Reducing losses in solution processed organic solar cells
Rahimichatri, Azadeh
DOI:
10.33612/diss.170159026
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date: 2021
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Rahimichatri, A. (2021). Reducing losses in solution processed organic solar cells. University of Groningen. https://doi.org/10.33612/diss.170159026
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
3
Chapter 3
Non-fullerene organic solar cells:
Recombination versus extraction
Summary
Inrecentyears, non-fullereneacceptororganicsolarcellshavedrawnattentionmostly due to their high absorption, tunable energy levels, and high efficiency. However, still a lot is to be understood about the physics of these solar cells. Earlier, our group has shown that the overall influence of different material properties and device parame-ters can be described by a single parameter θ. In this chapter, θ is shown to be directly related to the ratio of the rate of recombination to that of extraction. We achieve this by direct measurements of the rates of recombination and extraction in non-fullerene acceptor solar cells using ITIC as the acceptor, with varying active layer thicknesses, light intensities, and interfacial layers. We provide a simple experimental tool for un-derstanding how modification of different parameters can lead to further enhance-ment of fill factor. The results support that the competition between charge extrac-tion and recombinaextrac-tion determines the fill factor of non-fullerene based organic solar cells.
3
3.1 Introduction
Theelectrondonorandacceptormaterialsinorganicsolarcellsareselectedmainlybased on a proper matching of their energy levels to allow for efficient charge transfer and to obtain high VOC. [1–3] The fullerene based acceptor PCBM has been widely used with
polymer donors in bulk heterojunction organic solar cells. [3–9] One of the challenges in further improving fullerene-based bulk heterojunction organic solar cells is that PCBM easily crystallizes and aggregates, which affects long term performance of the device and hinders charge transport. [10–13] Furthermore, fullerene based acceptors are weak ab-sorbers in the visible region (380–740 nm). [14] Introducing non-fullerene acceptors has provided a new pathway to overcome the mentioned challenges. [15, 16] Due to their great potential for band gap engineering, and thermal as well as long-term stabil-ity, non-fullerene acceptor organic solar cells have gained great attention among organic photovoltaic devices. [17–26] In a detailed study on photostability of fullerene and non-fullerene acceptor ITIC based polymer solar cells in our group, it was found that despite higher performance of ITIC based devices, their photostability is in fact lower than that of PCBM based ones. [27] The observed photodegradation was attributed to the con-siderable loss in fill factor (FF) and changes in the mobilities upon light exposure. [27] Therefore, in order to further improve the long term performance of these solar cells, re-ducing FF loss is a crucial step. In another study of our group, it was shown that the the interplay between recombination and extraction of charge carriers determines the FF in OPV devices. [28]
While great enhancements in the performance and FF of non-fullerene acceptor based organic solar cells have been achieved, [21–23, 29–38] there have been few re-ports quantifying recombination and extraction in these devices . [18, 27, 29, 32, 39–45] Gasparini and co-workers have attributed the high FF observed in P3HT:IDTBR solar cells to the low Langevin recombination pre-factor. [29] Formation of large crystalline domains in these devices has been proposed to give rise to an ultrafast charge transfer, and hindered recombination due to large distance between photogenerated electrons and holes. [29] In another work by Zhang and co-workers, it has been found that imbal-anced transport and reduced electron mobility may not be limiting the performance of ITIC based solar cells, as PBDB-T:ITIC devices have shown high performance while hav-ing imbalanced mobilities. [39] In order to have a comprehensive understandhav-ing of the device performance in non-fullerene acceptor based organic solar cells for further im-provement of their long term stability, a universal tool to connect the FF with transport properties and recombination of charges is essential.
This study is motivated by our current understanding of competition between recom-bination and extraction which determines the FF of organic solar cells, reported else-where. [28] In this chapter, we determine the rates of recombination and extraction in BHJ solar cells with ITIC as the non-fullerene acceptor. We find a relation between the
3
3.2. Theoretical background 47
ratio of recombination rate (krec) to extraction rate (kex) and FF in these solar cells. The
experimental data follow the same trend as the drift-diffusion simulated points, reported earlier, [28] confirming that direct measurement ofkrec
kex is a simple approach for linking F F to the competition between recombination and extraction in non-fullerene
accep-tor organic solar cells. We show that the non-fullerene accepaccep-tor solar cells studied herein follow the figure of merit for the FF presented by our group, [28] leaving room for further developments in non-fullerene acceptor based organic solar cells.
