Degradation analysis of inverted PM6 (PBDB-T-2F):Y7 (BTP-4CI) solar cells with PDINO and MoO₃ as the ETL/HTL

Abstract

In this work, an analysis is presented of both the degradation and the photo-degradation of inverted organic solar cells (iOSCs), with the PM6 (PBDB-T-2F):Y7 (BTP-4Cl) blend without any additive as the active layer. PDINO and MoO₃ are used as electron and hole transport layers (ETL/HTL), respectively. The maximum obtained power conversion efficiency (PCE) of the devices is 9.14%. The long-term operational stability and the photostability aging tests of the unencapsulated iOSCs under continuous exposure of AM 1.5G illumination conditions (100 mW cm⁻²) are performed under dark and nitrogen environments for 3552 h, and following the ISOS-L-1 protocols for 220 h. The times to reach 90%, 80%, and 50% of the initial PCE of the iOSCs (T₉₀, T₈₀, and T₅₀) under dark and N₂ conditions are 2363 h, 2918 h, and >3552 h, respectively, and 17 h, 31 h, and 220 h, respectively, under ISOS-L-1. The analysis showed that V_OC is the least affected parameter by the degradation for both studies. The impedance spectroscopy (IS) measurements confirmed that the PDINO layer degradation was the main cause of degradation of the devices under dark and nitrogen environments, while the active layer showed the best stability. On the other hand, following the ISOS-L-1 protocols under continuous illumination, the IS results suggest that the performance degradation of iOSCs is mainly due to the photo-degradation of both the active and the PDINO layers, while the MoO₃ layer showed the best stability.

---

Introduction

During the last few years, n-type organic, low-bandgap non-fullerene acceptors (NFAs) have been used to replace fullerene acceptors in bulk-heterojunction (BHJ)-based organic solar cells (OSCs), achieving efficiencies well above 10%. These NFAs present strong absorption in the near-infrared region and it is possible to tune their energy levels to the contacting donor polymers, for example, P3HT or PM6 (PBDB-T-2F).^1–12 These small molecule acceptors, also called acceptor–donor–acceptor (A–D–A), contrarily to polymer acceptors, have definite chemical structures and molecular weights, with better reproducibility in device performance. The A–D–A structure, that is the alternating electron donating (D) with the electron accepting (A) unit, promotes intramolecular charge transfer (ICT), which results in lower bandgap values and wider absorption spectra.⁴ Both effects help in increasing the cell power conversion efficiency.

Many new NFA materials have been synthesized and characterized, each with its advantages and disadvantages. Among them, a very interesting group is the so called “Y family”, to which Y6, also called the (BTP-4F) material, belongs. In Y6, 2,1,3-benzothiadiazole (BT) was employed as the central core and its main characteristics and development achieved can be found in the reported studies.^7,9 Other modifications are being introduced into these new materials, such as by producing asymmetric molecules, and introducing A–DAD–A units as present in Y2 and Y1, which allow the PCE to be increased even more.

Y7, also known as BTP-4Cl, is also a highly conjugated organic semiconductor, composed of a fused thienothienopyrrolo-thienothienoindole (TTP-TTI) core base. The F atoms on the end units of Y6 are changed to Cl, so it is possible to say that Y7 is the chlorinated version of Y6. In Y7, the absorption range shows a redshift of about 20 nm in optical absorption spectra, which produced about 100 meV downshift in the LUMO level. A power conversion efficiency up to 15.7% with PM6 as the polymer donor has been achieved.^7,10

In general, cell performance depends on many factors, some are related to the properties of materials used in the active blend, ETL and HTL, others to the structure of the cell, and others to the processes used for its fabrication. One of these factors is the relation between the morphology of the active layer and the cell performance since it influences charge transfer and transport mechanisms in general.¹

With the synthesis of these new materials, this relation has also been studied. Upama et al. reported the effect of thermal annealing on the morphology of the NFA 3,9-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′]dithiophene (ITIC). The effect of introducing additives, such as 1-chloronaphthalene (CN), on the morphology of the active layer has been studied.^3,11 These authors also studied the effect of thermal annealing on the morphology of active layers prepared with PM6:Y7 as the donor polymer and NFA respectively. The effect on the layer morphology and cell performance of other additives can be found in ref. 12–15.

The effect of using different materials for the ETL and HTL, as well as standard or inverted structures, has also been reported for OSCs using different NFAs, including the ones belonging to the Y family.^{11,12,15–20}

As already mentioned, cell performance depends on many factors which are under study by many authors. However, it is important to remark that, if it is very important to improve the cell performance parameters obtained right after fabrication, it is also very important to study the long-term operational stability of these electrical parameters, especially under continuous illumination stability, since it is considered one of the major challenging issues to be confronted.

In this work, we study the degradation process (under darkness, and under continuous illumination) observed in inverted organic solar cells (iOSCs) using the donor polymer PM6 and the NFA Y7 for the active blend layer. No additives were used during the preparation of the blend, with the goal only to concentrate on the effect of the selected materials for their degradation study on the organic solar cells. In these cells, we used PDINO as the ETL material, reported as a promising interlayer that provides a highly photostable inverted non-fullerene organic solar cell.¹²

We selected MoO₃ as the HTL based on the reported information in the literature^21–25 and in our previously reported work,¹⁹ where better stability of the iOSCs is observed, in comparison with other materials used as HTLs.

The degradation study of main cell parameters was done under two different degradation conditions: dark and nitrogen (N₂) environments and, following the ISOS-L-1 protocols.²⁶ In the first case, the devices were exposed to light only during the electrical characterization and immediately returned to the dark conditions in N₂. This study was done for 3552 hours.

