Identifying the Impact of Surface Recombination at Electrodes in Organic Solar Cells by Means of Electroluminescence and Modeling

Reinhardt, Jens; Grein, Maria; Bühler, Christian; Schubert, Martin C.; Würfel, Uli

doi:10.1002/aenm.201400081

Cited by 78 publications

(98 citation statements)

References 38 publications

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“…30 Surface recombination will, however, become important for poorly injecting contacts, resulting in a significant decrease of V oc . 28 Indeed, we observe a strong drop of V oc when reducing the WF of the ZnO bottom electrode (Fig. 3(d)).…”

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confidence: 77%

“…27 On the other hand, experimental and simulation work by W€ urfel and coworkers on P3HT:PCBM based solar cells with different cathode and anode materials revealed a strong correlation between the electrode work function (relative to the majority carrier transport level) and the degree of surface recombination. 28 For ohmic contacts, surface recombination was largely suppressed. This is due to the fact that diffusion of majority charges from the electrode into the organic semiconductor results in the formation of a space charge layer near the electrode, accompanied by significant band bending, 29 which prevents minority carriers to reach the contact.…”

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confidence: 94%

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Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

Lange

Reiter

Kniepert

et al. 2015

Applied Physics Letters

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An approach is presented to modify the WF of solution-processed sol-gel derived ZnO over an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using P3HT:PCBM as the active layer. These devices compete with or even exceed the performance of the reference cell on ITO/PEDOT:PSS. Our finding challenges the current view that bottom electrodes in inverted solar cells need to be electron-blocking for good device performance.Transparent metal oxides (TMOs) are integral parts of today's optoelectronic devices, either as electron conducting electrodes or intrinsically-doped semiconductors. Among these, zinc oxide (ZnO) is becoming increasingly important, as it is composed of earth-abundant elements. Also, a wide range of vapor-and solution based methods is available for ZnO deposition, ranging from chemical or physical vapor deposition (CVD, PVD) for the preparation of epitaxially grown single crystalline ZnO films to sol-gel procedures for low cost solution-based deposition. ZnO is already intensively used as a low-cost, transparent electrode in inorganic devices, [1][2] but more recently also as a charge selective interlayer material in efficient organic solar cells (OSCs) or organic light-emitting diodes (OLEDs). [3][4] However, the moderate work function (WF) of untreated ZnO of about 4.3 eV causes injection barriers to almost all conventional organic semiconductors [5][6][7][8][9] and the WF of such ZnO layers is poorly reproducible due to the physisorption of contaminations. [10] Finally, its poor chemical stability in an acidic environment [11][12][13] makes the application particularly in organic devices challenging. Therefore, for some of the recent organic record solar cells, the WF of the ZnO forming the bottom electrode was

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confidence: 77%

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confidence: 94%

Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

Lange

Reiter

Kniepert

et al. 2015

Applied Physics Letters

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“…Therefore the increase of the work function of E-ZnO films by 0.4 eV does not degrade the built-in field of the inverted devices. [43] …”

Section: Electronic and Molecule Model Of Edt Passivated Zno Nanocrysmentioning

confidence: 99%

Ethanedithiol Treatment of Solution‐Processed ZnO Thin Films: Controlling the Intragap States of Electron Transporting Interlayers for Efficient and Stable Inverted Organic Photovoltaics

Bai

Jin

Liang

et al. 2014

Advanced Energy Materials

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169

153

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The surface defects of solution-processed ZnO films lead to various intragap states. When the solution-processed ZnO films are used as electron transport interlayers (ETLs) in inverted organic solar cells, the intragap states act as interfacial recombination centers for photogenerated charges and thereby degrade the device performance. Here we demonstrate a simple surface-passivation method based on ethanedithiol (EDT) treatment, which effectively removes the surface defects of the ZnO nanocrystal films by forming zinc ethanedithiolates.

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“…As we have previously shown [172], the logarithmic scale of the conventional iPC plot of and charge-transfer state (CT) dissociation, minority surface recombination losses of photogenerated charges or CT states that diffuse into the "wrong" electrode [32,33], and potentially first order non-geminate recombination.…”

Section: Figure 1 Steady State and Time-resolved Photocurrent Measurmentioning

confidence: 99%

“…Non-ideal electrodes can cause charge injection barriers to the donor's IP, and acceptor's EA, and minority surface recombination at the contacts can further reduce the OC and photocurrent [32,33].…”

Section: Ff Lossesmentioning

confidence: 99%

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

Stolterfoht¹

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Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability and mechanical flexibility. In contrast to traditional banded semiconductors, organic semiconductors are intrinsically disordered systems and exhibit therefore much lower charge carrier mobilities. They are also low dielectric constant systems -as such, their photo-excitations are excitonic at room temperature with the electron-hole pair remaining Coulombically-bound. Blending organic semiconductors with differing electron affinities, so-called electron acceptors and donors, creates molecular heterojunctions which deliver the required driving force for exciton separation. These so-called bulk heterojunctions (BHJs) are the dominant architecture for creating organic photovoltaic cells and photodetectors. However, not least because of an incomplete understanding of the underlying physical mechanism that control the conversion of photons to free charges, and the subsequent extraction of these free charges to the electrodes, these organic optoelectronic systems still lag behind their inorganic counterparts.Motivated by these factors, the work described in this thesis advances the fundamental understanding of the loss mechanisms associated with charge photogeneration and charge transport phenomena in BHJ organic solar cells and photodetectors, and presents new experimental methodologies to study these processes. The transport of photogenerated charge carriers in the percolated donor: acceptor pathways towards the extracting electrodes has been studied in-depth via existing and newly developed transient photovoltage and steady-state characterization techniques. A simple but conclusive understanding has been developed which allows for minimization of the detrimental recombination of free charge carriers for given device parameters such as film thickness, applied voltage and the mobility of the slower carrier type (either electrons or holes). The models' predictions have been experimentally validated for many different (> 25) BHJ solar cell systems. Inspired by the need to selectively optimize the processes which control the photocurrent output of the cell, such a charge photogeneration and extraction, a technique to quantify and disentangle both efficiencies has been developed as a next step. Corroborated by transient absorption spectroscopy, the dynamics of the photocarrier generation process was examined. The results suggest that the so-called charge-transfer state (CTS) separation limits the conversion of photons to free charges and the photocurrent output from short-circuit to the maximum power point, strongly depending on the slower carrier ii mobility. These findings were explained by the ability of the slower carriers to leave the donor: acceptor interface via 1) a high enough mobility, 2) a sufficiently large domain size, and 3) enough conduction pathways (entropy). This work also shows that the dynamics of charge extraction and CTS dissociation are simi...

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Identifying the Impact of Surface Recombination at Electrodes in Organic Solar Cells by Means of Electroluminescence and Modeling

Cited by 78 publications

References 38 publications

Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

Ethanedithiol Treatment of Solution‐Processed ZnO Thin Films: Controlling the Intragap States of Electron Transporting Interlayers for Efficient and Stable Inverted Organic Photovoltaics

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

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