Impact of Molecular Orientation and Spontaneous Interfacial Mixing on the Performance of Organic Solar Cells

Ndjawa, Guy Olivier Ngongang; Graham, Kenneth R.; Li, Ruipeng; Conron, Sarah M.; Erwin, Patrick; Chou, Kang Wei; Burkhard, George F.; Zhao, Kui; Hoke, Eric T.; Thompson, Mark E.; McGehee, Michael D.; Amassian, Aram

doi:10.1021/acs.chemmater.5b01845

Cited by 38 publications

(28 citation statements)

References 37 publications

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“…[67,68] Interestingly, in the case of PIPCP/ C 60 , the J SC in fact decreases with thermal annealing. This may explain the lower V OC in the bilayer devices, 0.76 V, and the greater E g -qV OC = 0.71 eV.…”

Section: Bilayer Solar Cells Achieve High Ffmentioning

confidence: 97%

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

Ran

Love

Heiber

et al. 2017

Advanced Energy Materials

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Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J SC ) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V OC ). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC 61 BM), that achieves a high V OC (0.9 V) with very low energy losses (E loss = 0.52 eV) from the energy of absorbed photons, a respectable J SC (13 mA cm −2 ), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from fielddependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC 61 BM. These results hold promise that given the appropriate morphology, the J SC , V OC , and FF can all be improved, even with very low energetic offsets.

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Section: Bilayer Solar Cells Achieve High Ffmentioning

confidence: 97%

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

Ran

Love

Heiber

et al. 2017

Advanced Energy Materials

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“…We have fabricated bilayer solar cells using a-rubrene, t-rubrene, and o-rubrene as the donors, and C 60 as the acceptor (glass/ITO-150 nm/rubrene-50 nm/ C 60 -50 nm/Bphen-12 nm/Al-100 nm, Figure 2b). The CuI layer, which has been shown to act as an effective seed layer for the growth of several organic donors, [36][37][38] had the benefit of maintaining the continuity of the rubrene films even after crystallization. The CuI layer, which has been shown to act as an effective seed layer for the growth of several organic donors, [36][37][38] had the benefit of maintaining the continuity of the rubrene films even after crystallization.…”

Section: Structural Order In Rubrene Thin Films and Its Influence On mentioning

confidence: 99%

Open‐Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge‐Transfer Bands

Ndjawa

Graham

Mollinger

et al. 2017

Advanced Energy Materials

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excited states are referred to as chargetransfer (CT) states and are notable for determining the mechanism of free carrier generation and recombination processes. [8] The CT state energy (E CT ), defined as the energy required to promote the D-A complex from its ground state to its vibrationally relaxed first excited state, has been found to strongly depend on structural and conformational factors, such as the aggregation state of the donor and/or acceptor near the D/A junction, and the relative conformation of D and A molecules forming the CT complex. [9][10][11][12][13][14] Recent studies have revealed a strong dichroic absorption of CT complexes at planar interfaces in bilayers with crystalline donors, thus providing experimental evidence for oriented CT complexes. [15] Several models have related E CT to the open-circuit voltage (V OC ) in organic solar cells. [2,[16][17][18] The fundamental and striking feature that emerges from these models is that V OC depends linearly on E CT and only logarithmically on other factors such as the number, oscillator strength, and lifetime of the CT states.Because of this linear dependence of V OC on E CT , strategies to improve V OC are often oriented toward increasing E CT . E CT can be roughly viewed as the energy difference between the donor ionization potential (IP) and the acceptor electron affinity (EA), corrected to the binding energy between a hole in D molecule(s) and an electron on adjacent A molecule(s) within their respective environments, as highlighted schematically in Figure 1. Because the energy levels of both the donor and acceptor depend strongly on morphological factors such as

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“…The evaporated molecules could diffuse into the bottom layer owing to their high kinetic energy, and a thin mixed interlayer could be formed between the two layers . The situation is complicated further by recent experimental and theoretical studies suggesting that the degree of diffusion might depend on the miscibility of the two materials, the orientation of the bottom layer, and the temperature of the substrate during the deposition . These factors should be considered in the construction of heterointerfaces.…”

Section: Preparation and Characterization Of Planar Heterojunctionsmentioning

confidence: 99%

“…More recently, Ngongang Ndjawa et al reported that in PHJs, the degree of intermixing of the donor and acceptor at the vacuum‐deposited interfaces can depend on the molecular orientation in the bottom layer ( Figure ) . They found that when C 60 was deposited on top of a ZnPc layer, the D/A interface suffered from greater intermixing of C 60 with the ZnPc face‐on orientation than with the edge‐on orientation.…”

Section: Photophysical Processes In Organic Photovoltaics Based On Plmentioning

confidence: 99%

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High‐Performance Solar Cells

2016

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Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

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Impact of Molecular Orientation and Spontaneous Interfacial Mixing on the Performance of Organic Solar Cells

Cited by 38 publications

References 37 publications

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

Open‐Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge‐Transfer Bands

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High‐Performance Solar Cells

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