Introduction

Polymer solar cells (PSCs) are extremely attractive candidates for use in next-generation solar cell technologies because of their mechanical flexibility, light weight and cost-effective production through solution-based manufacturing processes1,2,3,4,5,6,7,8. To reach commercialization and mass production, PSCs must exhibit high performance and special applications9,10,11.

A typical PSC is based on the bulk-heterojunction (BHJ) device configuration, which sandwiches a layer of polymer donor and fullerene acceptor blend between a transparent electrode (such as indium tin oxide (ITO)) and an opaque, reflective back electrode (such as aluminum (Al) or argentum (Ag))12. There are also interfacial layers existing between active layer and electrodes and the most common anode interfacial layer is poly (3, 4-ethylene dioxythiophene): poly (styrene sulfonic acid) (PEDOT: PSS) or MoO3 for hole transportation and the ZnO or TiOx are the most widely used for cathode interfacial modification7,12,13,14. A great effort is currently being exerted across a variety of research fields in the quest to achieve high-performance devices, for example, the synthesis of novel polymer donors, interfacial morphology control, optimized device structure and processing optimization. These devices have shown high power conversion efficiencies (PCE) approaching to 10%4,15,16,17.

Unlike traditional bulk solar cells prepared from silicon or polysilicon, organic absorbing materials with confined absorption band can selectively be either transparent or semi-transparent in different regions of the solar spectrum11,18. Accordingly, semi-transparent polymer solar cells (STPSCs), an extension of PSCs which utilize transparent conductive materials as both electrodes, have recently gained much scientific attention and are considered to be the highest priority market for PSCs9,19. The STPSCs offer an extensive spectrum of applications such as power-generating windows for buildings, foldable curtains, mobile terminals and clothes, etc. To date, although many transparent polymer-based organic solar cells have been reported previously18,20,21,22,23,24, the lack of appropriate device structure and suitable materials to use in interfacial layers and electrodes for PSCs has meant that these devices have exhibited either low efficiency (<5%, single junction) or unsatisfactory average visible transmittance (AVT) and color purity for large-scale commercial applications10,11.

Similar to graphene, other two-dimensional (2D) nanomaterials have been receiving great attention in recent years mainly due to their complementary electronic properties when compared with graphene25,26,27,28. Molybdenum disulfide (MoS2) is a 2D material consisting of hexagonal sheets of molybdenum (Mo) sandwiched between two hexagonal sheets of sulfur (S). MoS2 exhibits specific anisotropic mechanical and electrical properties due to the weak bonding between layers29,30. A mobility value of over 200 cm2 V−1 s−1 has been reported in a field-effect transistor architecture31,32. It has also been demonstrated that MoS2 could act as Schottky-barrier active layer or hole transport layer (HTL) in photovoltaic33,34,35,36,37. However, MoS2 has not been taken full advantages of its 2D plane properties, even though there are few traps on the layer surface due to its lack of dangling bonds.

We report here on materials and processes for reliable and cost-efficient processing of ITO-free semi-transparent polymer solar cells by introducing ultra-thin versatile MoS2 nanosheets as interfacial layers and semi-transparent electrodes with employing a silver nanowires–n-type MoS2 nanosheets (AgNW-MoS2 NSs) cathode and thin Ag metal anode. This is the first time to report a novel AgNW-MoS2 NSs/n-MoS2 NSs composite cathode can form excellent 2D morphology for interfacial contact and optical distribution in devices and p-MoS2 NSs/ thin Ag metal is found to achieve a suitable anode with high conductivity and transparency at the same time. The ultra-thin versatile MoS2 can effectively avoid aggregation of AgNW electrode and is beneficial for interfacial modification of devices, proposing the MoS2 can replace the traditional electrode and interface layer in organic optoelectronic materials. Incorporation of the AgNW-MoS2 NSs/n-MoS2 NSs cathode and p-MoS2 NSs/ thin Ag as the hybrid anode not only enhances fill factor of devices but also improves the light absorption in the BHJ layer and the transmittance of the devices simultaneously as shown in Fig. 1. After optimization of the devices, combination of material selection and device engineering facilitates the fabrication of ITO-free PSCs and STPSCs with good average performance of 8.00% (best value of 8.72%) and 6.02% (best value of 6.55%), respectively, based on poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T)38. (6,6)-phenyl-C70 butyric acid methyl ester (PC70BM) (PBDTTT-C-T:PC70BM) system. Furthermore, these good chrominance large-scale STPSCs module based on the novel device structure can light a LED chip under natural light illuminated, which shows great application possibility in power-generating windows for building integrated photovoltaic (BIPV).

