Crystalline Silicon Heterojunction Solar Cells Research Articles

Transition metal oxides as hole-selective contacts in silicon heterojunctions solar cells

This work reports on a comparative study comprising three transition metal oxides, MoO3, WO3 and V2O5, acting as front p-type contacts for n-type crystalline silicon heterojunction solar cells. Owing to their high work functions (>5eV) and wide energy band gaps, these oxides act as transparent hole-selective contacts with semiconductive properties that are determined by oxygen-vacancy defects (MoO3−x), as confirmed by X-ray photoelectron spectroscopy. In the fabricated hybrid structures, 15nm thick transition metal oxide layers were deposited by vacuum thermal evaporation. Of all three devices, the V2O5/n-silicon heterojunction performed the best with a conversion efficiency of 15.7% and an open-circuit voltage of 606mV, followed by MoO3 (13.6%) and WO3 (12.5%). These results bring into view a new silicon heterojunction solar cell concept with advantages such as the absence of toxic dopant gases and a simplified low-temperature fabrication process.

Optimization of the Antireflection Coating of Thin Epitaxial Crystalline Silicon Solar Cells

In this work we use an effective weighting function to include the internal quantum efficiency (IQE) and the effective thickness, Te, of the active cell layer in the optical modeling of the antireflection coating (ARC) of very thin crystalline silicon solar cells. The spectrum transmitted through the ARC is hence optimized for efficient use in the given cell structure and the solar cell performance can be improved. For a 2-μm thick crystalline silicon heterojunction solar cell the optimal thickness of the Indium Tin Oxide (ITO) ARC is reduced by ∼8nm when IQE data and effective thickness are taken into account compared to the standard ARC optimization, using the AM1.5 spectrum only. The reduced ARC thickness will shift the reflectance minima towards shorter wavelengths and hence better match the absorption of very thin cells, where the short wavelength range of the spectrum is relatively more important than the long, weakly absorbed wavelengths. For this cell, we find that the optimal thickness of the ITO starts at 63nm for very thin (1μm) active Si layer and then increase with increasing Te until it saturates at 71nm for Te > 30μm.

Application of n-type microcrystalline silicon oxide as back reflector of crystalline silicon heterojunction solar cells

We investigated the application of n-type hydrogenated microcrystalline silicon oxide (n-µc-Si1−xOx:H) as an alternative back reflector material for n-type heterojunction solar cells. The effect of the CO2 and PH3 flow rates on the refractive index, oxygen content, conductivity, and crystalline fraction (Xc) of n-µc-Si1−xOx:H films was evaluated. By controlling the film oxygen content, the refractive index could be widely changed, while the absorption coefficient at wavelengths exceeding 800 nm was practically zero. We found that the insertion of an n-µc-Si1−xOx:H layer between the back surface field and the rear electrode improves the reflectance at long wavelengths. The improvement in reflectance resulted in higher internal quantum efficiency in the IR range, suggesting that the application of n-µc-Si1−xOx:H as a back reflector is promising for reducing long-wavelength losses in heterojunction solar cells.

Influence of Periodic Surface Nanopatterning Profiles on Series Resistance in Thin-Film Crystalline Silicon Heterojunction Solar Cells

In the frame of the development of thin crystalline silicon solar cell technologies, surface nanopatterning of silicon is gaining importance. Its impact on the material quality is, however, not yet fully controlled.We investigate here the influence of surface nanotexturing on the series resistance of a contacting scheme relevant for thin-film crystalline silicon heterojunction solar cells. Two dimensional periodic nanotextures are fabricated using a combination of nanoimprint lithography and either dry or wet etching, while random pyramid texturing is used for benchmarking. We compare these texturing techniques in terms of their effect on the series resistance of a solar cell through a study of the sheet resistance (Rsh ) and contact resistance (Rc) of its front layers, i.e., a sputtered transparent conductive oxide and evaporated metal contacts. We have found by four-point probe and the transfer length methods that dry-etched nanopatterns render the highest Rsh and Rc values. Wet-etched nanopatterns, on the other hand, have less impact on Rc and render Rsh similar to that obtained from the nontextured case.

