pss 3D photonic crystal intermediate reflector for micromorph thin-film tandem solar cell solidi physica status

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1 phys. stt. sol. () 205, No. 12, (2008) / DOI / D photonic crystl intermedite reflector for micromorph thin-film tndem solr cell physic sttus pplictions nd mterils science Andres Bielwny *, 1, Johnnes Üpping 1, Pul T. Micle 1, Rlf B. Wehrspohn 1,Crsten Rockstuhl 2, Flk Lederer 2, Mrius Peters 3, Lorenz Steidl 4, Rudolf Zentel 4, Seung-Mo Lee 5, Mto Knez 5, Andres Lmbertz 6, nd Reinhrd Crius 6 1 Institute of Physics, µmd group, Mrtin-Luther-University of Hlle-Wittenberg, Heinrich-Dmerow-Str. 4, Hlle, Germny 2 Institute of Condensed Mtter Theory nd Solid Sttes Optics, Friedrich Schiller University Jen, Jen, Germny 3 Freiburg Centre for Mteril Reserch, University of Freiburg, Freiburg, Germny 4 Dept. of Chemistry, Phrmcy nd Erth Science, Johnnes Gutenberg University of Minz, Duesbergweg 10 14, Germny 5 Mx Plnck Institute of Microstructure Physics, Weinberg 2, Hlle, Germny 6 Institute of Energy Reserch, IEF-5 Photovoltics, Forschungszentrum Jülich GmbH, Jülich, Germny Received 5 July 2008, revised 22 September 2008, ccepted 22 September 2008 Published online 18 November 2008 PACS Qs, Jg, Bf, Bc, Jt * Corresponding uthor: e-mil [email protected] The concept of 3D photonic intermedite reflectors for micromorph silicon tndem solr cells hs been investigted. In thin-film silicon tndem solr cells consisting of morphous nd microcrystlline silicon with two junctions of -Si/µc-Si, efficiency enhncements cn be chieved by incresing the current density in the -Si top cell. It is one gol to provide n optimized current mtching t high current densities. For n idel photon-mngement between top nd bottom cell, spectrlly selective intermedite reflective lyer (IRL) is necessry, which is less dependent of the ngle of incidence thn stte-of-the-rt thickness dependent mssive interlyers. The design, preprtion nd chrcteriztion of 3D photonic thin-film filter device for this purpose hs been pursued stright forwrd in simultion nd experimentl reliztion. The inverted opl is cpble of providing suitble opticl bnd stop with high reflectnce nd the necessry long wvelength trnsmittnce s well nd provides further options for improved light trpping. We hve determined numericlly the reltive efficiency enhncement of n -Si/µc-Si tndem solr cell using conductive 3D-photonic crystl. We hve further fbricted such structures by ZnO-repliction of polymeric opls using chemicl vpour deposition nd tomic lyer deposition techniques nd present the results of their chrcteriztion. Thin film photonic IRL hve been prepred t the rer side of -Si solr cells. Completed with bck contct, this is the first step to integrte this novel technology into n -Si/µc-Si tndem solr cell process. The spectrl response of the cell is presented nd compred with reference cells. 1 Introduction Efficiency enhncement in photovoltic cells is not only mtter of electronic properties, but lso of vrious opticl properties. Anti-reflection cotings (ARC) nd grting couplers hve been investigted [1 3], even wter-repellnt low-indexed ARC for module glss [4]. Also engineered light-trpping for solr cells hs constntly been developed [5 7]. Severl pproches hve been exploring grting structures [8, 9] nd light trpping structures [10], lso for orgnic solr cells [11]. Also nonperiodic bckside reflectors from dielectric mterils [12] show promising results, while combined pproches of front nd bckside improvement re being discussed s well [13]. The growing reserch field of photonics nd photonic crystls [14] covers numerous pproches to improve solr cells [15, 16]. The opticl design of solr cells nd their components is therefore mjor tsk in photovoltic reserch lredy. The prllel development of electronic nd photonic properties nd their interctions nd possibilities of combined design is becoming n importnt issue. For n im-

2 Originl Pper phys. stt. sol. () 205, No. 12 (2008) 2797 provement of stte-of-the-rt 1st nd 2nd genertion PV cells nd modules, even smll improvements in efficiency on technicl level will contribute to the energy yield on short-term scle but only if such dvnces cn be introduced to estblished industril fbriction lines with cceptble effort. In recent yers, the growing demnd for solr energy nd thus efficiency nd cost effectiveness hve led to much interest in thin-film photovoltic devices. The principle need for PV technologies with short energy py bck time is obvious. Among the vilble technologies, the use of silicon s bse mteril is of interest, becuse of its nturl resources nd the environmentlly friendly chemistry of the mteril. Amorphous silicon thin-film technology hs been offering very low cost per Wtt pek in recent yers, but unstisfying efficiencies on stndrd conditions. The potentil of the micromorph tndem cell will be discussed in the following. 1.1 Micromorph tndem solr cell The micromorph tndem cell consists of two different silicon junctions. The bottom junction (or cell) is mde of microcrystlline Si (µc-si) nd bout 1 2 microns in thickness. The top cell is of hydrogented morphous silicon type (-Si:H), with thickness of typiclly microns. Both cells re thin-film devices, deposited from gs phse in PECVD or hot wire processes. Also, both re p i n devices, which in contrst to usul p n devices use built-in electric field to seprte generted chrge crriers. The cells re directly stcked on one nother, with just smll tunnelling lyer in between to void direct contct of n-type nd p-type mteril from seprte junctions. The cell is of so-clled superstrte type: the process is top to bottom, strting with the -Si cell on the front glss. The combined thickness of such tndem is bout two microns, reducing price per wtt pek, in terms of supply of resources nd in terms of fbriction time. Both junctions re bsed on industrilly vilble thin-film technologies nd cn cover lrge res quickly: The deposition cycle of industril modules cn deliver one 1.4 m 2 module bout every ten minutes. However, degrdtion of the top cell is n efficiency-limiting problem. Figure 1 shows the schemtic device. The front cell is deposited directly on textured ZnO front glss, which serves s trnsprent front contct nd superstrte. The etched texture is essentil for light trpping in these cells. In between the junctions, doped tunneling lyers terminte the n nd p regions of the junctions ginst ech other. The bottom cell follows conformlly nd is typiclly finlized with (TCO connected) metl bckside reflector nd contct. The mesured EQE of two junctions from the sme tndem re shown in Fig. 2. High energy photons re being bsorbed completely in the -Si:H top cell, while red nd infrred light is trnsmitted to the bottom cell, being bsorbed by the µc-silicon. In the spectrl rnge between 550 nm nd 700 nm the top cell suffers from decresing bsorption coefficient of -Si:H towrds longer wve- Figure 1 (online colour t: Schemtic of the micromorph tndem cell. lengths. The spectrl overlp of the EQE offers n opportunity to tilor the photon distribution between the two junctions by intermedite opticl filters. 1.2 Potentil Tndem cells re so-clled 3rd genertion PV technology. They fight fundmentl losses in bsorption nd thermliztion processes with the incorportion of more thn single electronic bnd gp within one PV device. Up to now, ll top-rted high efficiency cells re III/V triple junction cells [17]. The combintion of -Si:H nd μc-si:h cells to build tndem is motivted by the lrge difference in bndgp (~0.6 ev), s it contins the multi-junction dvntge. The micromorph tndem fetures bsorption edges of 1.1 ev (µc) nd 1.7 ev (). According to Green [18], the idel edges of bsorption for series-connected two cell tndem for use in diffuse sunlight re 0.97 ev nd 1.70 ev. The bsorption edge of -Si:H, however, is rther low t wvelengths bove 550 nm, s Fig. 2 shows. Since both cells within the tn- Figure 2 (online colour t: Sepertely mesured externl quntum efficiencies of the -Si:H nd µc-si cells of one tndem cell. Their overlp is the spectrl trget re of photonmngement.

