Electronic-beam evaporation processed titanium oxide as an electron selective contact for silicon solar cells

This work is dedicated to the study of electronic-beam (e-beam) evaporated titanium oxide (TiO_x) contact for polycrystalline silicon hetero-junction solar cells. A TiO_x material obtained by e-beam evaporation method is suggested as a possible alternative to the atomic layer deposition (ALD) process. The purpose is to achieve corresponding passivation efficiency between e-beam evaporation of TiOx and the ALD method. However, the TiOx in question achieved a relatively low passivation performance of S_eff = 113 cm⁻¹ in comparison to the reported ALD results. Nonetheless, as e-beam evaporation is well-established and an environmentally friendly deposition technology, e-beam evaporated TiO_x passivation layer has potential for improvement. What is clearly demonstrated in our work is how such an improvement in contact resistance dropped from >55 Ω/cm² to 2.29 Ω/cm². Indeed, our study established a correlation between the main process parameters of e-beam evaporation and their influence on the quality of electron selective TiO_x layer. Moreover, we reveal a possible scenario for the implementation of e-beam evaporated Titanium oxide as electron selective contact for asymmetrical hetero-junction solar cells.     

Effect of rapid thermal oxidation on structure and photoelectronic properties of silicon oxide in monocrystalline silicon solar cells

This paper concerns the topic of surface passivation properties of rapid thermal oxidation on p-type monocrystalline silicon wafer for use in screen-printed silicon solar cells. It shows that inline thermal oxidation is a very promising alternative to the use of conventional batch type quartz tube furnaces for the surface passivation of industrial phosphorus-diffused emitters. Five minutes was the most favorable holding time for the rapid thermal oxidation growth of the solar cell sample, in which the average carrier lifetime was increased 19.4 μs. The Fourier transform infrared spectrum of the rapid thermal oxidation sample, whose structure was Al/Al-BSF/p-type Si/n-type SiP/SiO₂/SiN_x/Ag solar cell with an active area of 15.6 cm², contained an absorption peak at 1085 cm⁻¹, which was associated with the Si–O bonds in silicon oxide. The lowest average reflectance of this sample is 0.87%. Furthermore, for this sample, its average of internal quantum efficiency and conversion efficiency are respectively increased by 8% and 0.23%, compared with the sample without rapid thermal oxidation processing.     

Efficiency characteristics of a silicon oxide passivation layer on p-type crystalline silicon solar cell at low illumination

Silicon dioxide (SiO₂) is widely used to improve the surface passivation properties of silicon solar cells. To minimize solar cell potential-induced degradation when the PV module is installed outdoors, a silicon oxide film is widely used as an insulator. However, experiments have confirmed that solar cells with a silicon oxide (SiO₂) film have a lower efficiency than solar cells without a silicon oxide (SiO₂) film at low illumination (<0.4 sun). Actually, the efficiency in the low illumination condition affects the average power output per day because the PV module mostly operates when the solar irradiation dose is less than 1 sun. To maximize the performance of the PV module, the output at a low light intensity level should also be considered. Shunt resistance (R_shunt) is known to cause a decrease in solar cell efficiency under low illumination conditions. PC1D simulation was used to analyze parameters, such as the series resistance, parallel resistance, and surface recombination, that affect the characteristics of the solar cell at low light intensity. In this study, we confirmed how the SiO₂ layer affected the low illumination properties of solar cells, even though these cells were more efficient at 1 sun. Silicon solar cells with a SiN_x/SiO₂ bilayer or a SiN_x single film were fabricated, and their characteristics were evaluated. Passivation characteristics were measured using the quasi-steady-state photoconductance (QSSPC) technique to evaluate the minority carrier lifetime and the implied open-circuit voltage (V_OC), and capacitance-voltage measurements were used to analyze the fixed charges. The values of the shunt resistance and series resistance in solar cells with different passivation layers were compared, and the cause of the decrease in the efficiency under low illumination was also analyzed via fill factor calculation.     

n-Type microcrystalline silicon oxide layer and its application to high-performance back reflectors in thin-film silicon solar cells

