Recent developments in non-fullerene-acceptor-based indoor organic solar cells

For over a decade, donor-acceptor blends composed of organic donors and fullerene acceptors dominated indoor organic solar cells (IOSCs). Numerous researchers have invested time to conduct extensive studies on developing new donor acceptor materials, interlayers, minimizing energy losses, and enhancing the open-circuit voltage (V OC) through device and material engineering, and optimizing device architectures to achieve highly efficient, environmentally stable, and commercially acceptable IOSCs. Through such efforts, the maximum power conversion efficiencies (PCEs) of IOSCs have surpassed 35%. In this regard, the transition from a fullerene to non-fullerene acceptor (NFA) is a useful strategy for enhancing the PCEs of IOSCs by allowing adjustment of the energy levels for compatibility with the indoor light spectrum and by improving photon absorption in the visible range, thereby boosting photocurrent generation and enhancing V OC. NFA-based indoor organic photovoltaic systems have recently drawn interest from the scholarly community. To compete with the standard batteries used in the Internet of Things devices, additional research is needed to enhance several characteristics, including manufacturing costs and device longevity, which must maintain at least 80% of their initial PCEs for more than 10 years. Further development in this field can greatly benefit from a thorough and comprehensive review on this field. Hence, this review explores recent advances in IOSCs systems based on NFAs. First, we explain several methods used to create extremely effective IOSCs, IOSCs based on fullerene acceptors are next reviewed and discussed. The disadvantages of using fullerene acceptors in IOSCs are noted. Then, we introduce NFAs and explore existing research on the subject. Finally, we discuss the commercial potential of NFA-based IOSCs and their future outlook.


Introduction
Because of the Internet of Things, thousands of different types of autonomous electronic devices that require low power consumption for indoor applications use large amounts of off-grid sources of energy [1][2][3][4][5].
IOSCs are considered as the best option for powering low-power-consumption devices because they can effectively convert low-intensity indoor light from the milliwatt to microwatt class of electrical power [6][7][8][9][10][11].
The irradiance spectra of frequently used indoor lights, including fluorescent lamps and LEDs, typically range from 400 to 700 nm and have irradiance power intensities less than 1 mW cm −2 , which are spectrally much narrower with weaker intensities (lower by more than 100 times) than the typical solar spectrum (AM 1.5G spectrum), thus, solar cells fabricated to harvest sunlight are not always optimized for indoor applications [12][13][14][15][16][17].Under artificial lighting conditions, crystalline silicon cells, for instance, have poor PCEs [18][19][20].Despite the fact that some newly developed solar cells like IOSCs and dye-sensitized solar cells, are less effective than crystalline silicon cells under standard solar illumination, they are promising candidates for indoor applications owing to the highly adjustable light-absorption properties of their photoactive materials [6,[21][22][23][24].Moreover, IOSCs have better commercial prospects than dye-sensitized solar cells because of advantages such as low-cost and large-area modules enabled by the solution printing and coating processes [25][26][27][28][29][30].However, the design of OSCs for indoor applications have both potential and difficulties.For instance, decreasing the light intensities can cause considerable changes in the functionality of solar cells [12].Currently, investigations lag behind the fast breakthroughs in material and technological innovation.Better knowledge of device physics is thus necessary for improving the performances of OSCs for indoor applications.Additionally, despite the significant growth in photovoltaic research towards indoor applications, new evaluation issues such as irregular light dispersion and diverging beams have resulted in less dependable device performances than the AM 1.5G condition.Apart from these, prior improvements paid close attention to outdoor applications, and molecular design approaches for high-performance narrow-gap materials (>800 nm) are being investigated [31,32].As a consequence, high-performance wide-bandgap photovoltaic materials (700 nm) principally used as acceptors are scarce for indoor applications [17,[33][34][35].In the first-generation IOSCs, fullerene-based acceptors were generally used [9,[36][37][38].The limited absorption of fullerene acceptors in the visible range hinders photocurrent production.
It is also impossible of tweak their electronic structures to achieve the required energy bandgaps (for different artificial light sources), making it difficult to change their V OC .To circumvent these issues, a new kind of NFA material was recently developed.In order to enhance the V OC (under low-intensity indoor lighting), NFA designs may be adjusted to provide strong light absorption in the visible zone and tunable frontier molecular orbital levels while reducing energy losses [39][40][41][42][43].Given these advantages, NFAs have emerged as viable candidates for IOSCs.For examples, the very first potential NFA-based IOSC (1 cm 2 ) with a PCE of 26.1% under 1000 lux was produced by synthesizing a unique NFA of IO4Cl to combine with PBDB-TF (donor) [41].Yan's group recently synthesized a novel NFA named FCC-Cl, that NFA based device demonstrated good exciton dissociation and charge transfer after mixing with the donor material D18, generating high J SC and FF values, thus, the PCE of the D18:FCC-Cl-based IOSC (0.85 cm 2 ) was as high as 28.5% under 1000 lux [40].The development of extremely effective photovoltaic materials for IOSCs is hampered by the lack of knowledge on the linkages between molecular structure, exciton dissociation, and charge transport, despite the fact that NFA-based IOSCs have much improved PCEs.In this regard, a comprehensive review of this topic may be quite beneficial for future progress.Therefore, we have summarized the progress achieved in the field of NFA-based IOSCs.First, we explore the techniques used to improve the performance of IOSCs in the past.The performances of fullerene acceptors in various IOSCs were then critically analyzed.Then, we introduce NFAs and discuss previous works on various NFA-based IOSCs.Lastly, we conclude this review by evaluating the market potential of NFA-based IOSCs and their future prospects.

