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Silicon chip with the basic integrity of the crystal lattice structure. Different directions of a different nature, is a good semiconductor material. Purity of 99.9999%, even up to 99.9999999% or more. For the manufacture of semiconductor devices, solar batteries. High-purity polysilicon used in the single crystal drawn from the furnace.
14TH EUROPEAN PHOTOVOLAIC SOLAR ENRGY CONFERENCE BARCLONA, SPAIN 30 JUNE-4 JULY 1997 A novel process for ultrathin monocrystalline silicon solar cells on glass Rolf Brendel Max Planck Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany Phone: +49-711-6891606, Fax +49-711-6891010, E-mail: firstname.lastname@example.org stuttgart.mpg.de ABSTRACT: We introduce the perforated silicon process (ψ-process) for the fabrication of ultrathin silicon lay- ers with efficient light trapping. A silicon layer grows epitaxially on the porous surface of a textured monocrys- talline Si substrate. Mechanical stress cracks the porous layer and thereby separates the epitaxial layer from the substrate. According to x-ray diffraction analysis, our Wf = 5.8 µm thick Si layer is monocrystalline. Reflectance measurements and ray tracing simulations predict a maximum short circuit current of jsc * = 36.5 mA/cm2 for the waffle shaped film when attached to glass. Transport simulations forecast cell efficiencies η = 16 to 19% for film thicknesses of Wf = 2 to 3 µm. Keywords: Thin Film - 1: Porous Silicon - 2: Ray Tracing - 3 1. INTRODUCTION substrate wafer prohibit the application of this technique to The challenge of thin film crystalline silicon solar cells photovoltaics for cost reasons.  consists of three major tasks: (i) The growth of a high In contrast, the process introduced in this contribution quality large grained crystalline Si layer on a cheap sub- is applicable to photovoltaics because the process facili- strate, (ii) the incorporation of a light trapping scheme to tates light trapping, avoids bonding, and saves the substrate compensate for the intrinsically weak near infrared absorp- wafer. Figure 1a to f illustrates step by step the process tion of crystalline Si, and (iii) an effective passivation of that produces a textured monocrystalline Si-film on glass: grain boundaries and surfaces. a) A monocrystalline Si substrate wafer receives a A textured monocrystalline Si layer on a float glass surface texture by any type of etching or mechanical grind- would contribute to all three tasks: (i) Monocrystalline ing. Textures much more complex than the regular inverted material has a potentially high volume quality and float pyramids of period p in Fig. 1a are possible. glass is a cheap substrate. (ii) Innovative film textures [2-4], b) The surface of the substrate is transformed into a such as the Pyramidal-Film texture , facilitate efficient porous Si layer (PSL) of thickness WPS. The orientation of light trapping. (iii) The monocrystallinity avoids grain the Si in the PSL mediates the information of substrate boundary recombination and enables efficient surface pas- orientation. sivation at low temperatures . Such a fabrication of thin c) Hence, Si grows epitaxially on the PSL. A low tem- and textured monocrystalline Si layers has not been demon- perature epitaxial technique is of advantage, since the sur- strated in the literature yet. face mobility of the Si atoms on the inner surface of PSL In this contribution we introduce the novel perforated leads to a sintering process at temperatures above 850 °C silicon process to fabricate textured monocrystalline thin . films on float glass. We study the light trapping perform- The front surface of the epitaxial layer is freely accessi- ance of such films experimentally and analyze the effi- ble at this stage. Any process at temperatures lower than ciency potential of the novel film structure theoretically. around 850°C may be used to form the cell emitter. An epitaxial emitter as well as an inversion layer or a hetero- junction emitter seems attractive. Surface passivation and 2. PERFORATED SILICON PROCESS grid formation should utilize innovative techniques de- Epitaxy on porous Si was