Chinese Chemical Letters  2019, Vol. 30 Issue (1): 217-221   PDF    
Effects of processing additives in non-fullerene organic bulk heterojunction solar cells with efficiency >11%
Shenkun Xiea, Jianqiu Wanga, Rong Wangb, Dongyang Zhangb, Huiqiong Zhouc,*, Yuan Zhangb,*, Defeng Zhoua,*    
a School of Chemistry and Life Science, Changchun University of Technology, Changchun 130012, China;
b HEEGER Beijing Research & Development Center, School of Chemistry, Beihang University, Beijing 100191, China;
c CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China
Abstract: Here we investigate processing additive-dependent photovoltaic performance and charge recombination in organic bulk heterojunction (BHJ) solar cells based on a polymeric donor of PBDB-T blended with a non-fullerene acceptor m-ITIC. We find that PBDB-T:m-ITIC solar cells exhibit good compatibilities with the utilized additives (DIO, CN, DPE, and NMP) in optimal conditions, can have a high charge dissociation probability approaching 100% (with DIO), leading to ultimate efficiency >11%. Regardless of additives, we observe a dominant 1st order monomolecular recombination with insignificant bi-molecular recombination or space-charge effects in these solar cells. Despite of impressive power conversion efficiency (PCE), it is of surprise that Shockley-Read-Hall recombination is identified to play a role in device operation. Thus, it points to the necessity to mitigate the influences of traps to further boost the efficiency in non-fullerene based organic solar cells.
Keywords: Organic solar cell     Solvent additive     Morphology     Recombination     Non-fullerene    

Organic solar cells (OSCs) are among the most promising candidates for sustainable energy resources [1-3]. The main advantages in OSCs include their low fabrication cost, light weight, high flexibility, and large-area processing capabilities using roll-to-roll printing etc. In recent years, the power conversion efficiency of polymer solarcells (PSCs), barely 1% fifteenyears ago, has exceeded 13% [4-13]. It is well accepted that utilizing bulk-heterojunction (BHJ) structure in which the donor and acceptor materials are blended to form bi-continuous inter-percolated networks with abundant interfacial regions warrants efficient exciton dissociation for desired photoconductivity [13]. Despite of recognized disadvantages such as weak absorption in the visible region, high synthetic cost, and limited tunability of energy levels, fullerene derivatives with large nonplanar spherical configurations have been predominant electron-acceptor materials in the past two decades [14]. Very recently, non-fullerene (NF) acceptors have attracted many attentions for OSC applications. The problems with fullerenes to large extend can be circumvented in NF-based OSCs. For example, NFs acceptors have been shown with the high tunability on energetics through chemical modification with potentially low synthetic costs. Through judicious choices or modifications of molecular skeletons and flexible side chains, the electronic and absorption properties are well tailored [15, 16]. Among the diversity of NF acceptors, ITIC with the fused ring structure has gained great popularity showing successes in delivering several record efficiencies in NF-OSCs [17]. Another successor ITIC derivative IT-M with a very small electron-donating substituent (methyl) has yielded attractive PCEs > 12% [18]. In addition to PCEs, in some cases ITIC-based devices can exhibit more supreme stability than their analogue fullerene-based OSCs [18]. Despite of these accomplishments, so far fundamental knowledge of the influences of processing additives on photovoltaic behaviors with these emerging NF acceptor materials still lacks, which may impede the advances in improving the PCE of NF-OSCs.

Here, we use low bandgap acceptor meta-alkyl-phenyl-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 (m-ITIC, Fig. 1), inspired by side-chain isomerism engineering on the alkyl-phenyl substituents of ITIC reported by Yang et al. [19], to blend with a medium band gap polymeric donor poly[2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1, 2-b:4, 5-b´]dithiophene)-co-(1, 3-di(5-thiophene-2-yl)-5, 7-bis(2-ethylhexyl)benzo[1, 2-c:4, 5-c´]dithiophene-4, 8-dione)] (PBDB-T, Fig. 1) [20, 21] as the test bed to investigate the photovoltaic performance and relevant opto-electrical properties. poly[(9, 9-bis (30-((N, N-dimethyl)-N-ethylammonium)-propyl)-2, 7-fluorene)-alt-2, 7-(9, 9-dioctylfluorene)]dibromide (PFN-Br, Fig. 1) as interlayer.

Fig. 1. Chemical structures of utilized materials.

