Non‐fullerene all small molecule OBHJSCs with profound device characteristics

1 Fluoro-Agro Chemicals Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, Telangana, India 2 Catalysis and Fine Chemicals Department, CSIR-Indian Institute of Chemical Technology, Hyderabad, Telangana, India 3 Academy of Scientific and Innovative Research, CSIR-Indian Institute of Chemical Technology, Sector 19, Kamla Nehru Nagar, Ghaziabad, Uttar Pradesh 201002, India 4 Department of Physics, The LNM Institute of Information Technology, Jamdoli, Jaipur, Rajasthan, India

, with further close energy levels will have implications on the efficiency of OBHJSCs.

K E Y W O R D S
all small molecules, ITIC-acceptor, naphthodithiophene-benzodithiophene-donors, organic photovoltaics and OBHJSCs INTRODUCTION In recent years, solution processed organic bulk heterojunction solar cells (OBHJSCs) have been emerged as one of the forefront and promising renewable energy technologies to meet the future energy demands worldwide. [1][2][3][4][5] This is because of the low cost, light weight, good flexibility, simple device framework, and easy fabrication of OBHJSCs and attained great interest. Over the past few decades great progresses in the research on organic solar cells (OSCs) is observed. [6][7][8][9] Because of the limitations associated with the fullerene acceptors (PC 61 BM and PC 71 BM), many researchers started making OBHJSCs involving non fullerene acceptors. Specially, small molecule non-fullerene acceptors (SM-NFAs) have gained utmost importance and extensive investigation has been taking place to develop simple and efficient SM-NFAs. [10][11][12][13][14][15] Unlike the intrinsic limitations of weak absorption, high synthetic cost, and purification difficulty of fullerene based acceptors (PC 61 BM and PC 71 BM), [16,17] SM-NFAs have several unique advantages, such as nearinfrared absorption, tuneable energy levels and easy synthesis which allow better photon absorption and morphology modulation. Owing to the endless efforts, increase in the PCE from 10% to over 18% reported for the OBHJSCs with non-fullerene acceptors (NFAs) and polymer-based donor materials. [18][19][20][21][22][23][24] However, the drawbacks like batch-to-batch variation in the molecular weight and polydispersity associated with the polymer donors limit their widespread applications and pawing way for the development of small molecular donor materials suitable for all SM-NFAs. The combination of SM-NFAs with small molecular donors provides various advantages like tuning the blend absorption, frontier molecular orbital energy levels, and crystallization characteristics. These advantages greatly attracted the researchers and the research area of NF-ASM-OBHJSCs has been rapidly growing. [25][26][27][28][29] Nevertheless, the best power conversion efficiency (PCE) reported for the NF-ASM-OBHJSCs is around 14% which is still lagging far behind their polymer donor counterparts. Hence it is worthy to develop new organic small molecular D/A materials with reinforcing characteristics to improve the PCE of NF-ASM-OBHJSCs. [30][31][32] Among the donor materials reported for NF-ASM-OBHJSCs, most part are based on benzodithiophene (BDT) internal core due to fused benzene ring with two peripheral thiophene rings of BDT induces strong intra and intermolecular interactions in its thin film state and improves charge carrier mobility in the OBHJ. [30,33,34] Extensive research has already done to improve the efficiency of BDT based materials by adopting various combinations, like alkyl chain modification, and incorporating various substituents on the BDT core. [35][36][37][38] This indicates the necessity to develop new and efficient donor materials for NF-ASM-OBHJSCs. Naphthodithiophene (NDT), a structural analogue of BDT, has been explored recently for photovoltaic applications. Polymeric materials using NDT as donor core are utilized for solar cell and thin film transistor applications. [39,40] These materials showed high hole carrier mobility due to their unique properties like coplanarity, highly ordered π-stacked structures and high charge density of the NDT core. However, small molecule donor materials with NDT core are very few. [41] Prominent structural and charge transport characteristics of NDT core motivated us to develop new small molecule donor materials for NF-ASM-OBHJSCs using ITIC as non-fullerene acceptor. Backbone structure is fixed from one of our recently reported, efficient donor material with replacement of dithienopyrrole (DTP) units with NDT core. Such modifications resulted in hypsochromic shift in the absorption spectrum leading to complementary absorption with ITIC acceptor. Moreover, HOMO energy level is well stabilized and is an additional advantage to provide high V oc values. Marks and co-workers was first reported NDT based donor material. [42,40] NDT exhibits their unique properties like rigid, coplanar molecular structure and facilitating the charge carrier transport. Further, two linear 2-ethylhexyloxy chains on the NDT units at the 5-and 6-position increased the solubility of the resulting donor materials and also minimized the formation of molecular aggregates which can cause the charge recombination in the OBHJSCs. [43][44][45][46][47][48][49] OBHJSCs are fabricated using the synthesized donors and ITIC acceptor materials by adopting simple fabrication procedures. The devices with SNAF/ITIC photoactive blend showed superior photovoltaic performance with