Basic difference between outdoor OSCs and IOSCs
Outdoor OSCs and IOSCs are designed to function in different illumination situations and environments, resulting in substantial changes in design considerations, performance requirements, and practical applications [44,45].First of all, there are considerable differences in the lighting conditions that IOSCs and outdoor OSCs experience [46].IOSCs usually work in lower light intensity artificial light sources, often between hundreds and thousands of lux.Outdoor OSCs, on the other hand, are exposed to natural sunshine, which has substantially greater light intensities, often between tens of thousands and hundreds of thousands of lux [46].To maximize energy conversion, this variation in lighting situations requires different design strategies and material selections.Based on their unique illumination conditions, IOSCs and outdoor OSCs have different design concerns [12].The incident light's spectral range also changes between IOSCs and outdoor OSCs.Indoor lighting sources with visible emission spectra, such as fluorescent or LED lights, have unique emission spectra.As a result, IOSCs may be designed to absorb light in certain wavelength ranges [46].In contrast, outdoor OSCs must cover a broader spectrum, including ultraviolet (UV), visible, and infrared (IR) regions, to effectively utilize the full spectrum of sunlight [47].In addition, the stability requirements and environmental factors experienced by IOSCs and outdoor OSCs vary significantly.IOSCs are typically shielded from harsh outdoor conditions such as temperature fluctuations, humidity, and prolonged exposure to UV radiation [48].As a result, they may have less stringent stability criteria and may be allowed to employ materials that are less stable in outside situations.Outdoor OSCs, on the other hand, must survive a variety of environmental conditions, including temperature extremes, wetness, and continuous UV radiation exposure, needing more durable materials and improved encapsulation techniques [49].Finally, the costs of IOSCs and outdoor OSCs are different.Simpler device topologies and eased stability criteria may help IOSCs, perhaps yielding in cost reductions in fabrication and manufacturing procedures.Outdoor OSCs, on the other hand, need better efficiency, improved stability, and more stringent encapsulation procedures, potentially resulting in higher material prices and more difficult production processes.
In summary, there are significant variations between IOSCs and outdoor OSCs in terms of design concerns, light intensity and spectrum, stability requirements, and cost considerations.IOSCs are designed to work in low-light environments, have simpler device designs, and have less severe stability requirements.Outdoor OSCs, on the other hand, are meant to catch sunlight effectively, use more complicated structures, and require increased stability and environmental protection.Understanding and managing these opposing characteristics is critical for building customized OSC solutions for various application contexts, assuring maximum performance and efficiency.

