Charge-carrier photogeneration and extraction dynamics of polymer solar cells probed by a transient photocurrent nearby the regime of the space charge-limited current
To understand the complex behaviors of photogenerated charge carriers within polymer-based bulk-heterojunction-type solar cells, the charge-carrier photogeneration and extraction dynamics are simultaneously estimated using a transient photocurrent technique under various external-bias voltages, and a wide range of excitation intensities are analyzed. For this purpose, conventional devices with 80 nm thick active layers consisting of a blend of representative P3HT and PTB7 electron-donating polymers and proper electron-accepting fullerene derivatives were used. After the correction for the saturation behavior at a high excitation-intensity range nearby the regime of the space charge-limited current, the incident-photon-density-dependent maximum photocurrent densities at the initial peaks are discussed as the proportional measures of the charge-carrier-photogeneration facility. By comparing the total number of the extracted charge carriers to the total number of the incident photons and the number of the initially photogenerated charge carriers, the external quantum efficiencies as well as the extraction quantum efficiencies of the charge-carrier collection during a laser-pulse-induced transient photocurrent process were obtained. Subsequently, the charge-carrier concentration-dependent mobility values were obtained, and they are discussed in consideration of the additional influences of the charge-carrier losses from the device during the charge-carrier extraction that also affects the photocurrent-trace shape.
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