H.J. Pauwels

On the thermodynamic limit of photovoltaic energy conversion

An infinite stack ofp—n junctions with smoothly varying bandgap from ∞ to 0 is considered. AnI —V characteristic is derived, which is more correct than the classical exponential characteristic. It is shown that open-circuit operation is a reversible process and leads to the Carnot efficiency, if one defines the efficiency in the way that is usual in the theory of thermodynamic engines. If instead one uses the definition of efficiency usual in photovoltaics, open-circuit mode gives rise to zero efficiency. Then operation at maximum efficiency equals operation at maximum power and is not reversible.

Determination of the maximum efficiency solar cell structure

Abstract A very general formulation of a semiconducting photovoltaic device is given. The efficiency of this device is maximized with respect to tis structural parameters. In this way it is shown that no solar cell structure can achieve a large efficiency than an infinite tandem cell, with bandgaps monotonically decreasing from infinity to zero, and of which each individual cell is a selective black body with its own appropriate bias voltage. This maximum efficiency is numerically calculated for various cell- and sun temperatures and concentration factors.

Comment on a thermodynamical paradox presented by P. Wurfel

For original paper see ibid., vol.15, p.3967 (1982). Recently, P. Wurfel presented a paradox, where an engine converts heat into work with an efficiency of 25%, although it functions between two heat reservoirs at identical temperatures. It is shown in the present paper that the paradox results from neglecting the entropy creation at surfaces of bodies that emit radiation without at the same time absorbing an equal amount of radiation. By correctly taking into account the entropy creation, it is shown that Wurfels engine has an efficiency of 0%, i.e. cannot produce work in a stationary (or in a cyclic) process. The opportunity is taken to extend the model to a wider field of applications, resulting in valuable formulae, e.g. the maximum efficiency of the conversion of solar energy into work by use of a single solar collector.

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

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.

Theoretical influence of surface states and bulk traps on thin film transistor characteristics

Abstract It is shown that the theoretical derivation of drain current versus drain voltage characteristics of thin film transistors (TFT) is obtained, in the gradual channel approximation, by integration of the conduction electron charge versus gate voltage curve. The influence of bulk traps and surface states on these curves is evaluated quantitatively by using specific models. It is shown that at high donor concentrations, bulk traps have much more influence on TFT characteristics and even make it difficult to obtain good characteristics. At low donor concentrations, surface states have more influence but still allow good characteristics except at unusually high concentrations. Finally, the importance is shown of measurements of the channel conductance at low drain voltage as a function of the gate voltage for gaining insight in the physical parameters that determine TFT behavior.

Collection efficiency of heterojunction solar cells

The dependence of the generation efficiency<tex>k</tex>, the collection efficiency<tex>Q</tex>, and the current efficiency<tex>\sigma = kQ</tex>of a heterojunction solar cell on the two semiconductor bandgaps<tex>E_{g1}</tex>and<tex>E_{g2}</tex>is investigated. It is assumed that the two semiconductors have ideal absorption characteristics. It is shown that a heterojunction solar cell can exhibit a better current efficiency σ than a homojunction solar cell, if the semiconductor with the larger bandgap is situated at the illuminated side of the junction.

Analysis of generation in space charge regions of solar cells

A mathematical formulation of carrier transport in space charge regions in the presence of photoexcitation is presented. The analysis is only useful if bulk recombination can be neglected or is an a priori known function of position. The physical insight offered by this formulation is illustrated for the case of carrier transport in the space charge region of a heterojunction, and in a surface field region.

Influence of an insulating layer on the efficiency of a semiconductor-insulator-semiductor (SIS) heterojunction solar cell

Abstract By theoretical analysis it is shown that inserting a thin insulating layer between the two semiconductors of a p + n solar cell, can improve the efficiency in the same way as for an MIS solar cell, if the discontinuity in the conduction bands constitutes a barrier for photon excited electrons. In the other cases, the insulating layer either plays no role or is disadvantageous.

Model for the frequency-dependence of the capacitance in CdSCu2S solar cells

Abstract It is shown, theoretically and experimentally, that deep donor levels play an important part in the structure of the space charge region of CdSCu 2 S solar cells. Both in the dark and under illumination, they are responsible for the frequency dependence of the capacitance, although their role in the compensation mechanism of diffused Cu-acceptors is different in both situations. These levels lie at about 0.5 eV under the conduction band, they show a spread in energy of about 0.2 eV, and have response times in the range of 10 −2 to 10 −6 sec.

The influence of tunneling effects on the efficiency of heterojunction solar cells

A theoretical model is developed which presents the transport properties through the space charge region of ap+n heterojunction solar cell, whereby not only recombination through interface states but also tunneling through potential barriers is taken into account. It is investigated whether tunneling can give rise to optimum heterojunction structures which have better efficiencies that without tunneling. It is found that only if the strongly doped semiconductor has an optimum bandgap and the weakly doped semiconductor a larger bandgap, tunneling can make the structure optimum. In all other cases of optimum structures, tunneling deteriorates the efficiency.