Solution-processed hybrid solar cells using lead surfide colloidal quantum dots and zinc oxide nanowires
Haibin Wang a, Jotaro Nakazaki a, Takaya Kubo a, Hiroshi Segawa a b
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Poster, Takaya Kubo, 035
Publication date: 7th November 2016

Colloidal quantum dots (CQDs) are promising absorbers and carrier transporter for photovoltaics. This is because of high degree of freedom for controlling absorption band positions, and because the CQDs are compatible with solution-based technology. The power conversion efficiency of depleted heterojunction solar cells made up of ZnO electron transporter and CQDs has been steadily increasing, and reached over 11%. However, short carrier diffusion length of CQD films limits their film thicknesses, namely, light harvesting efficiency. Thus we focused on solar cells composed of PbS CQD/ZnO nanowire structures to achieve efficient carrier transport and light absorption. We then developed air-stable and high efficiency solar cells yielding 60% EQE at the first exciton peak (1020 nm) (H. Wang et al., J. Phys. Chem. Lett.(2013)). Here we construct PbS QD/ZnO NW solar cells with different PbS QDs giving the exciton absorption peaks between 700 nm and 1500 nm. An EQE of 26% is obtained in the solar cell made up of the CQD giving a 1500-nm exciton peak and an EQE onset of 1700 nm (0.73 eV~ Eg). Although Voc values of the solar cell decrease as the exciton peak shifts to longer wavelengths, the Voc of the solar cell giving a 1500-nm exciton peak (0.73 eV ~ Eg) remains approximately 0.4 V. It is interesting to compare the Voc with those of inorganic solar cells such as Si solar cells. The Voc of inorganic solar cells is known to empirically scale as Eg/q-0.4 (V), where Eg is the optical band gap (eV), and q the elementary charge. On the other hand, the colloidal PbS QD/ZnO NW solar cells yield the Voc higher by about 20% than 0.33 V estimated from the equation (0.73 (eV)/q-0.4 (V)). A series of experiments carried out on our solar cells indicates that the colloidal PbS QD/ZnO NW structure is useful for developing the bottom cells of multi-junction solar cells.

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