thoroughly studied for the scribed elsewhere [5, 8, 9]. fabrication of thin monocrystalline Si layers on insulating d) A superstrate (e.g. glass) is attached to the front substrates . In this process, an epitaxial layer grows by surface by a transparent encapsulant. The temperature chemical vapor deposition at temperatures T > 1000°C on a resistance of the superstrate and the encapsulant deter- plane monocrystalline Si wafer with a porous surface. The mines the maximum process temperature of all subsequent epitaxial layer is then transferred to an insulator by wafer process steps. bonding. Thereafter mechanical grinding removes the sub- e) We utilize the low mechanical strength of the PSL strate wafer. Subsequent chemical etching of the residual compared to bulk Si to separate the cell from the substrate. porous layer completes the process. The lack of light trap- A variety of treatments seem appropriate: Shock heating, ping, the bonding process, and the consumption of the filling the pores with liquids or gases that are forced to 1354 14TH EUROPEAN PHOTOVOLAIC SOLAR ENRGY CONFERENCE BARCLONA, SPAIN 30 JUNE-4 JULY 1997 a) b) Fig. 2: Perspective view of a free standing Si waffle pro- Fig. 1: Perforated silicon process. (a) A moncrystalline Si duced by the Ψ-process: (a) view from top and (b) obliquely wafer receives a surface texture of period p. (b) A surface viewing at the cross section. layer of thickness WPS is transformed into a porous Si layer (PSL). (c) An epitaxial Si layer (epi-Si) of thickness Wf times until a new texturing of the substrate wafer becomes grows. (d) A glass superstrate is attached to the epitaxial necessary. layer. (e) The mechanically weak PSL functions as a perfo- ration within Si and allows separation of epi-Si and sub- EXPERIMENTAL strate. (f) A detached back surface reflector enhances light 3.1 Sample preparation trapping and forms point contacts simultaneously. A p + -type, 1019 cm–3 boron doped, (100)-oriented monocrystalline Si wafer of 4" in diameter receives inverted pyramids of period p = 13 µm by photolithography and expand, distortion of the PSL by compressive or tensile anisotropic etching with KOH. Anodic etching in diluted stress, or ultrasonic treatment. In any case the PSL layer HF produces a WPS = 6 µm thick porous silicon layer in functions like a perforation within silicon (psi), hence the approximately two minutes time. Prior to epitaxy we heat name Ψ-process. the sample to 850°C for 10 min to remove the native oxide f) The back side of the cell is accessible for surface from the PSL surface. An epitaxial, Ga-doped Si film of passivation and formation of a reflector. A detached reflec- thickness Wf = 5.8 µm grows by the ion assisted deposition tor may also serve to form point contacts, that are benefi- (IAD) technique  at 700°C. The growth rate is 4 µm/h cial for small minority carrier recombination. on flat surfaces. Transparent poly-(ethylen-phtalate) fixes The free accessibility of the back and the front surface glasses of 2x2 cm2 in size to the epitaxial layer. A 2 min is an intrinsic advantage of the Ψ-process over processes ultrasonic treatment destabilizes the PSL layer and facili- that deposit Si directly onto an insulating substrate. tates mechanical removal of the epitaxial layer without Formation of the PSL consumes a thickness WPS/cos(α) chemical etching. In some cases we separate the epitaxial of the substrate wafer that is textured with facets inclined layer and substrate even without the application of ultra- by an angle α relative to the macroscopic cell surface. After sound. removal of all residual porous Si, the substrate retains the initial surface morphology (Fig. 1a) provided WPS/p << 1. 