Some Kinds Of Donor Materials On Photovoltaic Performances For Organic Solar Cells

Organic solar cells have attracted great attention in the field of solar energy development and utilization as theirs advantages, such as low cost, simple device, easy fabrication, light weight, etc. In this article, for the designed a series of polymers and small-molecular donors, the electronic properties and optical absorptions were investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We explored the influence of electron-withdrawing or electron-rich units in polymer and small-molecular donors on their electronic properties and optical absorptions. In addition, the role of polymer or small-molecular donor in light-to-electricity of organic solar cells was also investigated. The calculated results indicated that rational adjustment on electron-withdrawing or electron-rich units of polymer and small-molecular donors is an efficient way to change the electronic properties and optical absorptions of donors for improving the organic solar cell efficiency. In this works, based on the investigations for properties in donor materials and effecting factors in light-to-electricity of organic solar cells, we attempted to provide some efficient strategies and theoretical foundation of molecular design for donor materials in organic solar cell applications. The major content and results in this paper are listed following.In Chapter 1, the developments on organic solar cell and donor materials were summarized briefly. Moreover, we analyzed the basic issue in the field of organic solar cells existed.In Chapter 2, the computational methods of DFT and TD-DFT which including fundamental and common functional were introduced briefly. In addition, the Marcus equation for electron transfer in photoelectric material applications and the theoretical characterization for light-to-electricity in organic solar cells were also introduced.In Chapter 3, taking the reported two polymers of DPPTT-T and DPPTT-S as a case, the structure, electronic properties, optical absorption of polymer donors and their performances in organic solar cells were investigated by DFT, TD-DFT and Marcus theory systematically. The power conversion efficiency of organic solar cells based polymer DPPTT-T as donor is larger than that of DPPTT-S based solar cell due to DPPTT-T donor-based solar cell have a larger short-circuit current. The calculated results indicated that the phenomenon of high short-circuit current for DPPTT-T donor-based solar cell may be caused by a fast electron-transfer rate of polymer DPPTT-T in light-to-electricity of organic solar cells. For the performances of polymer donors in electron-transfer, the results show that the arrangement between polymer donors and PC61BM acceptor is an important factor to effect on the electron-transfer and charge-recombination. For the performances of polymer donors in hole mobility, the results demonstrate that shortened the distance of ππ stocking and increased the projected degree of overlap for π-conjugated ring in neighboring molecules is a very efficient approach to enhanced the hole mobility. In this Chapter, investigations on properties of polymer donors and performances of polymer donors in light-to-electricity of organic solar cells can provide reliable theoretical model and guidelines for designing and screening high-efficiency donor materials in organic solar cell applications.Introduction of strong electron-withdrawing unit in donor-acceptor polymers is a very feasible approach to improve the electronic properties and optical absorptions of polymer donors and their performances in efficiency of organic solar cell. In Chapter 4, based on the reported polymers (PCPDT-BT and PDTPr-FBT) which consist of benzothiadiazole(BT) as electron-withdrawing group and different electron-rich units (dithienopyrrole (DTPr) and cyclopentadithiophene (CPDT)), we incorporated two strong electron-withdrawing fluoroine atoms or cyano on the BT to replace BT with fluorinated BT (FBT) and cyano BT(CNBT) in PCPDT-BT and PDTPr-FBT, respectively, and designed two series of donor-acceptor polymers as donor materials. According to the calculated results, the incorporation of strong electron-withdrawing units into donor-acceptor polymers not only can obviously decrease the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital energy levels (LUMO) of polymer donors which results in improving the open circuit voltage of organic solar cells, but also can improve the optical absorptions and hole mobility of polymer donors. Especially, the cyano-substituted polymers, PCPDT-1CNBT and PDTPr-1CNBT, exhibit the best properties, such as the lowest HOMO levels, narrowest band gaps, highest open circuit voltage, and largest hole mobility (3.67×10-3 cm2V-1s-1 and 8.05×10-4 cm2V-1s-1, respectively) in all corresponding systems. In addition, based on designed polymers PCPDT-1CNBT and PDTPr-lCNBT as donors, the predicted power conversion