Toward stabilized 10% efficiency of large-area (>5000 cm) a-Si/a-SiGe tandem solar cells using high-rate deposition

This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10⁶ for a-Si:H films has been maintained up to the deposition rate of 15 Å/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm² a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.     

The influence of operation temperature on the output properties of amorphous silicon-related solar cells

The influence of the operation temperature on the output properties of solar cells with hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon germanium (a-SiGe:H) photovoltaic layers was investigated. The output power after longtime operation of an a-Si:H single junction, an a-Si:H/a-Si:H tandem, and an a-Si:H/a-SiGe:H tandem solar cell was calculated based on the experimental results of two types of temperature dependence for both conversion efficiency and light-induced degradation. It was found that the a-Si:H/a-SiGe:H tandem solar cell maintained a higher output power than the others even after longtime operation during which a temperature range of 25°C to 80°C. These results confirm the advantages of the a-Si:H/a-SiGe:H tandem solar cell for practical use, especially in high-temperature regions.     

Development of high-efficiency a-Si solar cell submodule with a size of 30 cm × 40 cm

We have achieved a stabilized conversion efficiency of 8.9% in a single-junction a-Si solar cell and 10.6% in a double-junction a-Si/a-SiGe solar cell for a size of 1 cm², which are the world's highest values achieved so far for this size and structure. We have been investigating the improvement of stability in a-SiGe film with regard to the bottom cell i-layer, and the control of E_opt in a-SiGe film in order to confirm the tandem cell design. On the other hand, uniformity of ± 1% has been obtained in conversion efficiency for many small cells fabricated in a size of 30 cm × 40 cm, evaluated by using a-Si single-junction structure. As a result, we have achieved the stabilized high-effective area conversion efficiency of 8.64% in a 30 cm × 40 cm a-Si/a-Si tandem submodule. The combination of the above techniques and further optimization can be expected to achieve a stabilized conversion efficiency of more than 10% for a 30 cm × 40 cm double-junction a-Si/a-SiGe submodule.     

Progress on high performance multijunction amorphous hydrogenated silicon alloy based solar cells and modules

Multijunction solar cell modules based on a-Si:H and its alloys have been developed. Triple junction devices have been modeled to quantify parasitic optical losses. A laser patterning method for modules has been developed and 98% active/aperture areas demonstrated. An ad hoc module of light-induced stability is developed which suggests much improved stability is to be expected in the triple junction devices; this is verified by experimental data. Triple junction 939.6 cm² a-Si:H/a-Si:H/a-SiGe:H modules tested at SERI have demonstrated an aperture area efficiency of 9.27%.     

Optical modelling of a single-junction p-i-n type and tandem structure amorphous silicon solar cells with perfect current matching

Using the admittance analysis method, the optimal design of a single junction a-Si : H solar cell is suggested and its photovoltaic parameters are calculated. The technique is then extended to design a tandem structure of two cells stacked one on the top of the other and connected in series. The top cell is considered of a-Si : H and bottom of a-SiGe : H and the condition of current matching is applied to determine the tandem's optimal design. The efficiency of the single-junction cell with the optimal design is predicted to be 13.1% and that of the tandem cell with the perfect current matching is 20.8%. The results of our calculations are discussed in the light of the recent experimental results.     

A-Si:H buffer in a-SiGe:H solar cells

Profiled a-SiGe:H-buffer layers between the doped and the absorption layers of amorphous silicon germanium (a-SiGe:H) solar cells are routinely used to avoid bandgap discontinuities and high-defect densities at the p/i- and i/n interface. Here, we present a much simpler approach replacing the profiled a-SiGe:H-buffer layers at both interfaces by a-Si:H-buffer layers. It is demonstrated that for a-SiGe:H solar cells (thickness of the E_G=1.5 eV part is 54 nm) these structures yield similar open circuit voltage V_OC and fill factor (FF) compared to the bandgap profiled layer at the same short circuit current density j_SC. The influence of thickness, optical bandgap and position of the buffer layers on the solar cell performance is investigated.     