3.2 Theoretical background
Neglecting space-charge effects, the extraction rate of electrons or holes kexis
approxi-mated as [46]
kex= 2µn,pVint
L2 (3.1)
with Vintbeing the internal voltage across the active layer, µn,pelectron or hole mobility,
and L the active layer thickness. In the determination of the extraction rate, we consider short-circuit conditions as charge transport at short-circuit is dominated by drift, and therefore, charge extraction efficiency is close to its maximum. Vintis approximated by
the VOCat the corresponding light intensity. In Chapter 2, we showed that for a system
with a bimolecular recombination pre factor (γpre) larger than 10−3, electrodic induced
charge (EIC) recombination does not influence the recombination rate of charge carri-ers. [47] Under this condition, the recombination rate close to OC, where the maximum recombination occurs within the active layer, is written as [47]
krec= 2 Gγ, (3.2)
where γ is the recombination rate coefficient, and G is the generation rate. Dividing krec
by kexgives krec kex = t = √ GγL2 µn,pVint. (3.3)
Bymeansofnumericalsimulations, ourgrouphasdefinedasingleparameterθ, which includes both the contributions of recombination and extraction as [28]
θ = γGL
4
µnµpV2
int. (3.4)
Clearly, Equations 3.3 and 3.4 are related. By identifying µn,pwith the geometrical
3
Figure 3.1: Simulated FF versus θ, calculated by Bartesaghi et al. [28] Each point represents theF Fof a solar cell for which the J −V characteristic is calculated using a drift-diffusion model. For
each point, θ is calculated by Equation 3.4.
devices [48], we get
t =√θ. (3.5)
Previously, our group has shown that θ determines the fill factor of many different solar cells. [28] In that work, J − V characteristics of organic solar cells were simulated using wide range of parameters that represent a wide variety of BHJs under different op-erating conditions. The parameters used were thickness, effective bandgap, electron and hole mobility, recombination pre-factor, and the generation rate of free charge carriers and the resulting values of θ span a range of 10−6− 10−3(using Equation 3.4) with fill
factors spanning from 0.25 to 0.87. Based on the simulations and confirmed by experi-mental data, when θ is low enough (10−6), FF can reach its highest values. The resulting
plot of FF versus θ is depicted in Figure 3.1. Bartesaghi et al. interpreted θ as a measure of the ratio between recombination and extraction of charges. [28] However, they did not prove this experimentally. Equation 3.5 enables us to provide experimental evidence that the fill factor is indeed governed by this ratio (t) by measuring the recombination and extraction rates.
3.3 Results and discussion
As a means to verify the connection between FF and the ratio krec
kex, the extraction and
recombination rates in ITIC based non-fullerene acceptor solar cells were directly mea-sured. The molecular structure of ITIC is shown in Figure 3.2. We performed transient experiments under varying active layer thicknesses, light intensities, cathode interfacial
3
3.3. Results and discussion 49
Figure 3.2: Chemical structure of the nonfullerene-acceptor molecule used in this chapter, ITIC.
[49]
Table 3.1: Active layer materials, interfacial layers, light intensities, and thicknesses used in this
study.
Donor:acceptor interfacial layer LED light intensity thickness
[sun] [nm] 0.5 130, 160, 250 LiF 0.2 160, 250 PBDB-T:ITIC 0.05 160, 250 Ca 0.5 130 PFN 0.5 130 w/o 0.5 130 PDCBT:ITIC LiF 0.5 100
layers, and donor materials. The non-fullerene acceptor solar cells were fabricated with a conventionaldevicestructureofindiumtinoxide(ITO)/PEDOT:PSS/activelayer/interfacial layer/Al, where PBDB-T and PDCBT were used as donor materials, and Ca, LiF, and PFN as interfacial layers. These data are summarized in Table 3.1.
In order to measure the recombination rate we performed TPV measurements under open circuit (OC), using a small perturbation LED light intensity with a step function, which causes exponential decay of VOCdue to recombination of excess charge carriers.
[47] A high input impedance of the oscilloscope (1 MΩ) was used to provide OC. Figure 3.3 shows VOCdecays of a PBDB-T:ITIC solar cell using LiF interfacial layer under LED
light intensities of around 0.5 sun, 0.2 sun and 0.05 sun. At lower LED light intensities the recombination lifetime increases as less extra charge is present in the active layer when applying the small perturbation.
