Efficiency Enhancement in Organic Photovoltaic Cells: Consequences of Optimizing Series Resistance

Abstract

Here, means to enhance power conversion efficiency (PCE or η) in bulk‐heterojunction (BHJ) organic photovoltaic (OPV) cells by optimizing the series resistance (Rs)—also known as the cell internal resistance—are studied. It is shown that current state‐of‐the‐art BHJ OPVs are approaching the limit for which efficiency can be improved via Rs reduction alone. This evaluation addresses OPVs based on a poly(3‐hexylthiophene):6,6‐phenyl C61‐butyric acid methyl ester (P3HT:PCBM) active layer, as well as future high‐efficiency OPVs (η > 10%). A diode‐based modeling approach is used to assess changes in Rs. Given that typical published P3HT:PCBM test cells have relatively small areas (∼0.1 cm2), the analysis is extended to consider efficiency losses for larger area cells and shows that the transparent anode conductivity is then the dominant materials parameter affecting Rs efficiency losses. A model is developed that uses cell sizes and anode conductivities to predict current–voltage response as a function of resistive losses. The results show that the losses due to Rs remain minimal until relatively large cell areas (>0.1 cm2) are employed. Finally, Rs effects on a projected high‐efficiency OPV scenario are assessed, based on the goal of cell efficiencies >10%. Here, Rs optimization effects remain modest; however, there are now more pronounced losses due to cell size, and it is shown how these losses can be mitigated by using higher conductivity anodes.

Outdoor operation of small-molecule organic photovoltaics

“…However, as this contribution becomes small compared to the contact resistances at elevated temperatures (R S ¼ 0.71 ± 0.03 U at T ¼ 140 C), its impact on FF also is reduced [18,25,29]. Additionally, the FF is reduced by increased charge recombination and the changing ratio of maximum power point voltage to V OC due to decreasing V OC with temperature.…”

Origin of size effect on efficiency of organic photovoltaics

“…However, no systematic attention has been paid on the influence of the OPV cell area (size) on the key photovoltaic parameters of the devices. Experimental [25][26][27] and modeling [26][27][28][29] data on this effect are very limited and most of the published papers attributed the reduction of the OPV performance with increasing area to the power dissipation on the cell series resistance R s and in particular to the R s contribution by front electrode of transparent conductive oxide (ITO).…”

Interplay between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells

“…Extensive work has been carried out to develop and optimize materials which allow for efficient extraction of charge carriers and enhance both power conversion efficiency (PCE) and cell stability. In order to obtain efficient selective contacts, two key requirements must be fulfilled: low contact resistance between the cathode or anode and the organic layer, 4,5 and adequate matching of energy levels to enhance electron or hole selectivity. 6 Several materials are available to enhance the electron extraction selectivity at the cathode contact, including alkali metal compounds (Ca, LiF…), metal oxides (TiO x , ZnO…), and low-molecular weight organic materials.…”