High-efficiency Heterojunction Solar Cells Research Articles

Fabrication of Thermally Evaporated CuIx Thin Films and Their Characteristics for Solar Cell Applications

Carrier-selective contacts (CSCs) for high-efficiency heterojunction solar cells have been widely studied due to their advantages of processing at relatively low temperatures and simple fabrication processes. Transition metal oxide (TMO) (e.g., molybdenum oxide, vanadium oxide, and tungsten oxide) thin films are widely used as hole-selective contacts (HSCs, required work function for Si solar cells > 5.0 eV). However, when TMO thin films are used, difficulties are faced in uniform deposition. In this study, we fabricated a copper (I) iodide (CuI) thin film (work function > 5.0 eV) that remained relatively stable during atmospheric exposure compared with TMO thin films and employed it as an HSC layer in an n-type Si solar cell. To facilitate efficient hole collection, we conducted iodine annealing at temperatures of 100–180 °C to enhance the film’s electrical characteristics (carrier density and carrier mobility). Subsequently, we fabricated CSC Si solar cells using the annealed CuIx layer, which achieved an efficiency of 6.42%.

A Comparative Study on p- and n-Type Silicon Heterojunction Solar Cells by AFORS-HET.

Despite the increasing trend of n-type silicon wafer utilization in the manufacturing of high-efficiency heterojunction solar cells due to the superior advantages over p-type counterparts, its high manufacturing cost remains to be one of the most crucial factors, which impedes its market share growth with state-of-the-art silicon heterojunction (SHJ) solar cells demonstrating high conversion efficiencies from various configurations, the prospect of using an n-type wafer is debatable from a cost-efficiency point of view. Hence, a systematic comparison between p- and n-type SHJ solar cells was executed in this work using AFORS-HET numerical software. Front and rear-emitter architectures were selected for each type of wafer with ideal (without defects) and non-ideal (with defects) conditions. For ideal conditions, solar cells with p-type wafers and a front-emitter structure resulted in a maximum conversion efficiency of 28%, while n-type wafers demonstrated a maximum efficiency of 26% from the rear-emitter structure. These high-performance devices were possible due to the optimization of the bandgap and electron-affinity for all passivating and doping layers with values ranging from 1.3 to 1.7 eV and 3.9 to 4 eV, respectively. The correlation between the device structure and the type of wafers as demonstrated here will be helpful for the development of both types of solar cells with comparable performance.

Enhanced efficiency in hollow core electrospun nanofiber-based organic solar cells

Over the last decade, nanotechnology and nanomaterials have attracted enormous interest due to the rising number of their applications in solar cells. A fascinating strategy to increase the efficiency of organic solar cells is the use of tailor-designed buffer layers to improve the charge transport process. High-efficiency bulk heterojunction (BHJ) solar cells have been obtained by introducing hollow core polyaniline (PANI) nanofibers as a buffer layer. An improved power conversion efficiency in polymer solar cells (PSCs) was demonstrated through the incorporation of electrospun hollow core PANI nanofibers positioned between the active layer and the electrode. PANI hollow nanofibers improved buffer layer structural properties, enhanced optical absorption, and induced a more balanced charge transfer process. Solar cell photovoltaic parameters also showed higher open-circuit voltage (+ 40.3%) and higher power conversion efficiency (+ 48.5%) than conventional architecture BHJ solar cells. Furthermore, the photovoltaic cell developed achieved the highest reported efficiency value ever reached for an electrospun fiber-based solar cell (PCE = 6.85%). Our results indicated that PANI hollow core nanostructures may be considered an effective material for high-performance PSCs and potentially applicable to other fields, such as fuel cells and sensors.

Enhancing the photovoltaic performance of Cd-free Cu2ZnSnS4 heterojunction solar cells using SnS HTL and TiO2 ETL

