Stability and Efficiency in Amorphous Tandem Solar Cells
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STABILITY AND EFFICIENCY IN AMORPHOUS TANDEM SOLAR CELLS F.Galluzzi*, G. Conte*, L. De Angelis* and R. Peruzzi** ENIRICERCHE S.p.A 00015 Monterotondo (Roma), Italy ** PRAGMA S.p.A, - Via D'Andrea, 6, 00048 Nettuno (Roma),
*
Italy
ABSTRACT Theoretical evaluation of stability and efficiency of two-junction two-terminal amorphous tandem solar cells is performed using an analytical model which takes into account not only optical characteristics and transport properties but also photodegradation effects due to dangling bond formation under light exposure. Requirements for material quality and device design are particularly investigated. INTRODUCTION Stability on light exposure appears as a major problem for a widespread use of amorphous silicon solar cells. Recent results [1] show that recombination processes of photogenerated carriers are mainly responsible for degradation effects and it has been therefore suggested [2,3] that an increase in collection efficiency and a lowering in generation rate could give much higher stability. Multijunction structures are particularly suitable of this purpose [41 since reduction of single cell thicknesses allows to decrease cell illumination and recombination processes, maintaining at the same time high efficiency values. Here we present a model study of photovoltaic performance and photostability in two-junction two-terminal amorphous tandem cells, considering in particular the role of material quality and device design. CARRIER LIFETIME PHOTODEGRADATION Minority carrier lifetime T in a-Si:H and related alloys are mainly determined by recombination processes at dangling bond (DB) sites: T = 1/K N
(1)
r where K is a proper recombination coefficient and N is the DB concetration. Light exposure induces the formation of new DB centers and - according to the simple model of Smith et al. [2] - their generation rate is related to an electron-hole recombination process: dN/dt!K g p n
Mat. Res- Soc. Symp. Proc. Vol- 70. ý 1986 Materials Research Society
(2)
606
where K is a generation coefficient and n, p are electron and hole concentrAtions respectively. On the other hand, in steady state condition, average minority carrier concentrations in a p-i-n cell can be approximated by n G/(1/Tn + Kc (n) +K G( p +c "pG/(0/Tp
(P))
where G is the carrier photogeneration rate and the coefficients KC are expressly introduced for taking into account carrier collection processes. As I/K c is essentially the average drift transit time across the cell, we assume Kc= 4
• (Vb - U)/d
(4)
where 1 is the carrier mobility, V the buil-in potential, U the voltage drop across the cell, d the i-layer Phickness of the cell. From eqs. (l)-(3) we obtain the time-dependent behaviour for carrier lifetimes under light exposure. Assuming for simplicity equal mobilities and lifetimes for electrons and holes, we have: I/T(t) = [(I/T
+ Kc )3 +G 2 t] 1 / 3 -K
(5)
where T is the initial non degraded lifetime value and X= 3K Kr According to eqs. (4), (5) lifetime photodegradation is dependent not only on exposu
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