Effects of Alkyl Terminal Chains on Morphology, Charge Generation, Transport, and Recombination Mechanisms in Solution‐Processed Small Molecule Bulk Heterojunction Solar Cells

Abstract

Length of the terminal alkyl chains at dicyanovinyl (DCV) groups of two dithienosilole (DTS) containing small molecules (DTS(Oct)2‐(2T‐DCV‐Me)2 and DTS(Oct)2‐(2T‐DCV‐Hex)2 ) is investigated to evaluate how this affects the molecular solubility and blend morphology as well as their performance in bulk heterojunction organic solar cells (OSCs). While the DTS(Oct)2‐(2T‐DCV‐Me)2 (a solubility of 5 mg mL−1) system exhibits both high short circuit current density (J sc) and high fill factor, the DTS(Oct)2‐(2T‐DCV‐Hex)2 (a solubility of 24 mg mL−1) system in contrast suffers from a poor blend morphology as examined by atomic force morphology and grazing incidence X‐ray scattering measurements, which limit the photovoltaic properties. The charge generation, transport, and recombination dynamics associated with the limited device performance are investigated for both systems. Nongeminate recombination losses in DTS(Oct)2‐(2T‐DCV‐Hex)2 system are demonstrated to be significant by combining space charge limited current analysis and light intensity dependence of current–voltage characteristics in combination with photogenerated charge carrier extraction by linearly increasing voltage and transient photovoltage measurements. DTS(Oct)2‐(2T‐DCV‐Me)2 in contrast performs nearly ideal with no evidence of nongeminate recombination, space charge effects, or mobility limitation. These results demonstrate the importance of alkyl chain engineering for solution‐processed OSCs based on small molecules as an essential design tool to overcome transport limitations.

Star-shaped D–π–A oligothiophenes with a tris(2-methoxyphenyl)amine core and alkyldicyanovinyl groups: synthesis and physical and photovoltaic properties

“…20 The topographic AFM images of the blended films determined for the optimized OSCs are given in Fig. 9.…”

“…The replacement of the active hydrogen by an alkyl group was found to be an efficient tool not only to increase the electrochemical stability of donor-acceptor small molecules, 15,16 but also to control their solubility and a number of important solid-state physical properties by simple adjustment of the alkyl chain length in the DCV substituent. 17,18 To fine-tune the energy levels in small molecules, some additional electron-donating blocks such as dithienosilole, [19][20][21][22] benzodithiophene, 23,24 and dithienopyrrole, 25 and etc., are often used. For the design of star-shaped molecules, a triphenylamine (TPA) core is one of the widely used electron-donating blocks due to its low cost, high electrochemical stability and hole transporting ability.…”

Integrated molecular, morphological and interfacial engineering towards highly efficient and stable solution-processed small molecule solar cells

“…Besides bulk recombination, 1,4,17,18 surface recombination is a further limiting loss mechanism for high performance OPV devices and was recently proven to be much more relevant than originally suggested. 19 Suitable interface layers are necessary to enable the loss-free extraction of the respective carriers at a rate higher than the bimolecular recombination rate.…”

“…S28b †) is complex and not fully understood, although multiple studies reported that the solution evaporation rate does impact the crystallization kinetics of dominantly the donor phase. 18,35 It illustrates that a reasonable explanation of the device characteristics should consider simultaneously both the molecular solubility and morphology. Since these DTS-based oligomers show a similar HOMO energy level (see Table 2), the V oc values for all of the devices are closely distributed around 0.90 V, with DTS(Oct) 2 -(2T-DCV) 2 being the only exception.…”

“…We would like to stress that all solar cells were processed under environmental conditions. Processing under nitrogen may result in higher performance numbers of over 5%, 18,30 since the residual solvents in a glovebox may induce unexpected SVA. Various processing techniques such as thermal annealing, solvent additives and SVA treatment were used in order to modify the BHJ morphologies in these systems and thus to optimize further their photovoltaic performance.…”