of time. 
     Solvent annealing: Solvent annealing is a process where an organic layer, which has been deposited on top of a substrate that has a bottom contact deposited by solution processing, is allowed to solidify at a controlled slow rate to enhance the self-organization in the organic polymer film. This is achieved by dissolving the organic polymer(s) in a high boiling point solvent, such as dichlorobenzene or tricholorobenzene, for depositing the organic polymer film by solution processing. Due to the high boiling point of the solvent, the film is usually wet after it is deposited, which is then allowed to dry in a controlled manner to slow down the time it takes for the film to convert from liquid phase to solid phase. The desired solidification time is between 2 to 20 minutes. The longer solidification time allows the polymer chains in film to align in a highly-ordered crystalline phase which may result in increased efficiency of photovoltaic conversion in the film. 
     Adding additives to enhance carrier mobility: Adding additives is a technique used in polymer solar cells to improve the morphology and enhance the carrier mobility. One example is adding slight amount of poor solvent(s) (e.g. alkanedithiols, or nitrobenzene) into the dominant solvent used to make polymer solution (e.g. chlorobenzene or dichlorobenzene). Improved polymer aggregation and crystallinity has been achieved in some polymer systems and so has enhanced carrier mobility. Another example is the addition of electrolytes and salt into polymer blend solutions, which is also shown to improve photocurrent in polymer solar cells. 
     Thermal evaporation: Thermal evaporation is a common technique, one of the physical vapor deposition (PVD) methods, to deposit thin film materials. In thermal evaporation, the material is heated in a vacuum of 10 −5  to 10 −7  Torr range until it melts and starts evaporating. The vapor then condenses on a substrate exposed to the vapor, which is kept at a cooler temperature to form a thin film. The materials are heated by placing them in a crucible (or boat) which is made of high electrical resistance material such as tungsten, and passing high current through the boat. 
     Device Structure and Fabrication 
     The solar cell device structure, shown in  FIG. 1 , comprises an active layer  120  which absorbs sunlight and converts it into electricity. The active layer  120  is between two contacts  110  and  130 , both of which are semi-transparent or translucent and built on a transparent substrate  140 . The translucent solar cell can absorb sunlight from both sides, from the top or the bottom. The device may further include a metal mesh 150 to provide a high surface conductivity and to increase charge collection efficiency. 
     Based on the polarity of the cell, two configurations are possible: (i) regular device structure, and (ii) inverted device structure. In the regular structure the bottom contact is the anode  130 , which collects holes, and the top contact is the cathode  110  which collects electrons during the energy conversion process, as shown in  FIG. 1 . The polarity is reversed in the inverted cell configuration, as shown in  FIG. 2 , the bottom contact is the cathode  230  and top contact is the anode  210 . 
     Active Layer 
     The active layer  120  is typically a bulk-hetero-junction (BHJ) of a p-type donor polymer and an n-type acceptor material. In the donor polymer, the photons are absorbed and the excitons are generated upon photo-absorption. The generated excitons migrate to the donor-acceptor interface, where they are dissociated into free electrons and holes, which are then transported through a 3-dimensional (3-D) interpenetrated network of donors and acceptors in the BHJ film and are collected at the contacts. Many polymers can be used as the donor in the BHJ film, such as P3HT, poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV), or poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV). Other low-band-gap polymers can be used for the active layer as well. 
     By selecting the polymer, the color and transparency can be adjusted for specific applications. The most common candidate for the acceptor materials are PCBM or [6,6]-phenyl C 71 -butyric acid methyl ester (C 70 -PCBM). Other materials such as single-walled carbon nanotubes (CNTs) and other n-type polymers can also be used as the acceptor material as well. The active layer can be obtained by spin-coating from polymer solution in organic solvent(s). The film can also be obtained by several other solution processing techniques, such as bar-coating, inkjet-printing, doctor-blading, spray coating, screen printing etc. By using these techniques, a large area of substrate can be covered by a polymer solution with ease and without compromising the cost of the process. Also, flexible substrates can be used to substitute glass, resulting in a translucent and flexible plastic solar cell. 
