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.     

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.     

Crystalline silicon on glass (CSG) thin-film solar cell modules

Crystalline silicon on glass (CSG) solar cell technology was developed to address the difficulty that silicon wafer-based technology has in reaching the very low costs required for large-scale photovoltaic applications as well as the perceived fundamental difficulties with other thin-film technologies. The aim was to combine the advantages of standard silicon wafer-based technology, namely ruggedness, durability, good electronic properties and environmental soundness with the advantages of thin-films, specifically low material use, large monolithic construction and a desirable glass superstrate configuration. The challenge has been to match the different preferred processing temperatures of silicon and glass and to obtain strong solar absorption in notoriously weakly-absorbing silicon of only 1.4 μm thickness, the thinnest active layer of the key thin-film contenders. A rugged, durable silicon thin-film technology has been developed arguably with the lowest likely manufacturing cost of these contenders and confirmed efficiency for small pilot line modules already in the 8–9% energy conversion efficiency range, on the path to 12–13%.     

非晶硅太阳能路灯系统的设计

介绍太阳能路灯系统的组成,各部分的工作原理、工作特点,并以沈阳汉锋工厂内安装的太阳能路灯为例,来说明非晶硅太阳能路灯系统的设计方法。具有弱光性、低成本的非晶硅太阳能电池与高亮度、长寿命的LED光源相结合将是未来照明工程的发展趋势。     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.     

Process development of amorphous silicon/crystalline silicon solar cells

We have already investigated some crucial limiting process steps of the amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell technology and some specific characterization tools of the ultrathin amorphous material used in devices. In this work, we focus our attention particularlyon the technology of the ITO front contact fabrication, that also is used as an antireflective coating. It is pointed out that this layer acts as a barrier layer against the diffusion of metal during the annealing treatments of the front contact grid. The criteria of the selection of the metal to be used to obtain good performance of the grid and the deposition methods best suited to the purpose are shown. We were able to fabricate low temperature heterojunction solar cells based p-type Czochralski silicon, and a conversion efficiency of 14.7% on 3.8 cm² area was obtained without back surface field and texturization.     

The role of the buffer layer in the light of a new equivalent circuit for amorphous silicon solar cells

Although the beneficial effect of the buffer layer between the p- and i-layer of amorphous silicon solar cells has been known for many years, the role of this layer is controversial. This paper examines the effect of the buffer layer using a new equivalent circuit for these devices (Merten et al. IEEE Trans. Electron Dev. 45 (1988) 423–429 [1]). The parameters of this model can be easily assessed by variable illumination measurements (VIM) of the devices' I(V)-curve. With the model, collection of carriers in the bulk of the cell is easy and clearly separated from the diode behaviour of the device. The VIM-method allows for a complete analysis of the thin film cells, covering both technological and physical topics. It is shown that the dominant effect increasing the efficiency of the cells with buffer layer is the reduction of the hole injection from the p-layer which leads to a reduced diode term. The buffer layer only slightly reduces the recombination in the i-layer. This reduction mainly occurs in a region close to the p/i-interface and cannot be observed with red light (homogeneous carrier generation).     

Production technology for amorphous silicon-based flexible solar cells

We have developed a-Si-based solar cells with plastic film substrate and achieved a stabilized efficiency of 9% in a 40 cm×80 cm cell. The structure and fabrication process of flexible solar cells are presented. Then we discuss the merits and demerits of our process from the viewpoint of mass production, and clarify that the SCAF cell has a good adaptability to mass production.     

Low-temperature deposition of amorphous silicon solar cells

We develop amorphous silicon (a-Si:H)-based solar cells by plasma-enhanced chemical vapor deposition (PECVD) at deposition temperatures of T_s=75°C and 100°C, compatible with low-cost plastic substrates. The structural and electronic properties of low-temperature standard PECVD a-Si:H, both doped and undoped, prevent the photovoltaic application of this material. In this paper, we demonstrate how to achieve device-quality a-Si:H even at low deposition temperatures. In the first part, we show the dependence of structural and carrier transport properties on the deposition temperature. The sub-band gap absorption coefficient and the Urbach energy increase when the deposition temperature declines from T_s=150°C to 50°C, the conductivity of doped layers and mobility-lifetime product of intrinsic a-Si:H drop drastically. Therefore, in the second part we investigate the impact of increasing hydrogen dilution of the feedstock gases on the properties of low-temperature a-Si:H. We restore n-type a-Si : H device-quality conductivity while the p-type a-Si:H conductivity is still inferior. For undoped layers, we depict the hole diffusion length, the mobility-lifetime product for electrons, the Urbach energy, and sub-band gap absorption coefficient as a function of the hydrogen dilution ratio. We incorporate these optimized materials in solar cell structures of single and multilayer design and record initial efficiencies of η=6.0% at a deposition temperature of T_s=100°C, and η=3.8% at T_s=75°C. For prospective opaque polymer substrates we develop, in addition to our conventional pin cells, devices in nip design with similar performance.     