Figure 1
figure 1

Device configuration of common inverted PSC and MoS2 nanosheets (NSs) modified ITO-free STPSC.

Methods

Sample Preparation

Chemically exfoliated Molybdenum sulfide (MoS2) nanosheets (NSs) were synthesized through Li intercalation. 0.8 g raw MoS2 powders (Sigma-Aldrich) were immersed in 24 mL n-butyllithium (n-BuLi, 2.5 M hexane solution) solution. The mixture was stirred at room temperature for 48 h under an Ar atmosphere and then allowed to settle for hours. The mixture was centrifuged several times to remove LiOH and unexfoliated material. The obtained MoS2 NSs were n-type materials, for the p-type MoS2 NSs were then prepared after 30 min oxygen plasma treatment (PDC-32G-2 PLASMA CLEANERM, high level, 18W). The sample was diluted with isopropyl alcohol to ~0.5 mg mL−1 for device fabrication.

For the post-treatments, the AgNWs-MoS2 ink was manufactured by blending the Ag NWs dispersion (diam. × L 60 nm × 10 μm, 0.5% isopropyl alcohol suspension Sigma-Aldrich) and n-type MoS2 isopropyl alcohol solution (1:1,V:V). Preparation of the Ag NWs and AgNWs-MoS2 Films: The Ag NW films were prepared using the spin-coating process and were formed on precleaned glass substrates that were attached to a supporting glass substrate. The as-received dispersion containing Ag NWs and AgNWs-MoS2 were spin coated for 40 s at speeds ranging from 500–4000 rpm. The dispersion was sonicated for 30 min and shaken well before being the spin-coating process. The formed electrode films were annealed at ≈120 oC for 10 min in a glove box filled with nitrogen (the detail process was shown in support information in Supplementary Fig. S1).

Device Fabrication

The polymer solar cells were fabricated on Glass/ITO, Glass/AgNW or Glass/Ag NW-MoS2 electrodes. For the electron transport layer, the sol-gel derived ZnO film using the reported method39 or 40 nm thickness of n-type MoS2 was placed on the top of electrode. The thickness of ZnO film is approximately 30 nm, determined by a profilometer (Alpha-Step-IQ). Subsequently, the modified samples were transferred to the nitrogen-filled glove-box. The D-A copolymer-containing photoactive layer was prepared by spin coating (900 rpm) the dichlorobenzene solution of D-A copolymer (PBDTTT-C-T) and PC70BM (1:1.5 w/w, polymer concentration of 10 mg mL−1) with 3% volume ratio of DIO additive on the modified electrode. The thickness of the photoactive layer was about 100 nm. Then, the MoO3 precursor or p-type MoS2 solution with optimal thickness was spin-cast on top of the polymer:fullerene composite layer. Finally, the device was pumped down in vacuum (<10−7 torr; 1 torr~133 Pa) and a~100 or 30 nm thick Ag electrode was deposited on top. The deposited Ag electrode area defined the active area of the devices as 0.06 or 0.65 cm2 (all the areas were tested with an aperture).

Power-generating glass Fabrication

The power-generating glass was fabricated from eight 0.65 cm2 (effective area) Device E. All the devices were weld together and then encapsulating by UV curing glue. The LED chip was accessed in the circuit.