The extraction rates were measured by performing a transient experiment. [48] First, the device is under steady state condition at a higher light intensity. Then the light inten-sity is slightly reduced, while the bias voltage is kept constant, which results in extraction of the extra photogenerated charge carriers. Charge carrier extraction rate is calculated by fitting a simple exponential fit to the decay of the current, carried out under different applied voltages, [48] shown in Figure 3.4.
We have estimated the values of γprefor PBDB-T:ITIC and PDCBT:ITIC solar cells
3
Figure 3.3: TPV transients as a function of light intensity for PBDB-T:ITIC solar cell with LiF
inter-facial layer. Recombination lifetime is measured by fitting the exponential Vocdecay after applying the small perturbation of light by reducing LED light intensity.
Figure 3.4: Experimental results for a PBDB-T:ITIC solar cell using LiF interfacial layer, showing (a)
extracted current under different applied voltages, (b) extraction rate, obtained by fitting a simple exponential function to the decay of the extracted current.
γ = γpreq
(µn+ µp), (3.6)
where q is the elementary charge, and ε is the dielectric constant. The recombination rate coefficient γ is measured using Equation 3.2, with directly measured TPV recombination rate krec, and generation rate calculated by G = JqLSC. Considering the fairly balanced
3
3.3. Results and discussion 51
Figure 3.5: F F as a function of krec
kex =
√
θ. The grey dots are simulated data as calculated by
[28]. Devices 1-4 correspond to PBDB-T:ITIC solar cells with varying interfacial layers, device 5 is PDCBT:ITIC,devices6-11arePBDB-T:ITICsolarcellswithvariousthicknessesandlightintensities, as shown in Table 3.2.
Table 3.2: Characteristics of all the studied devices, under LED illumination.
# Donor Interfacial d I krec kex kkrecex F F Voc Jsc µ γpre
material layer [nm] [sun] [µ s−1] [µ s−1] [%] [V] [Am−2] [m2V−1s−1]
1 PBDB-T LiF 130 0.5 0.38 5.05 0.076 0.58 0.86 73.6 9.3× 10−8 9.40× 10−3 2 PBDB-T Ca 130 0.5 0.38 5.05 0.076 0.60 0.85 75.0 8.1× 10−8 1.70× 10−2 3 PBDB-T PFN 130 0.5 0.62 4.67 0.134 0.55 0.78 73.1 8.5× 10−8 2.80× 10−2 4 PBDB-T - 130 0.5 0.40 3.15 0.127 0.57 0.67 71.8 4.7× 10−8 2.07× 10−2 5 PDCBT LiF 100 0.5 0.37 4.65 0.079 0.61 0.85 49.2 1.4× 10−7 6.54× 10−3 6 PBDB-T LiF 160 0.5 0.30 2.98 0.102 0.58 0.85 77.3 6.2× 10−8 1.03× 10−2 7 PBDB-T LiF 250 0.5 0.26 1.63 0.157 0.51 0.84 69.7 3.9× 10−8 2.03× 10−2 8 PBDB-T LiF 160 0.2 0.20 2.98 0.068 0.66 0.83 27.9 6.2× 10−8 1.30× 10−2 9 PBDB-T LiF 250 0.2 0.17 1.63 0.106 0.60 0.82 30.0 3.9× 10−8 2.13× 10−2 10 PBDB-T LiF 160 0.05 0.08 2.98 0.027 0.71 0.78 6.7 6.2× 10−8 8.50× 10−3 11 PBDB-T LiF 250 0.05 0.07 1.63 0.045 0.68 0.77 7.1 3.9× 10−8 1.62× 10−2
µp 2µ, where µ is directly calculated from the extraction rate at open circuit, using the
formula [46]
kdiff= 8µkbT
qL2 (3.7)
where µ is the charge carrier mobility, kbis the Boltzmann constant, and T is the absolute
temperature. It should be noted that at OC, the charge transport is diffusion limited. [48] PBDB-T:ITIC devices with thicknesses of 130 nm, 160 nm and 250 nm (device num-bers 1, 6 and 7), using a LiF interfacial layer, are compared. At a constant light intensity, as the device gets thick (250 nm), the recombination rate slightly reduces due to G getting smaller, while the reduction of the extraction rate is more pronounced (Table 3.2). The
3
stronger reduction of kexcan be understood by comparing equation 3.1 and equation 3.2,
where the extraction rate varies with thickness to a larger extent than does the recombi-nation rate. We should note that the 250 nm device has a slightly lower VOC(see Table 3.2)
which may further reduce the extraction in this device. The overall picture of FF versus measured krec
kex values is plotted in Figure 3.5 (numbered symbols). As a consequence of
largerkrec
kex, the thickest device has a smaller FF.