Synthesis and thermal properties
The chemical structures of all three small molecules (ONAF, SNAF & ITIC) are displayed in Figure 1. All the target compounds were sytheized for previous modified literature procedures as displayed in Scheme 1 [33,47,50] and their synthetic procedures are provided in experi-mental section. ITIC was synthesized by previous modified literature procedure. [18] The reported small molecules ONAF and SNAF were synthesized Knoevenagel condensation pathway by treating with corresponding bisaldehyde intermediates (ONp-CHO & SNp-CHO, respectively for ONAF & SNAF) with n-butylrhodanine. The prefinal bis-aldehyde derivatives ONp-CHO and SNp-CHO were obtained by reacting with the corresponding di-stannilated derivatives (7 and 8, respectively for ONp-CHO and SNp-CHO) with compound 6 using palladium catalyzed Stille cross-coupling reaction. Intermediates 7 and 8 were synthesized as per our previously reported procedures. [9,51] The key intermediate 6 was obtained by reacting compound 5 with NBS. The compound 5 was synthesized by reacting 4 with Vilsmeier formylation pathway in the presence of POCl 3 and DMF. Compound 4 was obtained by treating with 3 in presence of anhydrous FeCl 3 and CH 3 NO 2 . Intermediate 3 was synthesized by reacting the tributyl(thiophen-3-yl)stannane and compound 2 using palladium catalyzed Stille cross-coupling reaction. Compound 2 was obtained from catechol by reacting with The thermal properties of ONAF and SNAF were measured by using TG Analyzer and DSC instruments. As depicted in Table 1 and T m . The SNAF molecule shows T c on its first cooling cycle, which is not observed for ONAF. This indicates better intermolecular aggregation of SNAF induced by alkylthiophene substituent on BDT donor unit. [52] The observed thermal properties of these compounds demonstrated their potential for various optoelectronic device applications.

Optical and electrochemical properties
Optical properties of ONAF, SNAF, and ITIC were measured in dilute CHCl 3 solution and in thin film cast from chloroform shown in Figure 3A and B, respectively, and corresponding data is provided in Table 2. Both the compounds displayed broad and intense absorption in UV-vis region in the range of 300-580 nm and it is complementary to the absorption of ITIC. The absorption bands located at shorter wavelength corresponds to localized aromatic ππ* transition of the A-D-D'-D-A system, while the bands at longer wavelength is mainly attributed to an ICT transition from donor to the terminal acceptor units. [7][8][9]53] In CHCl 3 solution SNAF displayed absorption maxima as 554 nm (molar extinction coefficient is 0.86 × 10 5 Mol −1 cm −1 ) at longer wavelength region and multiple absorption bands were observed in shorter wavelength region with good molar extinction coefficients, whereas, absorption maxima for ONAF was 548 nm (molar extinction coefficient was 0.71 × 10 5 Mol −1 cm −1 ). Compared to solution, the absorption spectra of these molecules were significantly red-shifted and got broadened. This is attributed to the better intermolecular π-π stacking in solid state ( Figure 3B), due to their planer structure. The absorption peak in thin film are located at 600 nm (absorption coefficient of 2.2 × 10 4 Mol -1 cm -1 ) and 616 nm (absorption coefficient of 1.67 × 10 4 Mol -1 cm -1 ) for ONAF and SNAF,  respectively. The optical bandgap of these two molecules estimated from the onset absorption edge observed in the thin film absorption spectra is 1.87 and 1.78 eV, for ONAF and SNAF, respectively. [54] The estimated optical bandgap of ITIC is about 1.52 eV, which are well matched with the value reported in literature. In Figure 3, that absorption profile of ONAF or SNAF showed complementary absorption with ITIC, indicating that blend of these small molecule donors with ITIC can show the absorption ranging from 300 to 850 nm, which is beneficial for the light harvesting efficiency of the OSCs. In order to get information about the exciton dissociation and charge transfer between the donors and ITIC acceptor in the BHJ active layers, we have investigated the photoluminescence (PL) spectra of pristine SNAF, ONAF, and ITIC thin films and SNAF:ITIC and ONAF:ITIC blended films excited at the wavelength corresponding to the absorption peak of the donors and ITIC acceptor and shown in Figure S6a and S6b. As shown in Figure S6, SNAF and ONAF showed a strong PL peak around 670-680 nm, which was quenched when blended with ITIC, but the quenching in more for SNAF:ITIC blend, suggesting the photoinduced electron transfer from SNAF to ITIC was more effective in the SNAF:ITIC blend. The pristine ITIC showed a strong PL peak around which was quenched in both SNAF:ITIC and ONAF:ITIC, indicating the efficient electron transfer from SNAF or ONAF to ITIC ( Figure S6). This is an interesting situation that absorption of light by either donor or acceptor results in electron transfer from donor to acceptor. Further the HOMO/LUMO energy levels of these donors (SNAF & ONAF) and acceptor (ITIC) are very well matched (Figure S7). We predict that the HOMO/LUMO levels of D/A falling in this category ( Figure S7) with further close energy levels will have implications on the efficiency of OBHJSCs.
Electrochemical properties of ONAF, SANF and ITIC were measured by using a standard three electrodes of cyclic voltammetry (CV) technique ( Figure 4A and Table 2). The HOMO energy values of all small molecules were obtained from onset oxidation potential observed in CV and calculated from the empirical formula: E HOMO = -e[5.1 + E OX ] eV based on Fc/Fc + energy level relative to vacuum. The first oxidation peak potential was used to calculate the HOMO and the values are -5.25, -5.43 and -5.78 eV for ONAF, SANF, and ITIC respectively. The LUMO values calculated from CV (onset reduction potential) and obtained values are -3.51, -3.54, and -3.95 eV for ONAF, SANF, and ITIC, respectively. The HOMO and LUMO energy levels of ONAF and SANF small molecules well matched with ITIC acceptor. The difference of LUMO level of acceptor and LUMO of donors are 0.44 and 0.41 eV for ONAF and SANF, respectively, which greater than the threshold value (0.3 eV) for exciton dissociation and subsequent electron transfer at D/A interface, efficient in both BHJ active layers ( Figure 4B). [55]

