Powering the Future:  Comprehensive Strategies for Enhancing Efficiency and Stability in Organic Photovoltaic Cells

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Abstract

Organic photovoltaic (OPV) cells hold great promise for sustainable energy generation, offering a low-cost and eco-friendly alternative to traditional solar cells. However, their efficiency and stability remain key challenges. This paper addresses these challenges through a systematic analysis of three optimization strategies: side-chain modifications, interfacial layer engineering, and morphology optimization. Data tables highlight the impact of these strategies on key OPV cell performance parameters, with the results showing that side-chain modifications improve photon capture and charge transport efficiency, interfacial layer engineering enhances charge collection, and morphology optimization refines material interactions. The integrated application of these strategies offers a promising pathway for advancing OPV cell technology. By shedding light on the interplay between these strategies and their collective implications, this research contributes to the development of higher efficiency, commercially viable, and long-lasting OPV solar cells.

Introduction

Organic photovoltaic (OPV) solar cells offer a promising alternative to traditional solar cell technologies. Unlike conventional silicon-based solar cells, OPV cells are made from organic materials, typically polymers or small molecules, which offer several key benefits. Firstly, OPV cells can be produced using low-cost and scalable printing techniques, enabling large-scale production at lower costs compared to traditional solar cells1. This makes the technology highly attractive for applications in flexible and lightweight solar panels, which can be integrated into various surfaces and portable electronics1. OPV cells also have the potential for tunable and customizable absorption spectra. By designing and synthesizing different organic materials, researchers can tailor the bandgap of OPV materials to capture specific wavelengths of light1. This flexibility opens up opportunities for designing multi-junction OPV devices that can efficiently harvest solar energy over a broader spectrum of light, leading to improved overall efficiency. Thirdly, OPV cells have a low embodied energy (the sum of all the energy required to produce any goods or services) in their production compared to traditional solar cells2. The manufacturing process for organic materials requires less energy and generates fewer greenhouse gas emissions, making OPV technology more environmentally sustainable. 3456. OPV cells exhibit excellent low-light performance, allowing them to generate electricity even under overcast or indoor lighting conditions.7,897

In the context of organic photovoltaic solar cells, the correlation between polymer structure and energy efficiency has garnered significant attention, with a vast body of research exploring this dynamic relationship. Investigations into polymer structure have revealed compelling connections to crucial aspects of solar cell performance. One such correlation lies in the absorption properties of different polymer structures, as they exhibit diverse absorption spectra, enabling targeted light harvesting techniques across specific solar wavelengths. This contributes to improved light absorption and enhanced energy conversion efficiency7. Moreover, polymer tunability plays a major role in charge transport and mobility, with well-ordered structures and extended conjugation promoting higher charge carrier mobilities, which facilitates efficient electron and hole extraction2. The mobility of these electrons and holes within the polymer layer are directly related to tunable aspects such as the polymer side chains, conjugation length, and morphology separation within the solar cell. Furthermore, the morphology of the active layer impacts exciton diffusion (the energy absorbed by a particular molecular site is transferred to another nearby molecular site), charge separation, and collection10. Researchers have demonstrated how controlling the polymer structure affects phase separation behavior, domain size, and donor-acceptor interpenetration, leading to improved cell efficiency10. Additionally, the influence of polymer structure on stability and degradation has been investigated, prompting strategies like molecular weight alteration and donor-acceptor copolymer designs to enhance solar cell durability11. In regard to efficiency, side-chain engineering and interfacial engineering have emerged as effective approaches to optimize polymer structure and improve charge transport and collection1213.

This review will explore the significant impact of polymer structure on multiple length scales and delve into various strategies employed to enhance the efficiency of organic photovoltaic solar cells.

Methodology

The cells examined in this study were evaluated over a range of metrics that are essential for assessing the overall performance, energy conversion capabilities, practicality, and peak power generation potential of the altered OPVs. Each metric has been thoughtfully selected to provide a comprehensive understanding of the effects of polymer structure on the solar cell’s efficiency and sustainability. Power conversion efficiency (PCE) is a fundamental parameter that quantifies how effectively a solar cell converts incident sunlight into usable electrical energy. By measuring PCE, we gain insights into the inherent efficiency of the altered OPVs in harnessing solar energy for use. Open-circuit voltage (VOC) signifies the highest voltage that a solar cell can deliver when not connected to an external circuit. It offers critical information about the potential electrical output of the altered OPVs under no-load conditions. In contrast to this, short-circuit current (JSC) measures the maximum current that a solar cell can deliver when its terminals are directly connected. Understanding this is pivotal for assessing the cell’s capacity to generate substantial electrical currents under optimal illumination conditions. Fill factor (FF) quantifies the quality of a solar cell’s electrical output, representing the ratio of the maximum power output to the product of VOC and JSC. This metric offers insights into the efficiency of charge extraction and power delivery, contributing to a holistic understanding of the cell performance. Lastly, the analysis includes an estimated cost of material as this directly impacts the scalability and economic feasibility of the technology. Incorporating this metric provides valuable insights into the cost-effectiveness and potential for widespread adoption of the altered OPVs.

Results

Functional Mechanism of Organic Photovoltaic Cells

The functional mechanism and working principle of Organic Photovoltaic (OPV) cells is illustrated in Figure 114.

Figure 1.  Functional mechanism of a bilayer organic photovoltaic cell (D = donor, A = acceptor,14.