     To improve the photovoltaic conversion efficiency of the plastic solar cell, the BHJ film may undergo specific treatments. For example, in P3HT:PCBM system, both so called “solvent annealing” approach and thermal annealing approach can be used. In the “solvent annealing” approach, the slow solidification rate of the active layer  120  allows the P3HT polymer chains to be organized into a highly ordered crystalline state, which improves the absorption of light within the polymer, enhances the charge carrier mobility, improves the exciton generation and dissociation efficiency, and results in a highly balanced charge carrier transport. Due to these effects the efficiency of plastic solar cells can be enhanced significantly. Thermal annealing has also been used to partially recover the polymer crystallinity as well as to improve the solar cell performance. Other possible approaches may include solvent mixing, where two or more solvents are used to dissolve the polymer blend, which is used to prepare the active layer  210 , or by adding an ionic salt into the active layer  120 , as well as other potential interfacial layer modifications known in the art. 
     Regular Device Structure 
     In a regular device configuration  100 , shown in  FIG. 1 , the active layer  120  is sandwiched between semi-transparent bottom (anode)  130  and top (cathode)  110  electrodes. 
     The regular device structure of the translucent solar cell  100  has a transparent substrate  140  and a translucent anode  130  on top of the substrate  140 . The anode  130  can be provided with a volume and a metallic mesh  150  embedded within the volume. 
     The translucent solar cell  100  has a transparent active layer  120  made of a substantially organic material and a translucent cathode  110 . The active layer  120  lies between the translucent anode  130  and the translucent cathode  110 . 
     Bottom Contact 
     A transparent conductive oxide (TCO), indium tin oxide (ITO), fluorinated tin oxide (FTO) can be deposited on a coated glass (or plastic) substrates to form the transparent anode  130 . The TCO films are obtained by solution processing, sputtering or thermal spray-coating. To enhance the performance of the organic solar cells, the TCO covered glass surface is coated with a thin layer of high conductivity polymer, such as poly(ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), or polyaniline (PANI). 
     In another variation, to form the bottom transparent electrode  130 , the TCO is covered with a thin layer of transition metal oxides (TMOs), such as vanadium pentoxide (V 2 O 5 ), molybdenum oxide (MoO 3 ), or tungsten oxide (WO 3 ). In this case, the metal oxides are either thermally evaporated or deposited through solution processes directly on top of TCO glass substrates, and form the anodic interfacial layer. The TMO layer, with a thickness of 3-20 nm, can replace PEDOT:PSS in the polymer solar cells without effecting the performance since it is transparent and reasonably conductive. The efficiency of polymer solar cells with a TCO/TMO bottom contact is comparable to or even better than those with a ITO/PEDOT:PSS bottom contact. Using TMO as the anode interfacial layer also prevents the unwanted chemical reaction between ITO and PEDOT:PSS, which can cause performance degradation resulting in poor organic solar cell lifetime. 
     Conductive polymers, such as PEDOT:PSS or PANI, can substitute the TCO layer as the bottom transparent electrode  130 . Since conducting polymers can be solution processed, this method results in an easy and low cost process that gets rid of high temperature deposition process such as sputtering of TCOs. However, the conductivity of even highest conductivity PEDOT is only about 100 S/cm, which is about an order of magnitude lower than that of ITO. To achieve efficient charge collection, the conductivity must be improved. To overcome this deficiency, very fine metal lines or mesh 150 are embedded into the PEDOT:PSS or PANI film to provide high surface conductivity and efficient charge collection at the interface. The metal lines are thermally evaporated on top of glass substrates though a photo-mask prepared by photo-lithography. Several high conductivity metals such as aluminum (Al), gold (Au), silver (Ag), copper (Cu), chromium (Cr) coated with Au, etc. can be used for metal lines 150. The high conductivity polymer film can be deposited from aqueous solution on glass substrates covered with metal lines, evaporated on top of the glass substrates, by using solution processing techniques, such as spin-coating, bar-coating, inkjet-printing, doctor-blading, spray coating, screen printing or other techniques known in the art. 
     Top Contact 
     The top contact  110  in the regular device structure has to be transparent. This transparent cathode  110  has to fulfill two functions. It allows the light that is not absorbed by the active layer  120  to be transmitted effectively and enables efficient electron collection at the cathode-polymer interface at the same time. 
     One of the methods for obtaining a semi-transparent cathode  110  is thermally evaporating multi-layered metals films. Such multi-layered metal films include: (i) lithium fluoride (LiF) and Au, (ii) LiF and Al, (iii) calcium (Ca) and Au, and (iv) LiF, Al, and Au. The total thickness of the multi-layered metal cathode is about 10-12 nm. The metal films are evaporated under high vacuum in succession. The transmittance of metal electrode is about 80-85%. 
