Ternary Active Layers for Neutral Color Semitransparent Organic Solar

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Ternary Active Layers for Neutral Color Semitransparent Organic Solar Cells with PCEs over 4% Takashi Sano, Shusei Inaba, and Varun Vohra ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02144 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Applied Energy Materials

Ternary

Active

Layers

for

Neutral

Color

Semitransparent Organic Solar Cells with PCEs over 4% Takashi Sano†, Shusei Inaba†, Varun Vohra†* † Department of Engineering Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu City, 182-8585 Tokyo, Japan * Email: [email protected]; Telephone: +81-42-443-5359 (Office)

ACS Paragon Plus Environment

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ABSTRACT.

Ternary blend active layers that include an additional electron donor or electron acceptor material provide the means to easily tune the transmission properties of semitransparent organic solar cells (OSCs) by simply changing the relative concentration of each active material. We added a non-fullerene acceptor (ITIC) into a well-studied donor:acceptor active layer (PCDTBT:PC71BM) that can be produced in air and demonstrates long term operational stability. We investigated the optoelectronic properties of the resulting OSCs and observed that partially replacing the fullerene electron acceptor, PC71BM, with ITIC produces uniformly absorbing active layers which, however, generate a slight decrease in photovoltaic performances compared to the reference binary OSCs. On the other hand, adding ITIC to an optimized PCDTBT:PC71BM ratio of 1:4 leads to a slight increase in short-circuit current density from these ternary OSCs with

respect

to

the

binary

ones.

In

semitransparent

OSCs

fabricated

with

a

PCDTBT:PC71BM:ITIC ratio of 1:4:1, power conversion efficiencies of 4%, average visible transparencies around 40% and color rendering indices of 97 are produced. As the addition of ITIC does not affect the long term operational stability of the unencapsulated PCDTBT:PC71BM OSCs, our study opens the path to the fabrication of stable semitransparent OSCs with balanced optoelectronic properties which could readily be applied as solar energy-harvesting photovoltaic windows.

KEYWORDS. Semitransparent solar cells; Ternary active layers; Organic solar cells; Conjugated polymer; Building-integrated photovoltaics.


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Introduction Over the past decade, organic solar cells (OSCs) have demonstrate their potential as one of the leading technologies for integrated semitransparent photovoltaics.1,2 Conjugated organic molecules can be designed to produce active materials with broad absorption properties extending throughout the visible and the near-infrared regions. Unlike the state-of-the-art photovoltaic devices employing silicon or other active materials such as gallium arsenide (GaAs) and perovskites, OSC active layers are composed of organic semiconductors that can be combined to form uniformly absorbing thin films and devices.1-3 Although their photovoltaic performances remain below those of standard semiconductors, conjugated organic molecules possess several advantages such as high molar extinction coefficients, absorption spectrum tuning through advanced chemical designs and good mechanical properties, which considerably increases their potential for photovoltaic window applications.1,2 In fact, several studies on active layers combining a single electron donor and a single electron acceptor, which either have complementary visible absorptions or absorb outside the visible, resulted in the formation of semitransparent OSC (ST-OSC) with high color rendering indices (CRIs).1,2,4-7 Ternary active blends can simplify the formation process and optical tuning of OSC active layers with enhanced CRIs by combining three organic semiconductors with potentially simpler molecular designs.8-11 Additionally, compared to binary active layers, ternary active layers generally yield higher short-circuit current densities (Jsc) and power conversion efficiencies (PCEs) by broadening the range of harvested sunlight,12,13 but also show great potential to improve the long-term stability of OSCs.14 Neutral color ternary ST-OSCs with PCEs over 8% have been presented over the past couple of years but they either suffer from relatively low average visible transparencies (AVTs) below 30% or low CRIs under 90.8-10 Before considering


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these devices for photovoltaic window applications, it is also essential to fabricate ST-OSCs that exhibit good operational stability possibility without requiring an additional encapsulation step. Poly(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT) is a conjugated polymer with a fairly simple molecular design which can be combined with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in binary active layers to produce OSCs with PCEs over 5% that demonstrate good stability in air.15-20 However, as PCDTBT:PC71BM

active layers strongly absorb at

wavelengths below 600 nm, they cannot readily be used to fabricate neutral color ST-OSCs with high

