Competition between recombination and extraction of free charges determines the fill factor of organic solar cells

Bartesaghi, Davide; Perez, Irene del Carmen; Kniepert, Juliane; Roland, Steffen; Turbiez, Mathieu; Neher, Dieter; Koster, L. Jan Anton

doi:10.1038/ncomms8083

Cited by 554 publications

(596 citation statements)

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“…In addition, the good photovoltaic performance of the J51 -based all-PSCs should be also ascribed to the lower series resistance ( R S ) of 7.65 Ω cm 2 and higher shunt resistance ( R p ) of 1.09 kΩ cm 2 of the devices in comparison to the J50 -based devices with R S = 11.25 Ω cm 2 and R p = 0.69 kΩ cm 2 . [ 28 ] For investigating the exciton dissociation and charge transfer behavior in the polymer blend of the polymer donor ( J50 or J51 ) and polymer acceptor N2200 , we measured the photoluminescence (PL) spectra of the polymers and the polymer blends, as shown in Figure 3 b. According to the absorption spectra of the polymer donors and the polymer acceptor (Figure 1 c), there is an absorption band in 425-650 nm for the polymer donor J50 and J51 fi lms, and there are two absorption bands in the range of 300-425 nm, and 650-850 nm for N2200 fi lm.…”

Section: Doi: 101002/adma201504629mentioning

confidence: 97%

“…[ 28 ] In general, J sc has a power-law dependence on P light ( J sc ∝ P light α ), where power-law component (α) should be unity when the bimolecular recombination of the charge carriers is negligible. As shown in The average PCE is obtained from over 15 devices; b) Calculated from the inverse slope at V = V OC in J -V curves under illumination; c)…”

Section: Doi: 101002/adma201504629mentioning

confidence: 99%

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All‐Polymer Solar Cells Based on Absorption‐Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%

et al. 2015

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Section: Doi: 101002/adma201504629mentioning

confidence: 97%

Section: Doi: 101002/adma201504629mentioning

confidence: 99%

All‐Polymer Solar Cells Based on Absorption‐Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%

et al. 2015

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“…Lastly, high-performance low-bandgap polymers should also exhibit high charge transport ability in order to achieve high fi ll factors (FF). [ 24,25 ] Among the reported low bandgap polymers, diketopyrrolopyrrole (DPP) has been a commonly used building block due to its excellent properties such as strong absorption covering near-infrared and visible regions, its electron defi cient nature and high charge carrier mobility. A polymer (DT-PDPP2T-TT) has been reported as the best low-bandgap polymer, which could achieve an impressive short-circuit current density ( J sc) of 20.07 mA cm −2 with diphenolether as additive.…”

mentioning

confidence: 99%

Efficient Low‐Bandgap Polymer Solar Cells with High Open‐Circuit Voltage and Good Stability

Jiang

Chen

et al. 2015

Advanced Energy Materials

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occupied molecular orbital (HOMO) level of the polymer, which has a direct impact on the V OC of the cell. In addition, it is important to achieve low-bandgap PSCs with great stability for the purpose of practical applications. While thermal stability of PSCs is related to thermal properties of the polymer, air stability can be affected by the HOMO level of the polymer. Polymers with deeper HOMO levels are generally more stable against the oxidation by air. Lastly, high-performance low-bandgap polymers should also exhibit high charge transport ability in order to achieve high fi ll factors (FF). [ 24,25 ] Among the reported low bandgap polymers, diketopyrrolopyrrole (DPP) has been a commonly used building block due to its excellent properties such as strong absorption covering near-infrared and visible regions, its electron defi cient nature and high charge carrier mobility. A polymer (DT-PDPP2T-TT) has been reported as the best low-bandgap polymer, which could achieve an impressive short-circuit current density ( J sc) of 20.07 mA cm −2 with diphenolether as additive. The open circuit voltage ( V OC ) of cell is still relatively low (0.67 V) due to the relatively electron-rich nature of thienothiophene-based donor unit. [ 13 ] Another high-performance low-bandgap polymer is C3-DPPTT-T (with a high effi ciency up to 8.8%), which offers an excellent J sc up to 23.5 mA cm −2 but still exhibits a V OC of 0.57 V. [ 17 ] To increase the V OC of DPP-based low bandgap polymers, an effective strategy is to copolymerize the DPP unit with donor building blocks that are less electron-rich than thiophene or thienothiophene. When a benzodithiophene unit was copolymerized with DPP, a signifi cantly higher V OC of 0.73 V was achieved, but the cell effi ciency was signifi cantly lower (6.6% for single junction cell). [ 10 ] Recently, a DPP-based D 1 -A-D 2 -Atype low bandgap polymer (PDPP3T alt TPT) was reported with the introduction of a phenyl ring between two DPP2T moieties. This polymer further increases the V OC to 0.75 V and can achieve an excellent PCE of 8.0%. [ 16 ] In this paper, we report a novel DPP polymer (named poly(difluorobenzothiadiazole-bisthiophene-alt -diketopyrrolopyrrole-bisthiophene) or PffBTT2-DPPT2) that combines a D-A 1 -D-A 2 copolymer strategy and our temperature-dependent aggregation polymer design. The chemical structure of PffBTT2-DPPT2 is shown in Figure 1 a. While the fi rst acceptor unit (A 1 ) is DPP, 5,6-difl uoro-2,1,3-benzothiadiazole (ffBT) is selected as the second acceptor unit (A 2 ), as it was shown in our previous studies to introduce excellent polymer properties for high-effi ciency PSCs. [ 8 ] Although the bandgap of PffBTT2-DPPT2 is small (1.43 eV), PSCs based on PffBTT2-DPPT2 can still achieve an impressive V OC of 0.81 V, indicating a very small V OC loss of 0.62 V. The bandgap and V OC loss of PffBTT2-DPPT2-based PSCs are very close to the optimal Polymer solar cells (PSCs) have been developed as a promising low-cost and environmentally friendly alternative to conventional inorg...

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“…Hence, given the rather standard transport characteristics observed, one is led to suspect that favorable recombination is at the heart of the thick junction performance. This is consistent with Bartesaghi et al [182], who recently pointed out that the overall charge collection efficiency (which determines the FF and the PCE) is a result of the competition between recombination and charge extraction. In what is to follow, we study the recombination coefficient by applying 4 independent methods in steady state and transient modes, dark and illuminated.…”

Section: Electron and Hole Mobilitiessupporting

confidence: 79%

“…Recombination is non-geminate if the two charges in the encounter originated from different photoexcitations, and is the most common explanation for photocurrent losses under operational bias [91,92,162,182,183]. In forward bias, the driving field for extraction decreases, which leads to a build-up of electron (n) and hole (p) density in the bulk of the heterojunction.…”

Section: Electric-field Dependent Geminate Recombination Losses Inintmentioning

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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Competition between recombination and extraction of free charges determines the fill factor of organic solar cells

Cited by 554 publications

References 48 publications

All‐Polymer Solar Cells Based on Absorption‐Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%

All‐Polymer Solar Cells Based on Absorption‐Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%

Efficient Low‐Bandgap Polymer Solar Cells with High Open‐Circuit Voltage and Good Stability

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

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