Elecrically Detected Magnetic Resonance in A-Si:H Pin Cells
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ELECRICALLY DETECTED MAGNETIC RESONANCE IN A-SI:H PIN CELLS KLAUS LIPS AND WALTHER FUHS Fachbereich Physik und Wissenschaftliches Zentrum fOr Materialwissenschaften, Universit~t Marburg, Renthof 5, D-3550 Marburg, Germany ABSTRACT We report on a detailed study of EDMR in pin-type solar cells. Like in films the signals are dominated by the contribution of the e-db resonance. It is found that the spectra depend on the applied bias and photon energy of the exciting light. The data suggest that the dark current is controlled by recombination in the bulk of the i-layer. The sign of the signal depends sensitively on the sign of the internal field. At high forward bias and illumination recombination at the pi-interface plays an important role. Degradation by both light exposure and high forward current results predominantly from an increase of the bulk defect density. INTRODUCTION The performance of a-Si:H solar cells is determined by the quality of the intrinsic and doped layers, the top and back electrodes and their interfaces. The cell design affects the current limiting processes such as space charges, internal fields, barriers at the contacts and recombination in such a complicated way that single aspects of these processes are hardly accessible for simple experiments. A technique which is particularly attractive for the study of such problems is Electrically Detected Magnetic Resonance (EDMR). This method has been sucessfully applied to a-Si:H films by numerous groups [1-4] and led to a widely accepted model for the recombination mechanism in undoped material. In an EDMR experiment the current through a sample is recorded when the sample is brought into microwave resonance. Since the spin selection rule enters the transition rates between localized paramagnetic states, recombination will be delayed if the spins in the initial and final state are parallel. A resonant microwave field increases the number of pairs with anti-parallel spin orientation and thus enhances the transition rate. This results in a change of the conductivity. The paramagnetic centers participating in the transition can be identified by their g-value. Unfortunately, the intensity of the signal (AI/I) depends in a rather complicated way on the characteristic times involved namely the spin-lattice relaxation time, the lifetime of the spin pair and the thermal emission time from localized states. Nevertheless, in case the experiment is carried out at constant temperature the signal intensity is a direct measure of the transition rate [5]. Whereas EDMR of a-Si:H films has been studied quite frequently only little is known about spin-dependent processes in pin-type solar cells [6,7]. Here we report on a more detailed investigation which aims at identifying the current limiting processes in the device. SAMPLE PREPARATION AND EXPERIMENT The devices studied in this report were a-Si:H pin-type solar cells fabricated in a capacitively coupled rf glow discharge deposition system. The structure is glass/TCO/pSiC/i(0.5pm)/n-Si/Pd with an active area of 6.5mm2
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