Characterization

X-ray diffraction (XRD) patterns of the MoS2 nanoparticles were carried out on a Bruker D8 Focus X-ray diffractometer operating at 30 kV and 20 mA with a copper target (λ = 1.54 Å) and at a scanning rate of 1 o/min. Sheet resistances of electrodes were measured by using a four point probe setup with a source measurement unit (Keithley 2400). Current-voltage (J-V) characteristics were characterized using Keithley 2400. The currents were measured in the dark and under 100 mW·cm−2 simulated AM 1.5 G irradiation (Abet Solar Simulator Sun2000). All the measurements were performed under ambient atmosphere at room temperature. The scan range is form 0 V to 1V and 6.7 mV for each step. All the JV curves are based on 150 points. The incident photo-to-electron conversion efficiency spectrum (IPCE) were detected under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp) and the calibration of the incident light was performed with a monocrystalline silicon diode. Transmittance spectra were analyzed by UV-vis spectroscopy (Perkin Elmer Lambda 750). The morphologies of films were investigated by atomic force microscopy (AFM) using a Digital Instrumental Nanoscope 31 operated in the tapping mode. The thicknesses of all the layers were measured by surface profilometer (Alpha-Step-IQ). XPS studies were performed on a Thermo-VG Scientific ESCALAB 250 photoelectron spectrometer using a monochromated AlKa (1,486.6 eV) X-ray source. All recorded peaks were corrected for electrostatic effects by setting the C−C component of the C 1s peak to 284.8 eV. The base pressure in the XPS analysis chamber was 2 × 10−9 mbar. For the UPS measurements, He I (21.22 eV) radiation line from a discharge lamp was used, with an experimental resolution of 0.15 eV. All the UPS measurements of the onset of photoemission for determining the work function were done using standard procedures with a −5 V bias applied to the sample. Raman spectroscopy was performed using an InVia Raman Microscope system (Renishaw, Inc.), with an Ar+ ion laser operating at 613 nm and 1.2 mW. All the simulation were calculated via Matlab program, the Matlab files were provided from McGehee group, Center for Advanced Molecular Photovoltaics, Stanford University. For the evaluation of the color rendering indices (CRIs), the experimental transmission of each device is folded with the AM1.5 spectrum to obtain the perceived transmission under solar illumination. The resulting data is coupled with the CIE 1931 2o standard observer color matching functions to obtain the corresponding xyY points.

Results

Properties of ultrathin MoS2 NSs

In order to obtain ultrathin MoS2 NSs, the raw MoS2 powders were exfoliated by chemical lithium intercalation–exfoliation method28,40. Generally, the synthetic procedure involves two steps: n-BuLi can intercalate bulk MoS2 with lithium atoms and form LixMoS2 (0 < x < 1) in the gram scale or more, at room temperature. And then LixMoS2 can react with H2O rapidly and release large amounts of hydrogen gas, which can push the MoS2 layers and cause them to separate from each other and form homogeneous ultrathin MoS2 NSs. The powder X-ray diffraction (XRD) patterns of raw and exfoliated MoS2 nanosheets (Supplementary Fig. S2, Supporting Information), after chemical exfoliation, show that only the peaks of (002) and (103) plane remain which confirms that the MoS2 nanosheets were successfully striped. And the disappearance of other peaks could proves ultrathin MoS2 nanosheets tightly lie on the substrate with preferred orientation35. The morphology and structure of the raw MoS2 and exfoliated MoS2 nanosheets were studied by Transmission electron microscope (TEM), High Resolution Transmission Electron Microscopy (HRTEM) and scanning electron microscope (SEM) characterizations. As shown in Supplementary Fig. S3a-d and S4, bending and folding of the material can be clearly observed from the basal plane of the MoS2 NSs, which indicates that the MoS2 NSs are very thin. The HRTEM image Supplementary Fig. S2e shows the typical hexagonal single crystal structure of MoS2 with a distance of 0.278 nm for the (100) Mo atoms41. The selected area electron diffraction (SAED) pattern in (Supplementary Fig. S3f) implies that the MoS2 NSs have a single crystal structure with hexagonal symmetry42.

Supplementary Fig. S5 exhibits the transmission of the raw MoS2 and exfoliated MoS2 nanosheets films which were dispersed in isopropyl alcohol (IPA). In the range of the solar spectrum, the raw MoS2 and exfoliated MoS2 nanosheets films are transparent with relatively high transmission (>80%). The transmission of exfoliated MoS2 nanosheets film is slightly higher than the raw MoS2 one, probably due to the thinner film of exfoliated MoS2 nanosheets layer. In addtion, Fig. 2 demonstrates a quite difference of 2D morphology bettween raw MoS2 and exfoliated MoS2 nanosheets films. The exfoliated MoS2 nanosheets exhibit a much more smooth and homogenous morphology (Rms: 0.91) than raw MoS2 film (Rms: 2.57), which could enhance the interfacial contact and favor the better morphology, resulting in improved performance for organic optoelectronic devices12,13.