Theoretical studies
We further tried to show the relationship between the structural variations optical and electrochemical properties using DFT and TD-DFT methods. Initial geometries of the molecules were optimizes using B3LYP, M06 and CAM-B3LYP functionals with a common 6-311G (d, p) basis set. Detailed DFT methodology followed is given in ESI. Degree of π-conjugation between the donor and acceptor segments is essential to analyze the extent of charge transfer via the overlap of orbital interactions. More planar is the molecule, greater the charge transport.
In these molecules, NDT-BDT-NDT segment was found with a torsional distortion of ∼3.02 To understand the photo-physical behavior of ONAF and SANF, we looked at the absorption spectra using three various density functionals (B3LYP, M06 and CAM-B3LYP) with a 6-311G (d,p) basis set. CPCM model has been chosen to describe the solvation of the molecules to mimic the real environment and CHCl 3 was used as solvent medium in this study. Of these, trend simulated by M06 functional was found to be in line with experimental observations albeit having a small overestimation ( Figure S3, ESI) and CAM-B3LYP functional showed a small underestimation. The simulations predicted using other B3LYP functionals were highly deviated. The predicted oscillation strength was exceptionally high computed at the TD-DFT/CAM-B3LYP/6-311G(d,p)/CPCM (CHCl 3 ) level of theory. Nonetheless, B3LYP predicted HOMO-LUMO gap showed a difference of 0.1 eV, followed a consistent trend with the experimental band gap (solution state). A little underestimation was noticed in HOMO of ONAF but SNAF showed a small overestimation, whereas, LUMO of ONAF showed almost same as experimental but for SNAF, it was little underestimated. Electrostatic surface potential (ESP) of all the compounds displayed in Figure S4 (ESI). The red, green and blue colors represent negative, neutral, and positive charge population on the segments respectively. Positive charge sites were delocalized over the -NDT-BDT-NDT-framework except O-atoms of NDT-BDT and terminal acceptor units bearing negative charge population sites on both sides. The yellow color of S-C = S functionality of Rh bears a large but moderate negative electron density cloud due to weak electronegativity and strong polarizability of sulfur atoms.

