OPTOELECTRONIC DEVICES

Abstract: An optoelectronic device comprising a photoresponsive region located between first and second electrodes such that charge carriers can move between the photoresponsive region and the first and second electrodes, the photoresponsive region comprising: a stack of alternate first and second layers, each first layer comprising a first photoresponsive material and each second layer comprising a second photoresponsive material, the first and second photoresponsive materials having different electron affinities; wherein each pair of second layers on opposite sides of a first layer contact each other via first holes defined by said first layer between said pair of second layers, and each pair of first layers on opposite sides of a second layer contact each other via second holes defined by said second layer between said pair of first layers.

Description: OPTOELECTRONIC DEVICES The present invention relates to optoelectronic devices which comprise a photoresponsive region and in particular to devices intended to detect light such as photodetectors and to devices intended to provide a source of electrical energy from electromagnetic radiation such as solar cells. A photodetector or a solar cell typically comprises a photoresponsive material located between two electrodes, wherein the electric potential between the two electrodes changes when the photoresponsive material is exposed to light. US Patent No. 5,670,791 discloses a polymeric photodetector comprising a layer of a blend of poly (2-methoxy-5 (2 ' - ethyl) hexyloxyphenylenevinylene) ("MEH-PPV") and poly (cyanophenylene-vinylene) ("CN-PPV") located between an indium-tin oxide (ITO) electrode and an aluminium electrode. Relatively good device efficiencies have been achieved with these devices, but there is a demand for photodetectors having even higher device efficiencies. It is therefore an aim of the present invention to provide a photoresponsive device having an improved device efficiency. According to a first aspect of the present invention, there is provided an optoelectronic device comprising a photoresponsive region located between first and second electrodes such that charge carriers can move between the photoresponsive region and the first and second electrodes, the photoresponsive region comprising: a stack of alternate first and second layers, each first layer comprising a first photoresponsive material and each second layer comprising a second photoresponsive material, the first and second photoresponsive materials having different electron affinities; wherein each pair of second layers on opposite sides of a first layer contact each other via first holes defined by said first layer between said pair of second layers, and each pair of first layers on opposite sides of a second layer contact each other via second holes defined by said second layer between said pair of first layers. According to one embodiment, the first electrode is adjacent one of said first layers and the second electrode is adjacent one of said second layers. The first holes defined by each first layer between each pair of second layers are preferably arranged in an ordered array, and the second holes defined by each second layer between each pair of first layers are also preferably arranged in an ordered array. Preferably, both the first and second photoresponsive materials are semi-conductive conjugated polymers. According to one embodiment, each first layer between a pair of second layers comprises a blend of the first and second photoresponsive materials, the proportion of first photoresponsive material being higher than the proportion of second photoresponsive material; and each second layer between a pair of first layers also comprises a blend of the first and second photoresponsive materials, the proportion of second photoresponsive material being higher than the proportion of the first photoresponsive material. According to a second aspect of the present invention, there is provided a method of producing an optoelectronic device comprising a photoresponsive region located between first and second electrodes such that charge carriers can move between the photoresponsive region and the first and second electrodes, wherein said photoresponsive region is formed by a process comprising the steps of: depositing an array of first regions on a substrate comprising the first electrode, wherein the array of first regions αefine holes therebetween exposing portions of the underlying substrate; and depositing an array of second regions in the holes defined between the first regions such that the second regions partially overlap the first regions, and αefine holes therebetween exposing the portions of the underlying first regions; wherein the first regions comprise a first photoresponsive material and the second regions comprise a second photoresponsive material, the first photoresponsive material and second photoresponsive material having different electron affinities. According to a preferred embodiment, the array of first regions define an array of distinct holes, and the array of second regions define an array of distinct holes. According