T. Mueller

High quality passivation for heterojunction solar cells by hydrogenated amorphous silicon suboxide films

In this letter, we report on our investigations of hydrogenated amorphous silicon suboxides (a-SiOx:H) used as a high quality passivation scheme for heterojunction solar cells. The a-SiOx:H films were deposited using high frequency (70MHz) plasma enhanced chemical vapor deposition by decomposition of carbon dioxide, hydrogen, and silane at a substrate temperature of around 155°C. High effective lifetimes of outstanding 4ms on 1Ωcm n-type float-zone material and a surface recombination velocity of ⩽2.6cm∕s have been repeatedly obtained. Optical analysis revealed a distinct decrease of blue light absorption in the a-SiOx:H films compared to commonly used intrinsic amorphous silicon passivation used in heterojunction cells.

Crystalline silicon surface passivation by high-frequency plasma-enhanced chemical-vapor-deposited nanocomposite silicon suboxides for solar cell applications

A passivation scheme, featuring nanocomposite amorphous silicon suboxides (a-SiOx:H) is investigated and analyzed in this work. The a-SiOx:H films are deposited by high-frequency plasma-enhanced chemical-vapor deposition via decomposition of silane (SiH4), carbon dioxide (CO2), and hydrogen (H2) as source gases. The plasma deposition parameters of a-SiOx:H films are optimized in terms of effective lifetime, while the oxygen content and the resulting optical band gap EG of the a-SiOx:H films are controlled by varying the CO2 partial pressure χO=[CO2]/([CO2]+[SiH4]). Postannealing at low temperatures of those films shows a beneficial effect in form of a drastic increase of the effective lifetime. This improvement of the passivation quality by low temperature annealing for the a-SiOx:H likely originates from defect reduction of the film close to the interface. Raman spectra reveal the existence of Si–(OH)x and Si–O–Si bonds after thermal annealing of the layers, leading to a higher effective lifetime, as it ...

Application of wide-bandgap hydrogenated amorphous silicon oxide layers to heterojunction solar cells for high quality passivation

Wide-gap (highly transparent), hydrogenated amorphous silicon oxide (a-SiOx:H) layers are investigated for heterojunction solar cell applications: Intrinsic a-SiOx:H(i) films are formed in order to prove their applicability for surface passivating buffer layers sandwiched between the crystalline silicon (c-Si) and the doped amorphous layer used for the formation of the emitter and the back-surface-field in heterojunction cells. The a-SiOx:H films are processed by high frequency (70 MHz) plasma decomposition using silane (SiH4), hydrogen (H2), and carbon dioxide (CO2) at the low deposition temperature of 155 °C. Quasi-steady-state photoconductance and transient photoconductance lifetime measurements have been carried out to determine the passivation quality of the intrinsic a-SiOx:H deposited on c-Si of different doping types and levels. A variation of the applied thickness of the grown a-SiOx:H films determines the impact on the performance of heterojunction solar cells. It will be demonstrated that excellent effective lifetimes as high as 4.7 ms on 1 Ωcm n-type float-zone (FZ) material (corresponding to a surface recombination velocity of 2.3 cm/s) and 14.2 ms on 130 Ωcm p-type FZ material (corresponding to a surface recombination velocity of 0.52 cm/s) can be achieved by surface passivation using our a-SiOx:H films. To validate the capability of the intrinsic and doped a-SiOx:H films separately, heterojunction solar cells consisting of (front to back) a-Si:H(p+)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n+), a-Si:H(p+)/a-SiOx:H(i)/c-Si(n)/a-SiOx:H(i)/a-Si:H(n+), and the reversed doping sequence have been analyzed. By incorporating a-SiOx:H(i) to the a-Si:H(p)/c-Si structure we find a drastic increase of the open circuit voltage (up to 655 mV for p-type substrates and 695 mV for n-type substrates) and accordingly, a higher conversion efficiency than obtained with standard a-Si(i).