Additionally, at a constant thickness, FF increases with decreasing light intensity (see Table 3.2 and Figure 3.5, device numbers 7, 9 and 11), which is caused by a lower recombination rate, while the extraction rate does not vary with light intensity (see Equa-tion 3.1). It has been suggested that high FFs at low light intensity is indicative of trap-assisted recombination. [50] However, considering Equation 3.2, the reduced recombi-nation rate at lower light intensities might be solely due to photogeneration of less excess charge in the active layer at lower light intensity, and does not necessarily represent trap assisted recombination as a dominate recombination process.
Further,krec
kex is varied by changing the interfacial layers of the non-fullerene acceptor
solar cells (devices 1, 2, 3 and 4). By inserting either Ca or LiF interfacial layers recombi-nation rate stays almost the same as the reference device with no interfacial layer. This is consistent with our previous study in Chapter 2, where we showed that in the solar cells with γprelarger than 10−3, inserting an interfacial layer does not influence the
recombi-nation rate. [47] The reason is that in those solar cells (γpre> 10−3) the contribution of
electrodic induced charges in the measured recombination rate is negligible. Therefore, the recombination rate corresponds to bulk recombination of photogenerated electrons and holes, irrespective of the interfacial layer used. However, if the energy barrier of an interfacial layer is too high for the electrons to be extracted, the recombination rate might increase. As can be seen in Table 3.2, using PFN as an interfacial layer with high electron extraction barrier [51] causes 1.55 times larger recombination rate compared to the refer-ence device with no interfacial layer. The increase of the recombination rate in this case might be due to piling up of electrons near the PFN/active layer interface caused by the large charge extraction barrier. [51]
Using PFN and either Ca or LiF, the extraction rate gets 1.48 and 1.6 times larger than the reference cell with no interfacial layer (see Table 3.2). With the explanations men-tioned above, the ratio of krec
kex gets smaller when using Ca or LiF interfacial layer, leading
to a higher FF (Figure 3.5).
Finally, we compare the experimental data presented in Figure 3.5 with the data ob-tained from numerical drift-diffusion simulations. [28] The experimental values of t (krec
kex)
in this chapter are in the range of 10−2− 10−1, where a sharp decrease in the FF, both
in simulated and experimental data, is observed. As t gets larger, a plateau at low FFs is expected. The simulated pattern clearly matches the experimental data of FF versus t, verifying that it is the ratiokrec
kex which determines the FF in organic solar cells.
3
3.4. Conclusion 53
the ratiokrec
kex, finding the relation between FF and krec
kex and comparing it with the
simu-lated data, we proved that FF is governed by the ratio of krec
kex. This direct proof was not
offered in [28], where sets of parameters were varied to investigate the behaviour of the devices over a range of θ.
3.4 Conclusion
We have directly measured the rates of recombination and extraction of charges in non-fullerene acceptor organic solar cells using ITIC, and found a negative correlation be-tween FF and the ratio of the rates of recombination and extraction in these solar cells (t). The results provide a clear understanding on how the extraction and recombination vary with varying active layer thicknesses, light intensities, and interfacial layers. Our demonstration of the relation between FF and t is supported by drift-diffusion simula-tions of organic solar cells under various condisimula-tions, published earlier. [28] The proposed approach underlines the role of extraction and recombination rates in further enhance-ment of FF, to obtain a better performance in non-fullerene acceptor solar cells.
