From ICMA to ICBA/ICTA: Structural Modulation and Device Performance of Indene-C60 Adduct Acceptors

1. Why Is ICMA Worth Attention?

ICMA, namely indene-C60 monoadduct, CAS No. 186682-36-0, molecular formula C69H8, is a C60 fullerene derivative. Literature reports indicate that ICMA can serve as a candidate material for organic field-effect transistors and exhibits unipolar n-channel transport characteristics; its electron mobility is affected by thin-film fabrication, molecular orientation, interfaces, and device architecture.

ICMA can be used as a representative molecule to discuss how one indene-related addition structural unit affects the energy levels, molecular packing, electron transport, and device performance of C60 fullerene electron acceptor materials. Taking ICMA as the entry point and combining it with control materials such as ICBA, ICTA, and PCBM, this article introduces the structural modulation principles and application evaluation points of indene-based fullerene acceptors.

2. Structural Basis of ICMA

2.1 C60 as the Electron-Acceptor Framework

C60 is a spherical conjugated molecule composed of 60 carbon atoms and has strong electron-accepting ability. Because of this characteristic, C60 and its derivatives have long been used as electron acceptor materials in organic photovoltaic devices.

In organic photovoltaics, the role of the electron acceptor is to accept electrons from the electron donor and provide pathways for electron transport. Although unmodified C60 has favorable electron-accepting characteristics, it has limitations in solution processing, film formation, and compatibility with polymer donor materials. Therefore, researchers commonly improve the processability and device compatibility of C60 through chemical modification.

2.2 ICMA as an Indene-C60 Monoadduct

ICMA is a monoadduct fullerene derivative formed from C60 and an indene-derived structure. The term “monoadduct” is particularly important here. It indicates that only one indene-related addition structural unit is formed on the C60 carbon cage, rather than two or three. This distinction defines the difference between ICMA, ICBA, and ICTA:

Material	English Name	Chinese Name	Structural Feature
ICMA	indene-C60 monoadduct	indene-C60 monoadduct	One indene-derived addition unit
ICBA	indene-C60 bisadduct	indene-C60 bisadduct	Two indene-derived addition units
ICTA	indene-C60 trisadduct	indene-C60 trisadduct	Three indene-derived addition units

ICMA, ICBA, and ICTA belong to the same class of indene-based fullerene derivatives. However, because they differ in the number of addition units, their energy levels, molecular packing, and device performance also differ.

3. How Does Indene Modification Affect Material Performance?

3.1 Modulating LUMO/HOMO Energy Levels

One of the important parameters of electron acceptor materials is the LUMO, namely the lowest unoccupied molecular orbital. The LUMO energy level affects the ability of a material to accept electrons and also influences the V_oc, or open-circuit voltage, in organic solar cells.

Kang et al. reported that, in the series of indene-based fullerene derivatives ICMA, ICBA, and ICTA, as the number of indene addition units increases, both the LUMO and HOMO, namely the highest occupied molecular orbital, energy levels of the materials are raised. In bulk heterojunction solar cells prepared using P3HT, poly(3-hexylthiophene), as the donor material, the V_oc values of P3HT:ICMA, P3HT:ICBA, and P3HT:ICTA devices were 0.65 V, 0.83 V, and 0.92 V, respectively.

This indicates that, in the P3HT:indene-C60 multiadduct system, the number of indene addition units can serve as an important structural variable for tuning fullerene acceptor energy levels and device voltage. As a monoadduct, ICMA is suitable for studying the fundamental effect of one indene-related addition unit on the energy levels of C60 and on device voltage.

3.2 Changing Electron-Accepting Ability and Voltage Performance

In indene-C60 adduct series such as ICMA, ICBA, and ICTA, increasing the number of indene-related addition units raises the LUMO energy level of the fullerene derivative, thereby changing its electron-accepting ability and device voltage performance. For organic photovoltaic devices, a higher acceptor LUMO energy level is often beneficial for improving V_oc, but this does not mean that device efficiency will necessarily increase at the same time.

Organic photovoltaic performance is jointly determined by multiple parameters, including:

① V_oc, open-circuit voltage;

② J_sc, short-circuit current density;

③ FF, fill factor;

④ PCE, power conversion efficiency;

⑤ EQE, external quantum efficiency.

