Patent Application: US-201113823929-A

Abstract:
a material design is provided for a light and carrier collection architecture in single junction and multi - junction photovoltaic and light sensor devices . the lccm architecture improves performance and , when applied to single or multi - junctions , can lead to solar cells on flexible plastic substrates which can be easily deployed and even draped over various shapes and forms . the device has an array of conducting nano - elements in electrical and physical contact with the planar electrode . a spacer of 0 to 100 nm in thickness may be used to contact the array of conducting nano - elements . one or more volume regions comprised of at least one light absorbing material is present with the first in simultaneous contact with said spacer to form an operating photovoltaic single - or multi - junction device with periodic undulations to enhance trapping of the impinging light and photocarrier collection throughout the absorber volume regions .

Description:
the present invention has utility in lccm single and multi - junction solar cells . in the examples presented , cells are made or modeled or both using silicon - based absorber materials . the present invention is not limited to these absorbers and may be applied to organic ( including dye ) absorbers as well as to inorganic absorbers including fes 2 , cu 2 znsn ( se , s ) 4 , cigs , cdte . iii - v semiconductors and their alloys , and lead - based materials . the basic lccm architectures are in general depicted in the attached schematics for a single junction p - i - n cell yet it is appreciated that the inventive structures are applicable to p - n and surface barrier cells also . these basic architectures are presented schematically in fig2 a and 2b where they are seen to include a two - dimensional array of unit cells 10 with a nano - element 12 at each unit center . light can enter these arrays either through the substrate 14 ( superstrate structure ) ( fig2 b ) or through the free surface ( substrate structure ) ( fig2 a ). in fig2 a and 2b , an electrode array nano - element 12 inherent in each unit cell 10 is seen and a metal ( e . g ., ag , au , cu and alloys ) layer 16 serves as the counter electrode and as a back reflector . doping of the photonic absorber 17 such as p + layer 18 and / or n + layer 20 , spacers 22 and 24 such as et / hbl and ht / ebl layer , respectively are also optionally provided . the deposited absorber is part of 17 and has a thickness t , the nano - element has a height h , and an inter - electrode element array spacing is l . carrier collection enhancement is attained by using the nano - elements 12 to ensure that photocarriers in the absorber in 17 are within a collection length of their respective electrodes . light collection enhancement can be obtained through photonics , plasmonics , and effective absorber thickness phenomena of fig2 a and 2b . the electrode array nano - element 12 inherent in each unit cell is clearly depicted in these figures . it may have a variety of shapes including cone - like and columnar . also seen is the metal ( e . g ., ag ), which serves as the counter electrode and as a reflector . spacer layers 22 and 24 are also shown . these may be present to aid in adjusting the optical electric field distribution — and therefore the absorption and photocarrier generation distributions — as well as to serve as hole blocking / electron transport layers at the cathode ( negative voltage ) electrode and electron blocking / hole transport layers at the anode ( positive voltage ) electrode [ 3 ]. the lccm architecture with its repeated ( in two - dimensions across the plane of the substrate ) unit cell , electrode nano - element array is fabricated by establishing the nano - element pattern on a planar electrode ( fig2 a and 2b ) and then depositing the absorber containing 17 , spacer 22 or 24 , and doped layers 18 and 20 in the sequence necessary to order the materials as shown in fig2 a and 2b . the sequential deposition onto the initial nano - element array produces the undulating shape seen in fig2 a and 2b during fabrication and does so without any intervention with selection of processing parameters giving the appropriate conformality . the nano - element array serves to aid in collecting photocarriers from every point in the absorber volume and may serve also as a photonic structure . the nano - element array , or some part thereof , serves as both a structured electrode ( for efficient photocarrier collection ) and a photonic ( and depending on the materials , a plasmonic ) structure . this inventive lccm architecture is operative for single junction cells and in multi - junction cells . the latter are attained by repeating the required deposition sequences and tailoring inter - cell light reflection and transmission ,. a process flow for lccm single junction cells using a - si : h as the absorber has resulted in our demonstration of an 8 . 2 % pce lccm cell ( without antireflection coating ), which is the highest pce reported for any nano - structured solar cell , and our development of a clear pathway to 11 % pce a - si : h cell technology at $ 0 . 