Figure 2
figure 2

Tapping-mode atomic force microscopy (AFM) images of films (a) raw MoS2 film height image, (b) exfoliated MoS2 NSs film height image, (scan range: 5 μm×5 μm).

The electric and optical properties of AgNW-MoS2 electrodes

The MoS2 NSs were firstly introduced to adding in AgNW ink for transparent electrode. After optimization of the MoS2 NSs component in AgNW ink, the optical and electrical properties of the AgNW and AgNW-MoS2 electrodes fabricated by different spin speeds are summarized in Table 1 and Fig. 3a and Supplementary Fig. S6. The optical and electrical parameters of pristine AgNW electrode are basically identical with the previous reports. It is well known that a competitively transparent electrode must has a transparency of at least 90% and a sheet resistance of less than 10 Ω/sq43, therefore only AgNW ink is hard to satisfy the two requirements at the same time44,45. Delightfully, the modified AgNW-MoS2 loading shows a relative low sheet resistance of 9.8 Ω sq−1 and the corresponding transmittance of the electrode also exhibits a high value of 93.1% at 550 nm. It is obvious to found that the transmittance of the electrodes via different speed after the addition of MoS2 NSs has a significant increase, mainly attributed to its high refractive index of MoS2 NSs and a well 2D plane structure. Moreover, the environmental stability of AgNW and AgNW-MoS2 electrodes (500 rpm spin-coating speed) were tested under 50 oC and 70 relative humidity (RH) conditions for 7 days, as shown in Supplementary Fig. S7. AgNW electrode is easy to oxidation under the atmospheric environment46,47, nevertheless, the incorporation of MoS2 NSs in AgNW electrodes show eminent oxidation resistance and moisture absorption stability.

Table 1 Summary of the key parameters of the AgNW and AgNW-MoS2 electrodes fabricated at different spin speeds.
Figure 3
figure 3

The optical property of AgNW composite electrodes and energy level alignment of devices.

(a) Transmittance of spin-coated AgNW and AgNW-MoS2 on glass deposited at different spin speeds. (b) the schematic energy diagram of the electrodes and interfacial layers involved in the STPSCs.

The pristine MoS2 NSs are n type semi-conductor materials34,48, several work has been reported that MoS2 could be changed as a p type semi-conductor materials with a relative high work function after UV-ozone plasma treatment34,37,48. Thus, the properties of MoS2 NSs before or after the UV-ozone plasma treatment were also investigated. Supplementary Fig. S8 is the X-ray photoelectron spectroscopy (XPS) profiles of n-MoS2 NSs (w/o plasma treatment) and p-MoS2 NSs (with plasma treatment). The Mo 3d spectra of pristine MoS2 NSs exhibit strong Mo4+3d5/2 and Mo4+3d3/2 bands at 228.5 eV and 231.8 eV, in agreement with the others work for n-MoS2 NSs35,36. However, the two strong peaks have a notable shift to 235.5 eV and 232.3 eV, respectively, which is consistent with the spectra of MoO349. Therefore, it is proved that n-MoS2 NSs can be successfully oxidized to p-type materials after plasma treatment35.

The work function (WF) of the n and p type MoS2 NSs was determined via UPS (Supplementary Fig. S9) and Kelvin probe (Supplementary Fig. S10). The WF of the n-MoS2 NSs (pristine MoS2 NSs) is found to be 4.3 eV, while that of the AgNW-MoS2 electrode is 4.1 eV, which is even lower than Ag electrode (4.3 eV), meaning more efficient electron collection for cathode12. As expected, after plasma treatment, the oxygen doping at the surface of the MoS2 NSs film and making the WF of p-MoS2 NSs upshifts to 5.0 eV, which is nearly to the WF of MoO3 (5.3 eV). The substantial increased work function is close to the HOMO level of PBDTTT-C-T, as illustrated in Fig. 3b, which can avoid the hole from capturing and gathering at the interface to form a recombination center, leading to more effective hole selection and transportation for PSCs12.

