Enhanced Performance of Pseudo-Bilayer Organic Photovoltaic Devices via Small Molecule Doping

Abstract

Controlling both the film crystallinity of the active layer for better charge transport and the interdiffusion between donor and acceptor materials for optimal bicontinuous networks is essential in producing pseudo-bilayer polymer solar cells. In this work, we investigated the influence of a doping solution-processable small molecule with high carrier mobility, 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT), on the performance of pseudo-bilayer polymer solar cells made of an underlayer of poly(3-hexylthiophene) (P3HT) and an upper layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). By analysis of the X-ray diffraction and UV−vis absorbance spectra of P3HT:TES-ADT blend films it was demonstrated that the film crystallinity was enhanced by TES-ADT doping in the P3HT underlayer. The hole mobility extracted from the current density−voltage curves of hole-only devices based on P3HT:TES-ADT demonstrated an optimized value with proper TES-ADT doping and thermal annealing. An intermixed photoactive layer was observed for the annealed device, indicating the occurrence of interdiffusion with a large interfacial area. With improved film crystallinity and interdiffusion, the optimal device performance was obtained when 5% TES-ADT was blended with P3HT and a thermal annealing treatment at 150 °C for 1 min was conducted. At that optimal condition, the mean crystallite size was increased by 35%, and hence the enhancement of 8% and 14% in power conversion efficiency and short-circuit current density was observed, respectively.

Layer‐by‐Layer Processed Organic Solar Cells

“…Since Ayzner reported the first OPV based on LL solution processed P3HT/ PC 61 BM active layer by using orthogonal solvents of dichlorobenzene (DCB) and dichloromethane (DCM), which resulted in a PCE of 3.5% comparable to the traditional BHJ architecture, [33] many other groups used this method to fabricate OPVs, which yielded PCEs similar to or even better than the BHJ control devices. Among the materials selected for LL solution processing, P3HT/PCBM (or ICBA) system was mostly stu died, [95,[97][98][99][100][101][102][104][105][106][107][108][109][110][111]113,115,119,120,[124][125][126] and a remarkable PCE of 6.48% was achieved for P3HT/IC 70 BA system. [107] Other polymer donors, [103,112,114,116,127] small molecule donors, [118,123] and even non-fullerene acceptors were also used.…”

Synergies and compromises between charge and energy transfers in three-component organic solar cells

“…The thickness of P3HT and PCBM films was maintained at about 50 and 30 nm, respectively by using so-called "orthogonal solvents" (solvents that behave as good solvents for the layer to be deposited but that do not affect the morphology of the polymer below). 10,17 Optical Characterization. UV-Vis absorption and fluorescence spectra were recorded on thin films deposited on PET/ITO substrates, using a Specord S 600 (Analytik Jena, Jena, Germany) and Fluoromax-4 (HORIBA Jobin Yvon, Edison, USA) spectrofluorometer equipped with a 150 W xenon arc lamp as the excitation source, respectively.…”

“…This approach has already been successfully applied to realize traditional bi-component PHJs but, to the best of our knowledge, this is again the first time that it is applied to realize tc-PHJ-like structures. 10,17 Moreover, XPS depth profiling has been carried on the devices layers by following the C 1s, O 1s, S 2p and In 3d5/2 species (Figure 7). Although the spin-coating approach should not guarantee well-defined interfaces between each layer, our analysis allowed showing that no significant intermixing occurred between the different layers, with the P3HT and F8BT layers being of about 25 nm and 6 nm in thickness, respectively.…”

“…From the process point of view, realizing a PHJ device would allow easier scalability to large area manufacturing since of a better morphological and structural control than BHJ, which need the appropriate organization of donor and acceptor materials during the spin-coating or the post-production process. [10][11][12][13][14][15][16][17] Moreover, in the PHJs the free charge carriers can benefit from a continuous pathway to the electrodes, contrary to what happens in the BHJs where the presence of a large number of "insulated" domains does not ensure the charge transport. In addition, the PHJ has the further advantage that the donor and acceptor phase contact the anode and the cathode selectively, thus favoring the correct charge extraction.…”

“…Nevertheless, in spite of these advantages, PHJs exhibit low PCEs than BHJs due to the restricted donor/acceptor interface, resulting in a limited exciton dissociation. 10,16,[18][19][20][21][22][23][24][25][26][27] This suggests that the donor/acceptor interface and therefore the exciton dissociation play a predominant role for the PCE. To set the following discussion into context, it is helpful to first introduce a simplified expression for the photocurrent efficiency (ηph):…”