Peculiarities of Space-Charge-Limited Photocurrent

Abstract

We demonstrate new advantages of the space-charge-limited photocurrent technique for the investigations of charge carrier recombination. Bimolecular recombination coefficient in a-Si:H estimated according to suggested method for both electrons and holes is presented.PACS numbers: 73.50. Fq, 73.50.Gr, 73.61.JcIn a time-of-flight (TOF) experiment, nonequilibrium carriers are generated by short electrical or optical pulse in thin semi-insulating sample, sandwiched between two electrodes with applied external dc electric field. As a result, transient current occurs, and from its dynamics peculiarities of carrier transport can be studied.The TOF technique can be used in the socalled "small-signal" regime, for which the charge of nonequilibrium carriers is smaller than the charge on the external electrodes (Q K CU0, where C is the sample capacity and U0 is the applied voltage). If Q > CU0 , space-charge-limited current (SCLC) results. Standard SCLC theory, describing injection current peculiarities is presented in [1]. Also, the SCLC regime is achievable for optical carrier generation by powerful laser pulses (photoSCLC regime), and can be used, with some limitations [2], to study the density-of-states [3] and bimolecular recombination [4] in disordered materials. However, it must be mentioned that comprehensive interpretation of the photoSCLC regime has not been worked out so far. In this contribution we continue our previous studies [4,5] and present detailed theoretical analysis of the photoSCLC regime. The results of our analysis are compared with data, obtained from photoSCLC measurements in amorphous hydrogenized silicon a-Si:H.In analyzing the photoSCLC regime, we use the following assumptions of the standard SCLC theory: the one-dimensional description of the sandwich-like sample geometry, uniform initial distribution of the electric field, and negligible influence of thermal carrier generation. Carrier trapping, monomolecular recombination and diffusion are neglected. The carriers are generated by instantaneous light pulse. The bimolecular recombination as well as different resistivities of the detection resistor are taken into account. We solve the total current equation

Ultrafast Charge Carrier Recombination in a-Si:H and μc-Si:H

“…1b it is evident that a charge reservoir is not created in the case of hole drift, as it was obtained also in earlier experiments [5,9]. Thus, extraction may be obtained for holes only if B`B L m p am n [10], but in a-Si : H this condition is fulfilled only when T b 300 K.…”