3.5 Experimental procedures
Device fabrication: The solar cells were fabricated using
poly[(2,6-(4,8-bis(5-(2-ethylhex- yl)thiophen2-yl)benzo[1,2-b:4,5-b ]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c ]dithiophene-4,8-dione)](PBDBT)andpoly[(4,4 -bis(2-butyl octoxycarbonyl-[2,2 -bithiophene]-5,5-diyl)-alt-(2,2 -bithiophene-5,5 -diyl)](PDCBT)as donor, and3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11, 11-tetrakis (4-hexylphenyl)-dithieno[2,3- d:2 ,3 -d ]-s-indaceno[1,2-b:5,6-b ]dithiophe- ne (ITIC) as acceptor (purchased from Solarmer Energy, Inc). Structured indium tin oxide (ITO) was used as substrate. All substrates were cleaned with soap water for 5 minutes, followed by rinsing with de-ionized water, and subsequently a 10 min treatment in an ultrasonic bath in acetone and isopropyl alcohol, separately. Finally, the substrates were spin dried and transferred into the oven at 140◦C for 10 minutes, followed by UV-OZONE treatment
for 20 minutes. A 50 nm poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PE-DOT:PSS) layer was then spin cast on the substrate, followed by 10 minutes oven drying at 140◦C to remove the residual water. To fabricate PBDBT:ITIC solar cells a solution of
blend (1:1 by weight) in chlorobenzene with a concentration of 10 gL−1was spin-coated
at 1000 rpm or 500 rpm for 50 s, yielding active layers of approximately 130 nm and 260 nm thick, respectively. The active layer was then annealed at 100◦C for 10 min. Finally,
an interfacial layer of either LiF (1 nm), Ca (10 nm), or PFN (5nm) was deposited followed by Al (100 nm) evaporation through shadow masks in a vacuum chamber at 10−6mbar,
3
from a chloroform solvent with a concentration of 10 gL−1, in N
2atmosphere. The
sam-ple was annealed for 10 min at 160◦C. Finally, LiF (1 nm) and Al (100 nm) were thermally
evaporated.
Measurements: Current-voltage characteristics of the solar cells were measured using a
computer-controlled Keithley source meter in N2atmosphere. For the transient
experi-ments, the sample was illuminated with a biased white light LED with rise/fall time of and frequency of 100 Hz, with a pulse width of 5 ms. The rise/fall time of LED was tested using a photodiode with < 200 ns rise/fall time. Subsequent transient signals were acquired us-ing a digital storage oscilloscope (Agilent DSO-X 3034A) with a 350 MHz bandwidth and input resistance of 1 MΩ.
3
Bibliography 55
Bibliography
[1] F. B. Kooistra, J. Knol, F. Kastenberg, L. M. Popescu, W. J. Verhees, J. M. Kroon, and J. C. Hummelen, “In-creasing the open circuit voltage of bulk-heterojunction solar cells by raising the lumo level of the accep-tor,” Organic Letters 9(4), pp. 551–554, 2007.
[2] C. Falkenberg, S. Olthof, R. Rieger, M. Baumgarten, K. Muellen, K. Leo, and M. Riede, “The role of en-ergy level matching in organic solar cells—hexaazatriphenylene hexacarbonitrile as transparent electron transport material,” Solar Energy Materials and Solar Cells 95(3), pp. 927–932, 2011.
[3] A. Armin, I. Kassal, P. E. Shaw, M. Hambsch, M. Stolterfoht, D. M. Lyons, J. Li, Z. Shi, P. L. Burn, and P. Mered-ith, “Spectral dependence of the internal quantum efficiency of organic solar cells: Effect of charge gen-eration pathways,” Journal of the American Chemical Society 136(32), pp. 11465–11472, 2014.
[4] D. Chirvase, Z. Chiguvare, M. Knipper, J. Parisi, V. Dyakonov, and J. Hummelen, “Temperature dependent characteristics of poly (3 hexylthiophene)-fullerene based heterojunction organic solar cells,” Journal of
Applied Physics93(6), pp. 3376–3383, 2003.
[5] G. Dennler, M. C. Scharber, and C. J. Brabec, “Polymer-fullerene bulk-heterojunction solar cells,”
Ad-vanced materials21(13), pp. 1323–1338, 2009.
[6] T. L. Benanti and D. Venkataraman, “Organic solar cells: An overview focusing on active layer morphol-ogy,” Photosynthesis research 87(1), pp. 73–81, 2006.
[7] S. Günes, H. Neugebauer, and N. S. Sariciftci, “Conjugated polymer-based organic solar cells,” Chemical
reviews107(4), pp. 1324–1338, 2007.
