Electroluminescence and Transport in a-Si:H p-i-n Diodes at Room Temperature
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ELECTROLUMINESCENCE AND TRANSPORT in a-Si:H p-i-n DIODES at ROOM TEMPERATURE R. CARIUS*, F. BECKER*, H. WAGNER* and J.-Th. Zettler** *Forschungszentrum Jilich, ISI-PV, Jilich; **TU Berlin; Germany ABSTRACT The transport and recombination of injected electrons and holes in a-Si:H p-i-n-solar cells is studied by simultaneous detection of electroluminescence and forward current, both steady state and time resolved. We have calculated the spatial distribution of the radiative recombination in the layer using an optical model that takes into account all layers of the solar cell as well as the substrate and the back contact. The validity of the results has successfully been tested by photoluminescence measurements using different excitation energies (penetration depths). By comparison between measured EL spectra and calculated spectra it is found that in a 0.61m thick cell at room temperature the radiative recombination occurs in a narrow region (90 nm) in the vicinity of the p-layer. Time resolved electroluminescence and forward current measurements show that at room temperature the forward current is first determined by the space charge limited current of the electrons subsequently followed by the much larger recombination current. The recombination current is determined by the injection of holes from the p-contact in agreement with results from device modelling. Radiative and nonradiative lifetimes of the injected carriers are determined. INTRODUCTION The understanding of transport and recombination processes in p-i-n diodes is of importance for the development of light emitting diodes, solar cells, multi-color sensors and other devices based on p-i-n structures. Previous studies have been carried out with emphasis on the investigation of the forward current under double injection conditions, both steady state and time resolved [1-71. Model calculations of the transient behavior of the forward current [5] have shown two distinct rises of the forward current related to 1) the transit of electrons and 2) the screening of the space charge by holes. More recently, investigations on the influence of the defect density, the defect profile and capture cross sections of deep states on the transient response of p-i-n and n-i-n devices have given more insight into the transition from a space charge limited to the bipolar recombination limited current flow [6]. In a previous publication [ 7] the simultaneous detection of the forward current and electroluminescence signal was used to investigate the correlation between the electroluminescence intensity and the forward current over a wide temperature range (20 K < T < 300 K). At room temperature the response time of the electroluminescence signal due to a square wave voltage pulse was found to be on the order of ten microseconds which is of interest for application in low voltage light emitting devices based on silicon. However, the detailed response of the forward current and the electroluminescence signal at room temperature has not been measured yet and is subject of this paper
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