Cu2ZnSnS4 chalcogenide semiconductor is a very promising absorber material for solar cells and no longer fulfilled its realistic goals due to the possible challenge of defect-free fabrication, non-optimized buffer layer alignment and device configuration. In this article, we proposed a new planner structure defined as (SnS/CZTS/TiO2/ITO) using Ni and Al as a back and front contact materials, respectively, and simulated its photovoltaic (PV) performance using SCAPS-1D. In the simulation, defect densities were added to each layer and the interface between the buffer layer and the absorber layer. This work demonstrated the promising role of SnS HTL and TiO2 ETL in reducing the Voc deficit of CZTS-based solar cells by forming more favourable band alignment with the absorber layer, reducing non-radiative recombination at the interfaces. In response to changes in the material properties of different layers (such as thickness, carrier concentration, defect density), working temperature, and back contact metal work function, the PV performance of the new architecture for CZTS solar cells was investigated and optimized. The study of PV performance optimization revealed that a relatively thicker absorber layer with low carrier concentration showed better PV performances, while ETL and HTL with a thinner and higher carrier concentration are required to enhance PV performances. Power conversion efficiency of >30%, Voc of >1.09 V, Jsc of >32 mA/cm2 and FF of >87% was predicted for CZTS solar cells with the new architecture. The findings of this study suggest that SnS and TiO2 are expected to become HTL and ETL, respectively, for fabricating low cost, high efficiency, and Cd-free CZTS-based heterojunction solar cell.

Complex Investigation of High Efficiency and Reliable Heterojunction Solar Cell Based on an Improved Cu2O Absorber Layer

This study is aimed at increasing the performance and reliability of silicon-based heterojunction solar cells with advanced methods. This is achieved by a numerical electro-optical modeling and reliability analysis for such solar cells correlated with experimental analysis of the Cu2O absorber layer. It yields the optimization of a silicon tandem heterojunction solar cell based on a ZnO/Cu2O subcell and a c-Si bottom subcell using electro-optical numerical modeling. The buffer layer affinity and mobility together with a low conduction band offset for the heterojunction are discussed, as well as spectral properties of the device model. Experimental research of N-doped Cu2O thin films was dedicated to two main activities: (1) fabrication of specific samples by DC magnetron sputtering and (2) detailed characterization of the analyzed samples. This last investigation was based on advanced techniques: morphological (scanning electron microscopy—SEM and atomic force microscopy—AFM), structural (X-ray diffraction—XRD), and optical (spectroscopic ellipsometry—SE and Fourier-transform infrared spectroscopy—FTIR). This approach qualified the heterojunction solar cell based on cuprous oxide with nitrogen as an attractive candidate for high-performance solar devices. A reliability analysis based on Weibull statistical distribution establishes the degradation degree and failure rate of the studied solar cells under stress and under standard conditions.

Passivation optimization and performance improvement of planar a-Si:H/c-Si heterojunction cells in perovskite/silicon tandem solar cells

Recently, the monolithic spin-coating perovskite/planar silicon heterojunction tandem solar cells with high performance have attracted attention mainly due to simple fabrication, low preparation cost and high efficiency, especially compared with fully textured multi-junction perovskite/silicon tandem device. As is well known, the excellent passivation of a-Si:H/c-Si interface is the key to achieving a high-efficiency planar silicon heterojunction solar cell, which further improves the performance of the corresponding tandem cell. Therefore, we investigate the elements affecting a-Si:H/c-Si interface passivation, including the c-Si surface treatment technique, a-Si:H passivation layer and P-type emitter layer and so on. In these experiments, we adjust the immersed time of diluent hydrofluoric acid and pre-deposited hydrogen plasma with different gas mixture flows. Also, the suitable deposition parameters of intrinsic a-Si:H passivation layer are regulated by varying hydrogen dilution and time, and variously slight silane content is embedded into i-a-Si:H /P-type (I/P) emitter interface by hydrogen-rich plasma treating which is for acquiring optimal experimental processing conditions to promote the chemical passivation. In addition, the p-a-Si:H and p-nc-Si:H are comparatively studied as buffer layers to further improve the I/P interface passivation by varying the hydrogen dilution in the gas mixture during deposition. It can be found that p-nc-Si:H buffer layer with high conductivity and wide bandgap can not only reduce the defect density at the I/P interface, but also increase the conductivity of P-type emitter, which further improves the field passivation effect. By the above- mentioned optimization, the highest minority carrier lifetime and implied open-circuit voltage (<i>iV</i><sub>oc</sub>) of the structure of P-type emitter/a-Si:H(i)/c-Si/a-Si:H(i)/N-type layer (inip) sample can respectively reach 2855 μs and 709 mV, which demonstrates authentically outstanding passivation performance. An efficiency of 18.76% can be obtained for the planar a-Si/c-Si heterojunction solar cell with a <i>V</i><sub>oc</sub> of 681.5 mV, which is 34.3 mV higher than that of the reference device. Regarding the optimized planar a-Si:H/c-Si heterojunction solar cell as the bottom cell, we also obtain an efficiency of 21.24% for perovskite/silicon heterojunction tandem solar cell with an open-circuit voltage of 1780 mV, which proves that the above strategies are very effective for improving the passivation optimization and performance of bottom cell in the tandem device.