Light trapping is a key issue in improving the efficiency of thin-film Si solar cells, and using a back reflector material plays a critical role in improving a cell's light-trapping efficiency. In this study, we developed n-type microcrystalline silicon oxide (n-μc-SiO_x) films that are suitable for use as back reflectors in thin-film silicon solar cells. They exhibit a lower refractive index and lower absorption spectra, especially at long wavelengths of >700 nm, than conventional ZnO:Al materials, which are beneficial for this application. The n-μc-SiO_x films were prepared by the PECVD (plasma-enhanced chemical vapor deposition) method and applied to the fabrication of back reflectors in μc-Si:H solar cells. We also characterized the changes in cell performance with respect to the refractive index, conductivity, and thickness of the n-μc-SiO_x back reflectors. The novel back reflector boosts the total current density by up to 3.0% with the help of the enhanced long-wavelength response. It also improves open circuit voltage (V_oc) and fill factor (FF), which may be attributed to the reduced shunt current caused by the anisotropic electrical characteristics of the n-μc-SiO_x layer. Finally, we could achieve a conversion efficiency for the hydrogenated microcrystalline silicon (μc-Si:H) solar cells of up to 9.3% (V_oc: 0.501 V, J_sc: 27.4 mA/cm², FF: 0.68) using the n-μc-SiO_x back reflector.     

Deposition of device quality amorphous silicon and solar cell from argon dilution of silane

In our present study hydrogenated amorphous silicon (a-Si:H) thin films and solar cells have been prepared in a conventional single chamber rf-PECVD unit from silane–argon mixture by varying radio frequency (rf) power densities from 6 mW/cm² to 50 mW/cm². By optimizing the properties of the intrinsic material we have chosen a material which is deposited at 6 mW/cm² rf power density, 0.2 Torr pressure, 175 ^oC substrate temperature and by 97% argon dilution. For this material minority carriers (holes) diffusion length (L_d) measured in the as deposited state is 180 nm and it degrades by 15% after light soaking. This high L_d value indicates that the material is of device quality. We have fabricated a single junction solar cell having the structure p-a-SiC:H/i-a-Si:H/n-a-Si:H without optimizing the doped layers. This set exhibits a mean open circuit voltage of 0.8 V and conversion efficiency of 7.7%. After light soaking conversion efficiency decreases by 15% which demonstrates that it is possible to deposit device grade material and solar cells from silane–argon mixture.     

ITO: Zr bi-layers deposited by reactive O2 and Ar plasma with high work function for silicon heterojunction solar cells

We report the influence of reactive oxygen (O₂) and argon (Ar) plasma based ITO:Zr bi-layers for silicon heterojunction (SHJ) solar cells. The purpose of reactive O₂ sputtered ITO:Zr was to improve the Hall mobility and work function while the Ar based ITO:Zr films play an important role to maintain good electrical characteristics. The thickness of reactive O₂ based ITO:Zr films was fixed at 15 nm while Ar based films was varied from 65 to 125 nm, respectively. ITO:Zr bi-layers with the thickness of 15/105 nm deposited by O₂ and Ar plasma, respectively, showed lowest resistivity of 2.358 × 10⁻⁴ Ω cm and high Hall mobility of 39.3 cm²/V · s. All ITO:Zr bi-layers showed an average transmittance of above 80% in the visible wavelength (380–800 nm) region. Work function of ITO:Zr bi-layers was calculated from the X-ray photoelectron spectroscopic (XPS) data. The ITO:Zr work function was enhanced from 5.3 eV to 5.16 eV with the variation of ITO:Zr bi-layers from 15/65 to 15/125 nm, respectively. Front barrier height in SHJ solar cells can be modified by using TCO films with high work function. The SHJ solar cells were fabricated by employing the ITO:Zr bi-layer as front anti-reflection coating. The SHJ solar cells fabricated on ITO:Zr bi-layer with the thickness of 15/105 nm showed the best photo-voltage parameters as; V_oc = 739 mV, J_sc = 39.12 mA/cm², FF = 75.97%, η = 21.96%.     

Analysis of current loss from a series-parallel combination of dye-sensitized solar cells using electrochemical impedance spectroscopy

Dye-sensitized solar cells (DSCs) have been proposed as a substitute for silicon crystalline solar cells which have a high manufacturing cost but it is still difficult to fabricate highly efficient DSC module assemblies. Therefore, in this work, an externally connected module assembly was investigated for industrial applications of DSCs. The equivalent circuit of a DSC was determined using typical electrical components and the cause of a current loss in the parallel connection was analyzed using electrochemical impedance spectroscopy. Also, an externally connected module has been constructed using 50 DSCs, where each cell has an active area of 8 cm² (4.62 cm × 1.73 cm) and a conversion efficiency of 4.21% under 1 sun illumination (P_in of 100 mW/cm²). As a result, the externally connected DSC module assembly has an output of 7.4 V and 200 mA, and shows stable performance, with an energy conversion efficiency of 4.44% under 0.45 sun illuminations.     