Various approaches for improving performance of IOSCs
Theoretically, the highest PCE of IOSCs illuminated by the light of widely used LEDs may reach 58.4% [8].The highest predicted PCE for FT light source is 46%, but experimentally we are able to obtain around 30% [8].Thus, there is a large discrepancy between the theoretically predicted and actual PCE.Various indoor light sources, on the other hand, emit light at several wavelength ranges.Therefore, it is extremely difficult to develop a single IOSC that can work efficiently regardless of the light source.The self-lifetime of the IOSC is an additional issue.Different research groups have thus far developed various techniques to address these problems, which we discuss in this section.
Several significant techniques have been used thus far to increase the performances of IOSCs in addition to the development of numerous high-performing donor acceptor materials.First, efficient interfacial materials for charge extraction and transfer, as well as restricting energy losses owing to interfacial recombination, are critical for improving device efficiency [50].In IOSCs, interfacial materials facilitate the extraction and transport of photo-generated charges, such as electrons and holes, from the active layer to the appropriate electrodes [51].Efficient charge extraction is crucial to minimize losses and ensure high device performance [52].Interfacial layers, such as ETLs and HTLs, are typically utilized to enhance charge extraction by aligning energy levels and providing suitable pathways for charge transport [50,52].In IOSCs, unintended charge recombination at the interfaces can cause large efficiency losses.To reduce interfacial recombination processes and enhance charge collection, interfacial materials are used.These substances operate as energy barriers, preventing the recombination of charge carriers and encouraging their selective extraction to the proper electrodes [53].The proper design and selection of interfacial materials contribute to reducing losses associated with charge recombination, thereby enhancing the overall device performance [54].Interfacial materials also play a crucial role in controlling the morphology and interfacial properties of IOSCs.Researchers can modify the active layer's nanoscale shape and enhance the close contact between various materials by carefully selecting and designing interfacial layers.This morphological regulation enhances the efficacy of exciton dissociation, charge transfer, and light absorption [55].The contribution of interfacial materials to device stability and environmental protection is another crucial factor.These substances can serve as barrier layers, keeping out oxygen, moisture, and other hazardous species that could damage IOSCs and reduce their performance and lifespan [56].By incorporating suitable interfacial materials, the device can be protected from external factors and exhibit improved long-term stability [57].Additionally, interfacial materials help align the energy levels at the interfaces, facilitating efficient charge transfer and reducing energy barriers.The WF of the cathode should be comparable to the energy associated to LUMO level of the BHJ to achieve ohmic contact, allowing electrons to be extracted successfully without encountering an energy barrier, using the ETL for example.However, owing to the significant variations in the BHJ's LUMO level and the cathode's WF, ohmic contacts are challenging to generate in practice.To improve energy alignment, many electron carrying materials have been created to lower the WFs of cathodes, similar to the HTL.As a result, several researchers are working to improve high-performance IOSCs by interfacial engineering.Until now, various ETLs and HTLs (figure 1 shows the chemical structures of some materials used as the ETLs and HTLs of various IOSCs) have been utilized for improving the performances of IOSCs.PEDOT:PSS is commonly employed as the HTL in IOSCs because of its ease of processing, water stability, superior hole transport ability, and desirable WF (∼5.2 eV).However, it has several severe drawbacks, including high acidity, susceptibility to UV radiation, and high cost.Because of its higher acidity and vulnerability to UV radiation, the device lifetime is drastically reduced.Hence, Biswas et al recently developed a low-acidity, low-cost, and less-hydrophobic HTL material named PANI:PSS [58].Later, they demonstrated that the indoor light energy harvesting performance of PANI:PSS-HTL-based devices (with P3HT:ICBA active layer) was substantially dependent on the HTL's doping level and at an optimal PSS concentration, this device demonstrated a PCE of 8.1% under a 500 lux LED lamp (figures 2(a)-(c)) [27].In addition to organic semiconductors, an inorganic semiconductor was used as the IOSC HTL.Shim et al investigated the possibility of WO 3 as the HTL of an IOSC [39].Owing to the high WF (∼5.5 eV), low electron affinity, and superior thermal stability, they regarded WO 3 as a suitable HTL.They observed that the good hole selectivity (induced by the lower electron affinity) of the WO 3 layer allowed the IOSCs (WO 3 -HTL-based) to demonstrate a very high PCE (13%) under illumination from a 1000 lux LED.According to a comprehensive review, relatively few attempts have been made to develop HTLs for IOSCs.However, several studies have been conducted on how various ETLs perform in IOSCs.For instance, Han et al incorporated GQDs into the ZnO ETL of a P(Cl−Cl)(BDD=0.2):IT-4Factive layer based OSC owing to its strong absorption in near-UV region and high emission in visible region.Under 1 sun condition, the GQD-doped ZnO-based device had an excellent PCE of 14.0%, whereas virgin ZnO ETL based device showed 12.5% PCE.Under a 1000 lux LED light, meanwhile, doped ZnO ETL based device exhibited a remarkable PCE (19.6%), outperforming devices with pure ZnO based ETL (17.2%) [59].Ma and coworkers tested the performance of various ETLs in IOSC.OSCs with PDINO and widely utilized PFN based ETLs exhibited similar PCEs in both PM6:Y6-O and P3TEA:FTTB-PDI4 system under the 1 sun condition.The PCEs of the PM6:Y6-O and P3TEA:FTTB-PDI4 active layer based devices using PDINO as the ETL were 30.31% and 26.87%, respectively, whereas the PCEs of the PM6:Y6-O and P3TEA:FTTB-PDI4 active layer based devices employing PFN as the ETL were 22.7% and 17.88%, respectively [34].Additional optoelectronic characterizations show that PFN ETL-based OSCs have greater dark current densities and lesser shunt resistances than of there PDINO-based equivalents owing to the deep HOMOs of PDINO, which could explain why PFN-based devices perform poorly under indoor lighting.Using various ETLs, PFNs, TiO X , and ZnO, Torimtubun and colleagues thoroughly examined the behavior of PTB7-Th:PC 71 BM-based OSCs.As a consequence, under the 1 sun condition, the PCEs of the PFN-, TiO X -, and ZnO ETL-based OSCs were 10.55%, 10.2%, and 10.03%, respectively.On the other hand, under 1750 lux white LED, the PCEs of the PFN-, TiOX-, and ZnO ETL-based OSCs were 14.27%, 16.41%, and 16.49%, respectively.According to additional study, the indoor PCEs of PFN-based devices were lower, which might be related to trap-assisted recombination and leakage current.This research shown that excellent ETLs are required for high-performance IOSCs [60].Shim and colleagues investigated how a PEIE treatment onto ZnO ETL influenced the performance of P3HT:ICBA active layer based OSCs.For 1 sun condition, the PEIE-, ZnO-, and ZnO/PEIE-based OSCs had identical PCEs of 4.3%, 4.6%, and 5.2%, respectively.However, the PCEs of the PEIE-, ZnO-, and ZnO/PEIE-based OSCs were 12.6%, 5.7%, and 14.4%, correspondingly, under 1000 lux indoor light.Additional research clarifies the dipole moments caused by PEIE adjustment, resulting in appropriate WF reduction and remarkable indoor performance (figures 2(d)-(f)) [61].Sung and colleagues examined the impact of introduction of ZnO ETL of the performances of PTB7:PC 71 BM active layer-based OSCs in both indoor and outdoor lighting environment.ETLs based on ZnO NPs surpassed those based on sol-gel ZnO (7.16%), with a PCE of 8.32% under the 1 sun condition.Under an indoor light source (1000 lux), the sol-gel ZnO-based devices had higher PCEs than the NP-based ones.Further investigation showed that in terms of charge transport, charge carrier collecting effectiveness, reduced bimolecular recombination, and decreased monomolecular recombination, ZnO NPs outperformed sol-gel ZnO [62].Goo and coworkers looked into the impact of ETL (PEIE) thickness upon that efficiency of a P3HT:ICBA active layer-based OSC.The best PCE of 5.0% was attained by 2.2 nm thick ETL based devices under a 1 sun scenario.The associated device, however, displayed a PCE of 13.9% under 500 lux LED illumination because to the 8.5 nm PEIE's huge shunt resistance and tiny series resistance, which was advantageous for reaching a high FF value (figures 2(g)-(i)) [63].Ylikunnari and coworkers developed flexible OSCs having PV2001:PCBM active layer and different ETLs.The test results showed that SnO 2 ETL-based OSC is better for indoor light harvesting than ZnO ETL-based one as SnO 2 -based devices had a larger R SH (shunt resistance) than ZnO.The SnO 2 /grid-based devices achieved maximum PCE via R2R processing owing to optimization of the top electrode [24].
To solve the shortcomings of binary IOSCs, adding a third or fourth element to the active layer is a reasonable approach.In general, ternary or quaternary IOSCs outperform their binary counterparts because improved power realignment results in greater V OC , higher carrier mobility enhances FF, and additional absorption improvements raise J SC [64,65].Singh et al used EP-PDI as an additional element in active layer to enhance PTB7:PC 71 BM OSC performance.The ternary devices had an exceptional PCE (15.36%) under 500 lux white LED when the EP-PDI ratio was 40%, however the binary devices had a poor PCE (8.85%) (figures 3(a)-(c)) [66].PDTSTPD was used by Yin and coworkers as an additional ingredient in the PCDTBT:PC 71 BM mix film.The PCEs of the binary and ternary devices were 5.3% and 6.0%, correspondingly, under the 1 sun condition.The performance of the ternary devices varied depending on the light source, for example, the highest PCE (16.5%) exceeded that of the binary device under fluorescent (2700 K) lamp, with solvent vapor annealing, the PCE could be increased to 20.8% (figures 3(d)-(f)) [67].As the third element of ternary OSCs, Cho and colleagues developed a variety of wide-bandgap acceptors.The PM6:Y6:Y-Th2 OSCs, in particular, surpassed the binary PM6:Y6 (15.04%)OSC, demonstrating a PCE of 16.01% under the 1 sun condition.Furthermore, under 1000 lux of LED light, the ternary devices showed an excellent PCE of 22.7% [68].In their fabrication of OSCs using the FBT:PC 61 BM system, Farahat et al also included PDI as the third component.The PCE of the slot-die-coated ternary OSCs was 7.7% under the 1 sun condition, which was slightly smaller than the PCE of 7.9% of their spin-coated counterparts.The PCE (15.5%) of the slot-die-coated FBT:PC 61 BM:PDI OSCs improved under white LED illumination [69].The wide-bandgap PM7 was used by Bai et al as an additional element in the J52-F:BTA3-based active layer, and it significantly reduced energy loss and suppressed carrier recombination [70].Therefore, under indoor illumination, the ternary devices had a large PCE (20%) and a considerable V OC (1 V) [70].Quaternary OSCs have also been explored in addition to ternary devices for high-performance IOSCs.Nam et al chose PBDB-T, PTB7-Th, PC 71 BM, and 3ITIC-Th as the four BHJ components.Under the 1 sun condition, the quaternary devices had a PCE = 9.46% [71].Furthermore, the device exhibited a PCE = 14.29% under indoor lighting, and the quaternary system exhibited excellent semitransparent properties, allowing a PCE of 13.7% and an average visual transmittance of 48.6% to be achieved at the same time.This work showed that this quaternary OSC could function both during the day and at night, which increased its number of plausible applications [71].