3 physic sttus 2798 A. Bielwny et l.: 3D photonic crystl intermedite reflector for micromorph thin-film tndem solr cell dem divide the incoming photon flux from the sun mong them, we hve to consider the nture of this spectrum splitting. Let us think of theoreticl tndem, consisting of two identicl, lossless coupled, series-connected cells of the sme electronic bndgp. Let us further sy, tht it shows the sme efficiency s (probbly thicker) single junction of the sme mteril, but s tndem delivering twice the voltge but hlf the current. If we now replce the top cell with cell of higher bndgp, leving everything else untouched, the voltge will increse. This is the impct of the multi-junction dvntge on the bsorption efficiency. Also, this leds to reduced thermliztion, decresing the losses to phonon excittion. Due to its higher open circuit voltge V oc the -Si:H top cell delivers higher electricl power for ech bsorbed photon. So, in principle, the morphous cell s bsorption should be mximized. Optimiztion of the micromorph tndem cell will led to unique combintion of 3rd genertion potentil with 2nd genertion effectiveness in one low-cost & high efficiency thin-film solr cell. 1.3 Current mtching Although mximiztion of bsorbnce by the -Si:H cell is key fctor towrds efficiency enhncement, further properties of the tndem hve to be tken into ccount. In order to eliminte losses from imperfect electricl conditions, the junctions of ny series-connected tndem cell hve to be current-mtched. Uncompensted mismtch will prevent power from being delivered to the contcts s the smllest current will lwys dominte the tndem output. Micromorph tndems suffer from current mismtch: the -Si top-cell is limiting the electricl current of the whole tndem. Here the need for higher currents (nd thus higher bsorbnce) in the top cell becomes obvious for second cuse. Tble 1 shows the electricl properties of both junctions of tndem in detil. The open circuit voltges (V oc ) s well s typicl vlues t the mximum power point (MPP) in the tndem re shown. Typicl short-ciruit currents re bout 11 ma/cmm 2. The currents differ bout 10% which cn prtilly be compensted by driving the -Si cell not on MPP long the j(v) curve, but t reduced voltge nd incresed current. However, this is trde-off, reducing efficiency t the electricl fill fctor. A second solution: to increse the thickness of the top cell is not fesible either, due to the rther poor trnsport properties of morphous silicon. Thickness is limited to few hundred nnometers in -Si. Repmnn discussed the current mtching t MPP [19], showing the initil nd stbilized currents. The pproch of Tble 1 Voltges of the -Si/µc-Si junctions. V oc [V] -Si:H µc-si V mpp,tn [V] opticl current mtching provides n dditionl wy of in cresing the tndem efficiency there. In the idel cse, the solr spectrum cn be spectrlly splitted between the junctions fter ing the top cell. A rtio of the high energy photons re then bck reflected nd bsorbed in the top cell. If the opticl current mtching is optimized, both junctions could be driven t their individul MPP, which would led to further enhncement of efficiency, since the top cell voltge could be incresed gin by severl percent. Opticl interlyers with pproprite filter properties re therefore investigted, both theoreticlly nd experimentlly with gret effort. 2.1 IRL stte of the rt The bsic function of ny intermedite reflective lyer (IRL) in tndem cell is the bck reflection of certin rtio of incident photons tht hve not been bsorbed in the top cell. The exct mesure of this rtio is defined by the ctul current mismtch tht hs to be compensted. The problem of current mismtch nd the concept of the intermedite reflective lyer for silicon tndems hve been investigted in mny wys in the lst decde. The micromorph tndem ws topic t Forschungszentrum Jülich, Germny [19 21], University of Neuchâtel, Switzerlnd [22 24] nd Knek corportion, Jpn [25]. Also micromorph cells on flexible plstic substrtes hve been reported, e.g. by Hug et l. [26], with incorported IRL of silicon oxide compound. They chieved current mtching up to 0.2 ma/cm 2 of remining mismtch t tndem currents of j sc = 11.2 ma/cm 2. The common pproch to ll reported work is the use of mssive TCO lyer. It leds to reflection t the interfce between top cell nd IRL, s well s between IRL nd bottom cell. This reflectnce is minly described by Fresnel s equtions nd strongly depends on the thickness of the interlyer. The use of finely tuned spectrlly nrrow thin-film oscilltion leds to significnt ngulr dependence of the interlyer spectrum splitting efficiency. Nevertheless, the mssive IRL is very ttrctive concept, becuse of its high comptibility with vilble deposition technologies. Also remrkble enhncements hve been chieved: Knek reported initil efficiencies of 13.4% in smll module nd 15.0% for lb cell of 1 cm 2, compred with 11.7% listed in the 2007 solr efficiency tbles [17]. Investigtions of such lyers by Krc et l. in 2006 reveled: Opticl nlysis of the interlyer reveled possibilities for significnt improvements of J SC,top (>25%) or thickness reductions Δd i,top (>50%) for interlyers with smll refrctive index (n inter < 2.0) [27]. Their results included lso the observtion tht using mssive interlyers, strong enhncements of the bsorption (red/nir) in the top cell will significntly reduce bottom cell illumintion even t optimized thicknesses. This is cused by the spectrlly wide overll reflectnce t the interfces, leding to unwnted bck-reflection losses lso in the long-wvelength regime. Low refrctive index of the IRL is in ny cse fvoured, s low s even n IRL =

4 Originl Pper phys. stt. sol. () 205, No. 12 (2008) 2799 An opticlly optimized tndem cell would be composed from highly efficient light trpping front, very thin -Si lyer tht is possible becuse of n intermedite reflective lyer (IRL) nd thin but efficient µc-si lyer, llowed by the good light trpping nd diffuse bck side reflector. 2.2 Requirements The requirements for ny simple nd lso for ny sophisticted intermedite filter cn esily be deducted from lredy discussed properties. It should provide the following three conditions: First, the IRL should deliver spectrlly limited bck-reflectnce for the -Si top cell where the EQE of both junctions is defining our spectrl region of interest. Second, the IRL should to be trnsprent for red nd infrred light. Third, the IRL must exhibit sufficient electricl conductivity, s the micromorph tndem is seriesconnected 2-terminl device. The quntifiction of these requirements is somewht fuzzy since there is no stndrd micromorph tndem cell. Also cn the thickness of both junctions be used within certin limits to tune bsorbnce to mtch the tndem to promising photon mngement provided by n IRL. However, the reflectnce hs to modify the spectrl photon distribution in order to chieve current mtching of certin tndem cell. The mismtch is typiclly in order of mgnitude of 10%. Red trnsmittnce hs to be s high s possible nd the resistnce of the integrted IRL should be clerly below ~2 Ω to keep the voltge drop within the criticl limits of the cell s diode chrcteristic. 