Si-rich SiNx rear passivated c-Si solar cell with a novel antimony Local Back Surface Field formed by Laser Fired Contact

Local Back Contact (LBC) crystalline silicon solar cell with novel antimony (Sb) Local Back Surface Field (LBSF) are reported. The Sb LBSF is formed at low temperature with a Laser Fired Contacts (LFC) process. To improve the solar cell parameters of Sb LBSF, the rear passivation layer with SiN_x is optimized by varying the refractive index. The Si-rich SiN_x with a refractive index (n) of 2.7 possesses high lifetime of 2 ms with reduced absorption at a longer wavelength. The increase in lifetime is analyzed with Si–H bond concentration by FTIR. A 100 nm thick Sb layer with low laser power of 44 mW resulted in a junction depth of 500 nm with a carrier concentration of 5 × 10²⁰ cm^?3. The improved rear passivation with Si-rich SiN_x, the optimized Sb thickness yielded the best electrical results, with open circuit voltage (V_oc) of 643 mV and efficiency of 19.25%, compared to the reference cell with V_oc of 625 mV and efficiency of 18.20%.     

Optimum design of InGaP/GaAs dual-junction solar cells with different tunnel diodes

We design the InGaP/GaAs dual-junction (DJ) solar cells by optimizing short-circuit current matching between top and bottom cells using the Silvaco ATLAS. The relatively thicker base layer of top cell exhibits a larger short-circuit current density (J _sc) while the thicker base layer of bottom cell allows for a smaller J _sc. The matched J _sc of 10.61±0.05 and 13.25±0.06 mA/cm ² under AM1.5G and AM0 illuminations, respectively, are obtained, leading to the increased conversion efficiency. The base thicknesses of top InGaP cells are optimized at 0.8 and 0.65 μm for AM1.5G and AM0 illuminations, respectively, and the base thicknesses of bottom GaAs cells are optimized at 2 μm. For the optimized solar cell structure, the maximum J _sc = 10.66 mA/cm ² (13.31 mA/cm ²), V _oc =  2.34 V (2.35 V), and fill factor =  87.84% (88.1%) are obtained under AM1.5G (AM0) illumination, exhibiting a maximum conversion efficiency of 25.78% (23.96%). The effect of tunnel diode structure, i.e, GaAs/GaAs, AlGaAs/AlGaAs, and InGaP/InGaP, on the characteristics of solar cells is investigated. The photogeneration rate in the DJ solar cell structure is also obtained by incident light of different wavelengths.     

Influence of SiNx:H film properties according to gas mixture ratios for crystalline silicon solar cells

The Hydrogenated silicon nitride (SiN_x:H) using plasma enhanced chemical vapor deposition is widely used in photovoltaic industry as an antireflection coating and passivation layer. In the high temperature firing process, the SiN_x:H film should not change the properties for its use as high quality surface layer in crystalline silicon solar cells. For optimizing surface layer in crystalline silicon solar cells, by varying gas mixture ratios (SiH₄ + NH₃ + N₂, SiH₄ + NH₃, SiH₄ + N₂), the hydrogenated silicon nitride films were analyzed for its antireflection and surface passivation (electrical and chemical) properties. The film deposited with the gas mixture of SiH₄ + NH₃ + N₂ showed the best properties in before and after firing process conditions.The single crystalline silicon solar cells fabricated according to optimized gas mixture condition (SiH₄ + NH₃ + N₂) on large area substrate of size 156 mm × 156 mm (Pseudo square) was found to have the conversion efficiency as high as 17.2%. The reason for the high efficiency using SiH₄ + NH₃ + N₂ is because of the good optical transmittance and passivation properties. Optimized hydrogenated silicon nitride surface layer and high efficiency crystalline silicon solar cells fabrication sequence has also been explained in this study.     

Influence of laser power on POCl3 diffused back surface field on n-type PERT silicon solar cells with local back contact

We report n-type passivated emitter rear totally diffused (PERT) silicon solar cells with local back contacts (LBC) formed by laser process. With passivated back surface field (BSF), the PERT cell design shows an improved open circuit voltage (V_oc) with reduced recombination at the rear due to improved optical confinement. The rear side was diffused by POCl₃ diffusion with low sheet resistance (R_s) BSF and passivated using SiN_x. Laser ablation was used to open the SiN_x on the rear for LBC. The Nd:YAG laser power (mW) parameters and POCl₃ doping temperature were varied to obtain the BSF with lower sheet resistance. Laser power of 44 mW with 10 kHz resulted in 30 Ω/sq BSF layer with effective lifetime (τ_eff) of 90 μs and a higher V_oc of 646 mV. With the optimized laser parameters the best electrical results yielded a short circuit current density (J_sc) of 36 mA/cm² and efficiency of 18.54%.     