efficiencys of organic solar cells are ～7.2% and ～6.8%, respectively. Conclusively, introduction of strong electron-withdrawing unit in donor-acceptor polymers is a very feasible way to obtain high-efficiency polymer materials as donor for organic solar cell applications.In Chapter 5, the goal of this work is to adjust the electron-withdrawing acceptor units in donor-acceptor copolymers for obtaining high-efficiency donors in photovoltaic applications. To achieve this goal, based on the reported a series of donor-acceptor copolymers PCPDTTPD, PDTSTPD and PDTPTPD (mark as Pa1, Pa2 and Pa3, respectively) which consist of electron-rich C-, Si-, N-bridged bithiophene and electron-withdrawing thienopyrroledione (TPD), we replace nitrogen with oxygen or sulphur atoms on TPD unit to obtain two series of new donor-acceptor copolymers (Pb1-Pb3 and Pc1-Pc3). Compared with Pa1-Pa3 donors, the designed polymers of Pb1-Pb3 and Pc1-Pc3 exhibit better performances, such as smaller band gap, lower HOMO level, better optical absorption, higher open circuit voltage and higher hole mobility. Moreover, based on the designed polymers (Pb1, Pb2, Pb3, Pe1, Pc2 and Pc3) as donors in organic solar cells, the predicted power conversion efficiencys were～8.8%, ～10.0%,～8.4%,～8.5%,～9.9% and 7.9%, respectively. Conclusively, adjusting the electron-withdrawing acceptor units in donor-acceptor copolymers is an efficient strategy for obtaining high-efficiency donors in organic solar cell applications.In previous Chapter 4 and 5, the influence of incorporated strong electron-withdrawing groups or adjusted electron-deficiency capability in donor-acceptor polymers on their electronic properties and optical absorptions was investigated. We proved the feasibility of these strategies for molecular design. In Chapter 6, other effective strategy to modulate the electron-donating ability in donor-acceptor copolymers for improvement of organic solar cell performances was presented. In order to prove the strategy, starting form a series of reported donor-acceptor copolymers ((PCPDTBT(Pcl), PCPDTFBT (Pc2) and PCPDTDFBT (Pc3)) which consist of the electron-donating cyclopentadithiophene (CPDT) and each electron-withdrawing groups of benzo[c][1,2,5,]thiadiazole (BT),5-fluorobenzo[c][1,2,5]thiadiazole (FBT) and 5,6-difluorobenzo[c] [1,2,5]thiadiazole (DFBT), we replace electron-donating CPDT with dithienogermolodithiophene (DTTG) in donor-acceptor copolymers Pc1-Pc3, respectively, and construct a types of new donor-acceptor copolymers Pd1-Pd3. Compared with Pc1-Pc3 polymers, the designed copolymers Pd1-Pd3 not only can yield a more red-shift of absorption spectrum which results in a larger absorption region and an enhanced light-absorbing ability, but also can exhibit better electron transfer rates between polymer donor and PC61BM acceptor, higher hole transport rates and larger open circuit voltage. In addition, based on the designed polymers as donors in organic solar cells, the predicted power conversion efficiency can reaches up to ～8.4%. Therefore, the presented approach on the basis of modulating the electron-rich ability in donor-acceptor copolymer is a feasible strategy to improve their intrinsic properties of polymer donors and thereby realizing the purpose for improvement of organic solar cell efficiency.Organic solar cells based on small molecule materials as electron donors attracted less attention because of their power conversion efficiency behind that of polymer-based organic solar cells. Actually, in comparison to polymer donor materials, small molecular materials in organic solar cell applications have many advantages such as well-defined molecular structures, easier purification, tunable electronic properties, high charge mobility and better batch-to-batch reproducibility. In Chapter 7, diketopyrrolopyrrole-based small molecules with acceptor-core-acceptor (A-core-A) type as donor materials have been successfully used in organic solar cells. In this work, based on an A-core-A type molecules SMI consisting of diketopyrrolopyrrole unit as acceptor and benzene as core, we replace the benzene core with more electron-withdraw groups in SMI and further designed four new small-molecular donors (SM2-SM5) in order to improve electrical properties and optical absorption. Compared with SMI, the designed small-molecular donors SM2-SM5 exhibit batter performance with lower HOMO, narrower energy gap, larger absorption range, better electronic transfer between donor and acceptor and higher hole mobility. Moreover, the decreased HOMO levels and transition energy of small-molecular donors in organic solar cell applications play an important role in the parameters of open current voltage, fill factor and short-circuit current. Consequently, adjusting the electron-deficient ability of cores in A-core-A type small-molecular donors is an efficient approach which is used for obtaining high-efficiency small-molecular donors in organic solar cell applications.