Thickness Optimization and Photovoltaic Properties of Bulk Heterojunction Solar Cells Based on PFB–PCBM Layer

We report on the fabrication and study of bulk heterojunction (BHJ) solar cells based on a novel combination of a donor–acceptor poly(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N0-diphenyl)-N,N′di(p-butyl-oxy-pheyl)-1,4-diamino-benzene) (PFB) and [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) blend composed of 1:1 by volume. indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate (PEDOT:PSS)/PFB–PCBM/Ag BHJ solar cells are fabricated by a facile cost-effective spin-coating technique. The thickness of the active film (PFB–PCBM) plays an important role in the efficiency of light absorption, exciton creation, and dissociation into free charges that results in higher power conversion efficiency (PCE). In order to optimize the PCE as a function of active layer thickness, a number of solar cells are fabricated with different thicknesses of PFB–PCBM films at 120, 140, 160, 180, and 200 nm, and their photovoltaic characteristics are investigated. It is observed that the device with a 180 nm thick film demonstrates a maximum PCE of 2.9% with a fill factor (FF) of 53% under standard testing conditions (STC) (25 °C, 1.5 AM global, and 100 mW/cm2). The current–voltage (I-V) properties of the ITO/PEDOT:PSS/PFB–PCBM/Ag BHJ devices are also measured in dark conditions to measure and understand different parameters of the heterojunction. Atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) absorption spectroscopy for the PFB–PCBM film of optimal thickness (180 nm) are carried out to understand the effect of surface morphology on the PCE and bandgap of the blend, respectively. The AFM micrographs show a slightly non-uniform and rough surface with an average surface roughness (Ra) of 29.2 nm. The UV-vis measurements of the PFB–PCBM blend exhibit a reduced optical bandgap of ≈2.34 eV as compared to that of pristine PFB (2.88 eV), which results in an improved absorption of light and excitons generation. The obtained results for the ITO/PEDOT:PSS/PFB–PCBM (180 nm)/Ag BHJ device are compared with the ones previously reported for the P3HT–PCBM blend with the same film thickness. It is observed that the PFB–PCBM-based BHJ device has shown two times higher open circuit voltage (Voc) and, hence, enhanced the efficiency.

Solution Aging Effects on the Performance and Morphology of P3HT: PCBM Bulk Heterojunction Solar Cells

In a bulk heterojunction photovoltaic device, the morphology of the active layer strongly affects the device performance. By comparing the device performance from P3HT:PCBM solutions that were aged for different times, we notice that aging time has an influence on device power conversion efficiency. We prove the morphological difference in P3HT:PCBM thin films is caused by different aging time, and demonstrate a relationship between the P3HT:PCBM phase behavior and BHJ device performance through UV-Vis absorption spectrum and atomic force microscope.

Influence of thermal and solvent annealing on the morphology and photovoltaic performance of solution processed, D–A–D type small molecule-based bulk heterojunction solar cells

Four new small molecules CSDPP9–CSDPP12 were obtained with appended electron donating units in the molecular terminals of a DPP core. On solvent and thermal annealing, for the CSDPP11:PC71BM blend, the BHJ device displayed a PCE of 5.47%.

A novel donor–acceptor alternating copolymer based on angular-shaped benzo[2,1-b:3,4-b′]diselenophene for bulk heterojunction solar cells

A new D–A copolymer PBDSe-DTBT, based on the angular-shaped benzo[2,1-b:3,4-b′]dithiophene (BDP) unit with a selenium substitution, having both a low optical band gap of 1.71 eV and a deep HOMO level of −5.37 eV, has offered a promising average BHJ device efficiency of 5.6%.