Controlling the c-Si/a-Si:H interface in silicon heterojunction solar cells fabricated by HWCVD

This paper deals with the engineering of the hetero-interface between intrinsic amorphous silicon (i-a-Si:H) layer and the n-type crystalline silicon (c-Si) wafer during the fabrication of the Silicon Heterojunction (SHJ) solar cell by the Hot Wire Chemical Vapor Deposition technique. It is known that this interface and the associated surface passivation of the c-Si is key to obtaining high efficiency heterojunction solar cells. The monitoring of this interface was carried out using high-resolution transmission electron microscopy (HRTEM). The HRTEM data of the c-Si/a-Si:H interface reveals a drastic dependence on the filament temperature (Tf) used during the deposition of the i-a-Si:H layer. Detailed analysis of the solar cell characteristics indicates that the cells where one has an abrupt crystalline/amorphous interface shows higher conversion efficiency compared to those where we have a rough and a defective interface or where there are indications of local epitaxy in the a-Si:H layer. The second parameter which was engineered is the bulk defect density of the intrinsic a-Si:H layer. Though the thickness of i-a-Si:H layer in case of SHJ solar cells is only around 5nm and serves the purpose of passivating the dangling bonds on the c-Si wafer, the bulk defect density of this layer cannot be ignored. We have achieved a-Si:H films with acceptable bulk defect density without dilution of the silane gas with hydrogen. The bulk defect density of the i-a-Si:H layer has been determined by the constant photocurrent method (CPM) and is correlated to the performance of SHJ solar cells. A direct consequence of these control parameters was observed in the improvement of the external quantum efficiency near 600nm wavelength region.

Improvement of parameters in a-Si(p)/c-Si(n)/a-Si(n) solar cells

We analyzed and discussed the influence of thickness and doping concentration of the different layers in a-Si(p)/c-Si(n)/a-Si(n) photovoltaic (PV) cells with the aim of increasing its efficiency while decreasing its global cost. Compared to the efficiency of a standard marketed PV cell, elaborated with a ZnO transparent conductive oxide (TCO) layer but without Back Surface Field (BSF) layer, an optimization of the thickness and dopant concentration of both the emitter a-Si(p) and absorber c-Si(n) layers will gain about 3% in the global efficiency of the cell. The results also reveal that with introduction of the third layer, i.e. the BSF layer, the efficiency always achieves values above 20% and all other parameters of the cell, such as the open-circuit voltage, the short-circuit current and the fill-factor, are strongly affected by the thickness and dopant concentration of the layers. The values of all parameters are given and discussed in the paper. Thereby, the simulation results give for an optimized a-Si(p)/c-Si(n)/a-Si(n) PV cells the possibility to decrease the thickness of the absorber layer down to 50 μm which is lower than in the state-of-the-art. This structure of the cell achieves suitable properties for high efficiency, cost-effectiveness and reliable heterojunction (HJ) solar cell applications.

Fabrication and Photovoltaic Effect of CdS/Silicon Nanopillars Heterojunction Solar Cell

AbstractA 200 nm thickness layer of Cadmium sulfide (CdS) film covers on the silicon nanopillars surface to heterojunction solar cell. The nanopillars with the large surface ratio can increase the heterojunction surface and the light absorption, which can improve the photovoltaic performance of solar cell. Silicon nanopillars are fabricated by Cesium Chloride self‐assembly lithography and inductively coupled plasma dry etching as substrate. CdS film covers the silicon nanopillars surface by the RF magnetron sputtering using CdS ceramic target. The X‐ray diffraction patterns of the nanopillar and planar substrates after CdS film covering indicate that all the deposited films are crystal. The nanopillars wafer has lower reflectance than the planar one both before and after the CdS film covering for the wavelength ranging from 350 to 1050 nm. With this method, the nanopillar based CdS/Silicon heterojunction solar cell has the better photovoltaic performance than the planar one. The Jsc increases from 7.12 to 13.92 mA/cm2 and the energy conversion efficiency increases from 1.32 % to 1.83 %. The large surface ratio nanopillars might be a potential substrate for high efficiency heterojunction solar cell.

