Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells

Melianas, Armantas; Pranculis, Vytenis; Xia, Yuxin; Felekidis, Nikolaos; Inganäs, Olle; Gulbinas, Vidmantas; Kemerink, Martijn

doi:10.1002/aenm.201602143

Cited by 86 publications

(137 citation statements)

References 40 publications

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“…First of all, with an electron mobility of about

10^{- 3} {cm}^{2} {normalV}^{- 1} {normals}^{- 1}

and a hole mobility of about

10^{- 4} {cm}^{2} {normalV}^{- 1} {normals}^{- 1}

, the mobility contrast is well documented . Second, because of the semicrystalline nature of P3HT:PCBM with a relatively low energetic disorder, charge extraction and recombination are well described by quasiequilibrium concepts, so that drift–diffusion simulations lead to meaningful results . Third, the bimolecular recombination strength can be adjusted via the preparation conditions, in particular, the drying rate of the solvent and the application of thermal annealing .…”

Section: Resultsmentioning

confidence: 99%

Watching Space Charge Build Up in an Organic Solar Cell

et al. 2020

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Space charge effects can significantly degrade charge collection in organic photovoltaics (OPVs), especially in thick‐film devices. The two main causes of space charge are doping and imbalanced transport. Although these are completely different phenomena, they lead to the same voltage dependence of the photocurrent, making them difficult to distinguish. Herein, a method is introduced on how the build‐up of space charge due to imbalanced transport can be monitored in a real operating organic solar cell. The method is based on the reconstruction of quantum efficiency spectra and requires only optical input parameters that are straightforward to measure. This makes it suitable for the screening of new OPV materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick‐film OPVs.

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“…First of all, with an electron mobility of about

10^{- 3} {cm}^{2} {normalV}^{- 1} {normals}^{- 1}

and a hole mobility of about

10^{- 4} {cm}^{2} {normalV}^{- 1} {normals}^{- 1}

Section: Resultsmentioning

confidence: 99%

Watching Space Charge Build Up in an Organic Solar Cell

et al. 2020

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“…This ensures that the starting point of the time-resolved measurement is an excited CT complex at the (isolated α-6T):C 60 interface, instantaneously creating an electron in C 60 and a hole in isolated α-6T. [25,26] Before we proceed with the detailed analysis of these kinetics, we highlight that these measurements were performed Figure 1. We have previously experimentally demonstrated that these traces represent the cumulative amount of photogenerated charge collected at the electrodes.…”

Section: Time-resolved Charge Extractionmentioning

confidence: 99%

Charge Transport in Pure and Mixed Phases in Organic Solar Cells

Melianas

Pranculis

Spoltore

et al. 2017

Advanced Energy Materials

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116

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mixture of electron donating and accepting materials-a bulk heterojunction (BHJ). Such interpenetrating donoracceptor mixtures form complicated multilength scale morphologies, often involving several phases such as ordered/ disordered donor, mixed donor-acceptor and ordered/disordered acceptor. The coexistence of neat and mixed donoracceptor domains is thought to be beneficial as it could provide an energy cascade [4,5] for charge separation, [6,7] followed by charge transport in the neat phases. However, the neat regions may be discontinuous, forcing charges to cross mixed domains multiple times during extraction. If the mixed regions are severely disordered or contain less than the percolation threshold of required material, charge transport is thought to deteriorate significantly, limiting device performance. [8] The effect of donor-acceptor mixing (or phase purity) on charge transport has been previously addressed using microstructural characterization, [9,10] steady-state, [11][12][13] and/or timeresolved [14][15][16] mobility measurements on BHJs with a varying donor-acceptor stoichiometry and/or processing conditions. Although such studies have revealed important trends, they remain mostly semiquantitative-the question of how pure the neat domains have to be and how detrimental domain discontinuities or donor-acceptor mixing are in relation to charge transport kinetics remains to a large extent unanswered. Not knowing which important charge transport features to optimize, limits the development of next generations of organic optoelectronic devices.Here, we address this by measuring photo-generated charge motion from the first hopping events (with sub-picosecond time resolution) to full extraction in complete solar cell devices based on coevaporated bulk heterojunctions of α-sexithiophene (α-6T) and buckminsterfullerene (C 60 ). We carefully vary the molar fraction of α-6T in C 60 from homogeneously diluted (<10% molar), to a point where α-6T begins to form isolated aggregates (>10%-25% molar) or is strongly aggregated (50% molar). We thus vary the distance between isolated α-6T sites and the level of disruption of the C 60 phase in a controlled manner-the α-6T:C 60 system may be viewed as a model for the mixed donor-acceptor phase in OPV. C 60 was chosen as the model acceptor since its use in organic electronics is In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor-acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor-acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor-acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured i...