     In one instance, a semi-transparent top electrode  110  is obtained is by spin-coating a thin layer of n-type material such as cesium carbonate, calcium acetylacetonate [Ca(acac) 2 ], cesium fluoride (CsF), CNTs, followed by evaporating a thin layer of transparent metal such as Ag or Au. The thickness of the metal layer, in this case, would be about 15 nanometers or less. 
     Another way to obtain a semi-transparent top electrode  110  is to spin-coat a thin layer of n-type material such as cesium carbonate, calcium acetylacetonate [Ca(acac) 2 ], cesium fluoride (CsF), CNTs, etc. followed by depositing a transparent conducting oxide layer, such as ITO or FTO, by sputtering or thermal spray-coating to form the semi-transparent top electrode  110 . 
     A method for fabricating a translucent solar cell  100  is represented as process flow operations  300  in  FIG. 3 . The method comprises providing a transparent substrate at initialization operation  302 . Control then transfers to operation  304 . 
     In operation  304 , a transparent anode  130  is formed on the transparent substrate  140 . The transparent anode  130  is formed of a transparent conducting oxide layer deposited on the transparent substrate  140 . In the present disclosure, the conducting oxide layer of the anode  130  can be, but is not limited to indium tin oxide or fluorinated tin oxide and can be either sputtered or thermal spray-coated onto the substrate  140 . Control then transfers to operation  306 . 
     In operation  306 , a transition metal oxide layer is deposited on the transparent conducting oxide layer of the transparent anode by solution processing. The transition metal oxide layer has a work function substantially similar to a lowest unoccupied molecular orbital level of the organic active layer  120  and can be, but is not limited to vanadium pentoxide, molybdenum oxide, or tungsten oxide, in accordance with the present disclosure. Control then transfers to operation  308 . 
     In operation  308 , an organic active layer  120  is formed on the transparent anode  130 . The organic active layer  120  has a mix of donor and acceptor molecules. Forming the organic active layer may further comprise thermal annealing, solvent annealing or adding additives for enhancing carrier mobility, where the transparent substrate  140 , transparent anode  130  and the organic active layer  120  are treated within a temperature range of about 70-180 Celsius, in accordance with the present disclosure. Control then transfers to operation  310 . 
     In final operation  310 , a transparent cathode  110  is evaporated on top of the organic active layer  120 . The transparent cathode  110  is made of at least one metal layer and has a thickness less than 20 nanometers. The metal layer(s) of the cathode  110  can be lithium fluoride and gold, lithium fluoride and aluminum, calcium and gold, and cesium fluoride and gold, cesium fluoride and aluminum, cesium carbonate and gold, and cesium carbonate and aluminum, lithium fluoride and gold, and aluminum and gold. 
       FIG. 4  represents process flow operations  400  for fabricating a translucent solar cell  100 . The method comprises providing a transparent substrate at initialization operation  402 . Control then transfers to operation  404 . 
     In operation  404 , a transparent anode  130  is formed on the transparent substrate  140 . The transparent anode  130  is formed of a transparent conducting oxide layer deposited on the transparent substrate  140 . The conducting oxide layer of the anode  130  can be indium tin oxide and fluorinated tin oxide and can be either sputtered or thermal spray-coated onto the substrate  140 , in accordance with the present disclosure. 
     Additionally, a transition metal oxide layer may be deposited by solution processing on the transparent conducting oxide layer of the transparent anode  130 . The transition metal oxide layer preferably has a work function that is substantially similar to a lowest unoccupied molecular orbital level of the organic active layer  120 . The transition metal oxide can be, but is not limited to vanadium pentoxide, molybdenum oxide, or tungsten oxide, in accordance with the present disclosure. Control then transfers to operation  406 . 
     In operation  406 , an organic active layer  120  is formed on the transparent anode  130 . The organic active layer  120  has a mix of donor and acceptor molecules. Forming the organic active layer may further comprise thermal annealing, solvent annealing, or adding additives for enhancing carrier mobility, where the transparent substrate  140 , transparent anode  130  and the organic active layer  120  are treated within a temperature range of about 70-180 Celsius. Control then transfers to operation  408 . 