Microcrystalline silicon solar cells fabricated by VHF plasma CVD method

A series of systematic investigations on microcrystalline silicon (μc-Si:H) solar cells at high deposition rates has been studied. The effect of high deposition pressure and narrow cathode-substrate (CS) distance on the deposition rate and quality of microcrystalline silicon is discussed. The microcrystalline silicon solar cell is adopted as middle cell and bottom cell in a three-stacked junction solar cell. The characteristics of large area three-stacked junction solar cells, whose area is 801.6 cm² including grid electrode areas, are studied in various deposition rates from 1 to 3 nm/s of microcrystalline silicon. An initial efficiency of 13.1% is demonstrated in the three-stacked junction solar cell with microcrystalline silicon deposited at 3 nm/s.     

高效非晶硅叠层太阳电池的优化设计

研究了高效ａ－Ｓｉ／ａ－Ｓｉ／ａ－Ｓｉ－ＳｉＧｅ三结太阳电池的优化设计。电流匹配是影响二端子叠层太阳电池填充因子的关键因素，在内电极的ｐ／ｎ界面外附加载流子复合是由少数载流子浓度、界面态和ｐ／ｎ界面处材料的几何因素匹配决定的。利用适当的带隙匹配和ｉ层厚度匹配来实现ａ－Ｓｉ／ａ－Ｓｉ／ａ－ＳｉＧｅ三结太阳电池结构的最佳化，同时采用改善ｎ／ｉ界面特性的缓冲层技术，获得了Ｖｏｃ＝２．４８Ｖ，Ｊｓｃ＝６．     

Reduction of the phosphorous cross-contamination in n-i-p solar cells prepared in a single-chamber PECVD reactor

A new approach to reduce phosphorous contamination in the intrinsic layer during the deposition of amorphous silicon (a-Si:H) n-i-p solar cells prepared in single-chamber reactors is presented. This novel process consists of a hydrogen etching plasma performed after the n-layer deposition, which prevents a recycling of phosphorous from the reactor walls when exposed to a hydrogen-rich plasma during the subsequent i-layer deposition. The implemented process reduces the phosphorous cross-contamination in the i-layer, as corroborated by secondary ion mass spectroscopy measurements. Furthermore, the end of the etching process can be easily monitored by measuring the DC bias voltage at the powered electrode. By applying this process, we were able to improve the fill factor from 70% up to 75%, without degradation in the other parameters of the cell, neither in the initial nor in the stabilized state. Finally, by implementing this process in a-Si:H/a-Si:H tandem solar cells we obtained an initial efficiency of 10.3% (V_oc=1.76 V, FF=74.5%, J_sc=7.8 mA cm⁻²); light soaking test resulted in a stabilized efficiency of 8.5%.     

Advances in a-Si:H/c-Si heterojunction solar cell fabrication and characterization

Our progress in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction solar cell technology and current understanding of fundamental device physics are presented. In a-Si:H/c-Si cells, device performance is strongly dependent on the quality of the a-Si:H/c-Si heterojunction. Four topics are crucial to minimize recombination at the junction and thereby maximize cell efficiency: wet-chemical pre-treatment of the c-Si surface prior to a-Si:H deposition; optimum a-Si:H doping; thermal and plasma post-treatments of the a-Si:H/c-Si structure. By optimizing these aspects using specifically developed characterization methods, we were able to realize (n)a-Si:H/(p)c-Si and (p)a-Si:H/(n)c-Si cells with up to 18.5% and 19.8% efficiency, respectively.     

a-Si:H/a-Si:H stacked cell from VHF-deposition in a single chamber reactor with 9% stabilized efficiency

In the present paper we present results on a-Si:H/a-Si:H stacked cells deposited in a single-chamber reactor by the very high frequency-glow discharge (VHF-GD) deposition technique at 70 MHz. Hydrogen dilution of the i-layer yields more stable amorphous p-i-n solar cells, similar to what is observed for RF deposition. High dilution ratios of the i-layer are found to enhance contaminations. This is, for the single-chamber reactor, due to the etching effect of the highly reactive H₂-plasma. Additionally, oxygen incorporation into the i-layer is favored by the high hydrogen dilution. Different means to suppress the contaminations are employed and discussed. Regarding the stacked cell design, we show by experiment and simulation that it is important to carefully adjust the current mismatch between the component cells such as to obtain a slight top-cell-limited behavior after degradation. We present an a-Si:H/a-Si:H stacked cell with an initial efficiency of 9.8% showing only 8% relative degradation which results in a stabilized efficiency of 9%. The deposition rate of the employed H₂-diluted i-layer material is 4 Å/s. It is therefore demonstrated that it is possible to make highly efficient stacked cells showing good stability also in a single-chamber system and employing the VHF technique to obtain higher rates.     

