n‐type silicon solar cells with surface‐passivated screen‐printed aluminium‐alloyed rear emitter

Aluminium‐doped p‐type (Al‐p⁺) silicon emitters fabricated by means of a simple screen‐printing process are effectively passivated by plasma‐enhanced chemical‐vapour deposited amorphous silicon (a‐Si). We measure an emitter saturation current density of only 246 fA/cm², which is the lowest value achieved so far for a simple screen‐printed Al‐p⁺ emitter on silicon. In order to demonstrate the applicability of this easy‐to‐fabricate p⁺ emitter to high‐efficiency silicon solar cells, we implement our passivated p⁺ emitter into an n⁺np⁺ solar cell structure. An independently confirmed conversion efficiency of 19.7% is achieved using n‐type phosphorus‐doped Czochralski‐grown silicon as bulk material, clearly demonstrating the high‐efficiency potential of the newly developed a‐Si passivated Al‐p⁺ emitter. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the current linearity at low illumination of high‐efficiency back‐junction back‐contact silicon solar cells

The relation between current and illumination intensity of three structures of high‐efficiency back‐junction back‐contact silicon solar cells was analyzed. Both, n‐type cells with non‐diffused front surface and p‐type cell with floating n‐emitter show a pronounced non‐linearity due to strong illumination dependence of the passivation quality of the non‐diffused surface and the floating junction respectively. Quantum efficiency (QE) of this cell type drops significantly for the illumination lower than 0.5 suns. In contrast the QE of n‐type cells with n⁺‐front surface field (FSF) is linear. Low illumination current characteristics of all three of the analyzed structures could be well described by physical models. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface passivation schemes for high‐efficiency n‐type Si solar cells

An effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n‐type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p‐type silicon than on n‐type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p‐type to n‐type surface allows the use of standard technologies which are used for passivation of highly doped n‐type surfaces. Emitter saturation current densities (J_0e) of 49 fA/cm² have been reached with this approach on SiO₂ passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open‐circuit voltage (V_oc) of 676 mV was achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effective surface passivation of crystalline silicon by rf sputtered aluminum oxide

In recent years, excellent surface passivation has been achieved on both p‐type and n‐type surfaces of silicon wafers and solar cells using aluminum oxide deposited by plasma‐assisted atomic layer deposition. However, alternative deposition methods may offer practical advantages for large‐scale manufacturing of solar cells. In this letter we show that radio‐frequency magnetron sputtering is capable of depositing negatively‐charged aluminum oxide and achieving good surface passivation both on p‐type and n‐type silicon wafers. We thus establish that sputtered aluminum oxide is a very promising method for the surface passivation of high efficiency solar cells. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excellent Passivation of p-Type Si Surface by Sol-Gel Al2O3 Films

Al2O3 films with a thickness of about lOOnm synthesized by spin coating and thermally treated are applied for field-induced surface passivation of p-type crystalline silicon. The level of surface passivation is determined by techniques based on photoconductance. An effective surface recombination velocity below lOOcm/s is obtained on 10Ωcm p-type c-Si wafers （Cz Si）. A high density of negative fixed charges in the order of 10^12 cm^-2 is detected in the Al2O3 films and its impact on the level of surface passivation is demonstrated experimentally. Furthermore, a comparison between the surface passivation achieved for thermal SiO2 and plasma enhanced chemical vapor deposition SiNx ：H films on the same c-Si is presented. The high negative fixed charge density explains the excellent passivation of p-type c-S/by Al2O3.     

Short‐circuit current improvement of CuGaSe2 solar cells with a ZnS/(Zn,Mg)O buffer combination

CuGaSe₂ (CGS) thin‐film solar cells were prepared with an in‐line co‐evaporation process and the established buffer combination CdS/i‐ZnO was replaced by ZnS/(Zn,Mg)O. We obtained functional CGS solar cells with a strong gain in the short‐circuit current density as compared to the CdS/i‐ZnO buffer reference cells. The enhanced current density is a result of improved transmission in the wavelength region between 330 nm and 550 nm of the ZnS/(Zn,Mg)O buffer combination as compared to CdS/i‐ZnO. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application

