Toward the Rational Design of Organic Solar Photovoltaics: Application of Molecular Structure Methods to Donor Polymers

Abstract

Conjugated polymers are promising candidates in the design of polymer solar cell materials with suitable electronic properties. Recent studies show that the use of different functional groups as side chain in thiophene-based polymers changes the electronic and conformation structures. Here we design new thiophene-based molecules by replacing the hydrogen attached to the backbone of P3MT with electron-donating and electron-withdrawing groups. We then calculate the HOMO, LUMO, and HOMO–LUMO energy gap to quantify the theoretical merit of the new polymers as solar absorbers and their inter-ring torsional potential to understand their suitability to link together in high conductivity, extended conjugated systems. Calculations are done with first-principles density functional theory (DFT), implemented using B3LYP with dispersion function and 6-31G(d,p) as basis set. Our results show that the HOMO–LUMO gap is sensibly lowered by donating groups and we found that the substitution of the hydrogen with −NH 2 , and −F gives an energy gap lower than the energy gap of P3MT. The lowest energy gap was found when substituting with −NH 2 . Electron-withdrawing groups lower the HOMO, with the overall lowest found when −NO 2 is used. −COCl, −CONH 2 , and −Cl give a steric hindrance greater than that of PTB7, which is set as reference. Our calculations show a possible approach to the rational design of donor materials when substituents are inserted systematically in a generic oligomer.

Alternative Functionalization of Hydrogen‐Bonded Diketopyrrolopyrrole Derivatives**

“…The measured values of frontier orbitals are in good agreement with structurally similar functionalized DPP previously reported by us and other research groups and within the theoretical values for efficient solar cells. [22,[36][37][38] The molecular design difference between TDPPT-C and TDPPT-N resulted in a distinct morphology for each derivative, as illustrated by the transmission electron microscopy (TEM) images presented in Figure 3. The images depict the structures of both derivatives in chlorobenzene and toluene at the same concentration.…”

Novel benzodithiophene-TTBTBTT copolymers: synthesis and investigation in organic and perovskite solar cells

“…Energy & Fuels PAPER of conjugated polymers could lead to a high open-circuit voltage (V oc ) and efficient photocurrent generation in OSCs. [8][9][10][11][12][13] Tuning the frontier energy levels of conjugated polymers is also crucially important for the development of polymeric holetransport layer (HTL) materials for efficient and stable perovskite solar cells. [14][15][16] In particular, it is necessary to align the HOMO energy of the polymeric HTL material with the perovskite valence band to ensure efficient and selective extraction of holes from the absorber layer of PSCs.…”

“…7 In particular, deep-lying HOMO energy levels and a small band gap of conjugated polymers could lead to a high open-circuit voltage ( V oc ) and efficient photocurrent generation in OSCs. 8–13…”

Synthesis and Characterization of Polymers Containing Ethynylene and Ethynylene-Thiophene Based Alternating Polymers Containing 2,1,3-Linked Naphthothiadiazole Units as Acceptor Linked with Fluorine as Donor: Electrochemical and Spectroscopic Studies

“…The chemical structure of polymers dictates their optical and electronic properties which determines their photovoltaic performance in devices [ 1 , 14 ]. It is well recognized that the tuning of the energy band gaps and energy levels of conjugated polymers by tailoring their chemical structure is an effective way to achieve low band gaps in polymers for high performance.…”