Cross‑Sectional KPFM Reveals Potential Dip and Schottky Barrier in Thick PCDTBT:PCBM Bulk Heterojunction Solar Cells

Abstract

Kelvin probe force microscopy (KPFM) was used to map the cross‑sectional potential distribution of a high‑open‑circuit‑voltage PCDTBT:PCBM bulk heterojunction (BHJ) solar cell. The potential drop is confined to the cathode interface, confirming the p‑type behaviour of the photo‑active layer. In the field‑free central region the measured potentials follow a log‑normal distribution, indicating a broad energetic disorder that can trap holes and enhance bimolecular recombination. A Schottky barrier at the anode interface further impedes charge extraction, while the potential gradient in the depletion region can mitigate the dip by lowering the barrier for trapped carriers.

Background

Organic photovoltaics (OPVs) promise low‑cost, flexible solar solutions, yet their commercial viability hinges on power‑conversion efficiencies (PCEs) above 10 %. Recent progress has been driven by novel donor/acceptor materials and optimized fabrication protocols. The classic P3HT:PCBM pair achieved 3–5 % PCE, but thicker active layers, necessary for higher light absorption, suffer from reduced charge‑collection efficiency due to increased recombination and energetic disorder.

Cross‑sectional KPFM offers nanoscale insight into internal electric fields and potential landscapes, yet most studies have focused on the P3HT:PCBM system. Here we extend this approach to the PCDTBT:PCBM BHJ, a material that exhibits near‑unity internal quantum efficiency in thin devices but suffers from performance loss in thicker films.

Methods

Materials and Device Fabrication

PCDTBT (donor) and soluble PCBM (acceptor) were spin‑coated to form active layers 70–150 nm thick on a 20 nm PEDOT:PSS hole‑transport layer (HTL) atop ITO. Aluminum cathodes were evaporated under high vacuum. For KPFM, a 200 nm active layer on a highly conductive PEDOT:PSS anode was cleaved in liquid nitrogen to expose a smooth cross‑section.

Electrical and KPFM Characterization

J–V curves were recorded with a Keithley 236 under AM 1.5G illumination (100 mW cm⁻²). KPFM (n‑Tracer Nanofocus) was performed in dry nitrogen; AFM topography and frequency‑modulated CPD were simultaneously acquired using a Pt/Ir‑coated cantilever (350 kHz).

Results and Discussion

Thickness‑Dependent Photovoltaic Performance

Increasing the active‑layer thickness shifts the short‑circuit current (J_SC) due to interference effects, while the open‑circuit voltage (V_OC) remains constant, indicating unchanged built‑in potential. However, the fill factor (FF) steadily decreases with thickness, reflecting poorer charge collection.

Cross‑Sectional Potential Mapping

The KPFM topography (Fig. 3a) shows a smooth cleaved surface with <200 nm RMS roughness. Phase imaging (Fig. 3b) delineates the PEDOT:PSS/PCDTBT:PCBM interface. CPD mapping (Fig. 3c) reveals a steep potential drop confined to the cathode side (≈70 nm), characteristic of the depletion region. The mid‑region near the anode is field‑free, confirming the BHJ behaves as an effective p‑type semiconductor.

A pronounced potential dip appears near the anode interface (Fig. 3c). To confirm its spatial extent, a larger area was imaged (Fig. 4). The CPD image shows random bright and dark spots across the BHJ, indicating a wide energetic disorder that follows a log‑normal distribution (Fig. 4d). The long tail of the distribution (σ ≈ 400 meV) suggests a high density of deep trap states, which can capture holes and increase bimolecular recombination.

Energy Band Diagram and Implications

The measured potentials translate into an energy band diagram (Fig. 5a) where the PEDOT:PSS anode forms a 0.4 eV Schottky barrier with the deep HOMO of PCDTBT. This barrier lengthens hole residence time, raising the likelihood of recombination. The potential dip in the field‑free region (Fig. 5b) is a manifestation of dipole‑induced band bending caused by the distribution of hole traps. If the active layer were thinner (≈70 nm), the depletion field would overlap the dip, effectively lowering the Schottky barrier (Schottky‑barrier lowering) and facilitating charge extraction.

Replacing PEDOT:PSS with a deeper‑workfunction HTL such as MoOx would convert the anode junction into an ohmic contact, further improving extraction efficiency. Nonetheless, the intrinsic energetic disorder remains a bottleneck for thick devices, underscoring the importance of thin, high‑quality active layers or advanced interfacial engineering.

Conclusions

Cross‑sectional KPFM of thick PCDTBT:PCBM BHJ cells revealed a Schottky barrier at the anode and a log‑normal distributed potential dip in the field‑free region. These features trap holes and enhance bimolecular recombination, explaining the observed drop in fill factor for thicker films. Mitigation strategies include using deeper‑workfunction HTLs to eliminate the Schottky barrier and designing devices with active‑layer thicknesses comparable to the depletion width to suppress the potential dip via Schottky‑barrier lowering.

Abbreviations

BHJ

Bulk heterojunction

CPD

Contact potential difference

EIS

Electrochemical impedance spectroscopy

FWHM

Full‑width at half maximum

HTL

Hole transport layer

IQE

Internal quantum efficiency

KPFM

Kelvin probe force microscopy

P3HT

Poly(3‑hexylthiophene)

PCBM

[6,6]-Phenyl‑C60‑butyric methyl ester

PCDTBT

Poly[N‑9′‑heptadecanyl‑2,7‑carbazole‑alt‑5,5‑(4′,7′‑di‑2‑thienyl‑2′,1′,3′‑benzothiadiazole)

PCE

Power conversion efficiency

TOF

Time‑of‑flight

Voc

Open‑circuit voltage