     In operation  408 , a transparent cathode  110  is formed on top of the organic active layer. The transparent cathode  110  can be made of at least an n-type layer that can be deposited by solution processing and preferably has a work function that is substantially similar to a lowest unoccupied molecular orbital energy level of the organic active layer. The n-type layer can be, but is not limited to cesium carbonate, calcium acetylacetonate, or cesium fluoride. Control then transfers to operation  410 . 
     In final operation  410 , a transparent conducting oxide layer is deposited on the n-type layer of the transparent cathode  110  by either sputtered or thermal spray-coating. The conducting oxide layer can be, but is not limited to indium tin oxide or fluorinated tin oxide. 
     Alternately, in final operation  410 , a metal layer consisting of either Ag or Au, having a thickness less than 15 nanometers, can be deposited by thermal evaporation on top of the n-type layer of the transparent cathode  110 . 
       FIG. 5  represents process flow operations  500  for fabricating a translucent solar cell  100 . The method comprises providing a transparent substrate at initialization operation  502 . Control then transfers to operation  504 . 
     In operation  504 , an anode  130  is formed on the transparent substrate  140 . The anode  130  is an organic layer deposited by a solution processing. The organic layer has a volume and a metal mesh  150  embedded in the volume. The metal mesh  150  can be, but is not limited to gold, aluminum, silver, copper, or chromium coated with gold. Control then transfers to operation  506 . 
     In operation  506 , an organic active layer  120  is formed on the transparent anode  130 . The organic active layer  120  has a mix of at least one type of donor and at least one type of acceptor molecule. 
     Additionally, the organic active layer  120  may further comprise thermal annealing, solvent annealing or adding additives to enhance carrier mobility. The transparent substrate  140 , transparent anode  130  and the organic active layer  120  can be treated within a temperature range of about 70-180 Celsius. Control then transfers to operation  508 . 
     In final operation  508 , a transparent cathode  110  is formed on the organic active layer  120 . The transparent cathode  110  is at least one metal layer having a thickness less than 20 nanometers and can be, but is not limited to lithium fluoride and gold, lithium fluoride and aluminum, calcium and gold, and cesium fluoride and gold, cesium fluoride and aluminum, cesium carbonate and gold, and cesium carbonate and aluminum, lithium fluoride and gold, or aluminum and gold. 
       FIG. 6  represents process flow operations  600  for fabricating a translucent solar cell  100 . The method comprises providing a transparent substrate at initialization operation  602 . Control then transfers to operation  604 . 
     In operation  604 , an anode  130  is formed on the transparent substrate  140 . The anode  130  is an organic layer deposited by solution processing. The organic layer has a volume and a metal mesh  150  embedded in the volume. The metal mesh  150  can be, but is not limited to gold, aluminum, silver, copper, or chromium coated with gold. Control then transfers to operation  606 . 
     In operation  606 , an organic active layer  120  is formed on the transparent anode  130 . The organic active layer  120  preferably has a mix of at least one type of donor and at least one type of acceptor molecule. 
     Additionally, the organic active layer  120  may further comprise thermal annealing, solvent annealing, or adding additives to enhance carrier mobility. The transparent substrate  140 , transparent anode  130  and the organic active layer  120  can be treated within a temperature range of about 70-180 Celsius. Control then transfers to operation  608 . 
     In operation  608 , a transparent cathode  110  is formed on top of the organic active layer. The transparent cathode  110  is made of at least an n-type layer that can be deposited by solution processing and preferably has a work function that is substantially similar to a lowest unoccupied molecular orbital energy level of the organic active layer. The n-type layer can be, but is not limited to cesium carbonate, calcium acetylacetonate, or cesium fluoride. Control then transfers to operation  410 . 
     In final operation  610 , a transparent conducting oxide layer is deposited on the n-type layer of the transparent cathode  110  by either sputtered or thermal spray-coating. The conducting oxide layer can be, but is not limited to indium tin oxide or fluorinated tin oxide. 
     Alternately, in final operation  610 , a metal layer consisting of either Ag or Au, and having a thickness less than 15 nanometers, can be deposited by thermal evaporation on top of the n-type layer of the transparent cathode  110 . 
     Inverted Device Structure 
     In the inverted device configuration  200 , the bottom contact  230  is the cathode where the electrons are collected and the top contact is the anode  210  where holes are collected during photovoltaic generation. Both of the contacts are again semi-transparent. 