Investigation of the emitter band gap widening of heterojunction solar cells by use of hydrogenated amorphous carbon silicon alloys

The hydrogenated amorphous carbon-silicon alloys [a-SixC1−x(n):Hy] and [a-Six(n):Hy] layers were investigated in order to prove the feasibility to widen the optical band gap in emitters of the heterojunction solar cells. The alloys were fabricated by decomposition of silane (SiH4), phosphine (PH3), methane (CH4), and hydrogen (H2), using a plasma enhanced chemical vapor deposition. Particularly, we focused on the incorporation of hydrogen and carbon within the resulting [a-SixC1−x(n):Hy] and [a-Six(n):Hy] films, which later form the emitter. The corresponding local vibrational modes of Si−Hx, C−H, and the corresponding network have been analyzed by μ-Raman spectroscopy. The addition of carbon degrades the photoelectronic properties in the emitter layer. This deterioration can be minimized by H dilution. The resulting optical band gap EG as well as the thickness of the emitter were determined by spectroscopic ellipsometry. It was confirmed that the band gap EG can be tailored by using an appropriate gas mi...

High efficiency silicon heterojunction solar cell using novel structure

A novel approach for heterojunction silicon wafer solar cell fabrication is being investigated: This approach features nanocomposite plasma deposited amorphous silicon suboxides (a-SiOx:H) for high-quality surface passivation combined with overlaying plasma deposited doped microcrystalline silicon (µc-Si:H(p+)/µc-Si:H(n+)) for use as heterojunction emitter and back-surface-field. Special attention is paid (i) to the front and back surface passivation of the wafer by high-quality wide-gap amorphous silicon suboxides (a-SiOx:H), and (ii) to the influence of wide-gap high-quality µc-Si:H for use as emitter and back-surface-field (BSF). The p+ µc-Si:H films are likely to be suitable for use as emitter and BSF in a heterojunction solar cell device. They feature high transparency to suppress absorption, and high conductivity when annealed at the optimum temperature. Heterojunction solar cells fabricated by combining the excellent surface passivation properties of the intrinsic a-SiOx:H and the doped highly-transparent µc-Si:H layers show a drastic increase of the open-circuit voltage (up to 702 mV for n-type substrates). These high open-circuit voltages can be consistently attributed to the excellent surface passivation by a-SiOx:H preventing surface recombination at the hetero-interface.

Lead-free electrical conductive adhesives for solar cell interconnectors

The electrical properties of various lead-free electrically conductive adhesives are investigated. They are intended to replace the solder, which is normally used to connect the interconnector tapes to the busbar. Compared to solder joints conductive adhesives offer the advantage of lower contact formation temperature with reasonable low contact resistances.

Application of plasma deposited nanocomposite silicon suboxides and microcrystalline silicon alloys to heterojunction solar cells

In this work, a novel approach for heterojunction solar cell fabrication, featuring plasma deposited amorphous silicon sub-oxides (a-SiOx:H) for high quality surface passivation combined with plasma deposited doped microcrystalline silicon (μc-Si(n+/p+)) is used to form the heterojunction. Special attention is paid (i) to the front and back surface passivation of the bulk material by high-quality wide-gap amorphous silicon sub-oxides (a-SiOx:H), and (ii) to the influence of wide-gap high-quality μc-Si:H at the front side for use as emitter to suppress absorption losses. Heterojunction solar cells fabricated by combining the excellent surface passivation properties of the intrinsic a-SiOx:H and the doped high-transparent μc-Si layers show cell performance up to 19.3 % efficiency (certified) so far. By incorporating a-SiOx:H(i) to the heterojunction structure a drastic increase of the open circuit voltage (up to 695 mV for n-type substrates) is found, and accordingly, a conversion efficiency higher than obtained with standard a-Si:H(i). These high open-circuit voltages can be consistently ascribed to the adequate surface passivation by a-SiOx:H preventing surface recombination at the hetero-interface and to the decrease of the optical absorption in the blue light region due to an enhanced optical bandgap of 1.95 eV. Heterojunction solar cells using textured substrates exhibit an expected gain of short circuit current of 5 mA/cm2, when transferring the optimized process parameters of cells using polished substrates to cells using textured substrates. For cell devices using textured substrates, efficiencies exceeding 19 % have been obtained, so far limited by a relatively low fill factor of 77 %, and a loss in the open circuit voltage compared to cells based on flat substrates. This deterioration can be attributed to process related issues, such as photolithography steps damaging the tips of the textured upside pyramids.