3.1 IRL design Between different possible IRL designs to improve -Si bsorbnce, we cn distinguish between simple (mssive) IRL nd 1D, 2D nd 3D structured IRL, belonging to the mteril clss of photonic crystls (PhC). Homogeneous IRL introduce Fresnel type reflectivity. Befittingly tilored thickness of the interlyer produces one thin-film oscilltion in the spectrl region of low bsorption. Despite their denomintion s homogeneous in this rticle, these lyers my lso be porous on the micro or nno scle but lwys ct s one single opticl lyer of effective refrctive index n eff. One dimensionlly periodic IRL, so-clled Brgg stcks, feture spectrlly limited bnd stop reflectivity. 2D periodic structures do not posess this bnd stop property, besides possible thin-film effects, but produce diffrction ptterns ccording to their lttice constnt nd type of lttice. They cn be prepred with lower refrctive index, if the periodicity is fbricted with ir holes. 3D PhC finlly hve the potentil to combine ll mentioned effects in one photonic device. They my lso possess very low effective indices, even less thn typicl 2D structures due to their volume filling frctions. They develop strong stop bnd reflectivity between lttice plnes (Brgg reflection) nd dd diffrctive properties from their 2D surfces nd vriety of lttice plnes or, s denoted in terms of photonics, higher bnd fetures. The incorportion of n internl reflector is preferbly relized s rel intermedite lyer, plced in between the two junctions nd therefore consisting of trnsprent conductive oxides 2D or 3D structuring of one of the silicon interfces will led to strongly incresed recombintion rtes. Preferbly, ny IRL should be fesible s dd-on to lredy estblished fbriction processes, leving both cell depositions unchnged. After mssive IRL hve lredy proven enhncement potentil experimentlly, nd with 2D grtings being discussed in vriety of functions, novel pproch to IRL designs is the use of 3D photonic crystls (PhC): the design of n interlyer of periodiclly structured refrctive index in ll three dimensions. The potentil of different IRL pproches towrds current enhncement in the top cell hve been investigted for mssive lyers, Brgg stcks (1D) nd inverted opl PhC (3D). High reflectivities with spectrl width smller or s wide s the overlp of the EQE should be trgeted. 3.2 Concept: inverted opl IRL An internl reflector hs to be comptible with the rndom roughness of stte of the rt thin film cells, or it should introduce strong diffrctive effects itself, justifying plnriztion of the -Si bckside. Dependent of the lttice constnt (sphere dimeter), 3D PhC will show strong diffrctive properties. The onset of diffrction t the interfce between silicon nd the PhC is given by Eq. (1): n ω PhC ª 0.3. (1) 2πc nsi The diffrction limit is lso shown in Fig. 3. The photonic properties of inverted opls cn be deducted from the photonic bnd structure (BS), shown in Fig. 3. Photonic bnds re summrized solutions to the photonic mster Figure 3 (online colour t: Photonic bndstructure (BS) of inverted opl of dielectric contrst 1.00/6.25 (ir/indium-tin oxide). In ΓL-direction, stop gp opens up t 0.58α/λ (rrow mrks). The onset of diffrction is inserted t bout 0.3α/λ [28].

5 physic sttus 2800 A. Bielwny et l.: 3D photonic crystl intermedite reflector for micromorph thin-film tndem solr cell equtions nd show the llowed frequencies for modes of certin k-vectors. The BS yields for perfect nd infinite PhC. Eigenfrequency clcultions hve been performed with MPB [24]. The stop gp in ΓL-direction leds to bck reflection of incident light in this direction, if the normlized frequency lies within the gp. The opticl modes cn not propgte in the gp: they will be reflected. The key prmeter to strong photonic effects is the refrctive index contrst or the rtio of permittivities. A slope of the bnds indictes spectrl shift, if the direction of incidence is chnged. The spectrl position of their centre frequency however, is ffected by the lttice constnt = 2 d. This is expressed by spectrlly limited reflectnce of the PhC, its width relted to the gp in the photonic BS. There re two effects combined in the 3D PhC, the selective speculr (photonic) reflectnce nd the diffrction grting. According to Brgg s lw, the spectrl position of the reflection (Brgg pek) from interference of 111 lttice plnes of colloidl photonic crystl follows Eq. (2): λ ( n Θ) 1/2 = 2( ) d - cos, (2) 2 1/2 2 2 (111) 3 fit where d is the dimeter of the spheres, n fit is n effective refrctive index, nd Θ is the ngle of incidence ginst the 111 plne. This effective index however, is defined within the photonic stop gp. It is not equl to, but mostly in order of mgnitude of the effective refrctive index in long wvelength limit, which is why we ddress this index s fit prmeter. The reflectnce wvelength of opline films s well s their replic depends on the refrctive index contrst between spheres nd the so-clled host nd the lttice constnt of their periodic nno-structure. Highly indexed host mterils surrounding hollow (ir) spheres led to stronger photonic properties thn low-index mterils with the sme lttice prmeters. In this pper, we focus on the photonic stop gp, which depends on the lttice constnt of the PhC. Also we discuss the influence of diffrctive properties t the chosen crystl prmeters in efficiency enhncement. 3.3 Simultion of opticl properties The opticl properties of finite nd thin films of (inverted) opline photonic crystls nd the influence of interfces hve to be investigted with other numericl methods. We used the scttering mtrix method (SMM) s proposed by Whittker nd Culshw [29] for the clcultion of the coefficients of reflection nd trnsmission. The PhC structure is therefore implemented s 3D unit cell infinitely reproduced within the x,y-plne nd divided in lyers, so-clled chunks, like the one sketched in Fig. 4, long the verticl z-xis. This method is extremely fst compred with rigorous methods, s is pproximtes the correct solution of the fields. Only with n infinite number of Fourier orders solved, the correct solution is obtined. Therefore, ll the clcultions concerning efficiency impct of the IRL hve been performed with rigorous Fou- Figure 4 (online colour t: 2D chunk of the fcc unit cell for SMM. rier methods, especilly the lyered Korring Kohn Rostocker method (KKR). It uses n expnsion of the fields to sphericl wves in surrounding geometries of sphericl symmetry, which mkes it very well suited for ll opline PhC. Reflectnce spectr obtined vi SMM re plotted in Fig. 5. Here, the three stges of the fbriction process re resolved: opl templte, infiltrted composite nd inverted opl. These processes re discussed in detil in Section 5. The SMM results show, tht compred with the typicl opl reflectnce, the inverted structure provides lrger spectrl width nd significntly higher reflectnce, which is cused by the higher index contrst. Spectr of (inverted) opls show only smll devitions for different polriztions. The composite structure shows nerly no polriztion dependence t ll. Bsed on these clcultions, Figure 5 (online colour t: Clculted reflectnce spectr from scttering mtrix simultions of 1.00/2.89 inverted opl, composite nd opl templte (6 lyers, ech) with d = 300 nm [30].