80 / watt . this price per watt value compares favorably with $ 2 / watt , which is the current best value for the price per watt ratio . the lccm architecture allows the attainment of this $ 0 . 80 / watt value for a - si : h single junction cells because the cell design requires less a - si : h absorber due to light and carrier collection management thereby saving deposition time and cost . it is important to underscore that the single and multi - junction designs presented herein are , however , in no way limited to a - si : h . they can be applied to organic ( including dye ) absorbers as well as to inorganic absorbers including fes 2 , cu 2 znsn ( se , s ) 4 , cigs , cdte . iii - v semiconductors and their alloys , and lead - based materials . fig3 shows the cross - section of an actual inventive superstrate a - si : h single junction lccm cell . in this device , the electrode nano - elements penetrating into the a - si : h absorber are formed from aluminum - doped zinc oxide ( azo ), a well - known transparent conducting oxide ( tco ). other electrode shapes and materials may be used . the array of nano - scale electrode elements seen in fig2 a , 2 b , and 3 is basic to the lccm architecture and constitutes at least a part of one electrode . in the particular device of fig3 , the counter electrode is ag coated with al . the periodic structure of our lccm design gives rise to photonic effects ( e . g ., light trapping and advantageous optical electric field distribution ) and can also produce plasmonic phenomenon in the electrode elements , counter electrode , or both , depending on material composition . the arrangement of electrode elements , their height choice , and the absorber thickness choice are picked to insure that photocarriers generated in the absorber are within a collection length of the electrodes . the basic pattern generation process used to produce an actual lccm structure can be based on techniques such as optical , holographic , nano - imprinting , stamping , probe , nano - sphere , block - copolymer , or beam lithography . the pattern generation process first creates the array of nano - elements . these may be conducting and may be comprised of an inorganic , or organic conductor ( e . g ., metal , transparent conducting material )) or inorganic , or organic semiconductor . these nano - elements may be created by directly depositing them as an “ ink ” using a nano - probe technique . in fact , as will be described , conducting cone - like nano - elements disposed on ag are a very effective substrate lccm design and such an array may be made with this “ nano - ink ” approach of pattern generation . these nano - elements may be created by imprinting a pattern into an organic conductor , as an example . they may be created by imprinting empty volumes into a resist , using these volumes as templates , and subsequently electro - chemically growing or depositing the electrode element material using the empty volume template . a lift - off step may follow to better define the nano - elements . alternatively , the deposition of a conductor onto the nano - element exposed material may follow and may even be done to a thickness level to ensure mechanical stability of the nano - element array . the latter can be used in an approach with transfers the nano - element array from an initial substrate to a final substrate for process sequence completion . in any case , disposition of the absorber , its junction forming , and optional spacer materials is then undertaken in the order seen in fig2 a , 2 b and 3 and followed by the counter electrode formation , if a single junction cell is the objective . if a two terminal non - planar multi - junction cell is the objective , disposition of the absorber , its junction forming , and optional spacer materials is undertaken in the proper sequencing order as defined in fig2 a , 2 b and 3 . this is then followed at inter - cell boundary by an inter - cell electrical and optical matching structure . this may be accomplished by the standard tunnel junction formation used in multi - junctions [ 3 ] but done with the simultaneous objective of optimizing the light transmission and reflection . this latter objective may be achieved , for example , by creating a bragg stack structure for the reflection of supra - band gap photons back into the wider band gap absorber . these steps are then repeated . that is , there is a repetition of the absorber , its junction forming , and optional spacer materials depositions sequenced as defined in fig2 a , 2 b and 3 . the electrical and optical matching structure formation and subsequent absorber , junction forming , and optional spacer materials depositions are done as many times as is necessary for a non - planar multi - junction cell . the electrical and optical matching structure formation and junction forming materials steps may be designed to be combined . the unit cell of a two - junction non - planar multi - junction is seen in fig4 . certain single - junction lccm design rules become apparent from these various examples of single junction superstrate and substrate lccm solar cells devices . the inventive substrate designs are superior and inter - dome scattering is present in the inventive devices and can be optimized . tco nano - element or coated nano - element arrays on a metal reflector / electrode ( e . g ., ag ) give excellent performance . this result is opposite to what is taught in ref . 