Highly stabilized protocrystalline silicon multilayer solar cell using a silicon–carbide double p-layer structure

We have investigated a pin-type protocrystalline silicon (pc-Si:H) multilayer solar cell fabricated by employing a silicon–carbide double p-layer structure and a layered structure of multilayer processing through alternate H₂ dilution. The initial conversion efficiency is drastically improved by incorporating a hydrogen-diluted boron-doped amorphous silicon–carbide (p–a-SiC:H) buffer layer at the p/i interface. Remarkably, the pc-Si:H multilayer absorber exhibits superior light-induced metastability to a conventional amorphous silicon (a-Si:H) absorber. Therefore, we have successfully achieved a highly stabilized efficiency of 9.0% without using any back reflector.     

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.     

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.     

Widegap a-Si:H films prepared at low substrate temperature

Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E_g ∼ 1.90 eV and σ_ph ≥ 10⁻⁴ Scm⁻¹ has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.     

Boron doped amorphous diamond window layer deposited by filtered arc for amorphous silicon alloy p-i-n solar cells

In order to improve the conversion efficiency of amorphous silicon (a-Si:H) alloy p-i-n solar cells, the original p-a-Si:H window layer is substituted by the boron-doped amorphous diamond (a-D:B) films deposited using filtered cathodic vacuum arc technology. The microstructural, optical and electrical properties as functions of the boron concentrations in the films were, respectively, evaluated by an X-ray photoemission spectroscopy, an ultraviolet-visible spectrometer and a semiconductor parameter analyzer. The photovoltaic parameters of the solar cell modules were also detected as functions of boron concentration. It has been shown that the conductive a-D:B films could be obtained and still remained a wide optical gap. The p-i-n structural amorphous silicon solar cell using the a-D:B window layer increased the conversion efficiency by a roughly 10% relative improvement compared to the conventional amorphous silicon solar cell because of the enhancement of short wavelength response.     

Improved quality a-SiC:H films prepared by photo chemical vapour decomposition of silane and acetylene

Hydrogenated amorphous and microcrystalline silicon carbon alloy films have been grown by photo-CVD using C₂H₂ as a source gas of carbon. The hydrogenated amorphous silicon carbon (a-SiC:H) film with a band gap of ~2.0 eV prepared at a very low hydrogen (LD) concentration exhibits better photo-electronic properties compared to that at high hydrogen dilution (HD) having a similar optical gap. Notwithstanding a high deposition rate, the high photosensitivity ( 10⁶), the low density of the defect states ( 6 × 10¹⁶cm⁻³) and the Urbach energy parameter (72 meV) for the a-SiC:H film prepared at low hydrogen dilution and pressure are impressive. On the other hand, low pressure along with high hydrogen dilution have been found to be conducive to microcrystalline silicon carbon alloy (μc-Si:H) formation. Interestingly, crystallites are of silicon while carbon remains in the amorphous and grain boundary regions.     

Single-junction a-Si solar cells with over 13% efficiency

Single junction hydrogenated amorphous silicon solar cells having a high conversion efficiency of 13.2% were developed by combining three approaches. First, a new type of p-layer, such as (a-Si/a-C)_n multilayers, was investigated. The high open-circuit voltage was obtained without lowering the short-circuit current and the fill factor. Second, alternately repeating deposition and hydrogen plasma treatment method was applied to the fabrication of an a-SiC or wide gap a-Si : H films for p/i interface layer. High photoconductive and wide bandgap materials were obtained applicable to the p/i buffer layers. Third, the relationships between defect density of films or fill factors of solar cells and hydrogen radical in plasma were investigated. It was suggested that the H^*/SiH^* ratio was an effective parameter to improve the defect and fill factor, and the excess hydrogen radical deteriorated quality of films and cells.     

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.     

