Ambient Air Processing Causes Light Soaking Effects in Inverted Organic Solar Cells Employing Conjugated Polyelectrolyte Electron Transfer Layer

Abstract

Inverted polymer:fullerene bulk heterojunction solar cells employing a conjugated polyelectrolyte electron transfer layer display light soaking effects as the oxygen adsorbed on indium tin oxide (ITO) during an ambient air device processing induces interface charge trap states in the conjugated polyelectrolyte layer and reduces its interface dipole. The light soaking populates the trap states with photoexcited electrons and reinstates the electric dipole, leading to a recovery of efficient charge extraction and normal illuminated current−voltage characteristics consequently. The identified effect of adsorbed oxygen not only enables a remedy of the light soaking issue of the inverted solar cells via hydrogen plasma treatment of ITO but also suggests the importance of properly handling adsorbed oxygen species on ITO for achieving high performance organic devices based on ITO substrates in general. ■ INTRODUCTIONWith the champion efficiency approaching over 12% 1,2 and continuing commercialization efforts in the field, the attention to the organic solar cell continues. The structure of organic solar cells has been largely based on an active organic layer sandwiched between a transparent metal oxide bottom anode (typically indium tin oxide (ITO) coated with a hole transfer layer) and a metal cathode on top. Reversing the order, the inverted organic solar cell uses the bottom ITO as a cathode after applying a certain electron transfer layer (ETL) before depositing the active organic layer. The inverted structure is more attractive for a large-scale manufacturing, 2,3 displays a good ambient stability, 3,4 and in the case of polymer:fullerene bulk heterojunction (BHJ) active layer utilizes the natural composition gradient of BHJ, where the concentration of fullerene electron acceptor tends to increase toward the bottom. 5 For the ETL, the material choice has been mostly metal oxides (e.g., TiO x and ZnO), 2,6−9 but recently organic and organic−inorganic hybrid composite materials were also used. 6,10,11 Particularly, 9.2% power conversion efficiency (PCE) has been demonstrated by applying a conjugated organic polyelectrolyte PFN (poly [(9,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) as an ETL, in conjunction with a low band gap polymer donor PTB7 (poly [[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno[3,4-b]thiophenediyl]]). 11Most of inverted organic solar cells adopting metal oxide ETLs display so-called light soaking effects, 2,7−9 wherein the device PCE increases with increasing duration of solar illumination due to increasing open circuit voltage (V OC ) and fill factor (FF). This gradual, light-sensitive change in photovoltaic (PV) characteristics is driven by a disappearance of the S-shaped current−voltage curve near the open circuit point, whose potential origins have been correlated to trap filling in the metal oxide layer and subsequent increase in photoconductivity; 7 energy barrier between ITO and the metal oxide ETL; 8 che...

Atomic-layer-deposited AZO outperforms ITO in high-efficiency polymer solar cells

“…Figure 3b confirms that the J-V characteristics of AZO-based PSC devices overlap upon forward and reverse biases, reflecting the absence of hysteresis. Figure 3c shows that the J-V characteristics of AZO-based PSC devices remain unchanged with illumination time, indicating that "light-soaking" effects 27 do not affect device operation. Often times, light-soaking effects arise from the presence of residual oxygen introduced in the ETL deposition step (e.g.…”

Overcoming Electrode‐Induced Losses in Organic Solar Cells by Tailoring a Quasi‐Ohmic Contact to Fullerenes via Solution‐Processed Alkali Hydroxide Layers

“…The current generation of inverted OSCs with n-type MO x semiconductors (TiO x , ZnO, and AZO) as an ETL frequently displays so-called light soaking effects, which is best described by the appearance of a S-shaped current−voltage curve near the open circuit point. [ 38,39 ] The origins of light-soaking effects in OSCs are multiply and were correlated to trap fi lling in the metal oxide layer; chemisorbed oxygen on the metal oxide surface that creates carrier depletion; SCLC effects in the metal oxide layer or to an energy barrier at the metal oxide/organic active layer interface. [ 39 ] The light-induced charge generation in the MO x is coming along with a fi lling of the defective trap sites and is enhancing conductivity (photoconductivity) in parallel to reducing injection barriers.…”

“…[ 38,39 ] The origins of light-soaking effects in OSCs are multiply and were correlated to trap fi lling in the metal oxide layer; chemisorbed oxygen on the metal oxide surface that creates carrier depletion; SCLC effects in the metal oxide layer or to an energy barrier at the metal oxide/organic active layer interface. [ 39 ] The light-induced charge generation in the MO x is coming along with a fi lling of the defective trap sites and is enhancing conductivity (photoconductivity) in parallel to reducing injection barriers. Light soaking therefore requires exposure to light with UV light ( λ ≤ 400 nm) and gradually enhances solar cell effi ciency over illumination time toward the value of maximum performance.…”

Squaraine based solution processed inverted bulk heterojunction solar cells processed in air

“…The devices were fabricated under ambient conditions; the adsorbed oxygen on the ITO caused the formation of the mid gap charge transfer states in the PFN layer near to the ITO/PFN interface. 31 These states increase the work function of ITO by reducing the electric dipole moment within the PFN layer. Upon continuous light exposure, the photon excited electrons fill the mid trap states and restore the reduced dipole moment of PFN.…”