编子函数：求解二阶微分方程后求导取值 subs函数运行部通，高手请进
 

function [JJ,EQE1]=Jphoton(GG1)
QQ1=1e10
c=299792458 % velocity of light
l1=10e-9 % exction diffusion length of CuPc is 10nm
l2=20e-9 % exction diffusion length of C60 is 20nm
Qe=1 % quantum efficiency of the exction generation
t=1e-9 % lifetime of exction approximately is 1ns
s=88.54187818e-13 % vacuum permittivity
y=dsolve('l1*D2y-y+t*GG1=0','y(20e-9)=0,y(0)=0','x')


% the short-circuit extion current density at the interface x=xd/a
%计算光电流，外量子效率子函数

s1=1 % s1 is the efficiency of extion dissociation at the interface
s2=1 % s2 is the efficiency of charge collection at the electrode
q=1.6e-19 % the unit of quantity of electricity
Jphoton=-q*s1*s2*l1*l1*diff(y)/t
x=20e-9
JJ=subs(Jphoton)
EQE1=2*JJ/q/c/s

end

报错
??? Error using ==> sym.subs
Elements of the substitution cell array must be of the same size.



GG1是输入的数组，要计算出JJ,EQE1

如何改下比较好？

十分感谢