Tungsten doped indium oxide film: Ready for bifacial copper metallization of silicon heterojunction solar cell

Tungsten doped indium oxide (IWO) film was deposited by reactive plasma deposition technology for a-Si:H/c-Si heterojunction (SHJ) solar cell. The average transmittance and absorption of IWO film from 350 to 1200nm wavelength was 88.33% and 2.16% respectively. The hall mobility is 77.8cm2/Vs, with a corresponding carrier concentration of 2.86E20cm−3 and resistivity of 2.80E−04Ωcm. Based on high-performance IWO film, the 5in. SHJ solar cell with efficiency value of 22.03% and power of 3.37W was obtained by electroplated copper metallization technology. Compared with screen printed SHJ solar cells, the fingers of electroplated cells show finer width, higher aspect ratio and lower series resistance, resulting in increased fill factor and higher efficiency. The long term stability test of copper electroplated cells reveal that IWO film has excellent stability at the solar cell modules׳ operating temperature and can prevent copper diffusion effectively, which makes copper metallization of high efficiency heterojunction solar cell possible.

Improverment of electrical and optical properties of doped amorphous silicon layers applied to heterojunction solar cells

The optimisation of electrical and optical properties of doped amorphous silicon layers (the emitter layer) is the key importance to obtain high efficiency heterojunction (HJ) solar cells. Desired properties for the emitter layer include wide bandgap, low surface and interface recombination, and good doping efficiency. In this study, we report the thinfilm properties of n-doped nc-Si:H emitter layers deposited using RF (13.56 MHz) PECVD, at different SiH4/H2 gas flow ratios, at the same RF power, pressures, and temperatures. Trends relating deposition conditions to relevant film characteristics such as thickness, wide bandgap, crystalline fraction and conductivity are discussed. Finally, the heterojunction solar cells using the optimised parameters for n-doped nc- Si:H layers are fabricated with high short circuit current (17 mA).

High efficiency heterojunction solar cells on n-type kerfless mono crystalline silicon wafers by epitaxial growth

We present a heterojunction (HJ) solar cell on n-type epitaxially grown kerfless crystalline-silicon (c-Si) with a conversion efficiency of 22.5%. The total cell area is 243.4 cm2. The cell has a short-circuit current density of 38.6 mA/cm2, an open-circuit voltage of 735 mV, and a fill factor of 0.791. The key advantages and technological tasks of epitaxial wafers for HJ solar cells are discussed, in comparison with conventional n-type Czockralski c-Si wafers. The combination of HJ and kerfless technology can lead to high conversion efficiency with a potential at low cost.

Ion doping of graphene for high-efficiency heterojunction solar cells

We demonstrated the p-type chemical doping by chlorine and nitrate anions to enhance the Schottky junction in the solar cell. Nitrate ions were found to be more effective for reducing the sheet resistance and enlarging the work function of graphene for effective charge separation and transport, and the efficiency was increased to 9.2% by a factor of 1.68 under AM 1.5 illumination.

IPA-free Texturization of n-type Si Wafers: Correlation of Optical, Electronic and Morphological Surface Properties

The application of an Isopropanol(IPA)-free potassium hydroxide (KOH) solution was evaluated in order to prepare random pyramids on as-cut crystalline n-type Si wafers to reduce reflection losses of substrates for high-efficiency hetero-junction solar cells. The influence of saw damage removal and texturization processes on the resulting pyramid morphology and the corresponding interplay between optical and electronic properties are revealed. It is shown that both the depth of the saw damage etching (SDE) and the duration of the texturization etching have crucial influence on the resulting pyramid size distribution. Reflection losses can be reduced with decreasing fraction of small pyramids. By intermediate saw damage removal and texture etching times in (IPA)-free KOH solution the densities of electronic interface states were found to be strongly decreased (Dit,min < 5 10-11cm-2eV-1), in comparison to pyramids prepared in IPA containing solutions. For the purpose of fabricating amorphous/crystalline (a-Si:H/c-Si) heterojunction solar cells the Si substrate surfaces were passivated with an intrinsic layer of amorphous silicon (a- Si:H(i)) leading to minority charge carrier lifetimes τeff of 2 to 4ms, depending on the preceding texturization process.

Deposition and Characterization of Low-cost Spray Pyrolyzed Cu2ZnSnS4 Thin-films for Large-area high-efficiency Heterojunction Solar Cells

not Available.

Impact of a-Si:H hydrogen depth profiles on passivation properties in a-Si:H/c-Si heterojunctions

We explore the (near-)interface structure of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions as employed in high-efficiency heterojunction solar cells. We make use of secondary-ion-mass-spectroscopy profiles and minority carrier lifetime measurements taken on undoped deuterated amorphous silicon [(i)a-Si:D] layers deposited on c-Si from deuterated silane at identical conditions as the hydrogenated layers we have analyzed previously [T. F. Schulze et al., Appl. Phys. Lett. 96 (2010) 252102]. We briefly discuss the implications of the local interface structure for the c-Si surface passivation as well as for the heterojunction band offsets, and identify a route towards optimization of (i)a-Si:H layers as passivating buffers in a-Si:H/c-Si high-efficiency heterojunction solar cells.