Photovoltaic properties
The photovoltaic properties of these two organic SM donor and ITIC as non-fullerene acceptor were investigated by fabricating the OSCs with conventional device structure of ITO illumination (AM1.5G, 100 mW cm -2 ) were displayed in Figure 5 and photovoltaic parameters were compiled in  Figure 4B).
The incident photon to current conversion efficiency (IPCE) spectra of the optimized OSCs was shown in Figure 5. The SNAF based OSC exhibit a broader photoresponse in the wavelength region of 300-850 nm, while ONAF based OSC showed a broad photo-response from 300 to 850 nm with a dip around 600 nm. Moreover, the maximum IPCE value of SNAF based OSC was higher than that of ONAF counterpart. The integrated value of J sc from IPCE spectra of the OSCs were about 15.63 mA cm -2 and 14.75 mA cm -2 for SNAF and ONAF, respectively, and were consistent with the J sc values obtained from the J-V characteristics under illumination. These results indicated that IPCE efficiency was higher in SNAF based OSC than that of ONAF.
The charge transport properties of the optimized SNAF:ITIC and ONAF:ITIC blended films were evaluated by space charge limited current model using the hole only and electron only devices ( Figure 6). We have also measured the hole mobilities in the pristine small molecules and are about 0.92 × 10 −4 cm 2 V -1 s -1 and 0.81 × 10 −4 cm 2 V -1 s -1 , respectively for SNAF and ONAF. The electron /hole mobilities of SNAF:ITIC and ONAF:ITIC were determined to be 1.25 × 10 −4 / 2.12 × 10 −4 and 1.02 × 10 −4 /2.19 × 10 −4 cm 2 V -1 s -1 , respectively, which corresponded to μ e /μ h ratios of 1.67 and 2.15, respectively. These results indicate that SNAF:ITIC blend film showed higher hole mobility and a more balanced charge transport than that for ONAF:ITIC blend film, which is in agreement with the higher values of FF and J sc observed in SNAF:ITIC based OSC.
In order to investigate the exciton dissociation and charge collection mechanisms, the variation of photocurrent density (J ph ) with effective voltage [56,57] was employed and shown in Figure 7. As shown in Figure 7 that in both the OSCs, the J ph increases linearly with V eff and reached to a saturation value (J sat ) at high value V eff , indicating that all the photogenerated excitons after the absorption of light were dissociated into free charge carriers and collected by the electrodes. The J sat value of the SNAF:ITIC OSC was about 16.56 mA cm -2 , which is higher than that F I G U R E 6 Dark current-voltage characteristics for (A) hole only devices and (B) electron only devices for the optimized blended films F I G U R E 7 Variation of photocurrent density (J ph ) with effective voltage (V eff ) for the OSCs based on optimized active layers for ONAF:ITIC (∼15.96 mA cm -2 ), indicating that the SNAF:ITIC based OSC exhibited enhanced charge generation and therefore, higher J sc . The exciton dissociation rate (η diss ) and charge collection efficiency (η coll ) were estimated from the J ph /J sat ratios under short circuit condition and maximum power points, respectively and were 0.96/0.74 and 0.93/0.69 for SNAF:ITIC and ONAF:ITIC based OSCs, respectively. Higher values of both η diss and η coll for the SNAF:ITIC than that for ONAF:ITIC indicates than former device had more efficient exciton dissociation and charge collections efficiency than that for later device.
In order to get more information about the charge carrier recombination behavior in the OSCs, we have measured the J sc values at different illumination intensities (P in ) and shown in Figure 8. The relation between the J sc and P in can be described by the expression: J sc ∝ (P in ) α , where the exponent factor α related to the extent of bimolecular recombination. [58] The values of α extracted from the Figure 8 are for SNAF and ONAF based OSCs, respectively, indicating that the bimolecular recombination in the SNAF based OSC is lower as compared to that of ONAF, and consistent with the balanced charge transport and resulting higher value of FF. The V oc follow a logarithmic relationship with illumination intensity as V oc = (nkT/q)ln(P in ), where k is Boltzmann constant, T is the temperature and q is the elementary charge. [59] As shown in Figure 8, the slopes of fitting lines are 1.27 kT/q and 1.34 kT/q for SNAF and ONAF based OSCs, respectively, indicating there more trap-assisted recombination in ONAF based device.
The transient photovoltage (TPV) and transient photocurrent (TPC) measurements were carried out to get information about the carrier lifetime and charge extraction times of OSC based on ONAF:ITIC and SNAF:ITIC based active layers and shown in Figure 9. The carrier lifetime under open circuit conditions were extracted from TPV decay dynamics using mono-exponential fits. [60,61] In the assessment of TPV, the SNAF:ITIC based device shows a carrier lifetime of 4.58 µs which is higher than that for ONAF:ITIC counterpart (3.98 µs), which disclose that the charge recombination in the SNAF:ITIC based device is more effectively suppressed. The value of charge extraction time estimated from the TPC measurements for the device based on SNAF:ITIC is about 0.82 µs, which is lower than that for ONAF:ITIC counterpart (0.93 µs) represents more effective charge extraction in SNAF:ITIC based device leading to higher value of J sc and FF. These collective results revealed that SNAF:ITIC blend shows superior charge generation and charge transport characteristics leading to improved photovoltaic efficiency for the SNAF based OBHJSC devises.
The energy loss (E loss ) is an important factor and plays a crucial role in the overall PCE for OSCs. The E loss is expressed as E loss = qV oc -E g , where E g is the effective optical bandgap of the active layer and estimated from the onset of the IPCE spectra of the OSCs. [62] The val-ues of E loss for the optimized OSCs based on optimized ONAF:ITIC and SNAF:ITIC active layers are 0.70 eV and 0.56 eV, respectively. The lower E loss for the SNAF:ITIC based OSCs may be due to the lower HOMO energy level of SNAF. The energy loss is divided into three parts: (i) radiative loss associated with the absorption bandgap, which unavoidable for any type of solar cells, (ii) additional radiative recombination originated from the absorption below the bandgap, and (iii) non-radiative loss, depends upon the LUMO and HOMO offsets between the donor and acceptor used in the active layer of BHJ OSCs. The low value of E loss for the SNAF:ITIC based OSCs may be due to the suppressed non-radiative loss which is due to small value of HOMO and LUMO offsets for SNAF:ITIC than that for ONAF:ITIC. [63,64] The transmission electron microscopy (TEM) images were scanned to get information about the morphology and phase separation in the active layers and shown in Figure S8 (ESI). Both the active layers showed proper interpenetrating phase networks and the SNAF:ITIC showed more appropriate interpenetrating networks and compared to ONAF:ITIC, which is beneficial for charge transport and suppressed recombination leading to improvement in the both J sc and FF values, resulting higher value of overall PCE of the OSCs. [65] We have recorded the X-ray diffraction (XRD) pattern of the pristine SNAF and ONAF as well as their blend with ITIC and shown in Figure S9 (ESI). It can be seen from Figure S9 that the both the pristine SNAF and ONAF showed similar (100) diffraction peak (lamellar stacking) at 2θ = 4.96 • and different (010) diffraction peaks (π-π stacking) at 2θ = 23.85 • and 24.29 • for ONAF and SNAF, respectively. The XRD diffraction of the blended ONAF:ITIC and SNAF:ITIC film showed same (100) diffraction peak located at 2θ = 5.38 • , which is related to the combined lamellar diffraction of SM donor and ITIC, but different (010) diffraction peaks located at 2θ = 22.68 • and 23.84 • , for ONAF:ITIC and SNAF:ITIC, respectively, indicated different π-π stacking for the blended films. The π-π stacking distance for ONAF:ITIC and SNAF:ITIC is about 0.393 and 380 nm, respectively. Moreover, the higher intensity for both (100) and (010) diffraction peaks for SNAF:ITIC blend also indicate that the this blend is more crystalline. The compact π-π stacking and more crystalline nature of the active layer are beneficial for the charge transport and resulting the higher values of J sc and FF for the OSCs based on SNAF:ITIC active layer.

