Theoretical design of low bandgap donor–acceptor (D-A) monomers for polymer solar cells: DFT and TD-DFT study

Figures & data

Figure 1. Molecular structures of carbazole

Figure 2. Building units as donor/acceptor moieties

Table 1. Dihedral angle (ɸ), bond length (d_BL), and dipole moments (μ) for studied D-A monomers calculated by DFT/B3LYP/6-311 G level

S.No	POLYMER monomer	ɸ (in ^o)		dBL		μ in debye
		Gas	Sol	Gas	Sol	Gas	Sol
1	3,6-CB-BCO	150.97	149.43	1.47900	1.47902	5.2248	6.3972
2	3,6-CB-BCT	146.05	143.58	1.48203	1.48226	3.2858	4.0307
3	3,6-CB-BCS	147.15	144.63	1.48259	1.48289	2.5213	3.0752
4	3,6-CB-OCP	153.84	152.27	1.47451	1.47358	5.1631	6.1115
5	3,6-CB-TCP	148.46	146.62	1.47795	1.47760	4.1281	4.9334
6	3,6-CB-SCP	149.52	146.87	1.47865	1.47851	4.1269	5.0780
7	3,6-CB-ODP	179.99	179.99	1.46392	1.46181	5.0268	6.2897
8	3,6-CB-TDP	179.97	179.99	1.47019	1.49931	4.8143	5.7728
9	3,6-CB-SDP	179.99	179.98	147,122	1.49715	5.1966	6.7039

Figure 3. Optimized Molecular structures obtained by DFT/B3LYP/6-311 G of the 3, 6 linked carbazole copolymer monomers (D-A) in the gas phase

Figure 4. Bond length between the donor and acceptor in gas and solvent for D-A copolymer monomers calculated by DFT/B3LYP/6-311 G level

_{Table 2.} Quаdruроle mоments (in Debye) оf 3, 6-саrbаzоle bаsed роlymer mоnоmers, саlсulаted by B3LYР/6-311 G methоd

Gas
Polymer monomer	XX	YY	ZZ	XY	YZ	XZ	Qii	Q
3,6-CB-BCO	−130.19	−109.22	−130.35	22.36	−5.87	3.83	−123.253	−20.97
3,6-CB-BCT	−130.72	−112.77	−139.74	6.70	−4.91	−3.91	−127.743	−17.95
3,6-CB-BCS	−122.64	−119.91	−143.51	−5.43	−3.80	0.36	−128.687	−2.73
3,6-CB-OCP	−122.85	−119.98	−128.60	−12.28	0.39	−1.42	−123.810	−2.87
3,6-CB-TCP	−115.41	−126.99	−135.77	−0.88	1.42	−1.35	−126.057	11.58
3,6-CB-SCP	−111.88	−132.61	−141.47	0.14	−2.35	−0.97	−128.653	20.73
3,6-CB-ODP	−113.37	−121.77	−126.96	−13.35	0.36	−1.47	−120.712	8.4
3,6-CB-TDP	−106.23	−128.86	−134.09	−2.85	1.29	−1.35	−123.062	22.63
3,6-CB-SDP	−104.38	−134.37	−139.74	−2.98	−1.94	−1.32	−126.163	29.99
Solvent
3,6-CB-BCO	−131.38	−106.70	−130.41	27.12	−6.93	4.02	−122.834	−24.68
3,6-CB-BCT	−131.45	−110.91	−136.54	13.40	−2.02	3.01	−126.312	−20.54
3,6-CB-BCS	−129.91	−114.33	−141.18	−6.48	0.89	−3.25	−128.473	−15.58
3,6-CB-OCP	−126.38	−117.75	−129.47	15.70	−3.20	6.83	−124.533	−8.63
3,6-CB-TCP	−123.11	−123.34	−135.92	3.02	1.34	6.44	−127.457	0.23
3,6-CB-SCP	−119.71	−129.39	−140.43	0.70	2.69	−6.44	−129.843	9.68
3,6-CB-ODP	−115.47	−120.32	−127.61	−15.37	0.00	−0.00	−121.133	4.85
3,6-CB-TDP	−109.49	−125.45	−135.59	3.25	−0.01	−0.01	−123.510	15.96
3,6-CB-SDP	−106.41	−131.55	−140.82	1.78	0.00	−0.00	−126.261	25.14

Figure 5. Calculated HOMO and LUMO energy values (eV) at the DFT/B3lYP/6-311 G level for 3,6 linkage carbazole copolymer monomers (D-A) in gas (a) and in Solvent (b)

Table 3. Calculated E_HOMO, E_LUMO levels, energy gap (E_g) values of the studied monomers obtained by DFT/B3LYP/6-311 G level

D-A polymer monomers	Gas			Solvent
	HOMO eV	LUMO eV	Eg	HOMO eV	LUMO eV	Eg
3,6-CB-BCO	−5.6318	−2.6662	−2.9656	−5.6079	−2.8083	−2.7996
3,6-CB-BCT	−5.4906	−2.6573	−2.8333	−5.5298	−2.7767	−2.7530
3,6-CB-BCS	−5.4070	−2.5612	−2.8458	−5.4759	−2.6681	−2.8077
3,6-CB-OCP	−5.8049	−3.1582	−2.6466	−5.7238	−3.2693	−2.4545
3,6-CB-TCP	−5.6530	−3.1237	−2.5294	−5.5926	−3.1999	−2.3928
3,6-CB-SCP	−5.5722	−3.0181	−2.5541	−5.5926	−3.0796	−2.5130
3,6-CB-ODP	−5.9347	−3.5161	−2.4186	−5.8702	−3.5841	−2.2861
3,6-CB-TDP	−5.7578	−3.4709	−2.2869	−5.6332	−3.7765	−1.8567
3,6-CB-SDP	−5.6751	−3.3648	−2.3103	−5.6013	−3.6377	−1.9636