3.2 Sample characterization Otherwise, edges and tips become rounded with a curvature Figure 2 shows scanning electron microscope images of of radius WPS, as indicated in Fig. 1e. Hence, for sufficiently a free standing Si waffle produced with the Ψ-process. We small ratios WPS/p the substrate may be re-used several used no cleaning other than ultrasonic treatment prior to 1355 14TH EUROPEAN PHOTOVOLAIC SOLAR ENRGY CONFERENCE BARCLONA, SPAIN 30 JUNE-4 JULY 1997 106 0.6 µs is caused by de-trapping of carriers in shallow levels. (400) We did not measure the electron mobility. However, con- 105 sidering the measured hole mobility µ = 186 cm2/Vs as a INTENSITY [a.u.] lower bound for the electron mobility we calculate a minor- 104 ity carrier diffusion length L > 11 µm, which is larger than the film thickness Wf = 5.8 µm. 103 textured Si film on glass For thin film cells light trapping is essential. Unfortu- 2 nately, the optical performance of the encapsulated waffle 10 structure, with an Al-mirror behind the sample as shown 101 schematically in Fig. 1f, cannot be measured without con- monocrystalline substrate tacting the sample. Therefore, we estimate the short circuit 30° 40° 50° 60° 70° 80° current potential of our sample from a comparison of a ANGLE 2 θ measured hemispherical reflectance and a ray tracing simu- Fig. 3: X-ray diffraction spectra of the epitaxial Si waffle 1 lation with the program SUNRAYS [ 3]. The detached shown in Fig. 2 and of the monocrystalline substrate wafer. reflector has proven to reduce the optical losses in the Al Note the logarithmic intensity scale. significantly . Figure 5 shows the measured (solid line) and the calcu- scanning electron microscope investigations. The perspec- lated (circles) hemispherical reflectance. The ray tracing tive top view of Fig. 2 a shows regular inverted pyramids simulation almost reproduces the measurement without that are replica of the initial surface texture of the substrate adjustment of optical parameters. Small deviations between wafer. Figure 2 b views obliquely at the cross section of the measurement and simulation are qualitatively explained by waffle. The pyramidal tips point downwards. We find no the micro-roughness of the pyramidal facets that was not cracks. The film thickness, normal to the pyramidal facets, 2 included into the simulation [ ]. SUNRAYS calculates a is Wf = 5.8 µm. The top surface shows pitches of less than maximum short circuit current jsc * = 36.5 mA/cm2 ± 0.5 0.1 µm in depth and diameter (not visible in Fig. 3) thus mA/cm2 from the simulated absorption (triangles) for the introducing a kind of micro roughness. These pitches are Wf = 5.8 µm thick waffle structure with a texture period p = related to the IAD-technique since they also appear on flat 13 µm under AM1.5G spectrum of 1000 W/m2. epitaxial films that grow on non-textured bulk-Si. Hall measurements of a film co-deposited onto a high resistivity non-textured monocrystalline substrate yield an electrically active Ga dopant concentration of 2x1017 cm–3 and a hole mobility of 186 cm2/Vs. Figure 3 shows the CuKα x-ray diffraction spectrum of the Si waffle on glass in comparison to the spectrum of the monocrystalline Si substrate. Note the logarithmic intensity scale. All peaks are at the same angles, thus our Si waffle is monocrystalline with the same orientation as the substrate wafer. Only the large (400) peak originates from Si. All other peaks are more than 2 orders of magnitude smaller and are artifacts of the x-ray machine. The larger back- 1000 REFLECTANCE DR [a.u] ground intensity of the epitaxial film is caused by the amorphous glass substrate. Consequently, the IAD tech- nique  allows eptiaxial growth on porous substrates. t = 0.27 µs ± 0.08 µs The bulk minority carrier life time is one of the crucial material parameters for a solar cell. The surface has to be 100 well passivated in order to measure the bulk lifetime. Therefore, we oxidize a free standing Si waffle on both sides at 1000°C and charge the surfaces with a corona discharge chamber  in order to repel the minority carri- ers from the recombination centers at the surface. 