The influence of absorbing donors and acceptors on the efficiency for a stacked and a monolithic organic tandem solar cell

In order for organic bulk heterojunction solar cells to compete with the traditional inorganic cells, power conversion efficiencies of more than 10 % are desirable. A way to improve the efficiency is to use a tandem configuration. In this article, we study the influence of the energy levels (HOMO and LUMO) of donor and acceptor on the efficiency for a stacked and a monolithic organic tandem cell. First, we consider the case where only the donor of each subcell is the absorber active material. Then, we consider the situation where both the donor and acceptor are good absorbers; the photons absorbed in donor and acceptor are contributing to the output power of the solar cell. For our calculations, we always take into account the organic nature of the photovoltaic cell by imposing a minimal LUMO-difference, necessary for exciton dissociation. Ideal material characteristics are obtained from these calculations. They give us an idea how the configuration of the energy levels of the active materials should ideally be for stacked and monolithic organic tandem cells, and this for 2 situations: (i) only the donors absorbs light (ii) both donors and acceptors absorb light. One result is that the requirements for an almost optimal configuration for the stacked tandem cell are quite broad, permitting that the values of the bandgaps for optimal cells are not that strict. This is not the case for the monolithic configuration; especially the value of the bandgap Eg1 of the first subcell is more critical than for a stacked cell. Another result is that when both materials absorb light, the highest maximum attainable efficiency reached is the same as in the case where only one material absorbs light, but higher efficiencies are reached for materials which have not optimal energy levels

Guidelines for the bandgap combinations and absorption windows for organic tandem and triple-junction solar cells

Organic solar cells have narrow absorption windows, compared to the absorption band of inorganic semiconductors. A possible way to capture a wider band of the solar spectrum-and thus increasing the power conversion efficiency-is using more solar cells with different bandgaps in a row, i.e., a multi-junction solar cell. We calculate the ideal material characteristics (bandgap combinations and absorption windows) for an organic tandem and triple-junction solar cell, as well as their acceptable range. In this way, we give guidelines to organic material designers

The upper limit for the efficiency of organic solar cells: will organic photovoltaics be able to compete with traditional solar cells?

status: publishe

Modeling of nanostructured photovoltaics: the network model and the effective medium model

status: publishe

Which type of solar cell is best for low power indoor devices?

Low power devices such as sensors and wireless communication nodes, focused towards indoor applications, face serious challenges in terms of harvesting nearby natural sources of energy for power. Nowadays, these wireless systems use batteries as source of energy. These batteries need to be replaced in due time and this factor plays a major role in determining the life of the device. Often, the cost of replacing the battery outweighs the cost of the device itself. Also from an environmental perspective, reducing battery waste is laudable. In order to obtain an “infinite” lifetime of the system, the device should be able to harvest energy from renewable resources in the device’s environment. Photovoltaic (PV) energy is an efficient natural energy source for outdoor applications. However, for indoor applications, the efficiency of classical crystalline silicon PV cells is much lower. Typically, the light intensity under artificial lighting conditions found in offices and homes is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum. Although the crystalline Si cell is still dominating the PV market, second generation solar cells, i.e. thin film technologies, are rapidly entering the market. The different PV cells are rated by their power output under standard test conditions (AM1.5 global spectrum and light intensity of 1000 W/m²) but those conditions are not relevant for indoor applications. The question therefore arises: which type of solar cell is best for indoor devices? This paper contributes to answering that question by comparing the power output of different thin film solar cells (CdTe, CIGS, amorphous Si, GaAs and an organic cell with active layer P3HT:PCBM) with the classical crystalline silicon solar cell as reference. This comparison is made for typical artificial light sources, i.e. an LED lamp, a “warm” and a “cool” fluorescent tube and a common incandescent and halogen lamp, which are compared to the outdoor AM1.5 spectrum as reference. All light sources (including the outdoor spectrum) are scaled to an illumination of 500 lux to obtain a correct comparison. The best artificial light source for all cell types is the incandescent lamp which, for Si and CIGS, improves the performance of the cell with a factor of 3 compared with AM 1.5. The LED lamp is the worst light source for indoor PV with a decrease in performance of a quarter for amorphous silicon to two thirds for crystalline silicon cells. The best solar cells for indoor use depend heavily on the light source. For an incandescent lamp, crystalline silicon remains the best. However, for an LED lamp or “warm” fluorescent tube, amorphous silicon is significantly better. For “cold” fluorescent tubes as light sources, CdTe solar cells perform the best

Including excitons in semiconductor solar cell modelling

Excitons are marginally important in classical semiconductor device physics, and their treatment is not included in standard solar cell modelling. However, in organic semiconductors and solar cells, the role of excitons is essential, as the primary effect of light absorption is exciton generation, and free electrons and holes are created by exciton dissociation. First steps to include excitons in solar cell modelling were presented by Green 1996 and Zhang 1998. Their model was restricted to an analytic treatment of the neutral p-region of a one sided n+p junction, and uniform exciton dissociation and recombination was considered only in the p-bulk. We now extended this model to also include the space charge region (SCL), and exciton surface dissociation and recombination at the contacts, or non-uniform bulk dissociation (e.g. field enhanced dissociation in the SCL). As we assume a preset hole concentration throughout, and electric field in the SCL, our model is still not general, but it covers most real semiconductor situations. The model leads to two coupled non-linear differential equations, which are solved numerically. A first result is that it is possible to apply the standard semiconductor device modelling frame to situations were excitons are dominant. In particular, normal solar cell behaviour is calculated when there is only exciton (and no free eh) generation, and when exciton dissociation is only at a contact surface, or only in the SCL. Further exploration of our model is necessary to cover also situations and parameters relevant for organic solar cells

Efficiency simulations of thin film chalcogenide photovoltaic cells for different indoor lighting conditions

Photovoltaic (PV) energy is an efficient natural energy source for outdoor applications. However, for indoor applications, the efficiency of PV cells is much lower. Typically, the light intensity under artificial lighting conditions is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum. In this context, the question arises whether thin film chalcogenide photovoltaic cells are suitable for indoor use. This paper contributes to answering that question by comparing the power output of different thin film chalcogenide solar cells with the classical crystalline silicon cell as reference. This comparison is made for typical artificial light sources, i.e. an LED lamp, a “warm” and a “cool” fluorescent tube and a common incandescent and halogen lamp, which are compared to the outdoor AM 1.5 spectrum as reference. All light sources (including the outdoor spectrum) are scaled to an illumination of 500 lux to obtain a correct comparison. The best artificial light source for all cell types is the incandescent lamp which improves the performance of the cell up to a factor 3 compared with the AM 1.5 spectrum. One remarkable result is that a CdTe cell outperforms a CIGS cell with more than 33 % in an indoor artificial lighting environment (except with an incandescent light source)