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.     

Silicon-based thin-film solar cells fabricated near the phase boundary by VHF PECVD technique

The light-soaked and annealing behaviors for silicon (Si)-based thin-film single-junction solar cells fabricated near the phase boundary using a very-high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique are investigated. The hydrogen dilution ratio is changed in order to achieve wide band gap hydrogenated amorphous Si (a-Si:H) and narrow band gap hydrogenated microcrystalline Si (μc-Si:H) absorbers. Just below the a-Si:H-to-μc-Si:H transition, highly hydrogen-diluted a-Si:H solar cells with a good stability against light-soaking and fast annealing behavior are obtained. In contrast, the solar cell fabricated at the onset of the μc-Si:H growth is very unstable and its annealing behavior is slow. In the case of μc-Si:H solar cells with the crystal volume fraction of 43–53%, they show the lowest light-induced degradation among the fabricated solar cells. However, it is very difficult to recover the degraded μc-Si:H solar cells via thermal annealing.     

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.     

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 novel route to a polycrystalline silicon thin-film solar cell

An alternative approach is described for the fabrication of a polycrystalline silicon thin-film solar cell on inexpensive substrates. In a first step amorphous silicon is recrystallized in an aluminum-induced crystallization process forming a large-grained polycrystalline silicon layer on glass or metal substrates. In a second step this layer is used as a template for epitaxial growth of the absorber layer (2–3 μm thick) at T<600 °C using ion-assisted deposition techniques. The third step consists of the formation of an a-Si:H/c-Si heterojunction by depositing an a-Si:H emitter from the gas phase. It will be shown that each of these steps has been successfully developed and can now be implemented in a solar cell process.     

Low-temperature growth of thick intrinsic and ultrathin phosphorous or boron-doped microcrystalline silicon films: Optimum crystalline fractions for solar cell applications

The growth kinetics and optoelectronic properties of intrinsic and doped microcrystalline silicon (μc-Si:H) films deposited at low temperature have been studied combining in situ and ex situ techniques. High deposition rates and preferential crystallographic orientation for undoped films are obtained at high pressure. X-ray and Raman measurements indicate that for fixed plasma conditions the size of the crystallites decreases with the deposition temperature. Kinetic ellipsometry measurements performed during the growth of p-(μc-Si:H) on transparent conducting oxide substrates display a remarkable stability of zinc oxide, while tin oxide is reduced at 200°C but stable at 150°C. In situ ellipsometry, conductivity and Kelvin probe measurements show that there is an optimum crystalline fraction for both phosphorous- and boron-doped layers. Moreover, the incorporation of p-(μc-Si:H) layers produced at 150°C in μc-Si:H solar cells shows that the higher the crystalline fraction of the p-layer the better the performance of the solar cell. On the contrary, the optimum crystalline fraction of the p-layer is around 30% when hydrogenated amorphous silicon (a-Si:H) is used as the intrinsic layer of p–i–n solar cells. This is supported by in situ Kelvin probe measurements which show a saturation in the contact potential of the doped layers just above the percolation threshold. In situ Kelvin probe measurements also reveal that the screening length in μc-Si:H is much higher than in a-Si:H, in good agreement with the good collection of microcrystalline solar cells     

Application of real-time spectroscopic ellipsometry for characterizing the structure and optical properties of microcrystalline component layers of amorphous semiconductor solar cells

Over the past few years, we have applied real-time spectroscopic ellipsometry (RTSE) to probe hydrogenated amorphous silicon (a-Si:H)-based solar cell fabrication on the research scale. From RTSE measurements, the microstructural development of the component layers of the cell can be characterized with sub-monolayer sensitivity, including the time evolution of (i) the bulk layer thickness which provide the deposition rates, and (ii) the surface roughness layer thickness which provide insights into precursor surface diffusion. In the same analysis, RTSE also yields the optical properties of the growing films, including the dielectric functions and optical gaps. Results reported earlier have been confined to p-i-n and n-i-p cells consisting solely of amorphous layers, because such layers are found to grow homogeneously, making data analysis relatively straightforward. In this study, we report the first results of an analysis of RTSE data collected during the deposition of an n-type microcrystalline silicon (μc-Si:H) component layer in an a-Si:H p-i-n solar cell. Such an analysis is more difficult owing to (i) the modification of the underlying i-layer by the H₂-rich plasma used in doped μc-Si:H growth and (ii) the more complex morphological development of μc-Si:H, including surface roughening during growth.     

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.     

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.     

