Determination of diffusion lengths in organic semiconductors: Correlation with solar cell performances

Abstract

Organic semiconductors are promising candidates for future applications in solar energy conversion.Recent investigations of bulk heterojunction (BHJ) semiconductors have suggested a density of states and transport mechanisms by multiple trapping close to those observed in disordered inorganic thin films. That is why we have applied to BHJ thin films experiments that are currently used for disordered semiconductors. In addition to the steady state photoconductivity we have tested the ability of the steady state photocarrier grating (SSPG) technique to provide information on the minority carrier diffusion length. We found that SSPG can be applied to P3HT:PCBM thin films leading, for the best sample, to a diffusion length of the order of 125 nm. From the comparison of the transport parameters obtained on thin films with the performances of the devices integrating the latter, we conclude that SSPG is a very powerful tool for optimizing the BHJ thin film properties before their incorporation in solar devices.

Deposition of a‐Si:H thin films using tailored voltage waveform plasmas: impact on microstructure and stability

“…The mobility-lifetime (μτ) product is conveniently utilized to illustrate the films electronic transport performance. We evaluate the μτ-products for majority carriers (μτ maj ) and minority carriers (μτ min ) by using the SSPC [16] and SSPG [17] techniques, respectively. Note that electrons are much more mobile in a-Si:H, so they are generally considered to be the majority carriers.…”

Plasmonics in Organic and Perovskite Solar Cells: Optical and Electrical Effects

“…Generally, solution‐processed organic semiconductors are more disordered than highly crystalline inorganic materials such as silicon or gallium arsenide. As such, the diffusion length, which is the distance charge carriers can travel before recombining is much smaller in these materials . This requires the thickness of the absorber layer be reduced so that more charge carriers can be successfully extracted before recombination occurs.…”

Study of transport parameters and defect states in thin film perovskites under different environments − air or vacuum − and after light-soaking

“…On the other hand, if a device presents a weak energy conversion with an absorber having a large diffusion length, the reason may come from other issues as bad interfaces, resistive contacts, etc. Such a behavior was also observed with organic thin films and devices where the best devices were those incorporating films with the largest L amb [15]. Therefore, as also underlined by Balberg [16], the SSPG technique seems to be an excellent technique to investigate on material transport properties before incorporating it into a device.…”

“…Since the sample position is fixed, it can be set into a cryostat, opening the possibility to study materials that have to be kept under vacuum such as organic blends (e.g. P3HT-PCBM) and perovskites [15,17]. Considering the range of L available, the range of L amb that can be investigated is of the order of 50-1500 nm.…”