Explaining the Fill‐Factor and Photocurrent Losses of Nonfullerene Acceptor‐Based Solar Cells by Probing the Long‐Range Charge Carrier Diffusion and Drift Lengths

Abstract

stability have taken OSC performance to new heights. The emergence of a family of nonfullerene acceptors (NFAs) have delivered a discontinuous advance in cell efficiencies, achieving the efficiency levels of first-generation silicon devices, and with the milestone of 20% now in sight. [1][2][3] Further technological challenges are related to facilitating the manufacturing process, mainly by increasing the active layer thickness, whilst preserving the device performance. [4][5][6] To this end, a straightforward and facile quantification of the charge collection performance and losses in transport-limited solar cells is required. [7][8][9] To generate an external photocurrent, free charges have to be extracted. This process is diffusive or driven by drift, where extraction (mostly) competes with nongeminate recombination of free charges. A manifestation of this competition is the fill-factor FF. Some attention has been previously focused on correlating the FF with the efficiency of charge collection in organic solar cells based on the competition between the free charge extraction and second-order bimolecular recombination. [10] The bimolecular recombination is described via a recombination coefficient which is based on the Langevin expression multiplied by a reduction factor that takes into account processes that suppress the recombination below the Langevin limit. [11] In this picture, the extraction efficiency, as characterized by the ratio of the extraction to recombination rates, Organic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their opencircuit voltage loss and fill-factor, with state-of-the-art OSCs rarely displaying fill-factor of 80% and above. The fill-factor of transport-limited solar cells, including organic photovoltaic devices, is affected by material and devicespecific parameters, whose combination is represented in terms of the established figures of merit, such as θ and α. Herein, it is demonstrated that these figures of merit are closely related to the long-range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady-state photoconductance measurements. This yields a straightforward way of determining θ and α in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices-including the high efficiency nonfullerene acceptor blends-and show that the diffusion length of the free carriers provides a good correlation with the fill-factor. It is, finally, concluded that most state-ofthe-art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor.

Singlet and Triplet Excited-State Dynamics of a Nonfullerene Electron Acceptor Y6

“…These NFAs are strongly crystalline and exhibit deep highest occupied molecular orbital (HOMO) energy levels, large absorption cross sections in the near-IR region, and optical bandgaps suitable for solar cell applications. Therefore, a lot of studies on Y6-based OSCs have been conducted. − In contrast, the excited-state properties of Y6 are still not fully understood. For example, the singlet exciton diffusion length L D , which is defined as L D = ( Dτ ) 0.5 , where D is the exciton diffusion constant and τ is the exciton lifetime, is an important parameter that affects the PCE.…”

Molecular Doping Increases the Semitransparent Photovoltaic Performance of Dilute Bulk Heterojunction Film with Discontinuous Polymer Donor Networks

“…[ 47 ] The parameter θ can be related to the drift length ( l dr ) of carriers driven by the V int under short‐circuit condition, which is expressed by θ = d 2 / l dr 2 . [ 46 ] The l dr of the undoped film is 1.27 × 10 3 nm, which is much higher than the film thickness of 85 nm (Figure 3d). Though either types of doping increase the l dr , the larger l dr from smaller σ in the n‐doped Y6 is more beneficial for the FF improvement.…”

“…The FF in OSC has been well related to figure of merits (FoM) θ and α. [ 46–48 ] Bartesaghi et al. proposed the parameter θ as a measure of the ratio of the recombination to extraction rates where k rec and k ext are the recombination and extraction rates, γ is the bimolecular recombination coefficient, G is the charge generation rate, d is the film thickness, μ n and μ p are the electron and hole mobilities, and V int is the internal voltage.…”

“…[ 48 ] Based on α, we are able to calculate the diffusion length ( l dif ) according to α = d 2 /2 l dif 2 . [ 46 ] The l dif enhances from 58 to 65 and 67 nm, respectively, via p‐ and n‐doping (Figure 3e). Both of the increased l dr and l dif undoubtedly point out that either type of doping increases the FF via enhanced charge transport, and n‐doping is more efficient for it decreasing the energetic disorder most.…”

“…The above analysis well explains the J sc enhancement via p-doping and points out that either doping polarity suppresses the nongeminate recombination in the dilute BHJ film, which might be the origin of better FF and V oc .The FF in OSC has been well related to figure of merits (FoM) θ and α. [46][47][48] Bartesaghi et al proposed the parameter θ as a measure of the ratio of the recombination to extraction rates…”