In the second case, a photostability aging test of the iOSCs under continuous exposure of AM 1.5G illumination conditions (100 mW cm⁻²) was performed, until 50% of their initial PCE values (T₅₀) was achieved, which occurred after 220 h. During this photo-degradation test measurements, the devices were kept under continuous light exposure and measured every 10 min for the first 60 min. Afterwards, measurements were done every 30 min for the next 180 min. After that, measurements were done every 60 min until reaching the T₈₀ photo-degradation time. Finally, measurements were done every 180 min until reaching T₅₀. In this study, to highlight the effect produced mainly by the photo-degradation, devices were kept under a N₂ environment, using a sealed sample holder. In this way, the possible reaction with H₂O and O₂ molecules present in the air environment was prevented.

Results and discussion

Electrical characterization – current density–voltage (J–V) measurements

The iOSC structure under study and the energy levels of the different layers in the structure are shown in Fig. 1a and b, respectively. The band energy and work functions were taken from ref. 27–31.

Fig. 1 (a) Cell structure; (b) energy band diagram of the materials used in the iOSC under study.

Table 1 shows a summary of the device performance parameters just after fabrication, such as open circuit voltage (V_OC), short circuit current density (J_SC), fill factor (FF), power conversion efficiency (PCE), and both series and shunt resistances (R_S and R_SH) of the iOSCs. The maximum PCE of the iOSCs was 9.14% and an average of 8.93 ± 0.16%. They also showed a J_SC maximum of 22.5 mA cm⁻² and an average of 21.14 ± 0.94 mA cm⁻², a V_OC maximum of 740 mV and an average of 726 ± 9.90, and a FF maximum of 60.51% and an average of 60 ± 0.95%. Similar results were obtained by Moustafa et al. in ref. 11 where the iOSCs were fabricated without additives using the same materials in the active layer (PM6:Y7). They also studied the effect of introducing 1-chloronaphthalene (CN), and the effect of thermal annealing on the morphology of the active layer.

Table 1 Device performance parameters for the fabricated iOSCs, measured under AM 1.5G at 100 mW cm⁻² intensity. All parameters were averaged over nine devices

J SCavg [mA cm^−2]	V OCavg [mV]	FFavg [%]	PCEavg [%]	PCEmax [%]	R Savg [Ω cm^2]	R SHavg [Ω cm^2]
21.14 ± 0.94	726.00 ± 9.91	60.00 ± 0.95	8.93 ± 0.16	9.14	2.14 ± 0.16	202.50 ± 25.00

---

Degradation analysis of the iOSCs under darkness and continuous illumination

Fig. 2a shows the current density vs. voltage (J–V) characteristics of the best-performing iOSC after fabrication, under light (1.5G spectrum, 100 mW cm⁻²), for devices kept under dark and N₂ environments. This figure also shows the experimental and modeled J–V curves at different times of degradation. Fig. 2b shows J–V curves under darkness for different times during 3552 h. The J–V curve named “fresh” was characterized immediately after the fabrication process at 0 h. Then, the devices were stored in the dark under a N₂ environment at room temperature (RT) and a controlled relative humidity (RH) of 20–22%.

Fig. 2 J–V characteristics of devices measured under (a) light (1 sun) and (b) darkness. (c) UV-vis optical characteristics of ITO, ITO/PDINO and ITO/PDINO/PM6:Y7.

As can be observed in Fig. 2a, the J–V measurements were modeled using an equivalent circuit of one diode (inset in Fig. 2a).³² The extracted parameters from the fitting of the selected J–V curves are shown in Table 2. As reported in our previous studies,^19,33,34 the ideality factor of the diode (n) represents the dominant transport mechanism in the device, where n = 1 (ideal diode) indicates that the diffusion mechanism is dominant in the device. However, a value of n = 2 indicates that recombination is the dominant mechanism, which is mainly caused by trap-assisted recombination.^35,36 In our case, it is observed that n increased from 1.31 (fresh cell) to 1.46 after 3552 h. This suggests that the increasing trap density is minimal, and therefore the device degradation caused by trap-assisted recombination is small. On the other hand, saturation current density (J₀) increased from 9.72 × 10⁻⁹ mA cm⁻² to 1.87 × 10⁻⁸ mA cm⁻² in the same period. The value of J₀, related to the saturation current in OSCs, is associated with the number of charges capable of overcoming the energy barrier at the donor/acceptor interface in the active layer of the bulk heterojunction (BHJ) OSCs. J₀ also represents the minority charge density near the interface of the BHJ. In this sense, if the device presents a greater J₀, there is a greater recombination in the active layer.^37–39 This slight increment in n and J₀ is consistent with the great stability of V_OC over time, observed in Fig. 3a. According to ref. 36–38, V_OC is directly affected by n and inversely affected by J₀.

Table 2 Modeling parameters obtained from the circuital model

Time [h]	J L [mA cm^−2]	n	J 0 [ mA cm^−2]
a Photogenerated current density. b Ideality factor of diode. c Saturation current density of a diode.
0	20.58	1.31	9.72 × 10^−9
312	19.66	1.32	9.84 × 10^−9
2280	18.67	1.38	1.09 × 10^−8
2712	17.22	1.46	1.77 × 10^−8
3552	15.77	1.49	1.87 × 10^−8

---

Fig. 3 Normalized performance parameters over time: J_SC, V_OC, FF, and PCE of the iOSCs degraded under (a) dark and N₂ environments, and (b) ISOS-L-1 protocols. Series resistance (R_S) and shunt resistance (R_SH) over time of the iOSCs degraded under (c) dark and N₂ environments, and (d) ISOS-L-1 protocols.

