Enhancing the structure and performance of P3HT/PC70BM polymer solar cells with n-dodecylthiol

Polymer solar cells (PSCs) have attracted much attention because of their low cost, high flexibility, lightweight, and large surface area compared to long-established silicon solar cells. In most cases, bulk heterojunction type solar cells (BHJ) use a phase-separated blend of organic electron donor and acceptor components, where a conductive polymer is the donor and a fullerene derivative is the acceptor. To achieve high performance BHJ-type PSCs, the electron-donating conjugated polymer needs to be strong absorbent of a wide range of solar light waves and possess good hole mobility. This results in short-circuit current (Jsc) and/or open-circuit voltage (Voc) at their optimum values.;In this research, n-dodecylthiol (0-5% vol.) was added to P3HT/PC70BM polymer solution to improve the crystallinity of P3HT and enhance the P3HT/PC70BM phase separation. Higher P3HT crystallinity reduces the amount of PCBM dissolved in the amorphous regions of P3HT, thus promoting the aggregation of PC70BM, which contributes to PC70BM/P3HT phase separation. Adding 2% n-dodecylthiol to the active layer resulted in forming the smallest polymer crystallites size L, which was nearly 11.2 nm at optimum annealing conditions at (150°C for 30 min in a vacuum atmosphere). The smaller crystallite size suggests a shorter path of the charge carriers between P3HT backbones, which increases the short circuit current (Jsc) and decreases the open circuit voltage (Voc) in the solar cells.;UV-Vis and EQE analysis showed enhancement of self-organization ability, which led to improved P3HT crystallinity and intensified phase separation of P3HT/PC70BM in polymer films. EQE increased due to enhanced hole and electron polaron mobility with n-dodecylthiol. AFM images showed increased surface roughness with adding n-dodecylthiol, yielding more spaces for P3HT crystallites to form, and hence resulting in higher crystallinity. DLS analysis of P3HT:PC70BM:n-dodecylthiol dissolved in chlorobenzene solution showed an increase of aggregate size by adding n-dodecylthiol; which confirms the SEM images. This also shows that n-dodecylthiol does not enhance the dispersion of P3HT:BC70BM in the chlorobenzene solution. Also, it shows that the more n-dodecylthiol is added, the more aggregation will be formed. In addition, increasing mixing time and temperature improves the mixing process and results in smaller aggregates.;Kinetics of cold crystallinity for P3HT:PC70BM using Avrami model showed an overall increase in crystallization rate (1/t0.5) with increasing the annealing temperature. The increase in phase separation balancing for exciton dissociation and charge transport and collection resulted in a 33% increase in solar cell efficiency when the volume fraction of n-dodecylthiol is 2%. The enhancement of cell performance after thermal annealing deteriorated at temperatures higher than 150 °C.