Effect of Active Layer Thickness on the Performance of Polymer Solar

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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of Active Layer Thickness on the Performance of Polymer Solar Cells Based on a Highly Efficient Donor Material of PTB7-Th Yue Zang, Qing Xin, Jufeng Zhao, and Jun Lin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03132 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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The Journal of Physical Chemistry

Effect of Active Layer Thickness on the Performance of Polymer Solar Cells Based on a Highly Efficient Donor Material of PTB7-Th Yue Zang*, Qing Xin, Jufeng Zhao, and Jun Lin College of Electronics and Information, Hangzhou Dianzi University, Xiasha Campus, Hangzhou 310018, P. R. China

Corresponding author. E-mail: [email protected]; Tel: (+86-571) 86919029.

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Abstract One of the challenges for the commercialization of polymer solar cells (PSCs) is the difficulty in fabricating a homogeneous and pinhole-free,

thin

active

layer

through

large-scale,

high

throughput, roll-to-toll manufacturing process. On the other hand, thick active layers in current PSCs generally result in low power conversion efficiency (PCE). Here, we reported the effect of active layer thickness on the performance PSC device based on the state-of-art blends of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2yl)benzo[1,2-b;4,5-b’]dithiophene-2,6diyl-alt-(4-(2-ethylhexyl)-3uorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]

(PTB7-

Th):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The results showed that although the short circuit current density (JSC) was increased in the device with thicker active layer, the PCE was decreased due to the drastically declined fill factor (FF), which offset the improved light absorption from using thicker films. Optical modeling using transfer matrix method (TMM) and analysis of the fitted Shockley diode equation of illuminated current density-voltage (J-V) characteristics indicated that the decrease of FF for thicker PTB7-Th:PC71BM solar cells was

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ascribed to the inefficient charge carrier transport and collection, which resulted from relatively low electron mobility and the increased interface defect states. Based on these results, the devices with higher PC71BM content were fabricated to facilitate the electron transport, which allowed an overall increase of the efficiency to 8.15% for the device with 270 nm thick active layer due to the significantly improved FF.

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Introduction Polymer solar cells (PSCs) with bulk heterojunction (BHJ) structure have been regarded as one of the most promising candidates for the next generation of photovoltaic technology to solve the impending energy crisis1-2. In the past two decades, PSCs has been rapidly developed with new advancements in novel light-harvesting and charge transport materials3-5 as well as advanced device structure6-8, with state-of-art power conversion efficiency (PCE) over 13% in single junction devices9-12. With these progresses, the researches currently come to the stage of fabricating efficient flexible, large-scale solar cells using a roll-toroll (R2R) coating system in order to finally realize the mass production and commercialization of low cost PSCs13. However, one of the major challenges for the PSC technology is the difficulty in controlling the morphology and homogeneity of thin BHJ film over a large area, leading to increasing density of pinholes and leakage paths. On the other hand, utilizing thick BHJ composite layers in current PSCs generally results in low PCEs due to the decreases in fill factor (FF) and short-circuit current density (JSC)14-16.

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For the majority of high efficiency PSCs, the optimal thickness of the photoactive layer for the best performance is often around 100 nm3-4, 9-11. Increasing the active layer thickness is expect to improve the device efficiency due to the enhanced light absorption and exciton generation. However, limited by the low charge carrier mobility of organic semiconductors, a slightly increase of the active layer thickness normally causes a dramatic drop in photovoltaic performance due to the serious charge recombination loss17. Therefore, high charge carrier mobility is necessary to prevent the recombination as well as to promote charge collection when the active layer thickness reaches several hundred

nanometers.

Poly(3-hexylthio-phene)

(P3HT),

a

commonly used donor material, is one of the high mobility polymers (hole mobility up to 0.1 cm2 V-1 s-1). It has been demonstrated that relatively high PCEs can be achieved in P3HT:[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) solar cells with thick active layer thickness just over 300 nm18-19. Murphy et al. have demonstrated that a high hole mobility (1 cm2 V-1 s-1) polymer, PDQT, comprising diketopyrrolopyrrole (DPP) and β-unsubstituted quaterthiophene (QT), allowed the fabrication