The absorbance measurements for the layers (ITO, ITO/PDINO and ITO/PDINO/PM6:Y7) are shown in Fig. 2c. In the UV-vis spectrum, it is observed that the absorbance of the ITO layer showed a peak centered at ∼460 nm.⁴⁰ For the ITO/PDINO stack, the maximum of the spectrum red-shifted toward 480 nm, which is due to the absorption characteristics of the PDINO layer deposited on top of the ITO layer.⁴¹ After the deposition of the PM6:Y7 active layer, the absorption spectrum of the structure within the interval from 300 nm to 900 nm includes two new maximum peaks, located at 625 nm and 830 nm, respectively, which is consistent with the results reported in ref. 16.

The normalized performance parameters (V_OC, J_SC, FF, and PCE) are shown in Fig. 3a and b, for devices degraded under dark and N₂ environments, and for devices photo-degraded following ISOS-L-1 protocols, respectively. As can be observed in Table 3, the times to reach 90%, 80%, and 50% of the initial PCE of the iOSCs (T₉₀, T₈₀, and T₅₀, respectively) are 2363 h, 2918 h, and greater than 3552 h. This degradation analysis time is the longest reported for an inverted structure of OSCs using an ITO/PDINO/PM6:Y7 structure, degraded under similar environmental conditions. On the other hand, following the ISOS-L-1 protocols, the T₉₀, T₈₀, and T₅₀ were 17 h, 31 h, and 220 h, respectively, where T₈₀ is 3.6 times longer than that obtained in the recent reported work by Moustafa et al.¹² using the same active layer and ETL, but V₂O₅ as the HTL. The photo-degradation analysis was done under the same illumination conditions. In our case, T₈₀ was 11.6 times longer than that for iOSCs with the same active layer, but using ZnO as the ETL.^12,42 This improvement in T₈₀ is attributed to the use of MoO₃ and PDINO^18,43–45 instead of V₂O₅ and ZnO.

Table 3 Time to achieve E₉₀, E₈₀ and E₅₀ for analyzed devices

Degradation conditions	T 90 [h]	T 80 [h]	T 50 [h]
Dark and N2	2363	2918	>3552
ISOS L-1 protocols	17	31	220

---

It is important to remark that, during the first 300 min under continuous irradiation, the iOSCs demonstrated a highly photostable behavior, showing only a slight drop in PCE of 2%. In this sense, the devices exhibited an insignificant “burn in loss” degradation, which is commonly presented during the first operation minutes of the devices under light illumination. This phenomenon is associated with the intrinsic light-driven degradation due to the material properties, and the photochemical reaction of the ETL and HTL with the photoactive layer.^19,46,47

The normalized PCE decays exponentially over time during the first 2400 h, and afterwards the decay ratio incremented. The J_SC degradation presented an initial increase during the first hours of operation. Afterwards, it presented a similar behaviour to the PCE, suggesting that the behaviour of the PCE is given by the behaviour of the J_SC. The slight increase in J_SC can be attributed to the photochemical reactions in the active layer during the first measurements under illumination, resulting in an increment of the polymer conductivity, as reported in ref. 33 and 48. The J_SC value reached 83% after 3552 h, see Fig. 3a.

The FF behaviour over time showed an exponential decay during the first hours of operation, and after 2280 h the decay ratio incremented. It is reported that R_S can affect the FF, and therefore the behaviour of FF can be associated with the changes of R_S over time observed in Fig. 3c.³⁵

The V_OC degradation behaviour was the most stable parameter. During the first 2500 h, it had a slight increase, keeping stable afterwards; similar behaviour is reported in ref. 19, 49 and 50. The V_OC losses may arise from light-induced trap states/defects as described elsewhere.^35,51 Therefore, the slight increase in V_OC suggests that the trap density remains low, and therefore the device degradation caused by trap-assisted recombination is small. This agrees with the values of n and J₀ shown in Table 2.

In general, compared to the other device performance parameters, V_OC seems to be the least affected by the degradation in PM6:Y7-based iOSCs, while FF and J_SC appear to suffer more by degradation, and consequently PCE. This suggests that the performance deterioration is probably associated with the charge extraction, which is correlated with the interlayer degradation of the device.

The values of the overall series resistance (R_S) and the shunt resistance (R_SH) were calculated from J–V measurements over time. Fig. 3c shows the normalized behaviour of R_S over time. It is observed that R_S increased exponentially as R₀ + A₀ × exp(R₀ × t), until around 2100 h. Afterwards, it continues increasing at a faster rate in the form of R₁ + A₁ × exp((t − t₀)/t₁), where R₀ and R₁ are the relative initial resistances of the first and the second region, respectively. A₀ and A₁ are the degradation power factors of resistance, and t₀ and t₁ are the initial reference times of each region. Similar behaviour is reported in ref. 49 and 52. After 3552 h, the value of R_S reached more than 3.5 times its initial value. R_S mainly consists of the sum of the resistance due to the electrodes, the bulk active layer, and the interfacial contact resistance between the active layer and the electrodes,³⁵ and it increases until 3336 h. After this, it started to decrease until 0.93 of the normalized initial value, decreasing from 202.5 Ω cm² to 187.94 Ω cm² after 3552 h.

On the other hand, the photo-degradation behaviour of the PCE, J_SC and FF of iOSCs under continuous illumination presented an exponential decay over time (see Fig. 3b), showing that the behaviour of the PCE is given by the behaviour of the J_SC. 49% of their initial PCE, and 59% of their initial J_SC value were obtained after 220 h.

As can be observed in Fig. 3b, the V_OC had a slight increase of 5% from the first operation hours under continuous light illumination of the iOSCs, after which it remained constant during 220 h. Since, according to ref. 12, 35 and 51, the V_OC losses are due to the light-induced trap states/defects in the devices, we can conclude that our devices have lower photo-induced defects than those reported in ref. 12. One possibility for the reduction of states/defects in our devices can be attributed to using MoO₃ instead of V₂O₅ as the HTL, which should avoid the photocatalytic reaction of the V₂O₅, contributing to the decomposition of the organic active layer.^12,42

The morphology changes in the active layer, particularly the mixed region, cause degradation predominantly in the FF under illumination. Electron traps are formed due to rearrangement of polymers and diffusion-limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps cause losses mainly in FF and J_SC.⁵³ In our case, the devices reached 80% of FF after 220 h.

