Design of thin film solar cells based on a unified simple analytical model

Abstract

Polycrystalline thin film solar cells made with absorber materials such as CdTe, CIGS, CZTS and metalorganic halides (perovskites) are currently important alternatives for the silicon solar cell technology, which still dominates the photovoltaic market. Then, it is important to have tools which can be used to design this kind of solar cells. For this purpose, we have developed a unified simple analytical model that can be applied to thin film solar cells. The model is based on the basic physics of hetero-junction devices, but it takes into consideration that the space charge region can extend along the major part of the cell length, particularly for very thin cells, causing important effects that typically are not observed in conventional junction devices. Photo-generated carriers are collected by electric field-drift instead of diffusion, and simultaneously strong recombination at this region may dominate the electrical I-V characteristic of the cell. Since the space-charge region width varies with the applied voltage, the illumination current density and the saturation dark current density are no longer independent of the voltage as is assumed for conventional solar cells. When the model is applied to CIS and CdTe solar cells as examples, it is found that it is possible to design very thin film solar cells (absorber less than 1 m thick) with high efficiencies, whenever the recombination velocity at the back surface becomes small (10 2 cm/s), instead of the high recombination velocities present at ohmic contacts (10 7 cm/s). This fact implies the cost reduction of thin film solar cells by reducing absorber material thickness, and therefore it poses a challenge to develop deposition methods for very thin CdTe and CIGS absorber materials without pinholes, so that improved efficiencies are obtained when the surface recombination velocity is made small at the back by having a p+ or an electron blocking region before the ohmic contact. This result also explains the high efficiencies achieved by very thin perovskite solar cells.

Analysis of the performance of InxGa1−xN based solar cells

“…The model includes the voltage dependence of the depletion region (W) and the effect of this on both the illuminated and dark current components. In very thin solar cells, such as the ones to be simulated here, these effects are very important and should be considered using a complete analytical model previously developed by our group [18], as explained below.…”

“…The following equations, from 11 to 15, were taken from references [18,23]. The photocurrent density J L is expressed by the following equation.…”

“…The well-known saturation current due to diffusion (J 0 ) is given in Ref. [18], and mainly depends on Eg and the diffusion lengths for the n and p type materials. The reverse saturation current due to recombination in the space-charge region is expressed by Eq.…”

Ultra‐Thin Perovskite Solar Cells Analytical Model Involving Radiative and Nonradiative Carrier Recombination Mechanisms

“…In other words, for thin film solar cells, a more complete model must be used, instead of the ideal text‐book model used for thick cells. Recently, taking these aspects into consideration, an analytical model has been developed and used for simulating CdTe, CIS (CuInSe 2 ), and CIGS (CuInGaSe 2 ) tandem solar cells by the author and his collaborators …”

“…Recently, taking these aspects into consideration, an analytical model has been developed and used for simulating CdTe, CIS (CuInSe 2 ), and CIGS (CuInGaSe 2 ) tandem solar cells by the author and his collaborators. [11,12] PSCs are intrinsically very thin film solar cells; therefore, the aspects discussed earlier for thin film solar cells are very important to understand their behavior. It is shown later that for extremely thin solar cells, a simplified model can be used to calculate the I-V characteristics of solar cells such as the PSCs.…”