In this paper we discuss and model the effects of the density of defects in hydrogenated amorphous silicon from an electronics point of view. To this end, we have created a SPICE model that accounts for the two main field effects, Poole-Frenkel and tunnel, responsible for the leakage current. The comparison between our model and the experimental data shows that our approach allows a quick evaluation of the quality of the device with no need to run a complete steady state measurement. Also, we have validated our SPICE photodiode model by implementing it into a three CMOS simple pixel structure.

The evolution of excess defects in hydrogenated amorphous silicon p-i-n solar cells, induced by a forward current in the dark, has been studied by modeling their measured dark and illuminated current-voltage and quantum efficiency characteristics at different stages of degradation. Our electrical-optical model is based on the solution of Poisson’s and continuity equations. Modeling reveals that metastable defects are mainly produced in regions where tail-to-tail recombination of injected electrons and holes is high. These regions are characterized by either high defect density or low electric field. Simulation of experimental characteristics after 1h of current injection indicates that the spatial generation of current-induced defects is highly nonuniform, with the main defect formation occurring near the p∕i interface, and to a lesser extent towards the n∕i interface. Few defects are generated over the bulk intrinsic layer. Modeling of the characteristics after a longer duration of current injection indicates a broadening of the current-induced defect zone from the interfaces to the bulk intrinsic layer. After prolonged current injection, the density of excess dangling-bond defects in the bulk intrinsic layer increases significantly, while the defect density near the p∕i interface actually decreases, resulting in a more uniform distribution of excess metastable defects. Evidence from modeling suggests that some metastable defects have migrated from the interfaces towards the bulk. We thus conclude that prolonged current injection not only produces excess metastable defects, but also causes these defects to migrate to regions of lower defect density.

The reverse bias dark currents in pin solar cells having standard amorphous silicon (a-Si:H) or polymorphous silicon (pm-Si:H) as the intrinsic layer have been studied using experiments and modeling. Our electrical–optical model, takes into account high field enhancement of thermal generation via the Poole–Frenkel effect. Polymorphous silicon films have a higher hole mobility and a lower bulk density of states (DOS) than standard a-Si:H. However this does not result in lower dark leakage currents, except in thick (∼2.5 μm) pin diodes. For the latter, we have measured dark leakage currents as low as ∼3 pAcm−2 at −3 V, which to our knowledge is the lowest reported for such devices. Modeling reveals that the dark leakage current is controlled by thermal generation through the mid-gap defects in the intrinsic layer for a-Si:H cells, while it is dominated by the p/i interface defects in pm-Si:H cells. We conclude by estimating from the model, the dark leakage current possible in pm-Si:H diodes, if their p/i interface DOS were to be as in standard a-Si:H ones.

We summarize our current understanding of the optimization of PIN solar cells produced by plasma enhanced chemical vapour deposition from silane–hydrogen mixtures. To increase the deposition rate, the discharge is operated under plasma conditions close to powder formation, where silicon nanocrystals contribute to the deposition of so-called polymorphous silicon thin films. We show that the increase in deposition rate can be achieved via an accurate control of the plasma parameters. However, this also results in a highly defective interface in the solar cells due to the bombardment of the P-layer by positively charged nanocrystals during the deposition of the I-layer. We show that decreasing the ion energy by increasing the total pressure or by using silane–helium mixtures allows us to increase both the deposition rate and the solar cells efficiency, as required for cost effective thin film photovoltaics.

We have studied how the increase of pressure in the deposition of polymorphous silicon can reduce the ion energy and the damage of the p-layer in p–i–n solar cells. By increasing the pressure during the deposition of the i-layer to 3 Torr, we have been able to increase the fill factor and the solar cell efficiency up to 8.4%. In an attempt to reduce the optical gap and increase further the deposition rate we have studied the use of He dilution. Changing the dilution from hydrogen to He leads to a higher deposition rate at lower rf power while keeping similar transport properties. Moreover, the lower hydrogen content of He diluted films results in a smaller band gap and a higher short circuit current in solar cells. Thus, silane-He mixtures allow to reduce the ion energy for a constant deposition rate and to minimize the damage at the p-layer and p/i interface. The cells show a fill factor of 0.69, Voc of 0.91 V, an initial efficiency of 9.3% on textured SnO2.

Polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film, with a lower defect density of states and better electronic properties than standard amorphous silicon. We have studied the reverse-bias dark current in PIN structures using this material as the intrinsic layer and compared the results to amorphous silicon PIN devices. All the structures were grown using a standard plasma enhanced chemical vapor deposition process. For thick pm-Si:H devices, we have achieved reverse-bias dark current densities about ten times lower than those obtained using amorphous silicon as the intrinsic layer. This is consistent with the lower defect density of states in polymorphous silicon, which is about 7×1014 cm−3 against 5×1015 cm−3 for amorphous silicon. For a 2.5-μm-thick pm-Si:H diode, the current density obtained is as low as 3 pA cm−2 at −3 V. However, for thinner structures (∼0.5 μm), polymorphous and amorphous silicon show nearly the same reverse-bias leakage current. The experimental dark as well as illuminated characteristics of the diodes have been simulated using a model that incorporates field enhanced thermal generation under reverse-bias conditions (Poole–Frenkel effect). Results reveal that in pm-Si:H diodes, the P/I interface is much more defective than in standard a-Si:H PIN diodes. This fact is shown to completely mask the advantage of the lower defect density of pm-Si:H, in thin PIN diodes. However, in thick samples the electric field in the device and, therefore, also the Poole–Frenkel enhancement of thermal generation are smaller. The effect of the lower density of states in polymorphous silicon is then dominant, and we have achieved a dark current density of 3 pA cm−2 at −3 V for a 2.5-μm-thick diode, as already stated.

UV–visible imaging systems can be used for various applications, including homeland security systems and mobile phones. Compared to conventional CCD technology, CMOS-based active pixel sensors provide several advantages, such as high integration, low voltage operation, low power consumption and low cost. In this approach, we have recently developed a novel technology using polymorphous silicon. This new material, fully compatible with above-IC silicon technology, is made of nanometer size ordered domains embedded in an amorphous matrix. The quantum efficiency of detectors made of this nano-structured material reach up to 80% at 550 nm and 30% in the UV range, depending of the design and the growth parameters. Furthermore, a record dark current of 20 pA/cm2 at −3 V for 5 μm square pixels has been reached, and the sensor is perfectly linear over more than 6 decades of photocurrent. In addition, this new generation of sensors is significantly faster than their amorphous silicon counterparts.