Solution-processed bulk-heterojunction solar cells have gained serious attention during the last few years and are becoming established as one of the future photovoltaic technologies for low-cost power production. This article reviews the highlights of the last few years, and summarizes today's state-of-the-art performance. An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic technology towards the magic 10% regime. A cost model supplements the technical discussions, with practical aspects any photovoltaic technology needs to fulfil, and answers to the question as to whether low module costs can compensate lower lifetimes and performances.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.200801283

Polymer‐Fullerene Bulk‐Heterojunction Solar Cells

Although organic photovoltaic (OPV) cells have many advantages, their performance still lags far behind that of other photovoltaic platforms. A fundamental reason for their low performance is the low charge mobility of organic materials, leading to a limit on the active-layer thickness and efficient light absorption. In this work, guided by a semi-empirical model analysis and using the tandem cell strategy to overcome such issues, and taking advantage of the high diversity and easily tunable band structure of organic materials, a record and certified 17.29% power conversion efficiency for a two-terminal monolithic solution-processed tandem OPV is achieved.

https://science.sciencemag.org/content/sci/early/2018/08/08/science.aat2612.full.pdf

Organic and solution-processed tandem solar cells with 17.3% efficiency

Two-dimensional hybrid perovskites are used as absorbers in solar cells. Our first-generation devices containing (PEA)2(MA)2[Pb3I10] (1; PEA=C6H5(CH2)2NH3+, MA=CH3NH3+) show an open-circuit voltage of 1.18 V and a power conversion efficiency of 4.73 %. The layered structure allows for high-quality films to be deposited through spin coating and high-temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI3] (2) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual-absorber tandem device. Compared to 2, the layered perovskite structure may offer greater tunability at the molecular level for material optimization.

/pdf/a-layered-hybrid-perovskite-solar-cell-absorber-with-2cwbte319e.pdf

A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability

We calculate the maximum power conversion efficiency for conversion of solar radiation to electrical power or to a flux of chemical free energy for the case of hydrogen production from water photoelectrolysis. We consider several types of ideal absorbers where absorption of one photon can produce more than one electron-hole pair that are based on semiconductor quantum dots with efficient multiple exciton generation (MEG) or molecules that undergo efficient singlet fission (SF). Using a detailed balance model with 1 sun AM1.5G illumination, we find that for single gap photovoltaic (PV) devices the maximum efficiency increases from 33.7% for cells with no carrier multiplication to 44.4% for cells with carrier multiplication. We also find that the maximum efficiency of an ideal two gap tandem PV device increases from 45.7% to 47.7% when carrier multiplication absorbers are used in the top and bottom cells. For an ideal water electrolysis two gap tandem device, the maximum conversion efficiency is 46.0% using...

Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers

Tabulated values of the Shockley–Queisser limit for single junction solar cells

The fundamental (detailed balance) limit of the performance of a tandem structure is presented. The model takes into account the fact that a particular cell is not only illuminated by part of the solar irradiance but also by the electroluminescence of other cells of the set. Whereas, under 1 sun irradiance, a single solar cell only converts 30% of the solar energy, a tandem structure of two cells can convert 42%, a tandem structure of three cells can convert 49%, etc. Under the highest possible light concentration, these efficiencies are 40% (one cell), 55% (two cells), 63% (three cells), etc. The model also allows us to predict the ideal efficiency of a stack with an infinite number of solar cells. Such a tandem system can convert 68% of the unconcentrated sunlight, and 86% of the concentrated sunlight.

https://iopscience.iop.org/article/10.1088/0022-3727/13/5/018/pdf

Detailed balance limit of the efficiency of tandem solar cells

Endoreversible engines are heat engines where possible irreversibilities can only take place in the heat transport and not in the conversion of the heat to power. It is shown that such engines can profitably be described by the current-efficiency characteristic, i.e. the relation between the rate of heat transport and the efficiency of heat conversion. The curve displays a point where the inscribed rectangle has maximum surface area. Under this condition of operation, the engine delivers its maximum power. At such a maximum-power condition the endoreversible engine operates in an irreversible way and the conversion efficiency is lower than the Carnot efficiency.

http://iopscience.iop.org/article/10.1088/0022-3727/20/2/014/pdf

Reflections on the power delivered by endoreversible engines

The formula for the maximum efficiency of photovoltaic energy conversion in tandem solar cells is discussed This formula is obtained by the use of a thermodynamically founded diode I-V characteristic, and is compared to a previously published formula which was based on the classical diode I-V characteristic within the framework of the theory of Shockley and Queisser (1961) Series expansions of the complicated efficiency formula are presented They enable an easy and accurate computation of the maximum efficiency and provide more insight in the basic limit of the photovoltaic effect

http://iopscience.iop.org/article/10.1088/0022-3727/15/10/019/pdf

On the formula for the upper limit of photovoltaic solar energy conversion efficiency

The director orientation of the ferroelectric liquid crystal molecules within a surface-stabilised is measured by means of Diamant capacitance bridge. It turns out that the polarisation versus voltage curve has a strongly frequency-dependent hysteresis loop. For example, the width of the hysteresis loop (i.e. the coercive voltage) increases with increasing frequency f like f12/, over a large range of frequencies. A simple mathematical model, which considers the director orientation to be uniform throughout the display, explains these experimental facts.

https://iopscience.iop.org/article/10.1088/0022-3727/22/10/014/pdf

Polarisation against voltage hysteresis loop of surface-stabilised ferroelectric liquid crystals

The efficiency of solar energy conversion in a photovoltaic device consisting of an infinite number of solar cells is examined in detail. A mathematical analysis of the complicated functional dependence of the efficiency on temperature and light concentration ratio is presented. Although an infinite tandem seems, at first sight, only of academic interest, it is shown that the various approximate expressions obtained for the complicated efficiency function can give some more insight into the basic limits of the photovoltaic conversion, and can enable easy predictions of an upper limit of the efficiency of tandem structures with a finite number of solar cells.

https://iopscience.iop.org/article/10.1088/0022-3727/14/5/020/pdf

A De Vos

Papers

Detailed balance limit of the efficiency of tandem solar cells

Reflections on the power delivered by endoreversible engines

On the formula for the upper limit of photovoltaic solar energy conversion efficiency

Polarisation against voltage hysteresis loop of surface-stabilised ferroelectric liquid crystals

On the upper limit of the energy conversion efficiency in tandem solar cells