Devices were fabricated with conventional structure of ITO/PEDOT:PSS/BHJ layer/PFN-Br/Al. The PEDOT:PSS hole extraction layer (~30 nm) was attained by spin-coating on pre-cleaned glass/ITO substrates and dried at 160 ℃ for 10 min in air. BHJ solutions were prepared by solubilizing the PBDB-T:m-ITIC blends in chlorobenzene with a total concentration of 20mg/mL with adding additives at selected concentrations. The photoactive layers were attained by spin-coating the BHJ solutions on PEDOT:PSS-coated ITO substrates at a spin-rate of 1500 rpm. The thickness of active layers was controlled at around 110 nm. After spin-coating, the BHJ films were thermally annealed on a hot plate at 160 ℃ for 10 min. Then the PFN-Br solution (in methanol) at a concentration of 0.5 mg/mL was deposited atop the active layers to form a PFN-Br cathode buffer layer with typical thickness of ~5 nm. At last, Al cathode (ca. 80 nm) was thermally evaporated on the PFN-Br buffer layer at a pressure of ~1.0×10-4 Pa. The effective device area was 4 mm2 defined by shadow masks.Based on 4 representative processing additives, including 1, 8-diiodoctane (DIO), 1-chloronaphthalene (CN), diphenyl ether (DPE), and N-methyl-2-pyrrolidone (NMP), we focused on their impacts on the photovoltaic behaviors and recombination in PBDB-T:m-ITIC BHJ solar cells. Of interest, the devices show a high compatibility with the choice of additives, revealed by the overall enhancements on PCE (compared to cells without additives) with non-substantial differences in device parameters at each condition. Based on mizing D-A ratio, additives concentration, and processing ents, the achieve the best PCE exceeding 11% using DIO. The films with the compared additives exhibit resembling surface phologies with only slight differences in surface roughness. le visibly changed surface morphology with increased ctural sizes and non-uniformity can be observed upon easing the concentration of additives. These features to some ee reconcile the observed variations on device parameters. d on irradiation-dependent analysis, all concerned NF-OSCs identified with a dominant monomolecular recombination ciated with insignificant bi-molecular recombination or space charge effect. Light-dependent photovoltage measurements sug-that Shockley-Read-Hall (SRH) recombination plays a noble in operation of studied solar cells, revealed by non-ideality ors > 1, which points to the necessity to further reduce the ence of traps. Our results not only provide an effective mization route to attain satisfactory device characteristics but enrich fundamental insights into the impacts of additives and vant processing parameters on the photovoltaic performance SCs with emerging NF electron acceptors.

Firstly, we examined the impact of D/A ratio of BHJ blends on r cell performance. Fig. 2d shows adopted conventional device itecture. To enable efficient electron extraction, a thin layer of -Br was utilized as the cathode buffer layer which was found to atically improve the FF (fill factor) and ultimate PCE. The rovements are probably due to the present Br- anions that can rfacially dope the electron acceptor, leading to improved charge extraction efficiency at the cathode interface [22], Fig. 2b ws representative J-V characteristics of solar cells with different D/A blend ratios (w/w) under standard AM 1.5G solar diation (100 mW/cm2). The corresponding EQE spectra are wn in Fig. 2a. Table 1 summarizes detailed device parameters different D/A ratios. All devices compared in Table 1 have an charge thickness of ~120 nm determined by a profilometer. As can een, the best D/A ratio was found at the 1:1 ratio with which highest PCE of 9.78% was achieved with an open circuit voltage (Voc) of 0.92 V, a short current density (Jsc) of 15.70 mA/cm2, and a factor (FF) of 69.30%. Fig. 2b compares external quantum efficient (EQE) spectra of solar cells at different D/A ratios. The ge of the spectral shape of EQE at different D/A ratios can be erstood from the thin film absorption of neat donor and ptor shown in Fig. 2c. At a balanced ratio if 1:1 (w/w), we ined a relatively high EQE in the spectral range of 620-760 nm. order in EQE between devices roughly follows that of Jsc wed in Fig. 2b. The deviation between the Jsc determined based he integration of EQE and that from J-V curves is less than 5%, ring the accuracy of extracted device parameters.

Fig. 2. (a) EQE spectra of according devices. (b) Current density versus voltage characteristics of PBDB-T:m-ITIC solar cells at different D/A blend ratios (w/w). (c) Thin film absorption of neat PBDB-T and m-ITIC acceptor. (d) Device architecture of solar cells.