Stability of a-Si:H solar cells deposited by Ar-treatment or by ECR techniques

Stability against light soaking was studied for amorphous silicon (a-Si:H) solar cells using three different i-layers; (a) device-quality a-Si:H (standard a-Si:H) with bandgap of 1.75 eV, (b) narrow bandgap (1.55 eV) a-Si:H fabricated by Ar* chemical annealing and (c) a-Si:H(Cl) fabricated from SiH₂Cl₂. Both the narrow bandgap a-Si:H and the a-Si:H(Cl) solar cells showed much improved stability than that of the standard a-Si:H solar cells: e.g., fill factor of the narrow bandgap a-Si:H cell only slightly decreased from 56% to 53%, while that of the standard a-Si:H cell degraded from 62% to 51%. In addition, mobility–lifetime products of the a-Si:H(Cl) cell also exhibited improved stability than that of the standard a-Si:H solar cell.     

Calculation of the performances of the a-Si:H/poly-Si multistacked solar cells

We investigated multistacked solar cells with a structure of metal/a-si: H (n-i-p)/poly-Si (n-p)/metal. This cell consists of two component cells; top n-i-p junction a-Si : H cell and a bottom n-p junction poly-Si cell. The solar cell conversion influencing factors were investigated in terms of film thickness, doping concentration, minority carrier lifetime, diffusion length, surface recombination, surface potential, AR coating, and circuit parameters of solar cells. The optimization of material and solar cell was carried out by using a PC-1D simulator. The main stream lines of the studies were the p-n junction poly-Si bottom cell, the p-i-n junction top a-Si : H cell, and the equivalent circuit examination. The optimized simulation results indicates that the 22% efficiency of multistacked solar cells can be achieved by optimizing parameters in each layer.     

Fabricating high performance a-Si solar cells by alternately repeating deposition and hydrogen plasma treatment method

The performance of a p-i buffer layer in pin amorphous silicon solar cell was improved by the “alternately repeating deposition and hydrogen plasma treatment method (ADHT)”. The optical bandgap of the a-Si film was increased by hydrogen plasma treatement. The wide optical bandgap and the high photoconductive a-Si:H films without carbon could be fabricated by the ADHT method. The conversion efficiency of the solar cell with a-Si:H buffer layer was almost the same as that using an a-SiC:H buffer layer. Second, the a-Si (ADHT) films were applied to the n-i buffer layer. The insertion of a-Si (ADHT) films between the i-layer and the n-layer was effective to improve the cell performance, especially the fill factor. With the use of high performance a-Si p-i and n-i buffer layer deposited by ADHT method, a cell conversion efficiency of 12.9% was obtained.     

Integration of a-Si:H solar cell with novel twist nematic liquid crystal cell for adjustable brightness and enhanced power characteristics

This study improves the output power and brightness characteristics of a translucent hydrogenated amorphous silicon (a-Si:H) solar cell by integrating the solar cell with a novel twist nematic (TN) liquid crystal (LC) cell incorporating a sub-wavelength metal grating polarization beam splitter (PBS). Although conventional TN-LC cells are widely used to adjust the brightness in many display applications, the sheet polarizers used in such cells decay when exposed to ultraviolet (UV) rays and have a low light efficiency. Accordingly, in this study, a sub-wavelength metal grating PBS is used to replace not only the sheet polarizers in the conventional TN-LC cell but also the upper and lower alignment layers and transparent electrodes. Therefore, a translucent a-Si:H solar cell integrating with the novel TN-LC cell with the sub-wavelength metal grating PBS could improve power efficiency and durability in UV ray environment. The experimental results show that the transmittance gap between the “on” and “off” states of the enhanced translucent a-Si:H solar cell/novel TN-LC cell is of the order of 26.6% (i.e. 4.3-30.9%) for incident light with a wavelength of 800 nm, 6.3% (i.e. 10.8-17.1%) for an incident wavelength of 400 nm and 2.7% (i.e. 0-2.7%) for an incident wavelength of 510 nm. Moreover, it is shown that the novel TN-LC cell increases the maximum electrical power developed by the translucent a-Si:H solar cell and improves its power conversion efficiency by 0.209% in the “off” state and 0.417% in the “on” state. As a result, the proposed device represents an ideal solution for building-integrated photovoltaic (BIPV) systems, automobile industry applications and many other adjustable brightness photovoltaic applications.     