[8] A. Cravino and N. S. Sariciftci, “Double-cable polymers for fullerene based organic optoelectronic appli-cations,” Journal of Materials Chemistry 12(7), pp. 1931–1943, 2002.
[9] R. Ganesamoorthy, G. Sathiyan, and P. Sakthivel, “Fullerene based acceptors for efficient bulk heterojunc-tion organic solar cell applicaheterojunc-tions,” Solar Energy Materials and Solar Cells 161, pp. 102–148, 2017. [10] P. Cheng and X. Zhan, “Stability of organic solar cells: challenges and strategies,” Chemical Society
Re-views45(9), pp. 2544–2582, 2016.
[11] K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D. D. Bradley, and J. R. Durrant, “Degradation of organic solar cells due to air exposure,” Solar energy materials and solar cells 90(20), pp. 3520–3530, 2006. [12] Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, and H. Yan, “Aggregation and morphology
control enables multiple cases of high-efficiency polymer solar cells,” Nature communications 5, p. 5293, 2014.
[13] A. Weu, J. A. Kress, F. Paulus, D. Becker-Koch, V. Lami, A. A. Bakulin, and Y. Vaynzof, “Oxygen-induced dop-ing as a degradation mechanism in highly efficient organic solar cells,” ACS Applied Energy Materials 2(3), pp. 1943–1950, 2019.
[14] S. Cook, H. Ohkita, Y. Kim, J. J. Benson-Smith, D. D. Bradley, and J. R. Durrant, “A photophysical study of pcbm thin films,” Chemical Physics Letters 445(4-6), pp. 276–280, 2007.
[15] C. B. Nielsen, S. Holliday, H. Chen, S. J. Cryer, and I. McCulloch, “Non-fullerene electron acceptors for use in organic solar cells,” Accounts of chemical research 48(11), pp. 2803–2812, 2015.
[16] Y. Lin, J. Wang, Z. Zhang, H. Bai, Y. Li, D. Zhu, and X. Zhan, “An electron acceptor challenging fullerenes for efficient polymer solar cells,” Advanced materials 27(7), pp. 1170–1174, 2015.
3
[17] W. Zhong, J. Cui, B. Fan, L. Ying, Y. Wang, X. Wang, G. Zhang, X. Jiang, F. Huang, and Y. Cao, “Enhanced pho-tovoltaic performance of ternary polymer solar cells by incorporation of a narrow-bandgap nonfullerene acceptor,” Chemistry of Materials 29(19), pp. 8177–8186, 2017.
[18] H. Cha, J. Wu, A. Wadsworth, J. Nagitta, S. Limbu, S. Pont, Z. Li, J. Searle, M. F. Wyatt, D. Baran, et al., “An efficient,“burn in” free organic solar cell employing a nonfullerene electron acceptor,” Advanced
Materi-als29(33), p. 1701156, 2017.
[19] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganäs, F. Gao, and J. Hou, “Fullerene-free polymer solar cells with over 11% efficiency and excellent thermal stability,” Advanced materials 28(23), pp. 4734–4739, 2016. [20] W. Chen and Q. Zhang, “Recent progress in non-fullerene small molecule acceptors in organic solar cells
(oscs),” Journal of Materials Chemistry C 5(6), pp. 1275–1302, 2017.
[21] X. Xu, T. Yu, Z. Bi, W. Ma, Y. Li, and Q. Peng, “Realizing over 13% efficiency in green-solvent-processed non-fullerene organic solar cells enabled by 1, 3, 4-thiadiazole-based wide-bandgap copolymers,” Advanced
Materials30(3), p. 1703973, 2018.
[22] H. Li, Y. Zhao, J. Fang, X. Zhu, B. Xia, K. Lu, Z. Wang, J. Zhang, X. Guo, and Z. Wei, “Improve the perfor-mance of the all-small-molecule nonfullerene organic solar cells through enhancing the crystallinity of acceptors,” Advanced Energy Materials 8(11), p. 1702377, 2018.
[23] W. Zhao, S. Li, H. Yao, S. Zhang, Y. Zhang, B. Yang, and J. Hou, “Molecular optimization enables over 13% efficiency in organic solar cells,” Journal of the American Chemical Society 139(21), pp. 7148–7151, 2017. [24] X. Du, T. Heumueller, W. Gruber, A. Classen, T. Unruh, N. Li, and C. J. Brabec, “Efficient polymer solar
cells based on non-fullerene acceptors with potential device lifetime approaching 10 years,” Joule 3(1), pp. 215–226, 2019.
