Role of Contact Injection, Exciton Dissociation, and Recombination, Revealed through Voltage and Intensity Mapping of the Quantum Efficiency of Polymer:Fullerene Solar Cells

Tzabari, Lior; Wang, Jian; Lee, Yun Ju; Hsu, Julia W. P.; Tessler, Nir

doi:10.1021/acs.jpcc.6b01239

Cited by 11 publications

(18 citation statements)

References 40 publications

(65 reference statements)

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“…The 210 nm thickness was chosen based on the established optimized thickness for the P3HT:PCBM system. The EQE spectrum of the ZnO device is similar to the P3HT:PCBM absorption spectra and typically reported EQE spectra in the literature, [18][19][20] i.e., with the maximum between 500 and 550 nm and vibronic features. More strikingly, Figure 1c reveals the dramatically different EQE spectra for these two types of devices.…”

Section: Introductionsupporting

confidence: 68%

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n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Wang

Zhang

et al. 2017

Adv Elect Materials

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into the active layer, [6,7] the background doping, typically p-type, is believed to arise from polymer synthesis and is difficult to control. [3,9] In a recent report, we demonstrate that OPV active layers deposited on top of a MoO x hole transport layer (HTL) are p-doped, with the background hole concentration depending on the work function of MoO x and the active layer material. [8] Here we show that the electron transport layer (ETL) could also induce doping in the OPV active layer. In particular, poly(3hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) films deposited on top of branched polyethylenimine (PEI) ETL convert from typical lightly p-doped to strongly n-doped, with devices displaying substantially lower short-circuit current density (J sc ) and a strikingly different external quantum efficiency (EQE) spectral shape when compared with devices made on ZnO ETL. Using drift-diffusion and transfer matrix method (TMM) simulations to elucidate the experimental results, we establish that a combination of carrier type and concentration in the active layer fully explain the observed J sc and EQE spectral differences. Finally, we show that the origin of n-doping is due to the high solubility of PEI in the active layer processing solvent. The understanding is confirmed by device behaviors for PEI and another polymeric ETL, polyethylenimine ethoxylated (PEIE), and the dependence on active layer thickness and the energy of negative integer charge-transfer state. This research provides a comprehensive understanding of the effects of doping type and concentration on OPV device behaviors. Specifically, it provides an explanation for the wide variation of EQE spectra reported in the literature for OPV devices made with the same active layers but different interfacial contact layers. [10,11] Results and DiscussionsPEI and ZnO have been reported to be good ETL candidates in OPVs. [12,13] ZnO is an effective metal oxide ETL in OPV because of its low work function that enhances built-in field and deep valence band maximum that blocks holes to the cathode. [13,14] On the other hand, PEI is a polymeric ETL that functions as an interfacial modifier to reduce the cathode work function. [12] Figure 1a and Table 1 show the device current density versus Doping in an organic photovoltaic (OPV) device can largely impact its performance. In this work, it is discovered that n-type electrical doping in the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer can be induced by a certain electron transport layer (ETL), particularly branched polyethylenimine (PEI). Consequently, OPV devices with different ETLs exhibit dramatically different current density-voltage behaviors and external quantum efficiency (EQE) spectra. Using drift-diffusion modeling, Hall effect, and capacitance-voltage measurements, it is shown that the difference in EQE spectra originates from the different background carrier type and concentration in the OPV active layer, dictated by the specific properties of ETL. Fa...

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Section: Introductionsupporting

confidence: 68%

“…[18,19] Therefore, the EQE measurements were conducted with a background white-light light-emitting diode (LED) (Thorlab, MWWHL3) at 100 mW cm −2 . The monochromatic illumination area was ≈1 mm 2 with an intensity in the range of 10 −4 -10 −3 mW cm −2 across 350 to 750 nm.…”

Section: Methodsmentioning

confidence: 99%

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Wang

Zhang

et al. 2017

Adv Elect Materials

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“…The difference in shape and absolute values is a first indication of the internal mechanisms having different weight between the devices. 26,27 Figure 9 shows the bias dependent quantum efficiency of the 40 nm, 60 nm, and 120 nm thick BHJ layer devices (for each device, the QE is normalized to the low intensity value of its zero bias response). The symbols are the measured data points, and the lines are best fits to the model described in Section IV C 1.…”

Section: Intensity Dependent Quantum Efficiency Measurementmentioning

confidence: 99%

“…This model, taken from Ref. 27, is based on rate equations that accommodate a range of processes that take place inside a solar cell. The lumped model consists of three steady state equations that take care of each of the following species-excitons, polarons, and charge transfer (CT)-excitons:…”

Section: Electronic Modelmentioning

confidence: 99%

“…As explained in detail in the published work, 26,27 the broad and detailed data enabled us to fit the model to the measurement in a step by step manner, thus addressing each physical process individually. The inner parameter extraction requires not only the normalized QE data shown in Figure 9 but also the data shown in Figures 8 and 10.…”

Section: Electronic Modelmentioning

confidence: 99%

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Thickness dependent charge transfer states and dark carriers density in vacuum deposited small molecule organic photocell

Shekhar

Tzabari

Solomeshch

et al. 2016

Journal of Applied Physics

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We have investigated the influence of the active layer thickness on the balance of the internal mechanisms affecting the efficiency of copper phthalocyanine - fullerene (C60) based vacuum deposited bulk heterojunction organic photocell. We fabricated a range of devices for which we varied the thickness of the active layer from 40 to 120 nm and assessed their performance using optical and electrical characterization techniques. As reported previously for phthalocyanine:C60, the performance of the device is highly dependent on the active layer thickness and of all the thicknesses we tried, the 40 nm thin active layer device showed the best solar cell characteristic parameters. Using the transfer matrix based optical model, which includes interference effects, we calculated the optical power absorbed in the active layers for the entire absorption band, and we found that this cannot explain the trend with thickness. Measurement of the cell quantum efficiency as a function of light intensity showed that the relative weight of the device internal processes changes when going from 40 nm to 120 nm thick active layer. Electrical modeling of the device, which takes different internal processes into account, allowed to quantify the changes in the processes affecting the generation - recombination balance. Sub gap external quantum efficiency and morphological analysis of the surface of the films agree with the model's result. We found that as the thickness grows the density of charge transfer states and of dark carriers goes up and the uniformity in the vertical direction is reduced.

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Verification of Exciton Effects in Organic Solar Cells at Low Temperatures Based on a Modified Numerical Model

Xiong

Sun

Wang

et al. 2018

Journal of Elec Materi

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Role of Contact Injection, Exciton Dissociation, and Recombination, Revealed through Voltage and Intensity Mapping of the Quantum Efficiency of Polymer:Fullerene Solar Cells

Cited by 11 publications

References 40 publications

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Thickness dependent charge transfer states and dark carriers density in vacuum deposited small molecule organic photocell

Verification of Exciton Effects in Organic Solar Cells at Low Temperatures Based on a Modified Numerical Model

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