Highly Efficient Solution‐Processed Poly(3,4‐ethylenedio‐xythiophene):Poly(styrenesulfonate)/Crystalline–Silicon Heterojunction Solar Cells with Improved Light‐Induced Stability

Highly Efficient Solution‐Processed Poly(3,4‐ethylenedio‐xythiophene):Poly(styrenesulfonate)/Crystalline–Silicon Heterojunction Solar Cells with Improved Light‐Induced Stability

Improvement in passivation quality and open-circuit voltage in silicon heterojunction solar cells by the catalytic doping of phosphorus atoms

We apply phosphorus (P) doping to amorphous silicon (a-Si)/crystalline silicon (c-Si) heterojunction solar cells realized by exposing c-Si to P-related radicals generated by the catalytic cracking of PH3 molecules (Cat-doping). An ultrathin n+-layer formed by P Cat-doping acts to improve the effective minority carrier lifetime (τeff) and implied open-circuit voltage (implied Voc) owing to its field effect by which minority holes are sent back from an a-Si/c-Si interface. An a-Si/c-Si heterojunction solar cell with a P Cat-doped layer shows better solar cell performance, particularly in Voc, than the cell without P Cat-doping. This result demonstrates the feasibility of applying Cat-doping to a-Si/c-Si heterojunction solar cells, owing to the advantage of the low-temperature (<200 °C) process of Cat-doping.

Silicon heterojunction solar cells with high surface passivation quality realized using amorphous silicon oxide films with epitaxial phase

The epitaxial growth of the i-layer of crystalline silicon heterojunction solar cells has been widely accepted as harmful to surface passivation. In our experiments, however, although a very rough epitaxial phase in the intrinsic a-Si1−xOx:H passivation layer was confirmed by transmission electron microscopy and spectroscopic ellipsometry, a high effective lifetime and an implied-VOC of over 720 mV were achieved with lifetime samples. The high passivation quality was confirmed by the obtained open circuit voltages of 728 and 721 mV for n- and p-type solar cells, respectively, with an a-Si1−xOx:H/p-µc-Si1−xOx:H stack rear structure. These results indicate that, contrary to the common knowledge, high surface passivation quality can be achieved even when the epitaxial phase is present.

Ultrathin Crystalline Silicon Heterojunction Solar Cell Integrated on Silicon-on-Insulator Substrate

To achieve power generation on IC chips, a hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction solar cell was designed and fabricated on silicon-on-insulator substrate, where a 9- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> epitaxial p-type c-Si layer served as light absorption layer and the buried SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as back surface passivation layer. It was found that a 1- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> heavily doped thin p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> layer was vital for improving the cell performances. Efficiency up to 12.7% with an open-circuit voltage of 679.7 mV was achieved on a 1.0-cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}$ </tex-math></inline-formula> square cell. The device performance was also investigated by annealing at different temperatures. The results suggested that a relatively large thickness of a-Si:H and transparent conductive oxide layers could improve thermal stability of the solar cells at temperature above 300 °C.

Achievement of More Than 25% Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell

The crystalline silicon heterojunction structure adopted in photovoltaic modules commercialized as Panasonic's HIT has significantly reduced recombination loss, resulting in greater conversion efficiency. The structure of an interdigitated back contact was adopted with our crystalline silicon heterojunction solar cells to reduce optical loss from a front grid electrode, a transparent conducting oxide (TCO) layer, and a-Si:H layers as an approach for exceeding the conversion efficiency of 25%. As a result of the improved short-circuit current (J sc ), we achieved the world's highest efficiency of 25.6% for crystalline silicon-based solar cells under 1-sun illumination (designated area: 143.7 cm 2 ).

Amorphous/crystalline silicon heterojunction solar cells with black silicon texture

Excellent passivation of black silicon surfaces by thin amorphous silicon layers deposited with plasma enhanced chemical vapor deposition is demonstrated. Minority charge carrier lifetimes of 1.3 milliseconds, enabling an implied open-circuit voltage of 714 mV, were achieved. The influence of amorphous silicon parasitic epitaxial growth and thickness, as well as of the texture depth is investigated. Furthermore, quantum efficiency gains for wavelengths above 600 nm, as compared to random textured solar cells, are demonstrated in 17.2% efficient amorphous–crystalline silicon heterojunction solar cells with black silicon texture. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Effect of the CO2/SiH4Ratio in the p-μc-SiO:H Emitter Layer on the Performance of Crystalline Silicon Heterojunction Solar Cells

This paper reports the preparation of wide gap p-type hydrogenated microcrystalline silicon oxide (p-μc-SiO:H) films using a 40 MHz very high frequency plasma enhanced chemical vapor deposition technique. The reported work focused on the effects of the CO2/SiH4ratio on the properties of p-μc-SiO:H films and the effectiveness of the films as an emitter layer of crystalline silicon heterojunction (c-Si-HJ) solar cells. A p-μc-SiO:H film with a wide optical band gap (E04), 2.1 eV, can be obtained by increasing the CO2/SiH4ratio; however, the tradeoff betweenE04and dark conductivity must be considered. The CO2/SiH4ratio of the p-μc-SiO:H emitter layer also significantly affects the performance of the solar cells. Compared to the cell using p-μc-Si:H (CO2/SiH4= 0), the cell with the p-μc-SiO:H emitter layer performs more efficiently. We have achieved the highest efficiency of 18.3% with an open-circuit voltage (Voc) of 692 mV from the cell using the p-μc-SiO:H layer. The enhancement in theVocand the efficiency of the solar cells verified the potential of the p-μc-SiO:H films for use as the emitter layer in c-Si-HJ solar cells.

