Degradation and annealing of amorphous silicon solar cells by current injection: experiment and modeling

In this paper we report in detail on the effect of current injection in amorphous silicon solar cells. A set of devices has been degraded and then annealed at different current intensities. Device performances during the whole experiment have been monitored by current–voltage characteristics and quantum efficiency curves. It has been found that annealing rate increases with current intensity, while stabilized photovoltaic parameters decrease. Time evolution of efficiency and short-circuit current during degradation has been reproduced by a numerical device modeling, resulting in a pronounced increase of defects near the p–i interface. The model also demonstrated that annealing results are not well reproduced if current-induced annealing is not energy selective.     

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.     

Changes in the temperature coefficients of the characteristics of amorphous silicon solar cells subjected to light degradation and recovery

Measuring changes in temperature coefficients is an effective way to estimate the performance of a solar cell. We investigated changes in the temperature coefficients of the I–V characteristics of amorphous silicon (a-Si) solar cells subjected to light degradation and recovery. There is a good correlation between change in the temperature coefficient (Ψ) and the degradation/recovery state of a cell’s conversion efficiency (η). This relationship can be expressed by Ψ_η=−0.0052Δη−0.45. Therefore, the temperature coefficient corresponding to the degradation/recovery state can be estimated.     

Activation of dopant in the p-layer of amorphous silicon solar cells under illumination

We have investigated the effect of light-soaking on the p-doped layer of amorphous silicon (a-Si:H) solar cells by low temperature (50–300 K) AC conductance measurements. The experimental results are interpreted on the basis of an equilibration model of the doped material. The model takes into account the finite dimension of the layer and its presence inside a complex structure. It is shown that the Fermi level shifts after light soaking, which can result in activation of the doping impurities.     

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.     

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.     

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.     

On the sensitivity of open-circuit voltage and fill factor on dangling bond density and Fermi level position in amorphous silicon p–i–n solar cell

A simple method for calculation of current–voltage characteristics of an amorphous silicon solar cell is described in terms of excitation current, J_G, and excitation voltage, V_G, the latter being defined in terms of separation of quasi-Fermi levels. Contrary to the usual method of calculating the short-circuit current and dark current separately and assuming a linear superposition, in the present method the calculations are done first in the open circuit where the neutrality of space charge can be assumed and then the current has been calculated in terms of a gradient in the quasi-Fermi levels. We find that depending on other parameters, the open-circuit voltage is a weak function of dangling bond density except in cases of very large degradation. The sensitivity of open-circuit voltage, V_oc, to light-induced degradation can further be reduced by moving the thermal equilibrium Fermi level above the upper dangling bond level. Fill factor deterioration is found to be mainly due to conductivity modulation and is higher for the lower values of thermal equilibrium Fermi level.     

Reversible increase of photocurrents in excimer laser-crystallized silicon solar cells

Excimer laser-crystallized silicon solar cells fabricated show a steady increment of the current densities with exposure to simulated sunlight, over a 30 min period. The current density of the amorphous silicon cell under identical conditions remains steady, with no significant change. The process was observed to be reversible upon cooling, and the performance increase is attributed to the energy barrier introduced by the enhanced bandgap of a nanocrystalline silicon middle layer, created as a result of the crystallization. It is suggested that the thermal energy due to prolonged illumination allows carriers to cross the barrier increasing output currents.     

Thermal recovery effect on light-induced degradation of amorphous silicon solar module under the sunlight

The thermal recovery effect from the light-induced degradation under the sunlight is experimentally investigated on the amorphous silicon photovoltaic module (a-Si PV module) for installing directly to the roof flames of wooden houses. To enhance the recovery effect, the heat-insulating material is attached to the back side of the module for increasing the module temperature under the sunlight: the heat-insulated module.The generated power from the heat-insulated module is compared with that from the normal module (without the heat-insulating material) for 2 yr, and it has been cleared that the generated power normalized at 25°C from the heat-insulated module is approximately 7.3% higher than that from the normal one with the average temperature increase of 4.2°C under the sunlight.     

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.     

TCO and light trapping in silicon thin film solar cells

For thin film silicon solar cells and modules incorporating amorphous (a-Si:H) or microcrystalline (μc-Si:H) silicon as absorber materials, light trapping, i.e. increasing the path length of incoming light, plays a decisive role for device performance. This paper discusses ways to realize efficient light trapping schemes by using textured transparent conductive oxides (TCOs) as light scattering, highly conductive and transparent front contact in silicon p–i–n (superstrate) solar cells. Focus is on the concept of applying aluminum-doped zinc oxide (ZnO:Al) films, which are prepared by magnetron sputtering and subsequently textured by a wet-chemical etching step. The influence of electrical, optical and light scattering properties of the ZnO:Al front contact and the role of the back reflector are studied in experimentally prepared a-Si:H and μc-Si:H solar cells. Furthermore, a model is presented which allows to analyze optical losses in the individual layers of a solar cell structure. The model is applied to develop a roadmap for achieving a stable cell efficiency up to 15% in an amorphous/microcrystalline tandem cell. To realize this, necessary prerequisites are the incorporation of an efficient intermediate reflector between a-Si:H top and μc-Si:H bottom cell, the use of a front TCO with very low absorbance and ideal light scattering properties and a low-loss highly reflective back contact. Finally, the mid-frequency reactive sputtering technique is presented as a promising and potentially cost-effective way to up-scale the ZnO front contact preparation to industrial size substrate areas.     