to a preferred embodiment, the arrays of first and second regions are deposited by an mk-jet printing technique. According to a third aspect of the present invention, there is provided an optoelectronic device comprising a pnotoresponsive region located between first and second electroαes sucn that charge carriers can move between the photoresponsive region and the electrodes; the photoresponsive region comprising at least first and second olend layers each comprising a blend of a first photoresponsive material and a second photoresponsive material having differing electron affinities, wherein tne first and second blend layers comprise different proportions of the first and second photoresponsive materials. Preferably, at least one of the photoresponsive materials is a semi-conductive conjugated polymer, and furtner preferably, both of the photoresponsive materials are semi-conductive conjugated polymers . According to a preferred embodiment, the pnotoresponsive region comprises a stack of alternating first and second blend layers. The pnotoresponsive region preferably further comprises a layer consisting essentially of the first photoresponsive material adjacent the first electrode, and/or a layer consisting essentially of the second photoresponsive material adjacent the second electrode. These layers facilitate the transport of the respective charge carriers into the first and second electrodes. Embodiments of the invention shall be described hereunder, by way cf example, only, with reference to the accompanying drawings, in which: - Figure 1 is a schematic partial cross-sectional view of an optoelectronic device according to a first embodiment of the present invention; Figures 2(a) and 2(b) show the relationship between adjacent layers cf the device shown in Figure 1; and Figure 3 is a schematic view cf an optoelectronic device according to a second embodiment of the present invention. With reference to Figures 1, 2(a) and 2(b), a method for producing an optoelectronic device according to a first embodiment of the present invention is described below. A glass base 2 is coated with a layer of indiuπ tin oxide (ITO) to provide an anode layer 4. Next, a continuous layer of MEH-PPV 6 is deposited on the surface of the ITO layer opposite the glass substrate oy a standard deposition technique such as blade- coating, spin-coating or an mk-jet printing technique. The MEH- PPV .aver 6 has a thιcκness in the range of 5 to lOOnm, preferably 5 to 20nm. Next, an ink-jet printing technique is used to deposit an array of droplets of a solution of CN-PPV in an appropriate solvent en tr.e surface of the MEH-PPV layer opposite tne ITO layer. The volume of each droplet and the spacing of the droplets are controlled such that after crying to evaporate the solvent, there remains a layer cf CN-PPV spots 8 left on the MEH-PPV layer which contact each other (as shown in Figure 2a) or overlap each other (as shown in Figure 2b) at their circumference but also define distinct holes 10 therebetween which expose portions cf the underlying MEH-PPV layer 6. The outline cf the CN-PPV spots 3 are shown by bro en lines in Figures 2 a) and 2(b) . Next, an ink-jet printing technique is used to deposit an array cf droplets of a solution of MEH-PPV in an appropriate solvent or. the layer of CN-PPV spots 8 such that after drying to evaporate solvent, there remains a layer of MEH- PPV spots 12 which fill the holes 10 defined by the underlying array of CN-PPV spots 8 and partially cover the underlying CN- PPV spots 8 and define an array of distinct holes 14 therebetween which expose portions of the underlying CN-PPV spots 8. The MEH-PPV spots 12 (as shown by full lines in Figures 2(a) and 'b) ) also contact with each other (as shown in Figure 2(a)) or overlap with each otner (as snown in Figure 2(b)) at their circumference. Examples cf suitable solvents for the solutions cf MEH-PPV and CN-PPV include xylene, toluene and THF. It is preferable to use the same solvent for both MEH-PPV and CN-PPV with a view to achieving some intermixing. Each layer should be dried before deposition cf the next layer. This procedure s repeated tc produce an alternating stack of layers cf CN-PPV and MEH-PPV spots of the desired thickness, with a layer cf MEH-PPV spots as the top layer. The stack preferably has a thickness corresponding to an optical density of between 1 and 3. The thickness of tne stack is preferably in the range of 100 to 400nm. Next, a continuous layer of CN-PPV 16 is deposited on the top layer of MEH-PPV spots 12 by a standard technique such as blade- coating, spin-coating or an mk-jet printing technique, and a cathode 18 is formed on the surface of the continuous layer of CN-PPV opposite the top layer of MEH-PPV spots. The cathode 18 preferably comprises a layer of aluminium or has a bi-layer structure comprising an underlying thin layer of a metal having a low work function such as calcium and an overlying thicker layer of aluminium. For the sake of simplicity, Figure 1 shows a schematic partial cross-sectional view of a device having only two layers of MEH- PPV spots 12 and two layers of CN-PPV spots 8 wherein the layers of spots