The reduction behaviour over time of J_SC (Fig. 3b) can be attributed to the increment in R_S, which can be related to a phase separation of the acceptor and donor domains, as well as to the photo-degradation of the organic layers, conducive to the formation of an isolation layer between the metallic contact and the active layer. The increase in R_S reduces the collection of charge carriers, and the mobility. The value of R_S after 220 h was nearly 3 times its initial value, as can be observed in Fig. 3d.^19,48,54,55

Fig. 3d shows the normalized behaviour of R_SH over time, which reached 63% of its initial value after 220 h. The reduction in R_SH also affects the FF (see Fig. 3b).¹⁹

Impedance spectroscopy analysis

Impedance spectroscopy (IS) measurements were done to study the transport degradation processes taking place in the cell. A first analysis of IS was done on devices under dark and N₂ environments. The measurements were carried out under dark conditions by applying a small AC excitation signal (50 mV) to the cell structure. The frequency in the IS measurements was swept from 1 kHz to 2 MHz. The data were collected at different time intervals to obtain the impedance spectrum at different times elapsed, to study the degradation issues related to the different interfaces present in the device. The first measurement was done right after fabrication. Samples were kept under dark and N₂ conditions.

The experimental data have been fitted by an equivalent electrical model using three resistor/capacitor (3 RC) circuits in series, as shown in Fig. 4a (inset). R₁ is associated with the series resistance of the ITO layer; L with external measurement cables, C₁ & R₂ with the resistance and geometrical capacitance of PDINO, C₂ & R₃ with the active layer (PM6:Y7), and C₃ & R₄ with the MoO₃ layer. In the ESI,† the fits of the IS measurements over time using the 2 RC and the 3 RC models are included, and the fitting parameters for both models are also presented (Tables S1 and S2†), showing a better adjustment with the 3 RC model, Fig. S4–S6.† The geometrical capacitance value of each layer was calculated according to C_g = ε_layerε₀A/L_n, where ε₀ is the vacuum dielectric permittivity, A is the device area, ε_layer is the relative dielectric permittivity of the corresponding layer, and L_n is the layer thickness. The calculated initial values for each layer are shown in Table 4.^11,56–59

Fig. 4 (a) Experimental (symbols) and fitting (lines) impedance spectra (IS) response over time for iOSCs measured under dark conditions at 0.78 V. (b) Capacitance, (c) resistance, and (d) Tau data extracted from the fitting of IS measurements.

Table 4 Dielectric constant, thickness, and capacitance

Layer	L n (nm)	ε layer	Capacitance (nF)
PDINO	41	5.00	9.72
PM6:Y7	100	3.50	3.98
MoO3	7.50	5.70	60.60

---

Fig. 4 shows the IS behaviour as the cell degrades. Fig. 4a shows Nyquist plots (experimental and fitting) for different degradation times, while Fig. 4b and c show the evolution of the extracted values over time of the capacitance and resistance associated with each layer, respectively. Finally, Fig. 4d shows the normalized behaviour of the time constant over time (Tau = R × C), using the extracted values for each layer.

As expected, the Nyquist plot diameter increases with degradation time (Fig. 4a), suggesting an increase in the overall resistance of the device.⁶⁰

After 3552 h, the capacitance over time for the PDINO and active layer increased from 11.44 nF to 22.98 nF, and from 6.54 nF to 7.99 nF, respectively, see Fig. 4b. This increase of the capacitance over time suggests an increase of the charge accumulation in the layer, which can be associated with a worsening of charge carrier extraction, leading to the device degradation.^61,62 This behaviour can possibly be attributed to deterioration of the interfaces which causes the accumulation of electrons at the PDINO/active layer, and/or at the ITO/PDINO interfaces.⁶³ In this sense, the capacitance of the PDINO layer showed an increase of 100% of its initial value, while the active layer showed an increase of only 22%, suggesting that the degradation of the PDINO layer is the one determining the overall cell degradation. On the other hand, in the same figure, the capacitance of MoO₃ shows a decrease from 59 nF to 47.2 nF, in the same period. This trend, together with the increase of the respective layer resistance, Fig. 4c, indicates that the film is losing gradually its conductivity, worsening the device behaviour.⁶¹ This behaviour of MoO₃ capacitance over time agrees well with the results obtained in our previously reported work.¹⁹

It is important to notice that all resistances show an increase with time. But a lower increment is presented for the resistance of the active layer, showing an increment from 33.4 Ω to 41.6 Ω, while the PDINO and MoO₃ resistance increased from 250.7 Ω to 443.4 Ω, and from 20.23 Ω to 68.91 Ω, respectively, as can be observed in Fig. 4c.

After calculating the Tau values over time for each layer, as expected, the greatest variation of Tau over time was observed for the PDINO layer, with an increment of 3.5 in the normalized value of Tau, followed by the MoO₃ layer with an increment of 2.5. Finally, the Tau of the active layer showed the smallest ratio of increment of 1.5. From the above analysis, we can conclude that the layer that suffered the greatest degradation over time was the PDINO, followed by the MoO₃ layer. It is important to mention that the PM6:Y7 mixture in the active layer showed a great stability, although contacting layers degraded significantly, and the active layer maintained its good initial characteristics throughout time.