Photovoltaic performance and characterization

Besides suitable energy alignment in the devices, excellent interfacial morphology is also critical for high performance PSCs13. The formation of the AgNW and AgNW-MoS2 films can also be verified by optical microscope (OM) and AFM images displayed in Supplementary Fig. S11 and Fig. 4a,b. Enormous aggregation of AgNW can be observed from large-scale OM and micro-scale AFM observation, while the AgNW-MoS2 film develops more smooth morphology with no apparent aggregation in Supplementary Fig. S11. Intriguingly, the AFM image of AgNW-MoS2 film demonstrates highly oriented Ag nanowires arrange uniformly in MoS2 NSs background, with a more smooth surface than AgNW one (RMS:1.68 vs 21.62,). These oriented Ag nanowires can provide a convenient pathway for electrons transportation and collection in AgNW-MoS2 electrodes, beneficial to the high conductive electrode. In addition, the interpenetrating network structure of AgNW and MoS2 NSs also render a great contribution to the homogeneous surface of AgNW-MoS2 film and high transmittance. The Ag composite electrodes could also have a great influence on active layer, as shown in Supplementary Fig. S12. Supplementary Fig. S12a shows the morphology of BHJ active layer which based on AgNW electrode with a rough surface (RMS: 6.46). However, the AgNW-MoS2 film can provide a 2D substrate for deposition of BHJ active layer with a reduced RMS of 1.21. The different BHJ active layers and p-MoS2 NSs have a great effect on the morphology of top Ag electrode as well. The AFM height and three-dimensional images of 30 nm Ag film deposited on bare BHJ layer (Fig. 4c,e), display a horrifically rough surface with a RMS of 20.07, leading to the poor device performance (discuss later). In contrast, the Ag film covered on p-MoS2 layer/BHJ exhibits an uniform and smooth surface morphology in Fig. 4d,f, which means p-MoS2 NSs can act as a soft mattress and provide an ingenious interfacial contact between BHJ layer and Ag electrode.

Figure 4
figure 4

Tapping-mode AFM images of electrodes (a) AgNW bottom electrode film height image, (b) AgNW- MoS2 bottom electrode film height image, (scan range: 5 μm×5 μm) height images of 30 nm thin Ag top electrode film (c) which based on BHJ layer (d) which based on BHJ/p-MoS2 layer. Three-dimensional images of 30 nm thin Ag top electrode film (e) which based on BHJ layer (f) which based on BHJ/p-MoS2 layer, (scan range: 3 μm×3 μm).

To determine the function of the versatile MoS2 NSs on the optoelectronics, the PSCs based on: (PBDTTT-C-T) :(6,6)-phenyl-C70 butyric acid methyl ester (PC70BM), Glass/Cathode/ETL/ PBDTTT-C-T:PC70BM/HTL/Ag) were fabricated with the solution-processed 2D n-MoS2 NSs films as ETL and solution-processed 2D p-MoS2 NSs as HTL. The current density-voltage (JV) characteristics of inverted cells with various buffer layers under AM 1.5G irradiation at 100 mW·cm−2 are shown in Fig. 5a and the related electrical parameters are summarized in Table 2. The Device A (Glass/ITO/ZnO/BHJ/MoO3/Ag(100 nm)) as the control device delivers a PCE of 7.62% with a short-circuit current density (Jsc) of 15.97 mA·cm−2, an open circuit voltage (Voc) of 0.76 V and a fill factor (FF) of 0.63. After the PSCs are modified by n-MoS2 and p-MoS2 interfacial layer, the FF of the Device B (Glass/ITO/n-MoS2/BHJ/p-MoS2/Ag (100 nm)) increases to 0.70 together with improved PCE and Jsc. Since the FF of PSCs is normally determined by the interfacial contact and morphology of the devices, the increase in FF can be attributed to the smoother interfacial contact between electrodes and active layer after introducing the 2D MoS2 NSs layers. Moreover, the relative high conductive MoS2 layers also tend to form favorable interfacial transportation to enhance the charge extraction and reduce the charge combination, leading to the improvement in the Jsc. When the ITO cathode was replaced to AgNW, all of the device parameters, e.g. PCE, Jsc, Voc and FF are substantially reduced in Device C (Glass/AgNW/n-MoS2/BHJ/p-MoS2/Ag(100 nm)), resulting from the poor transmittance of cathode and reduced light absorption in active layer. Intriguingly, after introducing AgNW-MoS2 electrode, the PCE of ITO-free device is further improved from 6.39% for Device C to 8.00% for Device D (Glass/AgNW-MoS2/n-MoS2/BHJ/p-MoS2/Ag(100 nm)), which is even comparable to the control Device B based on ITO cathode. The improved efficiency is mainly related to the increased Jsc (15.66 mA cm−2) and FF (0.66), revealing the function of AgNW-MoS2 electrode and in good agreement with AFM measurement (Fig. 4 and Supplementary Fig. S12).