It is universally believed that a variety of device factors, including as the thicknesses of multiple layers, the surface morphology of each layer, and the electro-optical characteristics of the active layer, have a significant impact on an OSC's performance.To maximize the PCE, various researchers have attempted to optimize this device architecture in recent years [72][73][74].In this regard, device modeling and simulation have assisted the exploration of the device operating mechanism and interactions between several layers of IOSCs based on fundamental physics ideas.This is also an important and cost-effective way to manufacturing more precisely designed devices for future development.Recently, some researchers have tried to improve the performance of IOSC devices by optimizing different parameters through modeling and device simulation studies using FDTD-based simulation [75][76][77][78].They used the Lumerical FDTD software for this, which is a powerful tool for optical modeling of photovoltaic systems.Through an optical simulation analysis, Vincent et al recently demonstrated that a standard OSC composed of P3HT:ICBA may be a viable alternative for indoor applications [74].Furthermore, they recommend maximizing the PCE of the P3HT:ICBA active-material-based IOSC by optimizing its photon absorption capacities [74].The same team further evaluated the potential of P3HT:ICBA active-material-based IOSCs for harvesting low-intensity LED light.They adjusted the device's active layer thickness for various incident light brightness levels using FDTD simulations.They also proved that with the optimal design, the device could achieve maximum PCE.Using a mix of optical modeling and testing, they aimed to establish a strategy for enhancing the OSC architecture to enhance device efficiency (figures 4(a)-(c)) [72].Similar research was done on the impact of active-layer thickness on the performance levels of many semiconductor-based IOSCs.For example, Shin et al discovered that the rates of reduction in ideal short-circuit current density values (thus, PCE values) of PPDT2FBT:PC 70 BM active-material-based OSCs with increasing active-layer thickness are very low for indoor applications, whereas the opposite behavior was observed under the 1 sun condition.They propose that modifying the illuminating agent can alter the influence of the crucial parasitic resistance on an OSC's performance level (figures 4(d)-(f)) [79].FDTD optical simulations have been employed to optimize the thicknesses of the other layers as well as many other aspects of IOSCs, in addition to the active layer's thickness.Lee et al tried to develop an exceedingly efficient and adaptive IOSC device by employing a quasi-amorphous transparent electrode (ZnO/Ag/ZnO) and P3HT:ICBA active material recently [80].They used an FDTD optical simulation study to alter the thicknesses of many components of an IOSC device's mechanically stable electrode.Using a combination of modeling and experimental inquiry, they were able to attain a higher PCE (12.1%) for illumination with a 500 lux LED source [80].Quaternary IOSC device characteristics have also been optimized using FDTD optical simulation studies.With the help of weight ratio modification, Shin et al significantly increased the photoenergy absorption capacities of the active layer (PCDTBT: PTB7:PC 61 BM:PC 71 BM) of quaternary IOSC devices.To determine the ideal active-layer thickness, they also investigated the impact of the active-layer thickness on the device's optimal short-circuit current density while operating under a 500 lux white LED light (figures 4(g)-(i)) [81].To determine the precise role of the quaternary combination during device operation, Vincent and coworkers carried out a thorough optical simulation analysis on a PCDTBT:PTB7:PC 61 BM:PC 71 BM active-material-based IOSC device.They observed that the appropriate short-circuit current density value for a quaternary IOSC might vary greatly depending on the active-layer thickness.Furthermore, because of interference between the incoming and reflected (from the rear electrode) light, the simulated electric field intensity value within the active range was extremely sensitive to thickness.This finding led us to suggest that the development of an active layer with optimum thickness is required for improved performance from an OSC, and optical  [79], with permission from Elsevier, 2019); (g) Quaternary OPV device structure fabricated for the study, (h) J sc,ideal simulated under 500 lux white LED illumination, (i) experimentally obtained J-V curves for different composition weight ratios under 500 lux white LED illumination (500 lux, 0.17 mW cm −2 (Reproduced from [81], with permission from Elsevier, 2019).
simulation is an excellent tool for this purpose.They also found that PTB7 serves as a cascade energy level and an additional light absorber while PCDTBT serves as the device's main light absorber [75].Even though precise balanced bandgap calculations show that tandem photovoltaics are feasible under the 1 sun condition, Biswas et al proved that single-active-layer-based photovoltaics may more efficiently capture the low-intensity light from white LED light sources [14].The maximum J SC and V OC of a tandem photovoltaic structure connected in series are constrained, which affects the device performance under low-intensity light illumination [14].They used optical simulation to estimate different device performance characteristics.
According to the literature review, different IOSC device parameters have typically been optimized using optical simulation studies, but the effects of the generation rate, recombination rate, charge carrier diffusion, and electron collection process on the performances of the devices have rarely been investigated.Electrical simulation studies might be a great alternative to better understand the effects of altering various device parameters on the operation of an IOSC device [82].Thus far, various electrical models, such as the effective medium model [83] and Monte Carlo model [84], have been used to perform electrical stimulations of various PV structures.Nowadays, some researchers have tried to estimate the effects of the morphologies of various layers [83], domain sizes [84], and weight ratios of donor to acceptor [84] on the charge transport mechanisms of various OSCs using the Monte Carlo model.It is emphasized that the formation of charge transfer excitons in BHJ OSCs does not automatically lead to the dissociation of excitons and generation of charge carriers due to the built-in electric field created by the difference in the WFs of the electrodes [85].As a rule of thumb, charge transfer exciton dissociates more easily than Frenkel excitons because of lower binding energy.A condition of the dissociation of charge transfer excitons is recently proposed and used to derive the rates of exciton dissociation in BHJ OSCs [85].For effective charge exciton dissociation and hence efficient photo charge carrier production, it has been proposed that the energy offsets of LUMO (∆E LUMO ) or HOMO (∆E HOMO ) at the D-A interface in a BHJ OSC must be more or greater than the binding energy (E B ) of charge transfer excitons (equation ( 1)).According to equation (1), condition of dissociation of charge transfer excitons can be given by, (1) This will allow efficient dissociation of both singlet and triplet excitons [85].Besides this, recently the same group have proposed an alternative approach to simulate PCE (η PCE ) of BHJ OSCs, as a product of efficiencies of absorption (η abs ), dissociation (η dis ), and extraction (η ext ) [86], because the operation of BHJ OSCs consists of three processes: (1) photon absorption and exciton generation, (2) exciton diffusion and dissociation, and (3) charge transport and collection (equation ( 2)), Although η abs and η dis do not directly contribute to the simulation of η PCE the approach enables us in understanding their roles in optimizing the PCE of BHJ OSCs.The method has been applied to simulate η PCE as a function of the thickness of active layer for three different BHJ OSC structures, one with a fullerene acceptor and two with two different NFAs.The results have been found to be in good agreement with those from the previous simulation and experimental works and are expected to be useful in optimizing the thickness of the active layer.Moreover, recently they explained (with the help of electrical stimulation) that the increased PCE seen in BHJ OSCs may be related to the additional contributions of both donor and acceptor excitons [87,88].They estimated the contribution of excitons produced in the acceptor material to BHJ OSC photovoltaic performance and compared it to the contribution of excitons created in the donor material.Most parameters, including absorption and dissociation rates and diffusion lengths for singlet and triplet excitons, have been shown to be almost of the same order of magnitude regardless of whether excitons originate in the donor or acceptor in the various types of BHJ OSCs investigated.
So, here it has been observed that diverse range of approaches undertaken to enhance the performance of IOSCs.By leveraging advancements in material engineering, device architecture optimization, light management techniques, and interface engineering, researchers strive to overcome the limitations and maximize the efficiency, stability, and practical viability of IOSCs.Moreover, a device simulation study may be considered a very effective method for enhancing the design of IOSC devices in this review.In the near future, a combination of experimental, optical, and electrical modeling may be carried out to develop a next-generation IOSC device that can be utilized in the marketplace.