6 . inventive devices can be used to simultaneously to : ( 1 ) reduce the amount of absorber material used , and ( 2 ) enhance pce . both advantageously affect the crucial cell cost / watt ratio . nano - element spacing l in the 400 to 1000 nm range can be optimal , depending on h , d , etc . and are readily determined . this spacing range is easily suited to pattern generation approaches such as optical , holographic , nano - imprinting , stamping , probe , or beam lithography and to roll - to - roll processing . in addition , the roles of nano - element height , back metal , optical spaceret / hbl or ht / ebl layer thickness have been shown to be important . all of this is done utilizing thin films of tcos and avoiding the use of thicker film , randomly textured tcos commonly employed in solar cells . all of this can be done in structures for which photogenerated carriers are within a collection length of their collecting electrode . as noted above , multi - junction lccm non - planar cells are fabricated by following the design sequencing inherent in the single junction non - planar structure . multi - junction cells having lccm non - planar cells on planar cells have the planar cell fabricated and then the lccm cell is disposed on top of the planar cell . the lccm architecture applied to multi - junctions gives ( 1 ) enhanced absorption in all layers , ( 2 ) enhanced long wavelength absorption , ( 3 ) the freedom to reduce absorber layer thicknesses ( less material is needed ), and the ability to employ less stable absorbers in thinner layers . there is another further extremely important point . the collecting electrode elements and thin absorber layer versatility also gives the designer the opportunity to use absorbers with lower carrier mobilities and lifetimes . finally , fig1 a and 12b point to one more additional advantage of the lccm approach to single junction and full spectrum multi - junction cells . this figure makes the point that computer simulation shows that a single junction lccm cell with its nano - structured array is less sensitive to the light impingement angle . still another advantage of the lccm architecture for single and multi - junction solar cells can be seen by directly comparing this approach to light management versus that of transparent conductive oxide ( tco ) texturing . this comparison makes the following points : texturing is a random process resulting in a range of feature sizes and shapes . random texturing can be inherently difficult to control in manufacturing . texturing feature sizes can be larger than cell layer thicknesses giving the potential for shorting sites . the lccm structure is based on an array layout . it is systematic with no randomness . in the case of non - planar multi - junctions , the systematic array pattern in the bottom cell is transferred to other cells by the fabrication process flow thereby giving a periodic structure in every layer . in the case of the hybrid cell design , the systematic array pattern is only used in the cell disposed onto the planar cell . with the lccm architecture , the wavelengths and magnitudes of the fabry - perot absorption changes can be advantageously shifted and adjusted by modifying the lccm design ( e . g ., by modifying l , r , h , t , and the spacer layers ). such flexibility is not possible in texturing . the present invention is further detailed with respect to the following non - limiting examples . these examples should not be construed as limiting the scope of the appended claims . plasma enhanced chemical vapor deposited ( pecvd ) a - si : h was used as the absorber in superstrate single junction structures . atomic layer deposition ( ald ) was first used to coat the indium tin oxide ( ito ) on a glass substrate with transparent , conducting aluminum zinc oxide ( azo ). this azo served as an optical spacing layer , as hole transport layer , and as protection for the hydrogen plasma - sensitive ito during a - si : h pecvd from silane type gases . these materials are appreciated to be exemplary and that alternative materials with similar optical and electrical properties are readily substituted by a routineer in the art . after applying a template material , void regions were created in the template by standard e - beam lithography - based processing and ald was used to produce azo nano - elements in each template void region , thereby resulting in an array of azo conducting , but transparent nano - elements protruding from the ito electrode . the array of such nano - elements can be discerned from the fesem cross - section in fig3 . an etch step after ald was used to remove any azo which grew onto the exposed template lateral surface and the template material was removed by standard removal procedures . the resulting nano - elements are essentially perpendicular to the ito planar electrode material . doped , intrinsic , and again doped pecvd a - si : h layers were then sequentially deposited onto the nano - element array using pecvd parameters known to produce conformal deposition this was followed by the ald deposition of an azo back spacer layer and the deposition of an ag / al counter electrode / reflector , as seen in fig3 . the superstrate lccm single junction devices produced in this manner have yielded the highest pce ( 8 . 