Efficiency Enhancement of Polymer Solar Cells Based on Poly(3‐hexylthiophene)/Indene‐C70 Bisadduct via Methylthiophene Additive

Photovoltaic performance of polymer solar cells based on poly(3‐hexylthiophene) (P3HT) as the donor and indene‐C₇₀ bisadduct (IC₇₀BA) as the acceptor is improved by adding 3 vol% 3‐methylthiophene (MT) or 3‐hexylthiophene (HT) as processing additives. The results of UV‐vis absorption spectroscopy, X‐ray diffraction analysis and atomic force microscopy indicate that with the MT or HT processing additive, the active layer of the blend of P3HT/IC₇₀BA showed strengthened absorbance, enhanced crystallinity and improved film morphology. The power conversion efficiency (PCE) of the PSCs was improved from 5.80% for the device without the additive to 6.35% for the device with HT additive and to 6.69% with MT additive. The PCE of 6.69% is the top value reported so far for the PSCs based on P3HT.     

Accurate Characterization of Triple‐Junction Polymer Solar Cells

Triple‐junction device architectures represent a promising strategy to highly efficient organic solar cells. Accurate characterization of such devices is challenging, especially with respect to determining the external quantum efficiency (EQE) of the individual subcells. The specific light bias conditions that are commonly used to determine the EQE of a subcell of interest cause an excess of charge generation in the two other subcells. This results in the build‐up of an electric field over the subcell of interest, which enhances current generation and leads to an overestimation of the EQE. A new protocol, involving optical modeling, is developed to correctly measure the EQE of triple‐junction organic solar cells. Apart from correcting for the build‐up electric field, the effect of light intensity is considered with the help of representative single‐junction cells. The short‐circuit current density (J_SC) determined from integration of the EQE with the AM1.5G solar spectrum differs by up to 10% between corrected and uncorrected protocols. The results are validated by comparing the EQE experimentally measured to the EQE calculated via optical‐electronic modeling, obtaining an excellent agreement.     

Controlling Molecular Orientation of Naphthalenediimide‐Based Polymer Acceptors for High Performance All‐Polymer Solar Cells

Molecular orientation, with respect to donor/acceptor interface and electrodes, plays a critical role in determining the performance of all‐polymer solar cells (all‐PSCs), but is often difficult to rationally control. Here, an effective approach for tuning the molecular crystallinity and orientation of naphthalenediimide‐bithiophene‐based n‐type polymers (P(NDI2HD‐T2)) by controlling their number average molecular weights (M_n) is reported. A series of P(NDI2HD‐T2) polymers with different M_n of 13.6 ( P_L ), 22.9 ( P_M ), and 49.9 kg mol^?1 ( P_H ) are prepared by changing the amount of end‐capping agent (2‐bromothiophene) during polymerization. Increasing the M_n values of P(NDI2HD‐T2) polymers leads to a remarkable shift of dominant lamellar crystallite textures from edge‐on ( P_L ) to face‐on ( P_H ) as well as more than a twofold increase in the crystallinity. For example, the portion of face‐on oriented crystallites is dramatically increased from 21.5% and 46.1%, to 78.6% for P_L , P_M, and P_H polymers. These different packing structures in terms of the molecular orientation greatly affect the charge dissociation efficiency at the donor/acceptor interface and thus the short‐circuit current density of the all‐PSCs. All‐PSCs with PTB7‐Th as electron donor and P_H as electron acceptor show the highest efficiency of 6.14%, outperforming those with P_M (5.08%) and P_L (4.29%).