Abstract
Due to the inhomogeneous distribution of donor and acceptor materials within the photoactive layer of bulk heterojunction organic solar cells (OSCs), proper selection of a conventional or an inverted device structure is crucial for effective exciton dissociation and charge transportation. Herein, we investigate the donor and acceptor distribution within the non-fullerene photoactive layer based on PBDTTT-ET:IEICO by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) and scanning Kelvin probe microscopy (SKPM), indicating that more IEICO enriches on the surface of the photoactive layer while PBDTTT-ET distributes homogeneously within the photoactive layer. To further understand the effect of the inhomogeneous component distribution on the photovoltaic performance, both conventional and inverted OSCs were fabricated. As a result, the conventional device shows a power conversion efficiency (PCE) of 8.83% which is 41% higher than that of inverted one (6.26%). Eventually, we employed nickel oxide (NiOx) instead of PEDOT:PSS as anode buffer layer to further enhance the stability and PCE of OSCs with conventional structure, and a promising PCE of 9.12% is achieved.
Highlights
Bulk-heterojunction organic solar cells (BHJ-OSCs) attract increasing attention due to their characteristics of light-weight, low-cost, flexibility, rapid energy payback time and high-throughput roll-to-roll manufacturing [1,2,3]
We firstly investigated the characteristic species of PBDTTT-ET and IEICO by TOF-SIMS, and the distribution maps of elements within IEICO and PBDTTT-ET are shown in Figure 1c,d is the local enlarged drawing of Figure 1c
The integrated Jsc calculated from external quantum efficiency (EQE) curves are 14.04, 16.49 and 17.52 mA/cm2 for the devices without any anode buffer layer, with PEDOT:PSS and nickel oxide (NiOx) as anode buffer layer, respectively, which is in good agreement with the Jsc obtained from J–V curves
Summary
Bulk-heterojunction organic solar cells (BHJ-OSCs) attract increasing attention due to their characteristics of light-weight, low-cost, flexibility, rapid energy payback time and high-throughput roll-to-roll manufacturing [1,2,3]. OSC as an example, it is found that P3HT tends to enrich in the upper section of the photoactive layer due to the relatively lower surface energy (γ) of 16.9 mN/m, and a small amount of PCBM phase (γ = 45 mN/m) accumulates at the PEDOT:PSS interface [18] This inhomogeneous distribution can be found in non-fullerene and all-polymer solar cells such as PBDTTT-C-T:PDI and P3HT:F8TBT blends [19,20], where donors gather at the bottom and electrons enrich on the surface. This kind of concentration distribution goes against the carrier transportation for conventional OSCs. While for inverted structure, the kind of component aggregation can offer continuous and unhindered pathways for carrier transportation to the corresponding electrode, enhancing device performance. To further improve the device performance, air stable solution-processed nickel oxide (NiOx ) and poly[(9,9-bis(3-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)]dibromide (PFN-Br) are employed as anode and cathode interfacial materials, and a promising PCE of 9.12% is achieved for conventional device with high stability
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