Table 2 Performance of PSCs (PBDTTT-C-T:PC70BM) with different devices under the illumination of AM1.5G, 100 mW/cm2.
Figure 5
figure 5

Current (J)–voltage (V) characteristics and Incident photon-to-current efficiency (IPCE) of cells based on different devices (a) JV result for Device A-E (b) JV result for Device E-H, (c) IPCE result for Device A-E (d) IPCE result for Device E-H.

To fabricate a semi-transparent polymer solar cells (STPSCs), the top Ag electrode directly decreased Ag thickness from 100 nm to 30 nm11, since the control device with 100 nm thick Ag anode was almost opaque (average visible-light transmittance: <0.05). When the thickness of top Ag layer decreases to 30 nm for a thin electrode, the total transmittance of the devices shows an average value of 0.21, as shown in Supplementary Fig. S13. Gratifyingly, the STPSCs based on the structure (Device E: Glass/AgNW-MoS2/n-MoS2/BHJ/p-MoS2/Ag(30 nm)) emerges an average PCE of 6.02%, with a Jsc of 12.66 m−2, an Voc of 0.75V and a FF of 0.64, when the light was illuminated from bottom side, the PCE is the highest values reported so far for ITO-free semi-transparent PSCs devices. Moreover, the PCE of the Device F with the light injected from the top Ag side also present a PCE of 2.77%. The effectiveness of Device E structure inspires us to apply it for large-scale STPSCs. The JV curves of the STPSCs which increase the effective area from 0.06 cm−2 to 0.65 cm−2 were shown in Fig. 5b and Table 2. the ITO-free inverted device with a structure of Glass/AgNW-MoS2/n-MoS2/BHJ (0.65 cm−2)/p-MoS2/Ag (30 nm)) (Device G) still shows a PCE as high as 2.71%, with a Voc of 0.54 V, a Jsc of 9.38 mA·cm−2 and an FF of 0.53 and parameters of the device tested from top Ag side also reach a high level (1.20, Device H). The IPCE spectra of different devices based on PBDTTT-C-T:PC70BM are shown in Fig. 5c,d and the values of Jsc via the IPCE measurement well match with the ones from I-V curves. These results demonstrate the universality of this novel and large-scale STPSCs.

Discussion

To find out the reason for higher efficiency of PSCs caused by introducing the MoS2 NSs interfacial layer, the simulation of the spatial distribution of the squared optical electric field (normalized to the incoming plane wave) for the devices with ZnO/MoO3 and n-MoS2 and p-MoS2 were shown in Fig. 6a. The increased Jsc of these devices based on MoS2 interfacial layers can be attributed to high refractive index of n-MoS2 NSs (3.25 from 300 nm to 800 nm) leading to the higher light intensity within the active layer, as shown in Fig. 6a. From the figure we also can see that there is a dramatic improvement of the optical electric field intensity of 500 nm wavelength in active layer of the devices with n-MoS2/p-MoS2 with respect to the devices with ZnO/MoO3. Furthermore, the simulation of the spatial distribution of the squared optical electric field for the Device C, D and E were presented in Fig. 6b, where the figures of 0.36, 0.37 and 0.28 in brackets mean the relative optical electric field intensity of active layer for different devices. It is obviously found that there is no big difference between Device C and D (0.36 vs 0.37, the relative area ratio). The thickness of the top Ag electrode sharply decreases from 100 nm to 30 nm, while the relative optical electric field intensity of active layer slightly reduces to 0.28 for Device E. In addition, to gain deeper insight into the performance of the large-scale STPSCs were illuminated under different light intensity, we studied a power law dependence of Jsc upon illumination intensity is generally observed in PSCs and can be demonstrated as:

Figure 6
figure 6

Simulated spatial distribution of the squared optical electric field |E|2 (normalized to the incoming plane wave) for different the devices (a) Device A and Device B for 500 nm, (b) Device C, Device D and Device E for 400 nm, 500 nm and 600 nm with the same y-axes scale. (The insert figures in brackets mean the relative area ratio in active layer. All the simulation were calculated via Matlab program, the Matlab files were provided from McGehee group, Center for Advanced Molecular Photovoltaics, Stanford University53).

where I is the light intensity and α is the exponential factor50,51. In Supplementary Fig. S14, the data are plotted on a log−log scale and fitted to a power law using eq 1: α = 0.955, higher than the reference device52, indicating that bimolecular recombination could be less in the cell with the MoS2 NSs transport layer. It is also found that the STPSCs based on Device E structure behave an excellent performance under weaker illumination, as shown in Supplementary Fig. S14.

To evaluate the transparency perception of the above STPSCs in human eyes, the tristimulus value (X, Y, Z) and the color coordinates (x, y) were calculated from the transmission spectra while the incident light source was replaced with the AM 1.5 spectrum. The color coordinates of the studied STPSCs with different thickness of Ag on CIE 1931 chromaticity diagram are illustrated in Fig. 7a,b. The corresponding coordinates of the ITO, AM 1.5 point, AgNW-MoS2 and Device E are (0.350, 0.326), (0.359, 0.355), (0.333, 0.347) and (0.313, 0.329), respectively. All these films show very nice transparency color perception that is close to AM 1.5 point. Fig. 7c,d show two digital photographs taken through and not taken through the STPSCs based on Device E structure. There is no big aberration between the two pictures, which means it could provide an opportunity for these STPSCs applying in power-generating windows. In order to further to prove the possibility, a power-generating glass was fabricated which is made up of eight 0.65 cm2 Devices E, as shown in Fig. 8. Fig. 8a delivers the power-generating glass illuminated under 1.5 AM solar condition and the LED chip basically can achieve the rated power. Miraculously, the solar cell glass presents an excellent performance out of doors condition (Test time: October 25th, 2014 PM 13:00 and Environmental conditions: Nanchang City, Jiangxi Province, P.R China, 25 oC Sunshine and 43% HR). The photocurrent of power-generating glass is enough to light the LED chip regardless of illumination direction from the front or back side. From these applied tests, it can be sure to say, this kind of power-generating glass provide a great opportunity for building integrated PV (BIPV).

Figure 7
figure 7

Color properties of the electrode (a) The representation of color coordinate of the studied different electrodes and Device E on CIE 1931xyY chromaticity diagram. (b) the picure of Device E. Two digital photographs (c) taken through and (d) not taken through the STPSCs based on Device E structure. (The photographs were taken by Xiaotian Hu).

Figure 8
figure 8

Power-generating glass images.

Power-generating glass was fabricated via combine eight 0.65 cm2 deivces E, the devices were encapsulated by UV curing glue. To test the power of this semi-transparent solar cell glass, a 5 mW LED chip was put in circuit. (a) the power-generating glass was illuminated under 1.5 AM solar condition (b) the power-generating glass was tested from back side out of doors (c) the power-generating glass was tested from front side out of doors.

Conclusions

In summary, we have demonstrated a versatile MoS2 nanosheets with well 2D structure that were introduced into Ag nanowires transparent electrode, anode and cathode interfacial layers in PSCs. High performance ITO-free PSCs based on Glass/AgNW-MoS2/n-MoS2/BHJ/p-MoS2/Ag(100 nm) achieve a high PCE of 8.00%.

Furthermore, a novel STPSC is incorporating the MoS2 NSs in interfacial layers and AgNW transparent electrode with admirable interfacial contact and superior light-harvesting abilities. The STPSCs exhibited good PCE and AVT with neutral color perception close to that of standard sunlight, which is the highest performance for ITO-free semi-transparent PSCs devices reported up to now. The delightful results demonstrate STPSCs do have great values in power-generating windows of building integrated PV in near future.

Additional Information

How to cite this article: Hu, X. et al. Versatile MoS2 Nanosheets in ITO-Free and Semi-transparent Polymer Power-generating Glass. Sci. Rep. 5, 12161; doi: 10.1038/srep12161 (2015).