Hydrogenated nanocrystalline silicon p-layer in amorphous silicon n–i–p solar cells

This paper describes an investigation into the impacts of hydrogenated nanocrystalline silicon (nc-Si:H) p-layer on the photovoltaic parameters, especially on the open-circuit voltage (V_oc) of n–i–p type hydrogenated amorphous silicon (a-Si:H) solar cells. Raman spectroscopy and transmission electron microscopy (TEM) analyses indicate that this p-layer is a diphasic material that contains nanocrystalline grains with size around 3–5 nm embedded in an amorphous silicon matrix. Optical transmission measurements show that the nc-Si:H p-layer has a wide band gap of 1.9 eV. Using this nanocrystalline p-layer in n–i–p a-Si:H solar cells, the cell performances were improved with a V_oc of 1.042 V, whereas the solar cells deposited under similar conditions but incorporating a hydrogenated microcrystalline silicon (μc-Si:H) p-layer exhibit a V_oc of 0.526 V.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

Characterization of heterojunctions in crystalline-silicon-based solar cells by internal photoemission

Internal photoemission (IPE) has been successfully applied to evaluate band offsets of heterojunctions (HJs) in crystalline silicon (c-Si)-based solar cells. Tunneling of carriers through the potential spike at HJ and the presence of a carrier conduction path in the wide-band-gap material of HJ can affect the IPE results. In other words, IPE measures the effective band discontinuity, including effects of the carrier conduction path. This feature of IPE is suited for the characterization of solar-cell structures. Results obtained for hydrogenated amorphous silicon/c-Si HJ and gallium phosphide/c-Si HJ are presented and discussed.     

Intrinsic microcrystalline silicon: A new material for photovoltaics

Microcrystalline silicon (μc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) has been investigated as material for absorber layers in solar cells. The deposition process has been adjusted to achieve high deposition rates and optimized solar cell performance. In particular, already moderate variations of the crystalline vs. amorphous volume fractions were found to effect the electronic material – and solar cell properties. Such variation is readily achieved by changing the process gas mixture of silane to hydrogen. Best cell performance was found for material near the transition to the amorphous growth regime. With this optimized material efficiencies of 7.5% for a 2 μm thick μc-Si:H single solar cell and 12% for an a-Si:H/μc-Si:H stacked solar cell have been achieved.     

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.     

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.     

Cell structures with low-high heterojunction of c-Si and μc-Si: H under rear contact for improvement of efficiencies

For a remarkable improvement of conversion efficiencies of single-crystalline silicon (c-Si) solar cells, we have been investigating rear surface structures. The structure has a highly conductive boron (B) doped hydrogenated microcrystalline silicon (μc-Si:H) film with a wide optical bandgap between a p-type c-Si substrate and a rear contact instead of a heavily diffused layer. The conditions of depositing the μc-Si:H film were investigated. Both short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the cell with the μc-Si:H film are much higher than those without the film. The V_oc obtained was higher than 650 mV and the efficiency was 19.6% for a 5 cm × 5 cm cell. It is confirmed that a low-high heterojunction of the c-Si substrate and the μc-Si:H film is very effective in preventing minority carriers near the rear contact from recombining.     

New materials and deposition techniques for highly efficient silicon thin film solar cells

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for μc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Å/s, respectively. These μc-Si:H solar cells were successfully up-scaled to a substrate area of 30×30 cm² and applied in a-Si:H/μc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%.     

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).     

Planar rear emitter back contact silicon heterojunction solar cells

A planar rear emitter back contact silicon heterojunction (PreBC-SHJ) solar cell design is presented, which combines the advantages of different high efficiency concepts using point contacts, back contacts, and silicon heterojunctions. Electrically insulated point or stripe contacts to the solar cell absorber are embedded within a planar hydrogenated amorphous silicon emitter layer deposited at low temperature on the rear side. The new solar cell design requires less structuring and allows large structure sizes, enabling the use of low-cost patterning technologies such as inkjet printing or screen printing. By means of numerical computer simulation the efficiency potential of back contacted heterojunction solar cells is shown to exceed 24%. First PreBC-SHJ solar cells have been realized and exhibit higher short circuit currents than our state-of-the-art front contacted silicon heterojunction reference solar cells.     

Reduction of light-induced defects by nano-structure tailored silicon solar cells using low-cost TCO substrates

In order to overcome the light-induced defects of hydrogenated amorphous silicon solar cells, we propose novel silicon material, called nano-structure tailored silicon, where nano-sized crystallites embedded in the amorphous silicon matrix homogeneously. We have applied low-cost TCO substrates to their solar cells, so we obtained a stabilized efficiencies (7.39%) higher than standard a-Si:H solar cells (7.25%) on a low-cost substrate.