6 Originl Pper phys. stt. sol. () 205, No. 12 (2008) 2801 Figure 6 (online colour t: Clculted reflectnce spectrum from KKR simlultion of 1.00/6.25 inverted opl (6 lyers) with d = 250 nm ir spheres. Zero (dot, dsh) nd first orders (short dsh) re plotted seprtely, the inset shows sufficiently high diffrction ngles s function of wvelength [28]. we re ble to follow the experimentl progress of smple preprtion by mens of opticl spectroscopy. The results from KKR simultions re shown in Fig. 6. The reflectnce of zero diffrction order (speculr) reflectivity nd first orders re plotted. Also the diffrction ngle is shown in the inset for first orders. Anlogous to the SMM model, the structure for KKR method ws implemented s 6 lyer thin film of perfect fcc inverted opl lttice. In contrst however, we used higher contrst in the permittivity 1.00/6.25 to estimte relistic limits of the photonic properties. The results show spectrlly limited but rther wide reflectnce pek centered t bout 620 nm. The FWHM is bout 170 nm, the bse width between the minim is 230 nm. Reflectnce reches 93%. From plnr interfces nd perfectly coherent excittion, strongly pronounced Fbry Perot oscilltions (FPO) rise. They re higher in mplitude thn in the presented SMM spectr, becuse of the difference in dielectric contrst Δε. The first order bck diffrction into the front hlf spce is comprbly wek but experiences lrge devitions from snell s lw. The diffrctions ngles re lwys lrger thn 30. Absorption is neglected, termintion of the PhC is defined by hlf spces ssumed s silicon, pproximting surroundings of the silicon tndem cell. 4 Top cell enhncement clcultion To estimte the influence of n inverted opl IRL t the bck of n morphous silicon top cell, we use the results from our KKR simultions. Focus is on the bsorption of light within -Si in the spectrl region of its low bsorbnce between 550 nm nd 750 nm. The thickness of the -Si:H junction is D. The bsorbnce A S in the top cell is incresed by photonic bck reflection in zero order A 0 nd higher order diffrction A 1+ becuse of the reflectnces R 0 nd R 1+ respectively. The stndrd bsorbnce of the top Figure 7 (online colour t: Schemtic opticl pth inside the top cell for speculr zero order reflectnce (left) nd diffrcted higher orders (right). The clcultion for plnr silicon lyers leds to trpping of diffrcted light within th -Si lyer nd to strongly enhnced bsorption of the smll diffrcted rtio light [28]. cell is simply defined s in Eq. (3): A= 1- T = 1-exp( - α D). (3) S Using the trnsmittnce T S through the top cell. For zero order n opticl pth of D fter bck reflection is ssumed. Not bsorbed reflected intensity is lost. For bck diffrcted light, pth increse ccording to the diffrction ngle Θ hs been clculted ccording to Eq. (4). m 2 A1 1 (1 R1 ( )) exp m α D + = - λ Ê ˆ Â (4) m= 0 Ë cosθ Totl internl reflection (TIR) t the front of the -Si top cell ws ssumed for the received ngles, m is the number of these reflections. Thus, the dominnt loss mechnism becomes bck diffrction by the PhC itself, coupling the Figure 8 Clculted intensity loss in the -Si top cell, cused by bck diffrction of light. About 84% of diffrcted intensity (of 4%) cn be trpped in the top cell, 16% of it re lost vi reciprocity of the PhC diffrction.

7 physic sttus 2802 A. Bielwny et l.: 3D photonic crystl intermedite reflector for micromorph thin-film tndem solr cell Figure 9 (online colour t: Absorbnce enhncement by 6 lyer inverted opl of 1.00/6.25 permittivity contrst. The sensitivity of the cell is extended towrds the red prt of the spectrum by direct nd diffrctive bck reflection of light. The impct of higher orders is remrkbly high lthough their reflectnce is of poor mgnitude [28]. first orders bck into zero order fter TIR. A= AS + A0 + A1+ = 1 - T + T R (1- T ) + T + T R A. S S 0 S S S This is sketched in Fig. 7. Although the reflectnce into higher orders is rther wek, the light trpping cn led to significntly enhnced bsorbnce of the diffrcted intensity. A monochromtic solution of Eq. (5) is shown in Fig. 8, where the remining intensity within the -Si cell hs been solved. Light of 696 nm wvelength experiences only bout 4% of diffrction, but 84% of this smll rtio re being cptured by the top cell fter being trpped in totl reflection. The shpe of the curve in Fig. 8 results from bck diffrction losses cused by the PhC itself, smoothed by continuous bsorption in the cell. The effect of this enhncement on the bsorbnce of stndrd morphous silicon thin-film cell is shown in Fig. 9. Here, the impct of the discussed opticl properties is obvious. The spectrl position of the reflectnce, both direct nd diffrctive is well plced in the spectrl region of low bsorption of -Si. As result, the bck reflection enhnces bsorbnce in the top cell there, which leds to red-shift of the bsorption function. The -Si cell sensitivity hs been extended towrds the red. The thinner the -Si top cell is, the less bsorption it cn provide. Since thinner cells hve less trnsport problems, the effect of our IRL concept on different thicknesses of the top cell is of interest. Figure 10 shows three different thicknesses nd the expected enhncement rtio of our model. In thinner cells, the rtio of diffrcted nd speculrly reflected light on the enhcement is chnged. Thinner cells still benefit from the pth increse of diffrction, but hve reduced increses from direct reflection. Since they hve only reduced bsorbnce nywy, more (5) Figure 10 Absorbnce enhncement rtio for different thicknesses of the -Si bsorber lyer: The impct of mtched photonic IRL increses with thinner top cells. light hits the interlyer nd is therefore bck reflected by the IRL, which pushes the device s importnce in this cse. The combintion of thinner top cells nd mtched intermedite reflector is therefore considered fesible nd tempting pproch. 5 Experimentl reliztion The preprtion of inverted opls is multi step process. After substrte preprtion, n opl templte is produced s thin film on glss or the bck side of morphous silicon. It is then tempered. Afterwrds, the templte is infiltrted with trnsprent conductive oxide (TCO), in our cse zinc oxide (ZnO). This step is referred to s repliction, since it is the formtion of structurl negtive of the templte. Third nd finlly, the templte is destroyed chemiclly or vi clcintion in furnce, leving the desired inverted opl film. We discuss the first nd second steps in more detil, s they re of importnce for the technologicl spects. The Process sequence: the different steps of smple processing, strting with bre microslide glss or -Si solr cells to finlized PhC filters t their bck sides or contcted solr cell prototypes.