Table 1
Device parameters ofPBDB-T:m-ITIC solar cells with various D/A blend ratios (w/w) under standard AM 1.5G solar irradiation (100mW/cm2).

Morphology and phase separation of photoactive layers are ortant concerns for achieving desirable devices performance in s. Next, we examine how the D/A ratio in BHJ blends affects iltant film morphology (Fig. 3). As seen from the surface topographic images captured by atomic force microscopy (AFM) in Figs. 3a and d, it appears that unbalanced D/A ratios cause an increase in surface roughness with larger structures. At more balanced D/A ratios, the surface smoothness of BHJ films increases (Rq = 4.35 nm at1:1 and 4.61 nm at 1:1.3, Figs. 3b and c), compared to that (Rq = 5.58nm at 1.5:1 and 4.77 nm at 1:1.5) in Figs. 3a and d. Of interest, the roughness determined by AFM can be correlated to the Jsc in respective solar cells. The unbalanced ratios shown in Figs. 3a and d tend to result in donor-or electron-rich surfaces that may cause accelerated recombination and thus the reduction in Jsc, consistent with the results in Table 1. However, when compared to the evident variation of surface morphology, the Voc FF and PCE are barely changed (Table 1). This seems to hint that the energetics and diode quality in solar cells (that basically determine the Voc and FF) are likely unchanged at different D/A ratios.

Fig. 3. AFM topographic images of PBDB-T:m-ITIC BHJ films at various D/A blend ratios (w/w) of (a) 1.5:1, (b) 1:1 (c) 1

Now, we examine the effect of solvent additives on solar cell performance and BHJ film morphology. For this purpose, we chose 4 representative solvent additives, namely DIO, CN, DPE and NMP. In the optimal conditions, all applied additives yield satisfactory photovoltaic performance that can be similarly ascribed to the identified mechanisms for improving the PCE in fullerene devices [23-26]. For simplicity, these additives were compared based on the same D/A ratio (1:1). Detailed solar cell parameters with variations on solvent additives and their concentrations are summarized in Table 2. After adding 0.5% DIO, we achieved the highest PCE of 11.09% with Voc = 0.915 V, Jsc =16.98 mA/cm2 and FF = 0.71. Following the DIO device, the cell using NMP also leads to a PCE reaching 10.8%.

Table 2
Device parameters of solar cells using various solvent additives and their concentrations under standard AM 1.5G solar irradiation (100mW/cm2).

It is interesting to note that the additives seem to affect the solar cell operation in distinct manners, e.g., changing the amounts of DIO or NMP was found to mainly affect FF and Voc, while different DPE doses result in different Jsc in solar cells. Detailed mechanism for this observation is yet not fully understood at this stage. It may be related to the nanoscale phase-separated morphology, phase purity or intermolecular stacking in respective BHJ films. In comparison to the other three additives, the behavior of device using CN is less sensitive to the additive concentration, possibly due to the influence of boiling points. We further examined surface morphology of BHJ films based on the optimal concentrations for these additives (0.5% for DIO, NMP, and 1% for CN, DPE) with corresponding AFM images shown in Figs. 3e-h. The main morphological differences lie in the size of structures and uniformity, which can be correlated to D/A phase segregations inside the BHJ with which charge separation and eventually the Jsc are mediated. Despite of the changed solar cell parameters with different additives, resembling surface morphologies the compared BHJ films are observed with slightly changed roughness. Consistently, the Rq values of BHJ films can be correlated to the Jsc in according devices.

To shed light on exciton dissociation and charge collection processes in m-ITIC based devices, we performed light-dependent (Plight) photocurrent/photovoltage measurements. Fig. 4a shows photocurrent density (Jph) as a function of effective voltage for solar cells measured under 100 mW/cm2 irradiation with the voltage sweep range of -1.5-2 V. Here Jph is defined as Jph =JLJD, where JL and JD are the photocurrent densities under illumination and in the dark condition, respectively. Veff is defined as Veff = V0 —Vbias with V0 denoting for the voltage at whichJph is zero and Vbias for the applied external voltage bias. With this plotting method, the charge dissociation probability Pdiss can be estimated from Jph with respect to the photocurrent in the saturation regime (Jsat) [27]. Under short-circuit condition, Pdiss is approximated according to the relation by Pdiss=Jph/Jsat [27]. In our case, we chose Jsat at bias = -2 V and Pdiss of 96%, 94%, 94%, and 93% was attained using DIO, CN, DPE, and NMP additives at respective optimal concentrations. This result indicates that the additive-processed solar cells can have a high exciton dissociation rate with efficient charge collection, possibly ascribed to favorable D/A phase segregations with interpenetrated carrier transport pathways. This is also in line with the observed large FFs with which strong recombination seems unlikely. From Fig. 4b, the EQE spectrum of DIO devices is well distinguished from those of remaining solar cells, which may be correlated to the used lower amount of DIO additive and resultant differences in absorbance and photoconductivity. The highest PCE with DIO is mainly ascribed to the gain of FF with the excellent diode characteristics. Besides, the champion performance with DIO may also be linked to the highest Pdiss, which can originate from the optimal balance between phase segregation, domain purity, molecular ordering and carrier mobility, eventually leading to minimizing the recombination losses [28-31].