CRIs.4

Here,

we

investigate

the

inclusion

of

3,9-bis(2-methylene-(3-(1,1-

dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-sindaceno[1,2-b:5,6-b’]dithiophene (ITIC)21,22 into PCDTBT:PC71BM active layers and study the performances of inverted OSCs prepared with reflective or semitransparent top anodes. As PCDTBT, PC71BM and ITIC have complementary absorptions, we explore the possibility to fabricate ternary ST-OSCs that transmit visible light uniformly. We demonstrate that addition of ITIC to the commonly employed PCDTBT:PC71BM (1:4 ratio) active layers produces a slight increase in Jsc and open-circuit voltage (Voc) which, despite a small reduction in fill factor (FF), leads to slightly enhanced PCEs in opaque ternary OSCs compared to the reference binary ones. The ST-OSCs prepared with the ternary active layers produce PCEs up to 4.02%, AVTs close to 40% and CRI values of 97 with relatively stable short-term operation and minor decreases in performances when stored for several days in ambient conditions. Results and Discussion The absorption spectra of 40 nm-thick active material films are presented in Figure 1(a) and the corresponding normalized spectra can be found in Figure S1(a). Reference PCDTBT:PC71BM active layers with a 1:4 ratio spin-coated in air produce an average PCE of 6.11% (Figure 1(b)


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and Table 1, Experimental details in Supporting Information). As the donor:acceptor ratio in PCDTBT-based OSCs strongly affects their performances,23 we prepared several ternary OSCs with various PCDTBT:PC71BM:ITIC ratios (Table S1 and S2). For the first set of experiments, we kept the same 1:4 donor:acceptor ratio as the reference binary OSCs (1:4:0). PC71BM was gradually replaced with ITIC by changing the PC71BM:ITIC ratio from 4:0 to 2:2 (Table S1). In these 1:4 donor:acceptor ratio OSCs, we can clearly observe a decrease of the Jsc and FF with increasing amount of ITIC. Nonetheless, for a PCDTBT:PC71BM:ITIC ratio of 1:3:1, active layers with neutral colors are formed. The second set of experiments was performed by maintaining a constant PCDTBT:PC71BM ratio of 1:4 and gradually increasing the amount of added ITIC (Table S2). The average Jsc of the OSCs increases from 12.02 mA/cm2 (no ITIC) to 12.61 mA/cm2 (PCDTBT:PC71BM:ITIC ratio of 1:4:2). For small amounts of additional ITIC up to PCDTBT:PC71BM:ITIC ratios of 1:4:1, the Jsc increase is associate with a Voc enhancement as well as a slight decrease in FF of the OSCs. However, for larger amounts of additional ITIC, a large drop in FF together with a mild reduction of the Voc can be observed. Unlike the neutral color 1:4:1.5 active layers, the ones prepared with a 1:4:1 ratio maintain high device performances and only exhibit a slight red appearance. Consequently, for the remaining part of this study, we selected the two active layers that have the most promising characteristics for the production of efficient neutral color ST-OSCs, namely, those with a PCDTBT:PC71BM:ITIC ratio of 1:3:1 and 1:4:1 (Table 1 and Figure 1(b)). Table 1. Average photovoltaic parameters from 8 binary or ternary OSCs. Jsc EQE Jsc Voc PCDTBT:PC71BM:ITIC FF (%) PCE (%) 2 2 (mA/cm ) (mA/cm ) (mV)

Rs (Ω.cm2)

1:4:0

12.02

10.73

875

58.1

6.11

3.95

1:3:1

11.73

11.03

907

49.4

5.25

7.27

1:4:1

12.50

12.01

906

54.9

6.21

4.63


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Figure 1. (a) Absorption spectra of 40 nm-thick PCDTBT, PC71BM and ITIC films; (b) J-V characteristics of opaque OSCs. The inset of Figure 1(b) corresponds to the energy levels of PCDTBT (data collected from Sigma-Aldrich), ITIC and PC71BM (data collected from Ossila). In standard PCDTBT:PC71BM active layers, charge separation occurs at the donor/acceptor interface and the photogenerated holes and electrons then percolate through continuous PCDTBT and PC71BM phases, respectively, to be collected at the anode and cathode, respectively. Several working mechanisms have been proposed for ternary OSCs which can be separated into based on charge or energy transfer processes, parallel device model and alloy model.24 The parallel device model implies that the ternary OSC Voc has the same value as the binary device based on the acceptor with the deeper lowest unoccupied molecular orbital (LUMO). As the Voc of PCDTBT:PC71BM:ITIC OSCs increases upon addition of ITIC to the PCDTBT:PC71BM active layers (Figure 1(b), Tables 1, S1 and S2), we can exclude the parallel device model. Li et al. demonstrated that, despite the small differences in LUMO levels of ITIC and PC71BM, electrons that separate at the donor/PC71BM and donor/ITIC interfaces in ternary blends can both percolate to the cathode without being trapped on the material with the lower LUMO.25 Their study also suggests that charges do not separate efficiently at the PC71BM/ITIC interface and thus a


