## 1 Introduction  
The carrier selection property of the amorphous/crystalline silicon (a-Si:H/c-Si) hetero-junction has been demonstrated for the first time more than three decades ago on npn bipolar junction transistors, which showed enhanced a-Si:H emitter efficiency and current gain [1, 2]. Such a property has then been exploited later to reduce the dark current and increase the open circuit voltage (\(V_{\mathrm{OC}}\)) of the a-Si:H/c-Si heterojunction solar (SHJ) cell [3, 4]. Extensive research and development have been devoted to the design and technology of such a cell, especially p\({}^{+}\) doped a-Si:H(p\({}^{+}\)) on n-type c-Si substrate, which led to a continuous improvement in the cell efficiency [5, 6] to presently exceed 26% [7]. The superior performance of the SHJ cell is mainly the result of the reduced saturation current of the p-type inversion layer emitter induced in the n-type c-Si side of the a-Si:H/c-Si hetero-interface [8, 9]. This results in a strong boost in \(V_{\mathrm{OC}}\) to reach values close to 760 mV, which cannot be achieved in any other type of high efficiency c-Si cell. Several factors contribute to minimizing the emitter dark saturation current and hence to the \(V_{\mathrm{OC}}\) boost in the SHJ cell. Minimum interface recombination is ensured by inserting an ultrathin intrinsic a-Si:H(i) buffer layer between the a-Si:H(p\({}^{+}\)) layer and the c-Si substrate [3, 4]. In addition, band-gap narrowing in the heavily populated inversion layer is expected to be less significant than in doped layers. Band-gap widening has even been reported in strong inversion layers due to carrier quantum confinement [10; 11], which results in a much smaller intrinsic carrier concentration in the p\({}^{+}\) inversion layer compared to heavily doped emitters. Finally, bulk recombination in the very thin inversion layer emitter is expected to be negligible. All these factors are usually lumped in the statement: \"excellent passivation property of the a-Si:H contact\" commonly used to justify the very small emitter saturation current of SHJ cells. It has been shown that the stronger the inversion, the better the cell performance [8], which means that the equilibrium band bending at the hetero-interface and consequently the difference between the work function of a-Si:H and that of c-Si should be large. Therefore, maximum free hole concentration in the p-type doped a-Si:H(p\({}^{+}\)) layer should be ensured. On the other hand, low a-Si:H doping levels or significant depletion of the layer results in a weak inversion layer with degraded cell performance [8; 12]. It has also been shown that using non-appropriate material with a relatively small work function for the Transparent Conducting Oxide (TCO) used at the front contact results in a front Schottky rather than ohmic contact [12; 13; 14; 15; 16; 17; 18; 19], which leads to further depletion of the Si:H(p\({}^{+}\)) layer and hence to weak inversion which substantially degrades the cell \(V_{OC}\) and \(FF\), as detailed in [12]. Therefore, extensive research is being carried out exploring novel TCO materials that are transparent, conducting, and having an adequately large work function, e.g., [20; 21; 22; 23]. In addition to a high \(V_{OC}\), and apart from slight optical absorption loss in the very thin TCO and a-Si:H layers, there are no fundamental constraints that limit the short circuit current, \(I_{SC}\), in SHJ cells to values smaller than that achieved in high efficiency c-Si cells. Such values could be achieved by optimizing the cell technology, design, and using proper materials. Optical absorption in SHJ cells is minimized by making the a-Si:H layer ultrathin and, as mentioned earlier, by selecting an appropriate TCO material. In general, values of \(I_{SC}\) comparable to those of high efficiency c-Si cells are steadily reached in present SHJ cells.