Simulation of High Efficiency Heterojunction Solar Cells with AFORS-HET

In this paper, the high efficiency TCO/a-Si:H (n)/a-Si:H(i)/c-Si(p)/uc-Si(p+)/Al HIT (heterojunction with intrinsic thin-layer) solar cells was analyzed and designed by AFORS-HET software. The influences of emitter, intrinsic layer and back surface field (BSF) on the photovoltaic characteristics of solar cell were discussed. The simulation results show that the key role of the intrinsic layer inserted between the a-Si:H and crystalline silicon substrate is to decrease the interface states density. If the interface states density is lower than 1010cm−2V−1 thinner intrinsic layer is better than thicker one. The increase of the thickness of the emitter will decrease the short-current density and affect the conversion efficiency. Microcrystalline BSF can increase conversion efficiency more than 2 percentage points compared with HIT solar cell with no BSF. But this BSF requires the doping concentration must exceed 1020cm−3. Considered the band mismatch between crystalline silicon and microcrystalline silicon, the optimal band gap of microcrystalline silicon BSF is about 1.4–1.6eV.

Optimisation of doped amorphous silicon layers applied to heterojunction solar cells

The optimisation of amorphous silicon layers (a-Si:H) is of key importance to obtain high efficiency heterojunction (HJ) solar cells. However, since many mechanisms take place in photovoltaic energy conversion, good electrical and optical properties of a-Si:H films do not always result in high efficiency HJ devices. This is principally due to the use of very thin layers were interfaces are of capital importance and bulk properties are not always the main guideline to best results on solar cells. In this work, we focus on the doping analysis of (n) and (p) a-Si:H layers directly and their impact on solar cell results. First, we have deposited and characterized simple (p) and (n) a-Si:H layers and then we have integrated them on full heterojunction solar cells. We have correlated the solar cells characteristics (Jsc, Voc and FF) with layer properties in order to understand the main mechanisms involved in the high performance of HJ devices. Finally, we have chosen the best layers to improve the efficiency of our heterojunction solar cells on (n) c-Si 125PSQ wafers up to 20% on an industrially-compatible process.

Toward a better physical understanding of a-Si:H/c-Si heterojunction solar cells

Despite energy conversion efficiencies exceeding 22%, current understanding of the physics behind heterojunction solar cells remains incomplete. The role of hydrogen and ion bombardment during the plasma deposition as well as the influence of an epitaxial layer remains a subject of debate. Our results suggest that hydrogen plays a key role in the fabrication of high efficiency heterojunction solar cells. We show that ion bombardment is not as detrimental as is often thought. Moreover we find that an epitaxial layer is not necessarily harmful to the cell’s Voc. We propose a criterion linking the layer’s epitaxy and the cells’ performance. To further investigate the role of the H2 plasma, we carry out in situ ellipsometry measurements on various kinds of c-Si wafers. The effects of this H2 plasma strongly depend on the resistivity of the c-Si wafer, suggesting that plasma conditions must be tuned to optimize cell efficiency according to the c-Si resistivity.

Ultra-high quality surface passivation of crystalline silicon wafers in large area parallel plate reactor at 40 MHz

We use a new in-house, large area and automated deposition system: the usable deposition area is 410 × 520 mm with RF-frequency of 40 MHz. We deposit intrinsic a-Si:H layer on flat p-type or n-type c-Si wafers after performing an HF dip. The overall recombination of these double-side passivated c-Si wafers is measured with an effective lifetime measurement set-up. We pay particular attention to the uniformity of the passivation obtained on the whole deposition area. We point out a major role of hydrogen dilution on quality of c-Si passivation. Excellent uniformity is obtained on the whole area with implied open-circuit voltages ( V oc) in a ± 1.5% range. We achieve excellent passivation with overall lifetimes approaching 7 ms (at Δ n ≈ 4.5·10 14 cm − 3 ) resulting in implied V oc of 708 mV on p-type c-Si; and lifetimes superior to 4.7 ms resulting in implied V oc of 726 mV on n-type c-Si ( S eff less than 2 cm/s for both). These results open the way to very high efficiency heterojunction solar cell fabrication in large area reactors.