Various acceptor materials for IOSCs
Appropriate donor and acceptor materials must be developed to provide ideal active materials for highly efficient IOSCs.To achieve relatively elevated (V OC ) indoors, it is necessary to take into account a number of factors, including good spectral match between the absorption spectra of the active layer and light illuminance spectra, adjusting the energy levels of the donor and acceptor components (to acquire the bandgap of the photoactive layer within 1.9-2.1 eV), and good electrical charge transport capacity.Different organic polymers and small compounds have been designed as donor materials to meet these requirements.Numerous acceptors based on fullerene and non-fullerene materials have also been produced.Because we are mainly interested in the acceptor materials, we will categorize all active materials of IOSCs into two groups: fullerene-acceptor-based and NFA-based IOSCs.

Fullerene acceptors for IOSCs
Fullerene acceptors have been frequently employed in IOSCs due to their high near-UV absorption and wide bandgap.The IOSC's V OC is proportional to the difference in donor HOMO and acceptor LUMO energy levels.In principle, higher V OC OSCs are perfectly applied as IOSCs.Choosing proper donor/acceptor combinations is thus critical for high-performance IOSCs.Additionally, high J SC requires fullerene-based devices with high EQEs.Finally, as light intensity falls, the FF drops since trap-assisted recombination dominates carrier recombination, hence, trap-assisted recombination must be suppressed to provide high FF.Thus far, only a few organic semiconducting materials, such as P3HT, PTB7, PTB7-Th, PCDTB-T, and PDTBTBz-2F anti , have been employed as donors (figure 5), whereas PCBM and ICBA have been used as acceptors.Figure 6 depicts the chemical structures of some of the most often utilized fullerene acceptors in IOSCs.Lee et al chose three BHJ configurations: P3HT:PCBM, PCDTBT:PC 71 BM, and PTB7:PC 71 BM.Under the 1 sun condition, P3HT:PCBM-based OSCs had a PCE of 2.4%, PCDTBT:PC 71 BM OSCs had a PCE of 6%, and PTB7:PC 71 BM OSCs had a PCE of 6.8%.When irradiated by 300 lux FL light, the P3HT:PCBM, PCDTBT:PC 71 BM, and PTB7:PC 71 BM-based IOSCs had PCEs of 5.8%, 16.6%, and 14.6%,The module has dimensions of 14 cm × 14 cm with a total active area of 100 cm 2 .A photo of the module is shown in the inset (Reproduced from [6], with permission from AIP, 2016); (d) Schematic of indoor BHJ OSC devices with an inverted configuration and their single-diode equivalent circuit model, (e) J-V curves for the optimized OPV devices and a-Si cell measured under white LED at an illuminance of 200 lux, (f) PCE of the optimized BHJ OSCs on the incident LED light intensity compared to the values obtained for an a-Si cell under the same lighting conditions (Reproduced from [30], with permission from ACS, 2019); (g) OSC device structure fabricated for the study, (h) J-V curves of all of the OSCs under standard LED indoor light (500 lux), (i) IPCE spectra of all the OSC devices (Reproduced from [91], with permission from ACS, 2019).
Additionally, at a luminance level of 800 lux, the highest PCE of 15.78% was achieved using TLD-840 FL lamps with varying irradiation intensities.Contrarily, the vacuum-processed DTCPB:C 70 OSCs showed better stability, retaining 87.5% of their initial PCE even after more than 150 h of photoaging [89].P1, a porphyrin-based donor developed by Yin and colleagues, consists of a porphyrin ring joined by two ethylrhodanine end caps through phenylene ethynylene bridges.Under the 1 sun condition, the P1:PC 71 BM OSCs beat PCDTBT:PC 71 BM-based OSCs with a PCE of 5.78%.The P1:PC 71 BM OSCs attained a PCE of 19.15% under LED light with a luminance of 300 lux, outperforming the PCDTBT:PC 71 BM OSCs (18.72%).Additionally, the P1:PC 71 BM OSCs performed much better than the PCDTBT:PC 71 BM OSCs in terms of thickness tolerance, achieving a PCE of 18.43% with a thickness of 200 nm, which was higher than that of the PCDTBT:PC 71 BM OSCs (13.21%) under the same conditions.This is because of P1's lower electronic abnormalities as compared to PCDTBT [2].Arai et al reported the invention of BDT-1T-ID, a novel tiny molecular donor.BDT-2T-ID displayed the best light-absorption efficiency.BDT-2T-ID:PNP OSCs showed a PCE of 16% under 200 lux white LED (8500 K).According to this theory, the researchers developed a BDT-2T-ID-based module with an output power of 111 µW under LED lighting (of 200 lux) and an active area of 9.5 cm 2 , which was adequate to power self-sustaining electronic apparatus (figures 7(d)-(f)) [30].In the presence of an indoor light source, Yang and colleagues investigated at the performances of three distinct BHJ configurations, including regio-regular P3HT:PC 61 BM, P3HT:ICBA, and PBDTTT-EFT:PC 71 BM.Under the 1 sun condition, the PCEs of P3HT:PC 61 BM, P3HT:ICBA, and PBDTTT-EFT:PC 71 BM were, respectively, 3.68%, 4.90%, and 6.95%.Meanwhile, the P3HT:ICBA OSCs outperformed the other two when lighted with LED and FTs at a 500 lux intensity because the absorption spectrum matched the emission spectra of the two indoor light sources exactly, resulting in the greatest PCE of 13.76%.As a result, raising the V OC might enhance the PCE of the IOSC further [90].The effect of substituents on the performance of BDT and WF3 and WF3S substituents was explored by Singh et al.The fluorine substituent WF3F on the BDT thienyl demonstrated a considerable increase in characteristics, with a larger optical bandgap.WF3:PC 71 BM, WF3S:PC 71 BM, and WF3F:PC 71 BM OSCs with DPE additive had PCEs of 7.71%, 7.91%, and 9.11% under the 1 sun condition, respectively.The devices had PCEs of 12.83%, 14.32%, and 17.34% under 500 lux LED, respectively (figures 7(g)-(i)) [91].
According to this review, very few fullerene derivatives, including PC 71 BM and ICBA, are often employed acceptor materials in IOSCs, while a large number of studies have concentrated on the development of extremely effective donors.However, fullerene-based IOSCs suffer from significant energy losses, making future advances in device performance extremely difficult.Furthermore, the absorption ability in the visible range is limited for fullerene-based acceptors.Indoor lighting conditions often involve narrower spectral distributions, with a significant portion falling outside the absorption range of fullerene acceptors.As a result, the utilization of fullerene-based acceptors can lead to suboptimal light absorption and reduced overall efficiency in IOSCs.This hinders photocurrent generation in the device.On the other hand, fullerene acceptors typically exhibit lower V OC .This lower V OC is primarily attributed to the energy offset between the acceptor and donor materials.In IOSCs, where lower light intensity is prevalent, achieving higher V OC is crucial to maximize energy conversion.The limited V OC of fullerene acceptors can result in reduced power output and lower overall efficiency.Moreover, fullerene-based acceptors are prone to morphological instability under thermal stress, which can lead to phase separation, aggregation, and crystallization issues.The performance of IOSCs relies heavily on achieving an optimal nanoscale morphology within the active layer.The morphological instability of fullerene acceptors can disrupt the desired interpenetrating network of donor and acceptor materials, impeding charge transport and increasing recombination losses.Additionally, fullerene acceptors can exhibit lower stability.They are susceptible to photo-oxidation and degradation upon prolonged exposure to light, including indoor lighting.This reduced stability can lead to a decrease in device efficiency over time, limiting the practicality and lifespan of IOSCs.Finally, it is not possible to modify or tune the chemical structures of these materials for improving the physicochemical properties according to need.Owing to their easily adjustable light absorption ability and energy levels, a novel class of acceptors known as NFAs has therefore been investigated in a number of IOSCs over the past several years.

