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.     

Effect of emitter layer doping concentration on the performance of a silicon thin film heterojunction solar cell

A novel type of n/i/i/p heterojunction solar cell with a-Si:H(15nm)/a-Si:H(10nm)/ epitaxial c-Si(47μm)/epitaxial c-Si(3μm) structure is fabricated by using the layer transfer technique, and the emitter layer is deposited by hot wire chemical vapour deposition. The effect of the doping concentration of the emitter layer S d (Sd =PH3 /(PH3 +SiH4 +H2 )) on the performance of the solar cell is studied by means of current density-voltage and external quantum efficiency. The results show that the conversion efficiency of the solar cell first increases to a maximum value and then decreases with S d increasing from 0.1% to 0.4%. The best performance of the solar cell is obtained at S d = 0.2% with an open circuit voltage of 534 mV, a short circuit current density of 23.35mA/cm2 , a fill factor of 63.3%, and a conversion efficiency of 7.9%.     

β-FeSi2 as the bottom absorber of triple-junction thin-film solar cells: A numerical study

Using β-FeSi2 as the bottom absorber of triple-junction thin-film solar cells is investigated by a numerical method for widening the long-wave spectral response. The presented results show that the β-FeSi2 subcell can contribute 0.273 V of open-circuit voltage to the a-Si/μc-Si/β-FeSi2 triple-junction thin-film solar cell. The optimized absorber thicknesses for a- Si, μ-Si, and/3-FeSi2 subcells are 260 nm, 900 nm, and 40 nm, respectively. In addition, the temperature coefficient of the conversion efficiency of the a-Si/μc-Si//3-FeSi2 cell is -0.308 %/K, whose absolute value is only greater than that of the a-Si subcell. This result indicates that the a-Si/μc-Si/β-FeSi2 triple-junction solar cell has a good temperature coefficient. As a result, using β-FeSi2 as the bottom absorber can improve the thin-film solar cell performance, and the a-Si/μc-Si/β-FeSi2 triple-junction solar cell is a promising structure configuration for improving the solar cell efficiency.     

Influence of p-layer on the performance of n-i-p μc-Si:H thin film solar cells

The high pressure radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) process was adopted to prepare the n-i-p microcrystalline silicon solar cells,the influence of p-type layers on the performance of the solar cells was investigated,and the optimum p layer suited to the n-i-p microcrystalline silicon solar cells was obtained.The experimental results demonstrate that the performance of the solar cells can be highly affected by the structural and optical properties of the p-layers,and the performance of solar cells can be greatly improved by optimizing p layers.We have achieved an initial active-area efficiency of 8.17% (V oc =0.49 V,J sc =24.9 mA/cm 2 ,FF=67%) for the μc-Si:H single-junction n-i-p solar cells and an initial active-area efficiency of 10.93% (V oc =1.31 V,J sc =13.09 mA/cm 2 ,FF=64%) for the a-Si:H/μc-Si:H tandem n-i-p solar cells.     

Microstructure evolution and passivation quality of hydrogenated amorphous silicon oxide(a-SiOx:H) on〈100〉- and 〈111〉-orientated c-Si wafers

Hydrogenated amorphous silicon oxide(a-SiOx:H) is an attractive passivation material to suppress epitaxial growth and reduce the parasitic absorption loss in silicon heterojunction(SHJ) solar cells. In this paper, a-SiOx:H layers on different orientated c-Si substrates are fabricated. An optimal effective lifetime(τ(eff)) of 4743 μs and corresponding implied opencircuit voltage(iV(oc)) of 724 mV are obtained on〈100〉-orientated c-Si wafers. While τ(eff) of 2429 μs and iV_oc of 699 mV are achieved on 111-orientated substrate. The FTIR and XPS results indicate that the a-SiOx:H network consists of SiOx(Si-rich), Si–OH, Si–O–SiHx, SiO2 ≡ Si–Si, and O3 ≡ Si–Si. A passivation evolution mechanism is proposed to explain the different passivation results on different c-Si wafers. By modulating the a-SiOx:H layer, the planar silicon heterojunction solar cell can achieve an efficiency of 18.15%.     

