| Nowadays, there is a pressing need for developing solar cell technologies, for either environmental or economic factors. Benifiting from their solution processing techniques, the compatibility with lightweight, flexible substrates and roll-to-roll manufacturing, organic polymer solar cells which emerged in the 1990’s have greatly promoted the development of this field. However, the organics suffer from their limited spectral absorption. Relatively, quantum dot(QD) based photovoltaic devices can matching the broadband solar spectrum through bandgap engineering.With the support of National 863 Program(2011AA050509), we synthesized a series of Pb Se QDs with different sizes, and employed them in solar cell applications. Comparing to other nanomaterials, Pb Se QDs are capable of absorbing entire visible, near infrared, and short-wave-length infrared, which is benifiting from their large Bohr exciton radius. The charge carrier mobility of Pb Se film is high, and multiexciton generation(MEG) was observed in Pb Se solar cells. At the beginning of our experiments in 2013, the published Pb Se solar cells have to be fabricated under inert atmosphere, with tedious and wasteful processing for active layers. Also, the fill factor(FF) was low(around 40 %), and the air stability was poor(lost their efficiencies in 5 min). Thus, aiming to facilitate the device producing process and improve their FF and air stability, we designed and performed the experiments, with some good results obtained. For the critical points of innovation in this paper, through device structure design, we have achieved the highest short-circuit current(JSC) of 32 m A cm-2 and FF of 61 %, and improved air stability(drop to 60 % of the maximum 12 days later). In detail, we developed several targeted works as follow:1. In order to facilitate the device fabrication process, all solar cells involved here have active layer thicknesses well-controlled below 100 nm;2. The QD surface plays an important role in their stability; when the QD size decreases, the proportion of selenium increases, leading to better stability. Based on this, we introduced smaller Pb Se QDs to photovoltaics, and enhanced the device lifetime;3. The charge carrier transportation within solar cells could dramatically affect the device FF; we employed bandgap engineering in device structure design, which introduced an electron driving force in the active layer, realized the FF improvement;4. Thinner active layer will absorb less photon; we introduced an optical spacer(transparent metal oxide) to the device, through thickness adjustment of the optical spcer, we optimized the light distribution within the device, leading to enhancement of the device absorption;5. The band-gap energies of QDs changes along with their sizes; by optimizing the band-gap energy of active layers, we got the device exciton separation efficiency improved, resulted in better performed solar cells.6. Charge recombination in solar cells will greatly harm the device efficiency; through modifying the Pb Se/anode interface, the device hole transportation was improved, leading to a high FF of 61 %, which is the highest FF in Pb Se solar cells;7. In polymer/quantum dot hybrid solar cells(HSCs), the electron donor and acceptor materials play important roles(the semiconductor heterojunction type and their charge carrier mobility) in the device performance. Through optimizing the band-gap energy and mobility of the Pb Se QD film, we got an enhanced of the device efficiency, which is a ten-fold increase compared to the previously reported value.To summarize, from the point of view of device structure design, we have overcame the complex device fabrication process, the low device FF, and the poor device air-stability. This study may provide references for further optimization of the performance of QD solar cells. |
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