Study on hole transporting layers and their interfaces for high performance perovskite solar cells

概要

In this doctoral study, we focused hole transporting mate materials (HTMs) and interface control methods, and conducted research as follows.

In Chapter I, we outlined current state of perovskite solar cells (PSCs).

In Chapter II, we studied on influence of O2 plasma treatment for NiOx layers in PSCs. This hydrophilic treatment made it possible to form the perovskite layer on the NiOx layer by the spin coating used precursor solution. We have revealed that NiOx films are sensitivity to O2 plasma treatment. When treating too strong, it can be assumed that the electronic state of NiOx layers were negatively affected. The power conversion efficiency (PCE) was reached to 12.3% in the case of optimized treatment was performed.

In Chapter III, we studied on influence of p-type dopants such as oxygen and tris(pentafluorophenyl)borane (BCF) for organic polymer hole transporting layers (HTLs) in mesoporous type PSCs. Mesoporous type devices which were employed dithiophene- benzene (DTB) copolymer without dopant as HTM were prepared under nitrogen environment. And then, these devices were measured J-V characteristics in air. As a result, initial PCE was shown very low value of 2.98% at 5 hours after exposed to the atmosphere. However, the device performance was gradually improved by keeping in dry air. And PCE was reached to 16.15% at 1 week after keeping. The cause of this phenomenon was guessed to be that oxygen in the atmosphere diffused into HTLs. It is considered that the carrier density increased due to the doping effect of oxygen and resistance of HTLs are decreased. The initial PCE was improved to 12% by the addition of BCF to HTLs because sufficient carrier density was obtained in initial stage. Further, the PCE was reached 16.89% by combining oxygen doping.

In Chapter IV, we studied on effect of charge recombination suppression due to inserting the passivation layer. When BCF doped poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6- difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT) was employed as HTM, the J-V curve was distorted and the fill factor (FF) was lower than when employed 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD). The highest occupied molecular orbital (HOMO) level of PPDT2FBT is higher than that of Spiro-OMeTAD. Therefore, charge recombination may have occurred on the HTL side because the conduction band level of the perovskite layer is close to the HOMO level of PPDT2FBT. To suppress charge recombination, we tried to insert the passivation layer which was formed by reacting the remaining PbI2 with formamidinium bromide dissolved in 2-propanol. As the result, the FF and the PCE were improved in the PPDT2FBT-based device by inserting the passivation layer. The valence band level of the passivation layer was slightly lower than that of original perovskite layer. For that reason, the passivation layer separates the conduction band level of the perovskite layer from the HOMO level of the PPDT2FBT layer. In addition, it is guessed that the frequency of charge recombination was reduced by making the flow of holes slightly gentler. The PCE of passivated PPDT2FBT device which was optimized composition of perovskite layer was reached to 18.4% comparable to Spiro-OMeTAD based device.

Finally, we consider the future prospects as follows. PCE of inverted planar type devices is lower than that of mesoporous type devices at the moment. However, inverted planar type devices possess more commercial advantages. Therefore, controlling wettability on the HTL of inverted planar type device is important when performing mass production using printing methods. In this doctoral study, it was found that increasing the carrier density by p-type doping and suppressing charge recombination by inserting the passivation layer were effective in polymer HTMs. By utilizing these results, it is expected that better devices will be obtained in the future.

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