Fabrication of c-Si：H（p）/c-Si（n） Heterojunction Solar Cells with Microcrystalline Emitters

The p-type microcrystalline silicon （μc-Si：H） on n-type crystalline silicon （c-Si） heterojunction solar cells is fabricated by radio-frequency plasma enhanced chemical vapour deposition （rf-PECVD）. The effect of the μc- Si：Hp-layers on the performance of the heterojunction solar cells is investigated. Optimum μc-Si：H p-layer is obtained with hydrogen dil u tion ratio of 99.65 %, rf-power of 0. 08 W/cm^2 , gas phase doping ratio of 0. 125 %, and the p-layer thickness of 15nm. We fabricate μc-Si：H（p）/c-Si（n） heterojunction solar cells without texturing and obtained an efficiency of 13.4%. The comparisons of the solar-cell performances using different surface passivation techniques are discussed.     

Indium–tin oxide films obtained by DC magnetron sputtering for improved Si heterojunction solar cell applications

The indium–tin oxide(ITO) film as the antireflection layer and front electrodes is of key importance to obtaining high efficiency Si heterojunction(HJ) solar cells. To obtain high transmittance and low resistivity ITO films by direct-current(DC) magnetron sputtering, we studied the impacts of the ITO film deposition conditions, such as the oxygen flow rate,pressure, and sputter power, on the electrical and optical properties of the ITO films. ITO films of resistivity of 4×10-4?·m and average transmittance of 89% in the wavelength range of 380–780 nm were obtained under the optimized conditions:oxygen flow rate of 0.1 sccm, pressure of 0.8 Pa, and sputtering power of 110 W. These ITO films were used to fabricate the single-side HJ solar cell without an intrinsic a-Si:H layer. However, the best HJ solar cell was fabricated with a lower sputtering power of 95 W, which had an efficiency of 11.47%, an open circuit voltage(V oc) of 0.626 V, a filling factor(FF) of 0.50, and a short circuit current density(J sc) of 36.4 m A/cm2. The decrease in the performance of the solar cell fabricated with high sputtering power of 110 W is attributed to the ion bombardment to the emitter. The V oc was improved to 0.673 V when a 5 nm thick intrinsic a-Si:H layer was inserted between the(p) a-Si:H and(n) c-Si layer. The higher V oc of 0.673 V for the single-side HJ solar cell implies the excellent c-Si surface passivation by a-Si:H.     

Improved performance of microcrystalline silicon solar cell with graded-band-gap silicon oxide buffer layer

Microcrystalline silicon(μc-Si:H) solar cell with graded band gap microcrystalline silicon oxide(μc-Si Ox:H) buffer layer is prepared by plasma enhanced chemical vapor deposition and exhibits improved performance compared with the cell without it. The buffer layer moderates the band gap mismatch by reducing the barrier of the p/i interface, which promotes the nucleation of the i-layer and effectively eliminates the incubation layer, and then enhances the collection efficiency of the cell in the short wavelength region of the spectrum. The p/i interface defect density also decreases from 2.2 × 1012cm-2to 5.0 × 1011cm-2. This graded buffer layer allows to simplify the deposition process for the μc-Si:H solar cell application.     

Significant Improvement of Passivation Performance by Two-Step Preparation of Amorphous Silicon Passivation Layers in Silicon Heterojunction Solar Cells

The key feature of amorphous/crystalline silicon heterojunction solar cells is extremely low surface recombination,which is related to superior passivation on the crystalline silicon wafer surface using thin hydrogenated amorphous silicon(a-Si:H)layers,leading to a high open-circuit voltage.In this work,a two-step method of a-Si:H passivation is introduced,showing excellent interface passivation quality,and the highest effective minority carrier lifetime exceeds 4500 μs.By applying a buffer layer deposited through pure silane plasma,the risk of film epitaxial growth and plasma damage caused by hydrogen diluted silane plasma is effectively reduced.Based on this,excellent passivation is realized through the following hydrogen diluted silane plasma process with the application of high density hydrogen.In this process,hydrogen diffuses to a-Si/c-Si interface,saturating residual dangling bonds which are not passivated by the buffer layer.Employing this two-step method,a heterojunction solar cell with an area of 239 cm~2 is prepared,yielding to open-circuit voltage up to 735 mV and total-area efficiency up to 22.4%.     

非晶Si1-xGex:H薄膜太阳能电池研究

The AMPS-ID program is used to investigate electrical and optical properties of the thin film solar cell of a-SiC:H/a-Si1-xGex:H/a-Si:H. The short circuit current density, open circuit voltage, fill factor and efficiency of the solar cell are investigated. The efficiency of the solar cell is 9.19% as thickness of a-Si1-xGex:H is 340 nm with Ge content x=0.1. In addition, we also discuss the factors which affect solar cell efficiency.     