NFAs for IOSCs
NFAs provide outstanding benefits in terms of bandgap regulation, decreased energy loss, NIR region absorption, thermal stability, and photostability.As a result, considerable efforts have been invested into fabricating high-performance NFAs for IOSCs (figure 8 shows the chemical structures of various NFAs used in different IOSCs and table 1 the summary of photovoltaic properties of previously reported NFA based IOSCs) [92][93][94][95][96][97][98][99][100][101][102].Cui et al developed a new NFA, IO-4Cl, with a bandgap of 1.89 eV in 2018.The device showed PCE, V OC , J SC , and FF values of 9.8%, 1.24 V, 11.6 mA cm −2 , and 68.1% at 1 sun condition when combined with PBDB-TF, respectively.One of the lowest in an IOSC, 0.6 eV, was found to be the energy loss for PBDB-TF:IO-4Cl.Under LED (2700 K) illumination, the PCEs of the devices were 22.2%, 24.6%, and 26.1% at 200, 500, and 1000 lux, respectively.It is worth noting that the BHJ area was 1 cm 2 .The PCEs at 200, 500, and 1000 lux were 23.0%, 23.4%, and 23.9%, respectively, when the devices were made via blade coating with a BHJ area of 4 cm 2 , proving that the PBDB-TF:IO-4Cl IOSCs were appropriate for large-scale processing.Furthermore, the stability of the PBDB-TF:IO-4Cl system was increased, after 1000 h of continuous indoor lighting, the device essentially retained its initial PCE (figures 9(a)-(c)) [41].In 2019, Ding and coworkers synthesized CD1:PBN-10 active layer based OSCs with an immense promise for IOSC.The active layer based OSCs had a PCE of 7.93% under 1 sun condition, while the V OC was a remarkable 1.29 V.This device had 26.2% PCE under 1000 lux FL light and a maximum 27.4% PCE at 2000 lux FL illumination, which was among the highest values recorded for IOSCs.Contrarily, CD1:ITIC OSCs underperformed under indoor light sources due to an inappropriate absorption spectrum [103].Cui and colleagues examined the use of ITCC, PC 71 BM, and IT-4F in IOSCs.The PCEs of PBDB-TF:PC 71 BM, PBDB-TF:ITCC, and PBDB-TF:IT-4F under the 1 sun condition were 8.43%, 10.3%, and 12.2%, correspondingly.Warm white LED light (2700 K) was irradiated on PBDB-TF:IT-4F-based OSCs at varying luminance of 200, 500, and 1000 lux.The PBDB-TF:ITCC devices performed better than the other two and showed similar PCEs under diverse indoor light sources, which may be due to reduced E loss and a matched absorption spectrum.Furthermore, the PBDB-TF:ITCC devices demonstrated significantly increased photo stability under low light.With a bandgap of 1.82 eV, their predicted hypothetical PCE limit was 40% under 2700 K LED light [28].Je et al investigated how chlorine substitution affected the BDT-Th unit's photovoltaic properties.Under the 1 sun condition, the PCEs of PBDB-TSCl:IT-4F, PBDB-TS-3Cl:IT-4F, and PBDB-TS-4Cl:IT-4F were 8.7%, 12.6%, and 12.7%, respectively.The PCEs of PBDB-TS:IT-4F, PBDB-TS-3Cl:IT-4F, and PBDB-TS-4Cl:IT-4F were respectively 5.3%, 20.4%, and 21.7% under FL (500 lux).This result showed that chlorination was essential for improving V OC and BHJ morphology  system).This device had a PCE of 8.3% under the 1 sun situation.However, the PCE increased by a factor of three under LED lighting, and the IOSCs demonstrated a better PCE at illumination levels ranging from 170 to 1650 lux.Furthermore, at a thickness of 200 nm, the devices maintained a PCE of more than 21% [106].

