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Green energy has attracted much attention in recent years. Among them, the least polluted solar energy is a bright spot in green energy. Compared with the current silicon-based solar cells, solar cells with polymer materials have the advantages of simple process and large-area production and those with quantum dot (QD) materials have low cost and easy to adjust their band gaps. Therefore, solar cells with polymers and QDs as light absorbing materials are important advanced research topic in solar cells. It is expected to become the latest generation of green energy. This study is based on the improvent of power conversion efficiency (PCE) in polymer and QD hybrid solar cells, which are divided into three parts.
The first part: A series of novel electron transport (ET) polymers composed of different conjugated main chains (fluorene, thiophene, and 2,7-carbazole) and crown ether side chain (crown ether, aza-crown ether and amine) is presented for bulk-heterojunction polymer solar cells with poly(3-hexylthiophene) (P3HT) or poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b’] dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7) as the active polymer and aluminum metal as the cathode. Unexpectedly, it is found that the main chain of ET polymers has a greater effect on the interfacial dipole than the side chain, even when attaching a high polarity group. The electron-rich bridge atom of the main chain may also contribute appreciably to the interfacial dipole. When used as the ET layer, all of these polymers can generate an optical interference effect for redistribution of the optical electric field as an optical spacer and, therefore, allow more light to be absorbed by the active layer, thus leading to an increase in short-circuit current density (JSC). They can also block hole diffusion to the cathode and prevent electron-hole recombination during the ET process. Among the five ET polymers investigated, PCCn6 is the most effective one, providing a remarkable improvement in the PCE (measured in air) of the device to 8.13% compared to 5.20% for PTB7:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM).
The second part: we propose new treatments on the PbS QD layer surface by exposing it to air, water vapor and oxygen environments at room temperature. For air, water vapor and oxygen treatments, the lead oxides PbSO3, and PbSO4 are formed at the surface, while for air and oxygen treatments, PbO is additionally formed. These oxides are able to prevent charge recombination between PbS QDs, which results in higher open circuit voltage (VOC) and fill factor (FF) than those without treatment. The absence of lower conductivity oxide PbO in the water vapor treatment leads to JSC higher than those of air and oxygen treatments. In the hybrid tandem solar cell with PTB7-Th:PC71BM as the top cell and PbS QD as the bottom cell, the water vapor treatment leads to a PCE of 9.12%, which is higher than that without treatment (7.37%), with air treatment (8.70%) and oxygen treatment (8.47%). The PCE 9.12% is the new record PCE among the reported polymer QD hybrid tandem solar cells.
The third part: we found that the weight ratio of polymer blended with QD could affect the morphology of active layer. At a low polymer concentration, the agglomeration of polymer is found to locate within QD aggregate domains. When the concentration of P3HT is high, PbS QD aggregates cannot wrap up P3HT chains and P3HT chains are distributed outside the PbS domains, which results in disturbance of carrier transport pathway in PbS QD phase leading to low PCE. Moreover, addition of 1 vol% anisaldehyde as an additive to active layer (P3HT:OA-PbS) can significantly increase PCE from 4.14% to 4.94%. We introduce the polymer, PCCn6, as the electron transport layer, the device efficiency is further increased from 4.94% to 5.26%. This PCE 5.26% is the world record in P3HT:PbS QD hybrid solar cells.
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