Development of thin film amorphous silicon oxide/microcrystalline silicon double-junction solar cells and their temperature dependence

We have developed thin film silicon double-junction solar cells by using micromorph structure. Wide bandgap hydrogenated amorphous silicon oxide (a-SiO:H) film was used as an absorber layer of top cell in order to obtain solar cells with high open circuit voltage (V_oc), which are attractive for the use in high temperature environment. All p, i and n layers were deposited on transparent conductive oxide (TCO) coated glass substrate by a 60 MHz-very-high-frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. The p-i-n-p-i-n double-junction solar cells were fabricated by varying the CO₂ and H₂ flow rate of i top layer in order to obtain the wide bandgap with good quality material, which deposited near the phase boundary between a-SiO:H and hydrogenated microcrystalline silicon oxide (μc-SiO:H), where the high V_oc can be expected. The typical a-SiO:H/μc-Si:H solar cell showed the highest initial cell efficiency of 10.5%. The temperature coefficient (TC) of solar cells indicated that the values of TC for conversion efficiency (η) of the double-junction solar cells were inversely proportional to the initial V_oc, which corresponds to the bandgap of the top cells. The TC for η of typical a-SiO:H/μc-Si:H was −0.32%/ °C, lower than the value of conventional a-Si:H/μc-Si:H solar cell. Both the a-SiO:H/μc-Si:H solar cell and the conventional solar cell showed the same light induced degradation ratio of about 20%. We concluded that the solar cells using wide bandgap a-SiO:H film in the top cells are promising for the use in high temperature regions.     

Fabrication of microcrystalline silicon solar cells on a SnO2 coated substrate using seed layer insertion

We fabricated hydrogenated microcrystalline silicon (μc-Si:H) solar cells on SnO₂ coated glass using a seed layer insertion technique. Since rich hydrogen atoms from the μc-Si:H deposition process degrade the SnO₂ layer, we applied p-type hydrogenated amorphous silicon (p-a-Si:H) as a window layer. To grow the μc-Si:H layer on the p-a-Si:H window layer, we developed a seed layer insertion method. We inserted the seed layer between the p-a-Si:H layer and intrinsic bulk μc-Si:H. This seed layer consists of a thin hydrogen diluted silicon buffer layer and a naturally hydrogen profiled layer. We compared the characteristics of solar cells with and without the seed layer. When the seed layer was not applied, the fabricated cell showed the characteristics of a-Si:H solar cell whose spectral response was in a range of 400-800 nm. Using the seed layer, we achieved a μc-Si:H solar cell with performance of V_oc=0.535 V, J_sc=16.0 mA/cm², FF=0.667, and conversion efficiency=5.7% without any back reflector. The spectral response was in the range of 400-1100 nm. Also, the fabricated device has little substrate dependence, because a-Si:H has weaker substrate selectivity than μc-Si:H.     

Development of n-μc-SiOx:H as cost effective back reflector and its application to thin film amorphous silicon solar cells

Development of doped silicon oxide based microcrystalline material as a potential candidate for cost-effective and reliable back reflector layer (BRL) for single junction solar cells is discussed in this article. Phosphorus doped μc-SiO_x:H layers with a refractive index ∼2 and with suitable electrical properties were fabricated by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique, using the conventional capacitively coupled reactors. Optoelectronic properties of these layers were controlled by varying the oxygen content within the film. The performance of these layers as BRL have been investigated by incorporating them in a single junction amorphous silicon solar cell and compared with the conventional ZnO:Al based reflector layer. Single junction thin film a-Si solar cells with efficiency ∼9.12% have been successfully demonstrated by using doped SiO:H based material as a back reflector. It is found that the oxide based back reflector shows analogous performance to that of conventional ZnO:Al BRL layer. The main advantage with this technology is that, it can avoid the ex-situ deposition of ZnO:Al, by using doped μc-SiO:H based material grown in the same reactor and with the same process gases as used for thin-film silicon solar cells.     