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of PSCs with thicker active layers (~100 nm to 800 nm) without undermining the photovoltaic performances20. However, the efficiency of PDQT based solar cells is relatively low. Hu et al. have demonstrated an efficiency of 8.62% in the inverted PSC with BHJ thickness of 280 nm by using a naphtha[1,2-c:5,6-c] bis[1,2,5]-thiadiazole (NT)-based, high-mobility, donor-acceptor conjugated polymer donor material (PBDT-DTNT) mixed with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as active layer, and solution-processed thermal cross-linkable conjugated polyfluorene

poly[(9,9-bis(6’-(N,N-dimethylamino)propyl)-2,7-

fluorene)-alt-2,7-(9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl)fluorene)] (PFN-OX) as electron extraction layer21. Remarkably, they observed that the efficiency of the inverted device can be as high as 7.42% with BHJ thickness around 1000 nm. Currently, novel polymer materials composed of twodimensional conjugated benzo[1,2-b:4,5-b']dithiophene (BDT) have attracted great interest due to their promising photovoltaic properties22-25. In addition to having a low bandgap for ensuring efficient harvest of solar light, many 2D-conjugated BDT based polymers process high hole mobility for facilitating charge

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transport. Typically, a highly efficient polymer of poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene2,6diyl-alt-(4-(2-ethylhexyl)-3-uorothieno[3,4-b]thiophene-)-2carboxylate-2-6-diyl)] (PTB7-Th) has been developed, which boosted the power conversion efficiency of polymer solar cells to levels that exceed 10% in single junction cells with PC71BM as acceptor26. Recently, non-fullerene acceptors have attracted great interests because of their potential advantages as enhanced absorption coefficients in visible region, tunable chemical and energetic properties, and cost-effective synthesis process. PTB7Th have been widely used as the standard donor material and shown great potential for high-performance non-fullerene based solar cells. By tuning the energy levels and broadening the absorption bans of the non-fullerene acceptors, the best efficiency over 12% has been achieved in PTB7-Th based non-fullerene solar cells27. These results demonstrate high potential of PTB7-Th donor material for the large-area PSC application28-29. Therefore, a comprehensive optimizing and understanding of PTB7-Th based solar cells with different active layer thickness is required.

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In this study, the effect of active layer thickness on the photovoltaic performance of state-of-art blends of PTB7Th:PC71BM based PSCs was systematically investigated. The PCE of the device with thicker film was decreased due to the dramatic decrease of FF. To find the reasons for the poor FF at large active layer thickness, optical modeling based on the transfer matrix method (TMM) has been employed to understand the optical distribution and exciton generation profiles, which has been demonstrated having great influence on the charge carrier transport process. In addition, the diode parameters were extracted by fitting the illuminated J-V characteristics with conventional Shockley diode equation to analyze the interface defect states in different devices. Based on these results, the PCE of device with 270 nm thick active layer was increased to 8.15% by increasing the fullerene content in the blend film to achieve a balanced electron and hole transport and decrease charge carrier recombination loss.

Experiment The structure of polymer solar cell in this study was indiumtin oxide (ITO) (120 nm)/zinc oxide (ZnO) (30 nm)/PTB7-

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Th:PC71BM (70-270 nm)/molybdenum oxide (MoO3) (8 nm)/Ag (100 nm). The solar cells were fabricated on ITO covered glass (15 Ω/sq), which were cleaned consecutively in ultrasonic bath containing detergent, acetone, ethanol, deionized water for 10 min each step, and finally dried by high purity nitrogen blow. Prior to the deposition of functional layers, the substrate was treated by UV light for 10 min. As an electron transporting layer, the ZnO precursor solution prepared using a method described by Sun et al.30 was spin-coated on top of the cleaned ITO substrates and then heated at 200 ℃ for 1 h in air. Then PTB7-Th:PC71BM active layer was spin-coated on top of the ZnO layer. The solution was prepared by dissolving the polymer and fullerene at weight ratio of 1:1.5 or 1:3 in a mixed solvent of o-dichlorobenzene (DCB) and 1,8-diiodooctane (DIO) (97:3 vol%) overnight and filtered through a PTFE (polytetrafluoroethylene) filter (0.45 µm). Finally, the substrates were pumped down in high vacuum (< 2×10-6 Torr), and MoO3 (8 nm) topped with Ag (100 nm) were thermal evaporated onto the active layer. The current density-voltage (J-V) characteristics under illumination were tested using a Keithley 2400 programmable voltage-current source. A light source

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integrated with a xenon lamp with an illumination power of 100 mW/cm2 was used as a solar simulator.