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PC71BM/ITIC alloy-like model may apply to the electron transport in the ternary OSCs. Note that addition of a material with lower electron transport properties would strongly affect the overall electron collection efficiency in the device. On the other hand, PCDTBT and ITIC have the adequate optical properties for resonant energy transfer from PCDTBT to ITIC to occur.26 Nonetheless, supposing that holes are efficiently transferred from ITIC to PCDTBT, direct electron transfer from PCDTBT to ITIC and a two-step mechanism (energy transfer from PCDTBT to ITIC followed by hole transfer from ITIC to PCDTBT) would result in the same interfacial charge separated states. Taking into account all the above observations, we propose a model for PCDTBT:PC71BM:ITIC ternary OSCs in which competitive charge transfer occurs at the PCDTBT/PC71BM and the PCDTBT/ITIC interfaces followed by a PC71BM/ITIC alloy-like electron transport to the cathode. Similarly to binary active layers, producing the adequate nanomorphology in ternary blends is a key factor to achieve efficient charge generation and collection.24 In 1:3:1 OSCs, replacing part of the PC71BM with ITIC resulted in a FF drop along with a small decrease in Jsc compared to the binary OSCs (Table 1). The lower Jsc of the 1:3:1 OSCs is consistent with the external quantum efficiency (EQE) spectra (Figure 2(a)) and the photoluminescence (PL) quenching measurements (Figure S1(b)), which indicate that photons absorbed by ITIC do not efficiently separate into hole/electron pairs in the 1:3:1 active layers that exhibit a PL quenching ratio of 75%. Note that there is a slight mismatch between the Jsc directly measured using the solar simulator (Figure 1(b)) and the value obtained by integrating the EQE (Figure 2(a)) which is referred to as EQE Jsc in Table 1. This mismatch is caused by the limited measurement range of our EQE equipment with no data collected from wavelengths below 380 nm. As the EQE of 1:4:0 OSCs has a larger contribution at short wavelengths compared to the ternary OSCs, their


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EQE Jsc is considerably reduced with respect to their Jsc compared to the relatively small decrease observed for 1:3:1 OSCs. The 1:3:1 OSCs also exhibit a large series resistance (Rs) compared to binary OSCs suggesting that inefficient charge transport occurs. In fact, the electron mobility of the 1:3:1 active layers is almost an order of magnitude lower than that of binary ones (Table S3).

Figure 2. (a) EQE spectra of OSCs and ST-OSCs; (b) Normalized absorption spectra of binary and ternary active layers. As the hole mobility is also reduced with respect to the reference active layers, we suspect that large electron donor-rich and electron acceptor-rich domains are formed in the 1:3:1 thin films, and the resulting OSCs only yield a PCE of 5.25%. On the other hand, when ITIC is added to the active layer without changing the PCDTBT:PC71BM composition, the generated Jsc is approximately 4% higher than the reference OSCs and the ITIC PL is quenched by 90%, indicating that a more intimate mixing of the three active materials is achieved compared to 1:3:1 thin films. The large PL quenching value measured in 1:4:1 active layers also confirms that, if the adequate thin film nanomorphology is produced, efficient hole transfer occurs from ITIC to


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PCDTBT. Despite the stronger relative contribution of ITIC to the active layer absorption in 1:3:1 thin films compared to 1:4:1 ones (Figure 2(b)), the inefficient charge transfer from ITIC to PCDTBT in 1:3:1 active layers results in the same spectral shape above 550 nm for the EQEs of 1:3:1 and 1:4:1 OSCs (Figures 2(a)). Additionally, the hole mobility of 1:4:1 active layers is similar to that of the PCDTBT:PC71BM binary active layers and the ITIC added to the PCDTBT:PC71BM (1:4) OSCs only mildly affects their electron mobility (Table S3). The 1:4:1 OSCs consequently display a minor decrease in FF with respect to 1:4:0 devices, which is compensated by their enhanced Jsc and Voc, thus yielding an average PCE of 6.21%. Unlike opaque OSCs fabricated with a 70 nm-thick reflective Ag anode, ST-OSCs employ thin semitransparent Ag electrodes (15 nm). Their Jsc have values approximately 30% lower than the corresponding opaque OSCs (Tables 1 and 2). Thin Ag electrodes (15~20 nm) have higher sheet resistances than thick ones (60~70 nm), which leads to increased Rs in the ST-OSCs compared to the opaque ones.27 The Rs of 1:3:1 and 1:4:1 ST-OSCs reach high values of 9.40 Ω.cm2 and 8.53 Ω.cm2, respectively, thus producing a larger drop in FF and Voc compared to binary OSCs which maintain relatively low Rs values of 6.05 Ω.cm2 (Figure 3(a)). As a result, the highest PCE obtained for ST-OSCs (4.25%) corresponds to the binary active layers. Table 2. Average photovoltaic parameters from 8 binary or ternary ST-OSCs. PCDTBT:PC71BM:ITIC