Silicon nitride/silicon oxide interlayers for solar cell passivating contacts based on PECVD amorphous silicon

This Letter demonstrates improved passivating contacts for silicon solar cells consisting of doped silicon films together with tunnelling dielectric layers. An improvement is demonstrated by replacing the commonly used silicon oxide interfacial layer with a silicon nitride/silicon oxide double interfacial layer. The paper describes the optimization of such contacts, including doping of a PECVD intrinsic a‐Si:H film by means of a thermal POCl₃ diffusion process and an exploration of the effect of the refractive index of the SiN_x. The n⁺ silicon passivating contact with SiN_x /SiO_x double layer achieves a better result than a single SiN_x or SiO_x layer, giving a recombination current parameter of ～7 fA/cm² and a contact resistivity of ～0.005 Ω cm², respectively. These self‐passivating electron‐selective contacts open the way to high efficiency silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Quantum Transport across Amorphous-Crystalline Interfaces in Tunnel Oxide Passivated Contact Solar Cells: Direct versus Defect-Assisted Tunneling

Tunnel oxide passivated contact solar cells have evolved into one of the most promising silicon solar cell concepts of the past decade,achieving a record efficiency of 25%.We study the transport mechanisms of realistic tunnel oxide structures,as encountered in tunnel oxide passivating contact(TOPCon) solar cells.Tunneling transport is affected by various factors,including oxide layer thickness,hydrogen passivation,and oxygen vacancies.When the thickness of the tunnel oxide layer increases,a faster decline of conductivity is obtained computationally than that observed experimentally.Direct tunneling seems not to explain the transport characteristics of tunnel oxide contacts.Indeed,it can be shown that recombination of multiple oxygen defects in a-SiO_x can generate atomic silicon nanowires in the tunnel layer.Accordingly,new and energetically favorable transmission channels are generated,which dramatically increase the total current,and could provide an explanation for our experimental results.Our work proves that hydrogenated silicon oxide(SiO_x:H) facilitates high-quality passivation,and features good electrical conductivity,making it a promising hydrogenation material for TOPCon solar cells.By carefully selecting the experimental conditions for tuning the SiO_x:H layer,we anticipate the simultaneous achievement of high open-circuit voltage and low contact resistance.     

Silicon heterojunction solar cell with amorphous silicon oxide buffer and microcrystalline silicon oxide contact layers

This Letter reports on the fabrication and characterization of silicon heterojunction solar cells with silicon oxide based buffer (intrinsic amorphous silicon oxide) and contact layers (doped microcrystalline silicon oxide) on flat p‐type wafers. The critical dependency of the cell performance on the front and rear buffer layer thickness reveals a trade‐off between the open circuit voltage V_oc and the fill factor FF. At the optimum, the highest efficiency of 18.5% (active area = 0.67 cm²) was achieved with V_oc = 664 mV, short circuit current J_sc = 35.7 mA/cm² and FF = 78.0%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

19.4%‐efficient large‐area fully screen‐printed silicon solar cells

We demonstrate industrially feasible large‐area solar cells with passivated homogeneous emitter and rear achieving energy conversion efficiencies of up to 19.4% on 125 × 125 mm² p‐type 2–3 Ω cm boron‐doped Czochralski silicon wafers. Front and rear metal contacts are fabricated by screen‐printing of silver and aluminum paste and firing in a conventional belt furnace. We implement two different dielectric rear surface passivation stacks: (i) a thermally grown silicon dioxide/silicon nitride stack and (ii) an atomic‐layer‐deposited aluminum oxide/silicon nitride stack. The dielectrics at the rear result in a decreased surface recombination velocity of S_rear = 70 cm/s and 80 cm/s, and an increased internal IR reflectance of up to 91% corresponding to an improved J_sc of up to 38.9 mA/cm² and V_oc of up to 664 mV. We observe an increase in cell efficiency of 0.8% absolute for the cells compared to 18.6% efficient reference solar cells featuring a full‐area aluminum back surface field. To our knowledge, the energy conversion efficiency of 19.4% is the best value reported so far for large area screen‐printed solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