Challenges and outlook
A golden period of IOSC expansion has arrived with the introduction of NFAs.Although NFAs are receiving more attention, there are still a lot of challenges to be solved before novel materials, processing techniques, device topologies, long-term durability, and manufacturing procedures for large-area devices can be developed [102].The NFAs have revealed enormous possibilities for IOSCs in terms of improved device efficiency and long-term operating stability [49].Currently, huge fused rings with multistep synthesis, such as ITIC and Y6, are employed to make effective NFAs.Typically, these compounds exhibit the following traits: these molecules have a planar main backbone, strong electron-withdrawing units that are exposed at two sides for molecular packing, and bulky substituents that are perpendicular to the main backbone in the electron-donating units.To achieve better electron mobility, effective intramolecular charge transfer and molecular packing are both facilitated by the main backbone's planar properties.Effective molecular interactions are enabled by the electron-withdrawing end groups, which also facilitate effective intermolecular charge transmission.To control molecular aggregation, the bulky substituents are useful [94,110].Based on this structure, several kinds of NFAs can also be used to produce high-performing IOSCs with significant adjustments [47,102].Future improvements to electron-donating units, electron-withdrawing units, bulky substituents, and other aspects may be developed to further enhance NFA performance [92,102].However, simple acceptor synthesis can be investigated immediately.We can focus on the following points: (a) green synthesis can be utilized to reduce the use of detrimental reagents made with tin and lithium, (b) adequately lowering the number of fused ring units aids in reducing the synthesis steps, and (c) rational use of aromatic bulky substituents and aliphatic alkyl chains to optimize the molecular structure.Overall, additional research into NFAs is needed to reach new advances in high electron mobility, low energy loss, and remarkable stability.Secondly, suitable energy-level compatibility should be established between donors and acceptors.The HOMO and LUMO of the polymer donor and NFAs, which may yield the most V OC while also acting as a strong driving force for charge production and transportation, should be carefully selected [92].There is little question that the energy loss for efficient fullerene-free IOSCs can be as low as 0.5 eV (or lower), but more research into innovative photovoltaic materials with lower energy loss is still required [95].Third, to optimize photon usage and therefore get a high current, high PCE performance depends on the complementary absorption of the acceptor and donor.In addition to these, the chosen blend shape is crucial for high-performance IOSCs.Exciton separation is favored by adequate domain sizes, high domain purity, and higher charge mobility.Additionally, the recommended face-on orientation of the film and appropriate film morphology will support vertical charge transfer.It should be emphasized that because of their short lifetimes, IOSCs have not yet been commercialized.The performances of IOSCs have now been maintained for 1000 h as a consequence of the numerous attempts to increase the device stability, including insertion of additives and interfacial layers, as well as encapsulation.Even though the IOSC's stability against atmospheric, thermal, and illumination stresses has been confirmed, the morphological and chemical stabilities of the NFAs against the interactions of numerous factors, such as oxygen, moisture, heat, and illumination, still need to be improved by controlling their molecular structure.IOSCs have gained large PCEs since the establishment of NFAs, and the yearly expectation for their commercialization is growing [97].However, there are very few data on actual situations, and the best performance of the NFA-based IOSC was only ever attained on the laboratory scale [47].The development of large-scale manufacturing procedures that do not impair IOSC performance is particularly important for the deployment of large-area IOSCs.For OSCs, a number of large-area manufacturing techniques, including slot-die coating and blade coating, have been suggested.Before feasible commercialization of IOSCs, there are still several challenges to be resolved, including the need for dual temperature control for solutions and substrates, use of halogenated solvents, and development of discontinuous interfacial layers.It is crucial to create acceptable NFA materials that are stable throughout the continuous production processes and soluble in green solvents at low temperatures to address these concerns successfully.

