F. A. Rubinelli

Amorphous Silicon Carbide/Crystalline Silicon Heterojunction Solar Cells: A Comprehensive Study of the Photocarrier Collection

We have studied the current–voltage (I–V) characteristics of p+ a-SiC:H/n c-Si heterojunction solar cells at different conditions. Under standard test conditions (300 K, 100 mW/cm2, AM1.5) these cells show normal I–V characteristics with a high fill factor (FF = 0.73) and a relatively high efficiency for their simple structure (η≈13%). However, below room temperature and at illumination levels above 10 mW/cm2 they exhibit an S-shaped I–V curve and a low fill factor. Simulation studies revealed that this effect is caused by the valence band discontinuity at the amorphous/crystalline interface which hinders at low temperatures the collection of photogenerated holes at the front contact. At low temperatures a high hole accumulation at the interface combined with extra trapping of holes inside the p+ a-SiC:H layer causes a shift of the depletion region, from the c-Si into the p+ a-SiC:H. This leads to an enhanced recombination inside the c-Si depletion region causing a significant current loss (S-shape). Tunnelling through the valence band spike can reduce these effects. For lower doped p a-SiC:H layers (Eact>0.4 eV) this S-shape can even occur at room temperature.

Performance of heterojunction p+ microcrystalline silicon n crystalline silicon solar cells

We have studied by Raman spectroscopy and electro-optical characterization the properties of thin boron doped microcrystalline silicon layers deposited by plasma enhanced chemical vapor deposition (PECVD) on crystalline silicon wafers and on amorphous silicon buffer layers. Thin 20–30 nm p+ μc-Si:H layers with a considerably large crystalline volume fraction (∼22%) and good window properties were deposited on crystalline silicon under moderate PECVD conditions. The performance of heterojunction solar cells incorporating such window layers were critically dependent on the interface quality and the type of buffer layer used. A large improvement of open circuit voltage is observed in these solar cells when a thin 2–3 nm wide band-gap buffer layer of intrinsic a-Si:H deposited at low temperature (∼100 °C) is inserted between the microcrystalline and crystalline silicon [complete solar cell configuration: Al/(n)c-Si/buffer/p+μc-Si:H/ITO/Ag)]. Detailed modeling studies showed that the wide band-gap a-Si:H buffe...

Microcrystalline n-i-p tunnel junction in a-Si:H/a-Si:H tandem cells

The kinetics controlling the electrical transport inside the μc-Si tunnel-recombination junction (TRJ) of a-Si:H/a-Si:H tandem solar cells was studied in detail with computer simulations. Trap assisted recombination tunneling and Poole–Frenkel mechanisms were included in our analysis. Three different μc-Si tunnel junctions were investigated: (a) n-p, (b) n-oxide-p and (c) n-i-p. The highest theoretical efficiencies in a-Si:H/a-Si:H tandem cells were achieved with the n-i-p tunnel junction structure. The impact of the μc-Si effective masses, mobility gap, and mobilities in the tandem solar cell efficiency is also studied in this article. Several a-Si:H/a-Si:H tandem solar cells were made with the μc-Si tunnel configurations of types (b) and (c). In all of these samples one extra oxide layer was needed at the i-a-Si:H/n-μc-Si interface. Both tunnel junctions lead us to comparable experimental tandem solar cell efficiencies. When the n-i-p structure is implemented as TRJ in the a-Si:H/a-Si:H tandem solar cel...

Significance of tunneling in p+ amorphous silicon carbide n crystalline silicon heterojunction solar cells

We used the internal photoemission (IPE) technique to accurately determine the valence and conduction band offsets at the a-SiC:H/c-Si interface and investigated with numerical simulations their effects on the photocarrier collection in p+ a-SiC:H/n c-Si heterojunction solar cells. The valence and conduction band discontinuities were found to be 0.60 and 0.55 eV, respectively. However, despite the large barrier at the valence band edge, 30 nm p+ a-SiC:H/n c-Si heterojunction solar cells show no collection problems due to blocking of holes (FF=0.73). Combined IPE measurements and simulation results indicate that tunneling of holes through this barrier at the valence band edge can explain the unhindered collection.

Improvement in the spectral response at long wavelength of a-SiGe:H solar cells by exponential band gap design of the i-layer

A new band gap profile (exponential profile) for the active layer of the a-SiGe:H single junction cell has been designed and experimentally demonstrated. In this paper we compare its optical and electrical characteristics with the two more common profiles: the U- and V-shapes. As predicted by the simulations, the new profile combines the advantages of both profiles. Like the V-shape, the exponential shape reduces the amount of Ge in the i-layer, decreasing both the space charge defect density inside the i-layer and the recombination losses. It also improves the electric field. At the same time, the exponential shape generates the same current density as the U-shape.