have the pattern generally shown in Figure 2 (a) . Figure 1 is a schematic partial cross-sectional view taken through line A-A in Figure 2(a). In an alternative modification, the MEH-PPV spots 8 could be replaced by MEH-PPV/CN-PPV spots comprising a blend of a major proportion of MEH-PPV and a minor proportion of CN-PPV. Similarly, the CN-PPV spots 12 could be replaced by CN-PPV/MEH- PPV spots comprising a blend of a major proportion of CN-PPV and a minor proportion of MEH-PPV. In the product device, the thickness of the spots is in the range of 5nm to lOOOnm, preferably 5nm to 20nm, and the diameter of the spots is in the range of 0.5 to 1000 microns, preferably 5 to 50 microns. The close proximity of photoresponsive materials having differing electron affinities ensures efficient charge separation when an exciton is formed within the photoresponsive region upon exposure of the pnotoresponsive region to light. The thιcκness of the spots is preferably made as small as possible to minimise the lateral diffusion length i.e. the distance tnat a charge carrier needs to travel before collection in an area of high or low electron affinity, as the case may be, to thereby maximise the device efficiency. The use of an mk-jet printing technique to deposit the ordered array of droplets means that droplets of very small size can be deposited resulting in an ordered array of regions of different photoresponsive materials having a very small size, thereoy making it possiole to produce a device naving a very good device efficiency . Furthermore, the feature that the array of CN-PPV and MEH-PPV spots is ordered ensures that the lateral diffusion length is uniformly small wherever the exciton is formed within the photoresponsive region, thereby ensuring that the device exhibits a uniform sensitivity across the whole area of the photoresponsive region. As described aoove, the alternate two-dimensional ordered arrays of MEH-PPV spots and CN-PPV spots are staggered, resulting m paths from each MEH-PPV spot to the anode which only pass through MEH-PPV, and paths from each CN-PPV spot to the cathode which only pass through CN-PPV. The provision of such paths increases the efficiency of collection of the respective cnarge carriers by the anode and cathode, respectively, thereoy furtner maximising the efficiency of the device. Next, with reference to Figure 3, a second embodiment of tne present invention will be described. A layer of MEH-PPV 34 is deposited on a glass base 30 provided witn an anode layer 32. The anoαe layer 32 is typically a layer of ITO. A layer of a CN-PPV/MEH-PPV blend 36 comprising a major proportion of CN-PPV and a minor proportion cf MEH-PPV is deposited on the MEH-PPV layer 34. The proportional ratio of MEH-PPV to CN-PPV is preferably m the range of 5:95 to 40:60. In this embodiment, the ratio is 20:80. Next, a layer 38 of a MEH-PPV/CN-PPV blend comprising a major proportion of MEH-PPV ano a minor proportion of CN-PPV is deposited on the underlying CN-PPV/MEH-PPV layer 36. The proportional ratio of MEH-PPV to CN-PPV is preferably in the range of 95:5 to 60:40. In this emoodiment, the ratio is 80:20. This deposition of alternating CN-PPV/MEH-PPV and MEH-PPV/CN-PPV blend layers is repeated until a stacK of the desired nu oer of layers is achieved with the top layer being a MEH-PPV/CN-PPV blend layer. Next, a layer consisting of CN-PPV is formed on the top MEH-PPV/CN-PPV blend layer, followed by the formation of a cathode 42 on the surface of the CN-PPV layer 40. The cathode typically comprises one or more metal layers. In a preferred embodiment, it comprises a thin layer of a metal having a low work function such as calcium and an overlying layer of aluminium. In tne final device, the MEH-PPV layer and the CN-PPV layer adjacent the anooe and cathode, respectively have a thιcκness m the range of 5 to 3000A, preferably 50 to 150 A, and the MEH- PPV/CN-PPV and CN-PPV/MEH-PPV blend layers have a thickness in the range of 5 to 3000 A, preferably 50 to 150 A. The stack preferably has a total thickness sufficient to absorb about at least 50% of lignt passing through the stack having a wavelength in the range cf 350 to 800nm. If, for example, the device is tnen provided with a nigh reflectivity cathooe, about 75- or greater cf light incident on the stack will be adsorbed as incident light wnich is not absorbed by the stack on the first pass is reflected back througn the stack. Preferaoly, the stack: has a total thic .ess sufficient to aosorb aoout 90% of the liσht. Any polymer deposition technigue such as spm-coating or blade- coating can be used to deposit the polymer and polymer blend layers. An mk-jet printing technique is preferred because it can be used to produce ravers of very small thicknesses. In the embodiments descπoed above, MEH-PPV and CN-PPV have been used as photoresponsive materials having different electron affinities. However, it will be understood to the person skilled in the art that the present invention is not limited to the use of tnese specific materials but is σenerally applicable to any appropriate combination of photoresponsive materials having different electron affinities.