CONCLUSION
In summary, two organic small molecules namely ONAF & SNAF were synthesized and used as donors for SM-OSCs with small molecule acceptor ITIC and their thermal, optical, electrochemical, theoretical and photovoltaic properties were systematically evaluated. Both the small molecule donor shows broad absorption in UV-vis spectrum and complementary absorption with ITIC acceptor, there by covering almost complete solar spectrum. The HOMO/LUMO energy levels of both donor materials matched with acceptor ITIC. OBHJSC devices of ONAF/ITIC and SNAF/ITIC displayed PCEs 7.48 % and 9.94 % respectively. Compared to ONAF, SNAF showed higher PCE due to the broader absorption, improved hole mobility and high FF resulted from the efficient inter-molecular packing of the active layer. The energy loss for the SNAF:ITIC based OSCs is about 0.56 eV, which is one of the lowest value for all small molecule OSCs. This efficiency is one of the best PCE for NDT-BDT based NF-ASM-OBHJSCs systems and ONAF & SNAF are a promising donor material for use in SM-OSCs. We predict that the HOMO/LUMO levels of D/A falling in this category, as in Figure S7, with further close energy levels will have implications on the efficiency of OBHJSCs.

C O N F L I C T S O F I N T E R E S T
The authors declare no conflict of interest.