The effect of charge transfer state on the open-circuit voltage of small-molecular organic photovoltaic devices: A comparison between the planar and bulk heterojunctions using electroluminescence characterization

Lee, Chih Chien; Su, Wei; Chang, Wen Chang; Huang, Bo Yao; Liu, Shun Wei

doi:10.1016/j.orgel.2014.10.040

Cited by 19 publications

(17 citation statements)

References 55 publications

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“…Recently, Widmer et al investigated a series of small molecule OPV devices, and they found that the maximum attainable V OC ( V OC‑MAX ) obtained by extrapolating the temperature-dependent V OC to 0 K is well consistent with E eff . Nevertheless, it is still in hot debate whether the maximum value of V OC should solely depend on the energy level alignment or the additional charge transfer (CT) states induced at the heterojunctions. − Undoubtedly, solving this important but controversial issue requires a reliable experimental approach to accurately determine the energy level alignment at organic heterojunctions.…”

Section: Introductionmentioning

confidence: 99%

Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices

Yang

Cheng

et al. 2016

ACS Appl. Mater. Interfaces

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Energy level alignment at the organic donor and acceptor interface is a key to determine the photovoltaic performance in organic solar cells, but direct probing of such energy alignment is still challenging especially for solution-processed bulk heterojunction (BHJ) thin films. Here we report a systematic investigation on probing the energy level alignment with different approaches in five commonly used polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PCBM) BHJ systems. We find that by tuning the weight ratio of polymer to PCBM the electronic features from both polymer and PCBM can be obtained by photoemission spectroscopy. Using this approach, we find that some of the BHJ blends simply follow vacuum level alignment, but others show strong energy level shifting as a result of Fermi level pinning. Independently, by measuring the temperature-dependent open-circuit voltage (VOC), we find that the effective energy gap (Eeff), the energy difference between the highest occupied molecular orbital of the polymer donor (EHOMO-D) and lowest unoccupied molecular orbital of the PCBM acceptor (ELUMO-A), obtained by photoemission spectroscopy in all polymer:PCBM blends has an excellent agreement with the extrapolated VOC at 0 K. Consequently, the photovoltage loss of various organic BHJ photovoltaic devices at room temperature is in a range of 0.3-0.6 V. It is believed that the demonstrated direct measurement approach of the energy level alignment in solution-processed organic BHJ will bring deeper insight into the origin of the VOC and the corresponding photovoltage loss mechanism in organic photovoltaic cells.

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Section: Introductionmentioning

confidence: 99%

Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices

Yang

Cheng

et al. 2016

ACS Appl. Mater. Interfaces

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“…The electron and hole wave function overlap increases as the volume containing the CT state decreases, resulting in a larger recombination rate and a smaller dissociation efficiency compared with that of the delocalized CT states . Previous studies have also shown that an increased CT state energy leads to a higher open-circuit voltage ( V OC ). , Thus, a low DBP concentration benefits OPV performance due to the existence of CT states with higher energies.…”

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confidence: 99%

Charge Transfer and Collection in Dilute Organic Donor–Acceptor Heterojunction Blends

2018

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Experimental and theoretical approaches are used to understand the role of nanomorphology on exciton dissociation and charge collection at dilute donor-acceptor (D-A) organic heterojunctions (HJs). Specifically, two charge transfer (CT) states in D-A mixed HJs comprising nanocrystalline domains of tetraphenyldibenzoperiflanthene (DBP) as the donor and C as the acceptor are unambiguously related to the nanomorphology of the mixed layer. Alternating DBP:C multilayer stacks are used to identify and control the optical properties of the CT states, as well as to simulate the dilute mixed heterojunctions. A kinetic Monte Carlo model along with photoluminescence spectroscopy and scanning transmission electron microscopy are used to quantitatively evaluate the layer morphology under various growth conditions. As a result, we are able to understand the counterintuitive observation of high charge extraction efficiency and device performance of DBP:C mixed layer photovoltaics at surprisingly low (∼10%) donor concentrations.

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“…The energy level alignment diagram of the ZnO/Au/ PEDOT:PSS/P3HT:PC61BM BHJ OPD is shown in Figure 12, which indicates that the sufficient energy offset ensures efficient electron transfer between the ZnO and In addition, in the fullerene acceptor, the low-band-gap polymer that needs to collect more photons usually shows a larger energy gap (ΔE LUMO and ΔE HOMO ), and at the same time, the energy downshift between the acceptor LUMO and donor HOMO is relatively small, which greatly contributes to energy and voltage loss during dissociation process of bound excitons. 48,49 Under light illumination, due to the different HOMO levels of PC61BM and P3HT, photogenerated holes tend to be trapped in the isolated P3HT surrounded by PC61BM. The obvious difference in the donor-acceptor combination between the E HOMO and E LUMO ensures the driving force for charge separation in the photosensitive layer.…”

Section: Resultsmentioning

confidence: 99%

Use of Organic Polymer P3HT:PC61BM as the Active Layer to Improve the Performance of ZnO Ultraviolet Photodetector

2021

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One of the most interesting structures of organic photodetectors (OPDs) is the bulk heterojunction (BHJ), which can be achieved by mixing electron donors and electron acceptors in a suitable solvent and then spin-coating. BHJs have proven to be a very useful method to overcome the short diffusion length of organic semiconductor excitons. However, this structure enhances disorder and hinders the collection of photogenerated carriers. Therefore, it is very important to prepare OPDs with this structure. In this paper, we show how to use conjugate polymer donor poly(3-hexylthiophene) (P3HT) and fullerene derivative acceptor 6,6-phenyl C61-butyric acid methyl ester (PC61BM) to form a binary BHJ active layer by spin-coating to improve the performance and detector wavelength of a ZnO ultraviolet (UV) photodetector (PD). Compared with the ZnO UV PD, the responsivity of the OPD can reach 0.58 A W–1, which is increased by about 7.14 times. Furthermore, the OPD shows improved absorption, high photocurrent to dark current ratio (1.63 × 104), and excellent detection capability (3.67 × 1012 Jones). The preparation of a new OPD by this work shows strong performance and provides a new idea for the research of OPD. Therefore, it is very important to design and manufacture ZnO UV PD to form a BHJ OPD with organic polymers P3HT:PC61BM.

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The effect of charge transfer state on the open-circuit voltage of small-molecular organic photovoltaic devices: A comparison between the planar and bulk heterojunctions using electroluminescence characterization

Cited by 19 publications

References 55 publications

Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices

Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices

Charge Transfer and Collection in Dilute Organic Donor–Acceptor Heterojunction Blends

Use of Organic Polymer P3HT:PC61BM as the Active Layer to Improve the Performance of ZnO Ultraviolet Photodetector

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