[25] J. Hou, O. Inganäs, R. H. Friend, and F. Gao, “Organic solar cells based on non-fullerene acceptors,” Nature
materials17(2), p. 119, 2018.
[26] C. Yan, S. Barlow, Z. Wang, H. Yan, A. K.-Y. Jen, S. R. Marder, and X. Zhan, “Non-fullerene acceptors for organic solar cells,” Nature Reviews Materials 3, p. 18003, 2018.
[27] N. Y. Doumon, M. V. Dryzhov, F. V. Houard, V. M. Le Corre, A. R. Chatri, P. Christodoulis, and L. J. A. Koster, “Photo-stability of fullerene and non-fullerene polymer solar cells: The role of the acceptor,” ACS applied
materials & interfaces , 2019.
[28] D. Bartesaghi, I. del Carmen Pérez, J. Kniepert, S. Roland, M. Turbiez, D. Neher, and L. J. A. Koster, “Com-petition between recombination and extraction of free charges determines the fill factor of organic solar cells,” Nature communications 6, p. 7083, 2015.
[29] N. Gasparini, M. Salvador, T. Heumueller, M. Richter, A. Classen, S. Shrestha, G. J. Matt, S. Holliday, S. Strohm, H.-J. Egelhaaf, et al., “Polymer: nonfullerene bulk heterojunction solar cells with exception-ally low recombination rates,” Advanced Energy Materials 7(22), p. 1701561, 2017.
[30] B. Qiu, L. Xue, Y. Yang, H. Bin, Y. Zhang, C. Zhang, M. Xiao, K. Park, W. Morrison, Z.-G. Zhang, et al., “All-small-molecule nonfullerene organic solar cells with high fill factor and high efficiency over 10%,”
Chem-istry of Materials29(17), pp. 7543–7553, 2017.
[31] S. Li, L. Ye, W. Zhao, X. Liu, J. Zhu, H. Ade, and J. Hou, “Design of a new small-molecule electron acceptor enables efficient polymer solar cells with high fill factor,” Advanced Materials 29(46), p. 1704051, 2017.
3
Bibliography 57
[32] B. Fan, W. Zhong, X.-F. Jiang, Q. Yin, L. Ying, F. Huang, and Y. Cao, “Improved performance of ternary polymer solar cells based on a nonfullerene electron cascade acceptor,” Advanced Energy Materials 7(11), p. 1602127, 2017.
[33] H. Yao, L. Ye, J. Hou, B. Jang, G. Han, Y. Cui, G. M. Su, C. Wang, B. Gao, R. Yu, et al., “Achieving highly effi-cient nonfullerene organic solar cells with improved intermolecular interaction and open-circuit voltage,”
Advanced Materials29(21), p. 1700254, 2017.
[34] L. Ye, W. Zhao, S. Li, S. Mukherjee, J. H. Carpenter, O. Awartani, X. Jiao, J. Hou, and H. Ade, “High-efficiency nonfullerene organic solar cells: critical factors that affect complex multi-length scale morphology and device performance,” Advanced Energy Materials 7(7), p. 1602000, 2017.
[35] L. Zuo, J. Yu, X. Shi, F. Lin, W. Tang, and A. K.-Y. Jen, “High-efficiency nonfullerene organic solar cells with a parallel tandem configuration,” Advanced Materials 29(34), p. 1702547, 2017.
[36] Y. Wang, W. Lan, N. Li, Z. Lan, Z. Li, J. Jia, and F. Zhu, “Stability of nonfullerene organic solar cells: from built-in potential and interfacial passivation perspectives,” Advanced Energy Materials 9(19), p. 1900157, 2019.
[37] R. Sun, J. Guo, C. Sun, T. Wang, Z. Luo, Z. Zhang, X. Jiao, W. Tang, C. Yang, Y. Li, et al., “A universal layer-by-layer solution-processing approach for efficient non-fullerene organic solar cells,” Energy &
Environ-mental Science12(1), pp. 384–395, 2019.
