The recently synthesized regio‐regular poly(3‐hexylthiophene‐2,5‐diyl) selectively deuterated on the main chain backbone and/or on the hexyl side chain have given the opportunity to record their infrared (IR) and Raman spectra, and to carry out a spectroscopic study supported by density functional theory calculations. The Effective Conjugation Coordinate associated with the collective C=C stretching mode, with Raman scattering and IR absorption near 1,450 cm−1, is used as a probe of the electronic structure and the molecular conformation of the chain backbone. With the help of the data collected from the deuterated species, the vibrational assignment for the structurally relevant 1,600–1,300 cm−1 region has been clarified. The excitation‐dependent wavenumbers and intensities of the C=C stretching Raman modes are discussed. Raman spectra excited in‐resonance or off‐resonance show the existence of a multimodal distribution of effective conjugation lengths that are ascribed to a phase hairy‐A with a practically flat backbone chain, a phase hairy‐B where the conformation of the backbone is slightly distorted, and an amorphous phase. IR spectra provide additional information on the Effective Conjugation Coordinate.

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Inverted organic solar cell (IOSC) devices with different volume ratios of In₂S₃ nanoparticles have been studied under local spectral irradiances in Malaysia with respect to that of AM1.5G. The J-V curves of encapsulated IOSC devices were measured outdoor using an Ivium Potentiostat and local spectral irradiances were acquired using an AVANTES spectrometer concurrently. All of the IOSC devices experienced significant improvement in power conversion efficiency (PCE) under the both local sunny and cloudy conditions with respect to the AM 1.5G, by 22-35% and 31-65%, respectively. From spectral analysis, the area under the graph of spectral irradiance in UV-visible region is significantly higher compared to infrared region for both local sunny and cloudy conditions, by 44.6% and 55.9%, respectively, while it is only recorded as 12.9% for AM 1.5G. Last but not the least, we have successfully verified the numerical analysis to predict device performance by comparing the simulated and measured PCE values for different irradiance intensities whereby the prediction of PCE is better under sunny condition with a deviation of 3.4-10.8% compared to cloudy conditions, with deviation of 28.9-30.5%.

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