10 × 10 cm2 Hit Solar Cells Contacted with Lead-Free Electrical Conductive Adhesives to Solar Cell Interconnectors

In the present study we have investigated the optical and electrical properties of 10times10 cm2 HIT solar cells with evaporated Cr/Ag grids. Contacts were done with low temperature lead-free electrically conductive adhesives to standard solar cell interconnector tabs (IT). Compared to solder joints conductive adhesives have the advantage of lower contact formation temperature with reasonable low contact resistances (less than 1 mOmegacm2). ITs glued directly on ITO are not long term stable if the applied pressure during manufacturing is too low. Their contact resistance increases by about three to four orders of magnitude after 50 temperature cycles (-40degC/85degC). Thin Ag layers beneath the glue avoid contact degradation. Depending on the contact gluing process parameters (temperature, time, pressure and glue material) especially on textured substrates the polymer constituent of the conductive adhesive can bleed out into the solar cell light exposed area and provoke a higher optical reflection

Blistering of implanted crystalline silicon by plasma hydrogenation investigated by Raman scattering spectroscopy

Czochralski silicon wafers were implanted with H+ ions at a dose of 1×1016cm−2 followed by hydrogen plasma treatments at different temperatures. The minimum hydrogen implantation dose required for silicon surface exfoliation of 3×1016H+∕cm2 without further hydrogen incorporation was reduced to one-third by subsequent plasma hydrogenation. The corresponding local vibrational modes of hydrogen molecules, vacancy-hydrogen complexes, and Si–H bonds on surfaces have been analyzed by micro-Raman scattering spectroscopy to investigate blistering and platelet formation. The surface profile has been studied by atomic force microscopy and scanning electron microscopy. The plasma treated samples were annealed to investigate the mechanism and applicability of the induced exfoliation. ⟨111⟩-platelet formation occurred below plasma hydrogenation temperatures of 350°C. At temperatures above 450°C, ⟨100⟩-platelet nucleation induced blistering.

Bulk and Interface Degradation of Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells Under Proton Irradiation

Two different types of n-type amorphous silicon/p-type crystalline silicon heterojunction solar cells-with and without insertion of a thin intrinsic a-Si:H layer-have been irradiated with proton doses between 5.1010 and 5.1012 protons/cm2 at 1.7 MeV. They have been investigated as well by classical measurement techniques like spectral response and current-voltage characteristics under illumination as well as by electroluminescence measurements of the forward biased solar cell. As another interface sensitive technique, admittance spectroscopy has been applied before and after irradiation. Recently we have shown under which sample preparation conditions this latter technique can be applied to large area solar cells without the need to prepare special test structures [1]. Regarding the insertion of a thin intrinsic silicon layer at the interface between the n-type a-Si:H top layer and the p-type c-Si substrate (HIT structure), we find that this layer does not change the degradation behavior of the effective minority carrier diffusion length (obtained from spectral response measurements) in the crystalline silicon. The resulting damage constant, kL, is 1.2 10-6. Solar cell efficiencies dropped to slightly less than 50% of the original values for irradiation doses of 5 1012 protons/cm2. These results are comparable to the degradation found for crystalline silicon homojunction solar cells [2]. Comparing admittance spectroscopy and electroluminescence efficiency measurements, we found that the latter technique is more sensitive to proton irradiation induced interface modifications. In particular we observed a stronger degradation after irradiation for the heterostructure with the insertion of the intrinsic a-Si:H layer. The electroluminescence is dominated by the crystalline silicon band-to-band recombination and decreases monotonically for increasing irradiation doses