     An inverted transparent solar cell  200  is shown in  FIG. 2 . The inverted solar cell  200  comprises a transparent substrate  240  having a bottom surface and a top surface. 
     A first translucent electrode, the cathode  230 , is on the top surface of the substrate  240  and made of a transparent conductive oxide. The cathode  230  is formed of a transparent conducting oxide with an n-type interfacial layer. 
     A second translucent electrode, the anode  210 , is made of a transparent conductive oxide and has an interfacial layer. A transparent active layer  220  is made of a substantially organic material and is between the translucent anode  210  and the translucent cathode  230 . 
     Bottom Contact 
     The role of the bottom contact  230 , the cathode, is to collect free electrons that are generated in the active layer  220  during photovoltaic conversion process. To achieve efficient electron collection several options can be used. Examples are given below. 
     A thin layer of an n-type material such as CsCO 3 , CsF, Ca(acac) 2 , CNT, or other materials with similar properties can be spin-coated on a TCO covered glass or plastic substrate  240  to achieve a transparent bottom cathode  230 , as shown in  FIG. 2 . The thickness of all these cathode interfacial layers is very small, only a few nanometers, and as a result, they are highly transparent. The work function of ITO is about 4.7 eV, which makes it a hole transport material. Therefore, the ITO surface has to be modified with a thin n-type interfacial layer, as mentioned above, to make it an electron collecting contact. For example, the work function of CsCO 3  is about 2.9 eV. 
     The ITO or FTO coated glass or plastic substrate  240  can be coated with a thin layer of titanium oxide (TiOx), zinc oxide (ZnO), or ZnO:Al and other electron transport materials, to achieve a transparent bottom cathode  230 . The thickness of the oxide layer in this case is about 10-20 nm. 
     Top Contact 
     The top contact  210 , the anode, collects the holes in the inverted device configuration. For the top contact  210 , several configurations may be used. 
     The first configuration is comprised of a high work function p-type interfacial layer coated with a high conductivity thin metal film. The materials used for p-type interfacial layer are transition metal oxides, such as V 2 O 5 , MoO 3 , or WO 3 . The thickness of the metal oxides are about 3-10 nanometers in order to maintain transparency. The oxide film can be obtained by thermal evaporation or solution processing, directly on top of the polymer film. Since the conductivity of metal oxides is not particularly good, an additional layer of high conductivity metal, such as Au, may be required to coat the metal oxide layer. The metal can be thermally evaporated and have a thickness usually not exceeding 15 nanometers, to maintain the transparency. 
     Another way to obtain top contact  210  is to deposit a transparent conducting oxide layer, such as ITO or FTO by sputtering or thermal spray-coating, in place of a high conductivity metal such as Au, since transparent conductive oxides have better transparency and comparable electrical conductivity. 
       FIG. 7  represents process flow operations  700  for fabricating a translucent solar cell  200 . The method comprises providing a transparent substrate  240  at initialization operation  702 . Control then transfers to operation  704 . 
     A transparent cathode  230  is formed on top of the transparent substrate  240 . The forming process includes the steps of forming a transparent conducting oxide layer, in operation  704 , and an n-type interfacial layer by solution processing, in operation  706  on the transparent substrate  240 . In accordance with the present disclosure, the n-type layer can be, but is not limited to cesium carbonate, calcium acetylacetonate, or cesium fluoride. Control then transfers to operation  708 . 
     In operation  708 , the transparent substrate  240  and the transparent cathode  230  are thermally annealed within a temperature range of about 70-180° Celsius. Control then transfers to operation  710 . 
     In operation  710 , at least one organic active layer  220  is deposited on the transparent cathode  230 . The organic active layer  220  can be deposited by solution processing and has a mix of donor and acceptor molecules. The organic active layer  220  has a lowest unoccupied molecular orbital energy level that is substantially similar to the n-type layer of the transparent cathode  230 . Control then transfers to operation  712 . 
     A transparent anode  210  is formed on the organic active layer  220 , the forming process including the steps of depositing a transition metal oxide layer by solution processing, in operation  712 . The transition metal oxide has a work function substantially similar to a highest occupied molecular orbital energy level of the organic active layer. The transition metal oxide layer of the anode  210  can be, but is not limited to vanadium pentoxide, molybdenum oxide, or tungsten oxide and is of a thickness less than 30 nanometers. Control then transfers to operation  714 . 