8 Originl Pper phys. stt. sol. () 205, No. 12 (2008) 2803 process of smple preprtion is summrized in the overview scheme below. The synthesis of the monodisperse PMMA colloids were done by surfctnt free emulsion polymeriztion [31] nd crystllized into opline films using verticl [32] nd horizontl crystlliztion pproches [33] s well s spry induced self-ssembly [34]. Airbrush experiments were performed with commercilly vilble pprtus from Sil.Air. For the horizontl crystlliztion coting-knifes nd lb-mde doctor blde pprtus were used s well s severl experimentl knife coting devices. We choose chemicl vpour deposition (CVD) nd tomic lyer deposition (ALD) methods for the purpose of infiltrtion with trnsprent conductive oxides. In both cses the opline film is exposed lterntively to gseous precursors, either Zn(Me) 2 (DMZ) or Zn(Et) 2 (DEZ) s first nd H 2 O s second one. The two regents dsorb to the surfce of the colloids nd rect there to ZnO. If fter every precursor deposition, the remining gseous chemicls re flushed out of the rection chmber, the surfce rection becomes completely self-terminting. Repliction vi chemicl vpour deposition (CVD) following literture procedure [35]. A solution of dimethyl zinc in toluene CDMZ, 2M, Aldrich) respectively deionized wter from Milli-Q-system (Millipore), which were kept in seprte glss flsks, were used s precursors. Nitrogen ws bubbled through the solutions t rte of ~200 ml/min lterntively to blow the precursor into the rection chmber with the opl smple, which ws kept t 90 C. After ech cycle nitrogen purge ws pplied to remove excess mteril. A typicl run ppers s follows: 4 minutes wter purge, 4 minutes puse, 90 seconds DMZ purge, 3 minutes puse, 3 minutes nitrogen purge. Purge times nd cycle number were vried. A sufficient filling ws observed fter 5 7 cycles, when the opticl ppernce of the opl chnged nd the brgg-reflectnce vnished due to refrctive index mtching. This ws verified using UV VIS opticl mesurements. Repliction vi tomic lyer deposition (ALD) ws performed in commercilly vilble Cmbridge Nnotech, Model Svnnh. For ALD process, pure undiluted DEZ precursor from Sigm-Aldrich hs been used s delivered. The temperture of the smple during rection ws kept s low s 60 C. The detils of the deposition cycles re shown in Tble 2. Since the resulting deposition is conforml, the desired deposition thickness defines the number of cycles to use. We choose 40 nm of coting thickness. While bout 23 nm re the limit for conforml infiltrtion with d = 300 nm spheres, little dditionl thickness on the outer surfces nd on top of the PhC film provides improved mechnicl stbility nd esier electricl contct procedures lter on. 5.1 Templte preprtion Artificil opls from monodisperse polymer or SiO 2 colloids re commonly used for the preprtion of templtes for inverted structures (opl replic). They re crystllized into the cubic densest pcking nd removed fter the filling of the interstitils leving behind the opl replic. It is thus necessry to strt with colloids, where this size cn esily be djusted to obtin replic with the desired selective reflection. In ddition it must be fesible to remove the colloids fter replic formtion without dmging the finl photonic structure. Therefore, we choose poly-methyl-metcrylt (PMMA) spheres for opl templte preprtion. This polymer cn be removed under mild conditions using orgnic solvents or oxygen plsm [33, 36] without ffecting the inverted structure or the Si-lyers. Also, the PMMA spheres cn be prepred by surfctnt free emulsion polymeriztion (SFEP) whereby the size of the resulting beds is controlled by the monomer to wter rtio t given rection temperture nd inititor concentrtion. In Fig. 11 the reltionship between the monomer concentrtion nd the size of the colloids is shown. An nlytic evlution of these dt llows the prediction of the colloid size within 20 nm using the following eqution [31, 37] log d = 1/3log [ M] , (6) where d is the colloid dimeter (in nm) nd M is the monomer concentrtion (mol/l). Monodisperse colloids between 200 nm nd 500 nm in size cn be produced. Opline films prepred from these spheres exhibit Brgg peks between 400 nm nd 1000 nm, s shown in Fig. 12. Tble 2 Deposition cylce of ALD. mteril pulse [s] exposure [s] N 2 purge [s] precursor A Zn(Et) precursor B H 2 O pulse: durtion of precursor influx, exposure: llowed rection time, purge: durtion of nitrogen flushing; t substrte temperture of T = 60 C. The low temperture mkes long purge times for wter necessry. Figure 11 Dependence of the sphere dimeter on the initil monomer to wter rtio for PMMA t n inititor concentrtion (potssium peroxodisulfte) of 12 mmol/l. The correltion between the bed dimeter nd the Brgg-reflectnce of the corresponding opl is lso shown.