氧化随机织构硅表面对单晶硅太阳电池性能的影响研究

热氧化生长的SiO\-2 薄膜经常在高效单晶硅太阳电池中被用作扩散掩膜,化学镀掩膜,钝化层或者基本的减反射层.在这些高效太阳电池中,经常使用碱性溶液对单晶硅表面进行处理,得到随机分布的正金字塔结构的织绒表面,减少表面的光反射.表面氧化后的正金字塔太阳电池暗反向电流-电压呈现"软击穿"现象,并联电阻明显下降.研究结果表明引起这些现象的原因在于氧化正金字塔表面会导致在体内形成位错型缺陷,这些缺陷能够贯穿整个pn 结,导致太阳电池的并联电阻下降,同时载流子在位错型缺陷在能隙中引入的能级处发生复合,导致空间电荷区 关键词： 热氧化 随机织构 位错 太阳电池     

The effect of anti-reflection coating of porous silicon on solar cells efficiency

Anti-reflection coatings of solar cells have been fabricated using different techniques. The techniques used include SiO₂ thermal oxidation, ZnO/TiO₂ sputtering deposition and porous silicon prepared by electrochemical etching. Surface morphology and structural properties of solar cells were investigated by using scanning electron microscopy and atomic forces microscopy. Optical reflectance was obtained by using optical reflectometer. I-V characterizations were studied under 80 mW/cm² illumination conditions. Porous silicon was found to be an excellent anti-reflection coating against incident light when it is compared with another anti-reflection coating and exhibited good light-trapping of a wide wavelength spectrum which produced high efficiency solar cells.     

Low-temperature synthesis of conducting boron-doped nanocrystalline silicon oxide thin films as the window layer of solar cells

Low-temperature synthesis of highly transparent conducting B-doped (p-type) nc-SiO_X:H films has been pursued by 13.56 MHz plasma-CVD, using a combination of SiH₄, CO₂ and B₂H₆, diluted by H₂ and He. Higher substrate temperature (T_S) encourages nanocrystallization in B-doped nc-SiO_X:H network by reducing bonded H-content, while bonded O-content also reduces simultaneously. At optimized T_S = 150 °C, p–nc-SiO_X:H film having an optical band gap ~1.98 eV, high conductivity ~0.18 S cm⁻¹, has been obtained via dopant-induced escalation of the electrically active carriers at a deposition rate ~5.3 nm/min. The p–nc-SiO_X:H film appears as a promising window layer for the top sub-cell of multi-junction silicon solar cells. A single-junction nc-Si:H based p-i-n solar cell of efficiency (η) ~7.14% with a current-density (J_SC) ~14.18 mA/cm², reasonable fill-factor (FF) ~66.2% and open-circuit voltage (V_OC) ~0.7606 V has been fabricated, using the optimum p-type nc-SiO_X:H as the window layer deposited at T_S = 150 °C.     

Synthesis and characterization of Cu2ZnSnS4 thin films by SILAR method

Semiconducting Cu₂ZnSnS₄ (CZTS) material has been receiving a great technological interest in the photovoltaic industry because of its low-cost non-toxic constituents, ideal direct band gap as a absorber layer and high absorption coefficient. CZTS thin films have been successfully deposited onto the fluorine-doped tin oxide/glass (glass/FTO) substrates coated glass substrates using successive ionic layer adsorption and reaction (SILAR) method and investigated for photoelectrochemical conversion (PEC) of light into electricity. The best solar cell sample showed an open-circuit voltage of 390 mV, a short-circuit current density of 636.9 μA/cm², a fill factor of 0.62 and an efficiency of 0.396% under irradiation of 30 mW/cm². Preliminary results obtained for solar cells fabricated with this material are promising.     

The p recombination layer in tunnel junctions for micromorph tandem solar cells

A new tunnel recombination junction is fabricated for n–i–p type micromorph tandem solar cells. We insert a thin heavily doped hydrogenated amorphous silicon (a-Si:H) p + recombination layer between the n a-Si:H and the p hydrogenated nanocrystalline silicon (nc-Si:H) layers to improve the performance of the n–i–p tandem solar cells. The effects of the boron doping gas ratio and the deposition time of the p-a-Si:H recombination layer on the tunnel recombination junctions have been investigated. The current-voltage characteristic of the tunnel recombination junction shows a nearly ohmic characteristic, and the resistance of the tunnel recombination junction can be as low as 1.5 ·cm 2 by using the optimized p-a-Si:H recombination layer. We obtain tandem solar cells with open circuit voltage V oc = 1.4 V, which is nearly the sum of the V oc s of the two corresponding single cells, indicating no V oc losses at the tunnel recombination junction.