     In operation  714 , a transparent conducting oxide layer is deposited onto the transition metal oxide layer. The conducting oxide layer can be, but is not limited to indium tin oxide and fluorinated tin oxide or can be sputtered or thermal spray-coated onto the transparent substrate  240 . 
     Alternately, in final operation  714 , a metal layer of either Ag or Au, and having a thickness less than 15 nanometers, can be deposited by thermal evaporation on top of the transition metal oxide layer. 
       FIG. 8  represents process flow operations  800  for fabricating a translucent solar cell  200 . The method comprises providing a transparent substrate  240  at initialization operation  802 . Control then transfers to operation  804 . 
     A transparent cathode  230  is formed on top of the transparent substrate  240 . The forming process includes the steps of forming a transparent conducting oxide layer, in operation  804 , and an n-type interfacial layer by solution processing, in operation  806  on the transparent substrate  240 . The n-type layer can be at least cesium carbonate, calcium acetylacetonate, or cesium fluoride. Control then transfers to operation  808 . 
     In operation  808 , the transparent substrate  240  and the transparent cathode  230  are thermally annealed within a temperature range of about 70-180° Celsius. Control then transfers to operation  810 . 
     In operation  810 , at least one organic active layer  220  is deposited on the transparent cathode  230 . The organic active layer  220  can be deposited by solution processing and has a mix of donor and acceptor molecules. The organic active layer  220  has a lowest unoccupied molecular orbital energy level that is substantially similar to the n-type layer of the transparent cathode  230 . Control then transfers to operation  812 . 
     A transparent anode  210  is formed on the organic active layer  220 , the forming process including the steps of depositing a transition metal oxide layer by solution processing, in operation  812 . The transition metal oxide has a work function substantially similar to a lowest unoccupied molecular orbital energy level of the organic active layer. The transition metal oxide layer of the anode  210  can be, but is not limited to vanadium pentoxide, molybdenum oxide, or tungsten oxide and is of a thickness less than 30 nanometers. Control then transfers to operation  814 . 
     In final operation  814 , at least one metal film is deposited on the transition metal oxide layer and can be, but is not limited to gold and silver. 
     In a variation, a thicker TMO film can be deposited on top of the polymer film, with thickness of about 20-50 nanometers. The large thickness of TMO does not have a significant effect of the device performance, while maintaining its interfacial properties. Once a comparatively thicker TMO film is deposited on the polymer film, it acts as a protective barrier for the polymer film. As a result a highly transparent conductive metal oxide, such as ITO or FTO may be evaporated or sputtered on top of TMO film to complete the device structure. 
     The work-function of Cs 2 CO 3  can be modified from 3.45 eV to 3.06 eV by a low temperature (less than 200° C.) annealing treatment, verified by ultraviolet photoelectron spectroscopy (UPS). With the inverted device structure (ITO/Cs 2 CO 3 /RR-P3HT:PCBM/V 2 O 5 /Al), the PCE improves from 2.31% to 4.19% by a 150° C. thermal annealing treatment of the Cs 2 CO 3  interfacial layer as shown in  FIG. 10 . Generally the decomposition temperature of Cs 2 CO 3  is around 550-600° C. However, preliminary X-ray photoelectron spectroscopy (XPS) results reveal that thermal annealing helps Cs 2 CO 3  decompose into a low-work function cesium-oxide. The lower work function of Cs 2 CO 3  matches better with the lowest occupied molecular orbital level of the organic polymer, thereby increasing the efficiency of polymer solar cell. 
     0.2 wt % Cs 2 CO 3  dissolved in 2-ethoxyethanol was spin-coated on pre-cleaned and UV-ozone-treated ITO glass substrates as the cathode  230 . Various annealing temperatures were carried out on the hot plate inside the glove box for 20 minutes. RR-P3HT and PCBM were separately dissolved in 1,2-dichlorobenzene (DCB) then blended together with 1:1 wt/wt ratio to form a 2.5 wt % solution. This RR-P3HT/PCBM solution was spin-coated at 600 rpm for 40 seconds, and the wet film was dried in a covered glass Petri dish. The dried film was then annealed at 110° C. for 10 minutes. 
     The active film thickness was about 210-230 nanometers measured by a Dektak 3030 profilometer. The anode  210  is 10 nm V 2 O 5  covered by 100 nm Al. The devices were tested in the glove box under simulated AM1.5G irradiation (100 mW/cm 2 ) using a solar simulator. The illumination intensity was determined by a NREL calibrated Si-detector with KG-5 color filter, and the spectral mismatch was corrected. 