9 physic sttus 2804 A. Bielwny et l.: 3D photonic crystl intermedite reflector for micromorph thin-film tndem solr cell Figure 12 Reflection spectrum of n opl of in verge 47 monolyers of PMMA spheres (bed dimeter: ~312 nm); bndwidth Δω gp /ω gp = Thus, the selective wvelength of the replic cn be djusted esily to vlues from 550 nm to 680 nm, which re needed for the intended ppliction for the micromorph tndem cell. The photonic properties of this replic depend on the in-filled mteril nd chievble filling frctions (see Section 3). The opticl properties cn be ltered lso vi post-processing (e.g. tempering) of the infiltrted mteril. Sphere sizes of 284 nm nd 294 nm (Brggreflectnce t 630 nm nd 654 nm, respectively) hve been used for severl templte experiments. Also 360 nm spheres hve been used for prototyping experiment in Section 7. Different crystlliztion methods were pplied to prepre opline films from the PMMA spheres. The im of these experiments ws twofold. First, thin opline films with high qulity (strong reflectnce nd nrrow bndwidth) should be prepred nd secondly, n pproch should be used, with which lrge re cn be covered with n opline film. Exmples re shown in Fig. 13. With verticl crystlliztion method we chieved thin nd homogeneous films with controlled thickness [16] s shown in Fig. 14. For this pproch the substrte ws drwn verticlly out of suspension of colloids in wter with speed of severl 100 nm/s, wheres crystlliztion tkes plce in moving meniscus. However, decrese in film thickness goes long with lowering of the reflectnce intensity. Also the bndwidth of the Brgg-pek is brodened distinctly below monolyer number of 15, which is close to the criticl thickness for nrrow bndwidth reported in [38]. Figure 13 (online colour t: Exmples of lrgescle opline films. (A) opl on glss substrte (10 10 cm) prepred by sprying, (B) horizontlly dried opl film (40 25 cm). Figure 14 SEM microgrph of thin opl templte on silicon with constnt thickness; A. Redler nd P.-T. Micle. Although the crystlliztion in verticl meniscus provides good results, this pproch is time-consuming nd limited to smller substrtes. Lrger opl films cn be prepred by horizontl crystlliztion. Here the thickness of the film cn be djusted by the mount nd the concentrtion of the suspension which is dried on given substrte re. E.g. with coting knife (doctor blding) it is possible to prepre lrge scle opline films of good opticl qulity nd homogeneous thickness. Opline films of bout 50 monolyer in thickness prepred by this method exhibit reflectnce close to 80% nd nrrow bndwidth (Δλ/λ) of bout 5% (see Fig. 12). The usge of sprying pprtus is lso promising method for the covering of lrge res with n opline structure. For this purpose, suspensions of high concentrtion were spryed directly on the substrte using irbrush equipment. Opl films re built fter few minutes of drying [34]. Figure 13(A) shows n opl film of 100 cm 2 produced with this pproch. Since there re few experimentl limittions for the re size on which the colloidl suspension is spred, substrtes of lmost ny size cn be covered with colloidl film. As exmple PMMA opl of 1000 cm 2 size prepred in this wy is shown in Fig. 13 (B). Thus the first gol to prepre lrge opline films s templtes for the inorgnic replic cn be reched. Preferbly, the templtes re tempered t bout 90 C for few minutes to strengthen the interconnects between the so fr singulr hrd spheres. This improves mechnicl stbility nd leds to the so-clled tubes, smll PMMA extensions between the nno spheres. The next steps re the infilling nd repliction of the PMMA opls. 5.2 Inversion As conductive replic is desired, it is the first step to fill the opline voids with ZnO, using vpour deposition methods. This deposition is conforml, which is setting limits to the possible filling frctions. Insted of complete inversion, conformlly inverted opls cn be fbricted with up to 22.3% of volume filling (here the TCO), insted of 26% in theoreticl repliction. After infilling, the PMMA-templtes were dissolved in THF nd the ZnO replic were obtined. Figures 15 nd 16 show SEM imges of ZnO-replic prepred with CVD. The indepth infiltrtion of the templte succeeds very well, s the vporous precursors fill even nrrow spces. On top of n

10 Originl Pper phys. stt. sol. () 205, No. 12 (2008) 2805 Figure 15 SEM microgrphs of ZnO-replic prepred with CVD from 292 nm PMMA sphere opl. (A) Prtilly uncovered surfce. (B) Mgnifiction showing the ~290 nm ir holes nd the ~25 nm ZnO frmework thickness. inverted smple or rther t ll outer surfces, there is still the ppernce of the originl opl templte, becuse of the conforml repliction. At lest unless the surfce is opened, s shown in Figs. 15(A) nd 17(A). It is importnt tht during this process crcking-voids in the primry opline film fill-up from bottom to top with ZnO (see Fig. 16). This is highly desirble s the primry crcks re thus trnsformed into n electriclly conductive micro-grid tht connects top nd bottom cell. Exmples of ALD processed thin-film smples re shown in Fig Chrcteriztion Structurl chrcteriztion hs been performed with SEM, s lredy shown in Section 5. The three key properties re two of opticl kind: reflection nd trnsmission nd the electricl conductivity. The results on trnsmission will be presented with the spects of integrtion in Section Opticl properties To verify the results obtined from SMM clcultions, we mesured reflectnce of opl templtes, composite smples nd inverted ones. Under n opticl microscope of type Olympus BX51, the lst process step of templte removl looks s shown in the two photogrphs of Fig. 18. The spectr of ll three smple types re shown in Fig. 19. They hve been mesured with lbbuilt spectrometer setup, using monochromtor of type Figure 17 SEM microgrphs of ALD processed inverted smples: opened surfce of the confoml coting shows 3D PhC structure (A). Cleved edges give insight into film thickness nd crystlline qulity (B). Here, the octhedrl voids re visible t cut perpendiculr to crystllogrphic X-direction. HB550i by Jobin Yvon nd 250 W hlogen light source. Reflectnce ws mesured t 10 ngle of incidence. The predicted behviour hs been verified for ll three process steps. The significnt reflectnce increse of the inverted smple domintes the opticl properties in comprison. The ngulr dependence of the reflection pek cn be mesured in 2Θ setup for the ngle of incidence, keeping the ngle djusted for speculr reflection. With incresing ngle, the pek is blue-shifted, which is shown in Fig. 20. This cn lso be foreseen from the photonic bnd structure. There the bnds below nd bove the stop gp bend towrds higher frequencies when the ngle turns wy from incidence long ΓL-direction. The ngulr shift however, is clerly pronounced, but flls below hlf the norml mplitude not until reching 45. Remrkble in ll reflectnce spectr of inverted smples is, tht the FPO re much wider thn in opl or composite sttes of the smple. This is n ttribute of the very low effective index of the inverted opls. 6.2 Electricl properties In order to mesure conductivity or resistivity, smples hve been contcted on top with interdigitted contct fingers. The fingers hve been sputtered with gold directly on the ZnO surfce. The obtined I(V) dt from source meter mesurements re shown in Fig. 21. The contct line of this smple mesures Figure 16 SEM microgrphs of CVD processed inverted smple: the view on cleved edges presents the CVD deposition down n opline drying crck on two thick smples. This surplus ZnO coting is deposited from top to bottom of the photonic IRL nd cn form electriclly conductive micro grid to connect top nd bottom cell: (A) inverted smple with surplus deposition; (B) composite smple with mssive surfce deposition, lso down to the substrte. Figure 18 (online colour t: Reflectnce microscopy photogrphs (A: M = 20, B: M = 10) of the sme smple before nd during templte removl. The golden impression develops fter removl of the PMMA spheres nd verifies the finl inversion process step.

