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Film morphology greatly influences the performance of bulk-heterojunction (BHJ)-structure-based solar cells. It is known that an interpenetrating bicontinuous network with nanoscale-separated donor and acceptor phases for charge transfer, an ordered molecular packing for exciton diffusion and charge transport, and a vertical compositionally graded structure for charge collection are prerequisites for achieving highly efficient BHJ organic solar cells (OSCs). Therefore, control of the morphology to obtain an ideal structure is a key problem. For this solution-processing BHJ system, the solvent participates fully in film processing. Its involvement is critical in modifying the nanostructure of BHJ films. In this review, we discuss the effects of solvent-related methods on the morphology of BHJ films, including selection of the casting solvent, solvent mixture, solvent vapor annealing, and solvent soaking. On the basis of a discussion on interaction strength and time between solvent and active materials, we believe that the solvent-morphology-performance relationship will be clearer and that solvent selection as a means to manipulate the morphology of BHJ films will be more rational. [ABSTRACT FROM AUTHOR]

The experimental current is scaled by the generation, transportation and recombination loss of the mobile carriers. In this paper, we show that a change of small molecule/perylene diimide (PDI) weight ratio from 1:1 to 3:1 does not lead to different transportation or different recombination loss in the solar cell, while it results in different generation of the mobile carriers. With the increase of the donor/acceptor (D/A) ratio, the blend film has an enhanced absorption because the donor has a stronger light-harvesting ability than the PDI acceptor. With respect to the contribution from the enhancement of the absorption of the solar photons, our data demonstrate that the phase size of the acceptor (donor) domains is the key factor that determines the generation of the mobile carriers. We observe a good relationship between the average short-circuit current density ( JSC) and phase size of the acceptor domains. Through fine-tuning the D/A ratio, the best compatibility between the small molecule donor and the PDI acceptor is obtained at a mediate D/A ratio of 1.3:1, at which the absorbed solar photons are exploited efficiently, yielding an average power conversion efficiency of over 5.07 ± 0.10%. Compatibility and charge separation are modulated by varying donor weight ration in small molecule donor:perylene diimide based non-fullerene solar cells, which affords improved exploitation of the solar photons absorbed by the photoactive layer. The acceptor phase size is a key factor that scales the photocurrent. An efficiency of 5.1% is obtained from this non-fullerene small molecule system as a result of modulation of compatibility. [ABSTRACT FROM AUTHOR]

We chose DPP-BDT-DPP {DPP=diketopyrrolopyrrole, BDT=4,8-di-[2-(2-ethylhexyl)-thienyl]benzo[1,2- b:4,5- b′]dithiophene} as a model backbone and varied the anchoring groups [C5H11, COOCH3, and SiCH3(OSiCH3)2] terminated on the N-substituted alkyl-chain spacer of the DPP units to study the effect of anchoring terminals on the morphology of blend film and on the device performances of bulk heterojunction solar cells. By replacing the nonpolar C5H11 anchoring terminal with the polar COOCH3 anchoring terminal leads to an enhancement in the short-circuit current density ( Jsc) (4.62 vs. 9.32 mA cm−2), whereas the value of Jsc sharply decreases to 0.45 mA cm−2 if the C5H11 anchoring terminal is replaced by a SiCH3(OSiCH3)2 group. The changes in Jsc are associated with changes in the π-π stacking distance (3.39→3.34 Å vs. 3.39→3.45 Å) and the phase size (50→20 nm vs. 50→>250 nm) through alteration of the anchoring group from C5H11 to COOCH3 versus from C5H11 to SiCH3(OSiCH3)2. Interestingly, the anchoring terminals bring about drastic changes in molecular orientations, which result in different out-of-plane hole transport. This is the first time this effect has been systemically demonstrated to improve photocurrent generation by manipulating the dipolar anchoring groups terminated on the alkyl-chain spacer. [ABSTRACT FROM AUTHOR]

Highly crystalline TiO 2 nanotube arrays (TNTAs) film is easily realized by treating its amorphous film under the hydrothermal vapor (HV) environment. The freestanding TNTAs film can be lift-off from the Ti foil, which serves as photoanode based on FTO substrate after the transplanting procedure. Compared to the thermal annealing (TA) treated TNTAs photoanode, the HV treated photoanode possesses some excellent properties, such as long optical path, fast charge transporting and easy adsorption of the sensitizer molecules, which can result in high short current density. Notably, the N719 dye sensitized HV treated photoanode prepared under 180 °C shows relatively high photovoltaic response in the visible range due to its hierarchical structure, which gives an impressive performance conversion efficiency of 8.11% without any surface modification. And the present work sheds light on the application of the mild crystalline strategy for performance improvement of the DSSCs based on TiO 2 nanotube arrays photoanode. [ABSTRACT FROM AUTHOR]