Claims: .AIMS 1. An optoelectronic device comprising a photoresponsive region located between first and second electrodes such that charge carriers can move between the photoresponsive region and the first and second electrodes,' the pnotoresponsive region comprising: a stack of alternate first and second layers, each first layer comprising a first photoresponsive material and each second layer comprising a second photoresponsive material, the first and second photoresponsive materials having different electron affinities; wherein each pair of second layers on opposite sides of a first layer contact each other via first holes defined by said first layer between said pair of second layers, and each pair of first layers on opposite sides of a second layer contact each other via second holes defined by said second layer between said pair of first layers. 2. An optoelectronic device according to claim 1 wherein the first electrode is adjacent one of said first layers and the second electrode is adjacent one of said second layers. 3. An optoelectronic device according to claim 1 or 2 wherein the first holes defined by each first layer between each pair of second layers are arranged in an ordered array. 4. An optoelectronic device according to any of claims 1 to 3 wherein the second holes defined by each second layer between eacn pair of first layers are arranged in an ordered array. 5. An optoelectronic device according to any preceding claim wherein each first layer between a pair of second layers comprises an ordered array of interconnecting first regions wnicr- define said first holes therebetween, and each second layer between a pair of first layers comprises an ordered array of interconnecting second regions wnicn define said second holes thereoetween. 6. An optoelectronic device according to any preceding claim, wherein each first layer between a pair of second layers comprises a blend of the first and second photoresponsive materials, the proportion of first photoresponsive material being higher than the proportion of second photoresponsive material; and each second layer between a pair of first layers also comprises a blend of the first and second photoresponsive materials, the proportion of second photoresponsive material being higher than the proportion of tne first photoresponsive material . 7. An optoelectronic device according to any preceding claim wherein the first photoresponsive material and second photoresponsive material are semi-conducting conjugated polymers . 8. A method of producing an optoelectronic device comprising a photoresponsive region located between first and second electrodes such that charge carriers can move between the photoresponsive region and the first and second electrodes, wherein said photoresponsive region is formed by a process comprising the steps of: depositing an array of first regions on a substrate comprising the first electrode, wherein the array of first regions define holes therebetween exposing portions of tne underlying substrate; and depositing an array of second regions in the holes defined between the first regions such that tne second regions partially overlap the first regions and define holes therebetween exposing the portions of the underlying first regions; wnerein the first regions comprise a first photoresponsive material and the second regions comprise a second photoresponsive material, the first photoresponsive material and secor-d photoresponsive materials having different electron affinities. 9. A method according to claim 8 wherein the array of first regions define an array of distinct holes. 10. A method according to claim 8 or 9 wherein the second regions define an array of distinct holes. 11. A method according to any cf claims 8 to 19 wherein the process of forming the photoresponsive region further comprises the step of forming an array of third regions comprising the first photoresponsive material in the holes defined by the array of second regions such that the third regions partially overlap the second regions and define holes therebetween exposing portions of the underlying second regions; 12. A method according to any of claims 8 to 11 wherein the process of forming the photoresponsive region further comprises the step of forming an array of fourth regions comprising the second photoresponsive material in the holes defined by the third regions such that the fourth regions partially overlap the third regions and define holes therebetween exposing portions of the underlying third regions . 13. A method according to any of claims 8 to 12 wherein the substrate comprising the first electrode further comprises a continuous layer of the second photoresponsive material, and the first regions are formed on tne continuous layer of the second photoresponsive material. 14. A method according to claim 13 wnerem tne pnotoresponsive region further comprises a continuous layer of first photoresponsive mateπa--- adjacent the second electrode. 15. An optoelectronic device comprising a photoresponsive region located betwee- first and second electrodes such that charge carriers can move between the photoresponsive region and the electrcoes; the pnotoresponsive region comprising at least first and second bleno layers each comprising a blend of a first photoresponsive material and a second photoresponsive material having differing electron affinities, wherein the first and second bleno layers comprise different proportions of the first and second photoresponsive materials. 16. An optoelectronic device according to claim 15 wherein the first photoresponsive material is a semi-conductive conjugated polymer. 17. An optoelectronic device according to claim 15 or claim 16 wherein the second photoresponsive material is a semi-conductive conjugated polymer. 18. An optoelectronic device according to any of claims 15 to 17 wherein the photoresponsive region comprises a stack of alternating first and second blend layers . 19. An optoelectronic device according to any of claims 15 to 18 further comprising a layer consisting essentially of the first photoresponsive material adjacent the first electrode. 20. An optoelectronic device according to any of claims 15 to 19 further comprising a layer consisting essentially of the second photcresponsive material adjacent the second electrode. 21. An optoelectronic device substantially as hereinbefore described with reference to Figure 1 and Figure 2(a) or 2(b), or Figure 3. 22. A method of producing an optoelectronic device substantially as hereinbefore described with reference to Figure 1 and Figure 2(a) or 2(b), or Figure 3.

Inventors: BURROUGHES JEREMY HENLEY, BURROUGHES, JEREMY, HENLEY
Applicants: BURROUGHES JEREMY HENLEY, CAMBRIDGE DISPLAY TECH, BURROUGHES, JEREMY, HENLEY, CAMBRIDGE DISPLAY TECHNOLOGY LIMITED
Application year: 2001
Main classification: H01L51
IPC codes: H01L27, H01L31, H01L51