Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation

Figures & data

Figure 1. Normalised (a) PCE, (b) V _OC , (c) J _SC and (d) FF (to the initial maximum values) of BTR:PC₇₁BM devices, with active layers which have undergone increasing SVA time, as a function of photo-ageing time under one-sun equivalent illumination at room temperature in dry nitrogen.

Table 1. Performance of BTR:PC₇₁BM devices with the active layers treated for increasing SVA time.

SVA time (min)	J SC (mA/cm^2)	V OC (V)	FF (%)	PCE (%)
0	12.2	0.992	53.7	7.12
1	12.7	0.969	67.7	9.16
2	13.1	0.944	74.7	10.14
5	12.5	0.933	76.4	9.81
10	9.5	0.932	67.8	6.59

Figure 2. (a) GI-XRD spectra and (b) normalised GI-XRD spectra, of BTR:PC₇₁BM films with increasing SVA time, before and after photo-ageing.

Figure 3. UV–vis absorbance spectra of the fresh and photo-aged BTR:PC₇₁BM films: 0 min SVA, 2 min SVA and 10 min SVA films (a) without normalisation and (b) with normalisation to the PC₇₁BM peak at 378 nm.

Figure 4. (a) Raman spectra of BTR:PC₇₁BM films with increasing SVA time, before and after photo-ageing, (b) calculated Raman spectrum of BTR using B3LYP 6311G(d,p) with all alkyl side chains simplified to methyl groups, (c) normalised Raman spectra of BTR:PC₇₁BM films with increasing SVA time, before and after photo-ageing (inset shows zoomed-in of peak C), (d) normalised calculated Raman spectrum of BTR with different dihedral angle between the BDT and thiophene 4 unit using the same simulation method and (e) the chemical structure of BTR with numbered thiophenes for Raman peak assignment. The main backbone has dihedral angles ranging from ~15° to 25°. The thiophenes numbered as 4 are ~61° out of the plane of the BDT core.

Table 2. Assignments of the Raman peaks shown in Figure 4.

^Figure 4Peak	Vibrational mode
A	C-C stretching mode in ring of thiophene 1 and 2 (and 3 most likely) (like P3HT)
B	C=C of thiophene 1 and 2 and 3 (like P3HT)
C	C=C of fused thiophene and thiophene 4
D	BDT phenyl stretch (contribution from the fused thiophene – impossible for the phenyl ring to vibrate and the fused thiophene not vibrate as well) – (very tiny contributions from other thiophene)
E	Whole BDT unit, phenyl and fused thiophene, C=C in thiophene 1 and 2

Figure 5. (a) PL spectra corrected by its absorbance at 532 nm (corresponding to the excitation wavelength) and (b) normalised PL spectra of BTR:PC₇₁BM films with increasing SVA time before and after photo-ageing.

Figure 6. PCE drop, absorbance peak drop, relative Raman intensity drop and PL peak shift after the photo-ageing as a function of SVA time. For the GI-XRD FWHM, it is measured for fresh films (no obvious change after the photo-ageing).