Study of an Amorphous Silicon Oxide Buffer Layer for p-Type Microcrystalline Silicon Oxide/n-Type Crystalline Silicon Heterojunction Solar Cells and Their Temperature Dependence

Intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) films were used as front and rear buffer layers in crystalline silicon heterojunction (c-Si-HJ) solar cells. The surface passivity and effective lifetime of these i-a-SiO:H films on an n-type silicon wafer were improved by increasing the CO2/SiH4ratios in the films. Using i-a-SiO:H as the front and rear buffer layers in c-Si-HJ solar cells was investigated. The front i-a-SiO:H buffer layer thickness and the CO2/SiH4ratio influenced the open-circuit voltage(Voc), fill factor (FF), and temperature coefficient (TC) of the c-Si-HJ solar cells. The highest total area efficiency obtained was 18.5%(Voc=700 mV,Jsc=33.5 mA/cm2, andFF=0.79). The TC normalized for this c-Si-HJ solar cell efficiency was −0.301%/°C.

Evolution of the Charge Carrier Lifetime Characteristics in Crystalline Silicon Wafers During Processing of Heterojunction Solar Cells

All process steps in the fabrication of amorphous / crystalline (a-Si:H/c-Si) silicon heterojunction solar cells affect interface recombination and thus have an impact on the open circuit voltage of the final solar cell. Transient photoconductance decay is a convenient method to assess the charge carrier lifetime which is a direct measure for recombination and thus the interface quality. In this contribution we present a step-by-step analysis and evaluation of the charge carrier lifetime throughout the processing of the solar cells starting from the saw-damage etched Si wafer to the complete solar cell device. Particular emphasis is put on the optimization of the random pyramid texture. For this, surface morphologies and a-Si:H/c-Si interface properties were quantitatively evaluated and optimized with respect to low recombination and large carrier lifetimes. Based on this optimization a-Si:H/c-Si heterojunction solar cells with conversion efficiencies exceeding 20% were prepared on c-Si wafers textured in isopropanol(IPA)-free alkaline solution.

Role of i-aSi:H Layers in aSi:H/cSi Heterojunction Solar Cells

The dependence of solar cell parameters on i-aSi:H (non- or lightly-doped hydrogenated amorphous silicon) layer thickness in an aSi:H/cSi (crystalline silicon) heterojunction solar cell was analyzed using numerical simulation. By considering the quantum confinement effect at interfaces between i-aSi:H and cSi, experimental data which had not been explained by simulation could be successfully interpreted. The mechanism of an open-circuit voltage increase was visually presented by analyzing carrier distributions and quasi-Fermi levels near cSi surfaces and in the i-aSi:H layers. The optimized thicknesses of the i-aSi:H layers in both front and rear junctions were suggested to obtain the maximum conversion efficiency. The influences of the quantum confinement effect on the simulation results were discussed.

Doped SiO x emitter layer in amorphous/crystalline silicon heterojunction solar cell

In this work we propose to replace the emitter layer of the n-type doped a-Si:H/p-type doped crystalline silicon heterojunction solar cell, with an n-type doped SiO x amorphous oxide layer. The n-type doped SiO x :H shows a lower activation energy and higher carrier mobility value with respect to the n-type doped a-Si:H. Moreover, higher transmission, below 500 nm of wavelength, and higher conductivity are measured. The relevance of transparency of the (n) a-SiO x :H has been studied using that film in solar cells. The electrical parameters revealed a solar cell efficiency of 15.8 %. Moreover, the effect of TCO as a front side cell electrode is considered and discussed on the base of its workfunction when applied on top of the n-type doped SiO x emitter layer using also numerical simulations.

Reformulation of solar cell physics to facilitate experimental separation of recombination pathways

Experimentally identifying the spatial distribution of recombination in a solar cell is challenging, with only semi-quantitative information available from conventional characterization techniques. We develop a formulation of solar cell physics, based upon well-justified analytic approximations, to quantitatively extract information about recombination in different cell regions. We derive the dependence of VOC on light-intensity, temperature, and strength of recombination in the space-charge, quasi-neutral, and interface regions. Expanding the scope and utility of commonly used characterization techniques, we apply this formulation to evaluate the spatial distribution of recombination in exemplary crystalline silicon heterojunction and polycrystalline Cu(In,Ga)Se2 solar cells.