MoO3/Ag/AI/ZnO intermediate layer for inverted tandem polymer solar cells

We report an MoO3/Ag/Al/ZnO intermediate layer connecting two identical bulk heterojunction subcells with a poly（3-hexylthiophene） and [6,6]-phenyl-C61-butyric acid methyl ester （P3HT and PCBM） active layer for inverted tan- dem polymer solar cells. The highly transparent intermediate layer with an optimized thickness realizes an Ohmic contact between the two subcells for effective charge extraction and recombination. A maximum power conversion efficiency of 3.76% is obtained for the tandem cell under 100 mW/cm2 illumination, which is larger than that of a single cell （3.15%）. The open-circuit voltage of the tandem cell （1.18 V） approaches double that of the single cell （0.61 V）.     

Research on the boron contamination at the p/i interface of microcrystalline silicon solar cells deposited in a single PECVD chamber

This paper studies boron contamination at the interface between the p and i layers of μ c-Si:H solar cells deposited in a single-chamber PECVD system. The boron depth profile in the i layer was measured by Secondary Ion Mass Spectroscopy. It is found that the mixed-phase μ c-Si:H materials with 40% crystalline volume fraction is easy to be affected by the residual boron in the reactor. The experimental results showed that a 500-nm thick μ c-Si:H covering layer or a 30-seconds of hydrogen plasma treatment can effectively reduce the boron contamination at the p/i interface. However, from viewpoint of cost reduction, the hydrogen plasma treatment is desirable for solar cell manufacture because the substrate is not moved during the hydrogen plasma treatment.     

Numerical simulation of a triple-junction thin-film solar cell based on μc-Si_(1-x)Ge_x :H

In this paper, a-Si:H/a-SiGe:H/μc-SiGe:H triple-junction solar cell structure is proposed. By the analyses of microelectronic and photonic structures (AMPS-1D) and our TRJ-F/TRJ-M/TRJ-B tunneling-recombination junction (TRJ) model, the most preferably combined bandgap for this structure is found to be 1.85 eV/1.50 eV/1.0 eV. Using more realistic material properties, optimized thickness combination is investigated. Along this direction, a-Si:H/a-SiGe:H/μc-SiGe:H triple cell with an initial efficiency of 12.09% (Voc = 2.03 V, FF = 0.69, Jsc = 8.63 mA/cm2 , area = 1 cm2 ) is achieved in our laboratory.     

GaInP/GaAs tandem solar cells with highly Te- and Mg-doped GaAs tunnel junctions grown by MBE

We report a GaInP/GaAs tandem solar cell with a novel GaAs tunnel junction(TJ) with using tellurium(Te) and magnesium(Mg) as n- and p-type dopants via dual-filament low temperature effusion cells grown by molecular beam epitaxy(MBE) at low temperature. The test Te/Mg-doped GaAs TJ shows a peak current density of 21 A/cm2. The tandem solar cell by the Te/Mg TJ shows a short-circuit current density of 12 m A/cm2, but a low open-circuit voltage range of1.4 V~1.71 V under AM1.5 illumination. The secondary ion mass spectroscopy(SIMS) analysis reveals that the Te doping is unexpectedly high and its doping profile extends to the Mg doping region, thus possibly resulting in a less abrupt junction with no tunneling carriers effectively. Furthermore, the tunneling interface shifts from the intended Ga As n++/p++junction to the AlGaInP/GaAs junction with a higher bandgap AlGaInP tunneling layers, thereby reducing the tunneling peak. The Te concentration of ~ 2.5 × 1020 in GaAs could cause a lattice strain of 10-3 in magnitude and thus a surface roughening,which also negatively influences the subsequent growth of the top subcell and the GaAs contacting layers. The doping features of Te and Mg are discussed to understand the photovoltaic response of the studied tandem cell.     

Emitter of hetero-junction solar cells created using pulsed rapid thermal annealing

In this paper, we use a pulsed rapid thermal processing (RTP) approach to create an emitter layer of hetero-junction solar cell. The process parameters and crystallization behaviour are studied. The structural, optical and electric properties of the crystallized films are also investigated. Both the depth of PN junction and the conductivity of the emitter layer increase with the number of RTP pulses increasing. Simulation results show that efficiencies of such solar cells can exceed 15% with a lower interface recombination rate, but the highest efficiency is 11.65% in our experiments.     