2 %) reported to - date for any nano - structure based a - si : h cell and this was achieved without any anti - reflection ( ar ) layer . the fabrication and modeling expertise that has developed in working with single junction lccm cells has shown that this architecture is very advantageous also for multi - junction configurations of the present invention . the unit cell of a 2 - junction substrate lccm non - planar multi - junction device is shown in fig4 . this figure shows a wider gap material optically in series with a narrower gap back material with light initially entering through the wider gap material at the free surface . such devices can be a two terminal tandem cell but an also be modified to function as three terminal devices . computer modeling work on both single junction and multi - junction lccm solar cell structures ( a - si : h , nc - si : h , and tandem a - si : h / a - si ( 1 - x )/ ge ( x ): h ) shows the benefits resulting from the incorporation of nanostructures according to the architecture of this invention ( i . e ., lccm approach versus planar controls ). in all cases , the lccm approach outperforms the planar controls . fig5 gives the absorption as a function of wavelength for a single - junction nano - crystalline silicon ( nc - si ) superstrate lccm structure and for the corresponding nc - si planar structure . these plots are computer modeling results obtained for an inventive design simulated with maxwell &# 39 ; s equations solver software and expertise available at the university of arkansas ( ua ). these results are for single junction cells and they drive home an important point : the lccm architecture greatly enhances absorption , particularly at long wavelengths — and it is doing so in fig5 for a 400 nm nc - si deposition . fig6 gives an lccm substrate configuration 1 single junction cell which , by definition , has the light entry through the top ( 80 nm azo ) anode . the cell has 100 nm diameter aluminum zinc oxide ( azo ) columns as the nano - elements which are sitting on a cathode composed of 5 or 30 nm of azo coated onto an opaque planar ag film . the transport function of this azo coating is to serve as an electron transport / hole blocking layer ( et / hbl ) at the cathode . it also has an optical function , as will emerge in our discussion of the j sc response versus nano - element spacing l obtained from modeling . this response is given in fig6 for the two et / hbl thicknesses . as seen , j sc decreases with decreasing l for l & lt ; l touch and has two maxima in the range l ≧ l touch one of which is an absolute maximum near l ˜ l touch . the quantity l touch is the specific l for which the domes just touch . the j sc dependence on l in the l & gt ; l touch range present in fig6 is very different from that of superstrate structures where the role of the effective absorber thickness causes j sc to monotonically decrease with increasing l . interestingly , the j sc behavior for l & gt ; l touch in fig6 points to scattering among domes in this configuration 1 lccm substrate cell . the two j sc plots in fig6 point out the sensitivity of the a ( λ ) behavior to the details of device layer thicknesses and composition . detailed examination shows that both geometrical scattering and metal scattering , at the cathode , play a role through changes in the long wavelength a ( λ ) behavior . the former manifests itself most clearly in shifts in the fabry - pirot absorption peaks present in a ( λ ) with changes in the et / hbl azo layer thickness . the metal scattering role shows up in the significant reduction in the long wavelength a - si : h absorption which occurs when ag is replaced by al for the 5 nm et / hbl case . this points to the thinner et / hbl case , which still has azo that is thick enough to block hole tunneling at the cathode , allowing an enhanced near - field at the ag surface to penetrate the absorber more deeply . the short circuit current density given by modeling a planar control cell with a 200 nm thick a - si : h absorber is j sc = 10 . 97 ma / cm 2 and it is 14 . 08 ma / cm 2 for a planar control cell with a 750 nm thick a - si : h absorber . these are useful comparison j sc numbers since the first is for the limiting control planar structure obtained as the spacing l →∞ and the second is for the limiting control planar structure obtained if the absorber had , everywhere , the a - si : h thickness seen at the peak of the domes in fig6 . the superiority of the lccm configuration can be seen in fig . 