An Organic Photovoltaic Cell (OPV) cell operates by absorbing sunlight through an active layer composed of a blend of polymer donor and acceptor  materials. The absorbed photons create excitons, which then diffuse within the polymer blend to reach the donor-acceptor interface15. Charge separation occurs at this interface, splitting the excitons into free electrons in the polymer acceptor and positively charged holes in the polymer donor. These charge carriers move through their respective materials, known as charge transport, and are collected at the electrodes as electric current15. The generated current flows through an external circuit, producing electricity that can be utilized by the device.

OPV cells offer a promising and eco-friendly approach to converting sunlight into electrical energy, making them valuable for renewable energy production. Efficient operation requires, minimizing recombination and losses to enhance overall performance. In recent years, the field has witnessed tremendous advancements, driven by the pursuit of sustainable and renewable energy sources. The quest for efficient excitonic solar cells has become a focal point in this domain, with researchers exploring the fundamental relationship between polymer structure and energy efficiency. Research has demonstrated the critical role of polymer design in determining the performance of these cells16.

Side Chain Modifications

Thirdly, side-chain engineering offers a viable tool for improving PCE efficiency. Through fine-modification of the alterable side chains of non-fullerene acceptors (NFAs), 17% PCE can be achieved for OPV cells, whilst incorporating a more industrially applicable and scalable method as compared to the more frequently used lab technique of spin-coating17. Recent progress in the field of OPV cells has been predominantly driven by the development and adoption of NFAs. These high-performance NFAs exhibit a wide absorption range from 400 to 900 nm, facilitating efficient solar photon capture and generating high output current densities. NFA-based devices demonstrate diminished radiative and non-radiative energy losses, which enables the achievement of elevated voltages for sustained periods of time. This has led to remarkable power conversion efficiencies (PCEs) surpassing 16% in NFA-based OPV cells17

Notably, many NFAs are constructed from fused five- or six-membered heterocycles, like ITIC and Y6, which possess extensive conjugated structures promoting ordered intermolecular \pi-\pi stacking and enhanced charge transport17. However, this characteristic hampers their solubility, posing challenges for solution-based fabrication processes. To address this issue, careful adjustment of the flexible side chains of NFAs is essential to strike a balance between charge transport and solution processability, particularly when scaling up the cell’s active area. Presently, large-area OPV cells produced via printing methods achieve a PCE of about 13%, lagging behind smaller-scale spin-coated devices18. Most evident was the fact that when employing a blade-coating technique to extend the active layer’s dimensions, a notable PCE of 15.5% is sustained, showing evidence of a balanced interplay of solution processability and charge transport18.

The academic community has also delved into the influence of polymer structure on the stability and degradation of organic solar cells as well. Some polymer structures are susceptible to degradation under environmental factors such as moisture, oxygen, and UV radiation, which can hinder the long-term performance of the solar cell. However, researchers have identified strategies to enhance the durability of solar cells, such as altering the molecular weight of the repeating units and adopting donor-acceptor copolymer designs. These innovative approaches mitigate degradation issues and contribute to the long-term stability of organic photovoltaic solar cells. With an abundance of optimization techniques and engineering mechanisms available, this paper aims to centralize the leading and most effective strategies to provide clearer solutions for the future of research and development in this field.

The Effect of Side Chain Alterations

Beginning with side-chain alterations 17 listed in Table 1, the length of the polymer chain plays a key role in photovoltaic performance among a variety of NFA (non-fullerene acceptors) based cells.

 These devices show both reduced radiative and non-radiative energy losses, can obtain high voltages, and are more adept at forming intermolecular \pi-\pi stacking to improve charge transport due to their large, conjugated structure19. The recorded PCE values for cells in which PBDB-TF:BTP-4Cl-8 or PBDB-TF:BTP-4Cl-16 was incorporated into the active layer were lower than those of cells that had an intermediary length with PBDB-TF:BTP-4Cl-12 as the active layer. BTP-4Cl-8 (with 2-ethylhexyl) and BTP-4Cl-16 (with 2-hexyldecyl) cells reached a maximum PCE of 16.3% and 15.6%, respectively17. Meanwhile, the more optimal polymer length BTP-4Cl-12 (with 2-butyloctyl) was able to reach 17.0%. Even despite having the lowest VOC for the 0.06 cm2 sized cell at 0.858 V, the BTP-4Cl-12 chain had the greatest JSC and FF values of 25.6 mA/cm2 and 0.776 respectively17. It is also important to note that these three OPV cells have similar band gaps, Elosss, and molecular energy levels. Ultimately, this demonstrates how polymer chain length can impact current generation and charge extraction efficiencies.

Side Chain Alterations
Active layerCoating methodVOC(V)JSC (mA/cm2)FF, %PCE (%)Area (cm2)
PBDB-TF:BTP-4Cl-8Spin-coating0.87225.274.316.3 (16.1 ± 0.2)0.06
Spin-coating0.86324.971.115.3 (14.8 ± 0.3)0.81
Blade-coating0.83821.763.511.5 (10.7 ± 0.5)0.81
PBDB-TF:BTP-4Cl-12Spin-coating0.85825.677.617.0 (16.6 ± 0.2)0.06
Spin-coating0.85325.477.216.70.06
Spin-coating0.84925.573.816.0 (15.5 ± 0.3)0.81
Blade-coating0.8332671.615.5 (14.9 ± 0.4)0.81
PBDB-TF:BTP-4Cl-16Spin-coating0.86224.274.815.6 (15.2 ± 0.2)0.06
Spin-coating0.8542471.814.7 (14.2 ± 0.3)0.81
Blade-coating0.80719.468.910.8 (9.81 ± 0.6)0.81
Table 1. Photovoltaic parameters of the OPV cells with side-chain modifications. VOC = Open-Circuit Voltage, JSC = Short-Circuit Current, FF = Fil Factor, PCE = Power Conversion Efficiencies17.