大面积集成型a－SiC：H／a－Si：H叠层太阳电池的研制

报道了大面积（２７９０ｃｍ２）集成型ａ－ＳｉＣ：Ｈ／ａ－Ｓｉ：Ｈ叠层太阳电池的研制及稳定性实验结果，讨论了限制该电池效率的一些因素。实验电池的性能参数：Ｖｏｃ＝４０．８Ｖ，ＩＳＣ＝５３０．４０ｍＡ，ＦＦ＝４９．４％，有效面积（２２８０ｃｍ２）光电转换效率ＥＦ＝４．６９％（ＡＭ１．５，１００ｍＷｃｍ－２，２５℃）。制备出光电子学性能优良的ａ－ＳｉＣ：Ｈ薄膜及解决电池内部ｎ／Ｐ结的接触问题是提高该电池性能的关键。     

a-Si alloy three-stacked solar cells with high stabilized-efficiency

a-Si alloy three-stacked solar cells have been studied to improve the stabilized efficiency of a-Si: H based solar cells. Based on the analysis by the individual characterization method of the component cells in stacked type cells, the a-Si :H middle cell was replaced with an a-SiGe :H cell. Furthermore, the optical confinement technology was improved to obtain a high-output current with thin i-layer thickness in the a-SiGe :H bottom cell. By this device design, the initial conversion efficiency was improved up to 12.4% and more than a 10% stabilized efficiency was obtained in a-SiC :H/a-SiGe :H/a-SiGe :H three-stacked cells. These cell characteristics were confirmed by measurements at the JQA Organization (the former JMI Institute).     

Recent developments in high eficiency photovoltaic cells

Enormous progress has been made in recent years on a number of photovoltaic materials and devices in terms of conversion efficiencies. Efficiencies in the range of 18%–24% have been achieved in traditional silicon-based devices fabricated from both multicrystalline and single-crystal materials. Ultrahigh-efficiency (>30%) photovoltaic (PV) cells have been fabricated from gallium arsenide (GaAs) and its ternary alloys like gallium indium phosphide (GaInP₂). The high-efficiency GaAs-based solar cells are being produced on a commercial scale, particularly for space applications. Major advances in efficiency have also been made on various thin-film solar cells based on amorphous silicon (aSi:H), copper gallium indium diselenide (CIGS), and cadmium telluride materials. This paper gives a brief overview of the recent progress in PV cell efficiencies based on these materials and devices.     

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.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

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.     

Monolithically series-interconnected transparent modules of dye-sensitized solar cells

We have realized a new type of dye-sensitized solar cell (DSC) modules. The monolithically series interconnected structure, which is similar to the structure of amorphous silicon solar cells (SCs), was employed so that the advantages of DSCs compared to conventional silicon SCs (low costs, low energy consumption in production processes) were fully exploited. To achieve other important features of DSCs (transparency and color choice) we have developed transparent counter electrodes (CEs) composed of Pt-loaded In₂O₃:Sn nanoparticles and separators composed of SiO₂ nanoparticles to replace conventional non-transparent ones used in the modules. The performance of the new CEs is significantly improved to be close to those of conventional ones during electric generation operations. In all 85% of the maximal conversion efficiency was maintained after 2000 h of a durability test under 1 sun light soaking at 60 °C.     

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σ_ph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p–i–n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm²) at room temperature.     

A practical field study of various solar cells on their performance in Malaysia

A practical field study has been carried out with the intention to analyze and compare the performance of various types of commercially available solar panels under Malaysia's weather. Four different types of solar panels, such as mono-crystalline silicon, multi-crystalline silicon, amorphous silicon and copper–indium–diselenide (CIS) solar panels are used for the practical field study. A number of performance related parameters have been collected using data logger over a period of three consecutive days in the hope that this would give some initial information on the real performance of different solar panels. Results show that mono-crystalline silicon and multi-crystalline silicon solar module perform better when they are under hot sun, whereas the CIS and triple junction amorphous silicon solar panel perform better when it is cloudy and has diffused sunshine. Furthermore, the efficiency of crystalline silicon solar panel has been found to drop when the temperature rises higher. This phenomenon does not appear in the CIS and amorphous silicon solar panels, which shows that the performance of CIS and amorphous silicon solar cells are better in terms of power conversion efficiency and overall performance ratio. Better performance of thin film solar cells like amorphous silicon and CIS are observed from the initial results, which draws attention over the selection of solar panels and also may encourage the usage of these in tropical weather like Malaysia.