Small-molecule Organic Materials As Cathode Buffer Layers Toward Efficiency Enhancement Of Polymer Solar Cells

Polymer solar cells (PSCs) have been attracting considerable attention as promising renewable energy sources owing to their advantages of low-cost, high-flexibility, light-weight, and easy roll-to-roll fabrication. To enhance the power convernsion efficiency (PCE) of PSCs is always the focus of PSC research. Several practical methods toward efficiency enhancement of PSC have been developed, including synthesis of new donor/acceptor materials, the micro-morphology control of the photoactive layer, new design of the device structure, incoporation of buffer layers, and modification of electrodes. In the thesis, we focused on enhancing PCE of PSC devices by incorporating new small-molecule organic materials as cathode buffer layer (CBL), and carried out the following two works:(1) For the first time, we applied three amino (-NH2)-containing small-molecule organic materials—Biuret (C2H5N3O2), Dicyandiamide (DCDA, C2H4N4), and Urea (CH4N2O)—as novel CBLs in P3HT:PCBM bulk heterojunction (BHJ) PSCs, resulting in obvious efficiency enhancement. The amino-containing small-molecule organic material CBLs were inserted between the photoactive layer and cathode (Al) by spin-coating. Under the optimized condition (solution concentration:1.0mg/mL, spin-coating speed:3000rpm), PCE of the CBL-incorporated BHJ-PSC devices are3.84%,4.25%and4.39% for biuret, DCDA and urea, which are enhanced by～15%,～27%and～31%respectively compared to the reference P3HT:PCBM BHJ-PSC device without any CBL.. The efficiency enhancement originates from the enhancements of short current density (Jsc) and fill factor (FF), and enhancement ratio of Jsc is13%,11%and15%, while the enhancement ratio of FF is2%,11%and12%, respectively. Upon inserting the amino-containing small-molecule organic material CBLs, an interfacial dipole layer with its negetive charge end toward the photoactive layer and the positve charge end toward the cathode (Al) forms, which may decrease the energy level offset between the LUMO level of the PCBM acceptor and the work function of Al and consequently facilitate the electron extraction and collecting by Al cathode. Furthermore, the difference on the enhancement effect of biuret, DCDA and urea is due to their difference on the work function matching with P3HT:PCBM, and is also proposed to partially originate from their difference on the chemical structure specifically the number of the amino groups.(2) Triethyl citrate (C12H20O7) was applied as CBL for PSCs based on P3HT:PCBM or PCDTBT:PC70BM, affording efficiency enhancement of PSCs. Our preliminary results revealed that, under the optimized spin-coating condition (solution concentration:1.0mg/mL, spin-coating speed:3000rpm), triethyl citrate formed CBL between the photoactive layer and cathode (Al), and the PCE of PSCs devices based on P3HT:PCBM or PCDTBT:PC7oBM was enhanced by25%or11%, respectively. Interestingly, for P3HT:PCBM-based PSCs, the PCE enhancement originated from the improvments of both short current density (Jsc) and fill factor (FF); while the PCE enhancement of PCDTBT:PC7oBM-based PSCs was attributed to from the increase of both open circuit voltage (Voc) and FF. Further study is need to understand the underlying mechanism.