It was also observed that a greater device area impacted measurements in regard to short-circuit current density and fill factor will slight reductions across all cells. However, a more significant change is seen when comparing the different polymer cells when they were manufactured via a spin-coating or blade-coating method. Those whose active layer was constructed with spin-coating experienced greater open-circuit voltage, short-circuit current densities, fill factors, and PCEs. The drop in these parameters from spin-coating to blade-coating production was most notable at the extreme ends of the polymer chain length with the BTP-4Cl-8 and BTP-4Cl-16 cells suffering the greatest losses17. The apparent decline in performance can most directly be attributed to decreased charge mobility when the cell was produced via blade-coating. As a result of this, more charge recombination and decreased JSC and FF greatly impacted the blade-coated cell’s efficiency. However, these side-chain modifications still allow for the devices to maintain upwards to 85% of their maximum PCEs even when the active layer thickness is elevated to 300 nm20. Furthermore, after 500 hours in the atmosphere these OPV cells were able to maintain 85-90% of their performance capabilities21. crystallinity22.

Interfacial Layer Engineering

Several papers232425 provide an overview of the recent progress in this field, specifically in regards to organic materials as interfacial layers (IFLs). The authors discuss the importance of interfacial engineering in achieving high power conversion efficiencies (PCEs) in OPV and highlight the underlying device physics and the origins of enhanced PCEs by IFLs. These IFLs between the BHJ (Bulk heterojunction) composite and the electrodes modify the interface and improve charge carrier transport properties23.

Figure 2. (a) Device structures of OPVs. (b) Working principles of OPVs, where LUMO = is the lowest unoccupied molecular orbital, HOMO = is the highest occupied molecular orbital, (c) I–V characteristics of OPVs, where VOC is the open-circuit voltage, ISC is the short-circuit current 23.

Different types of IFLs have been investigated, including PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate), metals, salts, metal oxides, and water/alcohol soluble organic materials23. The role and significance of each type of IFL are discussed, and their impact on enhancing PCEs is explored. These materials offer tunable electrical properties through structural modification and can be deposited on top of or underneath the BHJ composite without damage. The enhanced PCEs are attributed to the formation of dipole moments at the BHJ composite/electrode interface, doping effects, and improved surface electrical conductivities23

The literature also discusses the formation of ohmic contact between the organic materials and the electrodes23. An ohmic contact is a junction between two conductors that has a linear current–voltage and exhibits low resistance over a wide range of applied voltages. This is crucial for efficient charge carrier transport and reduced recombination, which is when an electron pairs with a hole and gives up the energy to produce either heat or light. The introduction of IFLs helps adjust the energy barriers and improve charge carrier selectivity, leading to enhanced PCEs in the cells. 

In both conventional and inverted configurations depicted in Figure 2, the BHJ composite is positioned between an indium tin oxide (ITO) electrode and a metal electrode, forming an ITO/BHJ composite/metal sandwich structure23. The electron donor (D) and electron acceptor (A) components within the BHJ composite are closely connected to the electrodes, leading to the separation of electrons and holes, which tend to recombine at the BHJ composite/electrode interfaces25. Energy barriers arise from the mismatch between electrode work functions, an important parameter that dictates energetic landscape at the interfaces and the density of charges formed upon the contact26, and D or A energy levels, impeding efficient charge carrier collection. To address these challenges, IFLs, known as electron extraction layers (EELs) or hole extraction layers (HELs), are introduced to modify the BHJ composite/electrode interfaces25. EELs and HELs create low-resistance contact points, reducing energy barriers and enhancing charge carrier selectivity at these interfaces, thus curbing charge recombination25. This integration of IFLs significantly improves power conversion efficiencies (PCEs) of OPV. Furthermore, the insertion of IFLs enables the adjustment of OPV polarity, facilitating the use of high work function metals like silver (Ag) or gold (Au) as anodes. They are also key from the viewpoint of sustainability as IFLs are known to be able to prevent oxygen and moisture penetration in the BHJ composite and electrode interface, prolonging the performance lifetime of these cells.

One prominent polymer example used as an IFL is PEDOT:PSS. It is a doped polymer and one of the earliest interfacial layers (IFLs) used in OPV development. Utilized in a cell, it can establish an ohmic contact between the BHJ composite and the ITO electrode, facilitating hole transport rather than electron transport. The presence of PEDOT:PSS thin film on the ITO electrode surface leads to smoother surfaces, reducing leakage current and aiding in the transport of holes. Additionally, its solution processability, transparency in the visible spectrum, and high electrical conductivity make it a popular choice in OPVs24. One drawback, however, is the acidic nature of PEDOT:PSS that can etch the ITO surface and negatively impact cell lifetime2728. Water/alcohol soluble organic materials such as conjugated polymers, fullerene derivatives, and graphene oxide offer creative solutions to this challenge via tunable structures and hence properties23291730.