Fig. 4. (a) Photocurrent (Jph) as a function of effective bias (see definitions in text) of solar cells using various solvent additives at their optimal concentrations. (b) Corresponding EQE spectra of BHJ solar cells. (c) Light-dependent Jsc and (d) Voc of additive-processed solar cells.

To further clarify the impact of additives, we investigated Plight-dependent Jsc and Voc to gain insights into charge recombination properties within the devices. Fig. 4c shows Jsc of OSCs with various additives as a function Plight. a slope equaling 1 indicates a dominant monomolecular recombination and the deviation from the power of unity can arise from a couple of reasons including bimolecular recombination, space-charge effect, or variations in mobility between the two carriers etc. [32]. We note that in all cases, the power is less than unity, indicative of the presence of bimolecular recombination or even space-charges [33, 34]. While the slope in the DIO and NMP processed devices approaches more to the unique, which may implying the decrease on bimolecular recombination with these two additives. The characteristics of Jsc versus Plight can be affected by the balance between charge carrier transport in BHJ films.

In Plight-dependent photovoltage measurements, a slope equal to thermal voltage (kT/q with k being the Boltzmann constant and q being the elementary charge) or larger than kT/q in the BHJ solar cell can respectively indicate the dominant trap-free recombination or the influence of SRH recombination [30]. Fig. 4d shows characteristics of the determined Voc against Plight for various solar cells. The slopes are 1.46 kT/q, 1.42 kT/q, 1.36 kT/q, and 1.46 kT/q for devices with DIO, CN, DPE, and NMP respectively. To our surprise, the SRH recombination tends to be present in all these devices, indicative of the influences of traps even at the optimal conditions. It can be foreseen that further boosting of PCEs may be attained through fine materials purification or device engineering with mitigated SRH recombination. At this stage, we have no clear clues if these traps are in the bulk of BHJ films or at the cathode interface with the presence of PFN-Br buffer layer. Further investigations may be required to elaborate.

In summary, we investigate the impact of solvent additives on photovoltaic characteristics in non-fullerene PTDB-T:m-ITIC BHJ solar cells. Based judicious optimizations, we attain the best PCE of 11.1% with using DIO additives. The attractive performance can be mainly attributed to the complementary absorption with favorable film morphology that enables to achieve high photo-harvesting and satisfactory diode characteristics (or FF). The concerned BHJ solar cells show a good compatibility with respect to the choice of additives and in the optimal conditions, high charge dissociation probabilities > 93% can be achieved, possibly due to pertinent phase segregation and interpercolated transport pathways. The surface roughness of BHJ films can be tuned by the D/A ratio. Of interest, balanced D/A ratios are found to be associated with enhanced surface smoothness, which somehow correlates to the increased Jsc in solar cells. Based on comparing the effects of four representative additives, we found that the recombination in operational devices is dominated by the monomolecular type along with minor bi-molecular recombination and/or space charge effects. Of note, in these optimized solar cells, pronounced SRH recombination is still observable, indicating the role of traps under device operation. The presented study not only provides a systematic route to attain satisfactory photovoltaic performance but also point to the necessity to mitigate the influence of traps to further boost the PCE in organic non-fullerene solar cells.


D. Zhou thanks the National Natural Science Foundation of China (No. 21471022). Y. Zhang thanks the National Natural Science Foundation of China (No. 21674006). H. Zhou thanks the Chinese Academy of Science (100 Top Young Scientists Program, No. QYZDB-SSW-SLH033) and the National Key Research and Development Program of China (No. 2017YFA0206600).

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