Results and discussion The chemical structures of PTB7-Th and PC71BM are shown in Fig. 1(a). In order to scrutinize the effect of active layer thickness on the performance of devices, PTB7-Th:PC71BM polymer solar cells were carefully designed. Inverted device architecture was chosen because it demonstrated better long-term ambient stability compared with conventional PSCs31. Moreover, it has been reported that the inverted devices based on the blend of PTB7-Th and PC71BM exhibited higher efficiency than the conventional structure due to the better optical filed distribution and exciton generation profile28. The device configuration is shown in Fig. 1(b). The photoactive layer thickness of the PTB7Th:PC71BM blends were varied from 70 nm to 270 nm. The current density-voltage (J-V) characteristics of the PTB7Th:PC71BM cells with different active layer thickness under simulated AM 1.5 solar illumination with an intensity of 100 mW cm-2 are shown in the Fig. 2. The corresponding device performance parameters are summarized in Table 1. The deviation

10


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of the photovoltaic parameters may be attributed to the shorts and shunt paths caused by particulates on the ITO-coated glass substrates32. It can be seen that the variation of the active layer thickness has a marked impact on the photovoltaic performance of the PSC devices. The JSC initially increases up to 16.55±0.12 mA/cm2 with increasing active layer thickness from 70 nm to 90 nm. Subsequently, the JSC is temporarily lowered, and has a bottom value at the thickness of 120 nm. Thereafter, the JSC begins to increase again with increasing the active layer thickness. When the active layer thickness increases over 120 nm, the open circuit voltage (VOC) shows a slight decline, while the FF drastically drops. As a result, a maximum efficiency of 9.68% is observed in the PTB7-Th:PC71BM solar cells with an active layer thickness of 90 nm. The decline of VOC at thicker active layer region causes an increase of energy loss (Eloss=Eg-eVOC), which is from optical bandgap of PTB7-Th (Eg=1.58 eV) to the VOC of the devices33. It is found that the devices with 70-120 nm thick active layer shows VOC of 0.80±0.01 V, corresponding to an Eloss of 0.78±0.01 eV. Further increasing the active layer thickness to 270 nm leads to

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the increase of Eloss to 0.80±0.01 eV. The increase of energy loss is related to the generation rate of the excitons (i.e. electron-hole pairs) and to recombination process since the VOC is described by the following equation34: 2 KT  (1 − P )( BL + BSRH ) N CV  − VOC =   q q  PG 

Egap

(1)

where P is the dissociation probability of bound electron-hole pairs, Egap is the effective energy gap, Ncv is the effective density of states, G is the generation rate of electron-hole pairs, and BL and BSRH are the Langevin and Shockley-Read-Hall (SRH) recombination strengths, respectively. As indicated in Eq. (1), the increased energy loss in the devices with thicker active layer may be attributed to the decrease in exciton generation rate and increase in the charge carrier recombination. As the thickness of active layer is increased, the JSC seems to increase since the absorption of PTB7-Th:PC71BM film continuously increases. However, the fraction of incident light intensity absorbed by the film is influenced by all layers in the device due to the interference effect. To analyze the reason why JSC exhibits the above behavior, optical simulation using a transfer

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matrix method (TMM) has been performed. TMM has been demonstrated as a useful tool for understanding the optical field distribution within the multilayer structured PSC devices to estimate the potential achievable photocurrent. Each composing layer is represented by its wavelength-dependent complex refractive indices (n+ik), which were measured by spectroscopic ellipsometry. Based on the n and k values for each material, the interference of reflected and transmitted light waves at each interface in the stack, and thus the fraction of light absorbed by the BHJ films in the PTB7-Th:PC71BM device are calculated and summarized in Fig. 3(a). The intensity of light absorption at wavelength between 400 nm and 500 nm is continuously increased with increasing active layer thickness. It should be noted that the device with 90 nm thick active layer shows relatively high absorption, in contrast, that of devices with 120 nm and 180 nm thick active layer is lower at long wavelength region over 600 nm. This is evident that the misfit of optical field distribution occurs. Particularly, the lowest light absorption at wavelength between 500 nm and 600 nm is observed in the device