The Effect of Interfacial Layer Engineering

Interfacial engineering has offered another viable route for increasing performance in organic photovoltaic cells as well.2323

The two primary materials considered in this IFL alteration were conjugated polymers and fullerene derivatives. When looking at the impacts of incorporating conjugated polymers such as PEDOT:PSS into IFLs, an increase in overall cell performance is observable. This thin film smooths the surface of the ITO and mitigates linkage current, which consequently helps with the transportation of holes through the cell23. The majority of the recorded data placed these OPV cells in the 6.2-6.9% PCE range which is a noticeable improvement from the standards table where commonly used PDI based cells report values anywhere from 0.57 to 3.45% PCE2. Without significant leaps in regards to measured open-circuit voltage, the greatest contributor to a larger FF and PCE in these conjugated polymer IFL cells was from the increase in short-circuit current density. JSC in these cells ranged from 9.89-16.76 mA/cm2. Charge carrier collection efficiency greatly improves with IFLs as EELs or HELs are implemented to optimize the BHJ composite/electrode interface and reduce existing energy barriers that limit transport through the cell. 

Figure 3. Molecular structures of organic materials used as the interfacial layers in OPVs23
Interfacial Layer Alterations 
Type of materialsMaterialsDevice structuresDevice Performance
VOC (V)JSC (mA/cm2)FF (%)PCE (%)
Conjugated polymersWPF-6-oxy-FITO/PEDOT:PSS/P3HT:PC61BM/IFL/Al0.629.89593.67
PF6NO25-pyITO/PEDOT:PSS/PCDTBT:PC71BM/IFL/Al0.9111.666.26.9
PFNSO-BTITO/PEDOT:PSS/PTB7:PC71BM/IFL/Al0.6516.7661.16.61
PF2/6-b-P3TMAHTITO/PEDOT:PSS/PCDTBT:PC71BM/IFL/Al0.8910.6676.2
Fullerene derivativesbis-PC61BMITO/PEDOT:PSS/PIDT-PhanQ:PC71BM/IFL/Al0.8811.19605.87
bis-PC61BMITO/PEDOT:PSS/PIDT-PhanQ:PC71BM/IFL/Ag0.8811.5616.22
ETL-1ITO/PEDOT:PSS/PIDT-PhanQ:PC71BM/IFL/Ag0.8711.28646.28
ETL-1ITO/PEDOT:PSS/PIDT-PhanQ:PC71BM/IFL/Al0.8611.17625.96
Table 2. IFL materials, device structures and OPV cell performance parameters of cells with IFLs. VOC = Open-Circuit Voltage, JSC = Short-Circuit Current, FF = Fil Factor, PCE = Power Conversion Efficiencies23.

Fullerene derivatives also aim to achieve the same results as conjugated polymer IFLs. According to the table, these fullerene-based cells tend to have more consistent open circuit voltage measurements that gravitate around ~0.88 V23. Similarly, their JSC and FF as a result are more consistent as compared to conjugated polymers that can produce a wide range of results and graphs at both ends of the spectrum. Due to their slightly lower short-circuit current densities of around 11.2 mA/cm2, cells that use fullerene derivatives for IFLs achieve slightly lower PCE recordings of around 5.87%-6.28%31. However, the precision and consistency of these results should not be disregarded in consideration of large-scale production. Overall, IFLs offer charge carrier selectivity, which reduces the likelihood of charge recombination at the BHJ composite/electrode interfaces. It has been suggested that the increase in PCE and specifically VOC values for IFL cells is a result of the formation of dipole moments and ohmic contact at the interface23. These improvements can result in major industrial gains considering how advantageous the low cost, flexible, clean, and high throughput nature of BHJ manufacturing is. Also, OPV cells generate significantly less waste and pollution when compared to their inorganic photovoltaic counterparts. As a result, incorporating different configurations in OPV cells that take advantage of IFLs is worth consideration24

Morphology Optimization

Another key modification technique used to enhance the PCE of many OPV cells involves morphology control. The optimization of all-polymer solar cells involves many factors such as phase separation, interpenetrating network structure, molecular stacking, and polymer chain orientation32. Effective interfaces between polymer donors and acceptors are proposed to mitigate recombination and enhance efficiency. In All-PSCs, the active layer’s phase-separated domain structures tend to be larger due to polymer chain entanglement and low mixing entropy, distinguishing them from fullerene-based and small molecules-based PSCs33. Charge transfer efficiency in All-PSCs relies on molecular stacking and polymer chain orientation, where face-on orientation enhances charge separation and transport3414353636.

Previous reports have suggested that the Flory-Huggins solution theory is suitable to describe polymer donor-acceptor mixing behaviors in distinguishing phase separation regions. Investigating the optimal donor-to-acceptor (D:A) ratio for creating a favorable phase separation structure is beneficial for enhancing the generation and diffusion of excitons33. The molecular weight of the polymer plays a crucial role in influencing aggregation tendencies, phase separation behavior, and the orientation of the main polymer chains. Manipulating the processing and engineering of the active layer is a simple and commonly employed approach to enhance the morphology of the blend film32. However, films produced through a single solvent-based spin-coating technique often exhibit inadequate morphology, including increased phase separation, reduced alignment of polymer chains, and compromised phase purity, resulting in unsatisfactory photovoltaic performance33. Fortunately, the application of solvent engineering, the inclusion of additives, controlled thermal annealing, and solvent vapor treatment have proven effective in optimizing the morphology of blended films343738. Furthermore, the adjustment of polymer side chains and regioregularity (the degree to which a polymer’s repeat units are derived from the same isomer of the monomer) offers a viable strategy for exerting control over the conformation, orientation, and stacking of polymers33. Scientists often use a 2D molecular weight optimization matrix to identify a target, intermediate blend of the two donor and acceptor regions, highlighting the need for a balanced aggregation strength between the two to achieve high-performance33

