High open-circuit voltage in heterojunction photovoltaics containing a printed colloidal quantum-dot photosensitive layer

[摘要] (cont.) The high VOC is remarkable for an architecture with symmetric electrodes and exceeds the offset between the highest occupied molecular orbital (HOMO) of the acceptor (near 5.2 eV) and the lowest unoccupied molecular (LUMO) orbital of the QDs (near 4.6 eV). The internal quantum efficiency (IQE) exhibits a strong dependence on QD lm thickness and reaches a maximum of 30% at a thickness of 3-4 monolayers, indicating that transport losses dominate photocurrent generation for QD thicknesses above 4-5 monolayers. From the bias-dependence of quantum efficiency, we identify an intensity-independent compensation voltage V0 of 1.5 V that represents the maximum attainable VOC. Investigation of the bias-dependence of the photocurrent decay transients identifies charge diffusion as the dominant mechanism responsible for photocurrent generation and reveals a vast discrepancy between the time constant associated with charge extraction (0.6 s, measured at 0V) and that of recombination (0.4 [mu]s, measured at 2 V). An alternative model for VOC is presented that considers the dark current in forward bias as the critical mechanism determining VOC. We conclude that suppression of recombination across the spiro-TPD heterojunction interface forces recombination to occur predominantly in the QD lm. Electroluminescence from the QD layer recombination that hole injection from spiro-TPD into the QD layer and recombination in the QD layer is, in part, responsible for current ow in forward bias. Because the device architecture is straightforward and the fabrication techniques are simple, QD tandem cells are easily attained, furthering the prospect for high conversion efficiencies coupled with the potential for scaleable manufacturability.

[发布机构] Massachusetts Institute of Technology