Therefore, the value of ICMA in energy-level modulation cannot be judged solely by V_oc. If the electron mobility of the acceptor material is insufficient, if donor/acceptor phase separation is unsuitable, or if interfacial charge collection efficiency is low, the final PCE may still be limited even when V_oc is improved.

3.3 Affecting Electron Mobility and Molecular Packing

The charge transport ability of fullerene derivatives depends not only on the energy levels of individual molecules, but also on how the molecules are arranged in the thin film. The more favorable the molecular packing is for electronic coupling, the easier it is for electrons to be continuously transported through the acceptor phase.

Ferguson et al. studied blend systems of P3HT with different indene-C60 multiadducts. The relevant results showed that when ICTA was used as the trisadduct, the short-circuit current density, fill factor, and open-circuit voltage of the device all decreased. The electron mobility in the ICTA acceptor phase was approximately one order of magnitude lower than those of ICMA and ICBA.

This result indicates that although increasing the number of indene addition units may raise the LUMO energy level and V_oc, it may also weaken intermolecular electron transport. The monoadduct structure of ICMA causes relatively limited disturbance to the electron-transport framework of the C60 carbon cage. Therefore, ICMA is suitable for use together with ICBA and ICTA to analyze the relationship among “number of addition units—energy level—mobility—device performance.”

4. The Role of ICMA in Organic Photovoltaics

4.1 As a Fullerene-Based Electron Acceptor

OPV, or organic photovoltaic, devices typically have an active layer composed of an electron donor and an electron acceptor. After the donor material absorbs light, excitons are generated. These excitons undergo charge separation at the donor/acceptor interface; electrons are transferred to the acceptor material, while holes remain in the donor material.

In OPV, ICMA mainly serves as an electron acceptor and electron transport component. Its C60 framework provides electron-accepting ability, while the indene monoadduct structure modifies its energy levels, solubility, and film-forming behavior.

4.2 Research Significance of P3HT:ICMA

P3HT is a classic polymer donor material and is often used to evaluate the structure–property relationship of fullerene acceptors. The significance of the P3HT:ICMA system lies in analyzing the following questions:

① Whether a monoadduct indene-based fullerene can effectively accept electrons from P3HT;

② How the LUMO energy level of ICMA affects V_oc;

③ How the compatibility between ICMA and P3HT affects the morphology of the active layer;

④ Whether changes in device performance mainly originate from V_oc, J_sc, or FF;

⑤ Whether ICMA exhibits different electron transport and morphological characteristics compared with ICBA and ICTA.

The study by Kang et al. showed that the V_oc values of P3HT:ICMA, P3HT:ICBA, and P3HT:ICTA increase with the number of indene addition units, but device efficiency is not determined by V_oc alone.

4.3 Key Points for Performance Evaluation Compared with PCBM

PCBM, namely [6,6]-phenyl-C61-butyric acid methyl ester, is a commonly used fullerene electron acceptor in organic photovoltaic research. In studies of fullerene acceptors, PCBM is often used as a reference material to compare the effects of different structural modifications on acceptor energy levels, active-layer morphology, and charge transport. The differences between ICMA and PCBM should not be judged solely by a single efficiency value. Instead, they should be comprehensively evaluated based on open-circuit voltage, short-circuit current density, fill factor, electron mobility, and thin-film morphology.

The study by Ferguson et al. showed that, in blend systems of P3HT with indene-C60 adducts, differences in device performance are closely related to electron mobility in the acceptor phase, active-layer morphology, and addition structure. The electron mobility of the ICTA acceptor phase is lower than those of ICMA and ICBA, adversely affecting short-circuit current density and fill factor, thereby limiting device performance.

5. Research Value of ICMA in Organic Field-Effect Transistors

5.1 n-Type Transport Characteristics

OFETs, or organic field-effect transistors, are important devices for evaluating charge-carrier transport capability in organic semiconductor thin films. Unlike OPV devices, which focus on photogenerated charge separation, OFETs more directly examine charge migration behavior in the channel.