The Effect of Cell Morphology and Architecture Alterations

Another key aspect in the enhancement of OPV devices lies in the cell morphology and molecular orientation of the active layer. From an architectural perspective, the cell’s active layer can exist in a binary structure with two distinct donor and acceptor polymers or it can exist in more complicated orientations with the addition of either more donor or acceptor polymers. Recently, the NDI-based polymer acceptor N2200 has emerged as a pivotal component in high-performance OPV cells in part due to its high electron affinity and excellent electron transport capabilities33. To ensure that optimal blend morphology is achieved, extensive research is done on solvents and how their interaction with the polymers impacts aggregation within the cell.3940414241  

Additionally, the pursuit of optimal molecular weight led to the discovery of a “sweet spot” which, when achieved for both polymer donor and polymer acceptor, resulted in improved PCEs and better intermixing32. Modeling has also shown that variation of polymer molecular weight affects intrachain and interchain interactions. In turn, this determines phase separation and morphology within the cell, both of which are key aspects for the generation and diffusion of excitons34. Thus, tuning donor and acceptor polymers to have optimal regioregularity and molecular weight are important considerations for engineers looking to improve crystallinity and orientation with OPVs33

Morphology/Architecture Alterations
AcceptorDonorArchitectureVOC (V)JSC (mA/cm2)FF (%)Highest PCE (%)
P(NDI2OD-T2)P3HTBinary0.521.41290.21
P(NDI2OD-T2)PBDTTT-EF-TBinary0.7941355.65.73
P(NDI2OD-T2)PPDT2FBTHBinary0.858.98478.74
P(NDI2OD-T2)J51Binary0.8314.1870.248.27
P(NDI2OD-T2)PTzBI-SiBinary0.851677.911
P(NDI2OD-T2)PTzBI-SiBinary0.8817.6275.7811.76
P(NDI2OD-T2)PTzBI-SiBinary0.86515.7673.7610.1
P(NDI2OD-T2) + PNTBPBTA-BOTernary0.8415.7774.910.09
P(NDI2OD-T2)PBTA-BO + PTzBI-SiTernary0.83615.6477.9210.12
P(NDI2OD-T2)J51 + PTB7-ThTernary0.8117.3763.79.29
Table 3. Blend compositions and operating metrics of recent N2200 based All-PSCs33.

Within Table 3, it can be seen that certain blends and compositions of donors and acceptors resulted in more favorable molecular stacking and order within the cells, thus boosting observable metrics, most notably parameters such as short-circuit current density and the PCE. For instance, an OPV cell consisting of an active layer of J51:P(NDI2OD-T2): was able to boost its JSC (14.18 mA/cm2 to 17.37 mA/cm2) and PCE (8.27 to 9.29%) with the addition of a PTB7-Th polymer in the donor blend33. However, such advancements achieved from turning a binary cell into a ternary resulted in drawbacks in fill factor measurements as well as a general lack of consistency among other polymer blends43. For example, the OPV cell PTzBI-Si: P(NDI2OD-T2) recorded the highest PCE at 11.76% among the rest of the tested devices despite having a ternary cell version PBTA-BO + PTzBI-Si:(P(NDI2OD-T2)), which only recorded a PCE of 10.12%33.

 The successful pairing of N2200 with various polymer donors, such as PTB7-Th, PTzBI-Si, and PBDB-T, has led to remarkable PCE enhancements. As such, the use of polymer donors with conjugated side chains has improved the molecular orientation in All-PSCs with respect to the D/A interface, while polymer donors lacking this structural feature have shown poorer performance as a result of favored edge-on orientation. Face-on orientation between the substrate and donor : acceptor interface has proven to be more efficient for charge separation and transport. These polymer blends, in addition to solvent engineering and morphological control strategies, have paved the way for efficient charge separation, favorable interpenetrating networks, and boosted All-PSCs performance, with PCEs reaching up to 11.76%33.

Discussion

General Discussion

Organic Photovoltaic cells present both strengths and challenges that impact their feasibility and potential widespread adoption. Key strengths include their sustainable cost production, reduced environmental impact during creation, lightweight and flexible nature, and versatility for various industrial applications. One of the most appealing aspects of OPVs is their potential for low-cost production using solution-based processes, which can lead to reduced manufacturing costs compared to traditional silicon-based solar cells. This advantage aligns well with the growing demand for affordable and accessible renewable energy sources. Additionally, the lightweight and flexible characteristics of OPVs make them suitable for integration into a wide range of applications, including wearable devices, building-integrated photovoltaics, and other unconventional energy-harvesting platforms. Furthermore, the absorption spectra of organic materials can be finely tuned through chemical modification, allowing for customization to match specific light sources or applications.

However, OPVs do come with certain drawbacks, primarily centered around their power conversion efficiency compared to traditional inorganic solar cells. While impressive strides have been made in increasing the efficiency of OPVs, they generally exhibit lower PCEs when compared to silicon-based cells. This lower efficiency can limit their competitiveness in high-power demanding applications and scenarios with limited available space for solar panels. Another challenge lies in their reduced durability and performance degradation when exposed to environmental factors such as moisture, heat, and UV radiation. The organic materials used in OPVs are more susceptible to degradation over time, leading to a shorter operational lifespan compared to their inorganic counterparts.

In the pursuit of overcoming these challenges, recent advancements in OPV technology have demonstrated the potential for improving their efficiency and stability through innovative engineering strategies. This paper aimed to systematically analyze the impact of various alterations and optimizations on the performance of OPVs, focusing on side-chain engineering, interfacial layer engineering, and morphology optimization. By investigating these aspects, the research provides insights into how altering the composition, structure, and processing methods can influence the key metrics that define OPV performance, as well as whether such changes are worth practical investment for large-scale applications.