Conclusion
IOSC are one of the most significant uses of organic semiconductors that have achieved excellent success and exceed their silicon counterparts because of their configurable bandgap through molecular design.
Numerous studies have stated that the IOSCs attain great performance using a variety of methods, and these optimistic findings show that this line of inquiry is worthwhile.Specifically, NFAs show great possibility for the development of highly efficient and commercially available IOSCs in the near future.However, the aforementioned difficulties are significant and must be resolved for meaningful applications to be achieved in the future.In summary, NFA-based IOSCs exhibit significant benefits in applications and more research on them are necessary.

Figure 1 .
Figure 1.Chemical structures of various ETLs and HTLs used in IOSCs.

Figure 2 .
Figure 2. (a) Schematic of IOSC device, (b) current density (J)-voltage (V) characteristics curves of different IOSC devices operating under 500 lux white LED bulb, (c) change in PCE value of IOSC device operating under 500 lux white LED bulb and 1 sun condition with the doping concentration of the HTL (Reproduced from [27], with permission from Elsevier, 2020); (d) Device structure of the employed IOSC with an inverted geometry and chemical structures of PEIE, P3HT, and ICBA, (e) representative J-V characteristics of OSCs with PEIE, ZnO NP, and ZnO NP/PEIE ECIs under an LED lamp with a luminance of 1000 lux, (f) power absorption ratios of all devices under LED illumination calculated by the FDTD method (Reproduced from [61], with permission from Elsevier, 2019); (g) Device structure of the P3HT-ICBA-based IOSC with a PEIE-modified ITO electron-collecting electrode and chemical structures of P3HT, ICBA, and PEIE, (h) energy-level diagram of the components of the OSC, (i) representative J-V characteristics of the OSCs evaluated under LED lamp (500 lux, 0.17 mW cm −2 (Reproduced from [63], with permission from Elsevier, 2018).

Figure 3 .
Figure 3. (a) Energy-level diagram of the materials used in the ternary blend system, (b) normalized UV/Vis absorption spectra of the PTB7 polymer, and PC71BM and EP-PDI small molecules in solid-state thin films, also shown is a spectrum of the light emitted from the indoor LED lamp, (c) J-V characteristics under indoor light conditions (Reproduced from [66], with permission from Wiley, 2019); (d) Normalized UV-visible absorption spectra of the binary and ternary BHJ films and individual materials.The thicknesses of films are about 85 nm, which is the thickness of the optimized BHJ device, J-V characteristics of binary PCDTBT:PC71BM and ternary PCDTBT:PDTSTPD:PC71BM OPV devices under different indoor illuminations ((e) FL and (f) LED) with a fixed illuminance of 300 lux (Reproduced from [67], with permission from RSC, 2018).

Figure 5 .
Figure 5.Chemical structures of numerous donor materials used in IOSCs.

Figure 6 .
Figure 6.Chemical structures of several fullerene acceptor materials used in IOSCs.

Figure 7 .
Figure 7. (a) J-V measurement of P3HT:PCBM (black square), PCDTBT:PC71BM (red circle), and PTB7:PC71BM (blue triangle) devices under 300 lux fluorescent lamp, (b) maximum power output density, Pmax, (solid symbols) and Vmax to VOC ratio (open symbols) of three OSC devices at different illumination levels of fluorescent lamp, (c) I-V curves of 8 pixels connected in series (red circle) and a pixel (purple diamond), respectively, of the PCDTBT:PC71BM module.The module has dimensions of 14 cm × 14 cm with a total active area of 100 cm 2 .A photo of the module is shown in the inset (Reproduced from[6], with permission from AIP, 2016); (d) Schematic of indoor BHJ OSC devices with an inverted configuration and their single-diode equivalent circuit model, (e) J-V curves for the optimized OPV devices and a-Si cell measured under white LED at an illuminance of 200 lux, (f) PCE of the optimized BHJ OSCs on the incident LED light intensity compared to the values obtained for an a-Si cell under the same lighting conditions (Reproduced from[30], with permission from ACS, 2019); (g) OSC device structure fabricated for the study, (h) J-V curves of all of the OSCs under standard LED indoor light (500 lux), (i) IPCE spectra of all the OSC devices (Reproduced from[91], with permission from ACS, 2019).

Figure 8 .
Figure 8.Chemical structures of different non-fullerene acceptor materials used in IOSCs.

Figure 9 .
Figure 9. (a) Molecular energy levels of the materials in this work, (b) plots of E loss against Eg, which are determined from the derivative of the EQE edge, the values marked in this panel are PCEs, (c) J-V curves of the PBDB-TF:IO-4Cl-based optimal cell under different indoor light intensities (Reproduced from[41], with permission from Nature, 2019); (d) Schematic illustration of the device architecture, (e) energy levels of polymers and acceptor, (f) J-V curves under 500 and 1000 lux FL illuminations of the champion cells (Reproduced from[105], with permission from ACS, 2020).

Table 1 .
Summary of photovoltaic properties of previously reported NFA based IOSCs.