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with 120 nm thick active layer which leads to the smallest JSC in this device structure. We then calculate the theoretical lossless photocurrent (Jph) of each simulated condition by assuming a unity photon-toelectron conversion, i.e., internal quantum efficiency (IQE)=100% at all wavelengths. The dependence of Jph on the active layer thickness is plotted in Fig. 3(b). The Jph curve clearly shows an oscillating characteristic due to the interference occurring at Ag and ITO electrodes, which results in the variations in optical field distribution inside the active layer35. Therefore, the oscillation of measured JSC is indeed, attributed to coherent interference effect. As shown in Fig. 3(b), the experimental value of JSC is lower than that of the Jph, indicating the IQE should be less than 100% due to the potential loss in charge recombination, transport, and collection. The calculated Jph values and ratio between JSC and Jph are summarized in Table 1. For the device with an active layer of 70 nm, the calculated Jph value is 17.24 mA/cm2, while the measured value JSC is 16.01±0.10 mA/cm2, which indicates that ≈93% of the generated excitons have been converted into charges and harvested by the electrodes. However, the devices with

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thicker photoactive layer show declined JSC/Jph ratio, implying the devices suffer increased charge recombination loss. In addition, the FF values exhibit almost constant at the thickness of less than 120 nm and drastically drop at thicker region. A strong decrease of FF with the thickness is strongly suggestive of inefficient charge transport and collection. Previous research demonstrated that the distribution of photo-generated excitons in different device structures has great influence on charge transport and collection due to the difference in the charge carrier mobility and the transport distance28,

36-37

. Based on the

TMM theory38, for the multilayer structured PSC devices, the total electric field (Ej(x)) at random position x inside layer j is given in terms of the electric field of the incident wave (E0) by

E j ( x) = E +j ( x) + E −j ( x) = (t +j e

iξ j x

+ t −j e

−iξ j x

) E0+

(2)

where ξ j = 2π ( n j + i ⋅ k j ) / λ , nj and kj are the optical index and extinction coefficient, t +j = E +j ( x) / E0+ and t −j = E −j ( x) / E0+ are the electric field propagating in positive and negative direction, respectively. The time averaged exciton generation rate Gj(x) as a function of position is then

15



Gj ( x) =

2 4π cε 0kjnj E j ( x) 2hν

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(3)

where c is the speed of light and the ε0 is the permittivity of free space. Fig. 4 shows the spatial distribution of exciton generation in the PTB7-Th:PC71BM layer with different thickness. The generated excitons in the devices with thinner active layer ((a), (b), and (c)) are distributed with a peak approximately centered at the BHJ layer while the excitons in the thicker devices ((d) and (e)) are broadly distributed with two higher proportion near each side of BHJ layer (one near ITO cathode and the other near Ag anode). Therefore, in the case of device with active layer thickness of 180 nm, the holes generated near ITO cathode travel a longer distance in the organic layer to reach the hole collecting MoO3/Ag anode, but the electron collection distance is much shorter. As a result, for the excitons created closer to the cathode, a more balanced charge carrier transport can be achieved since the electron mobility has been reported to be lower than the hole mobility in PTB7-Th:PC71BM film28. However, for the majority excitons generated near Ag anode, the conduction path for electrons and holes are reversed, with low mobility electrons traveling a longer

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distance. These unbalanced charge transport of holes and electrons increase the probability of charge recombination during their transport, and thus result in the decrease of FF. Increasing the active layer thickness further aggravates the situation, which results in a much lower FF observed in the device with 270 nm thick active layer. The result of this modeling is also supported by the slightly decreased FF from 0.71±0.01 to 0.70±0.01 with increasing the active layer thickness from 90 nm to 120 nm since there is a small shift of exciton generation profile toward anode, which is unfavourable for the electron transport as discussed above. Therefore, based on combined exciton distribution and charge carrier mobility, we consider the FF in the PSCs with active layer thickness over 120 nm is limited by the poor electrode transport and collection, leading to the increased photogenerated charge carrier recombination. Our results are consistent with the previous report that the distribution of generation rate should have a single narrow peak occurring around the middle of the photoactive layer to improve the PSC performance39. In order to get more insight of relationship between device performance and active layer thickness, a further analysis of diode

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parameters was performed by fitting the illuminated J-V characteristics with conventional Shockley diode equation as shown in Eq. (4)40

  V − J ⋅ RS A   V − J ⋅ RS A J = JS exp  − J ph (V )  − 1 + / nkT q R A   P  

(4)

where RP and RS are shunt resistance and series resistance, respectively, JS is reverse saturation current density, k is Boltzmann constant, T is the temperature in Kelvin, n0 is ideality factor, q is electronic charge, and A is the active area of solar cell. The fitting results are summarized in Table 2. For comparison, the dependence of photovoltaic performance and diode parameters on the active layer thickness of the PTB7-Th:PC71BM solar cells is presented in Fig. 5. It can be found that the RS is continuously increased with increasing the active layer thickness due to the low electron transport efficiency. In addition, the values of JS and n exhibit similar variation. they remain almost constant in the devices with thinner active layer thickness, but begin to increase with the active layers thickness further increased beyond 120 nm. Relevantly, the reverse saturation current density JS is