Side Chain Alterations Outlook

In the realm of non-fullerene acceptors (NFAs), strategic side-chain alterations have proven adept at capturing solar photons effectively, yielding high output current densities. These modifications have also exhibited potential in mitigating radiative and non-radiative energy losses, bolstering the efficiency of NFA-based devices. Turning to NFA BTP-4Cl, side-chain engineering emerges as a pivotal player, enhancing solution processability crucial for practical applications in the industry. The delicate balance achieved by tuning flexible side chains contributes to optimizing charge transport while retaining solution processability, a critical consideration for performance optimization.

As these modifications become more prevalent in the industry, so too will the deposition method, which encompasses spin-coating and blade-coating as prominent options. Spin-coating provides fine control over film thickness and uniformity, favoring research with its speed and precision. However, it can lead to material wastage and challenges in mass production uniformity. On the other hand, blade-coating suits large-scale manufacturing due to its roll-to-roll process, minimizing material wastage and simplifying production logistics. Yet, achieving consistent film thickness is more complex, requiring optimization for uniformity. The choice between these methods balances precision and scalability, influencing the impact of side-chain engineering on OPV performance and implementation.

Interfacial Engineering Outlook

Introducing interfacial engineering through the incorporation of either conjugated polymers like PEDOT:PSS or fullerene derivatives has demonstrated a promising avenue for enhancing organic photovoltaic cell performance. Further research and investment in this technique ought to continue as observable gains in optimizing charge carrier collection efficiency can be seen. This boost in performance is particularly notable in comparison to the relatively lower PCEs achieved by commonly used PDI-based cells. As compared to side-chain modifications, IFLs primarily focus on tailoring the interface between the active layer and electrodes to improve charge carrier selectivity. IFL engineering by nature is comparatively more limited in its ability to revolutionize material synthesis and process scalability. Challenges also exist in achieving uniformity and precision in the composition of each IFL. However, considering that IFL production is already in line with established large-scale fabrication techniques, continued exploration of novel materials and designs will further optimize charge transport and layer consistency, ultimately enhancing the overall stability and efficiency of these cells.

Morphology Outlook

Cell morphology and molecular orientation within the active layer of OPV devices have emerged as critical determinants of performance. The optimization of blend morphology through solvent interactions and the identification of the optimal molecular weight have collectively led to improved power conversion efficiencies and enhanced intermixing of materials. However, the transition from binary to ternary structures may result in trade-offs, impacting fill factor measurements and introducing inconsistencies among various polymer blends, thus emphasizing the need for greater and more specialized research on individual cell compositions. 

Comparing these findings to the prior strategies of side-chain engineering and interfacial layer alterations, it becomes clear that morphological optimization plays a complementary role in enhancing OPV performance.

Unification of the Three Strategies

While side-chain engineering and IFLs primarily target charge transport and carrier selectivity, morphology manipulation targets intermolecular interactions, phase separation, and donor-acceptor intermixing. The combination of these approaches could offer a comprehensive enhancement strategy, addressing multiple facets of OPV efficiency and stability: blade-coating techniques taken from side-chain engineering offer greater large-scale fabrication potential; IFLs grant the largest increases in PCE performance; Optimal morphology blends grant greater cell stability and phase separation. Furthermore, the field is witnessing advancements in sustainability, as ongoing research delves into materials that are both environmentally friendly and exhibit improved operational lifetimes. As such, manufacturers ought to weigh the considerable amount of steps often involved in optimization procedures in the grand scheme of their carbon footprints. Looking ahead, questions remain regarding the scalability of these techniques, as well as the long-term stability of the achieved improvements. 

Conclusion

In this study, we explored the potential and challenges of OPV cells, an emerging renewable energy technology. While OPVs offer sustainable, cost-effective, and flexible energy solutions, their lower power conversion efficiency compared to traditional solar cells and vulnerability to environmental factors remain obstacles. Our investigation focused on three optimization strategies: side-chain, interfacial layer, and morphology engineering. We found that subtle changes in polymer chain length impacted current generation and charge extraction efficiencies. Additionally, the choice of deposition method influenced performance, highlighting the balance between precision and scalability. Incorporating interfacial engineering with materials like PEDOT:PSS and fullerene derivatives demonstrated substantial gains in charge carrier collection efficiency and substantially high PCEs. Finally, optimizing cell morphology and molecular orientation enhanced power conversion efficiencies, albeit with less predictability. 

Acknowledgements

I would like to thank my mentor Dr. Avery Baumann for all her guidance and support while writing this paper.