Special sequences of wet-chemical oxidation and etching steps were optimised with respect to the etching behaviour of differently oriented silicon to prepare very smooth silicon interfaces with excellent electronic properties on mono- and poly-crystalline substrates. Surface photovoltage (SPV) and photoluminescence (PL) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were utilised to develop wet-chemical smoothing procedures for atomically flat and structured surfaces, respectively. Hydrogen-termination as well as passivation by wet-chemical oxides were used to inhibit surface contamination and native oxidation during the technological processing. Compared to conventional pre-treatments, significantly lower micro-roughness and densities of surface states were achieved on mono-crystalline Si(100), on evenly distributed atomic steps, such as on vicinal Si(111), on silicon wafers with randomly distributed upside pyramids, and on poly-crystalline EFG (Edge-defined Film-fed-Growth) silicon substrates.The recombination loss at a-Si:H/c-Si interfaces prepared on c-Si substrates with randomly distributed upside pyramids was markedly reduced by an optimised wet-chemical smoothing procedure, as determined by PL measurements. For amorphous-crystalline hetero-junction solar cells (ZnO/a-Si:H(n)/c-Si(p)/Al) with textured c-Si substrates the smoothening procedure results in a significant increase of short circuit current I_sc, fill factor and efficiency η. The scatter in the cell parameters for measurements on different cells is much narrower, as compared to conventional pre-treatments, indicating more well-defined and reproducible surface conditions prior to a-Si:H emitter deposition and/or a higher stability of the c-Si surface against variations in the a-Si:H deposition conditions.     

Decoupling crystalline volume fraction and VOC in microcrystalline silicon pin solar cells by using a µc‐Si:F:H intrinsic layer

Microcrystalline silicon thin film pin solar cells with a highly crystallized intrinsic μc‐Si:F:H absorber were prepared by RF‐plasma enhanced chemical vapour deposition using SiF₄ as the gas precursor. The cells were produced with a vacuum break between the doped layer and intrinsic layer depositions, and the effect of different subsequent interface treatment processes was studied. The use of an intrinsic μc‐Si:H p/i buffer layer before the first air break increased the short circuit current density from 22.3 mA/cm² to 24.7 mA/cm². However, the use of a hydrogen‐plasma treatment after both air breaks without an interface buffer layer improved both the open circuit voltage and the fill factor. Although the material used for the absorber layer showed a very high crystalline fraction and thus an increased spectral response at long wavelengths, an open‐circuit voltage (V_OC) of 0.523 V was nevertheless observed. Such a value of V_OC is higher than is typically obtained in devices that employ a highly crystallized absorber as reported in the literature (see abstract figure). Using a hydrogen‐plasma treatment, a single junction μc‐Si:F:H pin solar cell with an efficiency of 8.3% was achieved.

     

Effective silicon surface passivation by atomic layer deposited Al2O3/TiO2 stacks

In this work atomic layer deposition of Al₂O₃ and TiO₂ has been used to obtain dielectric stacks for passivation of silicon surfaces. Our experiments on n‐ and p‐type silicon wafers deposited by thin Al₂O₃/TiO₂ stacks show that a considerably improved passivation is obtained compared to the Al₂O₃ single layer. For Al₂O₃ films thinner than 20 nm the emitter saturation current density decreases with increasing TiO₂ thickness. Especially the passivation of ultrathin (～5 nm) Al₂O₃ is very effectively enhanced by TiO₂ due to a decreased interface defect density as well as an increased fixed negative charge in the stacks. Hence, the thin Al₂O₃/TiO₂ stacks developed in this work can be used as a passivation coating for Si‐based solar cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Passivation of structured p-type silicon interfaces: Effect of surface morphology and wet-chemical pre-treatment

The effect of both surface morphology and wet-chemical pre-treatment on electronic surface and interface properties was investigated for mono- and polycrystalline silicon substrates with special surface structures. Surface charge, energetic distribution, and density of rechargeable states on these surfaces were determined by surface photovoltage (SPV) measurements. These results were correlated to previously reported findings on atomically flat Si(111) and Si(100) surfaces of monocrystalline wafers. In this paper, a specially optimised sequence of cleaning, wet-chemical oxidation, and oxide removal procedures is described in detail for the first time. This method was successfully applied in order to remove contaminations and damaged surface layers and to obtain atomically flat areas on substrates with evenly distributed atomic steps, polycrystalline and monocrystalline substrates with randomly distributed pyramids. A significant reduction in surface micro-roughness, interface state density, and recombination loss was achieved. Using passivation by wet-chemical oxidation or H-termination, respectively, the optimised surface state can be preserved by the time of following preparation steps and during subsequent a-Si:H plasma enhanced chemical vapour deposition (PECVD).     