Jsc (mA/cm2)

EQE Jsc (mA/cm2)

Voc (mV)

FF (%)

PCE (%)

Rs (Ω.cm2)

1:4:0 (ST-OSC)

8.48

7.15

875

57.2

4.25

6.05

1:3:1 (ST-OSC)

8.13

7.23

898

50.7

3.70

9.40

1:4:1 (ST-OSC)

8.65

8.10

895

51.9

4.02

8.53


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Nevertheless, the 1:4:1 ST-OSCs produce an AVT of 39.2% (Figure 3(b)) and a PCE of 4.02%. The AVT calculated using the solar photon flux (AVTS) results in a slightly lower value of 38.4%. Details for the calculations of AVT and AVTS can be found in the Supporting Information (Experimental Details).

Figure 3. (a) J-V characteristics of the binary and ternary ST-OSCs under 1 sun. (b) Transmittance (T) of ternary active layers and ST-OSCs as well as Reflectance (R) of ternary ST-OSCs; (c) Photographs taken through the 1:3:1 and 1:4:1 ST-OSCs. The lighter areas at the outer ends of the substrates correspond to the electrode-free active layers. Despite the fact that in the photographs, the 1:4:1 ST-OSCs seem to transmit more light than 1:3:1 ST-OSCs, the AVT values for 1:3:1 and 1:4:1 ST-OSCs are 44.8% and 39.2%, respectively. The corresponding AVTS values 1:3:1 and 1:4:1 ST-OSCs are 43.8% and 38.4%, respectively.


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The photographs in Figure 3(c) clearly indicate that 1:3:1 and 1:4:1 ST-OSCs can be employed as photovoltaic windows with relatively neutral colors. In particular, the 1:4:1 ST-OSC (electrode-coated central area) and the 1:3:1 active layer (electrode-free area at the outer end of the substrate) do not generate any significant change in transmitted light color. However, the 1:3:1 ST-OSCs have a slight blue color aspect resulting from the non-uniform transmission of Ag. Nevertheless, both 1:3:1 and 1:4:1 ST-OSCs produce high CRI values of 93 and 97, respectively. For practical applications, it is essential to understand the impact of ITIC addition on the operational stability of PCDTBT:PC71BM OSCs. We compared the short-term (Figure 4) and long-term (Figure S2) stabilities of the unencapsulated binary (1:4:0) and ternary (1:3:1 or 1:4:1) devices. The normalized photovoltaic parameters in Figure 4 clearly indicate that a similar decrease in performances can be observed for 1:4:0 and 1:4:1 OSCs within the initial 30 min of operation under constant irradiation in ambient conditions. After an initial drop, the Jsc and Voc of OSCs with a PCDTBT:PC71BM ratio of 1:4 stabilize within 5 min to approximately 98% and 96% of their initial value, respectively. Their FFs undergo a larger decrease and stabilize only after approximately 20 min. The stabilized FF values for 1:4:0 and 1:4:1 OSCs correspond to 92% and 93% of their initial FF, respectively. After 30 min of constant light irradiation, the resulting PCEs for 1:4:0 and 1:4:1 OSCs measured decreased by 13% and 12%, respectively. Taking into account potential measurement errors, these results indicate that the addition of small amounts of ITIC does not affect the short-term degradation kinetics of PCDTBT:PC71BM OSCs.