References

  1. J. Yu, Y. Zheng, J. Huang, Towards high performance organic photovoltaic cells: a review of recent development in organic photovoltaics. Polymers. 6, 2473–2509 (2014). [] [] []
  2. Y. Kim, E. Lim, Development of polymer acceptors for organic photovoltaic cells. Polymers. 6, 382–407 (2014). [] [] []
  3. S. Lizin, S. Van Passel, E. De Schepper, W. Maes, L. Lutsen, J. Manca, D. Vanderzande, Life cycle analyses of organic photovoltaics: a review. Energy Environ. Sci. 6, 3136 (2013). []
  4. A. Maalouf, T. Okoroafor, Z. Jehl, V. Babu, S. Resalati, A comprehensive review on life cycle assessment of commercial and emerging thin-film solar cell systems. Renewable and Sustainable Energy Reviews. 186, 113652 (2023). []
  5. V. Muteri, M. Cellura, D. Curto, V. Franzitta, S. Longo, M. Mistretta, M. L. Parisi, Review on life cycle assessment of solar photovoltaic panels. Energies. 13, 252 (2020). []
  6. M. P. Tsang, G. W. Sonnemann, D. M. Bassani, Life-cycle assessment of cradle-to-grave opportunities and environmental impacts of organic photovoltaic solar panels compared to conventional technologies. Solar Energy Materials and Solar Cells. 156, 37–48 (2016). []
  7. E. K. Solak, E. Irmak, Advances in organic photovoltaic cells: a comprehensive review of materials, technologies, and performance. RSC Adv. 13, 12244–12269 (2023). [] [] []
  8. N. Nakao, M. Saito, T. Mikie, T. Ishikawa, J. Jeon, H. D. Kim, H. Ohkita, A. Saeki, I. Osaka, Halogen?free ??conjugated polymers based on thienobenzobisthiazole for efficient nonfullerene organic solar cells: rational design for achieving high backbone order and high solubility. Advanced Science. 10, 2205682 (2023). []
  9. Z. Wang, A. Tang, H. Wang, Q. Guo, Q. Guo, X. Sun, Z. Xiao, L. Ding, E. Zhou, Organic photovoltaic cells offer ultrahigh VOC of \sim 1.2 V under AM 1.5G light and a high efficiency of 21.2% under indoor light. Chemical Engineering Journal. 451, 139080 (2023). []
  10. B. Bonillo, R. S. Sprick, A. I. Cooper, Tuning photophysical properties in conjugated microporous polymers by comonomer doping strategies. Chem. Mater. 28, 3469–3480 (2016). [] []
  11. Z. Ding, J. Kettle, M. Horie, S. W. Chang, G. C. Smith, A. I. Shames, E. A. Katz, Efficient solar cells are more stable: the impact of polymer molecular weight on performance of organic photovoltaics. J. Mater. Chem. A. 4, 7274–7280 (2016). []
  12. N. Luo, P. Ren, Y. Feng, X. Shao, H.-L. Zhang, Z. Liu, Side-chain engineering of conjugated polymers for high-performance organic field-effect transistors. J. Phys. Chem. Lett. 13, 1131–1146 (2022). []
  13. J. Mei, Z. Bao, Side chain engineering in solution-processable conjugated polymers. Chem. Mater. 26, 604–615 (2014). []
  14. R. Alkarsifi, J. Ackermann, O. Margeat, Hole transport layers in organic solar cells: a review. J Met Mater Miner. 32, 1–22 (2022). [] [] []
  15. Y. Liang, D. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray, L. Yu, Highly efficient solar cell polymers developed via fine-tuning of structural and electronic properties. J. Am. Chem. Soc. 131, 7792–7799 (2009). [] []
  16. S. Brovelli, G. Latini, M. J. Frampton, S. O. McDonnell, F. E. Oddy, O. Fenwick, H. L. Anderson, F. Cacialli, Tuning intrachain versus interchain photophysics via control of the threading ratio of conjugated polyrotaxanes. Nano Lett. 8, 4546–4551 (2008). []
  17. Y. Cui, H. Yao, L. Hong, T. Zhang, Y. Tang, B. Lin, K. Xian, B. Gao, C. An, P. Bi, W. Ma, J. Hou, Organic photovoltaic cell with 17% efficiency and superior processability. National Science Review. 7, 1239–1246 (2020). [] [] [] [] [] [] [] [] []
  18. V. Vohra, N. T. Razali, R. Wahi, L. Ganzer, T. Virgili, A comparative study of low-cost coating processes for green & sustainable organic solar cell active layer manufacturing. Optical Materials: X. 13, 100127 (2022). [] []
  19. S. Kim, K. Hong, M. A. Saeed, T. H. Kim, H. Ahn, W. Lee, J. W. Shim, Y.-H. Kim, Outer sidechain engineering of selenophene and thiophene-based Y-series acceptors to produce efficient indoor organic solar cells. Applied Surface Science. 623, 157140 (2023). []
  20. C. Chen, W. Chang, K. Yoshimura, K. Ohya, J. You, J. Gao, Z. Hong, Y. Yang, an efficient triple?junction polymer solar cell having a power conversion efficiency exceeding 11%. Advanced Materials. 26, 5670–5677 (2014). []
  21. Y. Zhang, I. D. W. Samuel, T. Wang, D. G. Lidzey, Current status of outdoor lifetime testing of organic photovoltaics. Advanced Science. 5, 1800434 (2018). []
  22. Y. Yang, Z. Liu, G. Zhang, X. Zhang, D. Zhang, The effects of side chains on the charge mobilities and functionalities of semiconducting conjugated polymers beyond solubilities. Advanced Materials. 31, 1903104 (2019). []