Defect reduction and surface passivation of SiO2/Si by heat treatment with high-pressure H2O vapor

Heat treatment with high-pressure H₂O vapor was applied to improve interface properties of SiO₂/Si and passivate the silicon surface. Heat treatment at 180–420 °C with high-pressure H₂O vapor changed SiO_x films, 150 nm thick formed at room temperature by thermal evaporation in vacuum, into SiO₂ films with a Si-O-Si bonding network similar to that of thermally grown SiO₂ films. Heat treatment at 130 °C with 2.8×10⁵ Pa H₂O for 3 h reduced the recombination velocity for the electron minority carriers from 405 cm/s (as-fabricated 150-nm-thick SiO_x/Si) to 5 cm/s. Field-effect passivation was demonstrated by an additional deposition of defective SiO_x films on the SiO₂ films formed by heat treatment at 340 °C with high-pressure H₂O vapor. The SiO_x deposition reduced the recombination velocity from 100 cm/s to 48 cm/s. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

A study of interface characteristics in HfAlO/p-Si by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) and high-frequency capacitance-voltage (HF-CV) measurement are used for the investigation of HfAlO/p-Si interface. The so-called “slow” interface states detected by HF-CV are obtained to be 2.68 × 10¹¹ cm⁻². Combined conventional DLTS with insufficient-filling DLTS (IF-DLTS), the true energy level position of interfacial traps is found to be 0.33 eV above the valance band maximum of silicon, and the density of such “fast” interfacial traps is 1.91 × 10¹² cm⁻² eV⁻¹. The variation of energy level position of such traps with different annealing temperatures indicates the origin of these traps may be the oxide-related traps very close to the HfAlO/Si interface. The interfacial traps’ passivation and depassivation effect of postannealing in forming gas are shown by comparing samples annealed at different temperatures.     

Flexible a‐Si:H/nc‐Si:H tandem thin film silicon solar cells on plastic substrates with i ‐layers made by hot‐wire CVD

In this letter we report the result of an a‐Si:H/nc‐Si:H tandem thin film silicon solar mini‐module fabricated on plastic foil containing intrinsic silicon layers made by hot‐wire CVD (efficiency 7.4%, monolithically series‐connected, aperture area 25 cm²). We used the Helianthos cell transfer process. The cells were first deposited on a temporary aluminum foil carrier, which allows the use of the optimal processing temperatures, and then transferred to a plastic foil. This letter reports the characteristics of the flexible solar cells obtained in this manner, and compares the results with those obtained on reference glass substrates. The research focus for implementation of the hot‐wire CVD technique for the roll‐to‐roll process is also discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Oxygen deficiency dependence of magnetic and magnetoresistive properties for La0.5Sr0.5CoO3-x thin films prepared by pulsed laser deposition

The complete (001)-oriented thin films of La_0.5Sr_0.5CoO_3-x (LSCO) are deposited on (001) SrTiO₃ substrates by pulsed laser deposition under reduced oxygen pressure. It is revealed that the c axis of the film stretches with depleting oxygen. The magnetic, electrical, and magnetoresistive properties of the films are characterized by means of various techniques. Significant dependence of these properties on oxygen deficiency in the films is demonstrated, with enhanced magnetoresistance recorded for the samples deposited over a wide range of reduced oxygen pressure. Received: 9 July 1998 / Accepted: 15 January 1999 / Published online: 31 March 1999     

Monolayer passivation of silicon(0 0 1) surface by selenium

Monolayer passivation of the silicon(0 0 1) surface by selenium is investigated in an ultrahigh vacuum environment with a solid selenium source by reflection high-energy electron diffraction and residual gas analysis. It is found that precisely one monolayer of selenium is deposited on silicon(0 0 1) when the silicon substrate temperature is set slightly above the selenium source temperature and the passivation time ensures a little overdose of selenium above one monolayer. The temperature settings prevent selenium condensation on silicon(0 0 1), which makes selenium deposition on silicon(0 0 1) a thermodynamically self-limited process to exactly one monolayer.     