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proportional to the density of interface defect states (Dit)41, which can be expressed by the following equation J S = qA D it kT ω 0 e - E b / kT

(5)

where q is unit charge, A is the active area factor, Eb is relevant excitation energy, k is the Boltzmann constant, T is temperature, and ω0 is a rate prefactor. As shown in Fig. 5, when the active layer thickness increased over 120 nm, the JS drastically increased by one or two orders of magnitude from 1.01×10-7 mA/cm2 to 1.95×10-6

mA/cm2

and

3.48×10-5

mA/cm2,

respectively,

indicating an increase in interface defect states for thicker PTB7Th:PC71BM devices. In addition, according to the theoretical model developed by Saad et al42, the Dit also has a relationship with n as described by the following equation

Dit ∝ 2ε NVbi ( n-1) / qn

(6)

where ε is dielectric constant, N is carrier doping concentration, Vbi is built-in voltage, q is unit charge, and n is an ideality factor. Eq. (6) implicitly shows that the interface states is proportional to the ideality factor. Therefore, the increase of JS and n for thicker (180 nm or 270 nm) PTB7-Th:PC71BM solar cells indicate an increase in interface defect states, from which the electrons and

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holes can recombined and result in the reduction of FF with thicker active layer. In order to decrease the charge recombination, a balanced charge carrier transport is highly important especially in the case of low bandgap polymers due to their higher charge carrier densities43. Increasing the concentration of the acceptor materials (i.e. fullerene derivative) in the PSCs utilizing high mobility donors and thus increasing the electron mobility has been reported as an effective way to achieve the balanced charge transport44. Therefore, in order to improve the electron transport and collection, we increase the fullerene to polymer ratio to 3:1 in thicker PTB7-Th:PC71BM devices. The J-V characteristics of these devices are shown in Fig. 6 and the corresponding device performance parameters are summarized in Table 3. It can be seen that for all the thickness tested, the devices with higher fullerene content (PTB7-Th/PC71BM blend ratio of 1:3) shows a decrease of JSC, but an impressive enhancement of FF compared to the device with 1:1.5 blend ratio. The decrease of JSC for the same thickness films with higher fullerene content is due to the decreased contribution from the polymer absorption. On the other

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hand, the increased FF in the device with 1:3 blend ratio demonstrates that higher fullerene content can provide a betterconnected electron-transporting path, which is favored for better electron transport and collection. As a result, the best efficiency of device with 270 nm thick active layer was increased from 7.79% to 8.15% due to the significantly improved FF from 0.50±0.01 to 0.59±0.01. These results confirm that, by increasing the fullerene content, a more balanced electron and hole transport was achieved in thicker PTB7-Th:PC71BM layer for efficient charge collection. Moreover, with the rapid development of non-fullerene acceptor materials, this method can also be applied to the highperformance PTB7-Th based non-fullerene solar cells with thick active layers, although there are few reports on the effect of active layer thickness on the performance of polymer:non-fullerene devices.

Conclusions In summary, the effect of active layer thickness on the device performance of PTB7-Th and PC71BM based polymer solar cells has been investigated. The results showed that the JSC exhibited an oscillating characteristic with increasing active layer thickness

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due to the coherent interference effect, while the FF dramatically decreased when the active layer thickness increased over 120 nm. As a result, the highest PCE was obtained at an optimized active layer thickness of 90 nm. Optical modeling using a transfer matrix formalism has been utilized to explain the variation of performance in different devices. By analyzing the distribution of exciton generation, it was found that the decrease of FF at thicker active layer region was attributed to the unbalanced charge carrier transport due to the relatively low electron mobility. Moreover, the simulated diode parameters revealed that the interface defect states were remarkable increased for thicker PTB7-Th:PC71BM layer. Therefore, both the unbalanced charge transport of electron versus hole and the increased defect states decreased FF by promoting charge carrier recombination, leading to the inefficient charge carrier transport and collection. Finally, the FF of thick PTB7-Th:PC71BM

devices

was

drastically

improved

by

increasing the fullerene content, allowing an overall increase in PCE to 8.15% for the device with 270 nm thick active layer. Our results will be very useful for the development of new semiconductor materials, such as the non-fullerene acceptors with

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high electron mobility, and the design of device structures for fabricating large-area PSC devices by low-cost, high-throughput R2R process.