ICMA can exhibit unipolar n-type transport characteristics in OFETs. Here, n-type transport means that the material mainly transports electrons rather than holes. The study by Yu et al. pointed out that, for fullerene derivatives with different solubilizing groups and different frontier orbital energy levels, charge-carrier polarity is closely related to energy levels. C60 monoadducts such as ICMA and OXCMA exhibit unipolar n-channel behavior.

This characteristic makes ICMA a candidate n-type fullerene semiconductor material for comparing electron transport behavior among different fullerene derivatives under specific thin-film fabrication and device conditions.

5.2 Relationship Between Addition Structure and Charge-Carrier Polarity

OFET research can be used to determine the influence of addition structure on electron mobility and charge-carrier polarity. Yu et al. showed that the charge-carrier polarity of fullerene derivatives is closely related to their frontier molecular orbital energy levels. Different numbers of addition units and different solubilizing groups change the energy-level positions of fullerene derivatives, causing the materials to exhibit different transport characteristics in OFETs.

In that study, C60 monoadducts such as ICMA and OXCMA exhibited unipolar n-channel behavior, whereas bisadducts and trisadducts of indene-C60 and o-xylene-C60 exhibited ambipolar charge transport. This indicates that the addition structure not only affects the solubility of fullerene derivatives, but also influences their energy levels, air stability, and charge transport type.

For ICMA, the indene monoadduct structure is an important structural unit for modulating material performance. It preserves the electron-accepting characteristics of the C60 framework while changing the molecular energy levels and thin-film transport behavior. Therefore, it can be used to study the relationship between the number of indene addition units and the n-type transport performance of fullerene materials.

6. How Can ICMA Be Used in Experimental Research?

6.1 For Structure–Property Relationship Studies

ICMA is suitable as a reference material for indene-C60 monoadducts. Comparing it with C60, PCBM, ICBA, and ICTA can help researchers identify the dominant factors behind performance changes. Key questions for study include:

① Whether one indene addition unit is sufficient to change the LUMO/HOMO energy levels of C60;

② How the difference in addition number between ICMA and ICBA affects V_oc;

③ Whether increasing the number of addition units reduces electron mobility;

④ Whether active-layer morphology changes with acceptor structure;

⑤ Whether changes in device efficiency mainly originate from V_oc, J_sc, or FF;

⑥ Whether electron mobility in OFETs is consistent with charge collection capability in OPV devices.

6.2 For Donor/Acceptor Matching Studies

ICMA can form donor/acceptor blend systems with donor materials such as P3HT to study energy-level matching and charge separation processes. For new polymer donor materials, ICMA can also serve as one of the fullerene acceptor candidates for determining whether the donor is suitable for pairing with indene-C60 monoadducts. In such studies, attention should be paid to the following aspects:

① The HOMO and LUMO energy levels of the donor material;

② The reduction potential and LUMO energy level of ICMA;

③ Whether there is sufficient driving force for electron transfer between donor and acceptor;

④ Whether the active layer forms continuous electron transport pathways and hole transport pathways;

⑤ Whether the J_sc, FF, and EQE values of the device support effective charge collection.

6.3 For Optimization of Thin-Film Processing Conditions

As a fullerene derivative, the device performance of ICMA is sensitive to thin-film processing conditions. Solvent, concentration, donor/acceptor ratio, spin-coating conditions, annealing temperature, and annealing time may all affect active-layer morphology and charge transport. During experimental optimization, the following aspects should be closely examined:

① Whether the film is uniform and continuous;

② Whether the donor and acceptor form appropriate phase separation;

③ Whether the surface roughness is excessively high;

④ Whether the acceptor phase forms continuous electron transport pathways;

⑤ Whether the film morphology remains stable after thermal treatment;

⑥ Whether morphological changes correspond to changes in J_sc, FF, or electron mobility.

7. Recommended Characterization Methods

7.1 Material Verification

In ICMA studies, material purity and structure should first be verified. Fullerene derivatives may contain isomers or different addition byproducts, and differences in purity can affect device reproducibility.

Research Purpose	Recommended Method	Key Indicators
Purity verification	HPLC, high-performance liquid chromatography	Main peak purity, impurity peaks
Molecular weight verification	MS, mass spectrometry	Molecular ion peak, adduct peaks
Structural verification	NMR, nuclear magnetic resonance	Characteristic proton signals, structural consistency
Thermal stability evaluation	TGA, thermogravimetric analysis	Decomposition temperature, weight-loss behavior

Among these methods, HPLC is particularly important for ICMA because addition byproducts or isomers may alter thin-film morphology and charge transport.