[38] X. Zhang, N. Yao, R. Wang, Y. Li, D. Zhang, G. Wu, J. Zhou, X. Li, H. Zhang, J. Zhang, et al., “On the un-derstanding of energy loss and device fill factor trade-offs in non-fullerene organic solar cells with varied energy levels,” Nano Energy , p. 105032, 2020.
[39] X. Zhang, X. Zuo, S. Xie, J. Yuan, H. Zhou, and Y. Zhang, “Understanding charge transport and recombi-nation losses in high performance polymer solar cells with non-fullerene acceptors,” Journal of Materials
Chemistry A5(33), pp. 17230–17239, 2017.
[40] V. V. Brus, C. M. Proctor, N. A. Ran, and T. Nguyen, “Capacitance spectroscopy for quantifying recombi-nation losses in nonfullerene small-molecule bulk heterojunction solar cells,” Advanced Energy
Materi-als6(11), p. 1502250, 2016.
[41] Y. Qin, M. A. Uddin, Y. Chen, B. Jang, K. Zhao, Z. Zheng, R. Yu, T. J. Shin, H. Y. Woo, and J. Hou, “Highly efficient fullerene-free polymer solar cells fabricated with polythiophene derivative,” Advanced
Materi-als28(42), pp. 9416–9422, 2016.
[42] G. Ding, J. Yuan, F. Jin, Y. Zhang, L. Han, X. Ling, H. Zhao, and W. Ma, “High-performance all-polymer nonfullerene solar cells by employing an efficient polymer-small molecule acceptor alloy strategy,” Nano
Energy36, pp. 356–365, 2017.
[43] D. Baran, R. S. Ashraf, D. A. Hanifi, M. Abdelsamie, N. Gasparini, J. A. Röhr, S. Holliday, A. Wadsworth, S. Lockett, M. Neophytou, et al., “Reducing the efficiency–stability–cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells,” Nature materials 16(3), p. 363, 2017.
[44] S. M. Hosseini, S. Roland, J. Kurpiers, Z. Chen, K. Zhang, F. Huang, A. Armin, D. Neher, and S. Shoaee, “Impact of bimolecular recombination on the fill factor of fullerene and nonfullerene-based solar cells: A comparative study of charge generation and extraction,” The Journal of Physical Chemistry C 123(11), pp. 6823–6830, 2019.
3
[45] J. Vollbrecht, V. V. Brus, S.-J. Ko, J. Lee, A. Karki, D. X. Cao, K. Cho, G. C. Bazan, and T.-Q. Nguyen, “Quanti-fying the nongeminate recombination dynamics in nonfullerene bulk heterojunction organic solar cells,”
Advanced Energy Materials9(32), p. 1901438, 2019.
[46] L. J. A. Koster, M. Kemerink, M. M. Wienk, K. Maturová, and R. A. J. Janssen, “Quantifying bimolecular recombination losses in organic bulk heterojunction solar cells,” Advanced Materials 23(14), pp. 1670– 1674, 2011.
[47] A. R. Chatri, S. Torabi, V. M. Le Corre, and L. J. A. Koster, “Impact of electrodes on recombination in bulk heterojunction organic solar cells,” ACS applied materials & interfaces 10(14), pp. 12013–12020, 2018. [48] V. M. Le Corre, A. R. Chatri, N. Y. Doumon, and L. J. A. Koster, “Charge carrier extraction in organic solar
cells governed by steady-state mobilities,” Advanced Energy Materials 7(22), p. 1701138, 2017.
[49] National Center for Biotechnology Information, “Pubchem compound summary for cid 126843541.” https://pubchem.ncbi.nlm.nih.gov/compound/126843541. Accessed: 28.03.2021.
[50] A.Sharma, M.Chauhan, V.Bharti, M.Kumar, S.Chand, B.Tripathi, andJ.Tiwari, “Revealingthecorrelation between charge carrier recombination and extraction in an organic solar cell under varying illumination intensity,” Physical Chemistry Chemical Physics 19(38), pp. 26169–26178, 2017.
[51] T. Wang, Y. Hu, Z. Deng, Y. Wang, L. Lv, L. Zhu, Z. Lou, Y. Hou, and F. Teng, “High sensitivity, fast response and low operating voltage organic photodetectors by incorporating a water/alcohol soluble conjugated polymer anode buffer layer,” RSC Advances 7(3), pp. 1743–1748, 2017.