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.     

Productivity potential of an inline deposition system for amorphous and microcrystalline silicon solar cells

Amorphous and microcrystalline silicon single layers and p-i-n solar cells were produced dynamically using an inline deposition system called “line source”. A highly uniform deposition of thin-film silicon layers with layer-thickness variations of less than ±5% was achieved. Amorphous and microcrystalline silicon single junction solar cells were dynamically fabricated with initial efficiencies of 8.3% and 6.3%, respectively. The dynamic deposition rate of these solar cells is 6.75 nm m/min in case of a-Si:H and 3.3 nm m/min for μc-Si:H. In this work it will be shown that an enhancement of the deposition rate up to 15.6 nm m/min during the i-layer deposition of a-Si:H solar cells has only a weak negative influence on the initial efficiencies of the cells. Further on, the effect of substrate velocity on solar cell characteristics of a-Si:H solar cells is investigated. Finally, a productivity estimation of the line source concept is presented.     

Ray-trace simulation of light trapping in silicon solar cell with texture structures

We investigated the light trapping effect in a solar cell. We performed ray-tracing simulation for a light trapping structure in a silicon crystalline solar cell. By comparing theoretical and experimental values, the reliability of a simulation technique was evaluated. Using this simulation technique, we evaluated the light trapping effect in the silicon crystalline solar cell and glass with a V-shaped texture. Furthermore, we investigated the light trapping effect in a silicon thin film solar cell. In a silicon thin film with a thickness of 20 μm deposited on V-shaped glass, reflectivity which is comparable to that in a pyramidal texture structure was obtained. We concluded that the simulation technique used in this work is very effective for optimization of the structure in the enhancement of the light trapping effect.     

Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Current-voltage-temperature (I-V-T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I-V-T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J_sc and V_oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J_sc which improved by an order of magnitude. The treatment also affected the lower temperature I-V-T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.     

A high efficiency thin film silicon solar cell and module

A photoelectric conversion efficiency of over 10% has been achieved in thin-film microcrystalline silicon solar cells which consist of a 2 μm thick layer of polycrystalline silicon. It was found that an adequate current can be extracted even from a thin film due to the very effective light trapping effect of silicon with a low absorption coefficient. As a result, this technology may eventually lead to the development of low-cost solar cells. Also, an initial aperture efficiency as high as 13.5% has been achieved with a large area (91 cm × 45 cm) tandem solar cell module of microcrystalline silicon and amorphous silicon (thin film Si hybrid solar cell). An even greater initial efficiency of 14.7% has been achieved in devices with a small size (area of 1 cm²), and further increases of efficiency can be expected.     

The effects of enhanced light trapping in tandem micromorph silicon solar cells

Optical modelling is used to investigate the potential improvements in quantum efficiency and short-circuit current density of the top and bottom silicon cell in tandem micromorph configuration. The effects of enhanced haze parameter and different angular distribution functions of scattered light are presented and analysed. The role of an intermediate reflector (interlayer), located between the top and bottom cell, is studied from the optical point of view. The improvements in quantum efficiency of top cell are demonstrated for different types of interlayers. Potential thickness reductions due to enhanced light trapping in the solar cells are presented.     

Low temperature deposition of high open-circuit voltage (>1.0 V) p-i-n type amorphous silicon solar cells

P-i-n type hydrogenated amorphous silicon (a-Si:H) solar cells were deposited by the radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) process at a low substrate temperature of 125 °C, which is compatible with low-cost poly (ethylene terephthalate) (PET) plastic substrates. Wide band gap (E_opt>1.88 eV) intrinsic a-Si:H films were achieved before the onset of the microcrystalline regime by changing the hydrogen dilution ratios. On the other hand, the structural, optical and electrical properties of p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) window layers have been optimized at 125 °C. High quality p-a-SiC:H film with high optical band gap (E₀₄=2.02 eV) and high conductivity (σ_d=1.0×10⁻⁷ S/cm) was deposited at ‘low-power regime’ under low silane flow rates and high H₂ dilution conditions. With the combination of wide band gap p-a-SiC:H window layers and intrinsic a-Si:H layers, a high V_oc of 1.01 V (efficiency=5.51%, FF=0.72, J_sc=7.58 mA/cm²) was obtained for single junction a-Si:H p-i-n solar cell at a low temperature of 125 °C. Finally, flexible a-Si:H solar cell on PET substrate with efficiency of 4.60% (V_oc=0.98 V, FF=0.69, J_sc=6.82 mA/cm²) was obtained.     

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.