EES Blog

Current solar cell research focuses on increasing the power conversion efficiency (PCE) by employing multiple junctions. A new review by Adebanjo et al. provides an overview of the recent results.

The usage of multiple junctions to cover a broader range of the solar spectrum is used widely in III-V semiconductor solar cells. This approach has been transferred to organic polymer solar cells. By stacking several cells on top of each other, thermalisation losses are reduced and the open circuit voltage is improved. Achievable PCE values are calculated by the authors of the paper “Triple Junction Polymer Solar Cells”. Depending on the band gap of the materials used, maximum values are predicted to be 15% to 23%.

The article also reviews current materials used as active and interconnecting layers. Fullerenes are commonly employed as acceptor materials. Donor materials are most often polymers like P3HT. These can be tuned by structural modification for better absorption of different light spectra. Metal-oxides or transition-oxides are finally used for recombination and injection layers.

Current PCE’s do not reach the maximum value, e.g. 9.64% for a triple junction instead of 22.3%. Adebanjo et al. point out that the design of absorbing polymers, the selection of interconnecting layers and overall stack design are closely interconnected. In consequence, optimisation of the devices is achieved by tuning several parameters.

By Sebastian Axmann

Take a look at the review in EES:

Triple Junction Polymer Solar Cells
Olusegun Adebanjo, Purna Maharjan, Prajwal Adhikary, Mingtai Wang, Shangfeng Yang and Qiquan Qiao
DOI: 10.1039/C3EE42257G, Review Article

Researchers, led by Dr. Volker Presser in Germany, present a methodology to directly predict the desalination performance of carbon-based electrodes for capacitive deionization (CDI), an important first step towards directed CDI design.

Carbon-based electrodes offer an energy-efficient water desalination technology that involves removal of ions from water by electrosorption in charged porous carbon electrodes. The family of carbon materials ranging from activated carbons, carbon nanotubes, exfoliated grapheme electrodes to templated carbons and carbide-derived carbons have been extensively studied for desalination by CDI. To achieve optimal performance, components of the CDI system need to be tuned to achieve both high salt electrosorption capacity and fast kinetics, simultaneously. Therefore, tools to predict the performance of a certain carbon material and CDI design are essential for device design.

Given the inherent non-linearity of desalination by porous carbons, Porada et al. in their paper, Direct Prediction of the Desalination Performance
of Porous Carbon Electrodes for Capacitive Deionization, follow a two-prong approach: i) predict the desalination performance of a carbon material based on its pore size distribution and ii) use a two-dimensional porous electrode CDI transport model to predict the actual salt electrosorption kinetics. The authors have convincingly demonstrated that there is no direct relationship between salt electrosorption capacity and typically cited pore metrics and that the salt electrosorption capacity can be predicted by analysis of the pore size distribution and the pore volume correlated with incremental pore size ranges.

Despite the complexity of CDI, their work has shown the feasibility of predicting performance of different carbon-based materials in a passionately debated field. Looking ahead, the rational device design of carbon electrodes is certainly on the cards.  This paper is a must-read for anyone working in the field of carbon materials!

Read the paper here:

Direct Prediction of the Desalination Performance of Porous Carbon Electrodes for Capacitive Deionization
Slawomir Porada, Lars Borchardt, Martin Oschatz, Marek Bryjak, Jennifer Atchison, Karel J. Keesman, Stefan Kaskel, Maarten Biesheuvel and Volker Presser
DOI: 10.1039/C3EE42209G

By Prineha Narang

The “working horse” of current organic photovoltaics research are the polymers P3HT and PC(61)BM. Both materials have been characterized extensively in single layers as well as in devices. To date, a wide range of power conversion efficiency values have been reported and linked to various factors including material sources and processing conditions.

A new article in the RSC Journal Energy & Environmental Science by Westacott et al. examines P3HT and PCBM with respect to photon absorption. They compare the influence of a high and low average molecular weight P3HT (H-P3HT/L-P3HT type) and temperature treatments upon exciton dissociation yield. This comparison reveals that a highly intermixed phase of both materials improves charge generation which is promoted by a high average weight of P3HT. The underlying mechanism is found to be the formation of a finely intermixed phase between both materials.

A distinct increase in dissociated excitons was found for the high weight P3HT even under different concentration levels for both devices.

To distinguish between different mechanisms of phase formation in both types, pristine layers of each molecule type were deposited and laminated. Upon short heating (30s, 150°C), similar phase intermixing and subsequent reduction of crystallinity was observed for both types.

The L-P3HT is assumed to aggregate easier as the average chain length is smaller. Upon heating, separation of both materials is induced which in turn leads to a loss of the finely intermixed phase. The H-P3HT is assumed to incorporate less mobile amounts of PCBM due to the macromolecular structure of the folded molecules. Thus the H-P3HT type yields higher exciton dissociation rates and thus possibly larger efficiencies.

By Sebastian Axmann

Read the article in EES:

On the role of intermixed phases in organic photovoltaic blends
Paul Westacott, John R. Tumbleston, Safa Shoaee, Sarah Fearn, James H. Bannock, James B. Gilchrist, Sandrine Heutz, John deMello, Martin Heeney, Harald Ade, James Durrant, David S. McPhail and Natalie Stingelin
DOI: 10.1039/C3EE41821A

Researchers from China have found an effective strategy for filling the pinhole defects of organic dye-grafted titania films, enhancing the performance of organic dye-sensitized solar cells employing non-corrosive cobalt redox electrolytes.

Dye-sensitized solar cells (DSCs) use organic dyes to enhance the light-harvesting of inorganic solar cells, altering the wavelengths of incoming light to those better absorbed by the cell in a process known as up- or down-conversion. When dyeing a cell, a good solvent must be used to prevent the dye aggregating, the aim being to create only a monolayer of dye on the surface. However, this creates an incomplete coating of the inorganic surface with the dye, the gaps called ‘pinhole defects,’ which act as sites where the charge-hole pairs created by the absorption of photons recombine easily, significantly reducing current generation and thereby the effectiveness of the cell.

Zhang et al. in the paper Judicious selection of a pinhole defect filler to generally enhance the performance of organic dye-sensitized solar cells detail their quest to solve this problem, creating two bulky model dyes for coating a titania support and three very similar but less bulky ‘fillers.’ The dyes were coated onto the titanium with good solvents and the fillers with poor solvents to maintain the integrity of the dye coating. In every case, the filler reduced open-current voltage, a fundamental measure of efficiency. The net result was an impressive solar-to-electricity energy efficiency of 10.5% under non-concentrated light for their best cell.

The exciting results of this study make it a must-read for those involved with organic solar cells. Additionally, though this is a technical paper specific to dye-sensitized organic solar cells, anyone with interest in solar cells would find interest in this article because the general strategy of defect-filling could be used to reduce interfacial charge recombination in other types of solar cells.

Read the article in EES:

Judicious selection of a pinhole defect filler to generally enhance the performance of organic dye-sensitized solar cells
Min Zhang, Jing Zhang, Ye Fan, Lin Yang, Yinglin Wang, Renzhi Li and Peng Wang
DOI: 10.1039/C3EE42431F, Communication

By Benjamin Britton