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“…Melianas et al found ν 0 around 1 × 10 13 s −1 for electron transport in PCBM and N2200, whereas ν 0 around 1 × 10 10 to 1 × 10 11 s −1 was found for hole transport in a range of polymer OSC. [24,25] Hence, irrespective of the precise meaning of ν 0 , the implication of the findings in Figure 4 is that n-type OSC might eventually become superior to their p-type counterpart if one succeeds to reach the end of the model curves in Figure 4, that is, reach a relative doping concentration ≈10% without compromising morphology.…”

Section: Wwwadvelectronicmatdementioning

confidence: 99%

High Seebeck Coefficient and Power Factor in n‐Type Organic Thermoelectrics

Zuo

Wang

et al. 2017

Adv Elect Materials

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be attributed to the absence of an n-type equivalent of PEDOT, the overall scarcity of n-type OSCs, and the inefficiency of most n-type dopants. Since most good OSCs that are used as n-type materials like [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) and N2200 have a lowest unoccupied molecular orbital (LUMO) around −4.0 eV, [7,8] it is very difficult to realize a stable dopant with a HOMO above −4.0 eV as would be required for efficient electron transfer. Fortunately, Wei et al. have reported a promising material, (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl) dimethylamine (N-DMBI), that can be used as stable n-type dopant due to a two-step thermally activated doping mechanism. [9] Very recently, Huang et al. used this dopant to achieve record values for PF = 105 µW m −1 K −2 and ZT = 0.11 at room temperature for the small molecule A-DCV-DPPTT. [10] Some reports also have demonstrated that dihydro-1H-benzoimidazol-2-yl (N-DBI) derivatives can improve the electrical conductivity by several orders of magnitude in n-type OTE by solution-processing. For example, Schlitz et al. have shown that solution mixtures of P(NDIOD-T2) with 9 mol% N-DBI derivatives can achieve σ ≈ 1 S m −1 and a PF over 0.6 µW m −1 K −2 . [11] Yuan et al. reported a small-molecule 2DQTT-o-OD that, with 10 wt% of a novel N-DBI derivative incorporated in solution, acquires a PF of 17.2 µW m −1 K −2 at room temperature. [12] Shi et al. achieved a high electron mobility in FBDPPV with σ ≈ 1400 S m −1 and PF ≈ 28 µW m −1 K −2 , when adding around 5 wt% N-DMBI in solution. [13] Despite the positive effect of N-DMBI and N-DBI derivatives on electrical conductivity, it probably fails to further increase σ and PF upon further increasing the volume fraction in solution mixtures due to degradation of the blend morphology. [14] Liu et al. demonstrated a modified fullerene derivative, PTEG-1, that shows an increased miscibility at the nanoscale level and thereby achieved a PF of 16.7 µW m −1 K −2 with σ ≈ 205 S m −1 at 40 mol% of N-DMBI. [15] For p-type OTE, most reported experimental data seem to follow an empirical quasi-universal power law relationship between the Seebeck coefficient S and conductivity as S ∝ σ − 1/4 . [2,3] In view of the limited number of reported data, it is still unclear whether n-type OTE also follow this power law, and different power law slopes have been reported. [15,16] Here, we introduce a novel, inverse-sequential doping procedure that mitigates the need for solvent orthogonality in conventional sequential doping to investigate the potential of deposited on a previously cast dopant 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylaniline film to achieve a very high power factor PF ≈ 35 µW m −1 K −2 with a conductivity σ ≈ 40 S m −1 is introduced. It is also shown that n-type organic semiconductors obey the −1/4 power law relation between Seebeck coefficient S and σ that are previously found for p-type materials. An analytical model on basis of variable range hopping unifies these results. Th...

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Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells

Cited by 86 publications

References 40 publications

Watching Space Charge Build Up in an Organic Solar Cell

Watching Space Charge Build Up in an Organic Solar Cell

Charge Transport in Pure and Mixed Phases in Organic Solar Cells

High Seebeck Coefficient and Power Factor in n‐Type Organic Thermoelectrics

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