Achieving 6.7% Efficiency in P3HT/Indene‐C70 Bisadduct Solar Cells through the Control of Vertical Volume Fraction Distribution and Optimized Regio‐Isomer Ratios

Indene C-60 and C-70 bisadducts (IC(60)BA and IC(70)BA) have relatively high lowest unoccupied molecular orbital energies. In poly(3-hexylthiophene) (P3HT)-based polymer solar cells (PSCs), this produces an increase in open-circuit voltage (V-OC) and power conversion efficiency (PCE). However, ICBA synthesis produces a mixture of regio-isomers with different indene spatial orientations (2, 5, and 12 o'clock) that alter the IC(70)BA molecular packing when mixed with P3HT. In this paper, how the IC(70)BA regio-isomerism affects the PSC performance is examined by investigating the molecular packing of P3HT: IC70BA layers with different regio-isomeric ratios. For the first time, non-destructive spectroscopic ellipsometry is used to investigate the effect of the fabrication conditions on the P3HT/IC(70)BA vertical volume fraction distribution and the results are attributed to the spatial arrangement of the regio-isomers. It is demonstrated that this unambiguously affects the PSC performance. As a result, record device efficiencies are repeatedly attained for standard architecture P3HT: IC(70)BA PSCs with photoactive areas of 0.43 cm(2), achieving 5.9 (+/- 0.4)% PCE (n = 15). With control of the IC(70)BA constituent, device PCEs vary from below 2.2% to peak values above 6.7%, among the highest recorded PCEs for a P3HT combination, highlighting the importance of the molecular phase separation for high-efficiency devices.