It is important to mention that the strongest variation of all analysed layer parameters started approximately at the same elapsed time, around 2400 h, where performance parameters started to degrade more rapidly. The analysis after IS measurements confirmed that in the OSC using an ITO/PDINO/PM6:Y7 structure, the PDINO layer determines the overall cell degradation, while the active PM6:Y7 layer was the most stable over time.

A second analysis of IS measurements was done for devices under continuous irradiation (100 mW cm⁻²), following the ISOS-L-1 protocols to gain a better comprehension of the photo-degradation effects over each layer of the iOSC (Fig. 5). The measurements were done under similar conditions as for the devices under darkness. In this case, the devices were under illumination during each measurement. The experimental data were fitted using the same equivalent electrical model, consisting of three resistor/capacitor (3 RC) circuits in series, as shown in Fig. 4a (inset). This analysis also provides information regarding which layer plays the most important role in the behaviour of the device, in this case, under continuous illumination conditions.

Fig. 5 (a) Experimental (symbols) and fitting (lines) impedance spectra (IS) response over time for iOSCs measured under continuous irradiation (100 mW cm⁻²) at 0.78 V. (b) Capacitance, (c) resistance, and (d) Tau data extracted from the fitting of IS measurements.

Fig. 5a shows typical Nyquist plots (experimental and fitting) that were obtained for the fresh and photo-degraded iOSCs over time. Again, all plots exhibited a semicircle shape, the arc diameter of which increases over time for the photo-degraded devices. This behaviour is related to the increase of the resistance of the devices due to the continuous light exposure.¹²

Fig. 5b and c show the evolution of the extracted values over time of the capacitance and resistance associated with each layer, respectively. To further determine which layer is more affected by the photo-degradation under illumination, we calculate the time constant (Tau = R × C) for each layer over time, as shown in Fig. 5d.

After 220 h under continuous illumination of the iOSCs, the capacitance values over time for the PDINO, active layer and MoO₃ increased from 9.14 nF to 28.40 nF, from 5.50 nF to 7.11 nF, and from 61.81 nF to 70.30 nF, respectively, see Fig. 5b. The resistance values over time for the same materials increased from 21.22 Ω to 33.34 Ω, from 7.59 Ω to 34.70 Ω, and from 9.90 Ω to 19.00 Ω, respectively, see Fig. 5c.

A higher increment of the capacitance, 3 times its initial value, was observed for the PDINO layer. The capacitance of the active layer increased by 29%, while that of the MoO₃ layer increased only by 14%. As can be observed in Fig. 5c, all resistances of the layers increased with time. The lowest increment of resistance is presented for the PDINO layer of 57%, followed by the MoO₃ layer with an increment of 92%. The highest increment was presented by the active layer, showing 4.57 times its initial value.

From the Tau values over time, as expected, the greatest variation was observed for the active layer, with an increment of its normalized value up to 5.90 times. The normalized value of the Tau for the PDINO layer increased less, up to 4.88 times. The normalized value of Tau of the MoO₃ layer showed the smallest ratio of increment of 2.18 times. These results indicate that the performance degradation of iOSCs under AM 1.5G illumination was mainly due to the photo-degradation of the organic layers, that is, the active and PDINO layers. Such degradation has been related to photochemical reactions inside and between organic layers, activated by the continuous exposure to light, when the device is in operation.^64–66 In contrast the MoO₃ inorganic layer showed the best stability under continuous illumination.

Experimental section

Materials

ITO-coated glass substrates with a nominal sheet resistance of 10 Ω per square and 220 nm of thickness were purchased from PsiOTec Ltd. PM6, Y7, and MoO₃ were purchased from Sigma-Aldrich. PDINO was obtained from 1-material. Silver wire, with 99.99% purity, was obtained from Testbourne Ltd.

Device fabrication

ITO coated glass substrates were cleaned in acetone, methanol, and isopropyl alcohol for 10 min in each one, after which they were exposed for 30 min under UV light.

PDINO is diluted in methanol at a rate of 1.5 mg ml⁻¹ and left shaking for 3 hours at 1000 rpm, after which it is spin coated at 3000 rpm for 30 s onto the ITO substrate.

The active blend of PM6:Y7 is prepared at a 1 : 1 wt% dilution in CB, and left shaking at 1000 rpm, for 3 hours at 80 °C. Afterwards, the PM6:Y7 dilution is spin coated at 1500 rpm for 40 s onto the ITO substrate already covered by the PDINO layer.

The samples are annealed during 60 min at 90 °C, after which MoO₃ is deposited by vacuum evaporation, followed by the deposition of Ag also by vacuum evaporation.

Device characterization

The current density–voltage (J–V) measurements of the unencapsulated iOSCs under light and darkness were performed at room temperature using a Keithley 2400 source-measure unit and a solar simulator (Abet Technologies model 11 000 class Type A, Xenon arc). The light intensity of the AM 1.5G spectra was calibrated using an NREL certified monocrystalline silicon photodiode at 100 mW cm⁻².

Film thicknesses of the deposited materials were measured using a Gaertner Scientific L2W16S633 ellipsometer.

The study was carried out on devices under dark and N₂ environments and, following the ISOS-L-1 protocols.²⁶

Impedance spectroscopy characterization was performed near V_OC (0.78 V DC), superimposing an AC signal of 50 mV amplitude in the frequency range of 1 kHz to 2 MHz, to study the transport degradation processes taking place in the cell. The equivalent circuit consisted of three RC elements in series, each associated with one of the layers (ETL, active layer and HTL) present in the iOSC.

Conclusions

Inverted organic solar cells were fabricated using the PM6 (PBDB-T-2F):Y7 (BTP-4Cl) blend without any additive as the active layer, and PDINO and MoO₃ as the ETL and HTL, respectively. The maximum obtained PCE for these devices was 9.14%, consistent with data previously reported by other research groups. A degradation analysis of these devices, for 3552 h, was performed for devices under dark and N₂ environments, which as far as we know, is the longest performed for an inverted structure of OSCs using the above indicated materials. In addition, a photo-degradation aging test of the iOSCs under continuous exposure of AM 1.5G illumination conditions was performed following the ISOS-L-1 protocols.