The effect of spectral variations on the performance parameters of single and double junction amorphous silicon solar cells

We present the results of an experimental investigation into the effects of spectral variations in a maritime climate on the performance parameters of single and double junction amorphous silicon solar cells. Such considerations are important for accurate modelling of system performance. It is shown that one can distinguish between two effects: a primary effect that results from variations in the total irradiance in the spectrally useful range of the device, and a secondary (mismatch) effect observed in double junction devices that is related to details of device structure. Results showing the impact on the short-circuit current, the current at the maximum-power-point, the fill factor and the overall efficiency are discussed.     

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.     

Amorphous silicon solar cells

The perfectioning of the deposition techniques of amorphous silicon over large areas, in particular film homogeneity and the reproducibility of the electro-optical characteristics, has allowed a more accurate study of the most intriguing bane of this material: the degradation under sun-light illumination. Optical band-gap and film thickness engineering have enabled device efficiency to stabilize with only a 10–15% loss in the as-deposited device efficiency. More sophisticated computer simulations of the device have also strongly contributed to achieve the highest stable efficiencies in the case of multijunction devices. Novel use of nanocrystalline thin films offers new possibilities of high efficiency and stability. Short term goals of great economical impact can be achieved by the amorphous silicon/crystalline silicon heterojunction. A review is made of the most innovative achievements in amorphous silicon solar cell design and material engineering.     

Theory of amorphous silicon solar cell (b): a five layer analytical model

A theoretical analysis of recombination kinetics and space charge distribution in amorphous silicon is carried out with a view to bring out the underlying physics. A uniform excitation with a flat quasi-Fermi level and a constant np product has been used as a probe to estimate the relative importance of various parameters. Recombination rates have been calculated for various ratios of capture rates for Coulomb attractive and neutral traps. In practice a large ratio of capture rates exists and for this case two peaks of recombination maxima are found to lie in the space charge regions corresponding to transitions at the energy level E₁ (for D⁺–D⁰ transition) at the p–i edge and for E₂ energy level (corresponding to D⁰–D⁻ transition) at the i–n interface. A two independent level model therefore holds to a good approximation. The dangling bond density is found to determine both the space charge distribution and the recombination rate. Based on space charge density distribution i-layer can be divided in the five parts. The two recombination rate peaks are found to exist at the p–i and i–n space charge transitions respectively. This enables us to develop a simple model for the i-layer of the p–i–n diode.     

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.     

The application of grating couplers in thin-film silicon solar cells

As an alternative to randomly textured transparent conductive oxides as front contact for thin-film silicon solar cells, the application of periodic light grating couplers was studied. The periods and groove depths of transparent gratings made of zinc oxide were tuned independently from each other and varied between 1 and 4 μm and 100 and 600 nm, respectively. The one-dimensional grating couplers were realized using photolithography. We have analysed the optical properties of the gratings and the properties of amorphous and microcrystalline silicon solar cells incorporating these grating couplers. The achieved results are discussed with respect to the performance of cells deposited on flat and randomly textured substrates.     

Recent progress of amorphous silicon solar cell applications and systems

Fifteen years have passed since the first industrial use of amorphous silicon (a-Si) solar cells for consumer products. At present, a-Si solar cells are entering a new age of use in power generating systems at private residences and other outdoor applications. This paper reviews recent advances in amorphous silicon (a-Si) solar cells and their applications. Technological developments in the field of a-Si solar cells are discussed. Various applications and systems that take advantage of the a-Si solar cell are then introduced. Finally, future prospects are discussed, including a new concept of GENESIS system for worldwide energy generation and transmission.     