Direct current sputtered aluminum-doped zinc oxide films for thin crystalline silicon heterojunction solar cell

The high-power high-throughput direct current (DC) magnetron sputtering of aluminum-doped zinc oxide (AZO) film was investigated for fabricating amorphous silicon/crystalline silicon heterojunction solar cells. The AZO films were deposited using 1.0 wt.% and 2.0 wt.% Al-doped ZnO targets. The functional properties of the deposited films were characterized by measuring the electrical conductivity, Hall mobility, free carrier density and optical transparency. The influence of the sputtering power, oxygen concentration, substrate temperature, post-annealing and film thickness on the film properties were studied systematically. The oxygen in the sputtering gas deteriorates the electrical and optical performance of AZO films, which can be improved by post annealing. An optimal value of 1.0 × 10−3 Ω cm for the resistivity and 90% for the optical transmission was obtained at processing temperatures below 250 °C. Two types of thin Si heterojunction solar cells were prepared and compared using DC sputtered AZO film as the front electrode. Results show that DC sputtered AZO films meet the requirements for use as transparent front electrode in thin Si heterojunction solar cells.

Atomic Structure of Interface States in Silicon Heterojunction Solar Cells

Combining orientation dependent electrically detected magnetic resonance and g tensor calculations based on density functional theory we assign microscopic structures to paramagnetic states involved in spin-dependent recombination at the interface of hydrogenated amorphous silicon crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. We find that (i)the interface exhibits microscopic roughness, (ii)the electronic structure of the interface defects is mainly determined by c-Si, (iii)we identify the microscopic origin of the conduction band tail state in the a-Si:H layer, and (iv)present a detailed recombination mechanism.

Numerical Analysis of Electrical TCO / a-Si:H(p) Contact Properties for Silicon Heterojunction Solar Cells

In this paper we present a one-dimensional numerical simulation study concerning the electrical properties of the TCO / a-Si:H(p) / a-Si:H(i) / c-Si(n) hole contact in amorphous / crystalline silicon heterojunction (SHJ) solar cells. Simulations where performed with Sentaurus TCAD from Synopsys considering the implemented barrier tunneling model. The modification of the a-Si:H(p) / a-Si:H(i) / c-Si(n) p/n junction by the opposing TCO / a-Si:H(p) junction and its influence on the device recombination for the relevant working conditions are investigated. We demonstrate the relevance of the effective screening length in the amorphous silicon, which depends on the effective space-charge in the a-Si:H(p) and the work function mismatch at the TCO / a-Si:H(p) interface. We highlight the importance of an improved work function matching as this presents an additional degree of freedom for the design of the SHJ and is an important parameter to reach both a very high Voc and FF.

In-line deposition of silicon-based films by hot-wire chemical vapor deposition

Silicon-based films such as hydrogenated amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and hydrogenated amorphous silicon nitride (a-SiNx:H) can be deposited by hot-wire chemical vapor deposition (HW-CVD). The HW-CVD technology differs from conventional plasma-enhanced (PE)-CVD in a number of technological aspects, such as soft activation, high growth rates, and low system costs. To evaluate the HW-CVD technology for thin film deposition in solar industry an in-line hot-wire CVD system was used to deposit a-Si:H films for passivation of crystalline silicon solar cells as well as for the fabrication of thin film silicon solar cells. The HW-CVD system consists of seven vacuum chambers including three hot-wire systems with maximum deposition areas of 500mm by 600mm for each hot-wire activation source. The deposition processes were investigated by applying design of experiment to identify the effects and interactions of the process parameters on the deposition characteristics and film properties. The process parameters investigated were silane flow, deposition pressure, substrate temperature, film thickness, as well as temperature, diameter and number of wires, respectively. Growth rates up to 2.5nm/s were achieved for a-Si:H films. Intrinsic a-Si:H films for passivation of different crystalline solar cell types yielded carrier lifetimes of more than 1000μs for film thickness values below 20nm. For n-doped a-Si:H films prepared with PH3 as dopant gas, electrical resistivity is in the range of 102Ωcm. P-doped a-Si:H films prepared with B2H6 as dopant gas show electrical resistivity of about 105Ωcm. Crystalline silicon heterojunction solar cells with intrinsic thin layer (HIT cells) exhibit energy conversion efficiencies of more than 17% when fabricated with intrinsic HW-CVD amorphous silicon films as passivation layers.