11 physic sttus 2806 A. Bielwny et l.: 3D photonic crystl intermedite reflector for micromorph thin-film tndem solr cell Figure 19 (online colour t: Reflectnce mesurements of smples from ll three process steps: the predictions from prior SMM simultions (Fig. 5) re in good greement [30]. 25 mm in length cross the smple. The thickness is 2 µm, the distnce between the gold contcts is 1 mm. The contcts themselves show resistnces of few ohms per centimeter nd do not offset the results. The obtined vlue of R = 198 kω of resistnce leds to resistivity of R s = 10 Ω µm for bulk conduction through the whole smple. In cse tht only the uppermost ZnO lyer on top of the smple crried the current in these mesurements, the resistivity would be normlized by the 40 nm of this thickness, leding to R s = 0.2 Ω µm. For n IRL of 1.5 µm thickness (6 lyers t d = 300 nm), the resistnce cn therefore be estimted to mesure between 0.3 Ω nd 15 Ω. For the micromorph tndem cell, resistnce of 2 Ω hs to be considered criticl. The mesured conductivity of the undoped ZnO from ALD processing is close to the required order of mgnitude, but still requires improvement. Any dvnce in terms of conductivity will reduce the electricl losses of the common current in the series-connection between the two junctions. Figure 21 (online colour t: I(V) curve, mesured t room temperture with Keithley 2400 sourcemeter. The resistnce is constnt, its vlue 198 kω. Additionlly to mesurements of current nd voltge, electro chemicl impednce spectroscopy (EIS) hs been performed on the sme smple. EIS gives ccess on the impednce over wide frequency rnge nd lso resolves the phse shift of smple on the offered AC modultion. A constnt impednce in the EIS spectrum revels ohmic resistnce, while liner increse with decresing frequency is typiclly linked to cpcitive dielectric behviour. The results of our mesurement re shown in Fig. 22 nd revel constnt ohmic chrcter of impednce up to frequencies of khz rnge. In conclusion, only the vlue of resistivity hs to be decresed, the mteril is suited s processed. 7 Integrtion All investigtions on smples in free spce, i.e. not in or on silicon solr cell, did show the desired properties in ll steps of our work. However, to verify the potentil of n ppliction, we investigted three dditionl spects. The trnsmission onto hlf spce silicon solr cell is one of the criticl requirements nd will be presented first. The integrtion into solr cell process of the -Si µc-si tndem cell ws studied in prototyping pproch, in order to verify the comptibility of the so fr Figure 20 (online colour t: Angulr spectroscopy, performed on thin-film ZnO inverted opl on glss. The ngle of incidence hs been chnged from 10 to 55. The mplitudes hve not been cosine corrected for better survey [30]. Figure 22 (online colour t: Electricl impednce spectrum of inv. opl thin-film smple: The PhC behves clerly ohmic up to khz frequency rnge.

12 Originl Pper phys. stt. sol. () 205, No. 12 (2008) 2807 L A M Cell Glss Smple S Figure 23 (online colour t: Schemtic setup for the integrted cell trnsmission mesurement [30]. completely seprte processes. Also, combintion of typicl rndom rough superstrte top cell with the selforgniztionl growth of our opl templtes hs been explored. 7.1 Integrted trnsmission To obtin experimentl dt on the trnsmittnce of the fbricted IRL on the µc-si bottom cell, n integrted setup hs been built. Insted of microcrystlline cell, commercil silicon (mono crystlline) mini cell hs been used. This cell works s detector in the spectroscopic setup (Fig. 23). The illumintion from hlogen light source tkes plce behind the monochromtor slit (S). Mirror (M) nd lens (L) collimte the light, while n perture (A) reduces the spot size to the size of the smple. The spectrl nswer of the cell defines only the trnsfer function of the setup: s reference dt we use the trnsmit-tnce T int,0 of clened nd unprocessed sub-strte glss tht is plced directly on the solr cell. The ctul mesurement of T int,zno of ZnO inverted opl film on substrte glss of sme type is normlized with this reference by division of T int,zno /T int,0. The IRL smple is plced upside down, with the PhC in contct with the solr cell nd the glss substrte fcing the incident light. The glss is 1 mm thick, the IRL itself mesures bout two microns in thickness. The resulting spectrum is shown in Fig. 24. We cn clerly identify the min drop in trnsmittnce, cused by the photonic stop gp t bout 600 nm wvelength. The mplitude drops down to bout 52% there. Also the onset of bsorption from the zinc oxide cn be identified: t the short wvelength flnk trnsmittnce drops steeply, when the ZnO bsorbs high energy photons. The reduced trnsmittnce in t short wvelengths is not problem, s the bsorption in -Si is very efficient there. The long wvelength rnge shows high trnsmittnce, with n verge vlue of 95.7% between 700 nm nd 1100 nm. The thin-film oscilltions turn out to be much weker thn in coherent simultions nd even lower thn in speculr reflection spectroscopy. The reduction of trnsmittnce by the photonic stop gp however, turns out to be lso less pronounced thn one would hve expected from the reflectnce mesurements, but is still bout 40%. 7.2 Solr cell integrtion To combine the inverted opl IRL with micromorph tndem cell, we took the first step of growing the opl templte on the bckside of n -Si:H top cell. The fbriction process runs top to bottom, strting with the front glss, ending with the bck contct. The cell is therefore of so-clled superstrte type, directly deposited on the ZnO coted front glss. Our structure consists of d = 360 nm ir spheres surrounded by undoped ZnO from ALD. The PMMA templte removl ws forced by lternting cycle of bthing in heted THF liquid/vpour phse environment nd creful tempering. On the bckside of the finlized PhC interlyer ws then deposited thin lyer of silicon, to produce the correct opticl interfce with the bottom junction of tndem cell. The smple ws terminted with metl bck contct. This sndwich structure is shown in Fig. 25. To verify tht the fbricted device works properly, its quntum efficiency hs been mesured in the visible spectrum. The resulting grph is shown in Fig. 26. Figure 24 Integrted trnsmittnce of the smple ZnO inverted opl on glss onto silicon solr cell. Photonic bnd stop filtering is observed, very high trnsmittnce through the smple mesured between 700 nd 1100 [30]. Figure 25 SEM microgrphs of cleved edge from IRL prototype smple: (A) the inv. opl film covers the bck of the -Si cell. (B) The lyers consist of (1) glss, (2) p-type nd intrinsic -Si, (3) n-type -Si, (4) 3D photonic IRL of ZnO nd (5) Si bckside. The smple is terminted by thin metl contct.