Acknowledgements This work was partially supported by the National Nature Science Foundation of China (NSFC) (Grant No. 61705054), Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ17F050002, LY17B060012 and LQ16B070001), National Key Scientific Instrument and Equipment Development Projects of China (Grant No. 2016YFF0101908).

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Acceptors.

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doi.org/10.1002/adma.201800613. (12) Fei, Z.; Eisner, F. D.; Jiao, X.; Azzouzi, M.; Röhr, J. A.; Han, Y.; Shahid, M.; Chesman, A. S. R.; Easton, C. D.; McNeill, C. R. et al. An Alkylated Indacenodithieno[3,2b]thiophene-Based

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32


Page 32 of 39

Page 33 of 39

Figures and Tables

Fig. 1 (a) Chemical structure of PTB7-Th and PC71BM, and (b) Polymer solar cell configuration used in this work.

0

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PTB7-Th:PC71BM=1:1.5 70 nm 90 nm 120 nm 180 nm 270 nm

-4 -8 -12 -16 -20 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Fig. 2 J-V characteristics of PTB7-Th:PC71BM (1:1.5) solar cells with different active layer thickness.

33



Fig. 3 (a) Fraction of the total incident light absorbed in the active layer calculated by TMM for PTB7-Th:PC71BM (1:1.5) solar cells with different active layer thickness, and (b) Calculated photocurrent (Jph) and the measured short circuit current density (JSC) of PTB7-Th:PC71BM (1:1.5) solar cells against the active layer thickness.

34


Page 34 of 39

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Fig. 4 Distribution profile of exaction generation rate for PTB7Th:PC71BM (1:1.5) solar cells with (a) 70 nm, (b) 90 nm, (c) 120 nm, (d) 180 nm, and (e) 270 nm thick active layer.

35



Fig. 5 (a) Short circuit current density (JSC), fill factor (FF), open circuit voltage (VOC), and power conversion efficiency (PCE), and (b) Series resistance (RS), shunt resistance (RP), ideality factor (n), and reverse saturation current density (JS) of PTB7-Th:PC71BM (1:1.5) solar cells as a function of active layer thickness.

0

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 39

PTB7-Th:PC71BM=1:3 120 nm 180 nm 270 nm

-4 -8 -12 -16 -20 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Fig. 6 J-V characteristics of PTB7-Th:PC71BM (1:3) solar cells with different active layer thickness.

36


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Table 1 Photovoltaic performances of PTB7-Th:PC71BM (1:1.5) solar cells with different active layer thickness. Active layer

JSC/Jp VOC

JSC

(V)

(mA/cm2)

70

0.80±0.01

16.01±0.10

90

0.80±0.01

120

PCE

Jph

(%)a)

(mA/cm2)

0.71±0.01

9.44(9.13)

17.24

93%

16.55±0.12

0.71±0.01

9.68(9.42)

17.76

92%

0.80±0.01

15.62±0.11

0.70±0.01

8.94(8.76)

17.42

90%

180

0.79±0.01

17.67±0.15

0.64±0.01

9.22(8.96)

19.76

89%

270

0.78±0.01

19.70±0.09

0.50±0.01

7.79(7.68)

22.23

88%

thickness

FF

h

(nm)

a)

Average PCE in the brackets. Table 2 Derived fitting parameters of PTB7-Th:PC71BM (1:1.5) solar cells with different active layer thickness. Active layer RS thickness

RP

JS n

(Ω cm2)

(kΩ cm2)

(mA/cm2)

70

0.85

0.75

1.65

7.55×10-8

90

0.93

0.53

1.68

1.37×10-7

120

1.04

0.46

1.67

1.01×10-7

180

1.20

0.31

1.97

1.95×10-6

270

1.79

0.10

2.41

3.48×10-5

(nm)

37



Page 38 of 39

Table 3 Photovoltaic performances of PTB7-Th:PC71BM (1:3) solar cells with different active layer thickness. Active layer VOC (V)

JSC (mA/cm2)

FF

PCE (%)a)

120

0.80±0.01

14.53±0.10

0.73±0.01

8.73(8.51)

180

0.80±0.01

14.27±0.16

0.71±0.01

8.34(8.15)

270

0.80±0.01

16.96±0.12

0.59±0.01

8.15(8.01)

thickness (nm)

a)

Average PCE in the brackets.

38


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TOC Graphic

39


Effect of Active Layer Thickness on the Performance of Polymer Solar

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