6 since j sc exceeds these control values over much of the l range and can attain at least 17 . 3 ma / cm 2 for the configuration 1 architecture . the full advantage of substrate lccm approach can be understood by considering an areal mass density ( amd ) defined by expressing volumes in cm 3 and areas in cm 2 and taking 2210 mg / cm 3 as mass density of a - si : h allows amd to be calculated in mg / cm 2 and plotted for configuration 1 as seen in fig6 . using the j sc and amd data for configuration 1 and its controls allows us to determine that the peak j sc = 17 . 3 ma / cm 2 in fig6 is 58 % higher than the attainable with the 200 nm thick planar absorber control and yet the lccm device uses only 64 % more a - si : h . the latter point follows from noting that the amd for the lccm device giving the j sc peak is 0 . 072 mg / cm 2 and that for the 200 nm control is 0 . 044 mg / cm 2 . by comparison , the 750 nm thick planar absorber control has a 28 % increase in j sc compared to than that attainable with the 200 nm thick planar absorber control and yet it uses 275 % more a - si : h , since amd for this control is 0 . 165 mg / cm 2 . the increase in j sc for the substrate lccm structure means an increase in pce . the saving in absorber material for the substrate lccm structure means savings in deposition time and cost . turning to substrate configuration 2 seen in fig7 , it is seen that this substrate lccm architecture also uses columns as the nano - elements . these are composed of 5 nm azo ( et / hbl ) film on an ag coated sio 2 core . each of these nano - columns sits on a planar ag cathode and , in between the nano - columns , there is a 5 nm or 30 nm azo et / hbl residing on the planar ag . while the amd is the same function of l for both configurations 1 and 2 , the resulting a ( λ ) plots from our simulation design studies ( not shown ) for configuration 2 are inferior to those of configuration 1 in the middle wavelengths but even more so in the longer wavelengths . the impact of this poorer a ( λ ) performance on j sc is displayed in fig7 for the two thickness values of the planar inter - column azo layer . the thinner inter - column azo layer is seen to improve the j sc capability but not to the level seen in fig6 even though the amd dependence on l is the same . while the j sc curves of fig7 display an overall dependence on l which is similar to that of fig6 , there are two peaks present for the 30 nm but only one for the 5 nm inter - column azo planar layer . this points to a significant difference in the geometrical scattering interaction taking place when the inter - column azo layer thickness is changed from 5 to 30 nm . the simulations also suggest there is a metal cathode scattering influence involved in the thinner inter - column azo case since replacing ag with al for the 5 nm azo inter - column layer reduces the long wavelength a ( λ ) response ( not plotted ). interestingly , the fact that the peak in j sc clearly occurs for l & gt ; l touch again points to significant scattering interactions among domes in some substrate lccm architectures . fig7 shows that j sc exceeds the 200 nm and 750 nm thick planar absorber control values over much of the l range and can attain at least 15 . 9 ma / cm 2 with the configuration 2 architecture . this is not nearly as good as configuration 1 nor as good as the example presented by configuration 3 . the distinguishing feature of configuration 2 is that it has an ag film covering the nano - element . this approach is used in ref . 9 . configuration 3 is the same as configuration 1 except the azo nano - elements are now cone - shaped . as is the case for configuration 1 , these nano - elements are positioned on a layer composed of 30 nm of azo on planar ag . configuration 3 is similar to that studied in ref . 2 except the structure of that reference has an ag coating over the nano - cones . the a ( λ ) for this architecture ( not plotted ) again has variations in its magnitude and fabry - perot peak positions which depend on l thereby demonstrating the importance of the geometrical scattering . the resulting j sc as a function of l is given from simulation studies in fig8 . the data are shown for two values of the cone - shaped element height h ( 350 and 550 nm ) to convey the roll of this parameter in device performance . because two h values have been examined , there are two corresponding amd plots in fig8 . the general behavior of j sc as a function of l for l & gt ; l touch in fig8 is essentially that seen in fig6 and 7 . again the fact that j sc increases for certain l values indicates that scattering among domes is playing a role in this l & gt ; l touch range . fig8 shows that j sc exceeds the 200 nm and 750 nm thick planar absorber control values over much of the l range and can attain at least 17 . 1 ma / cm 2 with the configuration 3 architecture . using the j sc and amd data for configuration 3 and the controls allows us to determine that the peak j sc = 17 . 