  23. C. Yi, X. Hu, X. Gong, A. Elzatahry, Interfacial engineering for high performance organic photovoltaics. Materials Today. 19, 169–177 (2016). [] [] [] [] [] [] [] [] [] [] [] [] [] [] []
  24. J. H. Jeong, M. Jahandar, A. Prasetio, J. M. Kim, J. H. Kim, S. Kim, D. C. Lim, Multi-dimensional interfacial engineering for a practical large-area transparent flexible organic photovoltaics. Chemical Engineering Journal. 419, 129672 (2021). [] [] []
  25. Z. Yin, J. Wei, Q. Zheng, Interfacial materials for organic solar cells: recent advances and perspectives. Advanced Science. 3, 1500362 (2016). [] [] [] []
  26. K. Akaike, T. Hosokai, Y. Ono, R. Tsuruta, Y. Yamada, Increasing electrode work function using a natural molecule. Adv Materials Inter. 10, 2201800 (2023). []
  27. J. Cameron, P. J. Skabara, The damaging effects of the acidity in PEDOT:PSS on semiconductor device performance and solutions based on non-acidic alternatives. Mater. Horiz. 7, 1759–1772 (2020). []
  28. M. Jørgensen, K. Norrman, F. C. Krebs, Stability/degradation of polymer solar cells. Solar Energy Materials and Solar Cells. 92, 686–714 (2008). []
  29. C. B. Nielsen, S. Holliday, H.-Y. Chen, S. J. Cryer, I. McCulloch, Non-fullerene electron acceptors for use in organic solar cells. Acc. Chem. Res. 48, 2803–2812 (2015). []
  30. Y. Li, X. Huang, K. Ding, H. K. M. Sheriff, L. Ye, H. Liu, C.-Z. Li, H. Ade, S. R. Forrest, Non-fullerene acceptor organic photovoltaics with intrinsic operational lifetimes over 30 years. Nat Commun. 12, 5419 (2021). []
  31. Z. Liang, M. Li, Q. Wang, Y. Qin, S. J. Stuard, Z. Peng, Y. Deng, H. Ade, L. Ye, Y. Geng, Optimization requirements of efficient polythiophene:nonfullerene organic solar cells. Joule. 4, 1278–1295 (2020). []
  32. G. J. Hedley, A. J. Ward, A. Alekseev, C. T. Howells, E. R. Martins, L. A. Serrano, G. Cooke, A. Ruseckas, I. D. W. Samuel, Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells. Nat Commun. 4, 2867 (2013). [] [] []
  33. K. Zhou, K. Xian, L. Ye, Morphology control in high?efficiency all?polymer solar cells. InfoMat. 4, e12270 (2022). [] [] [] [] [] [] [] [] [] [] []
  34. K. Weng, L. Ye, L. Zhu, J. Xu, J. Zhou, X. Feng, G. Lu, S. Tan, F. Liu, Y. Sun, Optimized active layer morphology toward efficient and polymer batch insensitive organic solar cells. Nat Commun. 11, 2855 (2020). [] [] []
  35. M. S. Chen, J. R. Niskala, D. A. Unruh, C. K. Chu, O. P. Lee, J. M. J. Fréchet, Control of polymer-packing orientation in thin films through synthetic tailoring of backbone coplanarity. Chem. Mater. 25, 4088–4096 (2013). []
  36. T. J. Aubry, A. S. Ferreira, P. Y. Yee, J. C. Aguirre, S. A. Hawks, M. T. Fontana, B. J. Schwartz, S. H. Tolbert, Processing methods for obtaining a face-on crystalline domain orientation in conjugated polymer-based photovoltaics. J. Phys. Chem. C. 122, 15078–15089 (2018). [] []
  37. E. Pavlopoulou, C. S. Kim, S. S. Lee, Z. Chen, A. Facchetti, M. F. Toney, Y.-L. Loo, Tuning the morphology of all-polymer opvs through altering polymer–solvent interactions. Chem. Mater. 26, 5020–5027 (2014). []
  38. Z. Li, X. Xu, W. Zhang, X. Meng, W. Ma, A. Yartsev, O. Inganäs, Mats. R. Andersson, R. A. J. Janssen, E. Wang, High performance all-polymer solar cells by synergistic effects of fine-tuned crystallinity and solvent annealing. J. Am. Chem. Soc. 138, 10935–10944 (2016). []
  39. Y. Kim, H. Park, J. S. Park, J.-W. Lee, F. S. Kim, H. J. Kim, B. J. Kim, Regioregularity-control of conjugated polymers: from synthesis and properties, to photovoltaic device applications. J. Mater. Chem. A. 10, 2672–2696 (2022). []
  40. R. Hosseinabad, J. Kuebler, L. Fernandez-Ballester, Combined role of regioregularity and molecular weight on melt-crystallization and self-nucleation of poly(3-hexylthiophene). Polymer. 259, 125341 (2022). []
  41. R. Steyrleuthner, R. Di Pietro, B. A. Collins, F. Polzer, S. Himmelberger, M. Schubert, Z. Chen, S. Zhang, A. Salleo, H. Ade, A. Facchetti, D. Neher, The role of regioregularity, crystallinity, and chain orientation on electron transport in a high-mobility n-type copolymer. J. Am. Chem. Soc. 136, 4245–4256 (2014). [] []
  42. N. Yaghoobi Nia, M. Bonomo, M. Zendehdel, E. Lamanna, M. M. H. Desoky, B. Paci, F. Zurlo, A. Generosi, C. Barolo, G. Viscardi, P. Quagliotto, A. Di Carlo, Impact of P3HT regioregularity and molecular weight on the efficiency and stability of perovskite solar cells. ACS Sustainable Chem. Eng. 9, 5061–5073 (2021). []
  43. C.-Y. Liao, Y. Chen, C.-C. Lee, G. Wang, N.-W. Teng, C.-H. Lee, W.-L. Li, Y.-K. Chen, C.-H. Li, H.-L. Ho, P. H.-S. Tan, B. Wang, Y.-C. Huang, R. M. Young, M. R. Wasielewski, T. J. Marks, Y.-M. Chang, A. Facchetti, Processing strategies for an organic photovoltaic module with over 10% efficiency. Joule. 4, 189–206 (2020). []

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