Mobility and fill factor correlation in geminate recombination limited solar cells

Abstract

Empirical data for the fill factor as a function of charge carrier mobility for two different polymer:fullerene systems is presented and analyzed. The results indicate that charge extraction depth limitations and space charge effects are inconsistent with the observed behavior, and the decrease in the fill factor is, instead, attributed to the field-dependent charge separation and geminate recombination. A solar cell photocurrent limited by the Onsager-Braun charge transfer exciton dissociation is shown to be able to accommodate the experimental observations. Charge dissociation limited solar cells always benefit from increased mobilities, and the negative contribution from the reduced charge separation is shown to be much more important for the fill factor in these material systems than any adverse effects from charge carrier extraction depth limitations or space charge effects due to unbalanced mobilities. The logarithmic dependence of the fill factor on the mobility for such a process is also shown to imply that simply increasing the mobilities is an impractical way to reach very high fill factors under these conditions since unrealistically high mobilities are required. A more controlled morphology is, instead, argued to be necessary for high performance.

7.7% Efficient All‐Polymer Solar Cells

“…Although the higher electron mobility in the fi lm-aged blend solar cells leads to the enhanced fi ll factor (FF = 0.48) of these devices relative to those annealed at 175 °C (FF = 0.39), [ 17 ] the fi ll factor of these devices is still much lower than the corresponding fullerene/polymer photodiodes (FF > 0.6). [ 2c , 12 ] This result suggests that the bulk hole transport properties of PNDIS-HD:PBDTT-FTTE blend system remain to be fully optimized for balanced bulk charge transport and advances in this area could substantially improve the FF and thus effi ciency of these all-polymer BHJ solar cells.…”

An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems

“…This consideration was included explicitly because nite mobility, low collection length, low minority-carrier lifetimes, and non-radiative recombination losses of photogenerated carriers can reduce the ll factors in the J-V behavior of the light absorber. [25][26][27][28][29] (3) amorphous Si(Ge), organic PV and dye-sensitized solar cells. The FF, or the shape, of light absorber J-V curves depends on the specic parameters of the materials and devices used in the PV component.…”

Multiple state representation scheme for organic bulk heterojunction solar cells: A novel analysis perspective

“…[7] This CT state can either recombine nonradiatively (geminate recombination), or undergo charge separation leading to mobile electron and hole carriers. [7,10] However, because of both the low carrier mobility and the interpenetrated nature of typical BHJ blends, there is a non-negligible probability that dissociated free carriers recombine again at the large D-A interface (nongeminate recombination) before being collected at the electrodes. [7,11] These nonradiative recombinations can be a major loss mechanism p-1…”

“…[7] This CT state can either recombine nonradiatively (geminate recombination), or undergo charge separation leading to mobile electron and hole carriers. [7,10] However, because of both the low carrier mobility and the interpenetrated nature of typical BHJ blends, there is a non-negligible probability that dissociated free carriers recombine again at the large D-A interface (nongeminate recombination) before being collected at the electrodes. [7,11] These nonradiative recombinations can be a major loss mechanism p-1 that strongly reduces the power conversion efficiency in BHJ solar cells, [12][13][14][15][16] and are mainly influenced by the energy difference between the highest occupied molecular level ε D1 of D (called HOMO of D) and the lowest unoccupied level ε A2 of A (called LUMO of A).…”