Flexible amorphous and microcrystalline silicon tandem solar modules in the temporary superstrate concept

Encapsulated and series-connected amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) based thin film silicon solar modules were developed in the superstrate configuration using an aluminum foil as temporary substrate during processing and a commodity polymer as permanent substrate in the finished module. For the development of μc-Si:H single junction modules, aspects regarding TCO conductivity, TCO reduction, deposition uniformity, substrate temperature stability and surface morphology were addressed. It was established that on sharp TCO morphologies where single junction μc-Si:H solar cells fail, tandem structures consisting of an a-Si:H top cell and a μc-Si:H bottom cell can still show a good performance. Initial aperture area efficiencies of 8.2%, 3.9% and 9.4% were obtained for fully encapsulated amorphous silicon (a-Si:H) single junction, microcrystalline silicon (μc-Si:H) single junction and a-Si:H/μc-Si:H tandem junction modules, respectively.     

Influence of defects and band offsets on carrier transport mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells

We have investigated the carrier transport mechanisms in undoped a-Si:H/p-type c-Si heterojunctions with and without a μc-Si buffer layer, as well as their effects on the photovoltaic properties of the junction. The conduction behavior of the junction is strongly affected by the defect state distribution and band offset at the hetero-interface. The recombination process involving the interface states on the thin film silicon (a-Si:H/μc-Si) side dominates at low forward bias (V<0.3 V), whereas multistep tunneling capture emission (MTCE) dominates in the higher bias region (0.3<V<0.55 V) until the conduction becomes space charge limited (V>0.55 V). The MTCE process seems to be more closely related to the bulk defects in the thin film silicon than the interface states. In addition, the position of a trapping level, where the tunneling process occurs, seems to be determined by the hole energy at the edge of the c-Si and the trap distribution in the thin film silicon. Despite the domination of MTCE in the indicated voltage range, the reduced band offset at the interface increases current levels by the enhanced diffusion and/or emission processes. The insertion of a 200 Å thick μc-Si buffer layer between the a-Si:H (700 Å)/c-Si increases the solar cell efficiency to 10%, without an antireflective coating, by improving both the carrier transport and the red response of the cell.     

Material properties of microcrystalline silicon for solar cell application

The paper reviews the material requirements of microcrystalline silicon (μc-Si) in terms of the device operation and configuration for thin film solar cells and thin film transistors (TFTs). We investigated the material properties of μc-Si films deposited by using 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) from a conventional H₂ dilution in SiH₄. Two types of intrinsic μc-Si films deposited at the high pressure narrow electrode gap and the low pressure wide electrode gap were studied for the solar cell absorption layers. The material properties were characterized using dark conductivity, Raman spectroscopy, and transmission electron microscope (TEM) measurements. The μc-Si quality and solar cell performance were mainly determined by microstructure characteristics. Solar cells adopting the optimized μc-Si film demonstrated high stability with no significant changes in solar cell performance after air exposure for six months and subsequent illumination for over 300 h. The results can be explained that low ion bombardment and high atomic hydrogen density under the PECVD condition of the high pressure narrow electrode gap produce high-quality μc-Si films for solar cell application.     

Spectral effects on amorphous silicon solar module fill factors

The outdoor operation and monitoring of amorphous silicon (a-Si) solar modules present unique features when compared to the more traditional and quite well understood operation of the crystalline silicon (c-Si) technology. The peculiarities of a-Si contrast to such extent with those of c-Si solar cells that in the field, while the former performs better during summer, the latter is more efficient in winter. Concepts usually applied to describe phenomena in c-Si devices are often inadequate to describe the performance of a-Si cells. When looking at module performance, the fill factor (FF) can be regarded as one of the characteristic photovoltaic quantities of major interest. Under outdoor illumination, cells are seasonally exposed to different solar spectral contents and intensities, which vary considerably from summer to winter. The FF depends on both the quality (spectrum) and quantity (irradiation) of the incident light. In this context, we report results showing spectral effects on the FF of amorphous silicon solar modules deployed outdoors. While “blue” spectra improved the FF of a-Si devices, the contrary was observed for “red” spectra. The voltage-dependent spectral response of a-Si devices is also described and quantified. Our results reveal that a-Si modules can perform quite well at low irradiations and mainly diffuse spectra. We, thus, conclude that in system sizing programmes, the performance of a-Si modules should be treated more precisely with respect to spectra, to reveal their true operational characteristics and advantages.