7.2 Energy-Level and Optoelectronic Property Analysis

Energy-level analysis can be used to determine whether ICMA is suitable for a specific donor material. If the energy-level matching between donor and acceptor is unreasonable, charge separation efficiency may be low or voltage loss may be large.

Research Purpose	Recommended Method	Key Indicators
Comparing reduction potentials and estimating LUMO	CV, cyclic voltammetry	Reduction potential, estimated LUMO value; HOMO can be further evaluated by combining oxidation potential, optical bandgap, or UPS
Analysis of absorption characteristics	UV-Vis, ultraviolet-visible absorption spectroscopy	Absorption peak position, absorption edge
Determination of energy-level positions	UPS, ultraviolet photoelectron spectroscopy	Work function, HOMO energy level

7.3 Thin-Film Morphology Analysis

Thin-film morphology directly affects charge separation and charge collection in OPV devices and also influences electron mobility in OFETs.

Research Purpose	Recommended Method	Key Indicators
Surface morphology observation	AFM, atomic force microscopy	Roughness, phase-separation scale
Thin-film morphology observation	SEM, scanning electron microscopy	Surface continuity, cross-sectional morphology
Molecular packing analysis	GIWAXS, grazing-incidence wide-angle X-ray scattering	π-π packing of donor polymers, fullerene acceptor-phase packing, orientation, crystallinity, or amorphous characteristics

Morphological data should be analyzed together with device performance. For example, if PCE decreases, it is necessary to further determine whether the cause is reduced electron mobility, unsuitable phase separation, or poor electrode interface contact.

7.4 Device Performance Evaluation

The evaluation focus of ICMA differs depending on the type of device.

Application Direction	Recommended Test	Key Indicators
OPV	J-V curve	V_oc, J_sc, FF, PCE
OPV	EQE spectrum	Wavelength response, charge collection efficiency
OFET	Transfer curve	Electron mobility, threshold voltage, on/off ratio
OFET	Output curve	Contact characteristics, channel transport stability
Stability study	Air exposure or thermal treatment test	Parameter degradation, thin-film stability

8. Common Misunderstandings and Correct Interpretations

8.1 Misunderstanding 1: ICMA and ICBA Are the Same Material

ICMA is an indene-C60 monoadduct, while ICBA is an indene-C60 bisadduct. Both belong to indene-based fullerene derivatives, but they differ in the number of addition units. Their names should not be used interchangeably, nor should the performance data of one be directly applied to the other.

8.2 Misunderstanding 2: The More Indene Addition Units, the Better the Material Performance

Increasing the number of indene addition units usually raises the LUMO energy level of the fullerene acceptor and may improve the V_oc, or open-circuit voltage, of OPV devices. However, increasing the number of addition units may also affect molecular packing and electron mobility. Therefore, material performance cannot be judged solely by the number of addition units.

8.3 Misunderstanding 3: A Higher V_oc Means a Higher PCE

PCE, or power conversion efficiency, depends not only on V_oc, but also on J_sc, short-circuit current density, and FF, fill factor. If electron mobility decreases, thin-film morphology is poor, or charge collection efficiency is insufficient, an increase in V_oc does not necessarily lead to a higher PCE.

8.4 Misunderstanding 4: n-Type Transport in ICMA Means High Efficiency in All Devices

ICMA can exhibit n-type transport characteristics in OFETs, or organic field-effect transistors, indicating that it has electron transport capability. However, actual device performance is also affected by thin-film fabrication, interfacial contact, energy-level matching, molecular packing, and device architecture.

8.5 Misunderstanding 5: ICMA Can Simply Be Regarded as a Replacement for PCBM

ICMA is more appropriately regarded as an indene-C60 monoadduct electron acceptor material for studying the structure–property relationships of fullerene derivatives. It can be compared with C60, PCBM, ICBA, ICTA, and other materials, but it should not be simply defined as a universal substitute for PCBM.