Fabrication of amorphous silicon p-i-n solar cells using ion shower doping technique

We have studied the fabrication of amorphous silicon (a-Si : H) p-i-n solar cells using an ion shower doped n⁺-layer. The p-i-n cells with ion-doped n⁺-layer exhibited open-circuit voltage of > 0.8 V, fill factor of > 0.62 and conversion efficiency of > 8.4% when the ion acceleration voltage was between 3 and 7 kV. The a-Si : H p-i-n solar cell fabricated under an optimized ion-doping condition exhibited an open-circuit voltage of 0.84 V, a fill factor of 0.66 and a conversion efficiency of 9.9% which was very similar to those of conventional a-Si : H p-i-n cells fabricated in the same deposition chamber. Therefore, ion shower doping technique can be applied to fabricate large area, high performance a-Si : H p-i-n solar cells.     

An amorphous silicon random nanocone/polymer hybrid solar cell

This paper proposes and experimentally demonstrates an a-Si:H random nanocone/PEDOT:PSS/P3HT:PCBM hybrid solar cell to extend the absorption to near infrared and solve the difficulty of carrier transport through organic-inorganic interface. The internal electrical field inside a-Si:H random nanocone force holes move to the anode and electrons move to the cathode. The insertion of a layer of PEDOT:PSS conducting polymer between organic-inorganic interface could cause electrons and holes to partially recombine, thus establishing an electrically connected a-Si:H and P3HT:PCBM bulk heterojunction, which enables carriers transport through organic-inorganic interfaces efficiently. As compared to conventional polymer solar cells, the open-circuit voltage of hybrid solar cells was increased from 0.51 to 0.78 V. Additionally, the power conversion efficiency was increased from 1.73% to 2.22%, which demonstrates approximately 28% enhancement, indicating that the hybrid structure could largely increase the efficiency of polymer solar cells.     

Development of high efficiency p-i-n amorphous silicon solar cells with the p-μc-Si:H/p-a-SiC:H double window layer

A structure is developed to help improve the TCO/p contact and efficiency of the solar cell. A p-i-n amorphous silicon (a-Si:H) solar cell with high-conversion efficiency is presented via use of a double p-type window layer composed of microcrystalline silicon and amorphous silicon carbide. The best efficiency is obtained for a glass/textured TCO/p-μc-Si:H/p-a-SiC:H/buffer/i-a-Si:H/n-μc-Si:H/GZO/Ag structure. Using a SnO₂/GZO bi-layer and a p-type hydrogenated microcrystalline silicon (p-μc-Si:H) layer between the TCO/p-a-SiC:H interface improves the photovoltaic performance due to reduction of the surface potential barrier. Layer thickness, B₂H₆/SiH₄ ratio and hydrogen dilution ratio of the p-μc-Si:H layer are studied experimentally. It is clearly shown that the double window layer can improve solar cell efficiency. An initial conversion efficiency of 10.63% is achieved for the a-Si:H solar cell.     

Photo- and electro-luminescence of a-Si:H and mixed-phase alloys

In this paper, we review recent work on photoluminescence in the transition materials from hydrogenated amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H) and silicon-germanium alloy (a-SiGe:H). Also, the electroluminescence is reviewed as transport-controlled recombination. We focus on the new materials such as hydrogen-diluted a-Si:H, a-Si:H prepared by hot-wire chemical vapor deposition, high-growth rate films, and the new findings concerning the electronic structure in relation to the material microstructures and to the solar cell performance such as open voltage (V_oc).     

Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency

The p-type a-Si:H/n-type c-Si (P⁺ a-Si:H/N⁺ c-Si) heterojunction was simulated for developing solar cells with high conversion efficiency and low cost. The characteristic of such cells with different work function of transparent conductive oxide (TCO) were calculated. The energy band structure, quantum efficiency and electric field are analyzed in detail to understand the mechanism of the heterojunction cell. Our results show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work function, and the TCO work function should be large enough in order to achieve high conversion efficiency of P⁺ a-Si:H/N⁺ c-Si solar cells. With the optimized parameters set, the P⁺ a-Si:H/N⁺ c-Si solar cell reaches a high efficiency (η) up to 21.849% (FF: 0.866, V_OC: 0.861 V, J_SC: 29.32 mA/cm²).