Passivation of InP-based HBTs

The surface effects, the (NH₄)₂S and low-temperature-deposited SiN_x passivations of InP-based heterostructure bipolar transistors (HBTs) have been investigated. The surface recombination current of InP-based HBTs is related to the base structures. The (NH₄)₂S treatment for InGaAs and InP removes the natural oxide layer and results in sulfur-bonded surfaces. This can create surface-recombination-free InP-based HBTs. Degradation is found when the HBTs were exposed to air for 10 days. The low-temperature-deposited SiN_x passivation of InGaAs/InP HBTs causes a drastic decrease in the base current and a significant increase in the current gain. The improvement in the HBT performance is attributed to the low deposition temperature and the effect of N₂ plasma treatment in the initial deposition process. The SiN_x passivation is found to be stable. S/SiN_x passivation of InGaAs/InP HBTs results in a decrease in the base current and an increase in the current gain. The annealing process can cause the base current to decrease further and the current gain increase.     

Wet sulfur passivation of GaSb(1 0 0) surface for optoelectronic applications

A comparative analysis of the properties of the non-passivated and S-passivated GaSb(1 0 0) surfaces has been performed through PL, AFM and RHEED characterization. The samples treated with a 1 M Na₂S aqueous solution demonstrate an increase in the 5 K PL intensity. According to AFM data, the annealing of the S-passivated GaSb(1 0 0) leads to the formation of the clean flat (1 0 0) surface. Moreover, after annealing the PL intensity of the S-passivated GaSb(1 0 0) surfaces decreases by 20%, whereas for the non-passivated samples it drops by more than a factor of 4. The method of wet sulfur passivation has shown great effectiveness in pre-epitaxial processing for LPE and MBE growth of the GaSb-related materials for optoelectronics.     

Deep‐level characterization of emissive interface states in Alq3‐based OLEDs

We have succesfully investigated emissive interface states in fabricated indium‐tin‐oxide (ITO)/N,N′‐di‐1‐naphthyl‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′diamine (α‐NPD)/tris(8‐hydroxyquinoline) aluminum (Alq₃)/LiF/Al organic light‐emitting diodes (OLEDs) by a modified deep‐level optical spectroscopy (DLOS) technique. In the vicinity of the α‐NPD/Alq₃ emissive interface, a discrete trap level was found to be located at ～1.77 eV below the conduction band of Alq₃, in addition to band‐to‐band transitions of carriers from α‐NPD to Alq₃. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rapid crystallization of silicon films using Joule heating of metal films

50-nm thick amorphous silicon films formed on glass substrates were crystallized by rapid Joule heating induced by an electrical current flowing in 100-nm-thick Cr strips formed adjacently to 200-nm-thick SiO₂ intermediate layers. 3-μs-pulsed voltages were applied to the Cr strips. Melting of the Cr strips caused a high Joule heating intensity of about 1×10⁶ W/cm². Raman scattering measurements revealed complete crystallization of the silicon films at a Joule heating energy of 1.9 J/cm² via the SiO₂ intermediate layer. Transmission electron microscopy measurements confirmed a crystalline grain size of 50–100 nm. 1-μm-long crystalline grain growth was also observed just beneath the edge of the Cr strips. The electrical conductivity increased from 10^-5 S/cm to 0.3 S/cm for 7×10¹⁷-cm^-3-phosphorus-doped silicon films because of activation of the phosphorus atoms because of crystallization. The numerical analysis showed a density of localized defect states at the mid gap of 8.0×10¹⁷ cm^-3. Oxygen plasma treatment at 250 °C and 100 W for 5 min reduced the density of the defect states to 2.7×10¹⁷ cm^-3. Received: 3 April 2001 / Accepted: 9 April 2001 / Published online: 25 July 2001     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.