The Research Of Highly Efficient Small Molecular Bulkheterojunction Solar Cell

Organic bulk-heterojunction solar cells have attracted intensive research interest worldwide as they can be a potential supplementary source of potential low-cost, renewable energy. Compared to the polymer counterparts, organic solar cells based on small molecules have several unique advantages due to their well-defined chemical structure and definite molecular weight, such as ease of purification, reduced batch-to-bath variations. Recently, either the single junction small molecular bulk-heterojunction(SM BHJ) solar cells or tandem SM BHJ solar cells has reached a milestone power conversion efficiency(PCE) of 10 %. However, the overall device performance of SM BHJ solar cells is still inferior to polymer counterparts. Here the doctoral degree candidate investigated the influence of the film morphology on the performance of the SM BHJ solar cells through the use of solvent vapor annealing. It was found thatthe device performance can be effectively improved upon the treatment. The work of the thesis can mainly divide into three parts.In the first part, we reported that the efficiency of SM BHJ solar cells based on blend of small molecular donor p-DTS(FBTTh2): acceptor PC71 BM can be enhanced to 8.3 % by dichloromethane(DCM) solvent vapor annealing(SVA) method. The efficiency is among one of the highest values for SM BHJ solar cells reported to date. Compared to the control devices, the short circuit current(Jsc), fill factor(FF) and PCE of the optimized devices are significantly enhanced upon DCM solvent vapor annealing. By combining the results of optical absorption, film morphology and charge carrier transporting properties, we identified that preferable film morphology and improved charge mobility were major reason for the enhancement, preferable film morphology and improved charge mobility can facilitate exciton dissociation, charge transport and collection. Besides DCM, we also investigated the effects of CF, C2 S SVA and 1,8-Diiodooctane(DIO) as additive on the performance of SM BHJ solar cells. Among all of the approaches used in this study, the DCM SVA is the most effective method to boost the device performance.In the second part, the influence of SVA effects on the performance of SM BHJ solar cells from a series of small molecular donors, such as DPPBIT, DPPBIT4 F, BIT, BIT-4F, BIT-4F-T were investigated. Despite there only exists a minor difference between the molecule structure of DPPBIT and DPPBIT4 F, the performance of SM BHJ solar cells based on these two donors is very different after SVA. Compared to the control devices, the PCE of the solar cells based on DPPBIT4F:PC71BM(1:1, W/W) was enhanced by nine times after SVA treatment, leading to a high PCE of 5.4 %. In addition, we found that the DCM SVA method is very effective in enhancing the performance of the solar cells from a varity of donors, such as BIT, BIT-4F, BIT-4F-T while using CN or DIO as additive, the device performance undergo moderate increase. As a result, a high PCE of 8.1% was achieved in the SM BHJ solar cells based on BIT-4F-T, with a FF of 75.5 %, which is one of the highest values for SM BHJ solar cells. By combining the results of optical absorption, film morphology and charge carrier transporting properties, we identified that enhancement of optical absorption, preferable phase separation and define structure were major reasons for the enhancement.In Chapter 5, the reduction of the open circuit voltage after SVA process was investigated. The solar cells were fabricated from a series of small molecular donor materials DR3 TBDTT, p-DTS(FBTTh2)2, BIT-4F-T or polymer donor materials PCDTBT,P3 HT, PBDTTT-C-T. Compared to the non-SVA-untreated devices, the treated devices showed obviousdrop in Voc, as a result of depleted steady carrier density and the concomitant lower quasi-Fermi levels for electron and hole transport. On the other side, we demonstrated the loss in Voc can be recovered by a consequent thermal annealing(TA). Furthermore, we investigated the SVA effect on the ternary BHJ solar cell based on DR3 TBDTT,PBDTTT-C-T and PC71 BM, in which the SVA process was also found to be able to improve the performance of the ternary solar cell. Based on the results of the energy level of DR3 TBDTT, PBDTTT-C-T and PC71 BM, we found that the mismatch the enery level between DR3 TBDTT and PBDTTT-C-T is major reason responsible for the low FF of the ternary solar cell after SVA process.