8.6 Misunderstanding 6: Evaluating ICMA Only Requires Looking at Device Efficiency

The evaluation of ICMA should combine material purity, LUMO/HOMO energy levels, electron mobility, thin-film morphology, donor/acceptor compatibility, and device parameters. A single PCE value is insufficient to explain the true role of ICMA in a given system.

9. Material Selection and Control Design in ICMA Structure–Performance Studies

Table 1. Fullerene Frameworks, Core Acceptors, and Control Acceptor Materials

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Fullerene parent material	99685-96-8	F434635	Fullerene-C₆₀	Sublimed grade, ≥99.9%	The core carbon-cage framework source of ICMA; used to compare unmodified C60 with indene monoadduct-modified C60 in terms of energy levels, solubility, film-forming behavior, and electron transport.
Core indene-based fullerene acceptor	186682-36-0	I471729	ICMA	≥97% (HPLC)	Indene-C60 monoadduct electron acceptor material; used to study the influence of one indene-related unit on the LUMO level of fullerene, thin-film packing, electron mobility, and voltage performance in organic photovoltaics.
Same-series indene-based fullerene acceptor control	1207461-57-1	I157576	C60 derivative (ICBA)	≥98.5% (HPLC), mixture of isomers	Indene-C60 bisadduct acceptor material; used as a control against ICMA to compare the effects of the number of addition units on energy levels, open-circuit voltage, electron mobility, and device efficiency.
C70 indene-based fullerene acceptor control	1563175-93-8	I290271	Indene-C70 bisadduct, mixture of isomers	≥99% (HPLC)	Indene bisadduct acceptor material based on a C70 framework; used to compare the effects of C60 and C70 carbon-cage structures on light absorption, acceptor energy levels, and bulk heterojunction device performance.
Extended fullerene parent material	115383-22-7	F757064	Fullerene C70	≥99.5%	C70 fullerene parent material; used to construct C70-series acceptor controls and analyze the influence of carbon-cage size and absorption capability on the photovoltaic conversion process.
Classic fullerene acceptor control	160848-22-6	P400139	[6,6]-Phenyl-C61-butyric acid methyl ester	≥99.5%	Classic C60 fullerene acceptor material; used for comparison with ICMA to evaluate how different substituent structures affect energy levels, donor/acceptor compatibility, thin-film morphology, and device parameters.
C70 fullerene acceptor control	609771-63-3	P135215	[6,6]-Phenyl-C71-butyric acid methyl ester	≥97%, contains BHT stabilizer, mixture of isomers	Classic C70-series fullerene acceptor material; used for comparison with C60-series acceptors in terms of light absorption range, electron-accepting ability, and organic photovoltaic active-layer performance.
Bisadduct fullerene acceptor control	1048679-01-1	B405184	Bis-PCBM, mixture of isomers	≥95%	Bisadduct fullerene acceptor material; used to compare monoadduct and bisadduct fullerene acceptors in terms of LUMO level, open-circuit voltage, and electron transport.