     For the device without thermal annealing on Cs 2 CO 3  layer, the power conversion efficiency (PCE) is 2.31%. When Cs 2 CO 3  layers are treated by different temperature annealing process with different temperatures, all device performances improved. As the annealing temperature of the Cs 2 CO 3  layer increased from room temperature to 150° C., the PCE increases from 2.31% to 4.19%. In addition, all other device characteristics, such as Voc, Jsc, and FF, improved as shown in  FIG. 9  and  FIG. 10 . 
     The work-function of oxygen plasma-treated ITO substrate is 4.54 eV. When Cs 2 CO 3  is spin-coated on this ITO surface without thermal annealing, the work-function changes from 4.54 eV to 3.23 eV. The work-function of the Cs 2 CO 3  film further reduces to 3.13, 3.11, and 3.06 eV after annealing at 70° C., 120° C., and 170° C. for 20 minutes, respectively. 
     A highly efficient inverted polymer solar cell has been demonstrated by thermal annealing of a Cs 2 CO 3  layer. The UPS results show that the work-function of the Cs 2 CO 3  layer is decreased by thermal annealing, and preliminary XPS studies reveal that Cs 2 CO 3  decomposes intrinsically into a doped n-type semiconductor by the annealing process. This inverted cell can be applied to design a multiple-device stacked polymer solar cells or a tandem cell, which are widely accepted to further improve the efficiency of polymer solar cells. 
     Multiple-Device Stacking Scheme—Beyond Tandem Solar Cell Structure 
     Utilizing photovoltaic materials to cover different regions of the solar spectrum is effective in improving solar cell efficiency. Tandem solar cell structure, where two or more cells are connected in series, can be demonstrated in a polymer solar cell. Translucent solar cells with different solar spectrum coverage can be used to realize tandem solar cells with enhanced photo-voltage. In this scheme, two individual PV cells, each having their own substrate, are stacked on top of each other, shown in  FIG. 11 . The cells are connected electrically in series or in parallel, which can up to double the efficiency of the stacked system compared to a single cell. The multiple-device stacking may also improve the yield of solar cells. 
       FIG. 11  is the schematic of a multiple-device tandem structure showing two translucent PV cells  1100  stacked on top of each other. The unabsorbed light from the first cell is transmitted to the second cell through transparent electrode  1110  in the bottom cell. This light is absorbed by PV cell  2 . The PV cell  2  may or may not have a transparent top electrode  1110 . The cells may be connected electrically in series or in parallel to increase the performance of the tandem structure compared to a single cell. 
     Incorporation of Reflectors or Diffuser 
     The translucent solar cell can also be used in the situation where transparency is not required. In these situations, a light reflector or diffuser can be used behind the translucent solar cell to reuse the light passing through. This can improve the efficiency of the translucent solar cell due to improved light harvesting. 
     A Few Applications of Translucent Plastic Solar Cell 
     Unlike their inorganic counterparts, translucent polymer solar cells are inherently unique, with distinctive characteristics that are suitable for untapped applications in the building and transportation industry. There are three key characteristics that distinguish organic solar cells from inorganic cells: architecturally aesthetic, versatile and flexible, and low-cost. 
     The translucent solar cells have the ability to create architecturally aesthetic applications by integrating them onto glass, glass laminates, or flexible substrates of virtually any building and transportation windows, thus allowing triple functions of power generation, light filtration, and architectural element/aviation, automotive, and marine design. 
     Some building applications may include the commercial, industrial, institutional (educational and governmental), and residential markets. Commercial and industrial markets encompass, but are not limited to, offices, hotels/motels, skyscrapers, factories, power plants, and warehouses. Institutional and residential markets are comprised of, but not limited to, colleges/universities, hospitals, government buildings, houses, apartment blocks, and condominiums. In the transportation industry, the polymer solar cells can fit into practically any type of transports with windows in air, rail, road, and water. In particular, we can integrate our translucent solar cells from commercial or military aircrafts to ground and water transportation such as passenger/commuter trains, automobiles, buses, trucks, ships, and boats. 
     While the apparatus and method have been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the disclosure need not be limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. The present disclosure includes any and all embodiments of the following claims.