Photocarrier collection in a-SiC:H/c-Si heterojunction solar cells

Abstract We measured the temperature dependent current–voltage (J–V) characteristics of p a-SiC:H/n c-Si heterojunction solar cells with different doping levels in the p a-SiC:H layer. For heterojunction solar cells with an undoped a-SiC:H layer, S-shaped J–V characteristics are observed at room temperature leading to poor factors. Below room temperature, such S-shape also appears for the cells with a highly doped p a-SiC:H layer. Detailed simulation studies showed that the origin of this S-shape lies in a reduced electric field in the c-Si depletion region, which, in combination with a hole accumulation at the interface, causes an increase in recombination losses. As long as the p a-SiC:H layer in these heterojunction cells is highly doped (≥1019 cm−3), these collection problems do not occur under standard operating conditions (i.e., room temperature and 100 mW/cm2 illumination).

Transport in tunneling recombination junctions: A combined computer simulation study

The implementation of trap-assisted tunneling of charge carriers into numerical simulators ASPIN and D-AMPS is briefly described. Important modeling details are highlighted and compared. In spite of the considerable differences in both approaches, the problems encountered and their solutions are surprisingly similar. Simulation results obtained for several tunneling recombination junctions made of amorphous silicon (a-Si), amorphous silicon carbide (a-SiC), or microcrystalline silicon (μc-Si) are analyzed. Identical conclusions can be drawn using either of the simulators. Realistic performances of a-Si∕a-Si tandem solar cells can be reproduced with simulation programs by assuming that extended-state mobility increases exponentially with the electric field. The same field-enhanced mobilities are needed in single tunneling recombination junctions in order to achieve measured current levels. Temperature dependence of the current-voltage characteristics indicates that the activation energy of enhanced mobilit...

Using computer modeling analysis in single junction a-SiGe:H p-i-n solar cells

In this article we discuss basic aspects of single junction a-SiGe:H p–i–n solar cells by coupling computer simulations with experimental characteristics. We are able to fit the dark illuminated current–voltage characteristics and the spectral response curves of a-SiGe:H p–i–n structures in the initial state, modeling the density of dangling bonds in each device layer by using either uniform density profiles or the defect pool model. Although we can fit these experimental curves with any of these two electrical models, band gap profiling in the a-SiGe:H intrinsic layer leads to improvement of the solar cell performance only when the defect pool model is implemented in our simulations. A U-shaped band gap profile is tailored in our samples by a staircase band gap profile composed of (i) several front band gap graded steps, (ii) one lowest band gap region, and (iii) several back band gap graded steps. Only by using the defect pool model are we able to predict an optimum thickness for the front band gap grad...

Thermal ideality factor of hydrogenated amorphous silicon p-i-n solar cells

The performance of hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells is limited, as they contain a relatively high concentration of defects. The dark current voltage (JV) characteristics at low forward voltages of these devices are dominated by recombination processes. The recombination rate depends on the concentration of active recombination centers and the recombination efficacy of each of these centers. The first factor causes the ideality factor of the devices to be non-integer and to vary with voltage. The temperature dependence of the dark current can be expressed by its activation energy. For microcrystalline silicon solar cells the activation energy varies with voltage with a so-called thermal ideality factor of 2. This value was derived for devices with a spatially uniform defect distribution and reflects the recombination efficacy. Here we present results of a thickness series of a-Si:H p-i-n solar cells. We have matched the experimental curves with computer simulations, and show that the voltage-dependent ideality factor curve can be used to extract information on the cross sections for electron and hole capture. Also, the activation energy is used as a measure for the mobility gap, resulting in a mobility gap for a-Si:H of 1.69?eV. We find a thermal ideality factor close to 2 for all samples. This is explained with a theoretical derivation, followed by a comparison between the internal electric field strength and the spatial variation of the defect density in the intrinsic layer. The thermal ideality factor is shown to be insensitive to the defect distribution and the recombination profile in the device. It is, therefore, an appropriate parameter to characterize a-Si:H p-i-n devices, providing direct insight on the recombination efficacy.

Errors introduced in a-Si:H-based solar cell modeling when dangling bonds are approximated by decoupled states

Abstract In this paper we investigate in a-Si:H-based devices the accuracy of approximating dangling bonds by pairs of donor-like and acceptor-like states. We discuss the impact of using this approximation in device modeling by studying the dark current–voltage, the illuminated current–voltage and the spectral response curves. We find that the relative error introduced by this approximation in these characteristic curves can be tolerated when the correlation energy is assumed to be positive and when the capture cross-section of neutral states adopted is much smaller than that of charged states. A wide range of intrinsic layer-thickness values, density of states and temperatures has been investigated. This approximation fails when the correlation energy adopted is negative, and is not accurate enough when the correlation energy is assumed to be positive but the capture cross-section of neutral states adopted is higher than that of charged states.