Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

International audienceCrystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PC

Joule-Heating Annealing to Increase Organic Solar Cells Performance: A Comparative Study

In this work, we present our results on the influence of post-deposition treatments on the morphology and optical properties of photoactive films made of small molecules and their subsequent effect on the performance of photovoltaic (PV) devices. We have chosen DPP(TBFu)2:PC61BM as a benchmark model system and compared a novel joule-heating annealing (JHA) treatment with the widely used temperature annealing (TA) and solvent annealing (SVA) treatments. Detailed characterization of the morphology of the active layer and the performance of the devices suggests that JHA is a valuable post-treatment technique that provides fast information about the development of domains in the photoactive layer. Finally, in this context, solar cells on flexible indium tin oxide (ITO) substrates, made of polyethylene terephthalate (PET), were fabricated and analyzed

Solution processed organic solar cells based on A– D–D0–D–A small molecule with benzo[1,2-b:4,5-b0] dithiophene donor (D0) unit, cyclopentadithiophene

Solution processed small molecule A&ndash;D&ndash;D0&ndash;D&ndash;A, denoted as BDT(CDTRH)2, consists of 2-ethylhexoxy substituted BDT (donor D&#39; unit) as the central building block and 3-ethylrhodanine (RH) as the end capped terminal (acceptor group) unit, with a p-linkage of cyclopentadithiophene (CDT) (donor D). We have designed and synthesized it, and we have investigated its optical and electrochemical properties, finding that its energy levels are compatible with the energy levels of fullerene derivatives for efficient exciton dissociation. This small molecule has been used as an electron donor along with PC71BM as the electron acceptor for the fabrication of solution processed small molecule bulk heterojunction (BHJ) solar cells. The BHJ solar cell processed with BDT(CDTRH)2 : PC71BM (1 : 1 wt ratio) showed a power conversion efficiency (PCE) of 4.58% with Jsc &frac14; 8.66 mA cm_x0002_2, Voc &frac14; 0.98 V and FF &frac14; 0.54. The high Voc value of the device has been attributed to the deeper HOMO energy level of BDT(CDTRH)2. The overall PCE of the device has been increased up to 6.02% (Jsc &frac14; 10.42 mA cm_x0002_2, Voc &frac14; 0.94 V FF &frac14; 0.62) when the blend was processed with 3% v/v CN/CF solvent. This increase is mainly due to an increase in Jsc and FF, which was linked to the increase in crystallinity and favorable nanomorphology of the active layer improving exciton dissociation and achieving a more balanced charge transport in the device.</div

High open circuit voltage in efficient thiophene-based small molecule solution processed organic solar cells

International audienceWe have synthesized and fully characterized an oligothiophene small organic molecule for its use as electron donor moiety in solution processed bulk-heterojunction organic solar cells. Our results show that device solvent annealing process of the conjugated oligothiophene molecule leads to a light-to-energy conversion efficiency of 3.75% under standard illumination conditions. The solar cell presents open-circuit voltage and fill factors as high as 1.01 V and 63.05% respectively, which are among the highest values obtained for small molecule solution processed organic solar cells

Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells with 6.6% Efficiency Based on a Benzodithiophene Donor Core and Dicyano Acceptor Units

A novel semiconductor organic molecule, denoted as <b>VC89</b>, having A-D-D1-D-A structure, was synthesized and all relevant physical and chemical features for its application in solar cells were investigated. The structure comprises 2-ethylhexoxy substituted BDT (donor D1 unit) as a core and a dicyano acceptor unit (DCV) as the terminal acceptor group (A) linked through cyclopentadithiophene (CDT) (donor D) moiety. The BHJ OSC <b>VC89</b>:PC₇₁BM (1:2), processed with chloroform (CF) as solvent, showed an overall power conversion efficiency (PCE) of 4.63% with short circuit current <i>J</i>_SC = 9.28 mA/cm², open circuit voltage <i>V</i>_OC = 0.96 V, and fill factor (FF) = 0.52. When the active layer was processed using DIO as a solvent additive (3% v/v in CF), the corresponding solar cell showed a PCE of 6.05% with <i>J</i>_SC = 10.96 mA/cm², <i>V</i>_OC = 0.92, and FF = 0.60. The PCE was further improved to 6.66% with <i>J</i>_SC = 11.68 mA/cm², <i>V</i>_OC = 0.92, and FF = 0.62, when the DIO/CF (3% v/v)-processed active layer was treated with THF vapors (solvent vapor annealing, SVA). The increase in PCE was due to the enhancement in both the <i>J</i>_SC and FF due to the use of the dicyano groups as electron acceptor units. On one hand, <i>J</i>_SC is determined by the enhancement of the film light absorbance, which is reflected in a better IPCE and better charge collection. On the other hand, we show herein that the use of solvent annealing after treatment with chemical additives also leads to better nanomorphologies that substantially improve the solar cell efficiency