13 physic sttus 2808 A. Bielwny et l.: 3D photonic crystl intermedite reflector for micromorph thin-film tndem solr cell Figure 27 Microgrphs of PMMA opl templte grown on textured -Si thin film bck side: (A) morphously rrnged colloidl nno spheres t the bottom, (B) orgnized growth in the upper region of ~10 lyer film. Sphere dimeter is d = 298 nm. Figure 26 Experimentl EQE dt of the flt -Si:H cell with PhC reflector t the bckside nd two reference cells of textured -Si:H. The textured cells produce fr better EQE, while the flt cell shows typicl Fbry Perot oscilltions from the flt interfces. The fbricted device is obviously opticlly limited in its light-trpping cpbilities nd stys bout 10% 20% below the reference cell s EQEs. The EQE hs been mesured t reverse bis voltge. The bis influence hs been observed to sturte between 2 V nd 3 V, which ws interpreted s lrge series resistnce in the IRL. In comprison to two highly efficient reference cells with rndomly textured light-trpping front, the flt cell with the PhC suffers over nerly the complete bsorption region from reduced bsorbnce. Nevertheless hve these first experiments shown the comptibility of the proposed device with micromorph thin-film technology. Also they identified two importnt device prmeters: the conductivity of the IRL nd the need for light-trpping component in the cell. 7.3 Rndom rough superstrte As the rndomly rough surfce of HCl etched ZnO on the front glss is currently the best vilble light trpping structure for the tndem, the possibility to grow n opline IRL directly on this conforml roughness would llow direct introduction of our device in the fbriction process of the micromorph tndem. This is not only desired option, but necessry condition to void dditionl plnriztion steps of rough superstrte cell to llow the PhC growth: becuse of the importnce of textured front contcts in light trpping for thin-film cells, it not possible to bndone it for n IRL. Unfortuntely, opl films re typiclly grown either on ptterned substrtes to improve the lttice qulity or force certin orienttions or on perfectly flt substrtes. From experience, opl growth on rough substrtes usully leds to disordered, morphous colloidl films. This ppers t first sight to be fundmentl problem tht could prevent the ppliction of our self-orgnized PhC films to the existing tndem cell technology. Our growth experiments on the ctul superstrte top-cells however show, tht the selforgniztion process is cpble of heling out the distor tion cused by they silicon substrte roughness within cer- tin limits. After sufficiently lrge number of opl lyers, the fcc structure will previl. In Fig. 27, n opl film on textured -Si:H substrte (top cell bck side) is shown. The disordered bottom of opl growth in contct with the roughness cn be seen in Fig. 27(A), s well s the crystlline phse tht develops within only few lyers, Fig. 27(B). The fesiblility of opl thin-film growth on rndomly textured silicon surfce hs therefore been shown. It llows us to combine our photonic device with the existing lighttrpping technology, which is key feture to successful ppliction. To chrcterize the opticl qulity of this perturbed photonic crystl thin-film, we performed comprtive trnsmittnce mesurements under microscope, fibercoupled to photodiode-rry equipped spectrometer. At mgnifictions of M = 20 (NA = 0.3), we compred the trnsmittnce of the unprocessed textured -Si substrte with the trnsmittnce of the sme smple with the opl templte on top of it. Therefore, light hits the superstrte glss first, es through the textured ZnO front TCO nd the thin conforml -Si lyer (of only 80 nm thickness here), before it intercts with the PhC. The impct of the PhC in this sndwich structure cn be seen in Fig. 28. Figure 28 (online colour t: Trnsmittnce of the thin -Si cell with bck-side opl film (solid) nd without (dshed) for comprison. The stop gp t bout 640 nm is clerly observed.

14 Originl Pper phys. stt. sol. () 205, No. 12 (2008) 2809 The mesured trnsmittnce in the microscope is limited by the numericl perture of the objective to bout 0.5 for these smples, due to the highly efficient scttering t the rough texture. The onset of bsorption of the -Si film nd its chrcteristic slope re ffected by the opl bnd stop filtering round 640 nm in wvelength. The reltive decrese of trnsmittnce is 30% t 642 nm. Although the disorder introduced by the texture hs decresed the film qulity, s observed in SEM investigtions, the photonic stop gp hs nevertheless developed. Trnsmittnce experiences smll decrese t shorter wvelegths thn the stop gp position. This loss is ttributed to scttering processes t crystl defects in the PhC, s its rtio is incresing with wvelength. Although rndomness in photonic crystl is usully problem, we my gin benefit from certin disorder. The Brgg reflection peks of opl films re known to decrese in mplitude if film possesses less crystlline order. But it lso chnges its width to lrger vlues. With n pproprite control of the growth procedures, it should be fesible to ffect lso the spectrl width nd reflectnce mplitude by chnging the qulity of the templte. 8 Conclusion We hve investigted the potentil, impct nd experimentl fbriction of 3D photonic intermedite filters for tndem solr cells, with focus on micromorph tndem cells. Our pproch mkes use of selforgnized growth of colloidl crystls, which is suited for up-scling nd low-cost fbriction. An ppliction in between two junctions requires certin opticl properties. With use of polymeric opl templtes nd zinc-oxide inversion processes, we fbricted the proclimed structures. The desired spectrl peculirities were verified in opticl nlysis of our photonic crystls. They show the desired spectrlly selective reflectnce spectrum with limited, but sufficiently lrge spectrl width. The necessry long filtering in trnsmittnce hs been verified to 95% between 700 nm nd 1100 nm. Here, dditionl losses for the bottom cell cn be eliminted. All photonic properties cn be finely tuned by chnges in the templte lttice nd the TCO infiltrtion. Also, the smples show very low effective index of refrction becuse of the low filling frctions of the high indexed TCO. This cn be n dvntge for their tsk s reflectors between the silicon cells nd enhnces the internl reflection in the silicon junctions. With the use of (undoped) zinc oxide, typicl TCO in solr energy pplictions, we re lredy close to the desired order of mgnitude for electricl conductivity. Initil steps towrds integrtion into solr cell hve been explored nd the very first reliztion of n internl photonic reflector hs been demonstrted: prototype photonic crystl intermedite reflector t the bck of n -Si:H top cell hs been prepred nd chrcterized vi quntum efficiency mesurements. Thus, the technologicl comptibility hs been shown. 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