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Figure 4. Normalized (a) Jsc, (b) Voc, (c) FF and (d) PCE of OSCs extracted from their J-V curves measured under constant irradiation at 1 sun (AM1.5G, 100 mW/cm2) in air up to 30 min. On the other hand, the OSCs in which PC71BM was partially replaced with ITIC (1:3:1 OSCs) exhibit notable differences with the reference binary OSCs for their Jsc and FF evolutions during the short-term degradation measurements. Unlike the 1:4:0 and 1:4:1 devices, the Jsc of 1:3:1 OSCs remains almost constant for 30 min under constant irradiation (Figure 4(a)). Similarly, their FF maintains a relatively high value during these short-term constant irradiation measurements with a small decrease of less than 0.5%. Although the molecular structure of the employed non-fullerene acceptor is different, these results are consistent with previous findings


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by Gasparini et al. which suggest that replacing fullerene-based acceptors with non-fullerene ones considerably reduces the initial burn-in in OSCs.28 As a result, the degradation kinetics of non-fullerene electron acceptor-based OSCs follow the trend of their FF decreases. Our results for short-time measurement indicate that similar removal of burn-in can be found in the 1:3:1 OSCs despite the presence of a large amount of PC71BM. Partially replacing fullerene acceptors with non-fullerene ones thus seems to improve the short-term OSC stability. The long-term operation of OSCs could also be strongly affected by the addition of ITIC or the partial replacement of PC71BM with ITIC in the device active layers. We measured the 45 days shelf-life of 1:4:0, 1:3:1 and 1:4:1 unencapsulated OSCs and ST-OSCs that were kept in air in dark conditions between each characterization. The J-V curves of these devices were collected in air on the day they were prepared as well as 1, 3, 7, 14, 30 and 45 days after the initial measurement. From day 1, the OSCs exhibit relatively stable operation for 30 min and the photovoltaic parameters reported in Figure S2 correspond to the average values from repeated characterizations during 30 min on each measurement day. For the opaque OSCs (solid lines in Figure S2), only minor differences in Voc and FF evolution are found which cannot clearly be related to the presence of ITIC in the active layers. After 45 days kept in air and dark conditions, the Voc and FF of the opaque OSCs are reduced by 4~5% and 3~4%, respectively. Compared to the 1:4:0 and 1:4:1 opaque OSCs, the 1:3:1 opaque OSCs exhibit a slightly more stable Jsc throughout the 45 days experiment. As discussed above, this may be related to the lack of initial burn-in in 1:3:1 OSCs. After 45 days, the PCEs of 1:4:0, 1:3:1 and 1:4:1 opaque OSCs are reduced by 12%, 11% and 13%, respectively. Despite very similar trends observed for the Jsc evolution of opaque OSCs and their corresponding ST-OSCs (Figure S2(a)), the Voc and FF of the ST-OSCs drop remarkably faster than those of their opaque OSC equivalents (Figures S2(b)


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and S2(c)). Unlike 70 nm-thick Ag electrodes, the 15 nm-thick ones are prone to oxidation when kept in air for relatively long times. In fact, when no additional MoO3 layer is deposited on top of the 15 nm-thick Ag electrodes, the devices perform poorly (PCE of less than 4%) and their performances decrease quickly within hours. Although the top MoO3 layer partially protects the Ag electrodes from oxidation, the comparative results between opaque OSCs and ST-OSCs indicate that, after 45 days in air, the Ag electrodes may have oxidized to some extent. The shelflife measurements thus suggest that more stable ST-OSCs could be fabricated with alternative electrode materials such as transfer-printed graphene,29 a development that is outside the scope of the study presented here. Furthermore, the PCEs of ST-OSCs prepared with MoO3/Ag/MoO3 top anodes only decrease by 30%, 30% and 32% after 45 days, respectively, for active layer compositions of 1:4:0, 1:3:1 and 1:4:1. Considering that these devices are unencapsulated and kept in air, these long-term performances are well above those of most reported OSCs. Conclusions In summary, when the PCDTBT:PC71BM ratio remains unchanged in the OSC active layers, addition of ITIC slightly increases their PCE from 6.11% to 6.21%. Although charges are generated relatively efficiently at the PCDTBT/ITIC interface in 1:4:1 active layers, the lower electron collection efficiency in the ternary OSCs results in a fairly small increase of the Jsc and PCE of 1:4:1 OSCs with respect to 1:4:0 devices. On the other hand, partially replacing PC71BM with ITIC further disturbs the charge generation and transport in the OSC active layers deposited in air but can generate uniformly visible light absorbing thin films. As thin Ag anodes induce a change in spectral distribution of transmitted light, neutral color ST-OSCs are produced when active layers with a 1:4:1 PCDTBT:PC71BM:ITIC ratio are employed. These ST-OSCs exhibit enhanced short-term stability with respect to binary PCDTBT:PC71BM ones and yield average