Figure 6. The contour plots of HOMO and LUMO orbitals are calculated by DFT/B3LYP/6-311 G of the 3,6 linkage carbazole copolymer monomers (D-A) in the gas phase

Table 4. First singlet exсitаtiоn energy (E_oрt), exсitоn binding energy (E_B), аnd Triplet excitation energy (E_T) in eV

Polymer monomers	Gas			Solvent
	E^opt	EB	ET	E^opt	EB	ET
3,6-CB-BCO	2.1806	0.7850	1.4537	1.9092	0.8904	1.2728
3,6-CB-BCT	2.4057	0.4276	1.6038	2.2747	0.4783	1.5165
3,6-CB-BCS	2.4135	0.4323	1.6090	2.3250	0.4827	1.5500
3,6-CB-OCP	2.2869	0.3597	1.5246	2.0361	0.4184	1.3574
3,6-CB-TCP	2.1318	0.3976	1.4212	1.9977	0.3951	1.3318
3,6-CB-SCP	2.1504	0.4037	1.4336	2.0644	0.4486	1.3763
3,6-CB-ODP	1.4444	0.9742	0.9629	1.9197	0.3664	1.2798
3,6-CB-TDP	1.9692	0.3177	1.3128	1.8593	−0.0026	1.2395
3,6-CB-SDP	1.4146	0.8957	0.9431	1.9178	0.0458	1.2785

Table 5. The орen-сirсuit vоltаge V_ОС (eV) аnd LUMО-DОNОR(LD)−LUMОА-ССEРTОR (LA) of the studied D-A monomers оbtаined by B3LYР/6-311 G basis set

Polymer monomers	Gas		Solvent
	VOC (eV)/PC60BM	LD − LA(PC60BM)	VOC (eV)/PC60BM	LD − LA(PC60BM)
3,6-CB-BCO	1.0318	1.6338	1.0079	1.4917
3,6-CB-BCT	0.8906	1.6427	0.9298	1.5233
3,6-CB-BCS	0.8070	1.7388	0.8759	1.6319
3,6-CB-OCP	1.2049	1.1418	1.1238	1.0307
3,6-CB-TCP	1.0530	1.1763	0.9926	1.1001
3,6-CB-SCP	0.9722	1.2819	0.9926	1.2204
3,6-CB-ODP	1.3347	0.7839	1.2702	0.7159
3,6-CB-TDP	1.1578	0.8291	1.0332	0.5235
3,6-CB-SDP	1.0751	0.9352	1.0013	0.6623

Table 6. Eleсtrоniс trаnsitiоn dаtа оbtаined by the TD/DFT-B3LYР/6-311 G саlсulаtiоn fоr аll D-А mоnоmers in the gаs аnd sоlvent

Polymer	state	λmax	f	MO/character	% Contribution
Gas
3,6-CB-BCO	S0 → S1	568.59	0.1549	HOMO → LUMO	99.31
3,6-CB-BCT	S0 → S1	515.39	0.1221	HOMO → LUMO	99.09
3,6-CB-BCS	S0 → S1	513.71	0.1221	HOMO → LUMO	98.99
3,6-CB-OCP	S0 → S1	542.15	0.1510	HOMO → LUMO	99.32
3,6-CB-TCP	S0 → S1	581.60	0.1118	HOMO → LUMO	99.25
3,6-CB-SCP	S0 → S1	576.57	0.1113	HOMO → LUMO	99.16
3,6-CB-ODP	S0 → S1	584.96	0.1643	HOMO → LUMO	93.48
3,6-CB-TDP	S0 → S1	629.60	0.1633	HOMO → LUMO	97.59
3,6-CB-SDP	S0 → S1	624.92	0.1667	HOMO → LUMO	98.41
Solvent
3,6-CB-BCO	S0 → S1	529.43	0.2198	HOMO → LUMO	99.28
3,6-CB-BCT	S0 → S1	545.06	0.1660	HOMO → LUMO	98.97
3,6-CB-BCS	S0 → S1	533.27	0.1717	HOMO → LUMO	98.93
3,6-CB-OCP	S0 → S1	608.92	0.2174	HOMO → LUMO	99.50
3,6-CB-TCP	S0 → S1	620.65	0.1664	HOMO → LUMO	99.16
3,6-CB-SCP	S0 → S1	600.57	0.1676	HOMO → LUMO	99.08
3,6-CB-ODP	S0 → S1	645.86	0.3025	HOMO → LUMO	99.01
3,6-CB-TDP	S0 → S1	666.83	0.2811	HOMO → LUMO	99.58
3,6-CB-SDP	S0 → S1	646.49	0.2849	HOMO → LUMO	99.61

Figure 7. Simulated UV-Visible optical absorption spectra of the studied carbazole copolymer monomers (D-A) calculated by TD/DFT/B3LYP/6-311 G level in the gas phase

Figure 8. Simulated UV-Visible optical absorption spectra of the studied carbazole copolymer monomers (D-A) calculated by TD/DFT/B3LYP/6-311 G level in the solvent phase