10 Figure 4 shows the microwave reflectance transient -0.2 0.0 0.2 0.4 0.6 0.8 1.0 after a 20 ns optical puls excitation. We place the sample a TIME t [µs] quarter of the microwave wavelength above a metal reflec- Fig. 4: Transient microwave reflectance ∆R of the Wf = 5.8 tor to obtain optimum sensitivity . The decay is not µm thick Si waffle shown in Fig. 2 after optical excitation strictly mono-exponential but may serve to estimate the with a 20 ns laser puls. Minority carrier life time is τ = 0.27 lifetime τ = 0.27 µs ± 0.08 µs. The slow decay for times t > µs as deduced from the slope of the linear fit. 1356 14TH EUROPEAN PHOTOVOLAIC SOLAR ENRGY CONFERENCE BARCLONA, SPAIN 30 JUNE-4 JULY 1997 to optimize the cell thickness Wf in order to adequately DIFFUSION LENGTH L [µm] 100 assess the efficiency potential for fixed L and S . There- 24% 10 µm fore, the simulation varies the film thickness W for opti- 7 µm mum cell efficiency. We assume a Si cell with a 1019 cm–3 P- 5 µm 22% doped and 0.5 µm thick emitter and a base that is 1018 cm–3 20% B-doped. At thicknesses W < 1 µm, base and emitter are of 10 3µm equal thickness. The diffusion length L and SRV S are taken 2 µm 18% equal for the base and the emitter in order to reduce the 16% number of free parameters. We account for recombination 14% in the space charge region . Mobility values and band- 1µm 1 10% 12% gap narrowing parameters of c-Si are taken from Ref. . 10 0 10 1 10 2 10 3 10 4 105 Figure 6 shows the efficiency (solid line) at optimum RECOMBINATION VELOCITY S [cm/s] cell thickness (broken line) for a large range of parameters S and L. At a diffusion length L = 11 µm we calculate an Fig. 6: Theoretical energy conversion efficiency (solid energy conversion efficiency of 16 to 19% at an optimum lines) and optimum cell thickness (broken lines) for cells cell thickness of 2 to 3 µm, depending on SRV S (dots). An with surface recombination velocities S and minority carrier efficiency of 16%, corresponding to a SRV S = 104 cm/s, diffusion length L. The simulation assumes the waffle texture would be a great success for a 2 µm thin crystalline silicon of Fig. 2. For L = 11 µm efficiencies ranging from 16 to solar cell on glass. The deposition of a Wf = 2 µm thin film 19% are feasible depending on SRV S (dots). takes 50 min with the currently employed IAD-technique. 5. CONCLUSIONS REFLECTANCE, ABSORPTION 1.0 We introduced the novel perforated silicon process (Ψ- process). Epitaxy on a textured monocrystalline Si sub- 0.8 strate and mechanical separation of the epitaxial layer from simulated absorption the substrate yield ultrathin monocrystalline textured Si 0.6 films on any type of glass. Reflectance measurements demonstrate an optical absorption that corresponds to a 0.4 maximum short circuit current density jsc * = 36.5 mA/cm2. measured reflectance Theoretically, the material quality is sufficient for efficien- 0.2 simulated reflectance cies from 16 to 19% with at an optimum cell thicknesses ranging from Wf = 2 to 3 µm. 0.0 Further development of the Ψ-process aims at a small 300 500 700 900 1100 porous layer thickness WPS < 1 µm to reduce material WAVELENGTH [nm] consumption and to demonstrate frequent reusability of the substrate wafer. A further increase of deposition rate is also Fig. 5: Measured hemispherical reflectance of encapsulated important. We do not see obstacles to produce ultrathin waffle structure (solid line). Calculated reflectance and layers of 100 cm2 in size. absorption spectra (symbols). The simulated absorption spectrum corresponds to a short circuit current potential ACKNOWLEDGMENTS jsc * = 36.5 mA/cm 2. The error bars are due to the statistics The author thanks J. H. Werner (Institut für Physika- of the Monte Carlo simulation. lische Elektronik Univ. Stuttgart), H. J. Queisser, J. Küh- nle, and R. B. Bergmann for delightful discussions and their encouragement to leave beaten paths. Thanks to H. Art- 4. EFFICIENCY POTENTIAL mann and W. Frey (Robert Bosch GmbH, Zentralbereich We investigate the efficiency potential of crystalline Si Forschung und Vorausentwicklung, Abt. FV/FLD) for films with the shape shown in Fig. 2 by theoretical model- preparation of the porous Si and to S. Oelting (Antec) for ing. The optical model applies ray tracing by SUNRAYS as epitaxy. Technical assistance by B. Fischer and N. Jensen described above. The minority carrier generation rate is was of indispensable help. This work is supported by the taken spatially homogeneous in the Si film and is calculated German BMBF under contract no. 0329634, which is from jsc * and the cell volume. 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