1 ma / cm 2 in fig8 , which occurs for the taller nano - element case , is 56 % larger than the j sc attainable with the 200 nm thick planar absorber control . the lccm device giving rise to this peak uses only 48 % more a - si : h than that used in the 200 nm control . again we point out that the 750 nm thick planar absorber control has a 28 % increase in j sc compared to than that attainable with the 200 nm thick planar absorber control but uses 275 % more a - si : h . consideration of the geometry in fig8 makes another interesting point : this enhanced lccm optical performance is attained while keeping all the photocarries within 224 nm of an electrode ; i . e ., well within a collection length . multi - junction superstrate and substrate lccm cells are composed of some combination of two or more p - n , p - i - n , or surface barrier junctions and offer the following : ( 1 ) superstrate or substrate configurations ; ( 2 ) inter - cell electrical and optical matching structures which may comprise ( a ) a tunnel junction structure , ( b ) a tunnel junction and bragg stack reflector structure , and ( c ) a tunnel junction and a plasmonic reflector ; and ( 3 ) hybrid configurations using both an lccm cell or cells and using a planar cell or cells . an example of this last design is a substrate tandem cell composed of a lccm top a - si : h p - i - n cell such as that seen in the inset of fig7 positioned on top of a bottom planar p - i - n nc - si cell , this example structure uses the nano - element array in the top cell only . fig7 shows that a single junction lccm cell with only a 200 nm a - si : h absorber can generate a short circuit current density of 17 . 3 ma / cm 2 . a planar nc - si cell positioned under the lccm a - si : h cell can match this current density of the top cell by using a nc - si absorber thickness in the 1000 nm to 1500 nm range . if we take typical a - si : h and nc - si open circuit and fill factor values of v oc = 0 . 86 ev and ff = 0 . 63 for a - si : h cells and v oc = 0 . 54 ev and ff = 0 . 77 for nc - si , then this lccm / planar tandem design with short circuit current density of 17 ma / cm 2 should have a power conversion efficiency ( pce ) of about 16 . 6 % ( assuming v oc = 1 . 4 ev and ff = 0 . 7 ). it is important to realize that this will be a stable tandem and will not degrade appreciably since the a - si : h layer is so thin . the inter - cell interface region of the example just discussed may utilize a bragg stack reflector ( i . e ., a bragg minor ), a plasmonic reflector , or no reflector . fig9 shows the case where the inter - cell region contains a bragg stack reflector designed to reflect light within a bandwidth centered on λ back into a top cell . the geometrical thicknesses of the high - and low - index films are t h = λ /( 4 n h ) and t l = λ /( 4 n l ) respectively . here n h and n l , are the indices of refraction of the high - and low - index films , respectively . for the example of an lccm a - si : h cell on planar nc - si cell , this bragg structure can be taken to have high n layer to be a - si : h with n h ˜ 4 and the low n layer to be a tco with n h ˜ 2 , therefore t h ˜ 38 nm and t l ˜ 76 nm if λis taken to be 600 nm . a λ of about this wavelength is selected because photons in the 500 nm to 700 nm range can make it through the a - si : h to the bottom cell even though they are supra - band gap photons in the a - si : h . consequently , in this example tandem , photons in this wavelength range need to be reflected back into the top cell . this inter - cell structure could also function as a part of the tunneling interface between the two cells . for example , if photogenerated electrons are coming to the inter - cell region from the top a - si : h cell , then the first high n layer could be doped to be a n +/ p + tunnel junction and the last low n layer could be a tco contact to the p + contact of the bottom nc - si cell . fig4 shows an a - si : h on nc - si lccm tandem example in which both the top and bottom cells are non - planar due to the use of a nano - element array in the bottom cell . in modeling of this structure we took the electrical and optical matching structure to have a thin ( 5 nm ag ) metal layer as a plasmonic reflector . the absorption data of several different nano - element spacings are shown for the cell of fig4 in fig1 . as may be noted from fig1 too much of the light in the range from ˜ 500 to 600 nm is surviving to enter into the bottom cell and being absorbed there . light in this range has photons above the a - si : h band gap and should be adsorbed in the top a - si : h cell the photonic reflector must be adjusted to reflect this light back into the top cell effectively . alternatively , a bragg reflector - type structure could be utilized in the inter - cell region . this would be conformally deposited . any patents or publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains . these patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference . 1 . g . s . kinsey , p . pien , p . hebert , and r . a . sherif , “ operating characteristics of multijunction solar cells ,” solar energy materials & amp ; solar cells 2009 , 93 , 950 . 2 . 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