Table 2. Polymer Donors, Interlayers, and Electrode Materials

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Classic polymer donor	104934-50-1	P431318	Poly(3-hexylthiophene-2,5-diyl) (P3HT)	Regioregular, average Mw 85,000–100,000	Classic polymer donor material; commonly used with fullerene acceptors such as ICMA, ICBA, and PCBM to construct bulk heterojunction systems for studying donor/acceptor energy-level matching and active-layer morphology.
Low-bandgap polymer donor	958261-50-2	P1501430	PCDTBT	Mw 100–180 kDa	Polymer donor material; used in combination with fullerene acceptors to study how changes in acceptor structure affect photogenerated charge separation, charge collection, and device efficiency.
Low-bandgap polymer donor	1266549-31-8	P478306	PTB7	Average Mw 80,000–200,000, PDI ≤3.0	Commonly used polymer donor material; used in fullerene acceptor matching studies to analyze the effects of donor energy levels, acceptor energy levels, and thin-film morphology on organic photovoltaic device parameters.
Low-bandgap polymer donor	1469791-66-9	P486716	PTB7-Th	—	Polymer donor material; used to construct active layers with fullerene acceptors and study donor/acceptor compatibility, complementary absorption, and the formation of charge transport pathways.
Hole-transport and anode-modification material	155090-83-8	P475428	Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)	1%–1.3% in water, conductivity ≥850 S/cm	PEDOT:PSS can be used in studies involving anode modification, hole transport, or conductive coatings; its specific function depends on formulation, conductivity, film-forming conditions, and device structure.
Transparent conductive electrode material	50926-11-9	I398649	Indium tin oxide	≥99.9% metals basis, 50 nm	Transparent conductive oxide material; applicable to research on transparent conductive materials. In actual OPV device evaluation, ITO glass or flexible ITO conductive substrates are typically used, and the corresponding substrate material should be selected according to the device fabrication process.
Electron-injection and cathode interlayer material	7789-24-4	L434125	Lithium fluoride	PrimorTrace™, ultra-pure grade, ≥99.99% metals basis	Common cathode interlayer material; used to regulate electron injection and collection between the metal electrode and the fullerene acceptor layer, influencing fill factor and device stability.
Cathode/metal electrode evaporation source material	7429-90-5	A434752	Aluminum	PrimorTrace™, ≥99.999% metals basis, pellets, 3–8 mesh	High-purity aluminum pellet material; aluminum is commonly used as the cathode or electron-collecting electrode in organic photovoltaic devices. In actual devices, Al thin-film electrodes are usually formed by vacuum thermal evaporation, electron-beam evaporation, or related methods. A suitable material form should be selected according to the requirements of the equipment crucible or evaporation boat.
Low-work-function electrode material	7440-70-2	C110720	Calcium	≥99.5% metals basis	Common electron-collecting electrode material; can be used in combination with aluminum electrodes to reduce electron injection or collection barriers and to analyze the electron transport behavior of acceptor materials.

Table 3. Structural Precursors, Processing Solvents, and Morphology-Control Additives

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Indene structural precursor	95-13-6	I105568	Indene	≥98%	Structural source for indene-based fullerene derivatives; used to understand how indene addition units in ICMA, ICBA, and ICTA modulate fullerene energy levels and molecular configuration.
Active-layer processing solvent	108-90-7	C431386	Chlorobenzene	Anhydrous grade, ≥99.8%	Common processing solvent for organic photovoltaic active layers; used to dissolve polymer donors and fullerene acceptors, affecting film uniformity, phase separation, and charge transport pathways.
Active-layer processing solvent	95-50-1	D119675	o-Dichlorobenzene	Anhydrous grade, ≥99%	Common high-boiling-point processing solvent; used to regulate crystallization, phase separation, and the film-drying process of polymer donor/fullerene acceptor blends.
Morphology-control additive	24772-63-2	D473785	1,8-Diiodooctane	≥96%	Common active-layer morphology-control additive; used to optimize the phase-separation length scale, continuous transport pathways, and photovoltaic conversion performance of donor/fullerene acceptor blends.

Note: The above are representative Aladdin products. More product specifications can be searched on the Aladdin website using the product name, CAS number, or catalog number.

References

[1] Merck/Sigma-Aldrich. ICMA, 97% HPLC, CAS 186682-36-0, empirical formula C69H8, molecular weight 836.80.

[2] Kang H, Cho C-H, Cho H-H, Kang T E, Kim H J, Kim K-H, Yoon S C, Kim B J. Controlling Number of Indene Solubilizing Groups in Multiadduct Fullerenes for Tuning Optoelectronic Properties and Open-Circuit Voltage in Organic Solar Cells. ACS Applied Materials & Interfaces, 2012, 4(1): 110–116. DOI: 10.1021/am201075y.

[3] Yu H, Cho H-H, Cho C-H, Kim K-H, Kim D Y, Kim B J, Oh J H. Polarity and Air-Stability Transitions in Field-Effect Transistors Based on Fullerenes with Different Solubilizing Groups. ACS Applied Materials & Interfaces, 2013, 5(11): 4865–4871. DOI: 10.1021/am400618r.

[4] Ferguson A J, Larson J, Graf P, Kopidakis N, Nardes A M, Whitaker J B, Larson B W, Boltalina O V, Strauss S H, Maturová K. Beyond PCBM: Understanding the Photovoltaic Performance of Blends of Indene-C60 Multiadducts with Poly(3-Hexylthiophene). Advanced Functional Materials, 2012, 22(19): 4115–4127. DOI: 10.1002/adfm.201200336.

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