The performance parameters over time obtained from the illuminated J–V curves were analyzed. The T₉₀, T₈₀, and T₅₀, observed for these devices under dark and N₂ environments were 2363 h, 2918 h, and >3552 h, respectively, and following ISOS-L-1 protocols were 17 h, 31 h, and 220 h, respectively.

The analysis of the degradation of the device performance parameters under dark and N₂ environments and, following the ISOS-L-1 protocols, showed that, in both studies, V_OC seems to be the least affected. On the other hand, FF and J_SC, and consequently the PCE, were more affected by degradation. The analysis also indicates that the main cause of degradation seems to be associated with problems with the charge extraction, caused by the degradation of a device layer, or at its contact with the nearby ones.

The IS measurements further indicated that the PDINO layer is the one determining the overall cell degradation, while the active layer showed the highest stability during the elapsed 3552 h for cells degraded under dark and N₂ environments. The small increment observed in J₀ and n is reflected in the slight variation and good stability observed in the V_OC. This behaviour also suggests that the trap density remains low, and therefore the device degradation caused by trap-assisted recombination is small.

Under constant AM 1.5G illumination, following the ISOS-L-1 protocols, the IS results suggest that the performance degradation of the iOSCs is mainly due to the photo-degradation of the organic layers, that is the active and the PDINO layers, for which the normalized value in Tau increased up to 5.90, and 4.88 times, respectively. This work also confirms the higher stability observed for the iOSC, when a MoO₃ layer is used as the HTL, which in this analysis showed the smallest ratio of increment in Tau of around 2.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

Ángel Sacramento and Magaly Ramírez acknowledge Consejo Nacional de Humanidades, Ciencias y Tecnologías (CONAHCYT) for supporting Postdoctoral Scholarships, under grants BPPA-20220711103224986-2580943 and BPPA-20220624083033039-2364083, respectively. Victor Balderrama specially acknowledges CONAHCyT for the support by “Investigadores por México CONAHCyT” through Project 2 and Project Number 299061, “Laboratorio Nacional SEDEAM CONAHCYT”. The authors also acknowledge Ing. D. Nápoles, Ing. G. Alvarez, Ing. I. Vitales, Ing. L. E. Santos and Ing. B. Ruan for their assistance in measurements and other laboratory activities during this work.