Amorphous/Crystalline （n-n） Si Heterojunction Photodetector Made by Q-Switched 0.532-mm Laser Pulses with Novel Technique

Amorphous/crystalline n-n-isotype Si heterojunetions are made by a pulsed Q-switched second harmonic generation Nd：YAG laser. The process includes melting and subsequently fast resolidification of a thin front layer of monocrystalline Si by laser pulses to create an amorphous layer （phase transition）. Different laser energy densities are used to form the amorphous layer on a monocrystalline Si substrate, the results of the electrical characteristics of the heterojunctions are dependent strongly on the laser energy density. Optoelectronic properties such as current-voltage, capacitance voltage, and spectral sensitivity are measured in a-Si/c-Si hereto junctions （in the absence of anti-reflecting coating and frontal grid contact） prepared by different laser energy densities. The built-in-potential values extracted from current-voltage measurements are close to the published results of （n-p） amorphous/crystalline hereto junction made by glow discharge and plasma enhanced chemical vapour deposition. Furthermore, examination of the formation of amorphous pattern on Si surface is carried out with the help of optical microscopy. Best photovoltaic performance is recognized to be at ,5.6 J/cm^2. The photodetector shows a wide spectral response, and the peak response is at 780nm. On the other hand, this peak is independent of laser energy.     

A novel wide-dynamic-range logarithmic-response bipolar junction photogate transistor for CMOS imagers

In this paper, a new photodetector, bipolar junction photogate transistor (BJPG), is proposed for CMOS imagers. Due to an injection p+n junction introduced, the photo-charges drift through the p+n junction by the applied electronic field, and on the other hand, the p+n junction injects the carriers into the channel to carry the photo-charges. Therefore this device can increase the readout rate of the pixel signal charges and the photoelectron transferring efficiency. Using this new device, a new type of logarithmic pixel circuit is obtained with a wide dynamic range which makes photo-detector more suitable for imaging the naturally illuminated scenes. The simulations show that the photo current density of BJPG increases logarithmically with the incident light power due to the introduced injection p+n junction. The noise characteristics of BJPG are analyzed in detail and a new gate-induced noise is proposed. Based on the established numerical analytical model of noise, the power spectrum density curves a     

Insight into band alignment of Zn(O,S)/CZTSe solar cell by simulation

Cd-free kesterite structured solar cells are currently attracting attention because they are environmentally friendly. It is reported that Zn(O,S) can be used as a buffer layer in these solar cells. However, the band alignment is not clear and the carrier concentration of Zn(O,S) layer is low. In this study, the band alignment of the Zn(O,S)/Cu_2 ZnSnSe_4 p–n junction solar cell and the effect of In_2 S_3/Zn(O,S) double buffer layer are studied by numerically simulation with wxAMPS software.By optimizing the band gap structure between Zn(O,S) buffer layer and Cu_2 ZnSnSe_4 absorber layer and enhancing the carrier concentration of Zn(O,S) layer, the device efficiency can be improved greatly. The value of CBO is in a range of 0 eV–0.4 eV for S/(S + O)= 0.6–0.8 in Zn(O,S). The In_2 S_3 is mainly used to increase the carrier concentration when it is used as a buffer layer together with Zn(O,S).     

Electronic structure of Yb/H－Si (111) interface

Photoemission spectra are measured for Yb covered surface of wet-chemically-etched H－Si (111). The results reveal that the lattice structure of the H－Si (111) surface is stable against the deposition of Yb atoms. X-ray photoemission spectra indicate the formation of a polarized (dipole) surface layer, with the silicon negatively charged. Ultraviolet photoemission spectra exhibit the semiconducting property of the interface below one monolayer coverage. Work function variation during the formation of the Yb/H－Si (111) interface is measured by the secondary-electron cutoff in the ultraviolet photoemission spectral line. The largest decrease of work function is ～1.65eV. The contributions of the dipole surface layer and the band bending to the work function change are determined to be ～1.15eV and ～0.5eV, respectively. The work function of metal Yb is determined to be ～2.80±0.05eV.     

Electrical Property of Infrared-Sensitive InAs Solar Cells

InAs infrared-sensitive solar cells are fabricated by using the films grown by the liquid phase epitaxy technique. The film microstructures are characterized by x-ray diffraction and scanning electronic microscopy. The current-voltage characteristics of the solar cells in the dark and under AM1.5 illumination at 300 K and 77 K are discussed. The conversion efficiency of p-InAs/n-sub InAs cells decreases when the thickness of the p-type film changes from 1.7 μm to 3.5 μm, which is caused by the reduced effective photons near p？n junction. The p-InAs/n-InAs/n-sub InAs solar cell with the conversion efficiency of 7.43% in 1-2.5 μm under AM1.5 at 77 K is obtained. The short circuit current density increases dramatically with decreasing temperature due to the weakened effect of phonon scattering.