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PCEs of 4.02% together with AVTs of 39.2% and CRIs of 97. The 1:3:1 ST-OSCs produce lower PCE and CRI values of 3.70% and 93, respectively, but their AVTs reach close to 45%. The inclusion of ITIC into unencapsulated PCDTBT:PC71BM OSCs with active layers prepared in air does not negatively affect their short-term or long-term operational stability in air. Our results thus confirm that PCDTBT:PC71BM-based ternary active layers have a great potential for low-cost fabrication of air-stable photovoltaic window technology. Supporting Information. The following files are available free of charge. The Supporting Information file includes Experimental details (Materials and Methods), the comparative PL spectra of ITIC and the three active layers, the photovoltaic performances of devices with various PCDTBT:PC71BM:ITIC ratios, the hole and electron mobilities in the three active layers calculated from unipolar devices using space-charge limited current fittings, and the average long-term stability measurement of opaque and ST-OSCs. (PDF) Corresponding Author * [email protected] Author Contributions The manuscript was written by V.V based on equal experimental contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Japan Society for the Promotion of Science Grant-in-aid for Young Scientists (B) (Grant No. 17K14549).


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Notes The authors declare no conflict of interest. ACKNOWLEDGMENT The experimental work was supported by the Japan Society for the Promotion of Science through the Grant-in-aid for Young Scientists (B) program (Grant No. 17K14549). The authors are grateful to Prof. Qing Shen for providing access to PL and EQE measurement equipments. REFERENCES (1) Traverse, C.J.; Pandey, R.; Barr, M.C.; Lunt, R.R. Emergence of Highly Transparent Photovoltaics for Distributed Applications. Nat. Energy 2017, 2, 849-860. (2) Vohra, V. Can Polymer Solar Cells Open the Path to Sustainable and Efficient Photovoltaic Windows Fabrication? Chem. Rec. 2018, doi:10.1002/tcr.201800072. (3) Ziang, X.; Shifeng, L.; Laixiang, Q.; Shuping, P.; Wei, W.; Yu, Y.; Li, Y.; Zhijian, C.; Shufeng, W.; Honglin, D.; Minghui, Y.; Qin, G.G. Refractive Index and Extinction Coefficient of CH3NH3PbI3 Studied by Spectroscopic Ellipsometry. Opt. Mater. Express 2015, 5, 29-43. (4) Wong, Y.Q.; Meng, H.-F.; Wong, H.Y.; Tan, C.S.; Wu, C.-Y.; Tsai, P.-T.; Chang, C.-Y.; Horng, S.-F.; Zan, H.-W. Efficient Semitransparent Organic Solar Cells with Good Color Perception and Good Color Rendering by Blade Coating. Org. Electron. 2017, 43, 196-206. (5) Chen, K.S.; Salinas, J.F.; Yip, H.L.; Huo, L.J.; Hou, J.H.; Jen, A.K.Y. Semi-Transparent Polymer Solar Cells with 6% PCE, 25% Average Visible Transmittance and a Color Rendering Index Close to 100 for Power Generating Window Applications. Energy Environ. Sci. 2012, 5, 9551-9557. (6) Chen, C.-C.; Dou, L.; Zhu, R.; Chung, C.-H.; Song, T.-B.; Zheng, Y.B.; Hawks, S.; Li, G.; Weiss, P.S.; Yang, Y. Visibly Transparent Polymer Solar Cells Produced by Solution Processing. ACS Nano 2012, 6, 7185-7190. (7) Li, Y.; Lin, J.-D.; Che, X.; Qu, Y.; Liu, F.; Liao, L.-S.; Forrest, S.R. High Efficiency NearInfrared and Semitransparent Non-Fullerene Acceptor Organic Photovoltaic Cells. J. Am. Chem. Soc. 2017, 139, 17114-17119. (8) Xie, Y.; Huo, L.; Fan, B.; Fu, H.; Cai, Y.; Zhang, L.; Li, Z.; Wang, Y.; Ma, W.; Chen, Y.; Sun, Y. High ‐ Performance Semitransparent Ternary Organic Solar Cells. Adv. Funct. Mater. 2018, 28, 1800627.


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Conversion Efficiencies 10.1021/acsami.8b22337.

over

5.5%.

ACS

Appl.

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Mater.

Interfaces

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doi:

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Ternary Active Layers for Neutral Color Semitransparent Organic Solar

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