References

1. C. J. Brabec, M. Heeney, I. McCulloch and J. Nelson, Chem. Soc. Rev., 2011, 40, 1185–1199.
2. Y. Cui, H. Yao, L. Hong, T. Zhang, Y. Tang, B. Lin, K. Xian, B. Gao, C. An, P. Bi, W. Ma and J. Hou, Natl. Sci., 2020, 7, 1239–1246.
3. J. Lv, H. Tang, J. Huang, C. Yan, K. Liu, Q. Yang, D. Hu, R. Singh, J. Lee, S. Lu, G. Li and Z. Kan, Energy Environ. Sci., 2021, 14, 3044–3052.
4. Y. Tong, Z. Xiao, X. Du, C. Zuo, Y. Li, M. Lv, Y. Yuan, C. Yi, F. Hao, Y. Hua, T. Lei, Q. Lin, K. Sun, D. Zhao, C. Duan, X. shao, W. Li, H. Yip, Z. Xiao, B. Zhang, Q. Bian, Y. Cheng, S. Liu, M. Cheng, Z. Jin, S. Yang and L. Ding, Sci. China: Chem., 2020, 63, 758–765.
5. M. B. Upama, N. K. Elumalai, M. A. Mahmud, M. Wright, D. Wang, C. Xu and A. Uddin, Sol. Energy Mater. Sol. Cells, 2018, 176, 109–118.
6. M. B. Upama, M. A. Mahmud, G. Conibeer and A. Uddin, Sol. RRL, 2020, 4, 1900342.
7. C. Yang, S. Zhang, J. Ren, M. Gao, P. Bi, L. Ye and J. Hou, Energy Environ. Sci., 2020, 13, 2864–2869.
8. J. Yuan, Y. Zhang, L. Zhou, G. Zhang, H. L. Yip, T. K. Lau, X. Lu, C. Zhu, H. Peng, P. A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y. Li and Y. Zou, Joule, 2019, 3, 1140–1151.
9. G. Zhang, X. K. Chen, J. Xiao, P. C. Y. Chow, M. Ren, G. Kupgan, X. Jiao, C. C. S. Chan, X. Du, R. Xia, Z. Chen, J. Yuan, Y. Zhang, S. Zhang, Y. Liu, Y. Zou, H. Yan, K. S. Wong, V. Coropceanu, N. Li, C. J. BRabec, J. L. Bredas, H. L. Yip and Y. Cao, Nat. Commun., 2020, 11, 3943.
10. S. Zhong, B. K. Yap, Z. Zhong and L. Ying, Crystals, 2022, 12, 168.
11. E. Moustafa, A. A. A. Torimtubun, J. Pallarés and L. F. Marsal, Sol. RRL, 2022, 6, 2100480.
12. E. Moustafa, M. Méndez, J. Pallarés and L. F. Marsal, Sol. Energy Mater. Sol. Cells, 2022, 248, 111985.
13. Y. Cui, H. Yao, J. Zhang, K. Xian, T. Zhang, L. Hong, Y. Wang, Y. Xu, K. Ma, C. An. C. He, Z. Wei, F. Gao and J. Hou, Adv. Mater., 2020, 32, 1908205.
14. A. Karki, J. Vollbrecht, A. J. Gillett, S. S. Xiao, Y. Yang, Z. Peng, N. Schopp, A. L. Dixon, S. Yoon, M. Schrock, H. Ade, G. N. M. Reddy, R. H. Friend and T. Q. Nguyen, Energy Environ. Sci., 2020, 13, 3679–3692.
15. S. Bao, H. Yang, H. Fan, J. Zhang, Z. Wei and C. Cui, Adv. Mater., 2021, 33, 2105301.
16. E. Moustafa, M. Méndez, J. G. Sánchez, J. Pallarés, E. Palomares and L. F. Marsal, Adv. Energy Mater., 2023, 13, 2203241.
17. y. Chen, T. Liu, L. K. Ma, W. Xue, R. Ma, J. Zhang, C. Ma, H. K. Kim, H. Yu, F. Bai, K. S. Wong, W. Ma, H. Yan and Y. Zou, J. Mater. Chem. A, 2021, 9, 7481–7490.
18. M. Ramírez-Como, E. Moustafa, A. A. A. Torimtubun, J. G. Sánchez, J. Pallarés and L. F. Marsal, in IEEE Latin American Electron Devices Conference (LAEDC), IEEE, Cancun, Mexico, 2022.
19. A. Sacramento, M. Ramírez-Como, V. S. Balderrama, J. G. Sánchez, J. Pallarés, L. F. Marsal and M. Estrada, J. Mater. Chem. C, 2021, 9, 6518–6527.
20. M. Ramírez-Como, V. S. Balderrama, J. G. Sánchez, A. Sacramento, M. Estrada, J. Pallarés and L. F. Marsal, IEEE J. Electron Devices Soc., 2022, 10, 435–442.
21. Y. M. Sung, C. T. Chang, C. Tsao, H. Lin, H. Cha, P. Jiang, T. Liu, K. Chang, Y. Huang and J. Tsay, Nanoscale, 2023, 15, 3375–3386.
22. H. Pan, L. Zuo, W. Fu, C. Fan, B. Andreasen, X. Jiang, K. Norman, F. C. Krebs and H. Chen, Org. Electron., 2013, 14, 797–803.
23. Y. Kanai, T. Matsushima and H. Murata, Thin Solid Films, 2009, 518, 537–540.
24. N. K. Elumalai, A. Saha, C. Vijila, R. Jose, Z. Jie and S. Ramakrishna, Phys. Chem. Chem. Phys., 2013, 15, 6831–6841.
25. M. Y. Ameen, S. Pradhan, M. R. Suresh and V. S. Reddy, Opt. Mater., 2015, 39, 134–139.
26. M. O. Reese, S. A. Gevorgyan, M. Jørgensen, E. Bundgaard, S. R. Kurtz, D. S. Ginley, A. Elschner, M. O. Reese, S. A. Gevorgyan, M. Jørgensen, E. Bundgaard, S. R. Kurtz, D. S. Ginley, D. C. Olson, M. T. Lloyd, P. Morvillo, E. A. Katz, A. Elschner, O. Haillant, T. R. Currier, V. Shrotriya, M. Hermenau, M. Riede, K. R. Kirov, G. Trimmel, T. Rath, O. Inganäs, F. Zhang, M. Andersson, K. Tvingstedt, M. Lira-Cantu, D. Laird, C. McGuiness, S. Gowrisanker, M. Pannone, M. Xiao, J. Hauch, R. Steim, D. M. DeLongchamp, R. Rösch, H. Hoppe, N. Espinosa, A. Urbina, G. Yaman-Uzunoglu, J.-B. Bonekamp, A. J. J. M. vanBreemen, C. Girotto, E. Voroshazi and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2011, 95, 1253–1267.
27. Y. Cui, H. Yao, J. Zhang, T. Zhang, Y. Wang, L. Hong, K. Xian, B. Xu, S. Zhang, J. Peng, Z. Wei, F. Gao and J. Hou, Nat. Commun., 2019, 10, 2515.
28. J. Yao, B. Qui, Z. G. Zhang, L. Xue, R. Wang, C. Zhang, S. Chen, Q. Zhou, C. Sun, C. Yang, M. Xiao, L. Meng and Y. Li, Nat. Commun., 2020, 11, 2726.
29. K. Wang, C. Liu, T. Meng, C. Yi and X. Gong, Chem. Soc. Rev., 2016, 45, 2937–2975.
30. J. Wu, J. Lee, Y. C. Chin, H. Yao, H. Cha, J. Luke, J. Hou, J. S. Kim and J. R. Durrant, Energy Environ. Sci., 2020, 13, 2422–2430.
31. F. Tang, J. Wu, Z. Lin and X. Peng, ACS Appl. Mater. Interfaces, 2022, 14, 16487–16496.
32. F. J. García-Sánchez, B. Romero, D. C. Lugo-Muñoz, G. Del Pozo, B. Arredondo, J. J. Liou and A. Ortiz-Conde, Facta Univ., Ser.:Electron. Energetics, 2017, 30, 327–350.
33. A. Sacramento, V. S. Balderrama, M. Ramírez-Como, L. F. Marsal and M. Estrada, Sol. Energy, 2020, 198, 419–426.
34. A. Sacramento, M. Ramirez-Como, V. S. Balderrama, S. I. Garduno, M. Estrada and L. F. Marsal, IEEE J. Electron Devices Soc., 2020, 8, 413–420.
35. B. Qi and J. Wang, J. Mater. Chem., 2012, 22, 24315–24325.
36. N. K. Elumalai and A. Uddin, Energy Environ. Sci., 2016, 9, 391–410.
37. C. Waldauf, M. C. Scharber, P. Schilinsky, J. A. Hauch and C. J. Brabec, J. Appl. Phys., 2006, 99, 104503.
38. C. Li, Z. Song, D. Zhao, C. Xiao, B. Subedi, N. Shrestha, M. M. Junda, C. Wang, C. S. Jiang, M. Al-Jassim, R. J. Ellingson, N. J. Podraza, K. Zhu and Y. Yan, Adv. Energy Mater., 2019, 9, 1803135.
39. Z. He, C. Zhong, X. Huang, W. Y. Wong, H. Wu, L. Chen, S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636–4643.
40. N. Biyikli, I. Kimukin, B. Butun, O. Aytur and E. Ozbay, IEEE J. Sel. Top. Quantum Electron., 2004, 10, 759–765.
41. Z. G. Zhang, B. Qi, Z. Jin, D. Chi, Z. Qi, Y. Li and J. Wang, Energy Environ. Sci., 2014, 7, 1966–1973.
42. M. Cui, D. Li, X. Du, N. Li, Q. Rong, N. Li, L. Shui, G. Zhou, X. Wang, C. J. Brabec and L. Nian, Adv. Mater., 2020, 32, 2002973.
43. T. Kong, R. Wang, D. Zheng and J. Yu, Front. Chem., 2021, 9, 703561.
44. W. Wang, Z. Lin, X. Li, C. Zhang, T. Yang and Y. Liang, ACS Appl. Mater. Interfaces, 2022, 14, 18736–18743.
45. J. Yao, B. Qiu, Z. Zhang, L. Xue, R. Wang, C. Zhang, S. Chen, Q. Zhou, C. Sun, C. Yang, M. Xiao, L. Meng and Y. Li, Nat. Commun., 2020, 11, 2726.
46. L. Duan, X. Meng, Y. Zhang, H. Yi, K. Jin, F. Haque, C. Xu, Z. Xiao, L. Ding and A. Uddin, Mater. Chem. Front., 2019, 3, 1085–1096.
47. A. Manor, E. A. Katz, T. Tromholt and F. C. Krebs, Adv. Energy Mater., 2011, 1, 836–843.
48. V. S. Balderrama, F. Avila-Herrera, J. G. Sánchez, J. Pallares, O. Vigil-Galán, L. F. Marsal and M. Estrada, IEEE J. Photovolt., 2016, 6, 491–497.
49. A. A. A. Torimtubun, M. Méndez, J. G. Sánchez, J. P. E. Palomares and L. F. Marsal, Sustainable Energy Fuels, 2021, 5, 6498–6508.
50. V. S. Balderrama, M. Estrada, P. L. Han, P. Granero, J. Pallarés, J. F. Borrull and L. F. Marsal, Sol. Energy Mater. Sol. Cells, 2014, 125, 155–163.
51. N. Gasparini, S. H. K. Paleti, J. Bertrandie, G. Cai, G. Zhang, A. Wadsworth, X. Lu, H. L. Yip, I. McCulloch and D. Baran, ACS Energy Lett., 2020, 5, 1371–1379.
52. V. Singh, Curr. Appl. Phys., 2017, 17(11), 1450–1454.
53. M. B. Upama, M. Wright, M. A. Mahmud, N. K. Alumalai, A. M. Soufiani, D. Wang, C. Xu and A. Uddin, Nanoscale, 2017, 9, 18788–18797.
54. N. Grossiord, J. M. Kroon, R. Andriessen and P. W. M. Blom, Org. Electron., 2012, 13, 432–456.
55. X. Wang, C. X. Zhao, G. Xu, Z. K. Chen and F. Zhu, Sol. Energy Mater. Sol. Cells, 2012, 104, 1–6.
56. T. A. A. A. Torimtubun, J. G. Sánchez, J. Pallarés and L. F. Marsal, Sustainable Energy Fuels, 2020, 4, 3378–3387.
57. R. C. I. MacKenzie, V. S. Balderrama, S. Schmeisser, R. Stoof, S. Greedy, J. Pallare`s, L. F. Marsal, A. Chanaewa and E. von Hauff, Adv. Energy Mater., 2016, 6, 1501742.
58. S. Torabi, F. Jahani, I. V. Severen, C. Kanimoszhi, S. Patil, R. W. A. Havenith, R. C. Chiechi, L. Lutsen, D. J. M. Vanderzande, T. J. Cleij, J. C. Hummelen and L. J. A. Koster, Adv. Funct. Mater., 2015, 25, 150–157.
59. G. Garcia-Belmonte, A. Munar, E. M. Barea, J. Bisquert, I. Ugarte and R. Pacios, Org. Electron., 2008, 9, 847–851.
60. E. Von Hauff, J. Phys. Chem. C, 2019, 123, 11329–11346.
61. G. del Pozo, B. Arredondo, B. Romero, G. Susanna and F. Brunetti, Sol. Energy, 2017, 144, 105–110.
62. S. K. Gupta, L. S. Pali and A. Garg, Sol. Energy, 2019, 178, 133–141.
63. B. Arredondo, B. Romero, M. J. Beliatis, G. Del Pozo, D. Martín-Martín, J. C. Blakesley, G. Dibb, F. C. Krebs, S. A. Gevorgyan and F. A. Castro, Sol. Energy Mater. Sol. Cells, 2018, 176, 397–404.
64. J. Razzell-Hollis, J. Wade, W. C. Tsoi, Y. Soon, J. Durrant and J. S. Kim, J. Mater. Chem. A, 2014, 2, 20189–20195.
65. T. Liu, Q. C. Burlingame, M. R. Ivancevic, X. Liu, J. Hu, B. P. Rand and Y. L. Loo, Adv. Energy Mater., 2023, 13, 2300046.
66. H. Bin, J. Wang, J. Li, M. M. Wienk and R. A. J. Janssen, Adv. Mater., 2021, 33, 2008429.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3se00977g

---